hull speed constant

[newbs]

Doing a physics project on the realtion ships between beam, length and speed. need to know why the constant for hull speed is 1.34. why is it this number not any other?

[BrettM]

Contrary to popular belief, the hull speed formula has nothing to do with CB, CP beam, or displacement hence it only has the Length term in it.

It is derived from the speed (celerity) of a deep water wave of wavelength equal to the waterline length of the boat in question. When you run through the maths and sort out the conversions to feet and knots you should get 1.34.

FYI V=sqrt(g x L / (2 PI))
g = accelleration due to gravity
PI = 3.14...
L=wavelength

[Not A Guest]

The 1.34 works for medium sized sailboats.

The number ranges from .68 for large cargo carriers to over 2 for kayak shaped/sized boats.

[CDBarry]

No, BrettM is right, the hull speed is the speed at which generated wave length is equal to boat speed (in deep water). Resistance tends to increase substantially at this speed. It may or may not be a limiting speed for a given craft, but that is the "hull speed".

[Not A Guest]

I simply pointed out the "hull speed" constants that are used for various types of boats.

Some designers find it useful to modify "boat length" to account for bluntness (sharpness) of the bow and stern. That modification appears as a multiplier of the 1.34 and is incorporated into it.

you have to make poopoo on it

you fart on it, bitch

you know i hve this trouble pooping, i cant take it. THAT IS WHY I SUCK AT FOOTBALL SO MUCH. GOO PUSSSSSSSSY!!!!!!!!

[FAST FRED]

[quote=BrettM]Contrary to popular belief, the hull speed formula has nothing to do with CB, CP beam, or displacement hence it only has the Length term in it.

It is derived from the speed (celerity) of a deep water wave of wavelength equal to the waterline length of the boat in question. When you run through the maths and sort out the conversions to feet and knots you should get 1.34.


Actually the formula was devised hundreds of years ago by inspecting boats of the day. As most had a L/B ratio if 3 or 4 the number works , sorta.

The AYRS has a better formula (will dig it out ) that used the beam in the equation, and works for racing shells , destroyers and fat heavy boats too.


If you get 8K out of a kayak your doing fantastic , our 55lb ,18ft racing shell , with sliding seat and proper carbon fibre oars only does 6.5K in sprint mode.

FAST FRED

[Wynand N]

It is derived from the speed (celerity) of a deep water wave of wavelength equal to the waterline length of the boat in question. When you run through the maths and sort out the conversions to feet and knots you should get 1.34.

In a way I have to agree with BrettM.

The transverse wave system results from the interaction of the bow and stern wave systems, and varies with the boat's velocity. At max displacement speed, the hollow of the stern wave is reinforced, causing the boat to trim up at the bow. a condition that causes a very high resistance. (The boat's stern squats and the bow rises)

The typical Froude number representing max displacement speed is about 0.40, but this varies with hull form.
Example, an oldy styled yacht with long overhangs with the capability of extending its wave system well beyond its waterline endings when heeled, would achieve a higher Froude number than this.
Max disp speed Froude 0.40 is thus only a fair average.

Fair winds

Wynand Nortje

PS: Do we need unmannered foul mouths in this forum

[gonzo]

If your report those messages to Jeff he'll delete them.

[Ilan Voyager]

The hull speed constant is highly variable, and I do not understand this almost religious dogma of the 1.34. This cipher is only good for some old sailboats.

The most complex formulae use the LWL, BWL, form coefficient, prismatic coefficients forward and aft and other niceties. A mathematical analysis shows that within the range of "normally" shaped hulls the ratio displacement/length gives a good idea of the other different coefficients.

That has done Mr Gerr and his formula for calculating the hull speed is a very good approximation for small boats until ratios LWL/BWL < 5.

After a fineness coefficient must be added and Bill Roberts has calculated a good aproximation for very slim hulls.

The question has been raised several times in this forum and there many threads giving further explanations.

The problem in your question is what is the definition of "hullspeed" ?

If it is the speed at which a wave system with a wavelength equal to LWL, then BrettM definition is perfectly correct and has a true theoritical background.

If "hullspeed" is the speed going beyond that is too costly,definitions will differs because it is merely a rule of thumb. What is "too costly" ? if sufficient power light enough to fit in the hull , any hull form will go at any speed. Look at the first race around the world, where a swan 65, 1972 S&S design ( http://www.classicswan.org/pagine_htm/65.htm ) have gone well beyond estimed "hullspeed" although not light at all and not planning hull form at all. Just needed an optimistic (and experienced) crew with a big spinnaker in the Roaring forties.

[gsdickes]

Newbs:

What might not be clear from the earlier posts is that it's the open water waves that travel 1.34 times the square root of their length (as measured crest-to-crest, I think), NOT the boats. It's just that full bodied displacement hulls set up a wave train very similar to an open water wave train, hence the 1.34 applies pretty well. Get a hull that sets up a different wave train, or very little wave train at all, and the 1.34 no longer applies. If it did, your friendly neighborhood Hobie Cat 16 would only make about 5.4 knots no matter how much sail was up and how much breeze was blowing. And we all know they're way, WAY faster than that. So shape very definitley does matter and I would second Ilan Voyager's referral to the work of Messr.s Gerr and Roberts.

[Skippy]

Originally Posted by Ilan Voyager

... almost religious dogma of the 1.34.

The definition of hull speed that gets you the 1.34 value is like Fred said, the speed where the length of the bow wave is equal to the LWL, so as Wynand pointed out the stern is starting to drag in the trough between wave peaks. There's a short discussion of this at
http://mathforum.org/library/drmath/view/53491.html

And since it's just a definition, the cat's V/L doesn't have to be limited to 1.34 if the bow wave has a small enough amplitude.