(CLR) and (CE) questions, observations

Footy Class General Discussion (Footy)
1

I realize that for some of you, this will be another “Oh no, not the same questions Again!”, but bear with me. For those of you that have spent your lives in this, you know more than I will ever know and you should know that I will never want to know All that you already know. I want to know enough to be ablt to start building a model from scratch and have it come out right. I spent my life in Plastics Injection Moulding. I know it very well. I always enjoyed teaching someone who did not know. Even when the question was one that had been asked by a hundered others. So I know that some of you must still enjoy teaching newbies, to sailing, like me, who don’t know anything about sailing or RC…

Center of Lateral Resistance (CLR) or (Center of the Lateral Plane, CLP)

I believe I understand what it is, but the method and the variables seem to get me in trouble. The more I read, the more I see that lots of people disagree.

1.- Can it be tested on our little model sailboats? How can it be done Accurately? I have read that it can’t be done without lots of math calculations and software.

2.- If it can be done, doesn’t it have to be done with the keel and rudder in place? If those aren’t in place, how can the resistance be measured accurately? If so, do you have to be able to “temporarily” attach the keel so you can move it for testing it’s placement and how it affects the hull?

3.- Also dosen’t it have to be done with the correct “running weight” in the hull, including servos, batteries, sails, mast, boom, fittings. etc.? So would it be necessary to know all those weights and lay ballast approximately in the same locations in the hull?

4.- I also read that in heavy weather, the CLR will change as each wave comes and the ship’s displacement changes. Also it changes when the sails are full and the ship heels over from the force of the wind/sails. Definitely the hull below the waterline is always changing as the ship moves, so how do you account for that?

I ask because for each question, I am finding many answers. Some say you can't do it on your own. Some say you can, some say just to use the middle of the boat, some say make a pattern of the hull at waterline and find the center of the pattern using a needle for balance??? Some say it takes math and software that the average person can never do.
I see that some of the suggestions I have read, are totally wacked, even for a newbie/novice like me. Also I can see that I might not ever be able to really tell for myself, what the real CLR for any given boat will be, unless I find answers that I can understand and relate to.
I would appreciate your answers on how You find out what the CLR is for your sailboats, or do you check?  I could see that just putting on a keel about mid-point of the hull and makeing a "mast box" with a bunch of mast holes in it, could work by blind luck, just by moving the mast around enough to work, but that isn't good enough, right?

Center of Effort

I understand what it is, but how do you measure it accurately for any sail, for any type of sail (what is the proper method)?

1.- Does the Center of Effort in a sail not change in relation to the ship’s CLR as the sail is filled and the boom swings out? Is it necessary to take that into consideration when figuring out mast location?.

2.- I have read that the sail’s CE must be foreward of the ship’s CLR. I have also read that it must be aft of the ship’s CLR. What it the correct way and how much distance from the CLR? I have read anything from 2% to 12% from the CLR, or does that change with sail design as well?

I would appreciate in learning how You do your figuring for the relation of CE to CLR. I read enough to see that if this is not done correctly, it can really mess up the way the ship handles in the water and can turn a pleasureable hobby into a frustrating one. I prefer to "do it the right way first", before I mess it up, because I am not knowledgeable about ships, sails or water at all (except to drink and wash), so if I do mess it up, I won't know what I did wrong nor how to fix it. That's why I ask so many questions and I do so much research beforehand. I tend to beat a subject to death, so that when I start a project, I am fairly certain that I will be able to produce something that works properly the first time. If I cannot find acceptable answers, I don't take on the project, untill I do find them somewhere. That comes of my Production Management Philosophy from too many years in the plastics industry. 
I know some take the approach of try it and see what happens, but I have always believed in research first, answers beforehand and results that come out with less of "Murphy's Laws" at play during the construction. [u]Plus on a very limited budget, one has to figure it out first, to save on costs....[/u]

Thank you all in advance for helping solve these questions for me, in a way that I can understand.

Justin

2

Good question Justin and well couched.
I’m glad some one else is confused ~ Lets see what the panel have to say?

3

There is a lot of theory wrapped around this, but what the theory tends to leave out is that it’s not an exact science, because the CLR is not static and moves forward as the boat starts to move. (The C of E also moves, but in les predictable ways.) In boats with an essentially long chord, rectanguar, underwater profile the movement can be considerable. This is why 19th century plank on edge cutters have such long bowsprits to get somw sail forward to balance the movement of the CLR. In modern hull forms, with most of the lateral area concentrated in the fin, the movement is very small, typically from the centre of the fin to somewhere near the leading edge.

My practice is not to bother too much about theory and calculation but to determine the CLR by practical test. In still water, push te hull sideways, making several attempts until you find the position where the hull will move sideways without swinging the bow or stern toward you. This is the static CLR. This easy to do in the bath with a footy, but more difficult with an A boat.

Get over the C of E problem by making provision for the rig to be moved fore and aft. In the days of free sailing with Braine gear, boats went to windward with the rudder central and very fine adjustment of the C of E/CLR relationship was essential to get the best out of a boat. All serious models had a mast slide which permitted the whole rig to be moved by 1/16 steps over a range of a couple of inches or more. There was also provision to alter the mast rake. In a footy with a Macrig a range of mast sockets will serve the same purpose and avoid the need to calculate C of E positions for a whole range of rigs. A small amount of trial and error is worth a great deal of calculation. Similar considerations apply when creating a rig for an old hull. make something quick and dirty from old bin liners, sail it and chop it about on the pond side until the boat balances. THEN make a decent suit of sails.

Russell

4

Hi Justin

I have raced 12’ to 20’ boats for almost 40 yrs and as you sail a different boat, rigging (mast rake & position), sail tensions, crew position, and centerboard position are adjusted to achieve a small amount on windward helm (boat wants to turn upwind slightly). You then sail side by side w/like boats and continue to adjust for improved performance and talk to other skippers that sail faster for suggested improvements. There are a ton of variables and getting them all right takes years.

With regard to building your first boat (I built my 1st one in May), review and pick one of the free plans (Papaya, Cobra, Razor, Bob About or Harpy) which has the most complete instructions and build and start sailing it or better yet, start racing it.

From your post, it sounds like you have a basic understanding of the variables. If you decide to build a balsa Footy, it is fairly cheap to modify or build another hull (I built my 2nd hull in June). Even with the plans (sometimes not very detailed), I had a lot of questions for readers of this site. The second boat went much faster and turned out much better & had fewer questions., but I continue to read and learn and make new friends from this forum.

I do recommend going with a simpler sail rig (McRig etc…) vs. main & jib type. Good Luck.

5

It is also helpful to talk to skippers at a club in your area. It looks like the following may be close:

Tyler East Texas Area
East Texas MYC (#35)
Bruce Simmons, 903-825-6199
218 South Bay Drive
Bullard, Texas 75757
Sailing Site: Emerald Bay Club on Lake Palestine. Located 15 miles South West of Tyler.
Classes Sailed: Star 45 and IOM.

Check www.modelyacht.org for complete club listings.

Members sometimes will hand you the controls for a test sail.

6 
Do you test the CLR using the boat without keel or rudder? I would guess obviously without keel, because you wouldn't know where the keel was to be located yet, but what about the rudder?

Thanks for your answer…
Justin

Frank,
Thanks for the information about the club nearby. I haven’t looked at the AMYA site for clubs near here. I might try talking to them…

7

Hi Again

The CLR test needs to be performed with the hull assembled w/keel, ballast and rudder the heavier items like batteries and servos.

Most skippers use the soft eraser end of a pencil to find the point where it moves sideways. Push to fare or aft and boat turns.

Rudder needs to be fixed so it does not rotate. The rigging weight will not change CLR very much, so probably is not needed.

8

Sorry, I’m stumbling here. I thought a big part of the reason for CLR was to establish the actual location of the Keel. If the Keel is already in place, wouldn’t that state the CLR was already established?? I guess I’m not following the logic here and I must be missing a step. If I build a hull from scratch (not from a plan), how would I determine the CLR the first time, so I knew where the Keel should be??

Thank you,

9

Hi Justin -

I will jump in here as the “odd guy” - only for comparison reasons and to also create more confusion for you ! :smiley:

First off - the CLR as you already note is subject to argument and confusion. For every one who says a keel must be in place, I would counter with the simple question - "What about beach cats with asymmetrical hulls and NO daggerboard (keel) ? " Certainly they need to have a known CLR in order to position the front crossbeam on which to mount the mast. In a monohull model, one has the “ability” to some extent to shift the maast fore and aft to gain acceptable helm. As HistoryMan notes, (I think it was he) the ability to move mast and it’s CE in relationship with the CLR will provide weather or lee helm.

Back to a cat without keel - and keep in mind a lot of design software will offer predictions of where to place the keel - the entire side of the hull provides the lateral resistance. Pushing sideways is a quick way to find general location. Now on a cat, without the rudders in the “down” position, the boat will exhibit a tremendous amount of weatherhelm which at fist blush would indicate the mast location is too far aft. Drop the rudders into the down sailing position, you (as you guessed) hav added more lateral resistance to rear of boat (which - in a sense - “seems” to have move the mast forward) and the boat exhibits much less windward helm. Also very easily noted in a cat configuration, is the fact that when the main is sheeted in tightly, the boat will exhibit more windward helm. Tighten up the jib and the boat will again balance it’s helm witht he amount of windward helm either desired or designed in the boat. Reef a main and leave jib full and weather helm can be replaced with lee helm. Thus the need for a smaller jib if reducing size of mainsail area projected to wind.

I think this will give you an idea of why everyone says it isn’t an exact science and why answers can vary.

I would suggest that the rudder be down for your CLR test, and all you are really doing is spotting the “approximate location” for the keel. Thus the suggestion to allow for mast movement to compensate if your CLR point is off slightly.

Hope this doesn’t create more questions or concerns. By the way - CE ( center of effort) is usally calculated with both main and jib in a close hauled position with wind expected to be at right angles. Once you start sailing, you introduce apparant wind, and either sailing windward, or sailing off the wind will shift your CE greatly.

I’ll let the dinghy/Laser sailors introduce the concept of heeling boat/sail to windward when sailing downwind. “THAT” concept is another fun one to grasp.

Regards -

10

I have been yacht racing for over 40 years, and have won more than my share of races. I have never worried about CLR or CE or any of the other initials.

The important thing is the boat’s balance (or helm). It should have a slight weather helm. If it has lee helm, move the rig back ( or rake aft). If it has too much weather helm, move the rig forward or reduce rake.

That’s all you need to know to get started.

Think of CLR and CE as two sides of a teeter-totter. The idea is to move the fulcrum (the rig) until you get balance. The problem with these factors is that they are dynamic and change with angle of heel and changes in fore/aft trim and even with speed and waves. So don’t get lost in the details.

11

Justin! I started writing this just after you first posted. The replies that have appeared since assume that you already have a boat. As I suggest below, the right answer in that case is ‘suck it and see’. However, this assumes that the designer has already done a reasonable job of getting everything in the right place. Hence I port this without reference to anything posted between this and your initial post, except to say that, given where they are coming from, I don’t seriously disagree with any of them.

OK then!

First, have you heard the one about the pure mathematician, the statistician, the physicist and the engineer discussing the number of pints of beer on the table in front of them. The mathematician said ‘There is (integer) one pint on the table.’ The statistician said, ‘There may be any amount of beer, but there is a 99% probability that there is one pint.’ The physicist said, ‘There are between 0.95 and 1.05 pints on the table. The engineer said ‘Well, it’s about two, so we’ll call it four just to be on the safe side’.

The point is, why do you want to know? As you very correctly, point out, the centre of EVERYTHING changes on an encounter with a large wave (if the centre of gravity changes by much, it’s time to call for a helicopter: something just broke!). But just why do you want to know? What are you going to do about it if you do know that, for a fleeting moment, the centre of anything is in a particular position? Answer – nothing. Centres of lateral resistance and effort are drawing board tools and like most of their breed are empirical approximations. The trick is to understand why and in what way they are approximate.

I will interrupt myself here. The reason why we use things is to achieve helm balance. If we get this even slightly wrong (as we almost certainly will), we can modify the foils or the rig. This is most easily done by making the position of the rig and/or foils easily adjustable. Adjust and go sailing, adjust and go sailing until it is right. This will involve much less time (and error) than some incredible calculation.

Back to the main thread: Centre of Lateral Plane (or Centre of Lateral Area). This is not the same thing as the Centre of Lateral Resistance – although many people treat it as if it was, from which much confusion arises. CLP is precisely that. It’s the geometric centre of area of the lateral plane. You can determine this by balancing a cut-out of the underwater profile of the boat on a pin (a knife edge is actually much easier if all you want is the vertical plane on which the point lies). Alternatively, you can do it by dangling your cut-out and a plumb bob from a pin placed in various locations. Record the line taken by the plumb bob. Where all the lines cross in the middle is the position of the CLP. Finally you can get the fore-and-aft position by calculation using Simpson’s rule (this is junior high school maths suitable for pencil and paper). Because Simpson’s rule (itself an approximation) assumes shapes with reasonably, even contours – not a Footy with a dagger of a fin and a dagger of a rudder – I do not recommend using it at this point.

So where does this get us? Well, it gives us a more or less inaccurate proxy of the centre of lateral resistance – a very different number. This is the point through which the side force generated by the hull acts so as to oppose the sideforce generated by the rig. These are dynamic, not static. However, the relationship of CE directly above CLR must be EXACTLY maintained at all times, otherwise the boat will start to turn. The adjustment is made by trimming the sails or by adjusting the position of the rudder (i.e. steering). Since the CLR is dynamic and we already have the means to adjust it, there is no point in getting too carried away trying to locate it precisely. However, the better the ‘average guess’ we can arrive at, the less steering input we will need (less drag). So how do we do this?

Assume that we are talking about a normal model yacht with a salient bulb fin, deep draft and spade rudder. We can break the lift generated into components generated by four things – the hull itself (the ‘canoe body’), the fin, the rudder and the bulb. If the fin, rudder and bulb are reasonably standard aerofoil sections (or non-foil in the case of parallel-bodied low-lift slender bodies), their lift characteristics are easy to find you look them up as standard published data – just go browsing on the Internet for NACA sections and you will soon hit pay dirt. If they are not reasonably standard sections, they should be. This discussion assumes that things are being done reasonably properly using thought rather than the ‘whittle for victory’ school of design and, if not, I am causing you a degree of effort that is totally disproportionate to the effort required.

The published data has mostly been generated in big, expensive aerospace industry wind tunnels. It is pretty accurate. It also includes the position of the centre of lift at different angles of attack. Since you will inevitably be making some leeway, let’s go for an angle of attack of, say, 5 degrees. Find from the data on the section where the centre of lift is. Plot that on the foil and hey presto! If the fin/rudder is not parallel sided, interpolate.

We can do this for each of the ‘calculable’ elements – fin, rudder and bulb. We now need to find their areas in profile (planform if you were an aeroplane, since everything has been rotated through 90 degrees [usually!]). This can de done by very simple calculation (calculation of area of rectangle or trapezium, Simpson’s rule or – in extremis – counting squares on millimetre graph paper.

We are left with the canoe body. It is not practical to calculate where it’s centre of lateral resistance is, but we can probably use the centre of lateral area as a good proxy. This time the preferred method is to use Simpson’s rule and calculate as you would for the centre of buoyancy.

At this point we should have absolute lateral areas and absolute assumed centres of lateral resistance. We now have to put them together to get the CLR of the boat as a whole. This is where the bag of bones, the lucky rabbit’s foot, the black cat crossing the road and all the other things like that enter the picture. We have to assign a ‘weighting’ to each component. The fin is efficient at generating lift, so we will give it a coefficient of 1. The rudder is at least partly in the downwash of the rudder, so it will be less efficient. Let us give it a coefficient of 0.85. The bulb is difficult – the maths of endplates are fearsomely complicated but from rumour and myth we might guess 0.5. The hull is even harder – if for example it has a hard chine that digs in, the coefficient will be higher but let us take a guess at 0.3. So we multiply each area by its coefficient, to get a weighted area. We then take moments of the WEIGHTED areas about a convenient point (usually the forward waterline ending) to establish the calculated longitudinal position of the CLR.

Next thrilling instalment tomorrow: I got to fo to bed!

Cheers
:graduate::devil3::devil3::zbeer:

12

Angus -

I never got farther than 1 year of Basic Math in school. 2+2=4, after that I called someone else in to figure out what I needed to know. Since I was put into semi-retirement, I don’t have that someone to fall back on…

So what I’m saying is, you answered my big question, it’s more than I could figure out myself and I understand why so many times it’s more like “just make it and see how it works”… I will just make a mast box with several holes in it and see what happens… It’s way over my head. I’m sorry that you took so much time in working a response, but it’s way over my head. I do hope that you will find others reading it who will take more out of it than I will be able to and I do hope you understand my comments here.

I am in the process of building a Razor and I will use the information that I found on the website for Keel position and go from there.

I asked about all of this because my next try was going to be a sailboat that I draw up myself and I wanted to know how to figure out where the keel would go, but I think that in my position it might be better to just use plans from someone else and let it go at that… My ego says I want a boat that I can say I designed myself, but my common sense says that I do not have the skills, knowledge or the time left, in my lifetime, to do that… So a kit or a model designed by someone with the skills, will have to suffice. That’s OK, I just needed to understand what it really takes.

What I really take away with much of this is that it’s just a model sailboat, not a racing sailboat, with a crew and with millions tied up in it, so make what I can make, just try it and see what happens…

Yes? No?

Thank you,

13

Justin - if yor basic Math can get as far as 2 x 2 = 4, you ca manage Simpson’s rule! Look om Amazon for ‘Skene’s Elements of Yacht Design’. It sets out all the basic calulations in a ay that was very much designed for pencil-and-paper maths, If you’ll pm me yoir email adress, I’ll senfd you a draft of a translation of an old Norwegian book on modek yachts that might help.

:zbeer::zbeer::zbeer::zbeer:

14

even if a balance is a very complicated matter, there are two simple rules that - based on my experience - are a good starting point :

1 - on modern fin keeled boat center of effort is usually above center of fin keel ( or somewhere between keel leading edge, and fin geometric center )

2- distance between CE and CLR is usually not far from 10% boat lenght ( CLR considering hull+keel+rudder )

for practical use, center of sail plan can be found using - in a very simple manner - cutting a cardboard profile ( of sail plan or underwater lat plan ) and balancing it on a knife

no math, no equations, no computers needed !

last but no least for final tuning mast step and/or keel position should be made considering a possibility of longitudinal adjustment

and don’t forget that good balance is also a matter of a nicely shaped hull, not only a matter of centers

Flavio

Folgore ITA 5

15

jusval,

What the experts say about CLR, CLP and CofE or CofA moving when the boat is underway is no doubt correct – but hey, we’re talking Footys here not Americas Cup boats so lets try to be a bit more rational. The explanations below are taken from replies I’ve published previously on the Forums. I will apologise up front if I end up giving you too much detail, but it is amazing what some people know and others don’t – so just bare with me. The trouble with long winded explanations is that they bore rigid those who either don’t want or can’t be bothered to stick with it. All I can say is that the following worked well for my first own design Footy. My first Footy, like yours, was a Bill Hagerup Razor.

For the purposes of explanation I chose to refer to CofE for the sails and CLR for the underwater parts. In particular they will always be purely geometric versions ( even where I don’t specifically say so ) with no account being taken of aerodynamic or fluid dynamic factors. I guess it’s a case of it not being mathematically correct, but it does give you what you are looking for.

The main reason for needing to know the positions of the CofE and CLR is to ensure that the boat is balanced with respect to the position of the effort from the sails and the lateral resistance of the hull, the fin and rudder. If the CofE from the sails is too far forward of the CLR of the hull then the boat will tend to turn away from the wind – Lee Helm – and if it is too far rearward of the CLR then the boat will tend to turn into the wind – Weather Helm. It is generally considered best to aim for a small amount of Weather Helm. You can, to some extent, correct poor balance using the rudder but this will slow you down. Also, if the imbalance is too great you will find difficulty in tacking.

Getting a rough estimate of the sail CofE is relatively simple, if not a bit laborious. An individual sail geometric CofE can be found by using a cardboard pattern of the sail ( 750 gram Cornflake packets are nice and big ) If the cardboard isn’t big enough just join two pieces with CA’d butt join. Then make pin holes at the main corners and hang the pattern from a convenient door edge. Drop a plumb line from the pin (a piece of string with a small weight or a straight piece of wire with a hook will do) and mark the bottom end on the card. Repeat this for all three corners. Join the bottom mark to its respective pin hole for each position and, if you’ve been accurate, all three lines will cross at a point - this is the geometric CofE. If you have two sails such as with a Swing Rig or a traditional Bermudan Rig then just repeat this process for the second sail. Don’t be tempted to assume that the jib and the main sail are just one big triangle as there will always be a gap between the jib leech and the mast and this will effectively move the CofE of the jib further forward.

You now need the area of the sails. There is no real shortcut here, you just have to use a bit of simple maths by splitting up the areas into triangles and rectangles and adding them all together. Triangle areas are (half the base times the height) and rectangles just the product of the two side lengths. Squared graph paper can be used for this, although an accurate drawing will work just as well.

So, for the sails you’ll now have two areas and two CofE positions. You can combine the 2 using a process known as Area Moments which is simply taking a known area and multiplying it by a distance from some arbitary datum. You will know 2 areas and 2 individual offsets. You also know the combined area by just adding the 2 individuals together. What you are looking to find is the effective offset of this combined area.

First decide on your datum point. This can be anywhere along the axial centreline of the hull – I usually use the bow point. Take the area of the jib sail and multiply it by the distance, or offset, of your jib CofE from this datum. Do the same for the main sail, but using its area and its offset from the same datum point. Add these two products together and then divide the result by the total sail area. The result of this will be the offset of the combined jib and main about the same datum.

So, if your jib area is 45 sq ins with its CofE 8.43 ins from your datum, and your main is 128 sq ins with its CofE 13.66 ins from the datum, then the combined jib + main CofE will be :-

((45x8.43)+(128x13.66))/(45+128) = 12.3 ins from the datum.

This is a lot more difficult to explain than it is to do – honestly !

The hull CLR can be found in a similar way. With an existing hull, draw the underwater shape by using a set square to set up the hull on its side. Mark the Water Line at the bow and stern and then just use the set square to plot the hull shape onto a piece of card. You could of course use the hull plan side elevation if it is available. The fin and rudder will more than likely approximate to simple rectangles. Then proceed as for the sails in order to combine the three parts.

The received wisdom suggests the sail CofE could be anything up to 20% of the hull length forward of the CLR – I will query Angus’ claim for it to be exactly above later. Flavio’s diagram is spot on.

The purists would say that both the sail CofE and the Hull CLR will change when the boat is underway. This may well be true, but both of these changes are almost impossible to define. I have found that my own Footys work reasonably well with the sail CofE being about 16% of hull length – i.e. 48 mm – forward of the geometric CLR calculated as described above. For small changes to a ready built boat you could try moving the mast position – if you have made allowance to do this – or change the sails to put more or less area forward or back according to how the boat moves. Axially longer rudders will have the desired effect but are best remade from scratch, but small additions to the fin can be made by adding plastic card strips epoxied to the trailing edge. It is best not to reduce the chord of the fin to less than 40 mm or increase it to greater than 60 mm else you could have problems tacking in a decent breeze.

A finished boat can be measured in a water tank, but it must have its rudder and fin fitted. It doesn’t necessarily have to be fully fitted out inside, but it must have weights representative of the servos, batteries, receiver and sails in their correct axial positions and be sitting at its correct waterline. In this ecologically paranoid society of ours, using a full size bath takes an awful lot of water. I just use a small plastic central heating header tank and fill it up from the garden water butt – then tip it back in when I’m finished. ( Its something to do with being Welsh apparently). With a Footy set up like this, just poke it very gently sideways about 160 mm rear of the bow with a pencil or pointed stick. Keep repeating this at different positions until it moves bodily sideways with no tendancy to turn either one way or the other. Record this position as this is the CLR point. Done carefully I have found this agrees well with the fully theoretical method described earlier.

The advantage of using this technique is that you can mix and match the position of sails, fin and rudder to get the result you want, and I have found it to be remarkably accurate. But if this is to be your first boat build then I would strongly recommend starting with a known design such as Bill Hagerup’s Razor or Cobra, or the Kittewake K2 kit from Graham McAllister. You could then use these designs to verify the theoretical stuff I’ve just been talking about.

Finally, since changes to the sails are, at least on a Footy, the simplest way of adjusting the balance, it is a good idea to draw your sails in such a way that you can make alterations and know exactly what will happen to the CofE. You will be talking of only small changes of about 5 to 10 mm so you do need to be fairly accurate. I have mine set up in a spreadsheet – but that will have to be another thread.

P.S.

Both Angus and Flavio suggest balancing the cardboard cut-out sections on a knife edge or a pin. The trouble with both these methods is that they are both inherently unstable. It is far easier to use the hanging from a pin method outlined above, plus you get a check on your accuracy by examining the crossing of the lines.

I trust you are still awake. Stick with it, Footys are really fun and the racing competition can be intense.

Cheers,

firstfooty

16

Justin,
On a full size yacht, where an error on the part of the designer can be very costly, your questions become very complex and difficult to answer, but for the purposes of a Footy, you can reduce the issues to very basic ideas. All of this can be computed with math formulas from plane & solid geometry, but for simplicity, just consider the two concepts to be pretty much the same thing (centers of area), except that the CLR or CLP applies to all that is below the waterline, while the CE applies to what is above the waterline (for our simple purposes, only the sails are considered). As stated elsewhere, all of these numbers can change as the boat heels or enters into motion, but since the Footy is small, and we are just looking for approximate starting points anyway, we’ll just consider things as they are when the boat is sitting still, floating at its designed waterline. If you make some templates by cutting out a ‘shadow’ of the sail plan with all sails in proper position (or the hull below the waterline with all it’s appendages), from a single piece of rigid material, such as corrugated cardboard, you can find the approximate balance point of each template as described earlier, using a pin or the point of a sharp hobby knife. You can then transfer these points from your template to your drawing. Once this is done, you can adjust the position of the sails on your drawing so that their center of area is ahead of the underwater center of area by about 10-15% of the hull’s waterline length. This will give you the approximate starting location of the mast, but be sure to allow for an adjustment of at least 1/2" forward & aft on the first actual finished model. Once you have the model in water, you can refine the location by testing under various conditions. You can then make note of any changes on your drawing so that any future boats built from that plan will be easier to get right the first time.

Regards,
Bill Nielsen
Oakland Park, FL USA

17

Justin,
On a full size yacht, where an error on the pert of the designer can be very costly, your questions become very complex and difficult to answer, but for the purposes of a Footy, you can reduce the issues to very basic ideas. All of this can be computed with math formulas from plane & solid geometry, but for simplicity just consider the two concepts to be pretty much the same thing (centers of area), except that the CLR or CLP applies to all that is below the waterline, while the CE applies to what is above the waterline (for our simple purposes, only the sails are considered). As stated elsewhere, all of these numbers can change as the boat heels or enters into motion, but since the Footy is small, and we are just looking for approximate starting points anyway, we’ll just consider things as they are when the boat is not in motion, but is floating at its designed waterline. If you make some templates by cutting out a ‘shadow’ of the sail plan with all sails in proper position (or the hull below the waterline with all it’s appendages), from a single piece of rigid material, such as corrugated cardboard, you can find the approximate balance point of the template as described earlier, using a pin or the point of a sharp hobby knife. You can then transfer these points from your template to your drawing. Once this is done, you can adjust the position of the sails on your drawing so that their center of area is ahead of the underwater center of area by about 10-15% of the hull’s waterline length. This will give you the approximate starting location of the mast, but be sure to allow for an adjustment of at least 1/2" forward & aft on the first actual finished model. Once you have the model in water, you can refine the location by testing under various conditions. You can then make note of any changes on your drawing so that any future boats built from that plan will be easier to get right the first time.

Regards,
Bill Nielsen
Oakland Park, FL USA

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http://books.google.com/books?hl=en&id=rZbNvEWHbUoC&dq=‘Skene’s+Elements+of+Yacht+Design’&printsec=frontcover&source=web&ots=MMVjUtsj3s&sig=_9Cv-Xfrsql7YNDCbrjYL21dT2U&sa=X&oi=book_result&resnum=3&ct=result#PPP1,M1

It looks as if I can read Skene’s here, right online…

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Gentlemen,

After I take a few days to digest all of these posts, I will try to work on some of the suggestions. I need to do a lot of reading and thinking. I think that part of it is because, since I have never been near a sailboat, I don't have the necessary experience using one, to understand the principals. I can see that one really needs to be a "sailor" or at least have been sailing an "RC" before, enough to understand the reaction of these things firsthand.

I can see that the reality for the first time person is just like anything else. Find someone who knows and get them to teach you, using a pre-built boat and after you get proficient at it, then think about making your own.

I can visualize that even if I successfully make the Razor and stumble on a halfway decent build that floats and sails, I will still not know if it is right, because I have no actual experience in sailing and I won't "have a clue", what the boat is doing or why. I won't know if it's acting as it should at all.

Sort of putting the cart before the horse and I tend to do that. I have a room full of horses waiting for me to lead them around the other end of the carts;)

My personal opinion right now is that the best way for me would be to forego the build and look for the closest club to go talk to. Unfortunately that may not happen very soon. I tend to shy away from a lot of public contact since the stroke, so it may be a while. For sure, right now I need to rethink this and take a different approach, since I am really spinning my wheels here. I was originally going to do model aircraft but wasn't willing to go to the local club and deal with all of that, so I thought heck, how hard can a sailboat be?  I should have realized that it's like anything in life; it's more than meets the eye......   I do think maybe a store bought pre-built boat might be best, where I can just learn what I can on my own and see where it goes from there......

Thanks to all of you for all of your input. I will continue to read all of this and some books, to get some more knowledge of this…

20

Hi Justin,
I understand your desire to design your own Footy and the great feeling that a measure of success can bring. I would like to suggest that you do what is normal practice in model aircraft design, copy the important elements. That is, wing section, wing and tail areas, moment arm and incidence angles.

The footy equivalent would be to look at a known design such as one of the free plans. Copy the; below the waterline hull shape, fin position and area, rudder position and area, mast position, sail areas and distribution.

Now you can do pretty much anything to suit yourself with the hull above the waterline. You could deepen the fin or shorten it a little, change the shape of the rudder, round bottom, square edged, tapered… just keep the area close to the same. Add a cabin, play with the sail shape so long as you watch where the new shape is moving the centre of effort of the whole sailplan. Just have fun with it. Use ‘Harpy’ if you like, it was put out there as an experimenters hull so feel free to play with it.

An interesting thing with the several yacht design books I have on the shelf is that I initially thought that they would teach me how to design the perfect yacht. In reality they have taught me how to make calculations and understand better the yacht I have already designed. The core thing I understand now is that yacht design is an art rather than a science.

Graham