ventilating severely - '83 25 HP Evinrude

[Hezekiah]

Hello everyone:

New day - new problem...at least it's the weekend.

So I purchased an old 25 HP Evinrude, short shaft Tiller. I put it on a 1963 14' Richline.

When I get going in the water it ventilates severely when it gets up some speed. I took it back to the guy I bought it from; we did a water test and they said it was the prop. 1 guy there said it was a worn out prop that had slightly lost it's shape and that's what was causing it to ventilate. This was a sunday, on Monday - he discussed it with his boss and his boss said it was a spun hub...regardless....needed a new prop.

I ordered a new prop and it arrived yesterday. I put a OEM Johson/Evinrude, 11 pitch prop on it, as recommended by the owner of the shop where I got the motor (one of the props also recommended by **********).

Today - I take it out to water test it...and it is doing the same thing. You can definitely tell it has a new prop on it...it idles better, smoother in the water...but as soon as it gets up to go on plane it ventilates.

Here are some pictures of the setup and the lower unit. (MORE TEXT BELOW PICTURES)




These pictures to show how motor is sitting in relation to the bottom of the hull - seems perfect.





This picture to show how the motor is setting on the transom - it is also bolted with 2 bolts in addition to the clamps



This is part of the ventilation plate that is bent - could this be causing it?




Another slight bend in the ventilation plate.



When I talked to the guy at the shop where I got the motor from today (not the owner), he recommended a put a hydrofoil on it. I have one...and I'm ok with putting it on...but when I just purchased this motor...I would like to not have to rely on a hydrofoil.

Note: I just had a 15hp Evinrude mounted in the same exact position and it did great in the water....no ventilation.

Any input or recommendations?? I find myself once again scratching my head...


Edit: I took a video when I was on the water earlier - don't know how much help it will be...it's not that great...will try to take a better one tomorrow when I have someone with me....

https://www.youtube.com/watch?v=u3kwZjvroTY

 

[racerone]

That does not appear to be an original transom.----What does it measure from the top to the bottom ?---Should be no more than 15"

 

[Hezekiah]

That does not appear to be an original transom.----What does it measure from the top to the bottom ?---Should be no more than 15"

I restored the transom when I got the boat about a month ago. It measures 16 1/2" right now.

 

[oldboat1]

Motor height looks high to me in the first pic, but about right in the second one -- must be camera angle. It looks like the trim angle was changed between pics (not sure if changing trim angle helped). A hook in the bottom of the boat can cause issues, as there is the effect of a trim tab -- lifts stern and motor, which might mean the motor is too high on the transom in operation.

I'll guess you might have a ventilation issue, rather than cavitation. The bent anti ventilation plate might be an issue (air directed into the prop at some point in operation). I've straightened them out in some of mine, but you do it with a little fear and trembling as old aluminum can snap off. Think I would check the hull (should be straight), and carefully try to straighten the plate, then try it again testing out a couple of trim positions.

That transom looks a little questionable. It needs to be rigid (no flexing). If the interior plywood is rotted, whatever the configuration, it needs to be replaced. In some cases you can expand the wood area a little to improve transom rigidity. Can affect operation, but clearly a safety issue. Maybe you've got that handled.

(Edit. See the motor is mounted a little high on the transom -- looks like it can be mounted an inch or so lower. I would do that and try it out.)

 

[Hezekiah]

Thanks oldboat -

The transom wood is fresh - just replaced it about a month ago with marine grade ply (fir) - 3/4" on inside and outside of the transom - I put it back together just like it came apart..although it was my first transom rebuild...which is why it looks sloppy...its very solid.

And you're right...I took the photos of each side and adjusted the trim in between - sorry about that...was just taking pictures and trying to figure this out.

I will drop the motor all the way down on the transom tomorrow - the only reason I had it mounted like that with the previous motor was because I thought the anti-ventilation plate needed to be even with the bottom of the hull...??

 

[oldboat1]

Not locked in stone. Given problems, though, I think I would mount it as low as I could and try it -- can always block it up later.

 

[Hezekiah]

Not locked in stone. Given problems, though, I think I would mount it as low as I could and try it -- can always block it up later.

Ok...thanks for the advice.

Like I said, I will do that tomorrow - drop it down and try that.

Fear and Trembling might be an accurate description of how I feel about trying to straighten that out...I was thinking of putting a crescent wrench on it and trying to bend it a little...very slowly...someone recommended a hammer...but thats truly scary....

 

[oldboat1]

Yeah, might have to get creative. Maybe sandwiching the plate between a couple of pieces of angle iron, and clamping it up with a couple of big C clamps (maybe with big vise grips).

 

[jbcurt00]

I suspect some of the 'problem' is the swap to a larger hp motor. So start over w the motor down on the transom, adjust trim angle and height to the best combo for the 25. Not likely to be the same as the 15hp's best setup.

Is 25hp about the max hp for the 14ftr?

A tiny tach would help make sure you're prop'd right as well

 

[Hezekiah]

I suspect some of the 'problem' is the swap to a larger hp motor. So start over w the motor down on the transom, adjust trim angle and height to the best combo for the 25. Not likely to be the same as the 15hp's best setup.

Is 25hp about the max hp for the 14ftr?

A tiny tach would help make sure you're prop'd right as well

Good point about the premium setup for the 25 not being the same setup as the 15...again...I truly thought I was supposed to be aiming to make the anti-ventilation plate even with the bottom of the hull...but obviously something is not right...

The specs for the boat say 35hp is max

Will look into small tachs, thanks.

 

[Hezekiah]

Yeah, might have to get creative. Maybe sandwiching the plate between a couple of pieces of angle iron, and clamping it up with a couple of big C clamps (maybe with big vise grips).

What nightmares are made of

 

[jbcurt00]

Oddly, if ALL short shaft outboards were 15in and ALL short shaft transoms were 15in too, the vent plate even w the bottom of the hull still wouldnt be the best possible setup on all boats.

Test and tuning, good luck

 

[Hezekiah]

Oddly, if ALL short shaft outboards were 15in and ALL short shaft transoms were 15in too, the vent plate even w the bottom of the hull still wouldnt be the best possible setup on all boats.

Test and tuning, good luck

Ok...understood...best lessons are learnt the hard way, I suppose.

Will update the thread tomorrow once I do another water test.

If anyone else has any input to offer, please do...I like learning.

 

[jbcurt00]

Oh, and I'll change the title to ventilation rather then cavitation.

Many do interchange cavitation and ventilation but they are different, but for accuracy, you're experiencing ventilation of the prop

 

[flyingscott]

Be very careful straightening that plate out it will break easily. We have the same boat have never mounted a motor that high if you look at the keel you are above the bottom of the boat. Mount the motor as low as it will go you need to be closer to the bottom of the keel.

 

[Hezekiah]

Oh, and I'll change the title to ventilation rather then cavitation.

Many do interchange cavitation and ventilation but they are different, but for accuracy, you're experiencing ventilation of the prop

Thank you for clearing that up for me.

 

[Hezekiah]

Be very careful straightening that plate out it will break easily. We have the same boat have never mounted a motor that high if you look at the keel you are above the bottom of the boat. Mount the motor as low as it will go you need to be closer to the bottom of the keel.

I don't think I am going to try to straighten or bend anything until I see how it runs sitting all the way down on the transom.

I read somewhere on here that its best to make 1 change at a time...do a test run...then make another change if needed...so I'll definitely start with the easiest and less frightening option first.

 

[jasper60103]

I don't think I am going to try to straighten or bend anything until I see how it runs sitting all the way down on the transom.

I read somewhere on here that its best to make 1 change at a time...do a test run...then make another change if needed...so I'll definitely start with the easiest and less frightening option first.

I have the same motor but haven't run it in a while.
If memory serves me right, I believe the cave plate was slightly lower than the hull bottom. And it has a 11 " prop pitch like yours.
But do get a tiny tach knockoff for tuning your setup. They're cheap on ebay. Good luck.

 

[racerone]

??---Pour boiling water on that cavitation plate.----Give it a bump with a hammer.

 

[kbait]

Do not bend that plate w/vice grips! Same w/bending props. I re-bend w/rubber mallet, always using a piece of metal as a backstop on the other side of the metal your shaping/bending..large hammer face will work. If you vice-grip bend, or hammer w/o backstop, it'll invariably snap off. And +1 on lowering the motor to the transom. Having the anti-ventilation plate an inch or so below boat bottom won't slow you down noticeably, and will likely solve the issue.
Good luck.