Fully powered up (10-15 knot breeze) a sailboat generates about 1HP for every 50 sqr feet of sail.
That's the NET effect of the vector math. The F(sail) and F(keel) vectors are much longer.
Thinking too much about keel forces
- Thread starter Daveinet
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That's the NET effect of the vector math. The F(sail) and F(keel) vectors are much longer.
People: I was privileged to sail and race a class E inland scow. She is and was 28' long, about 8' beam, and had bilge boards. Its ballast are her crew, the boards were only to supply the appropriate resistance to stop side slip. The keel boats have to have a uniform profile on both sides, a bilge board does not. The boards on the E were curved like airplane wings. They had to be on the correct side to keep the boat on course. Yes, the boards do provide "LIFT". I know. One day because of conditions (short knowledgeable crew to man back stays, main and jib and boards) when tacking. We only were going down wind. I put both boards down just about one foot. The wind was strong enough to push the boat fast enough that the boards "lifted" 24' out of the water. Yes, I have told this story once before.
Old Salt
Old Salt
what keeps the boat from overturning? well, if i am to understand your description, you are talking about, basically, an upside down sailboat. one with no keel opposite of it's 'sail'.
to get to an answer to your question, you have to understand what causes heel on a sailboat. most people would say that the force of the wind in your sails causes heel but this is not entirely true. take a centerboard sailboat. sail across or up wind. the boat heels. raise the board. now, the boat moves down wind, instead of up or across, and heel will be reduced.
the keel, or other underwater foil, is responsible for heel. because the sails generate lift to lee above the water line and the keel generates lift to weather (in opposition to the sails, thus stopping leeway), there is a pinwheel movement generated around the waterline, which acts as the pivot point. this is heel. remove the lift created by the keel, which counters the lift created by the sails, and the boat simply goes the way the wind blows.
it's usually depicted as lever arms. one lever arm above the water and one below. the deeper the keel is the more heeling force it creates because it's like using a longer lever to lift something. sailboats are affected by a number of these 'arms'. ballast creates a righting arm that counters the heel created by the sal and keel.
fixed ballast on a keel acts differently that the moveable ballast of sailors on a dinghy. as the fixed keel boat heels, the righting arm of the ballast gets stronger as the heeling force of the wind gets weaker. in a dinghy, the righting arm gets weaker as the boat heels. it is strongest when the boat is flat or nearly flat. once the boat has heeled far enough the moveable ballast is directly over the center of the boat and has no righting force at all. a little farther over and it begins to get stronger; as force to help you capsize.
the boat you are talking about isn't acted on by forces on two different planes. it is only affected by one force. that force is the lift of the keel. the cable stops leeway. so, it won't overturn and needs no ballast.
Popeye ...
The forces generating lift is what got me thinking about this type of ferry, which is known as a Reaction ferry. They certainly aren't built as a foil and their usage pretty much dictates how the boats are built, many with 2 pontoons to traverse shallow rivers.
But it did make me wonder what it would take to make one designed for speed. I was thinking that horsepower would be improved by keel area, much as Jackdaw describes horsepower generated by sail area.
I think it would be cool to design one with a narrow, slippery hull form and relatively deep keel to see how fast it could zip across the water by increasing the angle with the current.
The forces generating lift is what got me thinking about this type of ferry, which is known as a Reaction ferry. They certainly aren't built as a foil and their usage pretty much dictates how the boats are built, many with 2 pontoons to traverse shallow rivers.
But it did make me wonder what it would take to make one designed for speed. I was thinking that horsepower would be improved by keel area, much as Jackdaw describes horsepower generated by sail area.
I think it would be cool to design one with a narrow, slippery hull form and relatively deep keel to see how fast it could zip across the water by increasing the angle with the current.
Ok just as an experiment I put a large rocket engine on the back of the boat, well I did not go any faster but that rocket fuel cost a lot $$$ so I put the sail up and decided that we should al go the same slow.I the scheme of all things if you want to go fast buy a speed boat if you want to sail just love it for what it is, FUN !!!!!!
Much of where I was coming from on this is illustrated on the PDF that Jibes posted on page 3. Looking at figure 6, most of the direction of pressure is straight sideways, with very little push forward. So that makes me ask if the whole aft end of the sail is providing any propulsion at all. Its seems the answer is some, because if the keel is angled away from the wind slightly, pushing the keel sideways results in forward motion .
Wouldn't a boat exceeding hull speed at such a gross rate just sink. Unless I misunderstand hull speed or you're just being fececious.
The notion that the lifting force of a sail varies in direction and strength over its area is well understood. The area of max strength is normally spread mostly vertically over the area just in front of max camber, as shown below. The NET of all the forces give a lift vector that is just forward of the chord of the sail. Likewise, the lift vector from the keel is just forward of the chord of the keel. The resulting addition of these vectors gives the boat its forward motion.
You're right.... but we COULD. Pikes Bay and the Bayfield channel are totally ice free. There is ice down in the shallow waters around Ashland. We are scheduled to get Kestrel launched the first weekend in May.
Lake Minnetonka is another story. Its been totally ice-free for 2 weeks and the WYC has been letting boat use the crane for the last 7 days. But I've got a punch list a page long, and am heading to France tonight. So BlueJ will be lucky if she beats Kestrel into the water!
Ironwood.... hmm.. While at Michigan Tech I killed way to many brain cells at the Iron Horse in Hurley......
Just not well understood by me, which is why I keep asking questions.
When I see it, I don't much forward force, most of the load is sideways. One begins to wonder at what point does the roach add more drag than forward force. It would seem like the higher the aspect, the better the sail would point.
I wrote a detailed and well-reasoned comment on this, and then my connection crashed and I lost it. So I'll try again with a shorter version:
I'm a semi-retired engineer with a specialty in fluid mechanics, and I'm afraid I have to burst your bubble a bit. A sharp leading edge is actually a bad idea. Have you noticed that airplane wings don't have sharp edges unless they are supersonic fighter jets (and that's a whole different discussion)? That's because unless the angle of attack is dead on the sharp edge (not going to happen on a boat keel) the drag is actually greater due to vortices that spin off the edge. On the other hand, if what you had was a really blunt edge, making a smaller radius would help.
The only way to see what would work best is the sketch the cross-section and add streamlines to it. The streamlines have to smoothly flow over both sides without having to make any really sharp corners.
This is actually a complicated subject that few people other than specialized engineers understand, so please don't feel picked on. The great bulk of the population would have done the same thing you did thinking they were doing the right thing. For the same reason that the average person would think that a flat plate would have low drag, which it very much doesn't.
i think you misunderstand hull speed. hull speed is not the maximum speed the structure of the hull can take before it breaks up. hull speed has to do with the bow wave.
as a boat moves through the water, the bow creates a wave. as the wave passes by the boat, another wave will be generated. what you end up with is two waves with a valley between. the forward wave will always be at the bow. the faster the boat goes, the longer the distance is between these waves. the boat will continue to gain speed until the rear wave is at the stern. at this point, the boat will sit between both of these waves. it will be unable to get out of this valley except in two ways.
one way is to plane. some boats have planing hulls with narrow entries that flatten out aft. these hulls, at a certain speed, actually rise up and plane on top of the water. by doing this, they can lift themselves out of the valley and accelerate beyond their hull speed. many dinghies can do this.
another way is to cut through the bow wave. this requires a lot of extra speed and a very fine entry. think cigar boat.
that's kind of simplified, but that's the gist of it.
the longer the boat is, the faster it can go before the rear wave hits the stern and it gets stuck in the valley. that's why it's called a hull speed and it's figured out according to lwl. at one time in history, racing sailboats had long overhangs so that they were within the specified hull length rules when level but had longer water lengths when heeled. it was a way to cheat the rules and get a greater hull speed when on the wind.