CB-BLR

I think that the 14.5+ inch diameter prop is just too much for a lot of small blocks. I really like the laser II series which are 13.75 inches - 13.5 inches. I will show my experience:

21ft boat
406 small block - aprox 450hp @ 5600 rpm
Volvo SX outdrive - 1.66 gear ratio

Prop #1 - Mirage 25 x 14.5 = 67 mph @ 5000 rpm

Prop #2 - Rapture 25 x 14.25 = 69 mph @ 5400 rpm

Prop #3 - Laser II 25 x 13.75 = 74 mph @ 5650 rpm

Prop #4 - Laser II 26 x 13.5 = 76 mph @ 5600 rpm

I read in a magazine article that to get the fastest prop, you should run the smallest diameter possible with the most pitch possible to attain your desired engine rpm. Think of it this way. How easy would it be to spin a 25 pitch prop that was 30 ft in diameter as opposed to one that was a foot in diameter. Too small and the slip would be too great. Too big, and you could not rotate it.

Just my two cents.

Chris

PS Rapture is made by Michigan Wheel

Full Force

iTrader: (5)

Makes sense to me, I am going to put on the stock 19P aluminum just to see what it does, I will go from there, who make s a small diameter 21?

CB-BLR

Mercury makes the Laser II prop

Pitches 19, 21, 23, 25 - Large hub 13.75 diameter

Pitches 20, 22, 24, 26 - Small hub 13.5 diameter

Chris

Full Force

iTrader: (5)

What is large hub and small hub? the splines?

Pat McPherson

Talk to Brett at BBlades; he will hook you up with some test props.
I have also asked about the Laser II and was told it does not have enough blade area for a 24' boat.
Believe me, I have tested a lot of props and the M+ will work on your boat.
The only other prop suggestion I have for you is a Turbo Vector. I ran one on my 24' Pantera and it was just as good as the M+.

CB-BLR

The tube which the blades are attached to is smaller on the even number Laser II props, so that they fit on small case outboard motors. The odd numbered props have the tube size for an alpha or bravo outdrive. Ii any case, they both work, I just use a plastic adapter when using the small tube props. Mercury makes it for just this situation.

Chris

oldandtired

Something is definitely up with the setup or the driving. A gain of 400-500 rpm with the same prop and no increase in speed leaves me scratching my head. Check the prop's hub and make sure that it's not spun (even though I can't imagine it lasting 5 minutes if it was). Also make sure that your engine coupler is ok. Bottom line is that your slip probably went up by 8-10% with the same setup and more power. That's not right, something else has to have changed.

Rene

Full Force

iTrader: (5)

It makes me scratch my head also.. I would think slip too but the gps kept going up until 4600 maybe 4700, after that is started to slip with more trim and gps started to drop off speed after that, I put the 21 on and it brought the rpm down and no more speed, I am nbot sure what is up, I may buy a smaller set of heads as my planing sucks and I know the 200cc runner heads are killing my torque, i tried them because I got them new for 500.00 I should have sold them and went smaller. I am looking into this now.

oldandtired

I don't know if I fully understand. The same prop at 400-500 more rpm is providing the same speed, right? The only thing that changes slip is changes in the configuration of the boat (weight, drive height and attitude, weight distribution). If none of those things have changed, you have to check the driveline to make sure nothing is wrong like the coupler, prop hub, splines.... You should try less wheel. You are not getting into the engine's powerband. A small block is probably going to need to spin around 5200-5400 to get to peak HP. If you can find a 17 to test, it will at least give you more data.

Rene

Full Force

iTrader: (5)

I am going to do a smaller head, I am finding out that I am not in the power band , I kind of knew that but I figured I would try it, I am looking at a 170-180 CC intake runner head now, and my cam guy said to advance it 4 degrees to bring the power band down.