Engine strainer

[Tristan]

A silly question - should the raw water engine strainer be below or above the water line?
Thanks for help
Tristan

[Loquat]

mine is below, near the seacock, under the sole

L

[AnchorageGuy]

Tristan, I have seen them in both places plenty of times. Our trawler strainer is actually mounted so that when the top is off it won't overflow because the top is above the waterline but the rest of the strainer is below the waterline. Chuck

[David M]

Doesn't matter, I have seen both and they both work. If below the waterline you simply shut off the sea cock to keep it from overflowing when you change out the strainer basket.

[Kiwi.]

Originally Posted by Tristan

A silly question - should the raw water engine strainer be below or above the water line?
Thanks for help
Tristan

Not a silly question at all; only silly if you don't ask it. I believe the ideal strainer is large tubular type with a clear lid that sits above the waterline and an intake and exit that sits below, I've only had this type on a motor yacht, it is not always easy to achieve especially on a sailing yacht. Which ever type you have, and whether or not it's above or below the waterline, think carefully about how you will re-prime the pump after you've cleaned out the strainer. If the pump is above the waterline you may have a problem in an emergency. Avoid putting a non-return valve into the line between the sea-cock and the strainer. Most non-return valves can block up with stuff that never reaches the strainer. My current boat has sea-cock then below waterline strainer with non return valve plumbed directly to the outlet of the strainer, then an above-water pump. When the strainer blocks up suddenly I can turn off the sea-cock, clean out the strainer and turn the seacock back on and not loose the prime to the pump. It's a 30 second job for a terrified skipper versus a 30 minute job (for a maybe shipwrecked skipper) if I loose prime to the pump.

[James S]

Top of my strainer is about a half inch above the water line...that puts the in and out of the strainer just under the water line.

[Kiwi.]

Hi James, congratulate the engineer that designed it that way; I've cleaned out a strainer like that without even turning the motor off.

[doug86]

It doesn't matter in terms of operation, but if mounted so the top is above the waterline, one can clear the strainer without having to close the seacock.

Someone mentioned "loosing the prime". Your raw water pump is a rubber impeller and doesn't need to be primed. The pump will empty the air out of the strainer all by itself regardless of where the strainer is mounted.

[Ziggy]

Originally Posted by doug86

Someone mentioned "loosing the prime". Your raw water pump is a rubber impeller and doesn't need to be primed. The pump will empty the air out of the strainer all by itself regardless of where the strainer is mounted.

Some rubber impeller pumps do lose prime, even with a brand new impeller.

[Laidback]

Originally Posted by Tristan

A silly question - should the raw water engine strainer be below or above the water line?
Thanks for help
Tristan

If it was above the waterline how would the water get into the strainer ?

[Boracay]

Putting a new strainer into Boracay the manual specified that the strainer had to be above the waterline.

So I went ahead and put it above the waterline. Didn't work!

So I put it below the waterline where it still works just fine.

Turning the the raw water seacock off after engine stop is my new religion.

[Kiwi.]

Originally Posted by Ziggy

Some rubber impeller pumps do lose prime, even with a brand new impeller.

Ziggy, I totally agree. What is not well understood is that there are a number of factors in play. They include:

1. Pump height above water line.
2. The size of the inlet line.
3. The size of the strainer.

Pump height matters: The vacuum side of a pump is entirely dependant on atmospheric pressure not on the power of the pump. Every inch above sea-level counts.
The size of the line between strainer and the the pump matters: A half inch line will typically not drain out while you're clearing the strainer, a one inch line will dump out the moment you open it. But you want a one inch line (at least) to cool your motor.
The strainer size matters: This is the killer of the "it will prime" argument. A decent strainer must be able to take you hand (let's say 3") into it and must be at least twice the depth of the intake plus the exit plus a gap between them. This volume will burn out many a "self priming" pump long before you see it on the gauge upstairs.

[Blue Crab]

Originally Posted by Boracay

Turning the the raw water seacock off after engine stop is my new religion.

Awhile back someone said on here that they attached a loop on the starter key and hang the key on the seacock.

[James S]

Originally Posted by Kiwi.

Hi James, congratulate the engineer that designed it that way; I've cleaned out a strainer like that without even turning the motor off.

That would be me (acting quite humble)...Thanks Kiwi

[HopCar]

If a Jabsco style rubber impeller pump won't self prime, when used for engine cooling, there's a problem. Either there is an air leak on the intake side or there is a problem with the pump. A bad impeller can cause the pump to not prime but so can a worn face plate, leaky seal, worn out wear plate, or worn out cam. I like Perko strainers mounted just above the water line. Groco ARG strainers are pretty good as well and cheaper than the Perko strainers.