Would like more speed, have current data for current prop. Can you help?

[roughshot]

Boat: Starcraft Mariner 210
Lenth: 22'
Beam: 8'
Engine: '79 150 Johnoson (WOT operating range between 4500 & 5500 rpm's according to badge)
Current Prop: OMC 15.5" X 15" alum. 3 blade
Load: 2 adults two kids, cooler = 600lbs
Boat weight: unknown, guesed with motor about 1500 lbs?

Data: (w/ GPS for speed)
on plane between 2500 rpm up to 3500 rpm.
After I'm on plane and trimmed out
3000 RPM = 20 mph
3500 RPM = 22 mph
4000 RPM = 24 mph
4500 RPM = 27 mph
5000 RPM WOT = 30 mph, 33 with just me in the boat.

So there is the current data. boat plans real nice and runs smooth on the water.
I'm happy with the holeshot, I don't need a rocket there but it seems to me like I should be able to pull some more mph, alot more. Seems to me this motor should push this boat well into the 50's

Please advise if you can assist/advise.

Thanks in advance.

 

[roughshot]

Re: Would like more speed, have current data for current prop. Can you help?

Ok. I've done some of my own research thus far and here is what I've found.
Currently my prop slip is about 21% at wot
If I want it down around 10% (optimal) and keep my rpm's at 5K
I will have to run a 21 pitch.
Here is my math. Formulas found on go-fast.com
Current Set up
15 Pitch
1.86 Gear Ratio
5000 RPM's

75000 Pitch x RPM
1964.16 Gear x 1056
38 Theo Speed
30 Actual speed
21% % Slip

Theoretical Set up
21 Pitch
1.86 Gear Ratio
5000 RPM's

105000 Pitch x RPM
1964.16 Gear x 1056
53 Theo Speed
48 Actual speed
10% % Slip

Given this, can anyone recommend a prop, stay with 3 blade, go to 4 blade, stay aluminum or go to SS?

Thanks,

 

[steelespike]

Re: Would like more speed, have current data for current prop. Can you help?

I have to do a little research but the problem with your idea is the motor can't turn the 15 beyond 5,000 a 21 will be much harder for it to turn so will lug the motor at about 3,800 rpm.
Problem with all the formulas is the accuracy of the input the hardest number is the configuration of the boat material and hull shape especially.Some aluminum boats do pretty well others struggle to make speed.more to follow.
A 79 Mariner is about 1200 lbs. I think with a 1200lb boat you would be running a 17" or 19" prop at about 5500 to 5700 rpm and yes speeds in the high 40s. Things to check: Is the antvent plate (just above the prop) about even with or slightly above the bottom of the boat? Does the throttle open all the way?Is the bottom clean and smooth?Does it have good compression?
At least 100 psi and even between cylinders.
Is it running on all cylinders?Is the prop in excellent condition.
Does the trim work and does the boat respond to up trim?

 

[roughshot]

Re: Would like more speed, have current data for current prop. Can you help?

I have to do a little research but the problem with your idea is the motor can't turn the 15 beyond 5,000 a 21 will be much harder for it to turn so will lug the motor at about 3,800 rpm.
Problem with all the formulas is the accuracy of the input the hardest number is the configuration of the boat material and hull shape especially.Some aluminum boats do pretty well others struggle to make speed.more to follow.

Thanks Steelespike, I know you'll come thru. I'm just doing some blind calculations.
30 mph is a good speed for cruising, but it seems like that should not be my WOT speed. If I don't have the kids in the boat, I would like to go much faster if possible.

 

[steelespike]

Re: Would like more speed, have current data for current prop. Can you help?

Check my earlier post. ^

 

[roughshot]

Re: Would like more speed, have current data for current prop. Can you help?

I have to do a little research but the problem with your idea is the motor can't turn the 15 beyond 5,000 a 21 will be much harder for it to turn so will lug the motor at about 3,800 rpm.
Problem with all the formulas is the accuracy of the input the hardest number is the configuration of the boat material and hull shape especially.Some aluminum boats do pretty well others struggle to make speed.more to follow.
A 79 Mariner is about 1200 lbs. I think with a 1200lb boat you would be running a 17" or 19" prop at about 5500 to 5700 rpm and yes speeds in the high 40s. Things to check: Is the antvent plate (just above the prop) about even with or slightly above the bottom of the boat? Does the throttle open all the way?Is the bottom clean and smooth?Does it have good compression?
At least 100 psi and even between cylinders.
Is it running on all cylinders?Is the prop in excellent condition.
Does the trim work and does the boat respond to up trim?

Antvent plate sits even with the keel, and the botom is clear of any dents. Throttle is open all the way but compression is around 95 all around. Motor is firing on all 6 and the trim works but is weak in the up position.
So given that info, should I just be happy with what I'm getting with the motor I have or can I milk some more speed out of this thing?

 

[steelespike]

Re: Would like more speed, have current data for current prop. Can you help?

You could try a decarb treatment with something like Sea Foam.Just follow the directions.
If there is carbon sticking the rings it may free them up.
You might carefully check the timing.If your prop is old and worn you might try a new one
in the same pitch.Might improve your slip numbers.

 

[roughshot]

Re: Would like more speed, have current data for current prop. Can you help?

Ok Steelespike, I swapped out props last night and ran my other which I believe to be a 14x19. Boat handled nicely and was able to top out at wot, 38mph with 5000rpms. Was also able to run 30mph down around 4K. Motor would not rev higher than 5K. I did have poor hole shot into the wind but with the wind I was on plane in 4 sec or less. Even at WOT the motor seemed to be working a lot less than with the 15.5x15 prop. Did not hear the prop slip durning hole shot that I have heard with the 15.5x15.
I have not decarb'ed yet but will be ASAP.
Any thoughts?

 

[steelespike]

Re: Would like more speed, have current data for current prop. Can you help?

Its pretty surprising that a 19 runs the same rpm as a 15.Perhaps the 15 had a lot of cup.
And the 19 cupped but less so.I think the 19 may be a good alternative.
A 5 mph gain is real nice.Considering the motor could have issues. A I have seen a couple of times here that they reduced the cup in a prop and had a big gain in rpm and speed perhaps the 19 had a dual effect of a pitch increase and cup decrease.Good luck with your decarb.