bens87241

with a 1.5 gear ratio for the prop to spin once my motor( and i should be more specfic here and say the crankshaft ) turns 1.5 time( which if looking at the dist rotor would be 3/4 of 1 turn) so your set up when the prop goes around once the crankshaft will go 1.32 so what it means is your prop is actualy spinning closer to engine rpm than a 1.5 ratio so yes your prop will spin more times at 1000 rpm that a 1.5 ratio at the same rpm which is why you had trouble with 21 pitch prop the extra pitch without the extra ratio was just oo much for the motor to handle......with the lower number gear ratio it puts more load on the motor. If you use the prop slip calculator plug in your drive and pitch you had at 4400 you would have a theoretical speed of 60mph with 0 slip if you want to compair apple to apple and say you do put a 1.5 drive in a 21.5 pitch would get you the same theoretical speed. now seeing how expensive stuff seems overthere i bet a outdrive is even more crazy. and alot of work really not needed cause you really wont gain anything. i think the gears ratio is right for that lighter boat, with the 17 pitch you gained cause again it was not overloading the motor .

The forged rods will allow you to turn a few more rpms safely but if the pistons are still cast i would still limit the rpms. If it was a car it would one thing you go there and then slow down on a boat we run those rpms ofr a longer time. and i really dont think the 4 blade prop will put more load on the drive than a 3 blade your boat weighs in the same no matter if there is a 3 or 4 blad prop on the shaft.....so the output shaft and gears are seeing the same weight and force load to move the boat the same speed. now the faster you go the more load there will be on the shafts and gears. i also repluged your info with the 19 pitch into the calculator and came up with almost 20% slip a little high.....now a 4 blad will help with this think of as that ever force load you boat is putting into the water to go 50mph all that force is now going through 3 blades. with a 4 blade the same amount of force will be spread among another blade lowering the force pounds each blade will have to carry. this is why slip goes down with a 4 or 5 blade prop. I have never heard of someone saying the blew a drive cause they put a 4 blade prop on. something to keep in mind that boat will be out of the water quite a bit at those speeds so a good driver shower to help keep the alpha cooler while going the higher speeds might be a good investment to help the drive live.

i still feel a rev 4 in 17 pitch would probable be the ticket.......if in the search bar on this site if you type liberator.....you should be able to scan down and find a thread from dtile he had the baja 260 that with a rev4 gained 10 mph and gained rpm over his mirage now he had 21 and used a 21 rev 4 he gives a good compair to speed and rpms on his boat. now each boat is differant but its food for thought. if i recal even with a no worked over rev4 he still gained 5 mph over the mirage at the same rpm

strega

I will definately look out for a rev4, sounds like the way to go. Don't worry about the drive shower, I fitted those before this boat ever hit the water!! Thanks for taking the time to help, it is greatly appreciated.

BBCLiberator

iTrader: (2)

Sometimes your advice is really useful, this in not one of those times.

My 211 never had a hook in the hull, straight as an arrow with a yard stick.

BBCLiberator

iTrader: (2)

I had an 87' 211 until a few years ago, I had the 350/260 combo and the best prop on it was 21" Ballistic @52.xxmph, the 454's were 3-5 mph faster, so 55-57 should be doable no problem. I believe they all came with a 1.5 ratio drive, but that doesn't mean somebody hasn't changed yours. A 21p prop should work well in the diameter you tried, but it obviously couldn't get on plane and that is why your speed was so slow. I would try another 21" prop and if that is to high for rpm, I would try a 19" 3-Blade 14-14.5 inches which will be easy on the alpha drive, but that should be to low a pitch for that combination.

I also agree with everyone, keep that thing under 4800rpm, 4600 is about perfect for that motor.

strega

I ran the serial of the drive and it is definately a 1.32. It was specifically made for the 454 alpha set up I believe.
Top end speed is not that important, I just want to get the best out of the engine and drive. That 15x17p is a lot of fun though!!

BBCLiberator

iTrader: (2)

Ok there you go, use this: http://www.rbbi.com/folders/prop/propcalc.htm

For that hull I always saw between 11-12% slip for the prop on my 87' 211 that puts you dead on almost for a 19" prop @ 55mph @4600rpm. Depending on the prop you may see a little higher, or lower slip. Good luck!!!

bens87241

if you are happy with your 17 prop now and it fits all your needs stay with it.....

bens87241

so what did you do or figure out for your prop?