help with outboard not cooling at high speed or load

[1114C]

Hi

I have a Bonwitco 449 with a Mercury 40hp from 1999, four stroke.

I have a problem and was wondering if any one here had any ideas or experience of this. When the engine is idling, the water flow out is great - flies out without any air, all is good. When on the plane though, the flow of water is less good, this means that if i go too fast or have more people in the boat, the engine over eats.

The engine has just been serviced and the impeller replaced so it appears the engine is fine, they tried to recreate the overheating when servicing the engine but could not (it was not on the boat), it appears therefore that it is a combination of boat and engine. The engine is a long shaft and that is what the boat requires.

I am guessing that there is air getting in where the water inlet is when planing and this is interrupting the water flow which in turn causes the engine to overheat?

Has anyone experienced this before, can anyone think of any possible solution?

Thanks in advance

Tom

 

[Beamishken]

Hi has the engine been on the boat all its life and worked fine before?

if it has its unlikely to be a set up problem more likely to be a salt build up somewhere in the cooling system I took the manifold off a honda a while ago & there was an internal anode which had corroded & swelled there are also very fine water galeries at the bottom of the block where the powerhead joins the leg they can corrode & build up salt scale

you can get a product designed to clean out the water galeries on inboard engines I think its called saltX might be worth googling that & running the engine in a barrel with some added to the water
I'd check the instructiond first though to be sure its suitable for aluminium engines

best of luck

 

[1114C]

thanks for that - will investigate that

boat and engine were bought separately, previous owners of the boat had no problems with other engine, 2 different people have serviced the engine and neither can find any problems

am wondering if those wider wing things people put on engines to help it plane early (not a problem i have) might help keep water in place, to be sucked up but this may just be very ill informed on my part

 

[volvopaul]

I'd run the engine out the water in a bucket with rydelime in 50/50 mix with water it will descale the whole block clean.

It does sound like you have an air leak, maybe who serviced it didn't assemble the lower leg correctly and displaced the seals, have you tested the thermostat.

 

[mlines]

Sounds to me like you need to adjust the way the engine fits the boat. Can you alter the angle the engine sits relative to the transom etc. I would guess that once you get up to speed the boat lifts onto the plane and air is getting into the water intakes. Our dinghy does this if too much weight up front.

 

[1114C]

thanks for the help

i think it is sitting too high in the water and i cannot work out how to lower it (have tried putting both people and fuel at the back), was hoping those wing things (I wish i knew the name) might force water under the intakes but it sounds like that might not work after all

 

[VicS]

thanks for the help

i think it is sitting too high in the water and i cannot work out how to lower it (have tried putting both people and fuel at the back), was hoping those wing things (I wish i knew the name) might force water under the intakes but it sounds like that might not work after all

Doel fins are what you are thinking of. I don't think they will solve your problem but Google will find info to read.

Check the height you have the motor mounted. The antiventillation plate should be about level with the bottom of the transom. There should be details in your owners manual

 

[Lakesailor]

You don't say if it a longshaft motor. That is the spec for the 449. What is the engine model number?

 

[1114C]

You don't say if it a longshaft motor. That is the spec for the 449. What is the engine model number?

Hi

Thanks for all the feedback

It is definitely a long shaft, am going to check all that has been said and will post again when i can get photos in a couple of weeks

Thanks

Tom

 

[Spi D]

There is no reason to believe there is an issue in the boat engine combo or that the engine suffers from air in the water intake.

Outboards work fine under all kinds of severe conditions and have no problems cooling on an air water mix. Your problem is bound to be in the water jacket somewhere. Descale as described and check gaskets & thermostat (flush the engine with the thermostat out but mind it will not cool properly if run without it).

Is there any chance that the old impeller broke and small rubber bits went into the circuit? Might sit somewhere and partially block a passage (enough to restrict the flow when most needed).

Not a DIY but to reproduce the problem on land you'd need a test prop or dyno to generate the load