What kind of MPH to expect?

[BRANDON24'IMP]

I have a 24' imp with 351 stroked to a 408 with a 15.25 bravo 1 4 blade prop with 28p prop has alot of rake to it now it twist 4600rpm's and will run 50mph on the dot does this sound right? or is their alot of slip? any recomendations to get some more out of it? thanks

 

[BaileysBoat]

Re: What kind of MPH to expect?

Check it out.

http://www.rbbi.com/folders/prop/propcalc.htm

 

[hwsiii]

Re: What kind of MPH to expect?

If the gear ratio on your motor is right at 2.17, you are running about a 10% Slip, and that is OK. But without knowing the horsepower and the gear ratio of your boat, and about what it weighs when you are testing it, it is almost impossible to tell. If you give us more information we can help more.

H

 

[Bondo]

Re: What kind of MPH to expect?

I have a 24' imp with 351 stroked to a 408 with a 15.25 bravo 1 4 blade prop

Ayuh,... How'd you manage to bolt a Ford to a Bravo,..??

 

[BRANDON24'IMP]

Re: What kind of MPH to expect?

Not A Bravo It's Alpha One Mercruiser Have Not Checked The Gear Yet In The Out Drive But I Think That V8 Outdrives Have 1.50 Gear Ratio I Think.

 

[hwsiii]

Re: What kind of MPH to expect?

Brandon if it is actually a 1.5 to 1 gear ratio you have 39% Real World Slip and that is not realistic. Get me the real gear ratio and I can tell you exactly how much slip you have.

H

 

[BRANDON24'IMP]

Re: What kind of MPH to expect?

went out this weekend and played with the trim a bit and bot the boat to 55mph with the bow up found out i have a 1.84 gear ratio

 

[hwsiii]

Re: What kind of MPH to expect?

At what RPM.

H

 

[hwsiii]

Re: What kind of MPH to expect?

Brandon, going by the 4,600 RPM and 50 MPH previous speed with a 1.84 gear ratio you are running 25% Slip, and that is way too high still. There is still something major wrong somewhere. When you will have time to discuss it, let me know and we will find the problem.

Here are the numbers.

H

 

[BRANDON24'IMP]

Re: What kind of MPH to expect?

what would you recomend looking in to or go after, now i have not put a gps on the boat to see what speed it is just go by the speedometer which i know is not 100% correct

 

[hwsiii]

Re: What kind of MPH to expect?

Do you know anybody that has a little hand held GPS, even some phones have them now days as part of the service features from the cell phone provider. You don't need anything exotic. Just be sure and make a run in two directions 180 degrees apart. If you will give me your email address I will send you a form to fill out, so we can figure out what is going on and what prop you need. I also need to know what manufacturer and model prop and Diameter and Pitch you are running right now, so I can figure out the geometry of the prop and see what elements in it are wrong for your boat.

H

 

[BRANDON24'IMP]

Re: What kind of MPH to expect?

will do on the gps this week and the prop is a 15 1/4 28p bravo 1 ss 4 blade part number on prop is 48-831916 prop has alot of rake but not much cupping i dont know if that matters or not.

 

[hwsiii]

Re: What kind of MPH to expect?

Brandon, Just got off work. Here is what will help me.
GPS speeds from 3000 to WOT in increments of 500 RPM.
What is your WOT RPM supposed to be.
The new alpha one drives are NOT supposed to use over a 15" prop, I don't know about the old ones do you.
Theoretically, it could be as simple as your speedometer is way off, unless we are way low on RPM, but I don't know what the RPM for that motor should be.

H

 

[BRANDON24'IMP]

Re: What kind of MPH to expect?

going to take the boat out tomorrow, funny you comment on the prop size it works but takes a special made trim tab anode to compansate for the prop dia. i'm not sure on the engine rpm either i did not build the motor bought the boat as a small project and someone else built the motor and did a really good job has studded heads arp bolts everywhere and put a set of roller rockers on it along with gt 40 heads i personally thing the motor could be turned up a bit more for wot like around 5500 to 5800 and going down a prop size to 14 3/4 same pictch may help it get to that point and may be what it needs i dont know prop pitch and all that is in the wind for me.

 

[hwsiii]

Re: What kind of MPH to expect?

Great, lets get those GPS numbers I asked for tomorrow, and we will be a wholw lot closer to an answer. WITH real speed numbers and I can see what slip we have at different RPM which will help us. I will talk to you tomorrow.

H

 

[wca_tim]

Re: What kind of MPH to expect?

it kinda sounds like you got way too much prop on there....

I've run up to a 28" hydromotive quad 4 (probably not to different in terms of blade area or diameter than your bravo 1) on an alpha drive behind a reasonably healthy 383 small block. that prop works fine for top end light load, but is a bit too hard on the drive train down low so i run a lower pitch prop most of the time.

I can run the 28" up to around 5500 rpms which on my fairly light, 17 1/2' boat is in the mid 70's... You've got a lot more boat to push...

Also, on a heavier boat, the higher the gear ratio and corresponding pitch on the prop, the more torque you subject the lower gears to under load... be carefull hitting the throttle out of the hole...

I'd try more like a 25 pitch prop and let it spin up a bit...