Propellers

[Joe E]

Greetings Bertram faithful,

It has been years since I have posted due the fact I had sold my beloved 31. Well as luck would have it we bought it back. It was to much boat for the gentalman that "had to have her". She was ridden hard and put away wet so we have some work to do.

One thing we never did was get the props right. Currenty the boat is running
twin Cummins 270Hp
1.5.:1 gears
1 1/2 shafts
3 blade 20 / 23 .5 cup
laoed cruise 19.5 @ 2350 PRM
WOT loaded 2600

I feel like I am leaving some top end on the table. Prop shop say i should be running 20 / 21P and cruis at 24 Knts.

does anyone have an extra set of 20/21 they are willing to to rent out for a coule weeks before drop money on props?

Thanks
Joe

[Tony Meola]

Joe

Don't have a set of props, but I am running 20 x 23 4 blade with cup. You can easily run 24 knots with this set up or you can go up a little, some of the guys are running 21 x 20 with cup 4 blades getting the same performance. It is just a matter of balancing the diameter and pitch to achieve the same results.

[Carl]

I believe something is off

I am running 20 x 20, med cup, 4 blade and have run 20 x 21, 3 blade, no cup.

With both sets of wheels I run 20 knots at 2300 rpm, and 2600 WOT

Big difference is I have the 210 Cummins with 1.5:1.



I'm an older 1962 Express coming in at 14,000lbs. Are you heavier or is bottom fouled? You should be going faster with the 20 x 23 cupped wheels, you should also be a bit over 2600 on the wot. (I have to take some out of my wheels as well, I just hit 2600, I'd like to be at least 2650 or 2700). Are you sure your tachs are reading correctly? Photo tachs are cheap to verify). Are you getting your no load RPM? Are the wheels in good conditon at a true 20 x 23 with .5 cup?

Spinning the wheels at 2350, you should be going faster, they may be marked wrong, flexing, unbalanced, tracking off.

Maybe shaft is not turning freely...

Bad cutless bearing- bearing binding, as they can swell if contaminated by petroleum product, not aligned to shaft n pinching or too loose and rattling. Shaft log rubbing?
Packing too tight?
Shaft alignment off? Shaft bent?
Rudders not aligned...should be a "little" toed in or out..."little" being 1/4-3/8" max. Much more is a drag.