Sunfish Hull Speed...?
- Thread starter Simon Sexton
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now class, turn to page 267 of your sailing bible and learn what the term "hull speed" means.
s.b. = high performance sailing by frank bethwaite
hull speed is a nautical term that in lay terms means that point when the stern of the vessel falls of stern wave.
s.b. = high performance sailing by frank bethwaite
hull speed is a nautical term that in lay terms means that point when the stern of the vessel falls of stern wave.
Submerged bodies do not experience hull speed.
Hull speed is a wave phenomenon. Simply stated, the waves generated by the hull moving through the water become longer as the speed increases. When the wave length equals the waterline length, the boat faces maximum bow and stern peaks of a single wave that require a large increase in power to overcome. This is hull speed.
Cavitation is a limit of underwater speed. Where a shape curves back toward the centerline underwater, low pressure is generated. Since water is not compressible (like air) this low pressure sucks water down the curve. At some speed, the pressure is so low that the water vaporizes, generating vapor bubbles with very low pressure inside. When these bubbles move to an area where pressure begins to rise, they implode forcefully, easily damaging the surface for the underwater body. This is cavitation, and it is loud, and can do serious damage (and, I suppose, pain in a living body, a tuna for instance). Cavitation does not slow an underwater body directly, but quickly creates enough damage to compromise performance at high speeds. Cavitation seriously pits high speed propellers. Submarine propellers are designed to postpone the onset of cavitation because of the easily detectable noise it makes.
Hull speed is a wave phenomenon. Simply stated, the waves generated by the hull moving through the water become longer as the speed increases. When the wave length equals the waterline length, the boat faces maximum bow and stern peaks of a single wave that require a large increase in power to overcome. This is hull speed.
Cavitation is a limit of underwater speed. Where a shape curves back toward the centerline underwater, low pressure is generated. Since water is not compressible (like air) this low pressure sucks water down the curve. At some speed, the pressure is so low that the water vaporizes, generating vapor bubbles with very low pressure inside. When these bubbles move to an area where pressure begins to rise, they implode forcefully, easily damaging the surface for the underwater body. This is cavitation, and it is loud, and can do serious damage (and, I suppose, pain in a living body, a tuna for instance). Cavitation does not slow an underwater body directly, but quickly creates enough damage to compromise performance at high speeds. Cavitation seriously pits high speed propellers. Submarine propellers are designed to postpone the onset of cavitation because of the easily detectable noise it makes.
Having been on a J33 at 10 knots, well over its hull speed of 7.1, I can tell you even those old hull shapes can still plane. The keel was also most assuredly still attached. We were going on a broad reach under Spinnaker, in probably close to 20 knots of wind and calm seas. It's just about having enough power that the force of the water running under the hull can lift the boat up a bit. Newer hull shapes and foils reduce the amount of energy required to do that, but enough energy will do it for pretty much anything.
When I was in my teens, I spent a lot of time on a sunfish. I took that boat out in all kinds of weather, even when the gale force flags were flying. I remember one particular day that the harbor patrol (Larry) followed me out to the mouth of the inlet, apparently thinking that I was about to be in need of rescue, only to shake his head & turn back when I ventured outside. After about an hour of beating upwind from there (& spilling a lot of wind on the way to keep from capsizing), I finally turned around & took off on a reach. That boat FLEW. I was bouncing from wave top to wave top as I hung as far off the stern as I could stretch myself. The bow dipped slightly under water several times, but somehow I managed to not pitchpole. When I got back to shore, I found that my spars were bent. I think that I would be conservative if I estimated my peak boat speed at 20 knots that day. It was a hoot. It was a frightening hoot, but it was a hoot. I feel confident in saying that Sunfish are not constrained by theoretical hull speed calculations if you have enough wind available.
Simon, what JS said. I'm not trying to say a sailor can make any hull plane. Only that with enough application of force, any hull will plane. Some designs plane easily, some near impossible. One has to consider buoyancy forces and lifting shapes also.
No.
Hull speed is a result of fighting gravity to climb your bow wave. Submerged objects don't have to do that. Submarines only have to deal with hull speed when on the surface. Under water, they go much faster. They do have a bow compression wave and lengthening wakes as they increase in speed. I don't know what the results are when they underwater wake begins to exceed their hull length. Logically, the "trough" of the wake is a low pressure area and so there would be that drag rearward versus the bow's high pressure. There may be some analogue to a sonic boom if the boat can begin out running the creation of its bow wake too. I've seen a nuclear submarine heading out of the Chesapeake, just the conning tower visible. Incredible the speed she was making. I blinked and she was gone, submerged. The spray still in the air from her wake. She had to have been going over 30 knots before disappearing.
Can a sub go 70 knots? That doesn't seem outrageous to me.
-Will (Dragonfly)
Oops here goes another conversation about whether sails “push” or “pull” the vessel to its hull speed. Sorry couldn’t resist stirring the pot <grin>
Simon. Just because you are a young whipper snapper and in school. Does not mean us old folks get confused when discussing math. One or two of us did have math in grade school. And yes in might have been a one room school house on the prairie. But we had chalk boards and we could do our ciphers.
I wasn't implying that, sir; my apologies for the misunderstanding. One of the reasons I joined this forum was because of the awe-inspiring level of knowledge everyone has.
Simon, JS knows that. He's just yanking your chain.
-Will (Dragonfly)
-Will (Dragonfly)