Why my boat can't move forward in strong wind???

[redmapleleaf]

Hi all, I have made the changes to the boat and now it is sailing quite nicely. Here is what I did
1. made a new centerboard that is 3 times bigger than before
2. made a new rudder that is 2 times bigger than before
3. add a jib

I took it out on a fairly windy day 12+mph and it sailed very well. I had to tie some heavy lumbers to the outrigger to prevent it from heeling and tipping over. Despite the heavy logs it still move well into the wind.

You can see some of the pictures of the boat at http://picasaweb.google.com/redmaple...riggerSailboat

Thank you for all your help.

[Fanie]

I told you you have a hull missing. It adds nice space if you put a trampoline on, no more logs and lumber passengers and will be faster too.

Well done, glad it's coming together.

[the1much]

well THANK-YOU for updating us,,and for the pics,,and glad to see things are working better,,,,,just wait till your next 1

[Boatpride]

The Bernoulli effect drives your boat through translation of wind pressure

changes on both sides of your sail. Without going any further in to physics

on this, i would suggest that the sail generates too much potential power for

the boat at the higher wind speeds. So as the guys have suggested, you

need more resistance to the lateral effects of the higher wind. IE redesign

your keel and make it deeper offering more resistance so you can sail in

higher winds.

I have finished a page on the topic of how sails work. You can read here if

you like. It goes in to the Bernoulli effect and explains the effect of the wind

on sails. http://boatpride.com/how-sails-work.html

Best Wishes

[marshmat]

Hi Chris,

May I recommend you take a look through any textbook on aerodynamics, or anything at all by C.A. Marchaj.

The flow over a sail, or any foil for that matter, is much more complicated than can be explained by the Bernoulli effect ( P0 = p +1/2 ρ V2 + ρ g Z ) alone. There is much more to aerodynamics than "flow speeds up, therefore pressure drops". To estimate the actual lift and drag on a foil requires, at a minimum, the potential flow field and net circulation of the flow (look up the Kutta-Joukowski theorem as a starting point). Net forces are the result of integrating the pressure distribution over the entire foil.

Computing the amount of lift an aircraft wing, sailboat keel or other foil gets from the Bernoulli effect due to velocity differential between the high- and low-pressure faces, compared to the lift calculated from the net circulation of the flow, is a standard exam question in third-year fluid mechanics. The answer, by the way, is typically on the order of 2 to 3 percent of the total lift.

[markdrela]

Computing the amount of lift an aircraft wing, sailboat keel or other foil gets from the Bernoulli effect due to velocity differential between the high- and low-pressure faces, compared to the lift calculated from the net circulation of the flow, is a standard exam question in third-year fluid mechanics. The answer, by the way, is typically on the order of 2 to 3 percent of the total lift.

That's not correct. You can calculate the lift by integrating the surface pressure obtained from the surface velocity via the Bernoulli equation, OR you can calculate the lift from the circulation, OR you can calculate the lift from the Kelvin impulse in the trailing vortex wake.

These are alternative lift-calculation methods --- they are not additive. Each one gives 100% of the lift all by itself.

[ancient kayaker]

Nice thread with a nice conclusion; the boat goes faster and better now. You've done a lot to improve the situation, but there's more to be had. A sailing canoe can be one of the fastest boats of its size around.

I had personal experience with a canoe rigged for sailing similar to your one, but a bit smaller sail. I sailed it sitting on the gunnel to keep the ama down and most of the time it plodded along in a pedestrian manner as an assymmetrical catamaran. Then one day things went very quiet; assuming I had slowed down for some reason I looked over the side, the water was ripping by and the ama was a foot above the waves. I'm embarassed to admit that I panicked, shifted my weight to bring the ama down and immediately lost half the speed. At that point I realised where I had been going wrong.

From your photos I see the ama is pressed into the water. With its displacement hull it would have a low hull speed because it is short so you get extra drag. A skimming style ama would plane at speed with less drag. I preferred two floats mounted high which are for insurance only, body weight is used to balance the boat so both are out of the water. Get it up man; live fast and dangerously! If you start to encounter a wet underwear condition you can always let out the sheet.

I would have thought that 50 sq ft was plenty of sail: most canoes seem to be over-canvassed; I think this is inherited from purpose-built sailing canoes of the early 1900's which were usually rich man's toys, lead ballasted with bronze centerboards and no ama. If you need more area I'm not sure I would add a jib; the classic rig from a canoe seems to be ketch or yawl, which is very easy to control and balance. Once balanced the weather helm vanishes, the rudder quits grumbling and the speed increases. But I haven't tried it. Yet.

[marshmat]

Mark,

You are correct that integrating the surface pressure distribution gives the total lift and drag. How you obtain this surface pressure distribution is up to you.

The description on the website Chris references is inaccurate in describing the difference in pressure as being due purely to the flow over the convex side having to go faster than the flow over the concave side. It makes no attempt to explain why this is true and tends to reinforce the pervasive, incorrect definitions of lift we all recall from grade school. This is why I suggested he research it more thoroughly.

When you take the difference in velocity between the upper and lower faces of a wing, assuming the (incorrect) equal-transit-time approach often taught in grade school, and calculate the 1/2 ρ V^2 term for each, you find that the lift due to this effect is a tiny fraction of the total. And when you go to measure the flow on an actual wing, you find that the actual velocities are nothing like that- the flow over the convex side is much, much faster than equal transit times would suggest. Applying Bernoulli's equation to the actual velocity distribution does work and does yield a reasonable approximation to the actual lift.

When you do the same for a sail, you find that a sail has negligible thickness- the convex and concave sides have the same path length. The Bernoulli effect doesn't contribute, in the sense of the (incorrect) "equal transit time" concept. But the Bernoulli equation is still valid, if you look at the true velocity profile of the flow.

Net circulation is not the only method of calculating lift. As Mark points out, the trailing vortex can also be used, among other methods. I reference the net circulation method frequently because, in my experience, it is the one that is misinterpreted least often.

Bernoulli's equation works. You just need to be careful how you apply it. The "Bernoulli effect" means that a flow speeds up, therefore the pressure decreases. Attempting to explain lift in this manner without also explaining the origins of the true velocity distribution (not the equal-transit-time thing) is at best incomplete and at worst tends to perpetuate misunderstanding.

Does this make more sense? I apologize if any of you had trouble following my logic in the earlier post.

[Boatpride]

Hi Marshmat,

yes i will read up on those books by C.A. Marchaj. I am aware that an eighteenth century concept is a bit leaky in this day and age! 2 or 3% is very low - wow!