310da bravo 3 drive ventilation problem

[Big blue]

can anyone help on this ongoing problem I have with what seems to be the starboard drive losing grip even in calm water, feels as if the prop is ventilating but cannot find why, the boat bottom is clean, no damage, new 4x3 props recently fitted with no improvement, any suggestions?

 

[Lazy Daze]

If there are no issues with the running gear (physically verify) such as line wrapped around the drive or seaweed (your signature doesn't mention location), and it works fine at lower RPM's, then I would suspect the coupler is shot. Installing those new props for the purpose of eliminating what you perceive as ventilation would not have solved the problem, unless the existing props were badly damaged.

 

[keokie]

Any new thru hulls in the bottom of the hull on the starboard side? Or anything caught on an existing thru hull?

 

[Big blue]

The boat is located in the uk, it’s a uk spec sea ray with D4.2 d tronics, I have owned the boat for 3 years and I inherited problem from when I bought it, boat has been out the water twice for drive and antifoul service, both times everything looks good, I tried toe drives in and out but just made the problem worse, so they are toe out,( 3/4” closer between the prop stafts), boat pulls up on plane great, runs at high speed great, but slow down to 25mph and it ventilates bad, it’s terrible in rough water, the factory ray marine thou hull fittings are on the starboard side and an additional ray marine down scan transducer

 

[Lazy Daze]

Since you can power up to full speed, that pretty much negates the coupler... which is a good thing.

Explain the situation where the over-revving occurs better, please.

Where/how is this down-scan ducer located? Thru-hull, transom mount, etc? If it's too close to the drive, that could very well be the issue. Why it doesn't happen more often... who knows.

Have tried trimming the drive down further?

 

[Big blue]

Mainly in rough water the problem is worse, the syncro gauge will swing back and forth as the props lose grip, it can start the port drive to lose grip too, I ran her this weekend in calm water and watched the gauges, Rev count would go up on starboard by about 100rpm, you can here the props lose grip, it will bite then settle down and start over again, the drives are trimmed in and neither drive likes any trim, I have reversed the celery sticks so the drives trim in more, but no improvement so moved the back to the original position.
The thru hulls are mounted thru the hull about 4’ forward of the starboard drive about 6” off center of the drive, the position is where they were installed from new.
My thoughts are either the thru hulls, or the thru hub exhaust pushing past the bearing carrier where it does have some corrosion present, various marine engineers have looked at my setup and say it looks good, anyone tried fitting the exhaust tubes instead of bellows to overcome this problem

 

[Lazy Daze]

Where is that down-scan ducer located? This would not have been done at the factory.

Assuming the "factory thru-hulls" are as you mentioned, they are not the cause. If it was, it would be a known issue.

The exhaust tubes/cup setup won't solve the problem. I've never heard of it causing the problem, either, for that matter. Even if the exhaust bellows had a huge hole in it, it shouldn't cause this issue.

If the exhaust is pushing pass the bearing carrier, you have much bigger issues to worry about.

Aside from what you see on the gauge, you are POSITIVE that you can hear and feel the props slip?

Does this happen consistently? Or is it more of a sporadic thing? When it does happen, does it continue to happen? If there's anything floating in the water, it can catch on the forward edge of the drive and cause ventilation. If you stop and/or reverse, the weeds/whatever will come off and you're good to go again.

 

[Big blue]

It happens constantly,this weekend in calm water the noise will start, the starboard motor will vibrate, revs go up by about 100rpm, for about 10-15 seconds then it drops to normal for about 1-2 minutes and starts again, in rough water it’s more dramatic and it was hard to tell which drive was causing the problem as if they took it in turn to rev up.
Another test was to trim each motor up whist at about 23-25mph til they started slipping, (same noise and rpm gain) when I trim down the port bites straight away but the starboard doesn’t, takes about 10-15 seconds to bite, both drive are in the same position when trimmed back down, it’s driving me mad, I have done nearly 300 hrs cruising with the motors like this, can’t see any cavitation burn on anything around the prop but it must be causing increased wear and tear to drives.
The down scan is about 6” aft of the factory ones, apart from the bow thruster there is nothing else on the hull that has been retro fitted, the downscan is a large ray marine plastic
Cpt-110 downvision transducer through hull

 

[Big blue]

Photo of drives before new props, starboard drive is trimmed out a little in this photo but normally trim in

 

[Big blue]

Port drive with new 4x3 24” props

 

[Thornton69]

Single or dual controls?
Do you have excessive free play in the shift linkages to the drives?
What is the condition of the drive lube?
Wonder if the cone clutches in the drives are slipping. They can give a bit of a chatter feel (vibration?) under load.

 

[BillK2632]

I have always heard the skeg doesn't really matter, but I noticed your STB drive has a chunk missing from the skeg. Has that always been that way? I am betting your problem is related to aeration in the water stream in front of the drive, ever been close behind another boat an gotten right in their propwash? I would be looking at the transducer and also at that skeg. The easy fix for the skeg is one of those SS skeg guards. As far as the transducer, the "boat" should not be mounted directly in front of the props, it can cause exactly what you are describing.

 

[scoflaw]

X2 on the slipping cone clutch

 

[Big blue]

I agree with aeration of the water in front of that drive, the downscan is a big lump of a unit, I might think about fitting a flush unit as I am not interested in the fish finder, just the depth.
The engines are single lever with new cables set up with the mounting jig,drive oil is changed every year, drives been striped recently with new bellows, upper swival shaft and seals, steering arms, trim senders,everything is tight, no slop, the toe in/out on drives do play a big part, setting them dead straight causes big problems on the props slipping, also shifting is perfect on both drives, no slipping of cones on full power when pulling up onto a plane.
I will tackle the skeg the next time it’s out, it’s always been like it, I’m a bit worried about the skeg plates as I will be adding more stainless to my galvanic corrosion problem that I suffer with on these drives.
Another note to add is that i also run the boat in a fresh water bay and it’s always calm water, i never have any problems with ventilation/cavitation in the bay, when I take it to sea, that’s when the fun starts. Any turbulent water sets it off as if the props are only just biting.

 

[Lazy Daze]

OK, that info (bay vs sea) is some more good info. The cone clutch is a consideration, but it seems unlikely since it hasn't gotten worse in the 3+ years this has been going on. Plus, you don't seem to have trouble getting up onto plane where the most stress is put on it. Add in the fact that you don't have any issues in calm water, at it even further eliminates the clutch - or any mechanical issue as we talked about near the beginning of this.

The bow thruster is a long way from the stern so it's "unlikely" that it's causing turbulence, but you can rule it out by removing the surrounding bottom paint and taping over the holes with a good tape, such as shrink/hull preservation tape. It should stay in place long enough. Unfortunately, there's no easy "test" with the downscan ducer that I can think of. You might have to remove it and do a temporary plug. Although if you're not concerned about needing/using it, then just remove and repair permanently.

I could just be that the downscan ducer is in just the right position. Can you call the previous owner and find out if the issue started after the ducer install? I've run twin 280/290/310/330/340 DA's with stern drives and I have never had this issue. Unless there's something else going on with the hull - such as an imperfection/damage from improper cradle storage, for example, creating a deflection in the hull...

I wouldn't worry too much about the broken skeg - it's not that big of a deal. Just file it smooth and paint it. One caveat about the skeg (in addition to your concern about corrosion) is that the keg guards need to be bolted on and the bolts are very close to the gear housing. New Al skegs can be welded on to fix a broken one, but only if the break isn't too close to the gear housing. Because of the mounting points of the skeg guard, and the fact that SS is harder than AL, you increase the chance of totally scrapping your lower unit in the case of a hit. The guard looks pretty, but from a functional standpoint, save your money and keep it off.

 

[Big blue]

I am leaning towards the downscan, it isn’t great neither and loses signal a lot, probably says it all really, I mentioned the bow thruster as it has a raised fairing on the forward edge, I attached a photo of another boat with identical looking thruster to explain better. But I assume the fairing helps more than hinders

 

[scoflaw]

Swapping drives wouldn't take long

 

[Big blue]

Swapping drives is a good point, at lease it will rule out any mechanical defects with them

 

[Thornton69]

The fresh water bay/sea thing is pretty interesting. Have you noticed much difference in your draft between the 2? When you are having this problem at 23-25 mph, have you tried to manipulate your hull position with the trim tabs to see if they have any influence? With sea water being denser than fresh, I would have thought it would be less likely to be a contributor to cavitation but only guessing. Regardless, would have thought the transducer theory would be a problem in both bodies of water.
Was the thruster added or is it OEM? Would be interesting to see what effects blocking it off like LD suggested would make.
You also mentioned corrosion in the starboard lower. If you are concerned with it venting the props then swap drives as Scoflaw suggested. It would at least help to narrow down which end to chase the problem.
You also talked about a vibration. I understood it that you were having difficulties identifying which side it was coming from. Not sure how cavitation of only 100 rpm can cause vibration. Is it at all possible that the port engine is getting a miss and dropping 100 rpm? Have seen older injection systems do funny things under specific loads/rpms. Especially if you add a little bouncing into the equation. I know you said you could hear it slipping so only an idea.

 

[Big blue]

The std drive definitely gains about 100 rpm as the syncro gauge swings over to the left, I also have vee three engine monitors so can watch the rpm closely, the trim tabs make the situation worse, putting trim tabs down is like putting the brakes on and it loads the props up, tipping it over the edge.
I do agree that trying to identify what’s going on when at sea with boat bouncing around is very difficult, I have done various tests to identify even which drive is the problem, in rough water it’s no chance, the syncro gauge swings left and right with both the motors reving up and back to normal, it’s worse in a following sea, not sure why unless the props are closer to the surface.
The bow thruster is about 5 years old, along with the downscan, the downscan is huge, it’s on a fairing block too and protrudes about 30-40mm off the hull, I have decided now to get rid of it and fit a flush bronze transducer instead.