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87 Force 250 prop help

Posted this on another board and received some sound advice but thought I would check here since this site gets more traffic. I have an 87 Baja Force 250 with the Alpha One / 454 combo. Gear ratio is 1.32:1. Motor is fresh and has had some work. I currently have an aluminum 21p 3 blade prop of unknown brand with a pressed hub. At 4000 RPM and about 36 MPH GPS, it feels like the hub has spun. RPM shoots up with no more forward drive. However back it off a little and it goes just fine, no slip. Had my mechanic ride along today and he thinks I have too much motor for that prop. He thinks I need to go to a 23P prop. He didn't have one so we bolted on a 25P to test. Boat ran 40 MPH GPS but would not spin over 3100 RPM. I had throttle left but even WOT, 3100 RPM. TableRocker (member here too I am sure) has been helping me over there. He said

Quote:

Well...sounds like you are making progress. It could be that your engine is hitting its torque curve about that 4000 rpm point and overwhelming the prop...COULD be. Especially being an aluminum prop.
If you can find a good used aluminum prop...or ANY decently priced 23" prop, and then test it, you'd have another data point against your baseline. My gut tells me a 23" is just going to be a little too much prop, or at least on the ragged edge. Your boat as it sits on the water is probably around 4400 pounds...so twisting a 23" is asking a lot unless you are making upwards of 350 hp.

I trust you brother, just looking for additional input before I drop more coin. Anyone else concur or have different ideas? Things done to the motor are:

block bore .070
valve job (ex valves, valve seats, HP valve springs)
cam bearings
VAT block
HP rod bearings
HP main bearings
marine head set
check align bore (ok)
replace brass freeze plugs
pressure check ex manifolds
marine conversion set gen IV
comp cam xe268-1 224/230 @ .050/.515/.520 "nitrided"
replace the crank, grind crank rods .010
balance assembly
hyd lifters
true roller chain set w/ thrust
.070 SRP flat top pistons
molly rings
HV oil pump
QuickFuel M650 Carb

The 1.32 ratio is a tall awkward ratio to use and a 21" aluminum prop is a poor choice. The hub is spun or the blades are merely flattening out. Do not go up in pitch yet. You have no solid baseline as of now.

I would START out with a stainless Mirage or Mirage Plus in 19" pitch. START there then see how things go. If you had a 1.5 ratio I'd start with a 21 or 23" Mirage.

I agree with Trash on trying a smaller and BETTER prop.
It probably won't matter much though because you will probably be destroying Alpha drives on a regular basis. When you go to replace one, change to a 1.47 or 1.62 ratio

Thanks guys. I am currently looking for a mirage 19 and 21 to test with.

https://www.offshoreonly.com/forums/...rops-sale.html

see this thread, 06superduty...a 21" Mirage is available.

Thanks! I am in the process of buying that 21p and BBlades is sending me a 19P mirage plus under the test program. I should have my answer before I have to put the boat away for the winter.....I hope lol

Quote:

Originally Posted by bajaman (Post 4764598)

https://www.offshoreonly.com/forums/...rops-sale.html

see this thread, 06superduty...a 21" Mirage is available.

Quote:

Originally Posted by 06superduty (Post 4765154)

If nothing else you'll have some more data points to ponder during the long winter.

Props can be "black magic" at times...I've seen cases where the exact same boat (hull, engine, etc) responds differently to the same prop...it's hard to explain. That's where the prop shop guys earn their keep.

My 208 has a Mirage 21" that is 'labbed', to use that term. Guy that did it must have made a deal with the Devil, it has a hole shot like a 19" and a top end like a 23". I've had that boat for nearly 30 years and have tried a half-dozen props early in its life to see if I could find one better, and NOTHING has come close.
I'm using a Revolution 4 prop on my 252, and am super happy with it...but I'm thinking of swapping that Mirage over to the 252 just for grins...see what it might do on that hull.

Good luck with your Project, don't give up!

Left this thread hanging last fall. I tested a stainless 19p 3 blade mirage plus. Only turned 3900 rpm and clocked 44mph on gps. Due to a ring gear issue I had to call it quits. But I’m ready to give it another try. I have a 23p mirage plus and a 17p mirage plus to test tomorrow. BBlades recommended the 17p after my run with the 19. We will see how things go. I also had a clogged fuel filter and trash in my tank last year. Now that that is cleared up, I’m hoping for better results.

44 mph @ 3900 rpm with your drive and 19" prop show a 17.2% slip. SOMETHING isn't right, still. There are a lot of engine builders on here that hopefully will chime in, but if you look at all the testing you've done, it seems to indicate you simply aren't making much power out of your engine. Maybe wrong cam for marine use?
But let's say you can get to 4200 rpm with the 17", that is still only 42 mph...4400 gets you back to about 45 mph. Your boat is probably running very 'wet' at those lower speeds so IF you can get it up on the pad more, lessen the wetted surface, you'll definitely see a marked increase in speed.
Just to eliminate this as a possibility...are you SURE you are trimming up the outdrive to maximum angle before prop blowout?
That is a pretty heavy boat but if you are putting say...300 ponies into the prop, it OUGHT to be able to push that 19" to 4400...maybe 4600, one would think.