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  #1  
Old 07-03-2007
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Hull Speed

Can a boat sail faster than hull speed and how good is the formula for calculation? If hull speed is calculated by the sqaure root of the lenght at the water line times 1.34 it does not seem right to me. My boat is 26 ft LOA but is 23 ft at the water line. The sqaure root of 23 is approximately 4.8 and if I multiply this by 1.34 I get a hull speed of 6.4. According to my knot meter I sail quicker than that on a regular basis. Is my formula wrong, is my math wrong, or is my knot meter wrong?
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Old 07-03-2007
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Quote:
Originally Posted by timangiel
Can a boat sail faster than hull speed and how good is the formula for calculation? If hull speed is calculated by the sqaure root of the lenght at the water line times 1.34 it does not seem right to me. My boat is 26 ft LOA but is 23 ft at the water line. The sqaure root of 23 is approximately 4.8 and if I multiply this by 1.34 I get a hull speed of 6.4. According to my knot meter I sail quicker than that on a regular basis. Is my formula wrong, is my math wrong, or is my knot meter wrong?
I don't know a thing about the math involved, but it is absolutely possible to sail faster than hull speed. Boats do it all the time when they "surf".
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Old 07-03-2007
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That is the quick math. To get your totally correct hull speed, you would have to know your exact amount of wetted surface. Combined with the pitch of your bow (Bruce Number ) . This speed would only be good in slack water with no waves, as any wave action would change the speed. So unless the quick hull speed is drastically lower then the real speed though the water, I would just assume that the 1.34 multiplier is a little Conservative for your boat
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Old 07-03-2007
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You can also check your speed on your gps.
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Old 07-03-2007
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Your knot meter probably registers a bit fast . You can compare to a GPS reading, but the two instruments are not measuring the same thing. Still, you can compare the two on a calm day with no current. Or look for a measured mile on the water in your area and check your knot meter against that. But conditions have to be perfect for any of these to be accurate.

And as a designer named Robert Perry told me once, the 1.34 multiplier is only a starting point.
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Old 07-03-2007
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Most of the time I sail above the hull speed.

What boat do you have? Boat speed depends on your hull shape and how fast it rides the wave.
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Old 07-03-2007
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Quote:
Originally Posted by timangiel
Can a boat sail faster than hull speed and how good is the formula for calculation? If hull speed is calculated by the sqaure root of the lenght at the water line times 1.34 it does not seem right to me. My boat is 26 ft LOA but is 23 ft at the water line. The sqaure root of 23 is approximately 4.8 and if I multiply this by 1.34 I get a hull speed of 6.4. According to my knot meter I sail quicker than that on a regular basis. Is my formula wrong, is my math wrong, or is my knot meter wrong?
A knot meter isn't the thing to use to check boat speed. It may not be calibrated accurately, it may be affected by tidal currents... Have you checked the speed via a GPS.

Also, what kind of boat do you have. There are many smaller boats that can plane or surf. A Foiler Moth can sail well above its hull speed, since it lifts up on a set of hydrofoils. The basic SQRT (LWL) * 1.34 rule really only applies to displacement hulls. It fails on multihulls, small dinghies, foil-equipped boats, and a few others.

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Old 07-03-2007
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Hull speed is the speed of the WAVES that a boat makes. Hull speed is when the bow wave and the stern wave are UNDER the boat and are equally supporting the boat.
When a boat goes faster than the hull speed (of the waves) the bow wave will move further aft under the bow (lifting it) and the stern wave will move AFT of the stern (no longer supporting the stern).
:-)
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Old 07-03-2007
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The boat is a S2 8.0B. I have noticed that the speed indicated by my GPS does not always agree with the knot meter, sometimes they are exactly the same, but usually it is reasonably close. I choose to believe which ever is faster. I would assume that each could be inaccurate under different conditions for different reasons, but I am curious to know what the maximum possible speed is that could be obtained.
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The difference between your knot meter and the gps is that your knot meter is reading the speed over water. Your gps is reading speed over ground.
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