[Solved!] New motor, first run... issue with.. ventilation?

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yardbird
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[Solved!] New motor, first run... issue with.. ventilation?

#1 Post by yardbird » Tue May 22, 2018 6:02 am

Old Landau pontoon. Swapped out an almost 23 year old 30hp 2-stroke for a brand new 40hp 4-stroke. Tohatsu with power trim/tilt
This boat has U-shaped pontoons filled with foam.
Motor mounts on a "transom pod" ... no center pontoon. Just a basically triangular... well you guys know what I mean...

We're on the Erie Canal. Speed limit generally 6mph, no wake. But we have access to a river and there are some undeveloped area where I can open it up a bit. So we launched yesterday to take it to the marina where our slip is, about a half hour boat ride.

Break-in for the motor says to keep it below 3,000 RPMs for the first 2 hours. Then below 4,000 until it has 10 hours on it. 1 to 2 minutes of full throttle permitted with 10 minutes between full throttle runs. I want to follow the break-in.

So we're on the canal headed for the marina. Motor sounds great. Very quiet. Running 6mph has it at about 600 RPM on the tach.

Our old 2-stroke... wide open... would give us about 12mph (this is all GPS) if we were lucky.
I advanced throttle on this one and at about 10 or 12 mph it starts... either cavitating or ventilating. I'm not totally sure which, but lots of air entrained. You can hear it happen. Here's the weird part though (to me)... the RPMs didn't run up when this happened. Couple times I heard it run up a bit. But I never got it over 2,000 RPMs.

The cav plate on this motor is where the old one was. I can not lower the motor on the transom. It's down as far as it goes.
I positioned the trust rod on this one such that the cav plate is not quite parallel to the bottom of the pontoons. By that I mean, when it's down-trimmed all the way, it is just slightly negative trimmed (tucked under... prop pointing very slightly down as opposed to being in a straight line with the line of the pontoons.) VERY slightly.

My wife likes to ride up front on one of the "fishing chairs" so I had weight on the bow.
It ALMOST seemed like it was sucking the back of the boat down and causing drag? Would that be why the RPMs didn't get over 2,000?

Here is what I want to try...
#1, put the thrust rod back in its lowest position so the motor has full range of trim. My transom is not straight vertical. It's angled. I had thought that letting the motor go that far negative would be bad. Maybe I was wrong. I had thought that letting it go that far negative would cause "plowing". Maybe I was wrong.

This motor also weighs 200 pounds where the old one was 100 pounds.

Appreciate any and all thoughts on this.
Last edited by yardbird on Wed May 23, 2018 12:43 pm, edited 1 time in total.

Bamaman
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Re: New motor, first run... issue with.. ventilation?

#2 Post by Bamaman » Tue May 22, 2018 9:44 am

Is the new motor the same length shaft as the old motor? Usually ventilation comes from the prop sitting too high. And sometimes having passengers in the front will lift the rear end causing ventilation. Sometimes the rubber in the prop breaks loose and the prop. is slightly spinning. Is it a new propeller?And
These issues can be trial and error.
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BobL
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Re: New motor, first run... issue with.. ventilation?

#3 Post by BobL » Tue May 22, 2018 9:59 am

Did you get a new prop with the new motor?

Since your revs are not changing it sounds like cavitation (engine height and/or weight distribution are my bets) BUT maybe you did not notice a rev change? Mis-matched prop, damaged/worn prop, and engine height usually point toward ventilation but you would see the RPM's jump.



From West Marine.....

Ventilation: This problem occurs when air from the surface or exhaust gas from the engine gets drawn into the prop’s blades. The boat’s speed drops, the engine over-revs and screams, and the prop sucks air. Ventilation results from excessively tight turns, a motor that is mounted too high on the transom, or an engine that is over trimmed. Ventilation can also occur from prop designs that are not matched to the application, poorly designed props, props with little or no cup, or props that are worn or have damaged edges or cup profiles.

Cavitation: Often confused with ventilation, cavitation results from water vaporizing or “boiling” due to the extreme lack of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation may result in “cavitation burn,” metal erosion or pitting of the prop’s blade surface. Causes of cavitation include incorrect engine height (outboards), dings or sharp corners in the leading edge, poor polishing, too much cup or crummy blade design. Cavitation can also occur from thru-hulls, sensors, or other turbulence-producing protrusions under the boat forward of the prop.
SOLD - 2009 20' Bentley w/4 Stroke 90hp Mercury
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2014 Xcursion 23RF XS package w/150 4 Stroke Mercury

yardbird
Posts: 133
Joined: Thu Jul 20, 2017 3:50 pm

Re: New motor, first run... issue with.. ventilation?

#4 Post by yardbird » Tue May 22, 2018 1:33 pm

New motor is same shaft length as the one it replaced.
Prop is new and came with the motor. From the list of available props, this one sits about in the middle.
Made sure the prop parts all went on in the correct order and orientation.

Up until I reach that 10 or 12 mph mark, everything seems perfect. I think I need to first give the motor back its full range of trim. Then do this same ride with wife at the bow... and then also have her come to the back of the boat. Try to see if this happens regardless of weight distribution. And maybe it NEEDS that negative trim to get going?. I don't know.

Went to the marina and lowered the thrust rod. It's kind of a crappy day. Tomorrow supposed to be nicer. Hoping to be able to take it for a test run tomorrow and see if I accomplished anything.

Well I did ... I didn't fall into the water and I didn't drop anything into the water. So... kind of an accomplishment right there. :)

yardbird
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Joined: Thu Jul 20, 2017 3:50 pm

Re: [FIXED!] New motor, first run... issue with.. ventilation?

#5 Post by yardbird » Wed May 23, 2018 9:30 am

That was it. The thrust rod had to go down.
I went out by myself today after moving the thrust rod down to give the motor its full range of tilt/trim.
Got it up to just over 20mph with no cavitation. Hadda play with the trim a little at certain points, but keep in mind I've never had a motor with power trim/tilt.
That was just under 3000 RPMs. Like... 2800-ish. Break-in procedure says to limit it to 3000 RPMs for the first 2 hours. I have another hour and 15 minutes. :)
Then I can go to 4,000 RPMS with bursts of full throttle for up to 2 minutes every 10 minutes. That lasts for 10 hours.

Anyways... seems to have a lot of down trim getting up to speed. Then I would kinda burp it up a little until I could hear the air start in the wash. Burp it back down a touch and quiet again. But it's never going to plane. Those U-shaped pontoons are displacement hulls.... not planing hulls.

I told my wife she can sit up front on the fishing chairs when we are just cruising the canal. That's all at mostly 6 MPH, but some places you can do 10.
Once we get into an area where we can go fast, we can experiment a little with how weight distribution plays into all this. But for now I think I'm pretty stoked.

20 mph is TWICE as fast as what we were getting out of the 24 year old 30hp 2-stroke that was on it. And I see the potential to squeak out a little more.

Thanks, guys for the advice, things to look for, etc.
OH! And I set my tach to the correct number of poles (duh). I THOUGHT it was reading kinda low. (can't believe I missed that one)

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bansil
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Re: [Solved!] New motor, first run... issue with.. ventilation?

#6 Post by bansil » Wed May 23, 2018 3:34 pm

:alright
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