Looking for help

[CFD156]

New to the site and have a problem which Im looking for help. Blew up 2000 40hp merc on duck boat. Replaced with 07 4stroke 40 Merc. Can't get boat on plane or get RPM to operating level. Tried numerous pitch props with no luck. Open for suggestions. Thanks

 

[DJ]

Re: Looking for help

How far off are you from the recommended operating RPM?

Are you sure the engine is running correctly? No fuel restrictions, etc?

Generally, one inch in prop pitch will give you 200 RPM, up or down.

As an example, a 11" pitch prop may turn 5500 RPM. A 13" will turn approximately 5100.

 

[CFD156]

Re: Looking for help

I ran a 13pitch on my 2 stroke/ Tried that and it was running like a tug boat. Went to a 12 pitch and got about 4600. Went to 11 pitch and bumped 5000, but the boat is not even close to getting up on plane. I can't find a 10 pitch to try and I don't want to sink money into a prop that may not work. I didn't think there was That Much difference between a 4 and a 2 stroke, but I am beginning to wonder if I have enough motor to even get the boat up on plane let alone get maximum performance out of my motor.

 

[MikDee]

Re: Looking for help

A silly question, the new motor is fully broken in right?

 

[rndn]

Re: Looking for help

What's the weight and how big is the duck boat?

 

[Silvertip]

Re: Looking for help

I suspect your tach is drastically incorrect. Verify it's accuracy. What size duck boat do you have? Sounds like you traded one problem for another. Is the 4-stroke a new motor? Either of those 40 HP motors would push a 15 or 16 foot fishing boat at 28 - 30 MPH with a 1.83 gearcase and a 13P prop. So either the engines are too small for the boat and load, or the engines simply are not making full power. A throttle that's not opening fully would be worth looking at.

 

[CFD156]

Re: Looking for help

Silvertip,
The boat is a Lowe Roughneck, 17ft. with 60floor. Maximum rating for tiller handle is 40hp. The motor is a brand new Mercury 40hp EFI. The blind is made of 3/4inch tubing with 1/4 plywood sides and brush. My two stroke would jump it on plane with 3 guys,gear and a dog. This one won't get it on plane with two people and no gear. ???????

 

[steelespike]

Re: Looking for help

Unfortunately the 40 4 stroke is not going to have the low end push of the 2 stroke.Propped right it should be close.Hard to believe your at 5,000 and not close to planing.Check the max reccomended rpm for the 4 stroke it may be close to 6,000 But I could be wrong.4 stroke needs to be at the top rpm with a light load.You may need to trim in a notch.Check the height. The anti vent plate needs to be about even with the bottom of the boat.And from there you should raise it about 1/4 inch at a time until just before venting becomes a problem.Be sure it pumps water and clamps have a good grip.The 4 stroke is probably heavier than the 2 stroke so you may need to move tank forward or battery/gear etc.