Flow rate at saltwater inlet

[GRAEME JACKSON]

Has anybody noticed the flow of water going to the saltwater pump ?
I removed the water trap filter and noticed that the flow was not very good. checked for any blockage all is ok
would it be advisable to fit some sort of scoop over the inlet under the hull to have water forced in rather than "sucked" in when the boat is under wayr??

[Jeff Kaplan]

graeme, what pump are you refering to,the water pump on the engine or the footpump at the galley sink?...jeff

[Stu Jackson]

Graeme

The scoop idea has been discussed as long as 18 years ago in the Tech Notes.  The consensus seemed to be that the scoop is not necessarily a good idea because if you have to clean out the inlet valve with a dowel the dowel can't be pushed through, and that the scoop may have more of a tendency to "collect junk" and block the inlet than if it was not there.

As far as flow, it's been discussed, but I admit that I don't remember where I could point you to a link, and it could be from another boat (i.e., C30 or C36) with the same engine.

Assuming that you're talking about the raw water side, I recommend checking the "system," since you have already started at the right place:  INLET, INLET VALVE, STRAINER, HOSE TO RAW WATER PUMP, RAW WATER PUMP (IMPELLER), HOSE TO HX and HX, HOSE TO EXHAUST NIPPLE, MUFFLER, EXHAUST HOSE.  If you have "little" flow to the strainer, you'd need to check the flow through the valve.  Many folks on our East Coast get growth inside the through hull hole, which restricts flow to the valve and the strainer.  One of older Tech Notes suggests the installation of a "T" above the inlet valve to allow doweling out the valve and through hull to assure a clean entry.

The pump, when the impeller and pump are working, are powerful enough to "suck" the water through the system and should need no help.

[Ron Hill]

Graeme : The important thing is not the number of gallons per minute pumped, but the engine temperature of about 160F.
If you are running hot, then you need to start checking the raw water system as Stu mentioned.
The most common cause is a clogged intake, loose hose, worn impeller cover plate and missing impeller blade/s.  I wrote an extensive Mainsheet article on the causes of hi engine temperature.
If the ocean water is warm, it's a hot day and the engine has been running a few hours; you can expect to see as much as a 5 degree engine temp increase!!  Under those conditions 165F is normal!!   

[GRAEME JACKSON]

just to put the record straight i was talking about the raw water intake to the engine
is it the impeller pump that sucks the water up and then pushes it through the engine
or is it the pressure of the sea water that pushes it to the pump
does not seem to be enough water flowing
temp sits arround 165-170
have the smaller heat transfer unit fitted not the 3in model

[Stu Jackson]

Graeme

165-170 with a 2 inch HX is GREAT.  You have NO reason to worry.

Here's the deal: the M25 with a 2 inch HX would keep an engine running at 180 degrees F at cruising speed and within 2000 to 2500 rpm (depending on how your tach was calibrated - in any event, 2/3rds to 3/4ers of max throttle in forward gear).  When "pushed" to max rpm, say in an emergency or heavy seas or strong current against) the engine would heat up to 200 or higher.  That's been my experience and others over the years.  The consensus is that the 2 inch HX was "marginal" at best, and needed to be thoroughly cleaned on a regular basis to provide adequate heat change to remove engine heat on the water exchange side.

The M25XP added 2 more HP, up to 23 HP, and put on the 3 inch HX.  Many of us with an M25 have replaced the 2 inch HX -- in my case I'm running at much less than 180 - 200 F and can keep the engine at max RPMs for extended times without overheating.

The pump is a variable volume impeller pump.   The variable volume is because it is tied to the belt, which varies speed based on the throttle applied to the engine.  All pumps,. except centrifugal ones - which this is NOT - can "suck" water TO them and then expel and pump water or fluids OUT.  In this case, since everything is below the waterline anyway, the water pressure outside IS pushing the water to the pump, but it could suck it to itself anyway, so this is NOT an issue.  Some folks prime the pump anyway in various ways, but Ron Hill's is the best with handy boy pump, to get water TO the pump before starting the engine after changing the impeller, to avoid the vanes running in a dry condition inside the pump until the vanes suck the water to the pump body.  Others use this or just some small amounts of lubricant jelly inside the housing.  Plenty has already been written on this subject, look around here and the Tech Notes Online or on your CD-ROM.  A search I just did on "2 inch HX" is helpful in popping up articles about this part of the subject, too.

The pump pushes the water through the HX, to the nipple on the exhaust riser under the heads sink, through the muffler and out the transom.

Given your reported engine operating temperatures, it appears that your system is working properly, and don't understand your concern about a lack of water flow.  It appears everything is working fine.