Beneteau - Shaft/Prop exit = ugly

I am hoping to be able to get some feedback on this. In the Beneteau threads there is common complaint of the 321 & 331 boats as being hard to back. Lots of people are putting on new props and so forth. Having a good look at the hull of both I see that FINOT has designed a stub or nacell to support the shaft in lieu of a cutlass. The shaft prop terminates about 6-8" aft of the stub/nacell. Visualize the sharp cut end of a 4x4 timber with the shaft prop cpming out of that.

I am not a engineer, but I do know how important smooth water flow is to the hull and prop. I seems that this configuration is creating a low pressure area just foward of the prop. I have always wondered why my 321 cavitates so easily. Could this be it? If so is there any way around this short of increasing the shaft length and installing a cutlass? Fairig te back end of the nacell?

You imput is appreciated.

Michael Puig

 

Where full keels with prop "apertures" are concerned, dramatic improvements have been realized by fairing the "deadwood" forward of the prop. I would think the same would be true in your case if the construction of the stub/nacell is such that it can be done without compromising it too badly.

I share your astonishment that designers have been so casual about this issue.

 

Update

here is a link to pictures of the stub nacell of the Beneteau, I am still trying to find soultions to a deeper keel. Please note keel and hull photos.


http://www.tten.com/745/745.html

 

Hmmm. This is a sailboat with a fixed blade prop, and you're worried about the drag of the skeg supporting the shaft?

If you stop the prop so one blade is on top, the wake from the skeg will reduce the drag you'd otherwise have on the prop. Fairing the skeg will improve efficiency when motoring, but may actually cut performance when sailing. If it were my boat, I'd take sailing performance over motoring.

Cheers,