cavitation/ventilation question-ADDED PICTURES

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yardbird
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Re: cavitation/ventilation question-ADDED PICTURES

#16 Post by yardbird » Sat Aug 04, 2018 5:04 am

I think these boats (pontoons) are very individual animals. What works for the round tubers may not be ideal to the U-tubers. And transom pods seem to come in a variety of shapes and sizes. I think the tri-tooners have it much easier in this regard. I just did a pretty careful comparison of the old motor to the new motor with photos from the back of the boat. Found one with the old motor and then duplicated the photo angle with the new motor.
The old motor's anti-cav plate appears to have been lower than the new one. The old 30hp could be opened up with no ventilation/cavitation issues. It would only get to about 10mph but... no issues. I think I'm gonna drop this one lower. Just a matter of whether I want to do it now or wait until I pull it at the end of the season.

Am I sure that'll fix things? No. But I can always raise it back up if it doesn't. I'm about as sure as I can be that I've got it propped right. Now I need to find the height. Raising it has negative effect. Logic tells me then that the other option is to lower it.

willy13
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Re: cavitation/ventilation question-ADDED PICTURES

#17 Post by willy13 » Sat Aug 04, 2018 8:54 am

I wonder if the wake of the side pontoons are disturbing the water the prop see's when the outboard is tucked forward of the pontoons ends?

yardbird
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Re: cavitation/ventilation question-ADDED PICTURES

#18 Post by yardbird » Sat Aug 04, 2018 11:02 am

willy13 wrote:
Sat Aug 04, 2018 8:54 am
I wonder if the wake of the side pontoons are disturbing the water the prop see's when the outboard is tucked forward of the pontoons ends?
I have a strong suspicion that that's the case. Going full down trim lets me push the throttle but with so much negative already built into the transom the boat plows so hard it doesn't steer well. And that's a lotta drag.

What's got me thinking is that trimming down like that puts the forward a little.. and down a little... and is that getting it out of the hull wakes?
There's just no reason I can come up with why you'd ever want as much down trim as I can get. So the setup isn't right somewhere.

But I did a bit of fixing on the bimini, changed the oil and filter, will do the lower unit maybe tomorrow. And still thinking about cutting down the transom to get me deeper. OR... since it's primarily a canal boat and doesn't get many opportunities to go fast unless we take it out to the river, maybe I'll finish up and get it back in the water and save the transom surgery for when we pull it at the end of the season.

Transom surgery would be something like...
Unbolt and lift motor a little (I have an engine hoist so could probably let it hang for a couple hours), unbolt and remove the transom, remove the aluminum channels that hold the wood part of the transom, cut down the aluminum pieces, cut the wood part of the transom, re-epoxy the cut, put the transom back together (will have to drill new top holes in the aluminum side pieces), reinstall transom, reset motor onto transom and redrill the mounting holes (would fill the unused holes with marine sealant. None of them would be sitting under water but ... just makes sense)

I'm guessing I could get it all done in a few hours

ORRrrr.... if it's wake turbulence... maybe the whale tale will shield it and I won't have to lower the motor at all.

I admit I just don't know.

r&s20
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#19 Post by r&s20 » Sun Aug 05, 2018 4:44 am

what kind of prop do you have, What i mean is is it a flat (for lack of better word) or is it cupped? hard for me to tell from pics
I had a similar problem when i went to a bigger motor, i thought i had a ventilation/cavitation problem.
What i really had was a prop slip problem, i had a little cup added to my prop and no more problem.
if it ain't broke-it ain't ours
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yardbird
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Re: cavitation/ventilation question-ADDED PICTURES

#20 Post by yardbird » Sun Aug 05, 2018 1:02 pm

It's not listed as being cupped. I started out with the prop that apparently comes standard with the motor. I think it was a 14 pitch (I'd have to look... still have it)

Dropped to a 9 pitch and it made a huge difference. But still sucking air at about 2500 RPM unless I trim full down.

I chopped the transom this morning and reinstalled the motor. I probably won't be able to get it back into the water until wednesday, but I'm dying to know if this fixes it.
The anti-cav plate is now even with the bottom of my pontoons. Pretty much where the old motor was. I can always raise it up a little at a time if needed, but without chopping it, I couldn't go lower.

And.. I'm not sure I'd know prop slip if it bit me.

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wwind3
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Re: cavitation/ventilation question-ADDED PICTURES

#21 Post by wwind3 » Mon Aug 06, 2018 4:16 am

mathematically I don't know how to calculate "slip"--but I have spun my hubs out several times with my toon--the rubber insert will wear out--esp on a toon--since there are tremendous forces at work pushing a barge thru the water. Never spun one out on my bassboat. The hubs are designed to give way when you hit something to keep from turning your gears into shrapnel. When my prop wears out I can go slow but when I try to speed up---engine revs up but boat wont speed up.. Doubt this is your case yet--you are probably on the right track with the turbulence problem.

As fa as cupping the tips--couldn't hurt to try to get a prop guy to do that. Not sure you can cup an aluminum prop--I cupped my stainless.....

….and if this wont drive you nuts--nothing will :biggrin2


http://www.propmd.com/propeller-basics.html#cup
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yardbird
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Re: cavitation/ventilation question-ADDED PICTURES

#22 Post by yardbird » Fri Aug 10, 2018 1:34 pm

Test run today with the motor lowered. Definitely took care of the problem. The anti-cav plate is even with the bottoms of the pontoons. I know a lotta people say to make it even with the bottom of the transom pod, but that just plain didn't work on this boat. I'm not getting splash back into the transom pod. The wakes look much more even. And at neutral trim I hit 20mph. That's with a 40 horse motor and a 9 pitch 3-blade. I'm happy with that. This old barge is like shoving a dumpster through the water. 20mph is almost double the best speed I ever got out of the old 30hp 2-stroke.

And call me crazy but it TURNS tighter at slow speeds. Has to be because I have more motor in the water, but still. An unexpected benefit. Anyways, I'm gonna call this a success. 6 hours of sweating in 90 degree heat was worth the difference I'm seeing.

And thank you for everyone who provided input. Y'all constantly gave me ideas of things to check, leads to look up more information, and without y'all's help I would likely still be struggling with this. Now.... to remove some of the negative trim built into the transom. I think a pair of 5 degree wedges will fix it. I am not aiming to remove all of it, but it's got WAY too much negative in it now.

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Marc K
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#23 Post by Marc K » Fri Aug 10, 2018 2:28 pm

Man, that sounds great - and well worth the effort! As you said, there are so many variations between boats.

Marc
Big and ugly but we love our 1999/2000 Crest II DL with a Yamaha F115

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wwind3
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#24 Post by wwind3 » Fri Aug 10, 2018 2:34 pm

:biggrin2 :biggrin2 :biggrin2 :biggrin2
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yardbird
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#25 Post by yardbird » Sat Aug 11, 2018 6:25 am

Aaaaand.... wow. I just got a good, accurate measurement of exactly how much negative angle is built into this transom.

Boat level. Pontoons level.
Using a digital angle gauge from my woodworking shop (used to get precise angles on saw blades, etc...) the transom is negative TWENTY ONE degrees!

Twenty one.... that's kinda... outrageous. Normal is like... 12. Some I've seen at 16 degrees negative. But generally everything I've seen is 12 to 15 degrees negative.
Mine is actually at -21.20, so if I add 5 degree positive wedges I can get it to 16 negative.

No real rush, I suppose. I'll probably just leave it as it is until haul out in the fall and add the wedges when I'm doing winterizing. But knowing this makes a lot of other stuff make sense.

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wwind3
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Re: cavitation/ventilation question-ADDED PICTURES

#26 Post by wwind3 » Sat Aug 11, 2018 8:38 am

yardbird wrote:
Sat Aug 11, 2018 6:25 am
Aaaaand.... wow. I just got a good, accurate measurement of exactly how much negative angle is built into this transom.

Boat level. Pontoons level.
Using a digital angle gauge from my woodworking shop (used to get precise angles on saw blades, etc...) the transom is negative TWENTY ONE degrees!

Twenty one.... that's kinda... outrageous. Normal is like... 12. Some I've seen at 16 degrees negative. But generally everything I've seen is 12 to 15 degrees negative.
Mine is actually at -21.20, so if I add 5 degree positive wedges I can get it to 16 negative.

No real rush, I suppose. I'll probably just leave it as it is until haul out in the fall and add the wedges when I'm doing winterizing. But knowing this makes a lot of other stuff make sense.

:o that is surprising....
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willy13
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Re: cavitation/ventilation question-ADDED PICTURES

#27 Post by willy13 » Sat Aug 11, 2018 10:30 am

Well Done! When I had a 40hp evinrude on our old pontoon it did the same thing. I was too lazy to try and fix it, lol.

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