# Dumbo needs help

1. Apr 10, 2005

### Tai

Hi everyone,

I'm hoping for some help in calculating the speed of a boat I am planning to build, I'm not looking for a precise speed just an indication of what it is likely to be to ensure the pumps I would like to use are powerfull enough.

The boat is a catamaran hull with the hull dimensions 150mmx100mmx620mm (per hull), the remainder of the boat will be above the water line. Overall the weight will be 8.5 Kilos, the propulsion system I would like to use is made up of two bilge pumps, each bigle pump is capable of moving 42 litres a minute so combined 84 litres per minute. The pump output pipe is 19mm in diameter, which i guess fits into the equation somewhere... :surprised

Is someone able to tell me an approximate speed in either kph or mph? I don't even have a clue where to begin

Tai

Last edited: Apr 10, 2005
2. Apr 11, 2005

### Speed

You need to determine how the drag varies with speed. Then for a first approximation, you relate that to the power output of your engines:

$$P = \frac{1}{2} \rho v^3 LW Cd$$

Bear in mind that your drag coefficient would have to include viscous, wave and profile drag terms.