Rethinking Hull Speed

[r.w.landau]

On the East coast of Fla.

Is the gulf stream. Add the speed of the boat to the speed of the current and you can make some time. The hull speed is moving thru water that is not moving add in the current and wow.If you have a tide, or coastal stream, the speed over the ground will change. Wave motion could play a role. However, the Knot meter should read near the hull speed.The hull speed formula 1.34 time the square root of the waterline line length is the same for the speed of waves. If you measure the length between two peaks of a wave you can calculate it's speed with the same formula.My thought is that you are in waves with large peak to peak lengths. These waves are actually carring you because of thier speed. The thing that puzzels me is that the GPS and the knot meter read the same.r.w.landau

 

[Dan McGuire]

Royce

I checked “Royce Sailing Illustrated Volume I” and as usual, I wasn’t sure exactly what he is trying to say. He uses the traditional hull speed and 1.34 factor extensively. He refers to the 1.6, but he appears to be saying at that point the bow dives under the bow wave. He says that in order to get to that 1.6 factor, it takes a tremendous power with the danger of demasting. At least I think that is what he is saying. As I said earlier, I think these discussions are great and don’t claim to have all of the answers. When I asked for the source of the 1.5 factor, I wanted to know so that I could understand the context. I remember in one of the earlier discussions, someone came up with a different equation which was based on a number of different factors. It sometimes gave higher numbers than the square root 1.34 factor equation.I don’t think the square root 1.3X equation is gospel, but it appears to work in the majority of cases and is a good first guess when someone asks you how fast a sailboat will go. Most non-sailors expect the answer to be a lot higher.

 

[John Nantz]

Interesting Article on Subject

The "Hull Speed" formula that everybody keeps quoting was apparently developed in or around 1976 with the advent of coal burning steel warships by Froude doing much of his work for the British Admiralty. He called it the Law of Comparison as he used it for model testing using models of 3', 6' and 12' in length. The ships he was studying had little horsepower compared to their length. Nowhere, the posting says, did Froude make a reference to a maximum speed.According to this article the hull speed is only a guide to a speed that should not be exceeded in the interest of fuel economy as the power required to increase speed increases exponentially.I hate typing on a laptop but one can read postings and the really good ones by Michael at boatdesign.nethttp://boatdesign.net/forums/showthread.php?t=1220Anyway, it is a really interesting read.

 

[Anchor Down]

I Agree

Dan, I completely agree. Just providing food for thought.

 

[r.w.landau]

The formula came from measuring the

speed of waves in the ocean. The formula came to be 1.34 times the square root of the distance from crest to crest of a wave.The formula works for displacement boats because, a displacement boat cannot climb the bow wave that it creates. So, a boat is running at hull speed when it's bow wave runs to the stern ( it's waterline length). From that point on, the boat tries to climb the bow wave and under non standard conditions, a boat will climb that wave and surf(plane) for a period of time. I do not doubt that some boats may have hulls that can plane. However, the formula is only good for true displacement boats. r.w.landau

 

[chuckwayne]

1876, not 1976!! (nm)

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