In message , John H.
writes

The Tees dumps into a harbor with a golf course on both sides. My kind of
place!

It does? All I see is industry north and south. Are we talking about
the same river?

That Alaska appears to be a serious fishing boat. Do you do well in the
North Sea? What kind of fish do you mostly catch?

It handles very nicely, planes at about 15 knots, feels as safe as
houses. If we're not under power, she bobs about in a swell and if I
put my head down (e.g. tying a knot) I feel sick pretty quickly. But
I've been out in a heavy rolling swell (say 15' top to bottom, sea up
above me) and had no cause for concern, she rides it nicely.

If you've the time, explain the problem with the engine a little more.
There are some very knowledgeable people here. I'm not one of them. From
what you've said, it does sound like the thermostat. I'd probably replace
it before doing anything else.

We brought her from Hartlepool Marina to a new berth on the River Tees
via the North Sea in March 2007. It's about 15 miles and the engine
didn't miss a beat. Pretty much full throttle all the way and she used
about 20 litres (that's about 5 US gallons or 4.4 Imperial gallons).
Well, that's presuming the tank was full when we set off, it may not
have been because I don't remember whether it was, but I had a 2nd tank
which was full as backup. Anyway, the engine was fine.

Then the boat was laid up for 12 months, or rather she simply wasn't
used. When we went back after 12 months, she wouldn't start. We had
charged the battery so that wasn't the problem. We went back later that
day armed with fresh petrol (gas), and a blow lamp. We removed the
spark plugs and heated them, pumped fresh fuel to the carbs, and it did
start and tick over nicely. Obviously we couldn't rev it up because we
were tied to a jetty. No sign of any problems, the engine idled for may
10 minutes or so but it didn't seem to get hot. It was pumping water
ok.

About a week later I had the trailer ready and she started after a bit
of persuasion. (It's very easy to flood the engine with too much
choke.) We let it idles for maybe 10 minutes and then untied the warps.
Once out in the river, it was my intention to give her a good thrash up
and down by way of a workout. But the engine would rev up to maybe half
speed, run for a short while (not more than a minute) and then cut out.
Turning the key to crank got it going again but it repeated this several
times. So we made out way to the jetty by the ramp and hauled her onto
the trailer. That's as much as I can say.

The engine, as best I can tell, is a 50D of 50DE (I have a workshop
manual). The engine plate is absent - the rivet shafts are still
present to it hasn't been drilled off (i.e. isn't stolen). It's a
3-cylinder 2-stroke, oil injection, electric start.

Another peculiarity, it won't rev up in reverse.

--
Jonathan

A good reputation is more valuable than money.
Publilius Syrus (~100 BC), Maxims