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 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

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!

Aa

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.

Thanks bajaman. I no longer have the 19 to test with as it was from the loaner program. I am going to run today and test the 21 and 17 props. I thought I had a 23 but it’s a 21. I agree something is off. Maybe wrong cam like you say. I’ve been looking into some EMI manifolds since someone tilde me about reversion with a cam like this. I’ll report back with numbers from both test today. I’m not 100% sure I was at a perfect plane last time. I grew up on boats but this is the first I’ve owned myself. I think I had the drive all the way down last time but now know that would have resulted in a negative angle. Or at least I think lol. I’ll play with the trim today to see if I can find the sweet spot.

Yeah...you'll get there...just will take some time. Next time out just keep trimming up (once on plane and running 25 + mph) until the prop blows out, i.e. you lose forward momentum and the revs climb quickly, then nudge it JUSSSST a bit back down.
I forgot to ask if your boats has trim tabs? Trim tabs and drive trim can work hand-and-glove together to optimize running angle.

I do not have tabs. Would like them though lol. I have a ss props now so I don’t think they will blow out. Pretty sure I was flattening the aluminum one. I can take the aluminum one and test that too. Think that is a good idea?

Oh, if you over trim you will blow the prop out, meaning break the surface/cavitate. An aluminum prop is not THAT bad, really...not for ultra-high performance, no...but will get most of us 'runabout guys' along just fine...as long as the hub isn't spun. The last thing you need is another variable to mess with you, lol!
That 21" may be just a bit too much with your 1.32:1 ratio. Then again, a strong 454 should be able to twist that thing pretty well.

Ok back from testing. Well testing one prop. Had some other issues with fuel today so I need to sort that out. Anyways I had the 17p Mirage plus mounted up so ran that first. Did like you said and got the boat up to about 25-28 then played with the trim. I could feel it ease up and hear the change in the motor so I think I had it on plane correctly. Then I eased into wot. Managed to get 49-50mph at 4500rpm. Might of had a little left but ran out of smooth water and had to turn. Was able to achieve those numbers twice with the 17p. My issues with fuel started after I changed to the 21p so I never got to run that again to get new numbers. According to BBlades prop slip calculator I’m at 8.89% slip so that is closer to what it should be?

Sounds like you are right there or damned close! I think that 17p is what is going to do it for you. I agree with your slip numbers:

https://cimg3.ibsrv.net/gimg/www.off...39aa2b8ddd.jpg

Man I sure hope so. Thanks for all of your help!! Now I need to find out why it dies in reverse and what the backfire is all about. Engine was pulled late last year. Maybe the cables need adjusted on the shift linkage.

Dying when shifting to reverse IS often just a case of cable adjustment...as for the backfire, that of course isn't normal and can be dangerous. I know NOTHING about that Quick Fuel carb but there could be an issue with it or in the timing.

I have a complete Bennet setup that I will sell for $250 shipped. The tabs are 8" x 12"