4 HP Mercury and Jon boat

[img]Hello, I am new to this forum and pretty much the boating community. Last year I wanted my own fishing boat so I picked up a 14ft aluminum jon boat with a 4HP Mercury long shaft, trolling motor and etc for a steal. Everything works great however I have some questions and an issue that I noticed today.

My transom I measured is 17". My Mercury appears to be a long shaft, my question is where am I supposed to measure the shaft to confirm, and will a short shaft work better for my boat based on the transom length? I plan to upgrade to a bigger motor anyway and the short shaft look more appealing to me and I feel I won't snag as much stuff in the water.

My second question is while heading out and coming in today, mainly at wide open throttle, i could feel the motor almost "slipping" and the revs were increasing. This happened almost every 2-3 seconds when wide open, and when not wide open it decreased for the most part. The performance was lacking and to me its a similar effect to if a car had a slipping clutch. Where should I start to look at to begin fixing this issue?

Attached are two images of my motor on the boat.

Thanks!

I don't know of your fabrication abilities..but I'd move up the transom for a 20" motor. When you decide to move to a 20hp a long shaft might be easier to come by.

I've caught a leaf on the prop and did the same thing. It stayed attached until I came into shore and then it fell off. I saw it fall off..if I hadn't I would have not known it was there.

I'm going to look into that once I fix the motor issue. How can I check the rubber or the hub of the prop? I pulled it off but I don't know what to look for.

livinloud11 said:

I'm going to look into that once I fix the motor issue. How can I check the rubber or the hub of the prop? I pulled it off but I don't know what to look for.

Click to expand...

Make a mark on the prop that won't get washed off straight across from the cotter pin loop. Use the motor until you feel that slip. Then check the mark on the prop. If it has moved in relation to the cotter pin loop it is a spun prop.

My 15hp kinda does that as I go over varying water temps. I live on a river with varying water temps and the prop does bite differently causing what seems like surging. I notice it more on my merc 4.5,1979 on my 1232jon.

The "slipping" that you feel could be a spun hub (you have received good advice above on verifying that malady)...could be cavitation due to the motor being mounted too high (I doubt that looking at your picture) but you have been supplied with a diagram to check the proper height out; or....you could have the motor tilted too far out.

If it persists, try moving the motor tilt control one hole closer to the transom. I call that "tucked under". The more a motor is "tucked under", the quicker it will get up on plane. The less it will cavitate. However, if it is adjusted too far in that direction, it can slow the boat down appreciably. Since you do not have hydraulic tilt and trim, you just have to find the best compromise position for the tilt of your motor.

regards, richg99

p.s. As far as the mounting height of your motor, I usually go for EVEN with the boat's bottom, as opposed to one inch below. There are lots of opinions for every setting on a boat, though. No one right way, I guess.