How Much power do I need?

[markrolf]

Hi All,

We have a 25ft Larson San Marino running a Volvo 4.3V6 Carb and a 290DP drive. We are going to repower her over the winter as it's only used on inland rivers in the UK.

The problem I have is understanding how much HP I need from the new engine. We will never go to sea, and I don't care about the residual problems of having a low powered engine.

Surely there must be a calculator for speed required x weight = HP needed?

I reckon we will only want 10 knots top speed and we weigh a max of 3 tonnes. I was thinking the Volvo D2-55 diesel with 55HP?

Thanks in advance

Mark

 

[Don S]

Re: How Much power do I need?

The D2 isn't setup for use with an outdrive, The D3-130 with an SX will give you about 20 Kts. But going to the D3-190 will give you a boat that does about 30Kts and you would have a lot better chance of selling it later on.
Here is the Volvo engine configurator.

http://www.volvo.com/volvopenta/global/en-gb/marineengines/Engine_Configurator/engine_configurator/

 

[markrolf]

Re: How Much power do I need?

The D2 isn't setup for use with an outdrive, The D3-130 with an SX will give you about 20 Kts. But going to the D3-190 will give you a boat that does about 30Kts and you would have a lot better chance of selling it later on.
Here is the Volvo engine configurator.

http://www.volvo.com/volvopenta/global/en-gb/marineengines/Engine_Configurator/engine_configurator/

Thanks Don, I just found that out too! The thing is, we currently run about 205hp, the D3 is just too much for the small rivers that we would use the boat on. I did think about the mercruiser 120hp.

Do you know of a formula to work out the ideal engine?

Thanks

Mark

 

[Don S]

Re: How Much power do I need?

Do you know of a formula to work out the ideal engine?

There is no such thing as the "ideal engine" for what you are trying to do.
Assuming that 120 hp Merc you are looking at is the 1.7L diesel, that would probably be about your best choice for a small fuel efficient diesel with an outdrive for what you want to do.
If you do a lot of very slow operation, underpropping the drive would help also, that way you can cruise at 4-6 kts at mayby 1000 to 1200 rpm instead of dead idle. Would be a lot easier on the engine that way. Then you could get your 10Kts at normal cruise rpm. It would be a trial and error type setup as planing hulled boats were'nt designed for what you are going to be doing.