Different setup for sea water intakes?

[andrew4895]

So i'm running an rlm 31, twin inboards. Currently out on the hard for a refit, stern drive overhaul, new engines, etc etc. One issue I have been having is whilst operating in weedy waters and occasionally canals the intakes become easily clogged with debris. The setup currently is just a valve, section of hose leading up to a strainer (vetus 150 style) and then onwards to the engine.

What I am thinking of doing is fitting a small tank inbetween the 2 engines and well below water line level. Both intakes will run into the top of this tank, running through a mesh in the top to filter any fine stuff. Access hatch in top to clean debris which would be done with both sea water inlets closed. There would then be 2 hoses from the bottom feeding directly to the engines.

Inside this tank I would have a float switch in the bottom of this tank (or maybe 2 for redundancy) set up backwards to turn an led on the dash on and sound a buzzer. I would then also have a water flow sensor just before each inlet feeds into the tank to tell me if no water is coming in. Where the inlets enter the tank they will pass through a non return valve to stop any back flow happening when the hose on the clogged inlet is disconnected.

My thinking is this would then allow both engines to stay running if one inlet gets clogged with weed, sticks or plastic bag etc. I would know when the inlet is clogged and be able to hopefully sort it out before the other one also becomes fouled.

My question is, do you think the one inlet would let enough water in to keep both engines happy? Any other thoughts around something like this working properly and being safe? Cheers for any help.

 

[spannerman]

What you are describing is a ‘sea chest’ as used on ships. It could work but its complexity with non return valves and float switches could create another set of problems. I assume the present intakes are through the drives, how about a second through hull intake and a ‘Y’ valve to select the other intake when required. You could still have flow sensors or exhaust temp sensors to warn when flow is restricted.

 

[andrew4895]

What you are describing is a ‘sea chest’ as used on ships. It could work but its complexity with non return valves and float switches could create another set of problems. I assume the present intakes are through the drives, how about a second through hull intake and a ‘Y’ valve to select the other intake when required. You could still have flow sensors or exhaust temp sensors to warn when flow is restricted.

Cheers for that. Non return valves and float switches wouldn't be required for it to operate correctly when not blocked. Water comes in from thru hulls in the bottom of the boat, not through the legs.

I have had a boat with a grate on the bottom of the inlet before, but that didn't help at all, the weed and stuff just got stuck in the grate and then was impossible to poke through without going under the boat and pulling it out. Not nice on a canal.

Another thought was to leave it how it is currently, instead putting a second valve just before the sea strainer, and a y piece just before that. Close the top valve and then pump water from a small tank at high pressure into the inlet to force the debris out of the bottom. Only requiring loss of engine for maybe 15/20 seconds instead of the several minutes it currently takes to sort out.