Beta BD722_thermostat_Kiwi prop

[RogerLS]

I trying to grt the right pitch for my Kiwiprop attached to a BD722 and am engaged in doing a lot of checks.
My problem is that if I run the engine at more than 2400 rpm, the engine temperature rises till it frightens me ( 95C max so far).
The cooling water flow rate is about 25 litres/min. ( above Beta's minimum of 20 l./min.) The maximum speed in neutral is 3500 as against the original 3600 rpm but I can only get 2900 rpm. with a boat speed of 6.1 kts. in forward gear with the temperature rapidly rising. This year the back of the boat is dirty with a sooty material I assume to be carbon from incomplete combustion.
The raw water impeller is new the inlet water filter clean and the heat exchanger clean as well.
The thermostat opens completely in near boiling water and closes properly when cooled. I've looked at the previos threads on BD722 thermostats and found them useful.
Now 6kts. is perfectly adequate for a boat with a waterline lenbgth of 25 ft. at rest and considerably more when running fast but not if the temperature goes up and up.
Has the forum any advice?

 

[Tranona]

You need to flatten the pitch. The overheating and smoke is a sure sign of an overpitched prop. You need to take at least 2" of pitch off to raise the revs by around 500 at least (rule of thumb 1" pitch change changes revs by approx 250). Ideally you should be able to get 3400 at WOT and speed of around 7 knots. You will wreck the engine if you run it in the way you are. No problem with running at 2400 cruising with the right prop as it will not be overloading the engine. You should be able to cruise at about 5.5 knots at those revs with the correct prop. If the cooling sytem is in good order it should not overheat even running at WOT.

 

[RogerLS]

Many thanks Tramona for your reply, which reinforces my own anxieties. I have been restricting the engine rpm to prevent overheating - or at least keeping the temperature below 90C in practice. The trials I reported lasted a couple of minutes or so.
There have been other threads on Beta BD722's overheating with a number of reasons suggested. There have been a number of possible contributors to work through including ttemperature gauge and rev.counter errors which I'm inclined to think trivial and which I cant measure directly while cruising.I have not noticed black smoke directly. At speed the exhaust port is underwater so the only evidence is soot like deposits on the stern.
I will reduce pitch when I lay the boat up on Friday but will not see the effect until next May when we start a new season cruising.