Impeller would be my first suspect, or even a thermostat in the motor not opening fully

The Wings couls be disrupting the water flow. Try the motor without them and see if it makes any difference.

And check the water inlet strainer grills are fitted and clear.

Is this a new thing or has it been happening for a while?
If you have made a change like adding the wings lately or is it gradually getting worse.If so remove or put what ever has been changed back the way it was and see if it fixes the problem.

Would suggest a new impellor anyway because with cavitation happening it will probably be stuffed.

Thanks guys, Will remove the wings and run the motor before I take it in for new impeller. Seems that the problem started when I put the wings on but cant see how that would effect water flow over the inlets?

Wings should be treated as a last resort to fix handling problems. Most times they can make a boats performance worse than what it was before fitting but some people persist with them because of the money spent buying them.

While taking the wings off check the tips of the blades on your prop, there is a chance of "feathering" where cavitation has occurred

Thanks carnarvonite, cavitation was probably the incorrect word. Still cant see how intake can be effected by wings when they are behind unless the back of the boat is lifting creating an air pocket between the transom and motor.

The foil wont make a real difference to the over heating. But will possibly help if the boat has a pod fitted as that is a big donk.
How old is the impeller? They need to replaced yearly but often they can go even quicker that that.

I had the same thing happen on my 130 yammaha.Was ok low to medium speed but once I got about 2/3rds throttle or higher the temp alarm would go off. I changed the water pump ,this helped a bit but it would still overheat. It turned out to be the thermostats.I did check they were opening by raising the temp in a pot on the stove, and they did open but obviously not enough or at the wrong temp.I would change the impeller but also put some new thermostats in it and it should be ok.

Thanks Rod P I'm going to start with the logical things like removing wings, replacing impeller, Impeller only just over a year old. Hope that is the problem. From what I have read the motors are reasonably simple and have few problems. If I cant fix it I will just have to travel slow and watch the temp gauge, not the ideal situation.

I'd be taking it to a qualified marine mechanic kingfisher549 if it gets to that stage.

Sounds ok going slow and watching the temp, but the sea doesn't always respect one's wishes and it would be nice to be able to reliably put the hammer down or travel at speed for however long is required.

not good news, but as B J said , get it to a mechanic. not worth the risks of getting stuck out there in rough weather (been there done that).it does sound like the impella or thermostat and not the wings .get it checked before the next time out

quick question?? do the new mercs run a poppet valve? (pressure relief) not up to date with my mercs anymore sounds very much the problem to me

i would be getting the pump pressure tested and flow rates, i would have thought the fins would not not effect it as they are fitted behind the intake , unless its lifting the motor to high and the water is below the cavitation plate and not allowing enough water to be sucked in, water impellor and thermostat would be first line but at a mechanics shop, yuo dont get many chances to get it wrong

Got it booked into the shop thursday, weather looks crap for a while so i'll let them play until the problem is fixed. Hope it is simple and back on the water soon, will keep you updated on what the problem is