Boat too slow.

[Erock]

I have a 84 4 Winns with a 2.5l Mercruiser. Top speed is supposed to be 35mph. I only can get to about 25mph. WOT Rpm is 3900 to 4300. I run about 3800-3900 wide open. The prop pitch is 15". I know going to a 14 will raise my Wot rpm , but will that make the boat go faster? There is some very small gouges in the boat bottom. I can't really see and kind of marine growth and anything like that on the bottom. There is no water in the false bottom, and the boat is not overloaded. I can get on plane pretty easily. I try to adjust my trim as I running to see if I can get faster(trim gauge inop but that's another post), but it doesn't seem to make a difference. I need some sugestions of what the problem coukd be. Thanks

 

[cooter2506]

Re: Boat too slow.

up in the adult sticky is a section for not reaching WOT. Follow that and do all tests you will find one or more of them causing your problem. But from personal experience do ALL tests.

 

[45Auto]

Re: Boat too slow.

What kind of shape is the motor in? It's over 25 years old. Is it worn out, needs a tune up, etc. Did it ever reach 35 MPH since you've owned it?

Do the checklist as Cooter2506 suggested before you waste money on props. Very small gouges in the hull won't make any difference at all to your speed.

 

[Erock]

Re: Boat too slow.

Engine runs very well. I just bought the boat this season so I don't know how long it's been since it hasn't reached max speed.

 

[marlboro180]

Re: Boat too slow.

Engine runs very well. I just bought the boat this season so I don't know how long it's been since it hasn't reached max speed.

I'm guessing that it runs smooth but not "well" if you can't reach WOT with a 15" pitch prop. 14 1/4" Diameter or other? Should easily be able to reach well past 4300 rpm if running properly. Something is holding the motor back.

A 14" prop will not make the boat speed faster, it will get up on plane faster but not top end. The motor, however, will be turning faster if it is tuned properly.

 

[bigskiohio]

Re: Boat too slow.

i say you need at least 17 probably 19 pitch. yours now would be for water skiing and gettting skiers up. if you dont ski try a 19

 

[searay3]

Re: Boat too slow.

You don't say how big the boat is. A 2.5ltr in a 19 footer would be a dog. Do a compression test and see what you are working with to start. Going to a higher pitch will dog it even more. Check the condition of the outdrive oil. Water in it? Weigh the boat and do an internet search on the boat model to see what it should weigh. Could be water logged in which case that little motor again would be heavily taxed to reach wot. Dropping pitch and/or diameter will increase rpm but not speed. About 200 rpm per inch of pitch (rule of thumb).

 

[bigskiohio]

Re: Boat too slow.

agreed on underpower but it is reving up to the right rpm so he should stepup on pitch. you would never hit 35 with a 15 pitch. fiberglass 18 and 19 ft.boats should be 30 t0 35 with 120. I am thinking sombody put on the 15 to overcome the missing power. People always want to get out of the hole. He will have to experiment with props.

 

[Don S]

Re: Boat too slow.

Bet the outdrive has been replaced, and the wrong gear ratio installed. If you can't get over 3900 with a 15" prop, then you either have a wrong gear ratio. You should be in the 17 to 19 range with most boats. Bet you have a 6 cylinder gear ration.

 

[cr2k]

Re: Boat too slow.

Not to mention boat speedos and tachs are not real accurate. GPS the speed.

 

[Erock]

Re: Boat too slow.

The drive was replaced that was on the boat. The 2.5L outdrive was replaced with 3.0L outdrive. Both gear ratios are the same, or so the plate on the side says(1.94). Wouldn't going from a 15" pitch to 17"-19" lower my WOT rpm? I'll see if I can find what prop originally came on the boat.

 

[45Auto]

Re: Boat too slow.

Going to a higher pitch prop will lower your max RPM. As Don says, if you can't get over 3900 RPM with a 15" prop, something is definitely wrong somewhere. When is the last time a compression check was done on the engine? What were the compression numbers?

 

[bigskiohio]

Re: Boat too slow.

might need throttle adjustment ,waterlogged.

 

[IVAZ]

Re: Boat too slow.

With the boat off push the throttle lever all the way forward. Check the carb to see if it opens up all the way.
I agree with the others that are saying to NOT go up on pitch. You cant reach the recommended WOT now, going up in pitch will make this worse.