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-   -   Sailboat maximum airespeed (http://www.sailnet.com/forums/general-discussion-sailing-related/82809-sailboat-maximum-airespeed.html)

patrickbryant 01-17-2012 02:38 PM

Sailboat maximum airspeed
 
Colleagues:

After completing a particularly challenging voyage on the S.F. Bay last Sunday, it occurred to me that there must be a reason why my boat progressively slowed down while I was on a close reach into an increasing wind (topping out at about 40 knots). Any comments on my explanation below will be greatly appreciated.

There appears to be a maximum air speed (wind speed) at which the sails of a sailboat on a reaching point of sail can no longer overcome the induced drag produced by the sails themselves plus the parasitic drag of the wind-exposed area of the boat (the combined exposure of the freeboard and all of the other wind-exposed areas). Above that speed, "the curves cross" and the sails can no longer produce enough lift (forward propulsion) to overcome the aerodynamic drag of the boat -- so an equilibrium is reached: the boat will go no faster. If the air speed increases, the boat will slow down.

Lift (propulsion) of an airfoil (wing or sail) increases as the square of the velocity. But the power needed to overcome drag increases as the cube of velocity, so at some velocity, drag overcomes lift (propulsion). The same effect is seen with airplanes: if an airplane that cruises at 100 knots requires 85 horsepower to maintain level flight (lift-to-drag equilibrium), the amount of power needed to make the same airplane cruise level at 200 knots (assuming you could add a bigger engine without increasing the airplane’s weight) would be 85 X (2 cubed) , or 85 X 8 = 680 horsepower. In the case of a sailboat penetrating forward (reaching) in the airmass, when the wind speed doubles, the drag increases by a factor of 8, while the lift (propulsion) produced by the sails only increases by only a factor of 4. Equilibrium is reached at some critical speed, and the boat will go no faster. Increase the wind speed any more, and the boat slows down.

The challenge of reaching into a high wind can be mitigated somewhat by reducing the sail area (reefing), which reduces its induced and parasitic drag more than the reduction in lift, but the parasitic drag produced by the freeboard and other exposed areas (cabin top, rigging, etc.) cannot be reduced. So, regardless of what one does, any given boat has a maximum wind speed into which it can reach before it begins to slow down. The more freeboard and other wind exposed surfaces, the lower is that maximum speed.

casey1999 01-17-2012 03:10 PM

I think you are correct. Take it to the extreme. Could you sail to windward in a 150 knot wind? I would say no, the drag from your boats "wing" (sail) and the drag on your boat (plus wave impacts) would slow your progress to the point I think you would go backwards.

mm2187 01-17-2012 03:47 PM

I am not sure that I agree with your way of looking at it. I think it comes down to the boats ability... not equilibrium(granted I do not know any boat that can safely sail in 150 knots of wind). I can not think of any boat that has as much freeboad/mast and standing rigging that equils the amount of sail area possible. therfore there will never be equilibrium from that. Your forward momentium is driven by your boats ability to stay perpendicular to the water as the wind increases your ability to stay up in return decreases your power in your sails which will slow you down.

casey1999 01-17-2012 03:57 PM

Quote:

Originally Posted by mm2187 (Post 817900)
I am not sure that I agree with your way of looking at it. I think it comes down to the boats ability... not equilibrium(granted I do not know any boat that can safely sail in 150 knots of wind). I can not think of any boat that has as much freeboad/mast and standing rigging that equils the amount of sail area possible. therfore there will never be equilibrium from that. Your forward momentium is driven by your boats ability to stay perpendicular to the water as the wind increases your ability to stay up in return decreases your power in your sails which will slow you down.

I used 150 knots as a outrages extreme so that one could clearly see no standard boat could make headway in such a storm, and there must be a point where a sailboat cannot make forward motion against a certain wind speed. As wind speed increses, you must reef (reduce sail) so there could be a point where the wind friction of the rig and boat is greater than the forward force gernerated by the sail. Also, the actual force acting to make a sailboat go forward is a small fraction of the total forces acting on the boat- that is why there is a keel, to counter the leeward forces.

CapnBilll 01-17-2012 03:59 PM

In any point of sail there reaches a point where the forces exerted on the sail needed to further increase boat speed will only blow out a sail.

casey1999 01-17-2012 04:01 PM

Sailing faster than the wind - Wikipedia, the free encyclopedia
A good link

curtcee 01-17-2012 04:40 PM

Quote:

Originally Posted by casey1999 (Post 817908)

That's a good article. Thanks for posting it.

Curtis

Faster 01-17-2012 04:51 PM

I think the greater hindrances are hydrodynamic drags... low drag multihulls, esp those on foils are going to go much faster than our lead bombs.

Could be those craft (and ice boats) may get a chance to run into the aerodynamic limits, but since we're working in two distinct elements I think the aerodynamic ones won't come into play for the average boat....

This question is probably a good academic one, though, when dealing with extreme winds in the 100 Knt + range...

GeorgeB 01-17-2012 06:02 PM

Patrick, you need to find a copy of the polar diagram for your boat. This will tell you the maximum speed potential for windspeed, point of sail and sail size. Remember, as a displacement boat, you are constrained on the top end by your theoretical hull speed. I donít know what boats you sail on the Bay, but I have had the experience of going into horizontal stalls a couple of times on light displacement boats where a gust pined us over and we didnít have enough displacement/ momentum to carry forward boatspeed. Wound up getting dragged sideways a little bit. On our family cruiser, we do not have the rail crew to hold the boat upright and the constant feathering and square waves/chop knocks some boat speed off.

Most importantly, where were you sailing and what type of boat? I had to take care of some family business and wasnít able to get out on the Bay. Sounds like I missed a good one.

-OvO- 01-17-2012 06:17 PM

Time to build a flying vertical wing...


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