Prop humming only at full speed

[intheworks1]

So Iv searched here and other places for days and cant find an answer. I have a 1973 Meyers semi v hull 12ft aluminum boat with a 1981 Johnson seahorse 7.5.....Motor and boat run great I just have one issue. Once the boat gets to full speed there is a very noticeable humming chopping sound (not a high pitch sound) that seems to be coming from the prop...sort of the noise you get when you drive over the line on the high way and hit the safety rumble strips only no vibration . The sound goes away at slower speed once water raises back up around the lower unit and it also is not effected by revs. Im wondering if the prop isnt low enough in the water or what. I look at it while going and it seems to be at normal height from what Iv seen online. Thanks

 

[Texasmark]

I had the same thing occur with a bass boat when I was running at or near WOT and was trimmed out pretty far. It's the prop blades hitting the water and I agree with your consensus that the prop isn't low enough in the water for whatever reason. Since props turn at something like half the speed of the crankshaft, you hear two distinct noises...engine singing and prop humming.

 

[GA_Boater]

Welcome aboard.

Does the humming sound like the theme from "Love Boat"?

The sound goes away at slower speed once water raises back up around the lower unit

What does this mean? On plane the lower unit should be in water up to the anti-ventilation plate - Always fully submerged.

This sounds more like a buzz in the dashboard of a car. Start putting a hand on parts of the boat in the stern area, safely of course, or move things around that are in the stern before taking off.

 

[intheworks1]

Definitely sounds like Love Boat just with a worse singer..LOL...no theres nothing in the boat vibrating. On plane the vent plate is a few inches above the water. There seems to create a pocket of lower water around the prop (if that makes sense). I am going to go out this weekend and get pics and video.

 

[WesNewell]

Lower the engine until the av plate is just above waterline (no more than 1 inch) at wot and see if that stops the noise. I assume this can be done in a few minutes.

 

[Sea Rider]

If it's a clamp motor and already sitting fully on transom won't be able to lower it any further down without a transom chop down modification for prop to work further down and achieve the max prop thrust while the motor runs at 90 deg angle.

To know real fast if motor sits at a proper height or not, once on plane at wot and deck load well distributed pull head sideways and visually check at which lower leg is water flow passing by, report your findings...

Happy Boating

 

[Texasmark]

Definitely sounds like Love Boat just with a worse singer..LOL...no theres nothing in the boat vibrating. On plane the vent plate is a few inches above the water. There seems to create a pocket of lower water around the prop (if that makes sense). I am going to go out this weekend and get pics and video.

Your engine must have a 15" mid section on a 20" transom. Sounds about the right dimensions for that. Get a ruler and measure the clamp bracket, engine vertical like installed on the boat, where the bracket sits on top of the transom down to your antivent. plate just above the prop.

Measure your boat from the same spot on the top of the transom down to the bottom of the transom at the center where the engine mounts. The engine length should be about 16" if your transom measures 15" and 21" if your transom measures 20 inches. If the engine measures 16 and the transom measures 20 you just found your smoking gun!

 

[intheworks1]

Transom measures 15” and mount to antivent plate is 16”. Something to note that I noticed tonight is the prop is very chewed up and has hit a lot of stuff since 1981. I ordered a new prop today.

 

[Sea Rider]

I ordered a new prop today.

If that motor has never been tached as loaded, how do you know if same ordred prop is best for your boat ? Motor must run at least middle to max wor rpm range factory stated for them and you don't know anything if that's happening.

Happy Boating