Construction techniques/ballast ratio

Footy Class General Discussion (Footy)
21

I assume, Claudio, that you are using the scale factor as a way to show that the light boat can’t maintain the same balance as the heavy boat while still having enough sail power for comparable speed. That could be true, and if you gave those specs to your friend, I can easily see why he said the light boat would not be good in heavy air.

But scaling down the specs of your boat doesn’t result in the specs of my boat, so let’s see if someone can help us with this. Let’s use the specs of our own real Footys:

Esterel is 546g total with a 260g bulb, ?fin length, and 1000cm2. My RangerDanger is 350g with a 215g bulb, 240mm fin length (bottom of hull to bottom of bulb) and ?sail area. Caculating the amount used for construction seems like sort of backing into the idea of ballast ratio. For Esterel the b/r is 49%, for RangerDanger it’s 61%.

Can any of you more mathematically inclined folks figure out the sail area RangerDanger would require to equal Esterel’s speed (I guess that would be maximum hull speed) in a 10mph wind? If someone can figure that out, then maybe we could go on to try to estimate the angle of heel for each boat. I wish I had the math skills…this could get interesting.

Thanks for any help…Bill

22

Hi Bill,

there is a nice explanation about forces on the sails : http://en.wikipedia.org/wiki/Forces_on_sails

All is based on the Bernoulli law.

Probably is too confusing, but I used the same simple formula to calculate the bulb weight as well the force on the sail and this was already presented here : http://www.rcsailing.net/forum1/showthread.php?6749-1-2-3-quot-esterel-quot at post 8.

The point is to find the equilibrium between the righting moment and the heeling moment.

Another potentially useful data, although not very precise for less then 1000cm² of sail area, can be extracted from the attached diagram :

…

Personally I always used the Bernoulli formula to calculate the bulb weight needed for a given wind speed and given sail area.

Sure Flavio will come up with more digestible comparative formulas for righting and heeling forces.

Cheers
Claudio

PS : the data sheet is transferred to the next post where both models are considered

23

Be able to compare apples with bananas is not easy.

it’s even difficult to tell if a boat is simply better or worse than another and, in many cases, the outcome of the comparison will depend on environmental conditions as well as by different design choices

Before we talk about models is perhaps appropriate to try to figure out what are the most reasonably foreseeable footy race conditions

Our models, you know, are small (very small. )
skippers must been within 50 m from them to be able to see them without a binocular
footies , by force of circumstances, will sail (at sea, or on a pond) near the shores

On the ground, usually, there are trees, houses, walls and other junk
Which means that the wind, in the majority of cases, it will be very disturbed, the direction and intensity annoyingly variables

Races in stronger winds were those of 2008 and 2009 in Liverpool ( gusts around 40/50 km / h - if I remember correctly)
During the races in wroclaw (2010) and Constance (2011) the wind was, I think, no more than 10/15 km / h.
This summer in Sestriere was the strongest wind around 20 km / h

So I think it’s more likely the windwill be between 10 and 15 km / h
But the strongest wind with which you can race is much stronger, and I think the English friends can give us some information more precise on this point (50 - 60 km / h?)

As regards the waves, however, in most cases the footy are racing in small ponds where the waves do not exceed 10-15 cm.

In Konstanz, unfortunately, the competition was on the lee side of a large lake and the waves were a little bigger and annoying.

In any case, the relationship between height and length of the waves, as usually happens was around 1:10 / 1:20, much more smoother than “cartoon” waves Claudio has posted here few days ago.

The effect of the waves was only to increase hull pitch , but did not cause any “pulse” of the wind.

Both in Konstanz in water there were a lot of debris and sometimes huge weeds (more than a meter long)

conclusions:The race course will most likely be as follow :

  • Wind from zero to more than 40 km / h

  • average wind speed between 10 and 15 km/h

  • Intensity and wind direction often changing due to the aerodynamic “noise” around obstacles (houses and trees)

  • Waves , usually very small and not “breaking”

  • In the water often, but not always, there will be garbage floating which will end up in keels and rudders

  • Often, but not always, ducks, and aggressive swans will be close to our small boats

This seems like a necessary starting point of before talking about displacement ballast and rocket science

Cheers

Flavio

24

Hi Bill,
I went this morning to recalculated the characteristics of two Footy of 510g and 350g displacement and 260g and 210g for the respective Bulbs.
Assuming the same wind speed 8/9kt, and same Heeling angle of 30°, it was necessary to find the Sail Area for the lighter model.

Hi appreciate the Flavio description on practical / environmental aspects .

Personally I still consider that the water environment is often similar to a heavy sea conditions as compared with real boats.

On my side I went to compare with theoretical calculations two Footy models of different displacement and bulb weight as from post 1.
The 1st boat is the Esterel with 510g and 260g bulb and 1070cm sail area.
The 2nd boat is a simulated one with 350g and 210g of bulb, Sail area to be found yet .
Sailing conditions : Wind speed 8/9 knots and Heeling angle 30°

Comparison image as results of calculations :

…

My conclusion from the theoretical calculations is that the lighter Footy with 350g of displacement and 210g for the bulb, shall use a smaller Sail Area of 790cm² against the 1070cm as used on the Esterel Footy, in order to meet the same Heeling angle of 30° with a wind speed of 8/9knots (~15/16km/h)

Here below the calculations Data sheets.

Cheers
ClaudioD

25

Claudio,

I firmly believe that a bit of common sense and healthy experience are worth much more than “partially” scientific theories
Your boats are certainly beautiful to see and are able to navigate properly, but unfortunately, the calculation method you proposed is completely wrong

As modeller I never managed to get in ahead of Bill, but in my professional life I have made ​​at least five thousand “real” stability calculations on all kind of ships and boats , from rowing boats up to aircraft carriers - both included -
I have quite clear ideas on this topic

Neglecting other details, the biggest mistake of your method is that - apparently - heeling angle appears to be independent of hull beam .
Obviously something is wrong.

Two models with exactly the same weight and the same ballast mass but different widths have huge differences in righting moment ( around 150% )
Your method is probably a little more reasonable on very slender and narrow boats , but not on footies where length/beam ratio is often two

Greetings

Flavio

26

Hi Flavio,
as ancient modeler since 1948, the experience proved that “form stability” cannot be considered on our models.

The formula I used (Bernoulli law) is the typical method to identify the Bulb weight against the Sail Area for a given Wind Speed, all the rest being equal.
This formula is not my invention, but passed to me by a qualified French Naval Engineer, him self modeler since 40 years.

It is essentially used as a “Static Tool”, no sea, no waves, more or less like a “balance” for tens of Class M, IOM, AC100, RG65, and more recently with my last design the Esterel-1.2.3… I never met bad surprises with lateral stability once in the water. The boat in that formula is a sort of floating body only .

Since, as you wrote, that my method is “completely wrong” I suppose that now the readers are expecting to know what could be the Sail Area of a Footy 305mm long with a bulb of 210g as described at the begin of this tread.

The only advise I was trying to give, refers to the risk run when reducing size and jeopardize the sailing performances.
My drawing presented above is showing the difference obtained with the formula I use, I would be surprised that a lighter boat could carry the same sail area as the heavier one.

I’m sorry and I do further apologizes for having put my nose in the Footy affairs !

Cheers
ClaudioD

27

Claudio, please don’t take offense to Flavio’s comment. Sometimes we all get a little worked up about our own opinions. You’ve expressed some pretty firm opinions, also, in challenging the idea that a light boat can perform in heavy air. My challenge back to you was to show the numbers, and I appreciate that you’ve shared the math that can help us understand things a little better, even if not perfectly. We all appreciate good discussion, and this can still be a good one.

The biggest concern I’ve had in your comparisons has been your assumptions about the measurements of the light boat. You’ve used real dimensions for Esterel as the heavier boat, and hypothetical ones that don’t reflect the reality of my designs for the lighter boat. Since you kindly shared the formula you use, though, I’ve plugged the real dimensions of my boat into your formula. Since I already know the bulb weight and wanted to see how much sail it could carry compared to Esterel, I solved the formula using Sail Area as the unknown.

RangerDanger is 325mm LWL, 80mm Beam, 253mm LD, 210mm LCV (for my McRig around the expected size) and the bulb is 215g or 2.11Nm.

Plugging those values into your formula gives 2.11 x .253 x .5 = .5 x 1.22 x SA x 20.25 x .210 x .866 which results in SA = 1000cm2.

The measurements may not be perfect, and I’m sure the results should not be taken too literally, but interestingly, that’s pretty close to the size of my B rig. In actual experience, I think I’d have gone to a smaller rig in 8/9 knots.

At any rate, that means, VERY theoretically, that RangerDanger can carry 93% of the sail that Esterel can carry in that wind, and with only 70% of the weight. That sounds promising to me, so I think I’ll continue my direction for a while.

So I guess this has been a rather long…but interesting, for sure, answer to your question, “What for, any rationale behind ?” that started all this :slight_smile:

I look forward to more discussion…I’m learning and enjoying it!

Bill

28

Hi Bill,
What it is disturbing is that it is written that the method of calculation I used is completely wrong , and underlined.

For me the real boat performance calculations are another story and the criteria used there cannot be transferred to our small models.

I do not think that all the modelers using that formula are all mistaking !

That simple formula do not consider the hull at all, since acting simply as a pivot centered on the supposed rotational point. For simplicity I used, as often done, the LWL to take the distance to the CE and CG. Another center can be used of course, but rarely the results will greatly change the purpose.

Once a bulb weight or sail area is found, the refinement can be carried on with tests on the water as necessary.
I never met lateral stability problems with my designs, at contrary !

The data obtained by the formula can be used as comparison as I did, or during the design definition because this is the purpose of that formula.

The bulb weight was given at your first message as 210g and the boat at 350g, I just used them ignoring the boat size since not part of the formula, but retaining only the bulb’s weights.

The aim, according to your intent was to find some one capable to do the same or even lighter.

My intervention was triggered by these intents highlighting another aspect linked to that, in the sense that going too light there is fee to pay !

If you apply correctly the formula with these parameters you will reach comparatively the value of about 790cm and 1070cm² being the reference point for the sail area.

Of course if I would have used a larger sail area for the heavy boat, and reduced the wind speed, the lighter boat would have reached more then 790cm².
As as said, the “form stability” for the models is always neglected since not producing substantial changes.

From the drawing I made of the two models you may pull some conclusions yourself !

With that, I do not think necessary to continue, I have other interest calling, you have a “formula” used by thousand of design modelers, you can use it or just trow it into the bin !

Cheers
ClaudioD

29

Wait a minute, Claudio…

“If you apply correctly the formula with these parameters you will reach comparatively the value of about 790cm and 1070cm² being the reference point for the sail area.”

I wasn’t questioning that you correctly used the formula using the dimensions you assumed for the light boat, but I was pointing our that those dimensions may not reflect the reality of 350g Footy construction. Remember, this thread started as a discussion of building techniques resulting in light boats with strong ballast ratios.

Are you suggesting that the calculations I made using my real boat measurements were an incorrect use of the formula you supplied?

It seems to me that, if you accept the accuracy of my calculations, then the idea that light boats can’t stand up in heavy wind may not be that simple…which probably leads us to the conclusion that this business of boat design is still as much art as it is science.

Bill

30

Bill,
My initial intent was to explain that reducing the overall weight will impose also a modification of the sail plan.
An example was taken to compare two boats of different weight using the same formula.
Comparative results will be proportionally be the same as depicted in my last drawing. I do not talk about performances but just physical combinations.
With my Esterel, I fixed the sail plan to a normal average of 1070cm and the mast length to 600mm. Right or wrong for that hull I will not know until in the water.
Of course it always possible to increase the sail surface with a lighter bulb so far is increased the righting arm within the limit of the Box, see sketch.

…

Is not my problem to judge if 500cm, 1000cm, 1300cm or 1400cm² is a valid choice, much depend on the boat displacement of course and weather conditions.

On the web there plenty empiric formulas derived from experimental observations made on real boats suggesting various combinations on how to match hull and sails, proving that is not a precise science in spite of sophisticated computer work, otherwise the ideal sailing boat, would exist since longtime. I agree with you a sailing boat is more art then science otherwise the Oracle AC72 would not have capsized as did it…

Having said that, I still believe that reducing the overall weight (bulb) will impose other changes in the sail plan. Of course one would think a mast height of 30cm or even 20cm, hoping to find some wind at the height, and making a wide sail area distribution, but the formula I used will still be a valid approach to compare bulb and sail area.

cheers
ClaudioD

31

formulas? you guys are using formulas… thats my problem I never use formulas…:slight_smile: Of course maybe thats why nonce of my footies every worked real well…:slight_smile:

32

This is my first time with this formula, Marc…I can see how it might be useful as another piece of the puzzle. The most interesting use I see for it is to determine the relative sizes of the A,B,C,and D rigs to span the wind conditions you expect to race in. Maybe this discussion will get your Footy itch started again? Since a lot of this discussion has been about Footys in heavy air, I’m reminded of the race in Orlando when you and Walt decided that the Drag Strip was better use of your time than Footy racing :slight_smile:

Claudio, I understand what you were attempting to show. What I think you demonstrated without doubt is that a 350g Footy built using the dimensions on your drawing will probably only carry 790cm2 of sail in 8/9 knots, as opposed to Esterel’s 1070cm2.

What I showed you, using your formula, is that a Footy built to the dimensions of my boat…a real boat…can theoretically carry 1000cm2 in that same wind. 93% of the sail area and only 70% of the weight.

I get the feeling that you don’t want to acknowledge my result because it seems to challenge your belief that light boats are only for light air. I can accept that amount of sail area is only one of many factors that govern heavy-air performance, but I think many people would say, “Gosh, that’s a surprising result…maybe the old thinking about light boats doesn’t entirely apply to Footys.”

Regardless of all that, what you might take away from all this is that lightweight construction techniques are valuable regardless of the total displacement you choose for your boat. Marc doesn’t need a formula to figure out that the more of the weight he can get in the bulb, the longer he can stay with his A rig in freshening conditions :slight_smile:

So back to the origin of this thread…anyone have any interesting construction techniques to share?

I tried a few depron hulls, but gave up on them after my hull cracked in that Orlando race. It just couldn’t hold up to the punishment of 2 rough days. The stress finally cracked the hull near the fin. The weight of Depron is still tempting, though. Have you Depron builders figured out neat ways of strengthening the stress points that don’t add much weight?

Bill

33

bill…

I remember that event… to the point of scissors…

drag strip was a lot of fun…

34

Hi Bill,
if you are right and no doubt about, then the Esterel , by applying the same formula or simple proportional rule, she could carry 1238cm².
How far I could go, I’m not sure !
Another American competent sailor was suggesting in a paper that if the boat is heeling at 30°/35° for more then 10 second, would be wise to change Rig.
Why that ? Simply because the drag effects will reduce the sailing speed.

Now I go back to my OneTwoTwo/ Esterel and gelcoat affair !

Cheers
ClaudioD

35

OK Claudio, now I’m really lost.

You put the dimensions of your boat into your formula, and it resulted in 1070cm2.

Now you say she could carry more.

I don’t understand how you can get her to carry more without changing at least one of the dimensions in the formula. Since the boat was built to the dimensions you already put in the formula, how can it change now???

Using YOUR favorite formula, using the actual dimensions of YOUR real boat, the result was 1070cm2 for 8/9 knots…period.

Using YOUR favorite formula, using the actual dimensions of MY real boat, the result was 1000cm2 for 8/9 knots…period.

Are you saying you don’t believe your own formula, or that you don’t believe my numbers?

36

Sorry for the misunderstanding Bill,

OK go back to the principle : the amount of sail surface that a boat can carry is based upon the ballast that the boat can load on board. I think you will agree on that.
Now, if the ballast is externally attached at the end of Fin blade, it become a Bulb as on modern boats, then it is possible to reduce the weigh of the bulb by increasing the length of the Fin, to maintain the same righting moment.

The problem is that the Wet area will increase and the fin length cannot be too long also for other reasons like strength and flexibility.

Now with the Footy Rules the limit is fortunately imposed by the 305mm of the Box.

[COLOR="#0000FF"]For the Esterel, I chose the sail surface of 1070cm², was just a guess based upon my readings about various Footy, no scientific calculations done at priori. ( probably very conservative)[/COLOR]

I do not know yet if the Esterel could carry more.

Now if you says that your boat of 350g and 210g in the bulb and Fin (?) could carry 1000cm² , then the Esterel using an heavier bulb of 260g and 200mm Fin, should be able to carry more sail area.

As a speedy example : (1000cm² / 210g) x 260 = 1238cm²

Hope is now OK for you !

Cheers
ClaudioD

37

Claudio, I said my boat could carry 1000cm2 of sail in 8/9 knots of wind because that is the result I got using the formula that you posted here.

I used that formula so that we could get a comparison of the sail capacity of our two boats in a given wind strength. In addition to bulb weight, your formula takes into account the leverage of the bulb by using it’s distance from the waterline, and the leverage of the push of the wind on the CE of the rig, also measured from the waterline. In spite of the fact that it does not consider beam, I accept that the formula is a reasonable predictor of sail size for a given wind strength.

You decided on a sail area, and used the formula to predict your bulb weight. I used my known bulb weight to calculate the sail area. Both of us used the same formula, we just solved it for a different unknown.

So based on your formula, your boat can carry 1070cm2 of sail in 8/9 knots. My boat can carry 1000cm2 in 8/9 knots. We have not done any calculations for other wind strengths.

Why you would post a reasonably sophisticated formula, defend its accuracy, tell us to use it, and then later imply that sail area would be simply proportional to bulb weight is completely beyond my comprehension.

Of course, if you increase your bulb weight, you can recalculate and see that you can use more than 1070cm2, though it won’t be directly proportional to the bulb weight increase. The same will be true if I increase mine and recalculate.

But that makes no sense, because what we were doing was comparing the real sail capacity of our real boats in the same wind.

I’ve lost patience with this. I give up. I trust any readers to figure it out for themselves.

Bill

38

Gentlemen, I have been watching this thread with interest. Thank you for your discussion; it has been of great value to me in understanding some of the factors in boat design.

It is my opinion that mathematics can only take you so far… there are so many variables - some relating to the design of the vessel, more relating to the adjustments available on the vessel, the dynamic (while sailing) adjustments (e.g boom, rudder, heading) and then the actual environment of non-flat water, and inconsistent wind speed and direction. And also add in the intention, the skill and the mood of the skipper… (grin)

I think any attempt to describe (mathematically) a sailboat actually sailing requires such a plethora of input variables that the mind boggles!! Also the results remain somewhat subjective because of the great number of input variables… Maybe this is why sailing holds our interest…

Looking forward to the Footy on the water - thank you for the thread!!!

39

Very nice of you, Penguin…glad you got something out of it.

After all this, I remain interested in the idea of performance predictors, even imperfect ones, as a design aid. I still believe pretty strongly, though, that since all Footys get to hull speed quickly, the primary predictor of performance will always be the skill of the skipper :slight_smile:

Bill

40

Here you go, Bill
Let’s get things back on track and give my answer to your question in post number 32 about beefing up Depron construction.
I’ve put a Box IV together in 5mm Depron to reinforce the whole hull rather than the just the stress points. For the eagle eyed amongst you, I know there is a slight twist in the aft sections and I’ll take a little more time and care over the next one. The main point of this experiment is that the bare shell feels significantly stiffer and stronger than the 3mm box III. I was very pleased to be putting on just a few g in upping to 5mm Depron so will pursue this idea to a working prototype.
This method of construction is so quick and easy I feel it’s perfectly appropriate to take more of a ‘suck it and see’ approach to design development. Cutting and sticking the four parts you can see in the pictures took less than half an hour, excluding glue drying time. The one thing that I did spend longer doing was shaping the 3mm MDF patterns, which took approximately one hours work with a band saw, half round file and some sand paper. Now all this has been done, I plan to cut and glue together a straighter hull and then spend somewhat longer fitting keel box, rudder tube, mast tube and servo tray etc.
Cheers
Phil