Raw water flow to closed cooling system

I am replacing a raw water cooled MCM 228 (with pre-alpha drive) to a full closed cooling MCM 250 and Gen 2 Alpha drive. I operate in salt water, the boat is a 1976 Wellcraft F-21 ("Airslot"). The "new" (lightly used) engine previously used blocking plates for the drive mounted sea water pick-up and a thru-hull for the belt driven pump. When flushing the engine, Mercruiser specifically recommends using two hoses, one at the drive and on for the ilet side of the belt driven pump. I am considering modifying the system to provide drive pump water to the inlet side of the belt driven pump, in addition to (and upstream of) the thru-hull. It seems as though this would allow for shutting off the thru-hull and allowing the drive pump to provide the entire system with sufficient flow when flushing, using only one hose. My question is whether the belt driven pump would potentially push water backward through the drive pump when under way, (is the belt driven pump a higher pressure pump or just higher volume?), and if so would incorporating a positive shut-off to the drive pump work? It just seems like a huge pain to figure out how to get flush water to the thru-hull, drag two hoses to the boat, etc. Any thoughts?

Re: Raw water flow to closed cooling system

Do not use both pumps, it always ends in problems. The engine mounted pump puts out more volume than the pump in the drive, so you would be starving the engine mounted pump.
The pump in the drive is used to cool the upper gear unit. The hose is cut to allow the water to circulate and the transom is blocked off.

Re: Raw water flow to closed cooling system

Hi, Don. I have read so many of your posts over the years that I feel like I am talking to an old friend. How 'bout this: In the scenario I am considering, the belt driven pump would use BOTH the flow from the thru-hull intake and any flow from the drive pump. That way there would be no "flow starvation," and I could simply shut-off the through hull for flushing (at idle speed only, of course). Thanks for your help!

Re: Raw water flow to closed cooling system

I wouldn't mess with it. If your impeller fails in the sterndrive, the engine driven pump can draw air from the upper connections of the drive. They are designed for water pressure, not holding suction. If that happens, the engine driven pump will also fail, because it will suck air before it sucks water from the thru hull fitting.

Re: Raw water flow to closed cooling system

Good. Now I know, and knowledge is good. Many thanks.

Re: Raw water flow to closed cooling system

The answer to dragging two garden hoses from the house.

Re: Raw water flow to closed cooling system

Yup, Uncle Willie, that's good too. The Mercruiser official installation manual suggests cutting the hose in the gimbal housing and removing the upper piece of the cut hose while blocking off the bib at the inner transom plate so that the drive pump flow is vented directly back into the ocean / lake. This allows the drive pump flow to continue to cool the upper gear housing assembly. Cutting the hose just seems so Mickey Mouse, though. What's the general consensus as to keeping the hose intact, and running an overboard discharge from the bib (similar to a bilge pump discharge) directed onto the drive similar to one of those drive "showers?" There could be a diverter in the system to use the flow as a raw water wash-down, too. Thoughts?

Re: Raw water flow to closed cooling system

I think your trying to make life way to complicated for yourself..........