Crank driven vs block mounted raw water pump pros/cons

IslandExplorer · May 1, 2025

Hey all, I was looking to start a thread with some ppl chiming in about possible pros and cons of using a crank driven raw water pump vs engine block mounted pump driven by a belt on an old stern drive like my Mercruiser 302 V8. I'm building a whole different cooling system with heat exchanger for my boat and am contemplating getting a block mounted pump vs the crank mounted kind and so far leaning to the crank driven type. I have plenty of room in front of engine for it.

Any feedback from experiences contrasting the two different types would be much appreciated so please comment away.

alldodge · May 1, 2025

Never had a crank mounted pump but would be much easier to work on then belt driven. Don't see either with an issue moving water better then the other

ROY WILLIAMS · May 1, 2025

IslandExplorer said:
Hey all, I was looking to start a thread with some ppl chiming in about possible pros and cons of using a crank driven raw water pump vs engine block mounted pump driven by a belt on an old stern drive like my Mercruiser 302 V8. I'm building a whole different cooling system with heat exchanger for my boat and am contemplating getting a block mounted pump vs the crank mounted kind and so far leaning to the crank driven type. I have plenty of room in front of engine for it.

Any feedback from experiences contrasting the two different types would be much appreciated so please comment away.

1970'S ENGINE MERCRUISER 302 V8 ....
DISTILLED WATER -ANTIFREEZE 50% ....

ROY WILLIAMS 1978 MCM470 HEAT EXCHANGER 1978-2025 YEARS ...

Scott Danforth · May 1, 2025

I use a crank driven water pump as they are standard on VP engines. you can go up in size with them as well. makes changing impellers easy. and removing sediment when you bury the outdrive because you are in skinny water.

cyclops222 · May 1, 2025

Either works just fine. Draining a crank pump is more difficult if you are going to a CLOSED COOLING system in a freezing location.

Scott06 · May 1, 2025

Would think how close the bulkhead/ Engine compartment is to the front of the crank would dictate this. If belt driven it would be behind the plane of balancer pulley

IslandExplorer · May 3, 2025

Thanks for the feedback guys! Seems like there are only a few drawbacks to the crank pump. I definitely like the idea of being able to go up a size if needed for extra cooling which I might need with my non standard setup... I also run in skinny water a lot so sand ingestion could definitely happen. Sounds like crank pump it is then. No pulleys or special brackets to match up, simple. My engine only runs 3 pulleys as is so would be nice to keep it that way too.

IslandExplorer · May 3, 2025

So, out of the crank pump users- do you use a stabilizer bracket? I have seen them run with only the hoses and the input flange holding it in place. Is that normal?

alldodge · May 3, 2025

Yes would be best with bracket to hold pump from twisting

Bondo · May 3, 2025

I also run in skinny water a lot so sand ingestion could definitely happen.

Ayuh,..... Have you considered a sea strainer,..??
Are you gonna run the drive's impeller to cool the drive,..??
What's yer plan to feed yer motor mounted raw water pump,..??

IslandExplorer · Sunday at 9:52 AM

alldodge said:
Yes would be best with bracket to hold pump from twisting

I was thinking it looks pretty weird to not have some sort of bracket... I'll definitely add one.

IslandExplorer · Sunday at 10:16 AM

Bondo said:
Ayuh,..... Have you considered a sea strainer,..??
Are you gonna run the drive's impeller to cool the drive,..??
What's yer plan to feed yer motor mounted raw water pump,..??

Strainer- Yes, was waiting to figure pump size first and then get a nice strainer for the intake plumbing. My boat will be in the shallows a lot and def around plenty of seaweed. I was looking at some dif sizes at the local marine consignment.

Drive impeller- yes, after some input from Scott on here previously I decided to keep the drive impeller operating to cool the drive and shaft seals and all that stuff. Was thinking it will be an additional raw water feed to main cooling? Or just dump it after running through drive, not sure..
The motor mounted circulator pump will have coolant circulating to a triple pass heat exchanger. Was planning to run crank driven raw water pump pulling from dedicated thru hull fitting to cool the exchanger and then dump into the risers.
That's the rough plan so far anyway. See any problems with that or recommendations? I bet a crank driven pump might be sufficient to cool exchanger solo but seems like extra flow from the drive pump would be a plus. What do you think?

IslandExplorer · Sunday at 10:18 AM

Kind of lost trying to figure raw water pump output needed for it. I was looking at a Johnson F6B-9.

alldodge · Sunday at 10:34 AM

Designed for left and right hand rotation
You have LH so front of motor rotates clockwise when facing it.

Intake right side of your pic

Scott Danforth · Sunday at 6:44 PM

Volvo uses an anti-rotation bracket

ROY WILLIAMS · Monday at 7:45 AM

IslandExplorer said:
So, out of the crank pump users- do you use a stabilizer bracket? I have seen them run with only the hoses and the input flange holding it in place. Is that normal?

MERCRUISER MODELS 225II-TR STANDARD COOLING.
ENGINE MCM302 CU IN. 1970'S. DWELL 26-31. OIL ENGINE 6 QUARTS. 5.7L V8 IDLE 550-600 RPM TIMING 10 BTDC.

IslandExplorer · Monday at 9:43 AM

Awesome, thanks for the input all, and the flow diagram. The pump rotation being compatible either way is nice, I think I will order one of those when I get paid. The anti rotaion bracket should be simple enough to make and I'lldefinitelybe making one of those.

Wish I measured the space between the engine and the bulkhead but it seemed like there was plenty of room.

As soon as I finish doing this full brake line replacement job I will be rebuilding that motor ASAP so cooling system can be built and tested.

IslandExplorer · Monday at 9:45 AM

This shows the space I'm working with. The area in front of it is an empty cargo hold.

IslandExplorer · Monday at 9:50 AM

Anyone here plumb in a separate expansion tank before?

My heat exchanger is very simple and does not have one built in. Prob have to make it myself so I have a rad cap and an overflow tank. Never seen one before so hoping to find a pic of a setup like that to emulate.

alldodge · Monday at 10:11 AM

Don't have a pic but my friends is mounted to the bulkhead away from the motor

Crank driven vs block mounted raw water pump pros/cons

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