AQ280/AQ260 Overheat On Plane

Hello all! I'm new to this great site and hope to contribute but right now need help getting my Skipjack 20 back in the water

Some background info - "Finatic" is a 1984 Skipjack 20 Open designed by the legendary Jack Cole. She's powered by a raw-water cooled VP AQ260 with AQ280 Outdrive which have seen only 1500 total hours of use divided about 50-50 between Salt & Fresh water. Motor was professionally rebuilt at 1100 hours when pulled to replace a rusting oil pan. Outdrive was R&R'd at the same time due to water intrusion from a damaged bellows. Parts very recently replaced include new Raw-water pump, Water neck connection on drive, Thermostat, and Exhaust manifolds & risers.

Overheat only occurs when boat gets up on plane, temps rise slowly from 160 to about 180-190 then spiking to 210 or so when throttled back and down off plane before slowly cooling back to 160. I removed the water neck connector thinking it wasn't sealing properly and found the gasket underneath was chewed up on one side of the o-ring like edge. Also, with the water connector removed, when you grab the lower unit and try to wiggle it, the tube (pivot tube?) underneath where the water connector mounts has considerable movement.

Is this movement the root cause of my overheat?

It shouldn't. The beaded gasket is supposed to seal around the tube. So unless the gasket is really bad, or the water neck screws not properly tightened, or the bearing around the tube is completely gone, there shouldn't be any air ingress.
Have you checked for barnacles or other obstruction inside the grids & water tube on the lower unit? Are the grids clear?

First, a warm welcome to you.

FinChaser said:

Outdrive was R&R'd at the same time due to water intrusion from a damaged bellows. Parts very recently replaced include new Raw-water pump, Water neck connection on drive, Thermostat, and Exhaust manifolds & risers.

Overheat only occurs when boat gets up on plane, temps rise slowly from 160 to about 180-190 then spiking to 210 or so when throttled back and down off plane before slowly cooling back to 160. I removed the water neck connector thinking it wasn't sealing properly and found the gasket underneath was chewed up on one side of the o-ring like edge. Also, with the water connector removed, when you grab the lower unit and try to wiggle it, the tube (pivot tube?) underneath where the water connector mounts has considerable movement. Is this movement the root cause of my overheat?

Click to expand...

Along with these parts that were replaced, should have been PDS bearing replacement, IMO.

As for the water neck fitting and the movement at the pivot tube........, if the upper pivot tube bushing is sloppy, the new beaded gasket will hold for a short time only. The pivot tube cannot have any slop/play at the top, or it will oscillate, causing the bead to eventually loose it's seal against the tube.
A breached seal at the water neck will supply sea water as long as it's submerged, but when up on step, it will rise out of the water, hence the suction breach, and possible over-heating.


Another cause of over-heating may/can be the exhaust riser to Y-pipe rubber couplers. The OEM couplers are always.... repeat, Always too short. When they are too short, one of the riser band clamps squeezes the rubber material into the scalloped areas where the spent sea water enters the exhaust flow.
The fix is new 95mm soft-wall marine exhaust hose that is approximately 2" longer than the OEM, moving the band clamps further up and away from the scalloped areas.

Although you say that these are new, the manifold-to-riser sea water transfer ports can become rusted and can restrict outgoing sea water. If the manifolds/risers are OK, these transfer ports can be cleaned and opened up, of which renews sea water flow.
If in salt water, you may want to do this annually.

Ok - what is the PDS bearing?

I checked my Y-pipe rubber couplers and the clamps are positioned such that they don't impinge on the exhaust riser outlets. Also, there are no obstructions of the intake grids or water tube on the lower unit. Boat is trailered 99.9 % of the time so marine growth is no issue.

As Albacore are now here off the coast, I'm inclined to just securely cap off the water intake on the transom and install a screened through-hull raw water intake scoop and a proper seacock to get the boat ready to run for tuna then address the outdrive issue later as the shops I'd consider doing the work are booked up for close to a month. If the pivot tube bushing is shot, will operating the drive with it that way cause any damage?

As to parts to do the thru-hull pickup, what size scoop and seacock would be adequate for water flow for a 350 V8? The existing raw water supply hose is 1-1/4" ID. Also, space in my bilge is fairly limited so instead of trying to squeeze in a Internal basket-type strainer, I'm considering an External South Bay screen-style strainer that is popular on the East Coast.

Info on External Strainers

http://www.thehulltruth.com/boating-forum/359247-external-sea-strainer.html

http://www.samsmarine.com/forums/showthread.php?13439-Strainer-scoops-instead-of-sea-strainers

FinChaser said:

............................ Also, with the water connector removed, when you grab the lower unit and try to wiggle it, the tube (pivot tube?) underneath where the water connector mounts has considerable movement.

Is this movement the root cause of my overheat?

Click to expand...

When the bushing no longer provides pivot tube centering, the pivot tube is now free to oscillate some within the suspension fork. This oscillation causes the gasket "bead" to loose contact with the very upper-most surface of the tube.
When the bead breaches seal, air will be pulled into to suction side of the sea water system while up on plane.
This is very likely what's causing your over-heating issue.

Here's the deal:
When you replace the pivot tube bushing, the transmission and the lower unit must be removed.
The main suspension fork "hinge pins" must also be removed (apply heat to the pin boss ears), so that the fork/Intermediate housing ends up on your work bench.
Removing the hinge pins and pivot tube is fairly user friendly, with a few precautions.

Good news is......, when you re-install, you now have all new O-rings between the trans and lower unit...... plus, you now have a fresh upper pivot tube bushing that will allow the water neck fitting "beaded gasket" to fully seal against, and stay sealed against!

What I'm suggesting is this; make the correct repair now....., rather than circumventing the real cause of the problem.
I'd be willing to bet that, time-for-time, the correct fix won't take you much longer.... and it be will less expensive.

FinChaser said:

Ok - what is the PDS bearing?
PDS = Primary Drive Shaft.
Depending on which flywheel cover is installed, it will be a single or double bearing PDS.
A common misnomer is that these bearings are OK, when indeed they may need to be replaced.
A failure here will result in a very expensive repair.


I'm inclined to just securely cap off the water intake on the transom and install a screened through-hull raw water intake scoop and a proper seacock to get the boat ready to run for tuna then address the outdrive issue later as the shops I'd consider doing the work are booked up for close to a month.
As said......, by doing this you'd be circumventing a correct repair.
I'd take that time that would otherwise be spent on the thru-hull pick up, the hose routing, and the sea water strainer, etc..... and spend it making the correct repair now.
Once done, you won't have this issue again for quite some time.

The OEM sea water pick-up (via the lower unit) sits down in the water. This location generally avoids debris, so the strainer is not necessarily required.
If you do a Thru-Hull pick up, you'll want the stainer.

If the pivot tube bushing is shot, will operating the drive with it that way cause any damage?
Depends on how badly worn the bushing is.

Click to expand...

Thank you, Rick

You've made me realize that it's foolish to try to fix/circumvent the problem in the way I was considering. Unfortunately, this is a repair I don't have the free time to attempt right now so will be taking it to a shop I had recommended to me this weekend. I will have them check the PDS bearing while they have the drive off. Wish I lived up that way or I'd bring the business to you.

Take care

David

FinChaser said:

Thank you, Rick

1..... You've made me realize that it's foolish to try to fix/circumvent the problem in the way I was considering. Unfortunately, this is a repair I don't have the free time to attempt right now so will be taking it to a shop I had recommended to me this weekend.

2.... I will have them check the PDS bearing while they have the drive off. Wish I lived up that way or I'd bring the business to you.

Take care

David

Click to expand...

1..... Get a quote from them, and then post it here. I can tell you if it's reasonable or not.

2.... The PDS is splined into the B/W coupler, so there is no way to spin it to check the bearings.
If you pull the shaft, it's silly to not replace $45 worth of bearings/seals.

Even if the engine is out, there is still no guarantee that they are truly OK by simply turning the shaft.
The AFT bearing must be in near excellent condition.