No raw water strainers?

Yes, I have seen it many times on a lot of SF from many manufacturers from Cabo to Viking to Post and many others. They use a screen on the intakes on the hull bottom which keeps most stuff out.

Don't be concerned. The screens work very well. Depending on the brand they are called South Bay Strainers or Algonquin Scoops. Thanks Pascal and others.

As a kid there were times when I'd spend almost as much time reversing off eel grass as going forward on Long Island's south shore bays. For the life of me I don't know why all boats don't have these. A little history on the subject:
"During the first half of the 20th century, the Columbia Bronze Co. of Freeport, LI developed the “weedless wheel”(an S shaped propeller that prevents the grass from becoming entangled), and the “South Bay Strainer” (protects the intake from cloging with grass) due to the large mats of floating eelgrass in Great South Bay. Many modern freshwater propellers follow this design, and are used in shallow lakes and rivers where weeds are troublesome. Below is an excerp from a manuscript depecting life in the "good old days" living on Oak Island, Great South Bay. As you can see, eelgrass was considered a nuisance, and many locals to this day still feel this way.
"While numerous improvements in motorboat power and design evolved in reaction to Prohibition, other improvements developed in response to more natural problems in our immediate vicinity. As the bays became choked with eelgrass and seaweed, the "S" shaped weedless propeller was developed for boats to successfully cross the Bay. The plague of eelgrass that descended on the entire Bay area sometime in the late 1930's remains as one of the unexplained phenomena of nature. I have heard many opinions as to its origins and disappearance but none of them were official pronouncements by responsible marine agencies of the government. Good as the eelgrass was for shellfish, crabs and fish life, I doubt that any of us today would like to see it return to these waters. Who can forget the sight of the Bay clogged and choked from shore to shore with enormous beds of eelgrass and seaweed? It became necessary for the ferries to have to reverse their engines two or three times during a crossing to clear their wheels of grass and weeds. This chapter in the life of the Great South Bay was a dramatic example of Nature gone awry, for a period, with no known cure. Eventually, Nature found its own cure when the eelgrass disappeared as quickly and mysteriously as it came. And following suit, the higher rpm 3 and 4 bladed propellers replaced the old "S" shaped two bladed "weedless wheels" which did not provide enough speed for ferries and pleasure boats in general. Today, there are a few isolated patches of grass that appear, but no vast impenetrable beds suck as then existed. One hopes that the Bay will remain clear of suck plagues as the uncontrollable spread of eelgrass clogging all channels, creeks and marshes.""

I first used one of these a few years back when the boat was spending summers in nantucket and the Watermaker strainer would clog up every few hours. Worked great, no more clogs so at the next haul out I had them install on our 4 air con intakes.

Since down here in sofl and bahamas neither the Genset or the Mains have any issues with clogging, I haven't converted these.

Note that if installing them on generator thruhull you must drill pressure relief holes in the back to ensure you won't scoop water in at speed if your genset(s) are off and drown them ...

On the boat I run the manufacturer installed standard slotted covers with 3 holes in the back. A while back, the holes got clogged with growth resulting in one generator ingesting water. It was caught in time and survived. I ve now instructed our diver to make sure the relief holes are clear

My 46 Post came with the South Bay strainers.
The Post did not have any internal sea strainers for the mains.

I installed internal sea strainers at a cost of about 3-4 K, not sure if they are necessary.

My previous boat was a 35 Viking and that boat did not have internal strainers either.

Sea strainers do help in addition to the screens if you dock in very shallow water where you stir up a lot of sediment. The swirling of the water in the strainer does allow some sand/sediment to drop out of the raw water. However, in most cases they're not necessary.

Yes I agree with Capt J, I don't think you need both.
It does make it slightly easier to winterize the mains.

The reason I installed internal strainers is because some small 1/4 muscles were found inside the gear cooler during maintenance and engine work.
I have no idea how they got past the South Bay Strainer.
At the time the boat was new to me, and I didn't realize, or recall from the hull survey and inspection that the boat had external South Bay Strainers.
After finding the muscles, it was presumed that there were no external strainers.

Now I have both.
The internal ones have never had anything inside them except clear water.

BTW, the muscles were way too large to pass through the South Bay Strainers.
Its a mystery how they got to the gear cooler.

Follow up on strainers:

My generator and A/C only have the slotted external strainers.

At anchor in certain location they both clog up with seaweed.
My A/C internal sea strainer clogs a lot in certain locations.
Its not an issue at my home slip so I haven't done anything about it, other than to monitor it when travelling.

I an aware of a "Marina Urban legend" that says you should not have the South Bay strainers on generator because it could cause water ingestion when running with the generator off.
I have no idea if that it true or not.

They started off as baby muscles that were able to pass through everything, found a home in your gear coolers and thrived and grew there. We are dealing with that down here, but mostly with A/C systems with pretty constant waterflow.

Gensets on this size vessel should have slotted strainers which are turned perpendicular from the flow so as to not force water into the system. If the water is forced into the system and the genset is not running then raw water can enter the combustion chamber which is either a quick or slow death, but it's death to the genset. An inside the ER full size strainer (Groco, Perko, etc) in addition on the genset only is a good idea. Strainers and hull scoops for a main engine intake are usually not installed together.

yup, I got the slotted ones on the generator through hull.
but im not sure if they are perpendicular to the flow?


Regarding water forcing its way into the engine with a non running generator:

If running on one main engine, Should the through hull be shut on the main not running?


I have never intentionally run on one engine, but I suppose if there was a failure or break down it could happen.
I understand that some long range cruisers do run on one engine at times.

Mains do not have the water lift mufflers found on gensets because their exhaust manifolds are higher

^^^^^^^^^^^^^^^^^
The bottom of that screen assembly - the screen itself- should have two set screw on the trailing edge to retain the screen which can be removed, although even with them out it's usually quite a tight fit. With the growth I see here you should have the screens removed/cleaned/painted every time you paint the bottom. Make sure they are back in place before you launch! When I first came aboard my current boat I took the boat from FTL to Palm Beach after a lengthy retrofit at Cable Marine. Within a week I took a dive to inspect the bottom and found two of the four screens feeding my mains to be missing. I do not believe they were ever reinstalled, and of course I ordered new ones asap.
No internal strainers are needed with this exterior one.

Running on one engine and shutting the other main engine seacock, I don't see a need. You usually don't start self pumping water into engines until 20 knots or above. You won't be doing that on one engine. You should lock the shaft down with a rope or other means on the non running engine if it has dripless shafts.

Generators are a different story if you're running at 25 knots and it's not running. Just like live bait wells with self fill on SF if you're running at over 25 knots a lot of times. I usually see the hull strainers with the holes drilled into it all over the place, on generators. I usually don't see the slotted ones.

SMR-PILOT- yes that is the setup. That screen with the holes in it slides out I believe and you can buy and replace the screen (and clean up in there)which is a good idea while it's out of the water if you can locate the screen.

Where do you keep the boat? That's a nasty looking strainer. Looks like it cuts down on quite a bit of water flow. If you don't take the screen off ( that will be a b*****), you at least should clean each of those holes with the proper sized drill. After you have them clean, if you start the engines once a weekend you should not see that result again. The water flow washes all the micro/embryonic growth off. Once they mature, you'll need a very conscientious diver.

Also IMO you really should have a groco or similar filter on the generator. If you're not running to port or to the local fishing grounds, you're usually sitting in a nice quiet cove or estuary. When the genny is in use in those areas, there is a lot of flotsam that will go by that intake at minimal tidal speed - slurp! I check my genny groco regularly for that reason. I don't have filters on the mains.

My Egg Harbor also did not come with internal strainers. I just have the South Bay strainers.

I've had good results spray painting both sides of the screens with Zinc marine paint. Not much growth after a season. But that is only 7 months up here.

Hello RT46: Not simply a "Marina Urban Legend", it is true. Generators should utilize a slightly bulged outward disc strainer with round holes, or the round disc with slots properly oriented as shown by Bamboo in post #11 above, over the thru hull intake in order to avoid forcing raw water into the generator's raw water system when it is not running. Such strainers for the purpose are readily available from Defender industries, or any serious marine supplier.