weight vs speed

[gasdok007]

Anyone have a rough idea how speed would vary simply by altering boat weight? Assuming same hull shape,wetted surface,power etc.I know this is not a simple question, but what is your estimate.If weight is cut in half does speed go up 10% ,20%?

[yipster]

somewhere on these forums are exell files that accept weight in the calculation also !
now where are those files again....

[KCook]

If the wetted surface doesn't change, then I wouldn't expect the speed to change much?

Kelly Cook

[marshmat]

Usually, reducing weight by half will also reduce submerged volume by half. Are we talking displacement or planing hull here? Loading a displacement hull heavier can sometimes lengthen the waterline, thus making flank speed a tad higher. But that's not true for all, and planing hulls tend to be the opposite- lighter is faster. The actual amount depends very much on the hull shape you are looking at.

[yipster]

right, but wsa does change asuming added weight
think i refer to savitski for planning boats, in a search you can find various predictions.

[yipster]

you beat me in time replying marshmat

[tom28571]

In general and assuming that the boat is well designed for the weight, the speed will vary as the square root of the HP divided by weight. However just halving the weight and thinking that all else is going to stay the same is not too useful.

[JonathanCole]

I think it might be more useful to think about wetted surface area which is directly related to the weight of the boat. The less overall weight, the higher any hull will sit, resulting in reduced wetted surface area and thus reduced resistance. But this may only be applicable to a displacement hull, because a planing hull can use horsepower to reduce the wetted surface area by getting the hull up and out of the water when planing.

[gasdok007]

Any thoughts along these lines?- 2 planing hulls same shape,size etc. Hull one Hits 30 kts with 200 HP. Hull 2 weighs half as much. Estimated HP for same performance on hull 2?

[tom28571]

100 hp

[Skippy]

gasdok007: Anyone have a rough idea how speed would vary simply by altering boat weight?

Dave Gerr came up with a formula for hull speed that includes displacement. You can find it in the second edition
(paperback, McGraw-Hill 96) of his book "The Nature of Boats", or on this web page (http://potter-yachters.org/manyways/hullspeed/).

The formula is expressed in terms of a speed/length ratio, which is in turn defined in terms of dispacement/length. If you plug those definitions into the formula, then you get hull speed directly in terms of length and displacement. If you also convert long tons to pounds, then this is the formula:

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

Since .311 is not too far from 1/3, you can roughly approximate the weight dependence as a reciprocal cube root. So if you halve the displacement while holding the waterline constant, this formula says the hullspeed should increase by 24 percent, 26 percent with my cube-root approximation. Of course, if your lighter boat is sitting higher in the water, the waterline will shorten somewhat.

Technically speaking, this isn't really the hull speed, which is defined as the point where the length of the bow wave approaches the waterline length. It probably approximates the speed a lighter or heavier hull can do with the same amount of power that would be required for a typical hull described by the traditional 1.34*sqrt(LWL) formula moving at its hull speed.

One thing that is not included in this formula is hull shape. The shape has to change if you change D/L, but it can go either way: A long, light hull can be very narrow or very shallow, with very different results in terms of wetted surface and wave drag (and stability! :) ). My vote for a third quantity to include in the next-generation formula would be beam, since that will approximate hull shape I think better than any other single number.

[RANCHI OTTO]

Rule of thumb..

increase or decrease in speed = 50% of ratio between the 2 displacements

basic displ= 10 t
new displ = 8 t
basic speed = 18.7 knots
new speed = 1/(8/10) = 1.25 > 50% = 1.125 > new speed = 1.125 x 18.7 = 21.0 knots

[kjell]

Any thoughts along these lines?- 2 planing hulls same shape,size etc. Hull one Hits 30 kts with 200 HP. Hull 2 weighs half as much. Estimated HP for same performance on hull 2?


A 30 knot boat must be a planing hull. 3.5 Ton 200 HP gives me 30.2 knots.
1.75 Ton 100 HP gives the same speed. I you continue with 200 HP you can make 42.8 knots.

[RANCHI OTTO]

> Kjell

If you have based your estimations on the ratio power/displ the speed remains the same but if you have less displacement the propeller will be less charged and increases the efficiency.

it would be better to use the coefficient KR = (BHP)^0.551 / (DISPL)^0.476

So doing the 2 KR coeff. would be 9.7 and 10.2, this means more speed with 100 hp and 1.75 t instead the version with 200 hp and 3.5 t.

It seams to be more logical.

[RANCHI OTTO]

This is an example of KR coefficient for screw on line propulsion type boats.

Each point corresponds to a measured top speed during sea trials of various boats.

[sujith]

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

in this formula

disp is mass displacement or volume displacement

[trouty]

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! :D

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?):D ;) Winks as good as a nudge to a blind man!

Cheers!

[Skippy]

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

That's weight in pounds.

[sujith]

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

[Skippy]

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.

[sujith]

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
breadth
depth
draft
KB
kG

[Eric Sponberg]

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

[gasdok007]

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