Foil Boats, Which Way From Here?

Technical discussion of ARC products
J Drew
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Re: Foil Boats, Which Way From Here?

Post by J Drew »

Bill,
The next question is, how much of an advantage is a foil in the long run?
A fixed foil has its disadvantages as you have pointed out.
With the aileron design, the aileron works like a governor to keep the lift under control when the speed gets to high. So theoretically, if it lifts out at 13.6 then what's the peak speed.
If it takes 15+ for the foil to be effective, how fast would a normal hull be going in that wind?
My point is, that in a perfect situation with a steady 18knot breeze, it will get up and go a few knots faster, but if it hits a lull then it slows down quickly and takes more to get going again. So, how much has it gained at the end of the day.
Bruiser
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Re: Foil Boats, Which Way From Here?

Post by Bruiser »

J Drew wrote:Bill,
The next question is, how much of an advantage is a foil in the long run?
A fixed foil has its disadvantages as you have pointed out.
With the aileron design, the aileron works like a governor to keep the lift under control when the speed gets to high. So theoretically, if it lifts out at 13.6 then what's the peak speed.
If it takes 15+ for the foil to be effective, how fast would a normal hull be going in that wind?
My point is, that in a perfect situation with a steady 18knot breeze, it will get up and go a few knots faster, but if it hits a lull then it slows down quickly and takes more to get going again. So, how much has it gained at the end of the day.
Hi, I am not Bill, however I have been in several races now where the foil boat had great expectations. Reality was, race starts in the morning. Very light wind. The race starts and everyone takes off. This particular race was out and back. On the way back to the finish, there is the foil boat still in the starting area as everyone finishes. I am sure that late in the afternoon, he was really flying, however the fleet was in on the beach by then.

So, in my view, the foil concept for us normal folks would be something that could be deployed if conditions were favorable. Just like a sail that has a certain sweet spot for a particular condition. If they can make the curved boards actually work well without penalty during light air then that might be something. However so far there has not been any real evidence of the advantage versus the costs.
Bill Roberts
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Re: Foil Boats, Which Way From Here?

Post by Bill Roberts »

Drew,
The peak speed for a well done foil beach cat should be in the 25 to 28 knot range. The foiling Moth can hit 28 knots in its best spurt. The Moth shows us something. It has tall legs. The hull is well above the water, at least 2 ft. Where I sail, rivers, bays and ocean, in a foiling wind, 15 to 20 knots, the chopp/waves are at least 2 ft high. A foiling beach cat must be capable of foiling 2 to 3 ft off the water to clear the wave tops.
There was a Rave dealer right here in Stuart, Fl 10 years ago. Eric and I would see the Rave out on the river on some windy days. It would get up and accelerate and the hull would begin hitting waves at speed and spray would fly and the boat would slow down and come off the foils. Then the cycle would repeat itself. The boat could get up on foils but it was not high enough or tall enough to clear the waves and the boat could not sustain being foil born on the river in foiling wind strength. The foil boat would go in spurts. Sometimes it would go 50 to 100 yards but that was it. The vertical legs to the lifting foil , T or L, must be tall enough to hold the hull clear of the wave tops and also tall enough to keep the T or L part of the foil totally submerged. A lifting foil won't work if it flies through air inbetween waves.
Some simple design work: To clear 2ft waves the boards need to be 2.5ft tall. The lifting foil should always be at least 0.5ft below the valley between waves. The CB trunk is 2ft tall/deep. We have just figured out that the vertical part of the CB must be at least 5ft tall. If you are going to sail in bays and the ocean, we better make that 6ft tall for more wave clearance. This is for a vertical foil that is exposed to a small sideforce, like sailing downwind. If you want to be foil born sailing to windward, you need another couple of feet of board underwater to generate side force. Now we are up to 8ft tall vertical boards. With the side force acting on the lower 2 ft of the board, that puts a very large bending moment on the board where it exits the hull and a very large side force on the trunk/hull where the CB exits the hull. Maybe we should forget about foiling to windward. Let's get the downwind foiling working first where the CB side force is small.
J Drew
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Re: Foil Boats, Which Way From Here?

Post by J Drew »

That's a lot of surface area. Drag is drag, whether its vertical or horizontal. We were talking about .5 sq ft being a big difference with foil area. When you add 2 or 3 ft to the board, then look at how much more sustained wind it will take to keep it up. Sailing on the river, I rarely see a constant breeze that high.
Bill Roberts
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Re: Foil Boats, Which Way From Here?

Post by Bill Roberts »

Drew,
In these simple simon calculations we estimated that the CB needed to be 5 ft tall for sailing downwind in 2 ft waves/chop.
Two feet of the CB length is in the trunk. One half of one foot is always underwater; underwater below the wave bottoms/valleys. That leaves 2.5ft of CB height operating in spray and passing through wave tops. This 2.5ft of board height
is not generating side force so no induced drag. It is wetted area drag that is 50% of the time submerged and passing through a wave and the other 50% of the time it is inbetween waves and being exposed to spray. This 30 inches of board height is not nearly as draggy per inch of height as one inch of the lifting foil length. The lifting foil is working hard and making drag carrying the total weight of the boat. One inch of CB leg height is not nearly as draggy as one inch of lifting foil length.
An all carbon beach cat or tri would run really good in the French Ditch where you have strong, steady winds and no waves.
The correct name for speed records set there is "speed in a test tube".
J Drew
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Re: Foil Boats, Which Way From Here?

Post by J Drew »

Bill,
Ok, now I understand the difference. I was thinking of the occasional times when the boat was not completely up on the foils and most of the boards would be submerged. At that point I assumed they would give a lot more drag then waves and spray.
By the "French ditch" do you mean the Indian river? From what I've seen, it does have strong constant wind without much wave action. That might be why they did this years AC in the SF bay, similar conditions?
The St. Johns doesn't normally see the same conditions.

I have a new question, regarding the location of the foils. Would the foils need to be farther forward than where the typical boards are? I was looking at the AC72, and it seems they are forward in proportion to ours
Bill Roberts
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Re: Foil Boats, Which Way From Here?

Post by Bill Roberts »

Drew,
There is a big difference in drag on a foil between a wet foil and a wet foil that is generating lift or sideforce. The drag on a foil roughly doubles when you make it generate lift or sideforce. That part of the foil drag is called "induced drag".
Where Eric and I sail and test on the St Lucy River the wind is all over the place. Some days it is so bad, shifty, that we cannot take any meaningful data.
The AC races were held on San Francisco Bay because of the almost guaranted wind strength. It wouldn't have been much of a show to have foil boats that could not get up on their foils for lack of wind. I doubt that we will ever see anything like the AC72s in the AC again.
We should take notice that in the ultimate sailing contest there are minimum and maximum wind strengths for racing. Maybe we in beach cat racing should consider this also. It doesn't make sailboat racing any better or safer for the race committee to say at the skippers meeting,"every boat must make their own decision as to whether to sail today or not".
(Winds gusting to 25 knots.) "There will be no chase boats or safety boats". "You are on your own". These comments also do not relieve the race committee of any liability for sending the boats out to race in conditions where there probably are going to be turnovers and breakdowns with no chase boats to help in emergency.
ee
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