can anyone help me figure this out!?

Koltar

Cadet
Joined
Mar 18, 2002
Messages
21
I have a 1981 17' Starfire Skeeter bass boat, and a 115hp Johnson V4 outboard on it. I just rebuild it and it has good compression and seems to run fine on the lake except one thing! I can’t get the boat to go faster than 40mph!<br /> <br />Everyone says that it should do at least 50, and I have talked with a guy that said he had one and it did 55. Right now I have a 13 3/8 X 17p Stainless prop on it. I get it on plane real nice but then when I trim it up the rpms go way up to 6500! So I cant trim it up when running WOT. So I bought a 13X19p aluminum prop and tried that, it seemed to plane out a little slower but the top end was still 40mph and my max rpm dropped down to 5800. So I went and borrowed a 4 blade 12 3/4 X 21p SS prop and it cause me to have terrible holeshot and still I could only get 40mph out of it. But with the 4 blades it did drop my rpm down to 5500 rpms.<br /><br />This motor has a hydrofoil on it and like I said seems to run good but I just cant figure out how to get the boat passed 40mph. I've talked with the local marinas and they reluctantly gave me advice to buy a new 450$ prop!<br /><br />I told them i'm not buying any more props until I can find out what is the reason for my slow boat.<br /><br />Could it be my hydrofoil? Could it be my boat is set down in the water too far? Or could it be I need a jackplate? Can I move the motor up even if I don’t have a jackplate to see if it helps?<br /><br />Any help I would really appreciate =), thanks<br />Koltar
 

Joe Reeves

Supreme Mariner
Joined
Feb 24, 2002
Messages
13,262
Re: can anyone help me figure this out!?

Koltar.... Back to basics. Are you sure that the compression is as it should be (100+ psi and even on all cyls), the spark (with the s/plugs out jumps a 7/16" gap on all cyls, the full spark advance is exactly where it belongs, and the carburetor throttle butterflies open fully at full throttle? If so, I would suspect that possibly the high speed jets in the carburetor(s) are somewhat fouled. The s/plugs on that engine should be either Champion QL77JC4 or L77JC4 plugs set at .040 . (The "Q" is a resistor plug).
 

rogerwa

Commander
Joined
Nov 29, 2000
Messages
2,339
Re: can anyone help me figure this out!?

How are you measuring speed. Have you verified it with a GPS?
 

ajp

Petty Officer 1st Class
Joined
Jan 6, 2002
Messages
280
Re: can anyone help me figure this out!?

Maybe your speedometer is incorrect. Anyway, 40 mph seems pretty fast. Slow down and enjoy the lake.
 

steve forsythe

Petty Officer 1st Class
Joined
Nov 7, 2001
Messages
245
Re: can anyone help me figure this out!?

I think you will find your hydrofoil slows you down a lot. Take it off first(the least expensive solution) and try that. Then do other suggestions.
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: can anyone help me figure this out!?

I agree with Joe. Lets check out the basics first. Did the boat ever go faster? What are you using for speed reference? If it's a boat speedometer try borrowing a GPS or at least try to get another boat to run along side. It wouldn't be the first time I've seen a speedometer stop at a certian point.<br /><br />You are right, something is out of whack. With the 19 you are in the correct RPM range but at 40 MPH you are at 25% slip. Thats a ton. Without rasisng the motor I would have expected the 4 blade to act as reported but should have been somewhat faster.<br /><br />Without a jackplate the foil may be slowing you down some but it's easy enough to take it off and see what happens.<br /><br />What about trim angle? Any effect from in to out? Is the bottom straight? Check it with a string from the back to about 6' forward.<br /><br />Good luck!
 

MGuckin

Senior Chief Petty Officer
Joined
Jun 10, 2001
Messages
760
Re: can anyone help me figure this out!?

Just my thoughts and I may be wrong!!!!!!,<br />Motor sounds like it is running fine but boat isn't. What are you checking speed with? With the reduction of rpm's you are getting, speed should increase. Have you compared with GPS or even radar or another boat? More pitch = slower hole shot. Higher the pitch, the more top end you should achieve as long as you are reaching your optimum operating range, which you seem to be. Do you have power T&T? Are you carrying excessive gear that would make you heavy? Is the x-factor (I just learned this term recently thanks to this forum) (mounting height on the transom)of the motor correct? Anti-cavitation plate (where the hydrofoil is mounted) should be even with the bottom of the lowest point of the hull at the transom or slightly above. Actually, hydrofoils do tend to give you more stern lift, but, also do creat a little more drag than just the plate itself. You might get a little more top end by increasing the x-factor using shims. Jackplate might help but is it worth the expense? <br />This doesn't make sense and I am curious as to the outcome.<br />I am still learning but I went thru this about 6 months ago. <br />Hopefully I have a given you a few ideas.<br /> ;) <br />Mike
 

MGuckin

Senior Chief Petty Officer
Joined
Jun 10, 2001
Messages
760
Re: can anyone help me figure this out!?

Sorry, I guess I should have refreshed before posting. And I do type slow.
 

Franki

Lieutenant Junior Grade
Joined
Feb 16, 2002
Messages
1,059
Re: can anyone help me figure this out!?

If your motor is mounted to low, and you have a hydrofoil installed, it can really take the sting out of you top end speed...<br /><br />I agree with purse snatcher... take it off and give it a go then... if you notice a huge improvment, then you need to raise your motor abit...<br /><br />rgds<br /><br />Franki
 

Koltar

Cadet
Joined
Mar 18, 2002
Messages
21
Re: can anyone help me figure this out!?

wow guys so many replies =), thank you for the suggestions.<br /><br />Yes i did the testing with a gps with the 17p and the 19p props and they both said 40mph, but i didnt with the 4 blade cause i forgot my gps =P.<br /><br />Hi Joe =), I did what you said and yes all cylenders are above 100psi, 2 are 120 and 2 are 115. Also I did the 7/16" spark gap, and I timed the motor like you said, i think its right =P.<br /><br />But I havnt gotten a chance to check out the butterflies so ill see if they are moving at the same time and same amount =).<br /><br />I cant wait to get home to try this all out and figure out what might be wrong, im also going to try to take the hydrofoil off and see if it helps my top speed, good suggestion =).<br /><br />Thanks again <br />Koltar
 

Ross J

Lieutenant Junior Grade
Joined
Nov 30, 2001
Messages
1,119
Re: can anyone help me figure this out!?

The book specifics for 115hp Johnson say...<br /><br />Prop supplied with motor - 3 blade, 13" dia, x 19" pitch<br /><br />Ross
 
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