Winglets on sails?

[tspeer]

I added a couple more lines to the graph I stole from Tom Speer to compare the effect of sealing the bottom to adding aspect ratio. ...

The bottom-left lines are the sealed-to-the-surface condition - no extra lines needed.

This graph was constructed using lifting line theory, which probably over-estimates the benefit of sealing a small gap. That's shown in the final graph from the same paper, which compares the lifting line prediction (diamond symbols) with empirical data from Hoerner on the drag of two wings side-by-side:

In real life, there is also the effect of the wind gradient, which would make the lift go to zero at the surface even if the rig were carried right down to the water. So one shouldn't ever expect to get the theoretical doubling of the effective span when the gap is zero.

 

[jimburden]

Thank you for the data. I have been a fan of turtle backing hulls and cabins and zero under boom gaps since about 1959 age 14 when I was drawing 12 meter cup boats with these features and zero chain to skin difference to the formula line and lead large bottom keel wings. I Did not know at the time the Aussies would do similar hulls but nobody sealed the gap. Why is this? The crew on Reliance and others were below decks on grinders anyway. Why did they never set and take the sails from below as well. get all off the decks?

 

[CT249]

The rules were changed after 1930 to ensure that primary winches etc were not situated below decks. There was no such rule initially in the 12 Metres, so the primaries etc were moved back down under the decks for a few years, until rules were made to ensure that the primaries and other winches were back on deck.

Why did they ban under-deck winches? Because from all accounts it was a horrible way of sailing. It was stinking hot a lot of the time, it made people nauseous a lot of the time, and it was impossible to arrange unless you had so many openings to the deck that the boats were dangerous as they could take on so much water.

I have no idea how you could "set and take the sails from below", but I've only run bow in one 12 Metre regatta. You already drop the kite into the forehatch.

Why did nobody seal the gap? Well, they were well aware of the theoretical advantages in the design of the Park Avenue boom etc. The idea of the low boom was part of the thinking behind the design of Intrepid, but tests by Halsey Herreshoff at MIT showed that there was almost no difference between dropping it as low as they did in reality, and dropping it all the way to the deck which was impossible anyway.

As Tom Speer notes, the wind gradient must also have an effect. Perhaps the effect of the windward gunwale and deck is often under-rated - some studies have shown lots of turbulence off the weather rail and if I recall correctly they indicate very little apparent wind around deck level aft. It may be a different area at the bow, where the boat is narrower.

Don't we also get down to the simple fact that about 50,000 gozillion hours of racing has shown that boats with slightly higher jib clews don't actually go slower than those where the jib clew is at deck level? If a vast amount of testing shows that having a significant gap at the jib clew higher doesn't matter in reality, surely it must be significant. After all, it's not as if the speed increase claimed by some theoreticians is so small that it could be ignored.

It's interesting that windsurfers used to talk a lot about "closing the gap" but these days there seems to be less emphasis on 'sealing' the gap between the foot of the sail and the top of the board. Looking at vids of the guys in the speed trench at Namibia, for example, shows them with what seems to be a wider gap than in the Rushwind sails of 25 years ago, for example.

 

[Gary Baigent]

Makes sense ... but you're talking about monohulls, that heel - so windward rail and crew create wind disturbance over the lower section of the main.
But in the more level "flight" of multihulls ... just look what C Class Groupama achieved with wing foot fitting, scraping right down to the tight trampoline. Also in the forthcoming AC50s you will see more of the same, going to be universal.
However this thinking is not new because to-trampoline main design was pioneered nearly 50 years ago by Steve Dashew with his D Class Beowulf.

 

[jimburden]

Thank you for the information and ideas.

 

[gggGuest]

just look what C Class Groupama achieved with wing foot fitting

I would be *very* wary of ascribing too much to that. I had a good look round all the boats at Falmouth, and there were many things different about Groupama. Never forget that 10 things 1% better are as good as 1 thing 10% better, and that boat was very refined in all aspects. Very impressive bit of kit with a lot of man hours behind it.

Its very easy to ascribe one obvious design feature as being a big advance, but that may not actually be the case. It may be of course, but it may not.