Prop and RPM question.

[Suctoes]

Hi everyone.... Well I'm new here and could not find a thread relating to my problem so here goes.

I just purchased a 1996 stratos 258, 15'-6" fiberglass bass boat with a 2000, Mercury 40ELPTO, 2 stroke, 2 cylinder motor. The person I purchased it from is a family friend and told me the motor on it, came off a pontoon boat and the prop was set up for "pushing not speed." He only fished small lakes and had no reason to change props. I took the boat out to the river and it starts great, seems to idle ok "does kind of woap woap woap woap at idle." but when I took it to WOT it would not get even close to plane and would only turn about 3500 RPM.
I don't doubt the rpms because motor sounds no where close to revved up, and my butt says we were only doing about 20MPH. Looking at the prop today the only number I could find was 27, it was on the inner hub.

1. Could my problem be just the prop?

2. What else could cause this that I could check with a DMM?

3. Does anyone have a prop pitch recomendation?

Thanks for any replies!

 

[usskydvr]

Re: Prop and RPM question.

Something else is wrong besides the prop. (assuming they really had the correct prop for pushing the pontoon). Take the prop off and look for p/n on the end of the hub. Nothing you can check with a DMM. I would start with a 10 3/8 x 14.

 

[Suctoes]

Re: Prop and RPM question.

Ok, checked the prop and it's a 13 pitch. Any more advice? it's got me stumped!

 

[usskydvr]

Re: Prop and RPM question.

Ok , the prop is good. Have to start looking at the engine. I suggest you get a Mercury Factory Service Manual. for a quick check you could pull off one plug wire at a time to see if it makes a noticable difference.

 

[CRFan1]

Re: Prop and RPM question.

I thin you may be overpropped (and potentially underpowered) with a 40 on that boat.....unless the motor is a bigfoot type motor with a different gear case ratio. Being fiberglass I am sure it is kind of heavy so I would try a 10" or 11" pitch prop assuming your motor is in good running condition.

 

[Suctoes]

Re: Prop and RPM question.

it has a doel-fin hydrafoil on it. Should I remove it and give it a try?

 

[usskydvr]

Re: Prop and RPM question.

It wouldn't hurt, but those are generally added to HELP get on plane quicker. Of course if your engine is mounted too low it will add a lot of extra drag. It should be mounted so the Doel-fin is right at (or slightly above) the bottom of your boat. Take a yard stick and lay it against the bottom right at the center and then extend it until it just touches the engine. It should be right at the anti-cavitation plate, which is what the doel-fin attaches to.