team Tickfaw

Have been trying to get my baja to ride a little better and changed props. It was recommended by Hydrotech that I replace my 25P Mirage plus 3 blades to 23P OFS 4 blades. Before the change I could only get 60-62 mph @ 4600 wot. Now I am getting 55 mph @ 4800 wot. I thought that by changing pitch that much should have brought rpms up to around 5000. Boat does ride completly different and cuts through wakes alot better. Dissapointed in how much mph lost though. Does this sound right? Was told that maybe at 4600-4800 rpm the bow could be breaking over due to the stern lifting. Boat is 30' around 7000 lbs ,21 degree deadrise, 850 total hp, bravo 1 at 1.50. Any idea's?By the way I now have a set of 25p mirage plus solid hub props that can go now.

Carder

If your boat is a '91, it would have a deep X dimension. The four and five blades are best up high, where they can run partially out of the water. Also, the four blades may be a stern-lifting design, whereas your M+ are helping to lift the bow to increase top end. Does the boat now feel like it is running on train rails? If so the stern lift is keeping the nose in the water, increasing wetted surface of the boat, yielding more resistance, so it goes slower. How about getting your M+ props labbed? Is there more RPM's above the 4600 you are getting? My $0.02 Carder

MOBILEMERCMAN

Like Carder said plus if you have a buddy with 23 Mirages try them they will turn around 5000. It will give you more info to tell your prop guy. 23's are bigger diameter than 25's see what your boat likes and lab from there.
Jim

monstaaa

try spining props opposite direction.

team Tickfaw

Thanks,
Yes the boat is a 91 and the engines sit low in the bottom. Have thought about trying to find some 23 mirages to try. The boat does feel like it is on rails.The 4 blades were sold to me as a stern lifting prop because of the bouncing I was getting when trying to trim. Have pics of the boat at speed and trimed and half or more of boat still in water. Lots of spray. Now less boat in water and more defined spray out sides of boat. As far as more rpms, I have checked to see if throttle blades are opening all the way on carbs and the do. Engines are mark 4 454 bored 40, balanced rotating assemby, non-forged pistons and crank. Converted to full roller setup with a cam set from RM builder. As I said I got it up to 4800 but that was it.
Thanks for any and all the help.

team Tickfaw

If I turn them in what do I have to do about throttles? What will this do to the ride?

Carder

The throttles will simply work backwards. The ride will either be better or worse than now... and either faster or slower. If it better, you can reverse the shifter cables so that they work in the normal fashion.

team Tickfaw

I know this sounds stupid but how do I reverse the shifter cables?

Carder

Don't worry about making a permanent change until you determine if spinning the props the other way increases your performance and handling.. If it does any Merc guy can walk you through the cable reversal switch. Carder

bbladesprops

Changing rotation may help you to get the speed back that you lost. It may also lessen the drivability you now have. It is a great experiment and should be done with some caution because of the boat attitude changes.

We ofer test propellers for $30 a pair if you wish to test the M+ 23's. Inward rotation with the M+'s at 23 P may be enough to carry the bow, preventing the porpoise and give substatial top end gains. Labbing will then play into the picture.

Determine the rotation first comparing 3 and 4 blade props in both rotations. Be aware that the idle in gear control will be decreased with inward rotation.