Probably a daft hydrodynamics question...

[Wiggo]

But if prop shafts and P brackets cause so much drag, why aren't they encased in a GRP? What I mean is would there be an advantage to creating a triangular moulding that encased the shaft and P bracket? My thinking is that it would keep 95% of the rotating shaft in the dry, which must reduce drag, and at the same time create two mini keels that would help keep the boat planted in the water and help low speed tracking. The forward facing profile would hardly be any wider than the existing shaft and bracket, and could even have a sharpened leading edge, if need be to further reduce drag.

I'll get my coat...

 

[sarabande]

likely to introduce significant tracking and directional balance problems by making the centre of lateral resistance (dynamic) move further aft.

 

[Wiggo]

Agreed, but you could allow for that with a new hull design. I'm not talking about retrofitting something here, this is just hypothetical...

 

[sarabande]

I see what mean, but large triangular shapes between the shaft exits and the screws are going to make it much more difficult for the boat to 'skid' round when the helm is put over. It will be like having a huge skeg and a tiny rudder.

I also suspect there is something about highly powered props needing an unrestricted flow of water otherwise the blades would be working in some water which is going along at boat speed, and another piece which is almost stationary in the shadow of the superskeg.

It would be interesting to find out, though, what the drag patterns are for a shaft which is rotating at an angle to the ambient water flow. Thousands of little vortices coming off the after face of the shaft and taking (? how long) to re-establish a laminar flow ? Intriguing avenue of thought. Beats moving wet wrapped big bales of sileage.

 

[Wiggo]

To be honest, it hadn't even occured to me to move bales of silage.

 

[sarabande]

at the moment it forms a hugely important part of my life - WOW !

 

[tico]

bugger, just typed a reply and 'pop' confuser crashed!
AFAIR a proportion of the drag is caused by the coriolis effect of the rotating shaft dragging a layer of water around with it. Hence the improvement by surrounding the shaft with a tube, altho that brings its own increase in x-sect area and hence additional drag.
I seem to remember that a forumite hereabouts (bradtarga?) sells such a shaft arrangement (h4 marine?)

 

[andy59]

A report on the Marex 370 aft cabin seemed to indicate that enclosing the shaft has some performance gains ,
http://www.h4marine.com/Seatorque01.htm
http://www.ybw.com/forums/showthread.php?t=222017

 

[oldsaltoz]

I suspect the introduction of a triangle to enclose the shaft would create more drag due to the increase in wetted area.

As for redesign to accommodate this change, I suspect this would be a deeper 'V' in the bow, another increase in drag.

 

[Pinnacle]

But if prop shafts and P brackets cause so much drag, why aren't they encased in a GRP? What I mean is would there be an advantage to creating a triangular moulding that encased the shaft and P bracket? My thinking is that it would keep 95% of the rotating shaft in the dry, which must reduce drag, and at the same time create two mini keels that would help keep the boat planted in the water and help low speed tracking. The forward facing profile would hardly be any wider than the existing shaft and bracket, and could even have a sharpened leading edge, if need be to further reduce drag.

I'll get my coat...

I already have a shaft encased in grp.....