New guy, Trying to figure which set of Duo Drops we need.

Long time lurker, 1st time poster. We have a 23ft 1977 Chris Craft Lancer Overnighter. It had a AQ280 s/p with a Chev 350. Looking thru CC specs on the boat they say it weighs about 4000lbs, we think its more than likely to be around 4500lbs. They are no scales local to really confirm its weight. The engine is a Chevy 350 with approx 260hp. We recently swapped the lower to a 290DP(1.95:1). The lower came with a mix set of dou props that are in bad shape. The front prop is a bent and nicked up B8, and the rear prop is a B7, which is in better shape. We did run these props to see at least where we stand. At WOT we hit 39/40mph(GPS) and maxed out at 4000rpm. Originally with the single prop, we were running 14.25 x 21 pitch getting 4500 rpms, and 39mph. I want to see what folks recommendations might be for "B" series Duo Props? Also, does anybody know what the RPM differences are between each set might be? Has anybody tried going from B series to J series props? Also has anybody, tried Power Tech Props vs V/P props. Thanks, Ken

When you installed the Duo Prop lower unit, did you take the A and B dimensions and did you adjust the shim value above the vertical shaft tapered roller bearing race?

Your 5.7L SBC engine dictates the 1.95:1 over-all reduction.
DO NOT use the 2.30:1.

As for Duo Props...... yes, run the B series and make sure that they are a matched set.
I would begin with B-3s. Both the FWD and AFT prop must be 3's.
If the WOT RPM is too high, move to a B-4 set.

DO NOT run the later series props with the thru-hub exhaust cavity.

Yes, the A and B measurements were taken and shimmed correctly, and the drive is 1.95:1.
Thanks, for the response. I'm hoping to guess right the 1st time.

..............

72fj40 said:

Yes, the A and B measurements were taken and shimmed correctly, and the drive is 1.95:1.
Good to hear that. (serious damage can occur had this not been done!)


Thanks, for the response. I'm hoping to guess right the 1st time.
Yes..... I hope so.
Just remember that the Duo Prop selection requires that the engine reach the OEM specified WOT RPM, just as with the single prop drives.
This "test" should be performed with the boat loaded as you normally would.

Also, whether or not you use the line-cutters, make sure that the little spacer/washers are installed at the FWD end of each propeller. It is mandatory that they be in place!

Click to expand...