Capsize Recovery and Bernoulli's Principle

I have been in discussion with a friend about the best boat-orientation for recovering a capsized 16. I know the recommended method is to point the bow into the wind, so that the mast runs perpendicular to the wind. The thought is that as the wind passes over and under the sail (while not upright, rather almost parallel) the sail creates an area of low pressure between the water and the sail, and this provides lift for the mast and should help you right the boat.
I am wondering how or if this works. My contention is that if you have the bow turned into the wind, the sail will luff and not create a pressure difference between the two sides of the sail; there is no 'pull' on either side. When capsized with the bow pointed into the wind the wind and sail are positioned the same way relative to each other as when you are stuck in irons, save that the mast and sail are now horizontal instead of vertical. Why would the sail assume the shape of a wing (and provide lift) while horizontal and not while vertical?

I am wondering if pointing the mast into the wind so that your trampoline is perpendicular to the wind might not help recovery even more. Aside from increasing your rate of drift, and running the risk of capsizing leeward immediately after recovering, could this orientation actually help getting the mast out of the water?
Wind pushing on the trampoline at a 90 degree angle might affect the top pontoon more than the bottom pontoon (because the bottom pontoon is anchored a bit by the water), and this could increase the force bringing the top pontoon back over?
As the mast come out of the water, instead of relying on Bernoulli's Principle to create lift, the sail would simply respond to the force created by the wind physically pushing on the underside of the sail, bringing the mast out of the water even more and assisting in recovery?
It is possible that with the mast angled slightly down from its attachment to the boat to the end of the mast in the water, with the mast pointed into the wind the wind might actually be applying a force downward towards the water until the mast is lifted parallel to the water and the wind can begin to push on the underside?

I am very curious about this and would like to know what other people have experienced, and if they have any explanation for any of this.

I think the idea of having the bows pointed into the wind is more to orient the rig into a "neutral" position both during and after the righting process. If the stern is pointed into the wind, the force of the wind will drive the mast down and also cause the boat to take off if you do manage to right it. If the mast is pointed into the wind, the boat will want to do an "up & over". If the wind is strong enough, the boat can even self-right.

Pointing the bows into the wind probably allows the rig to provide a little bit of extra lift and also orients the boat into a safe position once righted. Certainly people can and have righted Hobies from any orientation imaginable. Bows into the wind just provides the most consistent results.

sm

"Into the wind" may not mean what you think; it's about 45º to the wind as shown (and described) here:
viewtopic.php?t=1570

With the boat on its side, the sails are hanging down into the water. If the wind is forward of the mast it will push between the water and the sails and push the mast upward. In the orientation shown, the wind also pushes on the tramp to help the boat up.

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'00 H16 #104691
'78 H16 #32692 ex-rental [gone]
Old Holsclaw trailer
My Hobie 16 pages

I usually point the mast into the wind to right the boat but there is one trick I read in the Hotline years ago. With the mast at 45 deg off the wind, travel the jib car as far out as it will go. Then grab the jib sheet ahead of the cam clamps and pull tight. This will hold the jib down at an extreme angle and trap wind, lifting the mast. As you pull on the righting line, the extra lift from the jib will help. As the boat comes up, let go of the sheet and the jib will be uncleated.

One comment about Bernoulli effect. The low pressure is created on the high side of the airfoil and wouldn't be between the sail and the water. The pressure on the sail would be more from "angle of incidence" rather than Bernoulli. Wind on the tramp would generate more force to help right the boat than the airfoil affect. The former uses the full velocity pressure of the wind while the latter must create a difference in pressures related the decrease of static pressure as the wind speeds up on one side.

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Howard

Yes hrtsailor you are right, the low pressure area would have to located on the side of the sail facing the sky to provide any helpful lift. Thanks for your reply.
Interesting comparison between the force of the wind on the tramp and the force of lift that might be created by a pressure difference on either side of the sail.
I wonder if there is an easy way to measure these forces for comparison.
Maybe it could be tested land by attaching a pole to the top pontoon, running parallel to the ground, and adding weight to the pole until the boat begins to recover. The pole would simulate the forces applied by a sailor on a righting line, and the orientation that requires less force is a better method (at least in regards to using wind force).

Your user name fits you well!

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1980 H16

Hobies have HULLS not pontoons, also when the crew gets the righting line and leans to rite the boat the sail generally comes out of the water slightly, changing all your calculations, go buy a hobie and sail.

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Bodhisatfa

The force acting on the tramp is easy to calculate. Velocity pressure, in inches of water =( Velocity (fpm) divided by 4005) quantity squared.

So 2000 fpm wind vel. (about 23 mph) would create a presssure of 1/4" of water over the area of the tramp. Assuming a 7' X 8' area, the force would be about 72 lbs.

Calculating the Bernoulli effect requires knowing the difference in velocity on either side of the sail and the area involved. i don't know how to get the information.

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Howard

I suspect the flow is so turbulent that there's no point approaching it that way at all.

_________________
'00 H16 #104691
'78 H16 #32692 ex-rental [gone]
Old Holsclaw trailer
My Hobie 16 pages

Littlewing, you seem to have missed the spirit of the original question. You are right that when the boat begins to recover the angles between wind, mast, sail, water, etc change, and so too do the forces involved. However, the principles responsible for creating these forces do not change, so we are still able to examine them.
The original question came up as a result of a single sailor struggling to right a capsize on their own. In the absence of a water bag, any additional force that would assist the recovery is useful.

hrtsailor, good work! I don't have the technical know-how to independently corroborate your calculations, but it all sounds impressive! Do you think those 72 pounds of force would help you flip the boat back over, or wold it just cause drift? I am inclined to think that some, but not all, of that force would help you flip the boat, as the bottom HULL experiences more fluid friction due to its position int he water, and could act as a sort of fulcrum of a lever (a pivot point).

Anton, I was thinking that the airflow would be turbulent as well, or at least would not cause the sail to assume an airfoil shape that would produce lift. I am just not sure though.

Go out and flip a Hobie 16. Then you'll know...

_________________
'00 H16 #104691
'78 H16 #32692 ex-rental [gone]
Old Holsclaw trailer
My Hobie 16 pages

way to deep for me

Listen guys. You can calculate to your hearts content. As an engineer that is my preferred mode too. But from about 40 years experience sailing these wonderful boats I know that if you try to point the mast into the wind and right the boat, you will fail. You may be able to do this in light air with lots of weight, but with the wind above 12 knots and even 2 people that would be a real challenge. Just go out and try it. The wind is actually pushing down on the sail (which is much bigger than the tramp when in this position.
You can only use the area of the tramp that is above the centerline for an approximate force from the wind on the tramp (at least that is contributing to righting the boat). To get it correct you would have to have an equation that describes the varying forces contributed from different distances above the water and then integrate that equation to get a reasonable force. Way too much thought for me. Just go out and try it. There is a reason that the bows should be at least 45 degrees into the wind. Many years of experience has shown that it works best that way.
Fun to think about though and the experiments would be a blast to set up.

mdgann, nice post.

I wondered if having the mast pointing to the wind would actually result in the wind pressing the mast down until the mast was raised enough to get the wind under it.
Now I am wondering if instead of pushing down on the sail, the motion of the air across the top side of the sail would create a lower pressure area than the presumably more static air on the underside of the sail, and that this might provide lift.

All in all, those of you who say to just go out and try it are right. However, as someone else pointed out, you can successfully recover your boat in many different orientations, so knowing which one is best is hard to determine and becomes a subjective conclusion. For discussion and for fun, I want to come to an objective conclusion.

Thank you so far for the responses. I have enjoyed reading your thoughts and trying to work through this question.

No I didn't, as someone else has stated, go flip your boat in a blow, then come back to the forum, the rest is psychobabble to me.

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Bodhisatfa