Hullspeed of a sailboat

[hikingchrs]

What happends when you reach hullspeed on a sail boat, does any increase in wind speed only push the boat a little faster than before with lots more heeling. also what is the hull speed of a 15' boat, 4.5' beam, double ender?
Chris

[paladin]

about 1.5 times the square root of the waterline length......

[Meerkat]

Originally posted by hikingchrs:
What happends when you reach hullspeed on a sail boat, does any increase in wind speed only push the boat a little faster than before with lots more heeling.
ChrisAn increase in power causes a linear increase in speed up to hull speed. Above hull speed, power input must be exponential to see any increase in speed. If it takes 1 unit of power to reach hull speed, it will take 2 units to go 1 kt over hull speed and 4 units to go 2 kts over hull speed.

I was told by Bob Perry, noted NA, that hull speed is calculated as 1.41 * sqrt(LWL). It seems that the "magic" 1.41 (or 1.3, 1.5 etc.) varies by hull design and boat type (power vs. sail, planing hull vs displacement hull, etc.)

[WindHawk]

A sometimes more usefull formula (velocity ratio)for boat comparison purposes uses the hull speed as the denominator, while the numerator is biased to show a long waterline and light displacement equate to a faster boat:

VELOCITY RATIO =

1.88*lwl^.5*sailarea^.33/disp^.25 /(1.34*lwl^.5)

A good relatively fast boat will come out to about 1.0, an all-out race boat will be in the 1.5 to 1.8 range.

[Ian McColgin]

Back to the why: An object moving in water will produce a bow wave and one or more secondary waves. Depending on the displacement and shape of the object, these waves will be smaller or larger and will be spaced further apart the faster you go.

It works out about crest to crest in feet is the speed squared in knots divided by 1.69. Give or take. This is the same as saying that 1.3 times the square root of the wave seperation in feet will be the speed in knots.

1.3 is about typical for an ok displacement hull. Fatter gets down to 1.2 or so, really great hull design can get over 1.4.

As the boat goes faster, you can end up with only two waves near the hull: Bow and stern or wake. The faster you push, the more the stern wave is behind the transom and the boat must climb up hill, as it were, out of that trough and over its own bow wave.

Here's where the exponential increase in power is required and here is where simple wave formation outweighs other factors such as hull surface friction.

If a boat really climbs out fully, it's on a plane and the problems of wave formation limiting speed are no longer relevant. A displacement hull cannot climb out. Semi-displacement (sometimes called semi-planing) hulls climb to the point where the forward third of the boat are ahead of the bow wave. These typically have neither the power nor the surfaces aft to get fully on a plane.

[Hwyl]

Originally posted by hikingchrs:
What happends when you reach hullspeed on a sail boat, does any increase in wind speed only push the boat a little faster than before with lots more heeling. also what is the hull speed of a 15' boat, 4.5' beam, double ender?
ChrisFor a double ender it's less than 1.4. That's because a transom boat has an "an imaginary pointy bit" at the back. So you are looking at about 4 knots for your boat.

I was going to mention this in the other thread. Your boat could get awfully unstable in a breeze. The bow would tend to bury and you could have less control. Ask CLC directly, you paid enough for the kit.

Sailing lesson offer (see other thread) still stands.

[Tom Lathrop]

True that a displacement sailboat produces a bow wave but, perhaps more importantly, it creates a hollow behind the bow wave. A displacement boat is supported mainly by the bouyancy of its fat middle sections, just where this hollow in the water occurs. More power means more bow wave and more hole in the water and less support for the boat. The rockered shape of the hull also creates negative lift to suck the hull down into the hole even more. Adding still more power, as when being towed by a large powerful boat, will eventually drive the boat underwater and it will sink.

Even the USCG has been known to do this while on a "rescue".

[hikingchrs]

Thanks everyone,
To be honest I am not sure I am doing anything wrong I can get around the Lake in a good breeze tack and change course all I want moves along pretty good compared other sailboats on the lake,
this is my theory, that the wind is pushing the boat along at above the hull speed creating much resistance and lots of stain on the sail, when I go to tack I haul in the sheet to try ot help the tack along the yard is still hanging off the side of the boat and will not follow boom around since it is still full of lots of air. Bottom line is that I might need reef if I want to be out in these conditions... I have not spoken to CLC about my troubles becouse I am not using there sail design I have an additional 10 sq'. the additional sail area is great for the times I do most of my sailing in the evening afterwork when the wind has died down alot. I just got to figure out at what wind speed I need to reef...this weekend I will take out my GPS to figure out the speed I am getting.
Chris

[ 07-13-2005, 08:44 AM: Message edited by: hikingchrs ]