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 PalmettoSailor 07-24-2007 12:42 PM

Hull Speed?

Can someone explain hull speed to me? I thought I understood it but a thought occured to me as I was talking over a "Dark and Stormy" that caused me to question my understanding.

Does hull speed mean the maximum speed a boat (displacement hull) can travel while under power or the max speed under sail?

My 32' boat motors about 5.5 to 6 knots full power from it's 18 horse aux, but I've seen higher speeds (even touching 8 knots for a second)on the knot meter while sailing and the KM speed was consistent with my GPS.

I'm thinking when I hear the term "hull speed" it applies to the theoretical max speed while under power and not heeled over resulting in a longer water line allowing a higher speed.

 Giulietta 07-24-2007 01:03 PM

Midliefesailor

Yes, its correct, you can get higher speeds than theoretical hull speed, (which is a calculation based on hull lenght at water line, based on bow wave height and separation between the bow and aft waves), if your hull starts planing, meaning it rode the wave and is now "surfing" on it, so the wave is no longer an obstacle, or if as you well said, when healing, the water line increases due to the overhangs, but so does drag and other limitations.

HULL SPEED

GZ

and this site. In this site if you place the cursor over the rectangles, it explains what is what.

SAILCALCULATOR

Hope it helps

 tommyt 07-24-2007 02:34 PM

I was going to say that Alex or Jeff would be along shortly, but Alex beat me to it.

Alex, I get error messages on all the sites. Is it me?

I was looking this up the other day. I know that the theoretical hull speed on my C34 is in the mid to high 7's. I also know that I have surfed it at much higher speeds. I need to do a little more looking and I know that your references will probably give me some answers.

 SanderO 07-24-2007 02:44 PM

A displacement hull does not plane... that is a sailboat, It can slide down a wave or surf at more than hull speed.

The notion is that the displacement hull creates a bow wave as it pushes the water aside. The wave gets steeper and longer as it moves faster. At some point the vessel is in a trough and it would be trying to go uphill. I don't think a powered sailboat will create a longer and longer wave... steeper and steeper as more power is applied to drive it up the back of the wave.

I think the hull speed is 1.34 x square root of the LWL.. which does change a bit depending on the hull shape. So a 36' LWL would have a hull speed of 8 knots... 25' would be 6.7.

jef
sv shiva

 SailorMitch 07-24-2007 04:01 PM

Any formula you use will give you a "theoretical" hull speed. As some boat designer named Bob Perry told me once, the 1.34 x the sq. root etc. will get you in the ball park for most cruising boats.

 Johnrb 07-24-2007 04:01 PM

Note also that very narrow hulls can exceed the hull speed defined by the formula 1.34 X sq. root of LWL. High performance multihulls are a good example of this.

Destroyers are another example. The WW II HMCS Haida had a maximum speed of 36.5 knots - LOA is 377 feet (estimate LWL as say 20 ft. less) and you get a ratio of 1.93.

http://hmcshaida.ca/tourintr.html

 SanderO 07-24-2007 04:35 PM

Performance multihulls are not displacement hulls.

 USCGRET1990 07-24-2007 04:48 PM

Quote:
 Originally Posted by SanderO (Post 170943) Performance multihulls are not displacement hulls.

 SoOkay 07-24-2007 05:18 PM

I think the reason we can sail or motor faster than hull speed can be accounted for largely by considering the speed at which the current is moving us at.

I don't know if the asymmetrical shape of a heeled boat affects the formula or if it is only the waterline length, but I think its the latter only.

 SailorMitch 07-24-2007 05:25 PM

Quote:
 Originally Posted by Johnrb (Post 170933) Note also that very narrow hulls can exceed the hull speed defined by the formula 1.34 X sq. root of LWL. High performance multihulls are a good example of this. Destroyers are another example. The WW II HMCS Haida had a maximum speed of 36.5 knots - LOA is 377 feet (estimate LWL as say 20 ft. less) and you get a ratio of 1.93. http://hmcshaida.ca/tourintr.html
With two steam turbines generating 44,000 hp I certainly hope the Haida can exceed the 1.34 multiplier. I don't think that is not the kind of boat this discussion is about.

As for high performance multi-hulls, they most certainly do plane.

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