Prop rotation

Hi There. I have a 20m steel catamaran being built. The builder is about to install the engines, 2 X 220hp, with the props rotating inward. I was just informed by someone knowledgable that they should be rotating outward. If you could be so kind, I could use some of your expert advice.
Dan

 

Having a cat hull there is no flow interference btw the 2 props.
According to my own experience in towing tanks in selfpropulsion tests but with single hull models I would prefer props rotating inward.

 

Outward would definitely be no problem Inward it would depend on the configuration. For example, whether it is a displacement of planing hull, size of wake/rooster tail, hull spacing, etc.

 

Inward rotation (top blade in) increases the tendency of the boat to turn under the influence of the prop. This can be good or bad, because you have to be able to go in a straight line, at least, with only one engine, should you lose the other, and with wide separation, this can be a problem with cats.

 

Good point