hull speed

[Steve Dion]

It's only therotical

Your published hull speed on the HOW site is 6.2 (assuming it is correct). But many boats will exceed this speed. Especially when you have a lot of power like 25-30 knot winds and you have her reefed down.I have hit 8-9.5 on the knot meter running downwind in a following sea on our H'31.

 

[uncle.al@wcnet.net]

pretty fundamental question, i guess using the formula sq rt lwl x sq rt of 2, my 23.5 hunter has a hull speed of 6.5. i had it up to 6.4 the other day in fairly easy conditions -- 10 to 15, i guess, not over-trimmed, on a reach. yesterday, in 25, gusting to 30, i had the main reefed, no jib up, with the wind on the quarter, and the gps said 7.2. what the hell is hull speed? i remember reading about square-rigged ships that kept piling on the canvas in the 1800's and finally sailing under, and had the idea that the bottom of the boat was like the convex top of a plane wing and would tend to squat, the faster the water moved over the bottom's surface, but i don't recall knowing what are the consequences of exceeding hull speed. appreciate getting the real story on this good forum, of 'what does hull speed mean'?. thanks.

 

[Been there]

It's not what most people think

Hull speed, by definition, is the speed of a water wave whose length is the same as the hull's waterline. The formula is k times sqrt(LWL), where the constant k is close to 1.34. Like most physical "constants," k is a variable, in adequately broad context. It is a function of the properties of water and of acceleration of gravity at earth's surface. It does NOT a function of hull shape. For us, that makes it a constant.Hull speed is NOT a hard speed limit. It is true that the force required to maintain a displacement hull's speed through the water starts to increase very rapidly as speed increases. An early explanation was that the hull, once traveling at a speed that put it evenly between the crests of its bow wave and quarter wave, required ever more energy to "climb over" its bow wave, while its stern sank in the hollow between the two crests. If you imagine a boat in profile with wave crests at stem and stern and a trough in between, that wave is travelling at hull speed.This story leaves out several important details. (a) While the energy curve takes an upward bend near hull speed, it remains continuous. There is no wall. Powerful rigs on stable hulls can climb the curve higher than weak rigs. (b) Resistance is different for different hull shapes. As a result, some sailboats routinely exceed hull speed, while others will never see it. Most modern sailboats will exceed hull speed in good conditions. Catamarans and trimarans laugh at it. (And yes, these are displacement hulls.)

 

[Paul Akers]

Did you add the current?

GPS will show you speed over ground and not your speed through the water. So if you had a tail wind or the tide/current was running WITH you, you could have easily exceeded hull speed over the ground.

 

[Big T]

Squat

Just for trivia, there is such a nautical thing called squat. In fact, the Queen Elizebeth 2 (I'm almost certain that is the correct liner) ran aground with severe damage many years ago due to squat. She was trying to make time someplace where all charts showed adequate depth (though not with great margin). Unfortunately, "squat" lowered her hull to the bottom. Either the learning channel or discovery aired this.

 

[ED KNEBEL]

PLANING

On a broad reach in 20-30 knots of wind, you should be able to get the 23.5 on a plane, which will easily exceed displacement hull speed. The 23.5 does plane eaiser because of the wide rear end. Often you will feel a shutter, or different vibration in the tiller. Try to head off alittle while going down the wave. It makes my day!

 

[Paul Jacobson]

Hull Speed Explained!?

Hull Speed;When a boat goes through the water, it creates two waves. The first is the bow wave (self explanitory.) The second is the "quarter" wave. If you look over the side while your boat moving slowly, you will see that imediately behind the bow wave, the water dips down, like a wave in reverse. And just behind this is another wave, called the quarter wave. As the boat speeds up the quarter wave moves further and further back, with respect to the boat. Finally, the boat reaches a speed such that the quarter wave is formed by the stern. So, this means that there is a high point in the water at the front of the boat and a high point in the water at the back of the boat, with a low point in the middle, and everything is balanced. But, in order to go faster, the boat must start to also go "uphill', since in going faster the quarter wave will be left behind. The point of balance, or equalibrium, for any given boat, is the hull speed. Anything faster than that means the hull is starting to lift out of the water, or "plane". The hull speed, therefore, is a function of the boat's length and the specific gravity of the water. In practical terms, rthe following formula is used. Square root of waterline length of boat (in feet) times 1.34 = hull speed (in knots).Example: If you have a 28 or 29 foot boat, the waterline length is around 25 feet. Square root of 25 = 5 5 times 1.34 = 6.7 knotsThis is all very theoretical, but I hope it helps, a little.