hull and speed

[niels]

Everybody is always talking about fast hulls and slow hulls.
But when i'm on plane and trimmed out(WOT) there is only 1 meter(or less) hull left in the water.
So if you have two hulls with the same weight, length and weight distribution(and engine offcourse), how important can the shape of the hull be???
I'm very curious.
Can anyone tell me?
Greetings Niels.

[Seb]

The hull shape and thus hydrodynamics are VERY important in determining the top speed of a RIB.

Think about it; if you had two cars with the same engine and weight in them but one was a sleek sports model and one was a bloated 4x4, which would be faster?

Same type of idea. The design which is likely to cut its way through the opposing matter the best, be it air or water, will be the fastest.

Cheers

SEB

[niels]

But the hull nearly doesn't touch the water in both cases at wot.
the last meter in the water has almost the same hullshape.(both deep v)
Is it maybe only the wind resistance then?

[simmons0]

A 'slow' hull most likely won't lift out of the water in the same way as your revenger does

It might be on the plane but still have more hull in the water than a different hull shape.

[Swifty]

Another very important factor is how deep the Vee. In flat calm water a deep Vee hull will sit lower in the water for the same weight/speed than a shallow Vee. Hense if its lower theres more wetted surface which causes drag. So the shallow vee will be faster.
This is all fine until the water is not smooth, then the shallow vee will bounce around so hard it will have to slow to maintain control, Whereas the deep vee will be better able to cut through the wave.
Some top race boats have very deep vee's forward and rapidly flatten out at the stern to try and get the best of both worlds. I believe this is called dead rise??

As an example we paced an avon SR4 with 30hp and a SR5.45 with a 90hp. The SR4 won closely followed by the SR5.45. but when the wind blew up the SR5.45 just kept going whereas the other couldn't maintain control and had to back off.

In the overall equation fast hulls are rarely good in rough water. Its all a compromise between speed, stability and how well it will ride out a chop.

[Brambles]

Originally Posted by niels

But the hull nearly doesn't touch the water in both cases at wot.

Yeah, know what you mean.....

[gavin]

Hysucat

[Jonny Fuller]

Originally Posted by Swifty

Some top race boats have very deep vee's forward and rapidly flatten out at the stern to try and get the best of both worlds. I believe this is called dead rise??

It's actualy quite rare for this hull form in out and out racing.

The term 'Deadrise' when used as a generalisation, is refering to the depth of the 'V', usualy quoted 'at the transom'.

'Constant deadrise' refers to a hull with a deadrise that is substantialy the same at the transom as it is in the forward section. (by far the most common form for all out racing)

A 'Warped Deadrise' refers to form you described with a deeper 'V' in the forward sections, than at the transom. This form gives a very soft and 'pitch stable' ride.

[Swifty]

Jonny, many thanks for clarifying that.

That picture from brambles, if you look at the guy in the white t shirt, what was he trying to say?
Should we start a new thread "caption competition"

OK so landing gear hasn't come down.... brace brace brace
or
I told you there was a rattle from the dash when it does this.

[niels]

greetings
niels