# weight vs speed

Discussion in 'Boat Design' started by gasdok007, Sep 17, 2005.

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### sujithJunior Member

Hull speed = 1.24 * LWL^1.433 / Disp.^.311

in this formula

disp is mass displacement or volume displacement

2. ### troutyGuest

Yes

But first - are we talking Fresh or salt water here?:?:

The truth is - I haven't the slightest clue, whether it makes any difference to the answer, but I just hate to be left out of such important discussions, so I just think of something thought provoking to say and add my two bits worth, in the hope folks will (mistakenly) assume I must know what I'm on about and thus contributed something worthwhile!

Hey - sometimes I actually get away with it! Don't knock it till you've tried it!.

Now - back to the question - I actually have a propspeed caculator program I could probably email you if you like, that besides calculating prop pitches and resultant speeds - has a HP / weight / waterspeed calculator as well (for planing vessels).

It's lots of fun to mess with, come to think of it I think i have a prop slip calculator program as well - just holler if you want same.

Can't rememer where (who?) I swiped it from, anyone asks - you never heard of me - OK?) Winks as good as a nudge to a blind man!

Cheers!

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### SkippySenior Member

sujith: Hull speed = 1.24 * LWL^1.433 / Disp.^.311
disp is mass displacement or volume displacement

That's weight in pounds.

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### sujithJunior Member

I GOT THAT
DISPLACEMENT IS OF TYPES
VOLUME DISPLACEMENT=L*B*T*Cb
MASS DISPLACEMENT = L*B*T*CB*1.025
WHERE 1.025 IS DENSITY
I WANT TO KNOW WHICL DISP WE SHOULD USE

FOR BIG VESSELS WE USE THESE FORMULAS TO CALCULATE WEIGHT OF SHIP IN TERMS OF WATER IT DISPLACES

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### SkippySenior Member

It's definately mass rather than volume. Where do you get 1.025? Is that the metric density of seawater? The 1.24 coefficient applies only to speed in knots, length in feet, and mass displacement (weight) in English pounds.

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### sujithJunior Member

1.025 is metric density of seawater
thanks for ur replay
do u have any emperical formulas to calculate BMt(vertical distance between center of buoyancy and metacenter) of a ship
input i have is
lenght of vessel
depth
draft
KB
kG

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### Eric SponbergSenior Member

Gentlemen,

For planing powerboats, the most reliable formula I have used is Crouch's formula (George Crouch, 1920s or so):

V (in knots) = C/(Displ. x SHP)^0.5

C = constant depending on the type of vessel, and typical values are:
150 for average runabouts, cruisers, passenger vessels
190 for high-speed runabouts, very light high speed cruisers
210 for race boat types
220 for 3-point hydroplanes
230 for racing power catamarans and sea sleds.

Displ. is displacement in pounds
SHP is the shaft horsepower of the engine

For full-displacement vessels, the following formula works well:

V/(Lwl)^0.5 = 10.665/(Displ./SHP))^0.333

You will recognize the term on the left of the equals sign as the speed-length ratio.

On the right, Displ is again displacement in pounds
SHP is the shaft horsepower
The (Displ./SHP)^0.333 means the term in parentheses is taken to the one-third power, that is, the cube root.

For semi-displacement boats, you can use either equation and modify the coefficients accordingly. For some yachts when I know they are of a certain type or style, I have used the full-displacement equation and modified the numerator on the right, 10.665, to some other similar number, usually between 9 and 10, to make predictions to new designs of the same type more reliable.

All this material is discussed in depth in Dave Gerr's book, The Propeller Handbook. I use it regularly and have had very good estimates with these equations.

Eric

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### gasdok007Junior Member

Thanks,Eric.This seems consistent with my prototype's performance.

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### sandhammaren05Senior Member

Crouch can't be obtained from hydrodynamics. It's wrong because it implicitly assumes that drag is linear in the boat speed V, but drag goes like V^2 or worse. I used Froude scaling with one assumption to get
V'=V(wP'/w'P)^1/3 to compare rigs with the same drag coefficients, same gear ratios, comparable wetted areas. Dropping the gear ration drops the speed even if all other things are equal.

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### tom28571Senior Member

Granted that Crouch's speed formula is an empirical one, It is still very reliable for most planing boats. I have used it to estimate speed many times and it has always been very useful. True that drag increases exponentially with speed but other factors like decreased wetted surface and wave drag help offset that and the result still offers a very good and simple determinant of speed for a quick analysis.

Getting the constant right is the main issue and that requires some experience on the part of the user. Most use existing boat data to choose a reasonable constant for a new boat.

Saying that Crouch's formula is wrong is,well, wrong.

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### Mr EfficiencySenior Member

A Rip Van Winkle thread ?

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### tom28571Senior Member

Did he have a boat?

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