Sailboats should ideally use round, multiple hole type,
non-directional strainers, preferably one that is hinged and can be opened, if you insist on using one..
The image below is a Groco bronze RSC opening
sailboat / generator strainer..
From Groco:
"THE CONCEPT
Patented RSC series hull strainers are ideal for sailboats and generators by providing course primary filtration without exerting excess pressure to cooling lines which can flood engines that are not operating.
Access to the thru-hull fitting and seacock inlet beneath GROCO RSC hull strainers is made easy by the access door design. There is no need to remove the entire strainer to clear nuisance marine growth. Designed for systems where forced flow is not needed or wanted."
Groco and others do make them. However, if you already have a slotted/scoop type they should ideally face aft, per engine makers recommendations (see below). Otherwise you can run a risk of hydrolocking your engine. I have seen this more than a few times especially as impellers wear or when one has lost a vane or two unbeknownst to the owner.
Facing them aft can reportedly create a slight vacuum on the impeller circuit and
potentially shorten life but, I have not personally seen any noticeable or quantifiable change in impeller life when they aim aft vs. forward. When they aim forward however I have seen hydrolocked engines. The high point, of the water intake in engines on many sailboats, is often not very high above the waterline and it does not take surfing waves to cause a hydro lock on some boats especially with a bad impeller... Heck some old school CCA type boats had engines under the cabin sole.
Personally, I don't use an external strainer, stopped afer a very dangerous dive situation back in the late 80's, and never will use one again. We have far too much eel grass and debris here in Maine, often due to Moon tides, to deal with the stuff getting trapped in
no-mans land that can't easily be cleared without getting into the water. Getting into the ater adds a lot of increased danger especially with the water temps here in Maine.
From Westerbeke / Universal:
CAUTION: Do not use a high speed scoop-type through-hull fitting for the raw water supply for generators and auxiliary sailboat engines as it will tend to encourage siphoning. Water pressure against this type of fitting while the vessel is underway with the generator off or when sailing can push water past the raw water pump impeller and into the exhaust system, filling it and the engine as well.
From Yanmar:
"Scoop Strainer: Install the strainer so that the large area of the scoop strainer faces away from the direction of the ships forward movement, as shown in the figure."
If you design your raw water intake so that the hose to the internal strainer can come off and be held above static waterline, a
debris-rod (AKA a dowel) can be used to punch any clog that occurs before the internal strainer back into the Ocean from the safety of inside the boat.
Other considerations are sticking with a minimum intake ID of 1" between the seacock and internal strainer and to use smooth radius bronze sweeping elbows vs. 90 degree fittings, if or when they must be used. These tips help to minimize any clogging before the internal strainer.