35hp merc barely pushes boat

[dubaholic2]

a friend of mine just bought a 15' sylvan boat with a 35hp merc on it and hes having problems. he didnt water test the boat when he bought it, just hooked ears to it and heard it run. well, he puts it on the water for the first time and the waterpump isnt working. we put a waterpump in it and now it runs perfect, not a hiccup, but will only do about 5-10 mph. he literally got passed by a guy with a 10hp on an aluminum boat. anyway, what im trying to find out is what some of you may think could be causing the problem with no speed. we thought it might have been the prop, but we took it off and took it to a local marine repair shop and they put it on a driveshaft and put it in a vice and you couldnt turn it, so we believe the prop is good. any advice will be greatly appreciated!

 

[Bifflefan]

Re: 35hp merc barely pushes boat

Im going to guess that it is a fiberglass boat.
You just dont have the motor needed to go faster. The boat is just to heavy.
I used to have a 16' fiberglass and had an 85 HP on it.
Double the HP would be a good place to start.

 

[dubaholic2]

Re: 35hp merc barely pushes boat

i thought of that myself, but i just cant believe that that engine will only push the boat 5-10mph. i also had a 16' with an 85hp on it and it would do about 35, but i would figure this thing should do at least 20. its not a big boat...

 

[Bifflefan]

Re: 35hp merc barely pushes boat

My boat goes about 45 mph.
Per a formula on the internet to get from 45 to 55 mph, i would have to go from 170HP to 295HP for my hull design. That is almost double for 10 mph. barring all the other factors.

 

[jimmyzshack]

Re: 35hp merc barely pushes boat

This boat is almost 16' and i have a 25hp merc according to gps 22mph is the max so far. I'm still working on my motor hope to get a little more. does it have a rpm gauge on it. That would tell you a little more.

 

[dubaholic2]

Re: 35hp merc barely pushes boat

thats more of what i think this one should be doing. i may be wrong, but i just cant see the boat being so heavy that it will only do 10mph max.

 

[CharlieB]

Re: 35hp merc barely pushes boat

You need to figure out if the motor is getting up to its rated RPM.

Are all cyl's firing, compression good, carbs clean?

Does it sound like the motor pulls OK to a certain part of the throttle then races beyond that with no increase in boat speed?

Not being with you to see, hear, and feel what is happening, makes us rely on your observations.

We need a LOT more information before we can offer any better ideas.

 

[dubaholic2]

Re: 35hp merc barely pushes boat

like i said, the engine runs perfectly. when you have it in the yard it sounds like it would run 1000mph. checked compression and its at over 90lbs. on both cylinders. starts with first turn of the key. if the carbs were stopped up it couldnt run as good as it does. it seems solid all through the rpm range. no change between low and full throttle, as far as racing or anything. anything else you'd like to know just ask. thanks for the help, ian

 

[jimmyzshack]

Re: 35hp merc barely pushes boat

Might want to check the fuel to air ratio. I was messing with mine today and at one point couldn't get on plane rpms where up but not high enough.

 

[CharlieB]

Re: 35hp merc barely pushes boat

Unfortunately, reving the motor in the yard means little as even with one dead cyl the motor will rev like crazy.

You STILL NEED to verify a hot blue spark one each cyl.

You NEED a tach to KNOW what your WOT RPM is ON THE WATER.

Without knowing LOADED RPM we can't advise you anything other than to clean and adjust your carbs as spelled out in the OEM Service Manual.

 

[dubaholic2]

Re: 35hp merc barely pushes boat

we have checked that both cylinders are firing well. good hot spark. im not sure if it has a tach... if i have time ill go over and check today and see if i can get it in the water to check the rpms. thanks, ian