Effect of hull shape on speed

BogMonster · 11 June 2006, 11:44

Looking at the "speed survey" thread it is interesting to see the variations. In other words quite a few of you b&ggers are faster than me with a similar boat size/hp

I think some of it may be due to the prop (only a 17" prop on at the moment though a 19" is on order) but - how much difference does something like whether the hull is a shallow or deep V make? From what I have read in the few months I have been on here, it seems that hulls are either optimised for rough water (deep V) or speed (shallow V) and juggling the two is a bit of a trade off, but how much difference does it actually make in terms of knots? 5kt? 10kt? 2kt?

Just curious really, rough water is the dominant requirement for this part of the world anyway so no plans to change as the Humber seems to be the right sort of design for the conditions here - I have been out in something with a fairly shallow V hull and it was like going back to leaf springs

Discuss

jyasaki · 12 June 2006, 16:26

Quote:

Originally Posted by BogMonster

how much difference does something like whether the hull is a shallow or deep V make?

I'm thinking pretty substantial. A shallow V will plane better (so should be faster for a given power input), but will pound more in swell or chop.

Also, I believe that beam makes a pretty big difference, too. Narrower boats will be faster than wider ones.

Guesswork on my part, though.

jky

BogMonster · 12 June 2006, 20:43

Me too. I should have made the question more explicit I suppose.

Say for instance you had a 5.8m rib weighing about 600kg typical weight (ahem, a wee bit more with me on it

) with 115 hp on the back, mine does about 35kt, would a shallow V hull in that sort of size/weight/hp be expected to do 40, or 50, or ???

antiqueboy · 12 June 2006, 22:02

i know from an engineering friend that some boats have hull speeds
ie a raggi with a 20/30 footer and a small 40hp engine may not acheive any more with a 50hp due to the hull shape,sometimes it takes a big jump to take you to the next level, if the hull can take it,not sure how to apply that theory to ribs...

Nos4r2 · 12 June 2006, 22:04

I think that only applies to displacement speeds or displacement hulls-planing is a different matter?

mdt · 13 June 2006, 07:35

Quote:

Originally Posted by Nos4r2

I think that only applies to displacement speeds or displacement hulls-planing is a different matter?

That is correct as a displacement hull is unable to rise over the bow wave it makes.

For the ones who like numbers it is
1.34 X the Square Root of the Boat's Waterline Length.

But for a hull that plains me thinks the results of the 5.5-6m speed debate show there are many more
variables to be taken into account to work out what your theoretical top speed is.

brucehawsker · 13 June 2006, 19:01

One might be whether you take the tide into account!

Matt · 13 June 2006, 21:21

Hull shape is everything.
That's why all the lazy tossers without either the skill or knowledge to design their own just nick someone elses.

Dirk Diggler · 13 June 2006, 22:00

Quote:

Originally Posted by MadMat

Hull shape is everything.
That's why all the lazy tossers without either the skill or knowledge to design their own just nick someone elses.

and they also do a range of clothing!

Matt · 13 June 2006, 22:02

Are they splashproof?

Hightower · 13 June 2006, 23:45

My BWM hull is said to be a deep V (mabe on the shallower side of Deep) but I recon that my Hull has a maximum hull speed of about 42 knotts. She was getting very wobbly when only slightly trimmed out at this speed when conducting prop trials.

What I did notice though was that at 4000 rpm with different sized props I was getting quite a range of various speeds. But maxed out at WOT the top end was pretty much the same.

joakimhansson · 14 June 2006, 05:52

Hightower!

A "planing pad" should help you. People here say the planing pad should help you to get on plane, but we had a boat "hardhull" 21´. First we had a v8 inboard around 300hp.(50 knots), Then a outboard, johnson 225 on a bracket (53-55knots) and very wobbly. And after building a planing pad and no other chances we got 59-61 knots and no wobbling at all.

Rodan · 14 June 2006, 07:19

How do you build a planing pad on?

joakimhansson · 14 June 2006, 08:44

With a saw! Cut away and build a new bottom. It was a step like a stepped bottom to the planing pad. It was -94 and no pictures is aviable. I have some on the boat in my garage but it doesn´t help you. It was a friend of mine who did the work. Lenght around 120cm and wide around 15 cm. It was no problem to take sharp corners at all. Hmmmm I had to take a look in the garage after my photos.