I see what you're saying I'm thinking displacement hull when you say hull speed, when even a planning hull has a hull speed, buy can exceed that with the right conditions.
Thinking too much about keel forces
- Thread starter Daveinet
- Start date
Not always. If the hull is planing long before it reaches that theoretical number, than the number doesn't exist. My boat seems to do that. There really is no bow wave to climb over before it planes. It just goes faster and faster with more wind. I think your skows will do that as well.
Power boats use the motor to push the boat over the bow wave. I remember one time towing a Mercruiser with a dead motor. The harder we pulled, the higher the nose pointed in the air and the bigger the bow wave was. We probably had a 4 foot wave in front of the boat, with the rear of the boat sitting about 2 feet lower in the big hole that was being scooped out. The boat would not plane, no matter how hard we pulled.
Yes, the keel is a foil and any flow over it will create "lift"--but under normal circumstances the boat is moved forward by the lift created by the sail(s). A current over the keel could cause lift but it is doubtful that lift would be sufficient to move the boat forward against the current creating the lift. It is more likely that it would move at an angle to the current. The resolution of all these forces is pretty straightforward Vector math. It is the NET balance of force above and below that determine where and how fast the boat will go....
And where you point the rudder! I'm surprised no one has talked about the rudder. You know, the board thingy that hangs out the back and turns opposite from where you want to go. Explain THAT!
that one's pretty easy, actually. the rudder is a foil. when going straight, it has the same basic lift qualities as the keel. however, turn the ridder and it acts as the flaps on an airplane wing. you push the tiller to port, the rudder angles to starboard. this places the rudder's foil at a greater angle to the flow of water, creating lift and pulling the stern through the turn.
by the way, it is right to say the theoretical hull speed, as was said above. it is certainly no fast rule.
despite the jesting nature of your post: also not true. don't forget there are 60 mph sailboats, now.
Rudders are (or should be) foils. Just like keels. On our 367 the rudders generates HUGE lift. But you have to keep the AOA (angle of attack) very small. That's why we balance the boat with the trim of the mainsail and keep the angle of the rudder less than 5 degrees unless we are turning the boat. Turn too far, and the foil stalls. That kills steerage and boatspeed.
When it enters that mode, any boat that 'foils' is not longer a boat in the traditional sense.
Any foil that operates at an (small) angle of attack generates a force that is probably not perpendicular to the cord line of the foil. Generally when operating at small angles of attack the force has a forward (into the flow of the fluid) component soooooo
your keel is actually helping to push the boat along.
your keel is actually helping to push the boat along.
there is always a forward component. lift is perpendicular to the cord but, sails aren't usually sheeted in so far that the lift is only sideways as compared to the direction of the boat. sail lift is leeward and forward. keel lift is windward and forward. thus, the whole thing works to move the boat forward.
Bill,
We're already discussed this exact point. Both the lift vectors have a slight forward 'tilt' to them. They HAVE to for the boat to move forward at all. If they did not (meaning that they where both perfectly perpendicular to the cord), the vector math would not have a forward component meaning the boat would not move.
Wouldn't it help to clarify this thread if a simple drawing of the force vectors were posted? Anybody willing?
Gudenuph
Gudenuph