# Foiling boats weight sensitivity?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Jun 15, 2021.

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### Mikko BrummerSenior Member

I believe for the IQfoil (foiling windsurfer) both the sail area & foil area are specified. How much of a disadvantage say 10 or 20 kg of extra weight would then be? For the Formula kite, the foil area is specified, but they can choose from 4 kite sizes, 9 sqm to 15 sqm, depending on the wind & rider weight.

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### patzefranpatzefran

It is probably oversimplified, but assuming the total drag D is the induced drag of the foil, D ~ L² (squared lift) and weight W= L (lift of the foil).
the thrust T = D, as T ~ (Vapp)², square of apparent wind, you get dVapp/Vapp=dW/W, so you need more apparent wind to achieve the same velocity.
An accurate VPP wuold be necessary to get exact figure !

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### Doug HalseySenior Member

That is indeed oversimplified!

Induced drag is the largest contributor to the total drag at low speeds, but it gets smaller as the speed increases. Friction has an opposite trend, being small at low speeds but becoming the dominant contributor at high speeds.

The result is that curves of total drag versus speed look quasi-parabolic, similar to that shown below. The locations of the drag minima vary with foil parameters and lift, allowing designs to be optimized for low or high speeds, heavy or light skippers, etc.

Last edited: Jun 17, 2021
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### patzefranpatzefran

I agree, but the question was the disadvantage for an heavy skipper using same foil and same sail than a lighter skipper.
You should give the same curve for 210 Lbs of lift, which will show the drag penalty !

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### Doug HalseySenior Member

The drag curves only tell part of the story, and only for specific configurations & conditions.

I did some VPP calculations a few years ago for a Mach2 Moth showing that even in very moderate conditions, heavier skippers had a decided advantage. I suspect that the assumed value for the center of pressure of the sail was unrealistically high, but that just helps emphasize that many factors enter into the analysis, making simple answers difficult to accept.

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### SkyakSenior Member

I expand on this by saying that wind conditions are important to the question of advantage. A nation could send an excellent team of heavy sailors and get trounced in light wind. It would be wise for a country to make sure they cover the wind spectrum. The problem is highlighted by looking at takeoff time at different wind speeds -if you are heavy you will never make up the time lost getting on the foil. I would also note that these board classes where the craft weight is small compared to the skipper and sails are not at a a fixed angle allow jumping onto the foil in ways normal sailboats can not -with pumping and jumping. The NACRA 17 does not have this luxury, but it is a surface piercing foil which is different due to variable area.
An excellent example of foiling weight advantage was the last Sail GP series in Italy. For maximum advantage in light wind they dropped the grinders and sailed the F50s with a crew of 3. It's on Utube and I would recommend it.

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### CT249Senior Member

The sailor weights you gave prove the point precisely. Those sailors from the 1960 Olympics have an average weight of over 82kg. That's 4.6kg heavier than the average American 25-34 year old male at the time ( https://apps.dtic.mil/sti/pdfs/ADA433581.pdf ) - and Americans were on average 76mm taller than Europeans in the '60s.

The sailors in 1960 were much heavier than the typical male Olympian. The typical male Olympian was 74kg (http://www.randigriffin.com/2018/06/08/olympic-history-7-size.html) which was 10% less than the typical sailor.

As the facts that you gave show, the Olympic sailing classes used to favour heavier sailors. Even compared to Americans, the average Olympic sailor from your sample was inside the heaviest 20% of young men, which means that they were much heavier than average on a worldwide basis. If you add in the remainder of the small-boat medallists (like the 90kg Andre Nelis, the US Star sailors who were both 93kg, the Finn silver medallist of 85kg and even the suspicuously light Star winners) the average weight of sailing medallists gets even heavier.

My reference to 95kg may have been confusing and I apologise for not making it clearer. It was a reference to current body weights in those classes, because comparing Olympic sailor weight to the general population of those times would have required analysis of many years of data.

The reasons why the older classes favoured heavy sailors are irrelevant. The fact is that they did - and the heavy sailors don't ever seem to have said that there should be classes for sailors of all weights, as they do now. I can find no instance where Finn sailors of the 1960s, for example, said that the Europe should be brought into the Games to cater for light sailors. There seems to have been no thought to modifying the Finn, FD, Star or Dragon to make light and medium wieght sailors more competitive. The heavy sailors ignored the issue of weight bias when it suited their own interests.

Your comment about "no one complaining on internet forums 50 years ago" is silly. Nothing I said indicated that I was only talking about net forums. I specifically referred to an interview in the '60s with Elvstrom, for example. Back in the '60s there was a lot of press discussion about Olympic sailing. There were many reports on the selection of new or possible Olympic classes, like the FD, Tempest, Contender, Soling and Tornado. I've read many of those articles, and none of them ever seem to have said that it was important to cater for light sailors. People like Jack Knights would attend IYRU meetings and report on the discussion and voting about Olympic class selection. Knights himself brought up the question of fairness towards medium and light sailors in the Yachting World interview with Elvstrom, and Elvstrom dismissed the idea.

The fact, as the facts that you provided prove, is that the Olympic classes used to favour heavy sailors and none of the heavy sailors seem to have complained that it was unfair, or that people of all weights should be competitive in the Games. They have only started to make that claim now that it suits them.

Make no mistake - I believe that there SHOULD be a class in the Olympics for heavy sailors. In one of my sailing disciplines I used to be too heavy for the Olympic class myself. In my other sport I'm on the heavy side. I just don't agree that it's right for heavy sailors to ignore the issue of weight equity until it suits their own purpose, and then to make a big deal about it.

No one is fair if they only demand fairness when it suits them.

Last edited: Jun 17, 2021
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### CT249Senior Member

A survey of the (foiling) 2011 Worlds fleet showed that top sailors averaged 82kg. In contrast Mark Thorpe, multiple world champion before the foilers arrived, weighed around 68-70kg if I recall correctly.

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### Doug HalseySenior Member

As a skinny teenager in the middle 1960's, I was keenly aware that the Olympic classes were mostly for the heavier guys. The one exception, ironically enough, was the FD, where the best combination seemed to be a very light skipper with a huge crew.

Some of the most competitive teams in the US were husband-wife teams, with the wife skippering and the husband crewing. (Pat & Jack Duane, Barbara & Jack Pardee).

I never owned an FD, but several times I was recruited to skipper for heavier owners.

Paul Elvstrom's World Championship in FD's was as crew for the lighter Hans Fogh.

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### AlikSenior Member

I don't think this is fair approach, to take 'average' weight of Olympic sailors as You do it. My sample shows that quite few people around 75kg were around the winners. If You average this with 105kg crews, then you are getting 'average temperature in the hospital' - nonsense.

And the weights in my sample are far below 95kg which you claimed.

Regarding Elvstrom's statements - this is his personal opinion and can't be used as proof that 'none of heavy sailors complained'. At some point, they added 470 which is a light male boat.

The fact is: in older days, Olympic sailing was a competition for athletic-built males. Now it is becoming TV show for lightweight 'gender-equal' acrobats

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### CT249Senior Member

I explained why I used the 95kg figure. Even today, 95kg is significantly heavier than usual for fit adult males.

You are the person who brought up average weights, in your first post addressed to me. If talking about averages is unfair then why did you bring them up?

What is not "fair" about pointing out that the vast majority of Olympians were significantly heavier than the average adult male of their time? It's not nonsense to point out that the optimum weight for all the Olympic dinghies for many years was well above the average weight for fit males, even in a light wind Olympic venue. The simple fact is that exceptions do not disprove averages or optimums, and the average and optimum weight for Olympic dinghy sailors of the time was well over the average weight of fit young males. People knew that at the time.

I did not use Elvstrom's personal opinion as proof that none of the heavy sailors complained. I used it as evidence that at least some heavy sailors specifically said that they did not believe that the Olympics should cater for sailors of all weights. I notice that you have not provided a shred of evidence that the heavy sailors ever campaigned for equity for light sailors, although they now demand equity for heavy sailors.

Your "facts" about Olympic sailing having been "for athletic-built males" and now being for "gender-equal acrobats" are a classic example of bigotry, because you are inferring that people's athleticity and gender is linked to their weight. It's just a weird lie to imply that men like Tour de France winners, marathon runners, Nick Rogers, Nathan Wilmot and Nathan Outteridge are not athletic and are of dubious masculinity merely because they are lighter.

I said heavier sailors should be in the Games, and you insult light sailors. That in itself shows the difference in our attitudes towards fairness.

Last edited: Jun 18, 2021
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### AlikSenior Member

Becasue, say, we have a keelboat with 3 crew, with weights 75 + 105 + 110kg. What is the average weigth? 96.7kg. Does this mean, that light sailors are excluded from racing? No. Thus, light sailors were not excluded and heavy sailors were not favoured.
In today's Olympic program with current classes, there is no space for anyone over 85-90kg. Now look at population statistcs, and find out waht % is excluded from Olympic sailing.

I am operating avarage weigths of male population, to reflect the fact that considerable % of heavier population is excluded from Olympic sailing. And You are averaging weigth of sailors, however this makes no sense - need to look at range of weigths.

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### patzefranpatzefran

Thanks, Doug. Very interesting curves, and counter intuitive to me ! looks that the optimum weight with your hypothesis is well above 200 Lbs ! So the Righting Moment looks as critical parameter. I assume you accounted for windward heeling contribution to RM ? Looks like when you compare to a Floating Moth, heeling moment arm increase with flight height, and also sail is higher in the surface wind gradient which increase sail force !

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### AlikSenior Member

There is some quite interesting info on sailors weight:

SailJuice.com- Crew weights in the Olympic classes https://www.sail-world.com/Australia/SailJuicecom-Crew-weights-in-the-Olympic-classes/-38778?source=google
Note from here that all 'heavy-weight' classes have been removed for 2024 Games.

Analysing Weight Distribution of Finn Sailors https://finnclass.org/news/20-news/980-analysing-weight-distribution-of-finn-sailors - great review form Finn class:

With '470' in Olympic program for 45 years now, any talks about 'Olympic classes used to favor heavier sailors' are just baseless speculation. The typical helm+crew of '470' weight probably as much as me alone (but I am not targeting the Olympics, but my son should probably have a chance!)

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### DolfimanSenior Member

In the breeze, the weight needed to gain in Finn is 95-105 kg and you should remember that the lighter racers (in the 60s and early 70s at least) loaded themselves with wet sweaters that could provide up to 25 kg of additional ballast. It was torture to wear this but the only way to be competitive in breeze while having some advantage by light winds. That's may relativise your discussion about the optimal in those days.

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