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Sailormon6 08-01-2002 11:43 AM

Hull Speed
Lately, I have heard some sailors express the opinion that the design hull speed of a displacement boat only limits a boat''s speed when the boat is beating to windward, and that it is completely irrelevant when the boat is sailing off the wind and downwind. In the past, I was always led to believe that the design hull speed had a significant, limiting effect on the speed of a boat, no matter what direction it was going, and that the only thing that could make a displacement boat exceed hull speed is if conditions are such as to allow the boat to plane. Can someone enlighten me please?

dhartdallas 08-01-2002 12:38 PM

Hull Speed
You are right. For non planing hulls, the hull speed is limited by its waterline length. It has absolutely nothing to do with the wind or what point you are sailing on. Boats are slower to weather because they beat into the seas which slow them down and because they are going more directly into the wind.

Hull speed is a different thing. The speed of a wave is 1.34 times the square root of its length (distance between crests). The boat creates waves as it moves through the water. At 1/3 hull speed there are three waves formed along the windward side. At 1/2 hull speed, the waves decrease to two. At hull speed, the boat creates a wave a little longer than her waterline length and gets trapped between these two crests. Unless she has flat sections aft like planing boats and enough power to climb up on the wave and start planing on her flat sections, she can not exceed this speed. Roughly, and close, you can calculate your hull speed by 1.34 times the square root of the waterline length of your boat, for displacement, non-planing hulls.

Jeff_H 08-01-2002 05:24 PM

Hull Speed
Oh I am so sorry Dallas,I am afraid that is not the correct answer, Bert, tell him what he would have won!....OK seriously, Dallas has it mostly right. I am quoting here:

"Waterline''s affect on hull speed is theoretical and not absolute. As a hull goes faster, the bow wave stretches to the point where the bow and stern wave become on wave cycle, whose wavelength is equal to the waterline length. This brings us to wave theory. "

"The speed of a wave (in knots) is equal to the square root of the wavelength (in feet) multiplied by 1.34. If your boat has a waterline length of 32 feet, the theoretical hull speed is 7.6 knots. The waterline length is thought to limit the hull speed because if the boat goes any faster the stern waves has to move further back taking the trough between it and the bow wave along with it. As the trough moves aft, it causes the stern to drop, making the boat sail uphill."

"Except for planning designs, sailboats typically can''t generate enough power to go any faster and climb their own bow wave. But a boat with extra volume in the stern can exceed its theoretical hull speed because the extra bouyancy prevents the stern from dropping into the trough. By the same token, a fine-ended design might not achieve its theoretical hull speed if buoyancy in the stern is insufficient." (Written by Steve Killing and Doug Hunter)

In looking at the more recent data it is not all that unusual to achieve speeds that are 1.5 times the square root of the waterline. It is harder to achieve higher speeds upwind since there are generally less drive upwind than when reaching. My boat generally goes upwind in windspeeds over 10 knots at 1.5 to 1.6 times the square root of her waterline at wnd angles as close a s50 degrees true. Newer more efficient designs will often do better than that.



dhartdallas 08-01-2002 06:08 PM

Hull Speed
Duh. One assumes a standard hull form for discussion purposes. One assumes the boat isn''t round, square, diamond shaped, etc. Fat, skinny, the parameters shift, but they don''t change. Ocean racers have macro-dinghy hulls and surf in the roaring forties. A Tornado will do 1.7 times the windspeed on a broad reach. A ridable bow wave skews it. Trapped! Trapped like a rat! In a wave of its own making!

SailorMitch 08-01-2002 06:46 PM

Hull Speed
The time honored standard is 1.34 times the square root of the static waterline as a starting point for discussing hull speed. That doesn''t change with the point of sail. But the length of the waterline can change (lengthen) as a boat heels over, so yes, the boat can go faster than the standard 1.34 times it''s static waterline because guess what...........the waterline suddenly got longer!

And yes, some boats can do better than 1.34, and some can''t. My little 27 footer has exceeded it''s theoretical hullspeed for extended periods on several occasions, primarily downwind or reaching. The most recent time it turned out to be 1.7 times the static LWL.

Bottom line -- use 1.34 as the starting point, knowing up front that it may be high or low for a particular boat. Let''s don''t make this more difficult than it has to be.

DuaneIsing 08-02-2002 04:09 AM

Hull Speed
I don''t have the reference book in front of me, so forgive me if I am wrong here.

I believe that the factor we are discussing (1.34 plus or minus) has been expressed as a function of the displacement-length ratio, although I doubt that this is absolute, either.

I suspect (but can''t prove) that the design parameters found on most boats with low D/L ratios contribute to factors higher than 1.34 and conversely for the designs of the high D/L boats. Of course, SailorMitch''s point about the waterline length change can be significant, as well.

dhartdallas 08-02-2002 04:29 AM

Hull Speed
If a boat has a waterline length of 25 feet on her feet and you heel that boat such that there is now 27 feet in the water, you still have waterline length. The formula doesn''t specify how you get waterline length, just that you have it. When the boat heels, you change the waterline length. It is now 1.34 times the square root of 27 feet instead of 25 feet. Sail boats heel. The increase in waterline length with heeling is one thing that accounts for some people finding that they get more hull speed underway than they calculate sitting on the dock on the basis of the manufacturers stated designed waterline length. Of course there are factors that cause variation between boats such that the 1.34 may not produce exact hull speed of a given boat. But it is close. The boat is in the trough between the bow and stern waves and is trapped by it unless it can plane, no matter what that wave length is. The only way the D/L could influence hull speed is if you increase the D without increasing the sail area. It is assumed, no matter what the hull form, that you have enough power to reach the theoretical hull speed. Light boats are more easily driven, but they still make waves when the go through the water.

Sailmc 08-02-2002 04:30 AM

Hull Speed

DuaneIsing 08-02-2002 09:30 AM

Hull Speed
Greetings, Dean.

I did a quick google search on the topic and the best I could find was the snippet (from you''ll find below.

Please keep in mind that I''m not in a position to debate the subject on too technical a level since my fluid mechanics courses were long ago and not of much use here anyway. Since I have not read Gerr''s book, I can''t say what assumptions were used to derive the formula.

Fair winds,

[start snippet]
But just how fast can a monohull boat be expected to go? It all depends on displacement -- more specifically on the D/L ratio (i.e., how heavy the boat is compared to the LWL). Naval architect Dave Gerr worked out the relationship, one of the great accomplishments in modern naval engineering. (David Gerr: Nature of Boats, McGraw-Hill; Offshore, Dec. 94, pp 29-33)

D/L ratio = D[in long tons, 2240 pounds]/(0.01 x LWL)^3.

S/L ratio = 8.26 /(D/L ratio)^0.311

The formulas show that lower displacements permit higher speeds without actually planing. Everyone is familiar with Anthony Deane''s original formula for heavy displacment hulls, and people are slow to catch that non-planing boats go faster than Deane''s formula predicts, despite our observations that boats sometimes do go faster than they''re supposed to.
[end snippet]

Sailormon6 08-02-2002 11:17 AM

Hull Speed
Thank you all for your very informative responses.

Jeff H., you said: "The waterline length is thought to limit the hull speed because if the boat goes any faster the stern wave has to move further back taking the trough between it and the bow wave along with it. As the trough moves aft, it causes the stern to drop, making the boat sail uphill…[A] boat with extra volume in the stern can exceed its theoretical hull speed because the extra bouyancy prevents the stern from dropping into the trough." That seems to suggest that you should move crew weight forward whenever the boat is nearing hull speed. By doing so, you lift the stern slightly, making the boat ride in the water as if it has more buoyancy in the stern, and deterring the stern from dropping into the trough. Is that correct, or am I missing something?

In the materials submitted by Duane, the author says, "The formulas show that lower displacements permit higher speeds without actually planing." That really puzzles me, because if the speed of a displacement boat is limited by the fact that it can only climb up over its own bow wave when it is planing, then logically it seems that the only way that the boat could go faster without planing is if the bow wave itself moves faster through the water. So, does anyone know if the bow wave of a light displacement boat (which is presumably smaller than the bow wave of a heavy displacement boat of the same length) moves faster through the water than the bow wave of a heavy displacement boat? Or, is there some other explanation that I am missing?

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