Propellers or shaft spacing

[Kastally80]

I'm designing a 39' planing boat with 2 engines (v drive transmissions). I want to know if there's a rule to determine the best or the minimum distance between propellers or shafts.
Thank you

[CDK]

The engine width determines the distance between the shafts. Anything less than 30" or 75 cm for V8's will cause problems when the engines need service. It should at least be possible to get one foot between them. On a 39' boat you have sufficient space to put them farther apart, but don't overdo it: the boat must remain able to return home in case of an engine failure.

[Kastally80]

thank you CDK for your help
This is for engine accomodations but what about the propellers interaction

[CDK]

For conventional props a minimum hull clearance of 20% prop diameter is considered good practice, sometimes even less is acceptable. With insufficient clearance there is noise/vibration and in extreme cases hull damage.
It will be very difficult to place the props so close together that they interact.
The dominant factor really is the engine bay layout.
Of course the props should turn in opposite directions, SB clockwise, at least that's the way I did it. Glenn-L marine sells the book 'inboard motor installations', quite old-fashioned, but most principles still haven't changed and probably never will.

[mark775]

Much preferring single engine boats, partially for this reason, the closer stuff is to the center of the boat, the less stuff will hit it - in other words, it is worth while to get the props as far inboard as you can to let the lumber and other goodies slide by. V-drives open opportunities for engine placement. Engines can be angled inboard and at varying angles. One jackshaft, if remote V-drive, can be longer than the other, etc.. Engines need not be exactly side-by-side. Having engines far apart allows more leverage for spinning the boat (which has next to no practical purpose) but, believe me, tucking those props as far inboard as you can will pay huge dividends. I have always heard 20% blade tip to hull clearance, as well, and have not pushed the envelope on how close the prop tips can be but would, indeed, find it advantageous to find out. Please keep us posted - some race boater will know...