top speed

[wolfe99]

Ok,
I have a 1988 21 foot Bayliner Trophy. Its a pretty heavy boat and its loaded up pretty well with fishing gear. With a 17 pich prop Im only getting 28 MPH on the Gps @ 4900 RPM. This boat has a 4.3L OMC. Should it be faster, it was really slow untill I did a tune up, then it got better, could there still be a problem some where

 

[RubberFrog]

Re: top speed

Is that 4900 at WOT? 28 seems a little slow... how heavy is that old boat?

 

[RubberFrog]

Re: top speed

One other thing, how old is your tach and speedo? Are you sure they are accurate? What is the hole shot like?

 

[wolfe99]

Re: top speed

hole shot not real good, need to use the trim tabs, plus she is a heavy boat ha ha , hell it has a sink in ha ha, plus equipment.

 

[QC]

Re: top speed

Yeah, I think Frogger is on to it, something is wrong with your numbers. I believe the gear ratio is 1.68:1 . I ran the numbers and ended up with 40% slip. There is a number wrong in your deal there. My guess is the tach . . . With a still poor 20% slip those numbers come out to 37 MPH. Or another way to look at it, with your numbers (and the 1.68) and 20% slip I get 3600 RPM . . .

Also the gear ratio could have been changed . . .

Propeller Calculator

 

[H20Hound]

Re: top speed

just to give you a little perspective, my 23 ft Imperial walkaround weighs 4600plus me loaded down. 4.3l LX alpha one (1.81 great ratio 17P prop) Top speed of 37mph t 4800 rpm.

 

[wolfe99]

Re: top speed

what should I be looking for to find the problem then, I know that at 5100 at the tach with a 17p the motor sounds very smooth. Where should I trouble shoot, also this is GPS speed and I dont trim the motor up, thought it would over rev, but maybe my tach is that far off??

 

[QC]

Re: top speed

The first thing I would do is to look at the back of your tach for a little tiny switch, or plastic screw thing. Maybe with a pointer on it. There should be a way to set it for 4, 6 or 8 cyl engines. It could be simply set wrong. If not, then I would get some sort of cheap photo tachometer that you can use to verify it. I saw one on eBay today for $23.

 

[wolfe99]

Re: top speed

I think that tach might be pretty close, because at 5100 rpm with the 17p the motor sounds like it might be a little high, but to speed GPS is only 28MPH. This boat also has downriggers, Salmon Equipment, and many other fishing supplies adding weight. Could This be the problem maybe?

 

[Bondo]

Re: top speed

My boat is a Fully Equiped Lake Ontario Fishing Machine with All the stuff you've got,+ More........

It'll pull 43mph with alittle 4.3l......

Your boat has Issues,......... 1 of which is your Prop is Too Small if you're able to pull 5100rpms with your 4.3l.........

 

[QC]

Re: top speed

If he is puilling 5100 RPM @ 28 MPH with a 17P it's got 40% slip I don't believe that for a minute, unless he's got a 2.5:1 gear ratio . . . then this thing would be about 15% slip. Yes, I know there is no 2.5:1 ratio, which is the point . . .

 

[wolfe99]

Re: top speed

Ok, so what do I need to look for. What problem should I try to find?? This is what I have done so far:

New Plugs
New Cap
New Rotor
New Plug Wires
New Fuel fitler
Rebuilt Lower Unit (professionaly done by a Marina)
New Props ( 17 and 19 pich)
New Starter
New Alternator

The funny thing is if I go from a 17p to a 19p there is no top speed increase, and not really a RPM change untill its good and up on plane. But this is all GPS speed also. Could I have the wrong prop on there, Im running a Quick Silver I think.

 

[Bondo]

Re: top speed

The Only thing I can think of is,.....

You're moored in Saltwater,+ are dragging a couple tons of sea life around with you........

 

[wolfe99]

Re: top speed

No,
Great lakes bottom clean, maybe the motor is just tired now I guess and this all the boat can do. It is good for me all I do with it is fish, but some people on here are telling me it should do 40mph, I cant see this boat doing that NEW. I would think that if the motor was that tired it wouldnt pull RPM right? Oh and I think it is A 2bbl Carb, only 175 horse I think, and its a heavy boat

 

[steelespike]

Re: top speed

verify the tach with a known good shop tach.Do a compression check.