cavitation/ventilation question-ADDED PICTURES

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cavitation/ventilation question-ADDED PICTURES

#1 Post by yardbird » Fri May 25, 2018 6:27 am

Older Landau 2 toons, u-shaped toons, 18ft.
Previous motor was 30hp nissan 2-stroke.
Repowered with a 40hp Tohatsu 4-stroke.

With the 2-stroke, my top end speed was about 10-12mph gps.
With the 4-stroke, I'm still in the break-in period but I can get it up to a touch over 20mph and still be under the allowed 3,000rpm max for this stage of break-in.

Once I pass that 10mph mark I'm having to trim DOWN almost all the way. If I don't, it cavitates or ventilates hard. I'm guessing that by trimming down that hard I'm forcing the bow down and plowing... causing a lot of drag.

Shaft length on the new motor is same as the old. Cav plate in same location height-wise, just below the bottom of the center pod.

Would one of the aftermarket cav plate add-ons help with this?
I'm looking at

I did my testing with just me on the boat after finding it would ventilate so hard with my wife sitting up on the bow in one of the fishing chairs. Today I think we're going to take it out with her plus my son (adult). So I may try again with everyone seated at the back of the boat. We're on a canal, so 90 percent of the time this boat will be below 10mph anyways, but I want to be able to USE the additional power when we venture out onto the river.

So... thoughts and suggestions appreciated.
Last edited by yardbird on Thu Aug 02, 2018 10:53 am, edited 1 time in total.

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Re: cavitation/ventilation question

#2 Post by Bamaman » Sun May 27, 2018 6:38 am

First of all, what rpm's are you turning vs. the engine's posted redline? If you appear to be propped about right, look at the engine height. Propeller might need to run lower in the water.
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Re: cavitation/ventilation question

#3 Post by yardbird » Sun May 27, 2018 5:29 pm

RPMs about 3,000 (still in break-in period)

The engine is as low as it can go and is at the same height as the previous engine. I might be able to cut the transom down a couple inches, but I'd have to do a lot of checking to make sure I could still use the tilt/trim without running into stuff if I do that.

I think before I go chopping the transom (I JUST made this one!) I might see if I can get a jack plate to lower it ... as long as I can still use the tilt/trim

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Re: cavitation/ventilation question

#4 Post by ron nh » Mon May 28, 2018 8:48 am

I'd say you can't tell yet. 3000 RPM's with that HP will not lift the front enough to give you the answer.
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Re: cavitation/ventilation question

#5 Post by yardbird » Sat Jun 23, 2018 9:09 am

Ok... been doing several tests as I'm able. I had my son run the throttle up and drive while I looked at what's going on with the motor.

At neutral trim with just the 2 of us aboard, it'll start sucking air at about 2200 rpm.

When I look at the motor as we're up at around 2000 rpms, the plate ABOVE the anti-cav plate is just at the surface. But the anti-cav plate is well below the surface obviously. This makes me wonder and this may be a dumb question but I have to ask...
Can I get this blowout with the motor DEEPER in the water than it should be? I mean normally when I read about cavitation/ventilation issues the advice is to drop the motor DOWN a couple holes.

Run it up to 2200rpm and as it starts sucking air, trim DOWN. That stops the ventilation but causes the boat to plow and full throttle doing that won't get it over 3000 rpms if it gets there at all.

Symptom 2:
Running at 2100 rpms, I had the wife walk forward to the front of the boat. That caused us to have the blowout at lower rpms. Which tends to make me believe that by doing that, we lowered the bow, raised the motor up higher and it blew out. THAT suggests it needs to be lowered, which I can't do unless I cut my transom down and I think that would put my motor too deep in the water at rest.

It really looks like the wash off the u-shaped pontoons is "folding" over. Turbulence?

I'm going to pull the boat out of the water, probably at the end of this week for its first oil change and stuff. I've ordered a Whale Tail XL and if it looks like my mount height is ok, I'll install the whale tail and see if that helps.

Boat is rated for a 50hp so someone thought it could go faster :)
Prop right now is.... I think a 14 pitchx11 diam 3-blade. Might be the 13 pitchx11 diam. I'll look again when I pull it. I swear I thought I wrote it somewhere.

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Re: cavitation/ventilation question

#6 Post by yardbird » Sun Jun 24, 2018 11:40 am

OK... no responses so I'm talking to myself for the benefit of anyone having this issue in the future (or now)

It seems I'm horribly over-propped. Everything I'm finding SPECIFICALLY about pontoons with smaller motors is talking about a pitch as low as 9!
The whale tail is ordered. But I may be seriously looking to try re-propping before I go drilling to install a hydrofoil I may not need.

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Re: cavitation/ventilation question

#7 Post by Aluminum Maiden » Fri Jul 06, 2018 7:22 am

I realize the boat in my sig is different and I'm still learning but to give you some perspective on propping. My T60 has the large gear case which makes a larger prop work better than a small. Is your's like that? It came stock with an aluminum 14*11 on it. I replaced it with a used Mercury Vengeance stainless 14*10 (60$ :nana ). May have lost a hair top speed but gets to plane instantly. Depending on the river current, I may have to trim down a lot but never all the way down at the highest speed attainable weather permitting
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Re: cavitation/ventilation question

#8 Post by Marc K » Wed Jul 25, 2018 7:47 am

Bringing this back up to see if you figured it out?

I'm still learning..........

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Re: cavitation/ventilation question

#9 Post by yardbird » Thu Aug 02, 2018 8:45 am

Actually.... no. Not entirely.
Getting it propped right helped a ton, but it still won't go over about 2500-2800 RPM without blowing out.
It's not dire as we're on a canal where the highest speed imit is 10mph and a lot of it is 5mph no wake.

BUT... I know it's not doing what it's supposed to yet. So I pulled the boat out to change the oil and stuff and set up to really measure what all is going on. I have to resize some pictures, but should be adding to this soon.

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Re: cavitation/ventilation question

#10 Post by yardbird » Thu Aug 02, 2018 9:21 am

So I set it up to measure stuff...

View from the back. The horizontal stick represents the bottom of the pontoons.
rearViewSM.jpg (104.56 KiB) Viewed 622 times
Another shot from an angle...
rearangleviewSM.jpg (171.33 KiB) Viewed 622 times
What this is showing me is that my anti-cav plate is about 2-1/4 inches above the bottom of my pontoons. It's below the bottom of the transom pod, but well above the pontoon bottom. I've watched this a lot when running. The splash plate will run just at or ever-so-slightly above the water surface, but the anti-cav plate remains submerged.

I think this motor needs to be lowered. I think the only reason I don't see the anti-cav plate at the surface when running is because of the wakes coming off the pontoons.

Transom height...
transheightSM.jpg (98.51 KiB) Viewed 622 times
I have 3 and a half inches of transom sticking up above the surrounding metalwork. So... I should be able to cut this down by 2 inches with no problem. I mean other than... lifting the motor, pulling the transom out, cutting the aluminum and then cutting down the wood part of the transom, re-epoxy the cut, reassembling it and redrilling some holes.

And because this transom has SO much negative angle on it, I am consider adding wedges to give it some positive trim.
Here's what my trim gauge indicates when the motor is actually neutral trim (anti-cav plate parallel with toons)...
trimneutralSM.jpg (102.66 KiB) Viewed 622 times
If I add some 2 degree wedges, it should be closer to indicating correctly. There are 5 degree wedges available but they would basically make it indicate just as far off but in the other direct when neutral.

So... after all this... thoughts? You think I'd be on the right tracking lowering the motor 2 inches?

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Re: cavitation/ventilation question

#11 Post by yardbird » Thu Aug 02, 2018 9:25 am

I should add that I do have a whale tail here that I could put on and try that instead of cutting the transom down. I think I just dislike the idea of drilling my anti-cav plate if it ends up not being the solution I'm after

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Re: cavitation/ventilation question-ADDED PICTURES

#12 Post by riplipper » Thu Aug 02, 2018 4:13 pm

they make a whale tale that requires on drilling. You could check on the Hull truth and see what peoples experiences are. Most likely you will have a mixed bag, some love them and some that bitch about them....usually the negative people have never tried it :lol3
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Re: cavitation/ventilation question-ADDED PICTURES

#13 Post by wwind3 » Fri Aug 03, 2018 3:43 am

Lots to digest here..

First--you should always run rated HP if possible.
2nd--Too much wt toward front of boat and you will blow out. At any speed with a small motor...
3rd Cavitation plate should be even with bottom of mounting pod--don't lower it more...
4th Hydraulics going on around your lower unit are like what an airplane experiences in a stall--the smooth slipstream you should have around the prop is being broken up. Any turbulence around the prop will cause it to blow out--prop will be pushing "air" instead of water.
5th--Like someone said--each setup is different--I can tilt my motor way up before it blows out if I am by myself--but I only have 2 fishing seats and a trolling motor up front--and I try to avoid folks up front if I'm trying to get some speed.
6th Not sure about using a whale tail--no experience there.
7th Re: Trim gauge- not sure about this one. Looks like to me you have a lot of built-in negative trim at "neutral" because of transom slant---If you run it "tucked" with tilt all the way down--it will force the nose of the boat down-so you have a lot of negative trim to overcome. Hope I'm right about this one-I think I am.

Good luck..
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Re: cavitation/ventilation question-ADDED PICTURES

#14 Post by yardbird » Fri Aug 03, 2018 4:34 am

1.) Rated for 50hp max. Running a 40hp.
2.) I have 2 fishing seats up front but all test runs are done with nobody up front. Usually me and wife or me and brother. Passenger on rear couch.
3.) Cav plate height: If it's even with the pod now or just slightly below, then I still have a turbulence problem that I'm no at all sure how to resolve.
- if I go full negative on the trim I can get more throttle, but the boat is plowing so hard it feels like driving on a flat tire. Essentially am I not getting the prop into "cleaner" water by tucking it under that far? That's why I thought lowering it just a bit might be the ticket.
4.) Yes on the hydraulics around the lower unit. Familiar with laminar flow. But how to resolve this is the issue. The thought on the whale tail is that by broadening the cav plate (which is essentially what any of those devices do) I might protect the area below the cav plate from the folding wakes of the hulls converging on the prop location which would be an enormous amount of turbulence.

5, 6, already covered

7.) I think the wedges to reduce negative trim will offset that transom's negative angle. This is more of an "it bugs me" thing.

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Re: cavitation/ventilation question-ADDED PICTURES

#15 Post by Marc K » Fri Aug 03, 2018 12:08 pm

My first post here was about this subject. Coming from deep V boats, my pontoon boat just didn't feel right.

If I trim way up, I would just begin to see some front lift, then it blows out. My maximum speed occurs at neutral tilt at 5800-5900 RPM wide open. Now, I am convinced that the HP/weight ratio of mine is doing exactly what it was designed to do - but I still feel like I'm plowing and not gliding - because I am plowing :lol3

It is difficult for me to see where the cavitation plate is at full speed because of the massive water funnels shooting out between the transom pod and the tubes, but I think that mine sits much lower than what I see suggested here. I really need to have someone else drive so I can look at the angles and attitude from another boat.
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