Outboard engine cooling issue

[1114C]

I posted about this before and people were helpful but it was pointed out that without photos it was harder for people to help so am going to post again in the hope that someone knows something

I have a Bonwitco 449 with a long shaft engine. The engine has been serviced and the coolant system checked and it run for over an hour and no issues found. When i run it in a bucket, the water outlet comes out fast and if i rev it in neutral it comes out even faster. When i am moving slowly on the boat the water stream still comes out fast but (and this is the problem), once i start to plane the water comes out more slowly and in time the over heating alarm goes off.

I am told by 2 engineers that the engine is fine, the boat is fine but between them they are not. The water stream when in a bucket appears to confirm to me that the engine is fine too (but I am more used to sails than engines).

I noticed at the weekend when looking over the back of the boat when planing that the plate above the prop is coming above the water when planing

I am not sure what i should do - I was planning to put Rydelime through it but am not convinced due to the bucket testing that this will make any difference. Another option is to cut away the transom a small amount but this feels pretty serious - does anyone have any ideas.

I have tried altering the tilt of the engine but this has not helped as yet either

Thanks in advance

Tom

 

[Kawasaki]

Couple of things tom
Is the engine on its lowest 'holes' ?
The 'cav' plate looks to be about right ref its position, it should be about level with the keel at the stern/transome bottom
Not sure what year your engine is but some merc/mariners arouind 2000/2003 had a problem with water circulation
Your symptoms are very similar
Where the powerhead sits on the base of the motor there is a grommett which seals the water pipe that circulates the water around the block
Copper pipe and ali block and the grommett perishes and the block corrodes an blocks the flow of water underload
Has the impeller been replaced recently, or checked even?
Seems more like a water flow prob than engine position probs, looking at your picture, imho

 

[kashurst]

I have experienced something similar caused by the engine being too high. Not entirely clear from the pics but if you put a ruler along the keel you can determine how far below the keel the cavitation plate is (the flat surface just above the propeller). The cavitation plate needs to be an inch or so below the keel. I think the cooling water inlet on your engine is on the under side of the cavitation plate so if the prop is clearing the water so is the cooling water inlet.
I don't think rydelime will make any difference as your cooling system seems ok in a bucket and at lower speeds

 

[1114C]

Thanks for getting back to me both

Kawasaki - the engine is from 1999 so will try to see if that issue you describe is happening here

kashurst - the plate is not below the hull but instead just slightly above

i do wonder therefore if it could be a combination of the two - if i cannot locate the blockage described i may have to cut away the transom an inch although this fills me full of dread as it seems quite drastic and i do not want to ruin an otherwise okay boat

thanks again for the help

Tom

 

[kashurst]

if the cavitation plate is higher than the keel than you will have trouble. Is it a wooden transom or fibreglass ? Before you cut anything phone a mercury/mariner distributor and have a chat with them. I've used Bill Higham Marine in the past they are usually helpful. It doesn't sound like you have a cooling blockage because water runs through it very well at low speeds and in the bucket test.

 

[Kawasaki]

if the cavitation plate is higher than the keel than you will have trouble. Is it a wooden transom or fibreglass ? Before you cut anything phone a mercury/mariner distributor and have a chat with them. I've used Bill Higham Marine in the past they are usually helpful. It doesn't sound like you have a cooling blockage because water runs through it very well at low speeds and in the bucket test.

Dead right kashurst if the plate is higher than it should be probs will occur
However the fact that the motor pumps ok in the tub etc doesn't rule out a water flow problem
4 engines I have fixed with o ring problem
All of which ran OK ashore and at low to medium 'speed'
What happenes when the engine gets up to temp and more throttle is applied
The o ring swells and blocks the flow.
Plus it leaks over time and water gets to the ali and degrades it, therefore the seal is compromised
This is difficult to replicate ashore.
Anyway Tom you have to get the plate to line up properly
Most outboards have a selection of holes on the mounting bracket to raise and lower the motor to the correct height, are you sure you are on the lowest?

 

[1114C]

I am on what i think is the lowest, i found the screw on which the engine rested and i moved it down (and then removed it completely) but to no avail, this seemed to put the engine more vertical (rather tilted with the lower part of the leg further out from the hull than the upper part if that makes sense). Photo below shows the position of the tilt as it has been over the last while

I think i will ask a local engineer to have a look at what you describe and see if that fixes it before cutting away the transom (it is beyond my mechanical ability)

thanks very much for taking the time to help

tom

 

[Spi D]

It is not true that mounting the engine higher than anticavitationplate = keel line will give you trouble. It may under some conditions, but far from a rule.

How should this one work at all if high mount engines was bound to give trouble?

To test under max load and with the prop plenty below surface go forward at the speed where the bow is high and the engine is struggling.

If the impeller and the thermostat are as new and you still get an overheat, the water jacket / cooling channels must be blocked or reducing flow.


Every performance boat with an outboard runs the engine higher to minimize drag and until going extreme there will be no water pickup issues. You'll even see engines mounted on a powered jack plate to be able to raise or lower it while running. When raising high enough for the water intake to get air, a pickup is used.

 

[Kawasaki]

I am on what i think is the lowest, i found the screw on which the engine rested and i moved it down (and then removed it completely) but to no avail, this seemed to put the engine more vertical (rather tilted with the lower part of the leg further out from the hull than the upper part if that makes sense). Photo below shows the position of the tilt as it has been over the last while

I think i will ask a local engineer to have a look at what you describe and see if that fixes it before cutting away the transom (it is beyond my mechanical ability)

thanks very much for taking the time to help

tom

View attachment 31264

Tom, it looks from the picture you have located the engine mounting bracket/saddle at it's lowest available
The cav plate looks near enough
As Spi D has posted engines can run ok with their 'height' or lack off and 'cool' ok
Performance will differ in various ways but it's not 'performance' you are after!
I purchased a 'perfomance' rib last year
I didn't need the performance I wanted the boat to perform differently
So I lowered the engine on the transom
As Spi D says, it aint the be all and end all
On a Bonwicto like yours Tom, it aint no racing machine but the cav plate will make a difference where it sits in relation to the keel/transome.
Still think you have a water circulation snag here

 

[macnorton]

I am on what i think is the lowest, i found the screw on which the engine rested and i moved it down (and then removed it completely) but to no avail, this seemed to put the engine more vertical (rather tilted with the lower part of the leg further out from the hull than the upper part if that makes sense). Photo below shows the position of the tilt as it has been over the last while

I think i will ask a local engineer to have a look at what you describe and see if that fixes it before cutting away the transom (it is beyond my mechanical ability)

thanks very much for taking the time to help

tom

View attachment 31264

The cavitation plate should be below the hull or it will create a negative pressure area by the water pick up at high speed.
The answer imho is a longer shaft, cut the transome or go at disp speed

 

[[2068]]

The cavitation plate should be below the hull or it will create a negative pressure area by the water pick up at high speed.
The answer imho is a longer shaft, cut the transome or go at disp speed

Or fit the outboard to a bracket which is lower?
The way to find out if the water level is the problem is to run it in a big tank at full whack to see what happens.

 

[Lakesailor]

How about trimming it out? This will raise the bow and dig the stern in. Not ideal, but it will keep the inlet below water level. Also have a couple of bodies in the stern.

This isn't a suggested normal running configuration, but just to see if the overheat still happens. If not then it would seem the motor needs lowering. I did it on a 15ft boat. Not a difficult job, but maybe get a boatbuilder to do it to make sure the strength is retained.

If the O ring has swollen it needs replacing. As it pumps water when in a tank it may be only marginal if it needs prolonged running to make it bad enough to set off the alarm.

Try digging the stern in with trim first. It'll cost you nothing and may shed some light.

That bit in the middle like a horse's arse is a swollen grommet on the water tube on a 4hp Johnson.

 

[AndieMac]

Not sure what year your engine is but some merc/mariners arouind 2000/2003 had a problem with water circulation
Your symptoms are very similar
Where the powerhead sits on the base of the motor there is a grommett which seals the water pipe that circulates the water around the block
Copper pipe and ali block and the grommett perishes and the block corrodes an blocks the flow of water underload
Has the impeller been replaced recently, or checked even?
Seems more like a water flow prob than engine position probs, looking at your picture, imho

I think you are on the money K, been reading on other forums similar issues with this grommet and the 1999-ish models

 

[1114C]

I think you are on the money K, been reading on other forums similar issues with this grommet and the 1999-ish models

Thanks everyone for your help

I have been thinking about this and I am now convinced this way too

I have tried people sitting at the back and it made no difference, on a lightish boat that it is that should have made a difference i think
There is another Bonwitco of the same type based at the same place and they have never had anything similar happen - same shaft length but different engine

I think (and i am not certain at all) it might be getting worse which would tie in with this too

Am finding now a motor engineer who can go and have a look at it

Thank you everyone