Prop Issue Question

[sabmgb]

Well I'm having an issue with cavitation grind or whatever you want to call it, since I put my new 24V electric on bow. So I went from 1, 27 series battery to 2, 31 series batteries at the back of the boat. Boat has been sitting low in the water since the install. Just notice that the prop is now scored on all 3 blades next to the hub.

I'm not much for trimming the motor very much as no need to, but since the install I really can't do any trim. I haven't notice the sound of cavitation or even felt it. I have a 2072 jon boat, with a 90hp on it. I doesn't draw a lot of water, but more now.

So does anyone have any ideas how to resolve this? The shop guys are scratching their heads and are still looking for an answer.

Any help would be appreciated.

[skypilot]

So the 90 hp prop has cavitation damage? Of the 5 top holes on the motor transom mount, which hole is it mounted to the transom with?
Number 4? or higher?

I have some other questions when you answer back.

[sabmgb]

Sky, if I'm on the same wavelength as you on the holes, the pin (bar) is in the 2nd from the bottom, closest to the boat.

[Sinker]

Can you move your batteries forward? Its better for the trolling motor to have a shorter run of wire, as well as shifting the load around. 2, 31 series deep cycles are easily 150lbs, moving them would make a big difference.

S.

[skypilot]

Sky, if I'm on the same wavelength as you on the holes, the pin (bar) is in the 2nd from the bottom, closest to the boat.

Just to prevent confusion, are you talking about the 90 hp prop or the trolling motor prop as having an issue?

If the 90 hp prop is the issue, which set of the 5 holes on the 90 hp motor mount, is attached via the holes drilled in the transom? From the top hole counting down, is the 4th motor mount hole the one used to bolt the 90 hp motor to the transom?

Most dealers, in my experience mount O/Bs way too low for any kind of reasonable performance. Thus my question on mounting holes.

I have never seen an issue with trolling motor set up from a dealer other than maybe an adjustment to lower the trolling motor shaft in its bracket in order to prevent blowout caused by unique load distribution.

[sabmgb]

I think I ve got you now, it is the 90hp. I was talking the tilt holes in the bracket not the mounting holes. I will have to check the boat later.

[skypilot]

I think I ve got you now, it is the 90hp. I was talking the tilt holes in the bracket not the mounting holes. I will have to check the boat later.

Ok, good.

As a reference/starting point(when you have time to do so), level the boat/trailer on level ground(if what I suspect is "off", it is actually better to do this on soft or normal grassy ground, like in your yard for example.)

Anyway, level the boat/trailer and observe how the cavitation plate on the motor foot(motor lower end) relates to the precise centerline and the very bottom of hull or "pad" if you have a pad AND how many water pump holes are above and below the aforementioned centerline/very bottom of hull.(use a straight edge to determine an accurate horizontal line from centerline/very bottom of hull to past motor foot/lower end.
That will give us an idea of how deep/shallow the prop/cavitation plate/motor foot is, with regards to the very bottom of the hull @hull centerline.

Also, alum. or stainless prop and any holes in the prop hub and are they plugged or unplugged or any ridge ring on prop?
Also make(mercury, etc)

More to discuss, check out and explain later as you provide answers so we can determine cause and solution.

[sabmgb]

sky, thanks for the replies. Since I was out and about for lunch, buying new prop, I went and checked holes. I'm in the first hole from the top, so motor is as deep as it goes. I have an Envinrude 90hp. I will check out the other info tomorrow after fishing. I will also try and take pictures if I can get help holding things.

Thanks again

[skypilot]

sky, thanks for the replies. Since I was out and about for lunch, buying new prop, I went and checked holes. I'm in the first hole from the top, so motor is as deep as it goes. I have an Envinrude 90hp. I will check out the other info tomorrow after fishing. I will also try and take pictures if I can get help holding things.

Thanks again

If you are in the 1st hole from the top you may be too high with the motor on the transom.
Anyway hard to remember all the nuances between motor mounts.

Nothing under the hull protruding from the hull like a transducer?

Yes, take a look at all we discussed, post back and maybe we can resolve it.

[rick_iles]

Skypilot is on the right track. Years ago, I saw a similar issue with a 115 Merc. The cause was narrowed down to the motor being mounted too high on the transom.
Usually this also causes an increase in RPM.