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.

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]

Originally Posted by 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?

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.