Mast Position ?

Rcsailing.net Technology Discussions
21

Hi Anders,
you are not an expert ! hum !!! but use technical and sound arguments !! heheh !!

If you read the first Alan introductory post at the second coma :

" I’m aware how aero (sail CE) the hydro (Hull/Fin CLR) dynamic forces work and how to find them using cardboard cut-outs of sail plan and BWL hull/Fin profile and pinning to a wall using pendulum effect to find their geometric centres, no calculations or science involved in this area thank goodness "

My understanding was : no “calculations” involved only observations between the “basic setting” and the sailing tuning !

More about can be taken from here : http://www.boatdesign.net/forums/sailboats/center-lateral-resistance-1898.html

About large size pictures, I use Imageshack, upload an image with the browser , select the size and collect the forum “code” url… when presented and “copy and paste” here in the text

Cheers
ClaudioD

22

Alan,

I was trying to say that information on where to put the mast and the fin as a “combination” (both slightly forward, or both aft etc) was very hard to find, whereas information about the position of the mast relative to the keel (“lead”) is well written about. (mast only forward or aft etc)

“I’ve heard this saying so many times I honestly cannot believe, it seems more like an excuse than an answer, until some-one can explain how this is possible of course ?”

I have made about 20 international “A class” hulls out of the same female mould, using the same keel and mast positions. Identical boats, identical keels and rudders, sail profile set by class rules. Sometimes the keel (or mast) needs to be moved slightly to get the boat balanced, and on a couple of occasions the keel was moved over 50mm to balance that set of sails (different sail shapes).

A full mainsail with a flat jib will produce more weather helm than a flat mainsail and a full jib. For the same depth mainsail, a more rounded lower aft section will produce more weather helm than a flatter aft section. Sail shape is part of the reason for on water tweaking to get the balance correct.

Jon

23

Hi Jon,
Your description is just another proof that each boat has his own caracteristics like people.

Sail form, shape, hull material elasticity, fine adjustments, are almost not significants or visibles to us but not for the wind and waves !
Too many variables that often escape from construction control and the final tuning is a sort of “identity card” of that boat as well could be the “Curve of Areas” or the “CF balance” !

Probably, it is my opinion, that manual laminations, local temperature and RH variations may play a role from one boat made in April and one made in August, producing different hull elasticity under wave pressure.

I’m not surprised that a large Class A may exibit different micro elasticity in the hull from one model to another compared then with a more “potentially sturdy” IOM hull.

That’s make sailing a fantastic sport as well at model level and again this is why the “ideal sailing boat” is not yet invented !

I have appreciated your post !

Cheers
ClaudioD

24

Hi Jon,

Thanks for clarifying “my misunderstanding” of what you were saying, the lead % mystery seems to be is the “guess-it-mate” factor for those of us not savvy with all the variables involved not to mention the mathematics involved, so we need to settle with the range of 6-12% when considering the position of basic centres out-lined in post # 3

Building twenty A class boats :icon_smil …Respect !, I’m not questioning your experience just saying there must be a reason for why exactly the same boats need different settings to sail the same, I can understand different sail plans having different CE positions that require moving the mast to maintain same helm balance.

Maybe the point here is that some of us confuse ourselves (as I do too often) when we look at the Fin/Bulb/Mast subjectively e.g The mast was moved forward or the fin had to moved back etc etc. when we should be thinking more about these things more objectively rather than subjectively by measuring and locating the "basic centres” involved and then coupling them so that they work into a well balanced overall sailing boat package.

Armed with this basic information and high degree of faith :rolleyes: I could understand how two boats of the same boat designs can have different Keel fin positions, but I would bet their “centres would be very close to being the same” … Claudio high-lighted this point perfectly in post # 15 , you can move fin position but the C.G needs to remain in the same place to maintain its same basic balanced performance.

Then again, maybe I’ve been living in Germany too long and finally gone native :help: with their engineering culture of high precision and quality control :magnify: that its reputation is built on. There is NO-WAY a German would/can accept that two of the same model Mercedes Benz cars can have different driving characteristics … Das ist nicht möglich !

Cheers Alan :zbeer:

25

This is interesting :rolleyes: leads me to the question, What is the best position to have the Keel/Rig coupling along the LWL ?

Cheers Alan

26

The method of hanging or balancing the sail shape produces a geometric CE, which is only an approximation for the aerodynamic CE.

The aerodynamic CE is quite moveable. The obvious case is changing the sheeting angle – to go downwind, let the sails out, and the aerodynamic CE moves forward and the boat should change direction. Conversely, sheet in, and the aerodynamic CE moves aft and the boat changes to weather. See diagrams below that show the sail profiles for centred, 45 degrees and 90 degrees. Changing mast rake or position obviously will have a similar effect of moving the geometric CE.

What is not so obvious and is mostly neglected, are changes to the aerodymic CE due to sail shape or trim. If we change the twist of the mainsail of a boat going upwind by increasing tension on the mainsheet, mast ram or in most cases by tensioning the vang, we would barely change the sail profile as shown on the left, hence barely move the geometric CE, but we would move the aerodynamic CE aft and increase weather helm. Similarly, if we made a fuller mainsail, we would again barely change the sail profile shown on the left, hence barely move the geometric CE, but we would move the aerodynamic CE aft and increase weather helm.

See http://en.wikipedia.org/wiki/Forces_on_sails

Similarly, the method of hanging or balancing the underwater shape of the hull will give an approximation for the hydrodynamic CLR.

If we have 2 identical underwater profiles of a hull, then obviously the “geometric” CLR’s are the same. However, if the hull shapes are different (one has hard flat sections aft, soft and rounded forward – the other soft and rounded aft, hard V forward) then I would think the swimming pool method of pushing/pulling the hull sideways (which again is only an approximation) would show a difference. See diagrams below.

If our boat is sailing forwards, then it is heeling. As our hull heels, the underwater shape usually changes, and the hydrodynamic CLR usually changes position also.

Therefore yacht designers who use both these “geometric” methods are just using approximations of the actual centres – hence “on water tuning” is often required.

27

Do you want it to go downwind well? forward.
Upwind well? aft.
Maybe better in rough conditions? possibly aft

Probably also depends on hull shape, heights of CE and CG, …

Jon

28

Hi Jon,
all correct what you say, but would be nice to see some practical figures derived from your considerations.

These are some additional points I personally use to find out a “preliminary” mast position for the sail plan considered.
Different sail plan = different mast position unless the CE remain at the same relative position (distance from back face of the mast)
My way to proceed is that the carton silhoutte take into acount the following aspects when the boat is tilted :

a) the hull immersed surface is reduced because of the roundness is less efficient against drift
b) I use only the projected dimensions for the fin and rudder
c) the Bulb is escluded for the same reasons as point a).

With the above exercice and pendulum search method, I fix the “static” CLR position. This is the only way I found to start from somewhere.
Books are also reporting that the “Lead” on real boats is a variable parameter based upon empirical approaches and observations and often we can read 6% to 8 % for keel boats and up to 12% for 3/4 sloop rig and separated rudder.
Based on that I can assume a wide range where the “lead” can vary from 6% to 12% as function of various parameters like mast height, beam width, fin/ rudder ratios , sail ratio, etc.

I also assume on my design drawings that the Fin surface shall be in the range of 5 to 6 % of the Sail area and the Rudder in the range of 2 to 2.5% of sail area.

The original graph of S Crepaz is indicating a LEAD varying from 10% to 20% of LWL. My extrapolation was estended for different BWL/LWL ratios as common on our model sizes. Thus, according to the above, the Beam is of paramount importance !
In the above diagramme we can observe 3 curves , the central one being the “expected nominal” and the lateral curves suggesting more headig up or bear away attitudes.

All above, I repeat, are “STATIC” conditions that allow the first attempt of the modeler to position the mast somewhere along the Central Datum line.

Having said that, I would very much appreciate Jon any comments/ critics or additional infos that may help to answer to the tread question : “mast position ?” with more numerical data if availables ! Thank you .

Cheers
ClaudioD

29

Claudio, I have one of those questions that no-one seems to ask, but I always wanted to know the answer :confused: hopefully you can help me on this point.

I understand how different forms (round hull vs to U shaped hulls example) can have different side force efficiencies, but the BWL hull profiles are included regardless if the hull shape is “Round or U shaped” in the pendulum search for right CLR position.

A flat rudder has surface area of 275 cm2, if I take the rudder off the pendulum CLR search method, it’s position (CLR) changes.

A round bulb at 2,850 gms has roughly 280 cm2 immersed area, and I’m thinking this must also influence CLR position ?

Then why is the bulb profile not included ?

Cheers Alan

P.S Actually post #12 got me thinking is the post build CLR check. The “push” test shall be carried out with rudder,fin and bulb attached in the position that the design drawings are suggesting. where as the pre-build design the pendulum method is without bulb, is what got the grey matter burning & wondering if the two methods will have the same result ?

30

Hi Alan,
you should try to cut out the full silhoutte of the emmersed area, use the pendulum method and fix the CLR with the first. Often is falling out of the trailing edge of the fin, glueing a little piece of paper will help, this is why is better to cut two cartons. In the second one you cut the bulb and check it without.
Probably the CLR will shift only on vertical position therefore not affecting very much the longitudinal position .

With the “push test” in the water tank, the bulb will help to stabilize the boat, with little effects on the overall wet surface dynamics, also because is well centralised along the LWL.
Cheers
Claudio

31

Hi Claudio, all is clear thank you ! with so many dynamics in boat design where some aspects are critical & others excluded … it is little like playing 3D chess :slight_smile:

I thought about doing pendulum tests myself, but using your plans they’ve already been done so thought why waste the time.

Out of curiosity when I do push tests after construction I shall compare static CLR results with bulb on and off the keel.

Back to previous question best position to have the Keel/Rig coupling along the LWL, have been reading Eric Sponberg notes and he says:

[b]“There is no one right answer. There is a “central area” where the rig-fin couple can go, and there are practical limits that keep it within this central area. Balance for trim so that the weight of the rig and the keel, in combination with the bulb, produces level trim. We tend to make sure that the aft edge of the keel blade is not too far aft of the lowest point of the canoe body of the hull, or that the leading edge of the fin is not too far forward of this same point on the canoe body.

This keeps the rig-fin pair in a good position with respect to both the center of buoyancy and the center of flotation, and again, allows for a favourable location of the bulb in relation to the fin as well as the rest of the boat for level trim. It is all really an art, as much as science. I could probably design a boat and hull and a separate rig-fin pair, and find any suitable range, a certain mid-body length, where the rig-fin pair will work. There is no hard and fast rule prior to considering the practical considerations of weight and balance”[/b]

Or in other words, build it & test to find out ! :slight_smile:

Eric published the “Naval architects dozen design ratios” which makes for interesting reading for those interested.

Cheers Alan

32

Hi Alan,
As most architecs told me, no one offer a precise answer !
Some condiderations shall also be added and, as usual, a drawing will try to explain what are the limits and drawbacks that I experimented myself some years ago on a Class M.

Cheers
Claudio

33

Hi,
it is an interesting discussion, because I am just trying to figure out what the right keel/rudder configuration for good steering behaviour in stronger winds might be. My special problem is that my last RG65 design (No.3) is going well in light winds and smooth water. It is still quite fast in a breeze, but with increasing wind + waves I am getting difficulties when tacking. In many cases the boot starts to turn fast until the wind is coming directly ahead. Then turning stops abruptly and the boat starts moving backwards. This happens even when tacking on a crest… (no, the boat is not too light!)
It might be a problem of keel/rudder configuration or one of the sails. I am still looking for the reasons but I cannot test anything in the moment, because the water is too hard. I have to do it in a theoretical way.
Looking at your methods and recommendations, I have not made any real mistakes in my design. The boat is also well balanced when sailing a straight course.
My configuration is similar to the example “curved rocker line” in the drawing above.

Any idea?

34

Greetings from cold Heidelberg Joachim :shake:

Have you measured boats centers yet ?

Having problems in “light winds when tacking” and when bow crosses the wind boat goes into “irons” & stops ! but helm is balanced going up wind ok ?Is there enough boat speed going into the tack ?

I found need to be little more gently on the stick in light winds and not tack it over too quickly.

Another possibility is the bulb, do you have very much oscillation when you flick the tip of the bulb with your finger sitting boat on stand, I seen some bulbs that have big oscillations which can’t good for tacking as it flicks over in a tack and stalls angle of attack on the fin, acts like a brake !

Also have been playing around with different rudder profiles & positions and noted if the rudder is angled too far aft (CLR moves back) and tacking speed is slower, but unsure if two are related, need more on water testing time be conclusive.

Prost Alan :zbeer:

35

Hi ,
there is another possibility , when the rudder is making a too large angle, there is the risk to enter into stall conditions and for a while will not respond to commands.
Another point is the proportion for the Fin and Rudder surfaces. Small boats tend to requires larger appendix surfaces.
My experience is that I consider as “rule of thumb” that the appendix area shall be around 5 to 6% of sail area and the rudder shall occupy 1/4 of that, the ramaing being used for the fin.

Cheers
ClaudioD

36

Hi,
thanks for the quick reply, however …

Alan - there are no difficulties when tacking in light winds, but in stronger winds. You are right that in light winds it is not good to tack too fast. It is more important to maintain speed.
The keel is stiff enough, there are no oszillations.
I have not “measured” the centers, I took them from the CAD.

Claudio - in light winds, a small rudder and moderate rudder angles are fine, in stronger winds a bigger rudder has helped a bit as a bigger angle did. But anyway, even when turnig fast on a crest sometimes the boat stops turning when the bow points directly into the wind. I will check for the areas during the weekend. It is an intersting hint, but I have to measure the different rudder blades first.

I am not sure if it is really a problem of the appendices, may also be a problem of sails?!?

37

Hi Joachim,
The rudder lift efficiency will raise with the speed, that’s means that a small rudder will be better when the boat is going fast.
This is why multihulls have very small rudders when compared with monohulls.

To be efficient, the rudder shall turns few degrees only to avoid creating too many turbulences and drag that slow down the boat speed and by consequence the rudder efficiency !.
On real boat the tiller is keept between to fingers and feeling the pressure variations…just the one is needed !

With RC models sticks is impossible to be so sensitives.

The rudder blade maintain the efficiency up to a certain angle, then drop drastically when the “stall angle” is reached and this depends also on the profiles used. A ticker chord is more tollerant to higher attack angles. Generally less then 15° anyhow.

It is very significant to trace on paper an angle of +/- 15° to realise how small is .

In a windy day the boat is assumed to go faster, therefore a small rudder surface and lower angle (4/5°) will be more efficient and will reduce the drag and stall risks; at the contrary, in a calm day, the boat will be slower and the rudder surface need to be larger to produce the lift effects and still keeping low angle ( 4/5°) and reducing stall risks.

Polar Curves are very usefull for better understanding.

With RC models, stalling is the most common effect with rudders and also the most ignored one, expecially when turning around the buoy or changing sudden and sharp direction !!
Very often we can hears ’ I lost control of my boat…’ or ’ is not turning anymore, I have a servo problems !! ’ or was just simply a “stall” effect ?

Among rudder problems, what about the Lead CLR-CE ?
As, ‘rule of thumb’, the CE shall fall on the vertical front edge of the Fin +/- x mm
Where is positioned the bulb CG compared with the Hull CB ?

Cheers
ClaudioD

38

Ops, have misunderstood your original post sorry.

I had experience in stronger winds where boat had better helm balance on one tack compared other, upwind. was not so obvouis in lighter air but had me checking to see if mast was straight, checking shroud tensions, fin alignment and even if the hull was straight :scared: then after couple days of frustration saw that the sheeting ring on the jib boom was not quite centred, causing Jib sheeting to be off centre by only 2-3 mm in one tack compared to other, after correcting it, helm balance was then perfect … smallest basic detail was over-looked, never again !

Being an old school engineer I don’t have complete faith in CAD details, always need to physical test final construction just to be sure mother nature agree’s with computer “force & balance” computations.

Good luck with trouble shooting, please let us know what caused and how you solved the problem.

Cheers Alan

39

How do you measure this on RC models … onboard GPS Telemetry ?

40

I have experienced this, many years ago with my first model yacht from Graupner. (which I still have!)
I solved the problem by raking the mast further aft as the wind increases, this gives more weather helm, but also aids the tacking. Also there maybe a chance that the jib is slightly over sheeted. What happens in this instance is: The main is looses pressure before the jib as the boat heads into wind, the jib momentarily counteracts the input from the helm and with the obvious loss in speed during tacking, you easily overcome the rudder and become stuck in irons.
It could also be trying to carry too much sail for the conditions, or the sails are not flat enough, and any yacht will become unruly then… My money is on a sail balance issue some where…

Have a nice weekend all!