Posted: June 04 2008 at 10:40 | IP Logged

Hi Suwanneered,

You may have this info already but check out this link:   http://www.perfprotech.com/store/App_Themes/PPT/images/tech- specs/VD_Inline.pdf

It describes the 71 vs. 72.

The only thing I cannot locate are the implications of changing the ratio.  I would guess that it is similar to a car's differential/rear end.  If that is the case regarding automotive, tire size would come into play (larger or smaller diameter tire would change the final drive ratio) so would the props pitch (which is equivalent to distance traveled) may need to be reviewed.

I wrote to one of the marine tranny shops on line asking them on the CC application what the differences are between the 71 and 72 and the different gear ratios.   I'll post their reply as soon as received.

How close s the prop to the rudder?  I know on mine it is pretty close and 3/4" may not make the two meet, but it would be too close for my comfort as mine are about 1.5" apart.

Tom

Posted: June 04 2008 at 12:10 | IP Logged

Tom,

Your right on the implications for the propeller pitch. The 1.9 to 1 ration means for every 1.9 turn on the engine, there's one turn on the prop. This means more torque with higher RPMs across the board . Adding pitch to compensate is really the only variable. Props on 280s are pretty close to the hull, so be sure there's no cavitation at the highter RPMs that your setup might experience- loud bubbly/popping sounds at speed usually means cavitation. We had this problem and used a smaller diameter prop so it wasn't as close to the hull. We increased pitch to compensate. The complication is that your using a transmission out of spec, so trial and error to get the right prop setup may be called for. Driveline alignment is critical. If out of the water for some time, wooden boats were allowed to sit in the water so the hull could settle B4 alignment. Fiberglass is more rigid but doing the algnment in the water is always a good idea, especially if the drivetrain is newly installed- I would recommend a pro do this. 

On the singles at least, the rudder is located so the prop can easily be removed. You will note the rudder flange is slightly off center.

The diffenence between the 71&72 seriesis horsepower rating. The 72s being rated for higher Hp. The 71s handle the 350. From my research, the newer 71 models (10-17-006) also a higher pressure transmission fluid pump, resulting in more efficient power flow.

Enough of my gab...hope this helps...

Roy