no speed with mercruiser 3.0l

[rayray1]

Hello, new to boating. I have a 1985, 16ft Melhart with a 3.0l 140hp I/O mercruiser I'm get 5000 rpms at wot and only going 29/30 mph. Is this normal? I have 3 people in the boat at that time and 1/4 tank of fuel. I have a 3 blade, 15 pitch prop. I have done a check compression and readings are 114/110/112/110. Other friends seem to be getting around 40 mph with the same engine, length of boat, and prop. Is there any improvments i could make to increase speed or anything else to check. I have thought about changing the prop to a 17 pitch and adding a hydrofoil. Does compression seem normal or worn out. The engine is not the original engine. It came from a late 70's boat and also outdrive is a 1 drive, also from the late 70's.

 

[cr2k]

Re: no speed with mercruiser 3.0l

Your problem may lie in the flotation foam under the deck. Often it gets waterlogged and adds a bunch of weight to the boat.

Could also be how it is balanced and/or trimmed. Is your hull clean? See if you can borrow a 17p prop and try it.
Your compression is good.
Do you smell burning rubber when underway? If so would indicate possible coupler slipping. Also check your prop hub for slippage.

 

[fishrdan]

Re: no speed with mercruiser 3.0l

That 15P prop is too low, thus only going 30 MPH while over revving the engine. A 140HP Mercruiser of that vintage should be propped to turn 4200-4400RPM at WOT. You can try a 17P prop which will drop the RPM and increase the MPH, but I'm thinking you'll need to go to a 19P. I doubt you'll get 40MPH unless you go to a 21P prop, though your engine might not get to the WOT RPM with a 21P.

As cr2K suggested, the flotation foam might be soaked, weighing down the boat. My old 16' SeaSwirl with a 140HP Mercruiser had soaked foam, but would still turn a 19P prop to WOT with several people on board.

You compression isn't great, but it's within 10% across the board, so I wouldn't worry.