Need help with Prop. size-Problem Solved--

[ringmaster72]

I have a 1990 19 foot Raven Bowrider boat. I have a 350 chevrolet motor with 375-400 hp and an Alpha One drive. It currently has a Stilleto S/S 14.25 21p prop. At Wot, I am going about 55mph at around 4400RPM. I would like to see WOT hit closer to 5500 rpm, so I was thinking of going with a lower pitch to get the revs up.

My question is: Can i drop the pitch, which will make the revs higher and still gain top speed since the motor will be turning higher? Or will the drop in pitch be more than the additional rpms can overcome to go faster mph-wise?

 

[junior1113]

Re: Need help with Prop. size

Re: Need help with Prop. size

you pretty much got it. speed will be close to same though. why do you want 5500? 4800-5000 would be more ideal. a 19 will probably give 300-400 rpms with the same style prop. hole shot will be better. top end will be close to same. your #'s sound good maybee a 20 would be more ideal.

 

[hwsiii]

Re: Need help with Prop. size

Re: Need help with Prop. size

Ring, I show the ideal prop for you boat should be an 18" pitch and I show you should be turning between 5,100 and 5,200 RPM with that, you should be about maxing out on horsepower around there. The 18" will give you a much better hole shot and power band throughout the whole range of RPM with that. You should not lose any speed but gain everywhere else with holeshot and acceleration. Your prop slip at this time is 6% and that is great. Dhadley or Ken may chime in and tell you what exact prop to buy for maximum performance. I will only give recommendations on props after they have had a couple of days to give you an answer, as they are the prop performance gurus here.


Prop Picker

Prop Slip

Prop Change

H

 

[ramster]

Re: Need help with Prop. size

Re: Need help with Prop. size

I don't mean to burst your bubble, but theres no way your making 375 HP and only spinning a 21" Stilletto 4400 rpm. My 5.0 220 HP spins a 21" Stilletto 4400 rpm @ 55 MPH, in a 19 ft Glastron. With 375 HP you easily over rev that 21.

 

[junior1113]

Re: Need help with Prop. size

Re: Need help with Prop. size

agree 375-400 hp on a 19' should turn a 21" hell i had a 140hp evinrude pushing a 21" on a 19' four winns @5600 rpm. 400hp should have like a 23-25pitch and do 60+mph. seems more like 250hp

 

[ringmaster72]

Re: Need help with Prop. size

Re: Need help with Prop. size

It is a 355 chevy with World Products S/R Torquer heads, Comp Hyd roller cam 495/502 lift and roller rocker arms. It has about 9.8:1 compression, Edelbrock Performer Intake and Holley Dual feed 650 Carb. It runs great.

My boat must be a plow or something then. My Uncle has a 1985 Scarab with a 496 Fuel Injected Vortec and it goes 61 mph with a 23p, so i figured I was just about right. Now, I am really starting to wonder.....

 

[ringmaster72]

Re: Need help with Prop. size

Re: Need help with Prop. size

agree 375-400 hp on a 19' should turn a 21" hell i had a 140hp evinrude pushing a 21" on a 19' four winns @5600 rpm. 400hp should have like a 23-25pitch and do 60+mph. seems more like 250hp

Your drive gear ratio is different so we are not comparing apples to apples. Mine is a 1.5 final drive, I am sure your Evinrude was a different ratio.

Ramster, i am not sure about yours, do you know if you have the 1.5 ratio? Maybe you have a 1.65??

 

[ramster]

Re: Need help with Prop. size

Re: Need help with Prop. size

Yes I have a 1.60, depending on your cam duration, sounds like everything else should put you at a healthy 325 HP, did you use a marine cam? Any healthy 350 will make peak HP around 5500 rpm, but what you want is a strong flat torque curve for a boat. The higher your cam duration #'s the higher you shift your peak power in the rpm range, and you lose bottom end pull. I built a 383 small block chevy with 11.1 comp, aluminum heads, and forged pistons, with a fairly mild cam profile ( 224/230 duration and .488 lift) and that made 400 HP@5200 rpm and 460 ft.lbs.@4300rpm. That's in my car but what a boat motor that would make! But I still think that even at 300 HP you should spin that 21 Stilletto to 4800 even with your slighty taller 1.5 ratio, your 355 should make a good bit more torque than my stock 305. Is your boat very heavy? All that aside I was in the same position with my 21 Stilletto, I could only spin to 4400rpm, so I switched to a 20" four blade Apollo, and now I'm at 4800rpm @ 55MPH. I still want to try a BayPro II four blade 20.

 

[Tail_Gunner]

Re: Need help with Prop. size

Re: Need help with Prop. size

Yes I have a 1.60, depending on your cam duration, sounds like everything else should put you at a healthy 325 HP, did you use a marine cam? Any healthy 350 will make peak HP around 5500 rpm, but what you want is a strong flat torque curve for a boat. The higher your cam duration #'s the higher you shift your peak power in the rpm range, and you lose bottom end pull. I built a 383 small block chevy with 11.1 comp, aluminum heads, and forged pistons, with a fairly mild cam profile ( 224/230 duration and .488 lift) and that made 400 HP@5200 rpm and 460 ft.lbs.@4300rpm. That's in my car but what a boat motor that would make! But I still think that even at 300 HP you should spin that 21 Stilletto to 4800 even with your slighty taller 1.5 ratio, your 355 should make a good bit more torque than my stock 305. Is your boat very heavy? All that aside I was in the same position with my 21 Stilletto, I could only spin to 4400rpm, so I switched to a 20" four blade Apollo, and now I'm at 4800rpm @ 55MPH. I still want to try a BayPro II four blade 20.

The 4 blade 20 bay pro will turn like the 3blade 21 stilletto maybe loose 1 mph but gain quite a bit of midrange punch

 

[Tail_Gunner]

Re: Need help with Prop. size

Re: Need help with Prop. size

It is a 355 chevy with World Products S/R Torquer heads, Comp Hyd roller cam 495/502 lift and roller rocker arms. It has about 9.8:1 compression, Edelbrock Performer Intake and Holley Dual feed 650 Carb. It runs great.

My boat must be a plow or something then. My Uncle has a 1985 Scarab with a 496 Fuel Injected Vortec and it goes 61 mph with a 23p, so i figured I was just about right. Now, I am really starting to wonder.....

You need to check your tach timing or Carburetion..Something is clearly amiss..Sound like a 65 mph + motor with a 19' boat maybe 70

 

[fabrimacator21]

Re: Need help with Prop. size

Re: Need help with Prop. size

RINGMASTER72: If you can give me the part # and manufacturer of your cam I can tell you your hp...

I ran it through desktop dyno using off the top of my head durations, intake centerline, and lope seperation and got 326hp at 5k. Can't really tell what it is though until I know the right cams specs.

What does your boat weigh?

 

[fabrimacator21]

Re: Need help with Prop. size

Re: Need help with Prop. size

You need to check your tach timing or Carburetion..Something is clearly amiss..Sound like a 65 mph + motor with a 19' boat maybe 70

depending on weight... theres someone on here with a 400hp stroker running a 23 pitch and he's only getting 62 out of a 3000lb boat.

 

[ringmaster72]

Re: Need help with Prop. size

Re: Need help with Prop. size

RINGMASTER72: If you can give me the part # and manufacturer of your cam I can tell you your hp...

I ran it through desktop dyno using off the top of my head durations, intake centerline, and lope seperation and got 326hp at 5k. Can't really tell what it is though until I know the right cams specs.

What does your boat weigh?

Here is a link to the cam specs:

http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=158&sb=2

 

[MikDee]

Re: Need help with Prop. size

Re: Need help with Prop. size

That's a decent cam, & knowing the mods on your engine, I think your tach is off, I think you're turning more rpm then it shows, and could prob. use a prop with even more pitch.

 

[ringmaster72]

Re: Need help with Prop. size

Re: Need help with Prop. size

It is a new tach and the motor does not sound like it is revving higher than 4400.

 

[captainloud]

Re: Need help with Prop. size

Re: Need help with Prop. size

My bet is the floation foam is saturated, & your pushing an extra 1,000lbs of water weight in your boat.
Sounds like you're either not running right or try to push an elephant through the water................

I have a Bayliner 1850 Capri w/ a 4.3 V6, Edelbrock 3714 cam, 2114 manifold, 2002 Vortec Head Ported by L&B Engine Development, and a Weber/Edelbrock 1409 600cfm carb, albeit I have a 1.84:1 outdrive ratio I turn a 14.25" X 23P Merc Stainless Prop right to 5200rpm and chine walk all the way to 57-58mph...................
and by the way ramster's car is fast............and our boats are very similar in top speed.

 

[hwsiii]

Re: Need help with Prop. size

Re: Need help with Prop. size

Loud, if he had a 1.84 gear ratio he would be turning a 22" prop.

H

 

[wca_tim]

Re: Need help with Prop. size

Re: Need help with Prop. size

it couuld also be that the low to mid range power on that engine build is a little soft (a good thing in front of an alpha drive) and that by going down a little in pitch you'll hit closer to peak horsepower and pick up a good bit of speed in the process. If your peak horsepower (wot should be right at peak horsepower for top speed performance) is something like 5500 then you're going to go quite a bit faster if you get it propped it right (we're talking single digit miles per hour here)..

Your horsepower numbers are too high as well - especially if your heads aren't ported and and your running stock exhaust manifolds. as cast S/r torquers aren't going to make any more power than vortec heads, overall i think folks above are about right on with 325 hp. which puts your numbers in a little better perspective. I'm betting you'll be able top turn a 19 closer to where you want to and will pick up a few mph.

 

[fabrimacator21]

Re: Need help with Prop. size

Re: Need help with Prop. size

The fact that your motor is a roller really helps with numbers... you had it pretty close on your estimate. Here is your hp and tq numbers...

Peaks were 363 horsepower at 5k and 438 ft lbs at 4k.

 

[ramster]

Re: Need help with Prop. size

Re: Need help with Prop. size

If you have a boat shop in your area with an outdrive dyno, that's the best way to see what kind of power your making. That cam isn't ideal for a boat but it will still make a good spread of power well over 300 hp @ 4400 rpm, and probably 400 ft.lbs. of torque. Even if your boat was a rotting saturated mold slug like Capt. Loud's Bayliner you should still turn that 21 more than 4400rpm, I think this a now an engine problems thread.