How do you calculate the power required for propulsion?

[slipshot]

OK, another newby question that may have hundreds of variable,s but I wondered if there is a simple, or fairly simple application I could use that would allow me to work out, on a given craft that is underwater as say 3 metres, I know the weight and buoyancy, what power output of the motor would need to be to achieve a speed of say 2 knots?

Is there an easy way to do it, or can someone PM with an offer of their supreme intelligence to help me learn.

Am happy to be guided or even ridiculed if necessary (although I'm used to the latter!_.

Thanks and apologies for the ignorant question.

Andy

[Jango]

Take a look at the Formula,s developed by Naval Architect George Crouch

http://continuouswave.com/cgi-bin/crouchcalc.pl

These are primarily for planing Hulls, but can be used for Disp.Hulls also. I believe Hull factors for Displacement Hulls are Tipically 100 - 120.

[slipshot]

Thanks Jango

Helps but is still hard to determine since this provides for a hull rather than a craft fully submerged in water. But it's a start so my tahnks for this...

Andy

[Guest625101138]

Originally Posted by slipshot

OK, another newby question that may have hundreds of variable,s but I wondered if there is a simple, or fairly simple application I could use that would allow me to work out, on a given craft that is underwater as say 3 metres, I know the weight and buoyancy, what power output of the motor would need to be to achieve a speed of say 2 knots?

Is there an easy way to do it, or can someone PM with an offer of their supreme intelligence to help me learn.

Am happy to be guided or even ridiculed if necessary (although I'm used to the latter!_.

Thanks and apologies for the ignorant question.

Andy

Are you interested in something like a submarine?

There are three regimes for water craft.
1. Fully submerged so there is no wave drag or it can be neglected.

2. A displacement hull where wave component is likely to be a significant factor.

3. Planing or hydrofoiling where the boat is supported largely by dynamic forces.

I am taking it that point 3 does not apply because you are aiming for a slow speed.

If you care to give some indication of the shape and the operating regime then you might get a usable answer.

If the body is fully immersed and a simple shape then drag coefficients may be used per the attached:
http://www.aerospaceweb.org/question...cs/q0231.shtml

Rick W.