I'd start with the strainer gasket (or other air leaks BEFORE the raw water pump). If air is being sucked in the temperature will go nuts like that. Why? Aerated water does a lousy job of cooling, and the water flow is greatly diminished as well since it's far easier to pump air than water!

Jeff

The strainer filters raw water brought in from the river (bay/ocean), keeping out crud like grasses, tiny pebbles, dead fish. It's located right after the raw water seacock. Gasket is a big washer that fits between the strainer cover and the surface under the cover...just like a garden hose washer. I replaced mine last year. Though the old gasket looked good, it wasn't sealing correctly (probably had dried out) and putting a new one in increased water flow immediately.
One tip given on this board was to wet down around the cover and see if the pump tries to suck that water in as the engine runs.
Two questions: can you see the water flow thru the strainer (some have a clear top) and if so, does it appear vigorous? Water being ejected from the muffler at a good flow?
(actually..more questions) Finally, do you know what kind of strainer you have? If it's the clear top, Buck Algonquin sells the gasket (I think it's been posted on the board as a source).

SOunds like you have a raw water cooled engine with a "double-pocket" pump requiring two impellers. The mechanics who hang out here think it's easy to tackle the impeller change and yes, there have been posts on changing them. It can be a tad tricky to get the new one in but not difficult (I'll let the experienced impeller changers speak up).

Impellers impel with a fan like motion. The blades fill the chamber which has an opening for incoming water and another for outgoing. The chamber is designed to squeeze each blade as it reaches a point in its revolution which creates a little oomph to push the water thru the engine. Or something like that. You know, hypotenuse of the square root, etc. Hope that is clear enough.

Yes..it can be done while in the water

Thanks for the help. I found a guy who knows a lot more than me about Marinettes and engines locally.
He took a look and said my old pumps were leaking and that the shafts looked like someone had been hammering on them over the boat's 41 years!
Not sure if these were the original cases but they were beat.
So as part of my 5 year plan to update the boat, new water pumps, gaskets, impellers etc are in order. Cha ching! Hope that's the only big item this year!
next year, new carbs...oh boy can't wait!

Thanks, everyone for chiming in.

Jim

Jeff - when you say "update your engines" do you mean an actual engine swap or just a pump update? Replacing the engine to fix a pump sounds sorta radical.

Jimbo:
I have a pair of Sherwood dual pocket pumps that you can have for the cost of shipping.
They appear to be in good condition. The two sides have 1" ports so they use the 09959 impellers.