# length of hull

Discussion in 'Boat Design' started by sidthecoolguy, Sep 20, 2007.

1. Joined: Sep 2007
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Location: Arlington, Tx

### sidthecoolguyJunior Member

Hi rick
I want to know if there is any specific ratio of Lenght Vs Width Vs Height for a Deep V Hull

Also is there any standard ratio of the same for each of the catamaran???
Also is there any equation for calculation the distance between the 2 hulls of the catamaran

2. Joined: Sep 2006
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Location: Baltimore MD

### Hotel LimaJunior Member

For what exactly? A certain speed? Economy?

3. ### Guest625101138Previous Member

There are no specific ratios for a deep "V" hull. Some generalisations are:
1. The flatter and wider the stern, the less power to plane for a given overall weight.
2. Typical length to beam ratio is 3:1.
3. A longer narrower boat will have a smoother transition to plane - there is less wave resistance so the trim angle to get on the plane is less.
4. The deeper the "V" the less inclined the hull to slam into waves.
5. A flat hull with a square chine can trip on the chine in a tight turn and the boat will flip. So you need to have a "V" hull or at least a curved transition from the bottom to the sides. (Like the front of ski)

The catamaran hull type and spacing details I previously supplied were optimised for your top speed of 10mph to have minium overall drag. This was determined using the analytical method for wave making and the empirical formulas for viscous drag inherent in the Michlet software that I previously referenced.

Viscous drag is quite simple to determine using a couple of hull parameters and proven formulas. Determining wave drag is a complex problem. As far as I know all computational methods use Michell's integrals. This is complex maths that I achieved a fleeting understanding of in my years at university but now long forgotten. I just rely on the software. Michlet is highly regarded and used by many.

A catamaran has wave interaction between hulls so there is an optimum spacing for any given speed. However the relationship between spacing and drag it not all that significant.

You will find some information on components of hull drag here:
http://www.cyberiad.net/library/rowing/misbond/misbasic.htm

You can do calculations for planing using this site but it does not get down low enough for the size of boat you are considering:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html

The Savitsky formulas take a bit of study but you can find them on the web. The formulas do not give you power required to get onto the plane which can sometimes be a large hump in the power curve. I find a drag of 1/8th boat weight is not far off in most circumstances.

Rick W.

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