Sizing engines for a cruising catamaran

[bluenomads]

I know the OP is talking sailing cats, but I did a little study of larger power cats and their engines, including fuel consumption. Now the figures used are published by the manufacturers (eek!) or through real life where I could find/get them.

Essentially the formula 11.27 x LOA - 280 gives a good approximation for the size of each engine. You can also use displacement (even dodgier numbers from manufacturers) and have 0.6501 x displacement - 14.637 for each engine's size. These both give >80% correlation with the data points.

Fuel economy is all over the place though.

Comments welcome!

[Zzmeyer]

Interesting. Far less correlation than I’d expect but as you said these are manufacturer numbers. For the smaller Yanmar and Volvo marine engines there is pretty much a straight line correlation between horsepower and fuel consumption over a broad range of rpm.

[Chotu]

Originally Posted by bluenomads

I know the OP is talking sailing cats, but I did a little study of larger power cats and their engines, including fuel consumption. Now the figures used are published by the manufacturers (eek!) or through real life where I could find/get them.

Essentially the formula 11.27 x LOA - 280 gives a good approximation for the size of each engine. You can also use displacement (even dodgier numbers from manufacturers) and have 0.6501 x displacement - 14.637 for each engine's size. These both give >80% correlation with the data points.

Fuel economy is all over the place though.

Comments welcome!

What units are we using here for LOA and displacement?

Thanks!

[BigZ]

Hi everyone. I am about to replace 2 14.5 HP Nanni Diesels on my 38" catamaran. I was offered to change to Beta engines (14HP) because of big delay in supply times. Did anyone have experience with small Beta motors on Catamarans? The Nanni worked for the first 15 years of my boat life but had salt water cooling and suffered from internal corrosion. Any take on the difference between Nanni and Beta small motors will be appreciated (yes...i am aware they are both based on a Kubota motor block).
Thanks in advance
BigZ
S/Y Touchwood