Gearbox Advice

Klinsman · 11-10-2007, 02:16 PM

Hey all, i'm doing a 3rd yr dissertation project on gearbox shape and ratio and how they affect performance, basing it on the XR2 gearbox. Any help, advice or other information you could help me with would be greatly appreciated.

Cheers
Alex

Matt · 11-10-2007, 02:49 PM

Just out of interest, who's your lecturer and are you based in the fitzroy building?

Klinsman · 11-10-2007, 03:00 PM

Not based in fitzroy, as am in the faculty of technology in smeaton. Module leader is Peter Dyson, but i've got Ming Dai as my supervisor

Matt · 11-10-2007, 03:11 PM

Ah! Yong Ming Dai! Please send him regards from Matt Yorke (doubt he'll remember me though).

Matt · 11-10-2007, 03:29 PM

Fwiw, here's my 2 penneth worth.

Gearbox flow is almost as difficult to model as surface piercing propellers because it is running in a similarly turbulent flow.
Not quite as bad as props though.

But, if you make a number of baseline assumptions based on ideal scenarios, you could get some interesting results.

Consider the gearbox running with the waterline exactly level with the leading point, and 0 degrees angle of attack in a laminar flow, and also

At that point, the fastest gearbox will be the shortest one that that doesn't blow out. So technically, I reckon blow out speed is going to be a largely a function of length vs width & speed. A blow out is nothing more than high speed stall, same as cavitation is. IIRC a CLE is known to be faster than a SM.

You can exceed the blow out speed of a box by lifting it higher. This effectively reduces the AOA at the contact point with the surface of the water.

The blow out ring is also always contraversial. The idea is that the ridge creates a high pressure zone at the trailing end of the box helping to reduce the likehood of the box blowing out - but of course it also adds drag.

Possibly an easy way to model it would be by looking at it as a finite thickness trailing edge aerofoil. This means you could reasonably model flow over it in 2d to start with.

The skeg also plays an important role since in an ideal world you wouldn't have one. On a single engine boat, the fin is rarely running straight ahead, so creating pretty turbulent flow onto the prop.

If you start running the box at angles of attack other than 0, a more needle nosed box will drop if in terms of performance very quickly, wherease a rounder nosed one will have the varying angles of attack better.

The lift generated from the box runnig high makes a significant contribution to the handling of the boat.

Prop tip speeds seem to be fairly consistent. Myself and Jon have dome a few fag packet calculations and Fwiw, here's my 2 penneth worth.