External Strainer Orientation

[Gaiamar]

Greetings,

After reading many other threads I remain in need of knowledge and advise.

All 18 thru hulls have been replaced with new bronze. The problem is
that the contractor reversed the slotted strainers to face forward.
Original orientation was aft. Now they are glued and screwed in. There
were some words and now a standoff of wills.

The boat is a Beneteau 42CC and we sail at 6 or 7 kts. There are 3
external strainers 1 A/C (Not so worried here) 1 Gen and 1 Yanmar.

What is tha truth of the matter? Fore or aft.

Thanks Very Much

[Maine Sail]

Originally Posted by Gaiamar

Greetings,

After reading many other threads I remain in need of knowledge and advise.

All 18 thru hulls have been replaced with new bronze. The problem is
that the contractor reversed the slotted strainers to face forward.
Original orientation was aft. Now they are glued and screwed in. There
were some words and now a standoff of wills.

The boat is a Beneteau 42CC and we sail at 6 or 7 kts. There are 3
external strainers 1 A/C (Not so worried here) 1 Gen and 1 Yanmar.

What is tha truth of the matter? Fore or aft.

Thanks Very Much

Neither. Sailboats should ideally use round, hole type, non-directional strainers. Groco and others make them. However, if you already have them they should face aft otherwise you can run a risk of hydrolocking your engine. Facing them aft can create a vacuum on the impeller circuit and potentially shorten life but I have not personally seen any change in impeller life when they aim aft. When they aim forward however I have seen hydrolocked engines..

[Dsanduril]

I bet this has as many (and as strong) opinions as anchors

FWIW, ours face forward and have for 30+ years. Presumably on the theory that water gets pushed into the strainer when underway, but I can't recall a conscious decision at the time.

[Ziggy]

Actually, I think external strainers are a bad idea altogether. Much better to have a strainer like this that you can clean out from the inside of the boat.

[Gaiamar]

I do have an internal strainer and siphon break. These however do not
provide peace of mind. So would a surveyor flag this as a problem?

Thanks

[Maine Sail]

Originally Posted by Dsanduril

I bet this has as many (and as strong) opinions as anchors

Pretty strong opinions from engine makers too. They bolded it and made it a boxed warning in the manual, not me...:


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.

[delmarrey]

Power boats should face forward only, so when they go fast they don't loose suction in the air bubbles forced under the boat by speed or lift. Sailboats it doesn't matter too much but it's better facing aft to keep out the seagrass. Anchoring/docking is the worst time where one picks up junk out of the water.

[donradcliffe]

Can't figure out how to post a graph, but the dynamic head (how far above the static waterline that water will rise in a forward facing scoop) is 1.6 ft at 6 knots, and 4.4 ft at 10 knots. This neglects any wave effects.

[Ziggy]

Originally Posted by Gaiamar

I do have an internal strainer and siphon break. These however do not provide peace of mind.

As far as I can tell, if you have an internal strainer, an external strainer can provide peace of mind but does not add any safety or serve any practical purpose.

An internal strainer basket can be cleared in a minute if it gets clogged. The only way to clear an external strainer is to go into the water.

You can get more peace of mind by removing the external strainer and fitting a tee to the seacock (with a plug in the straight up branch.) That way if the thruhull gets obstructed with some debris, you can remove the plug from the tee and poke an icepick or a screwdriver through the seacock and remove the obstruction.

[Stumble]

External strainers were a bad idea from the beginning, and scooped strainers have sadly been taken from the narrow realm they should be used in, to general purpose.

Scooped strainers are designed to be used on high speed power boats (over 70kn) to force water into the system, other than that narrow purpose they have no useful application on boats.

[delmarrey]

Originally Posted by Ziggy

You can get more peace of mind by removing the external strainer and fitting a tee to the seacock (with a plug in the straight up branch.) That way if the thruhull gets obstructed with some debris, you can remove the plug from the tee and poke an icepick or a screwdriver through the seacock and remove the obstruction.

My sea strainer is removable with 4 screws for cleaning. But.......



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[Gaiamar]

I heard verbally that the ABYC standard indicates aft orientation. I'm
unable to confirm. Thanks for everybody's help with this issue.

[Maine Sail]

Originally Posted by Gaiamar

I heard verbally that the ABYC standard indicates aft orientation. I'm
unable to confirm. Thanks for everybody's help with this issue.

The ABYC defers to the manufacturers instructions on this. I gave you Westerbek/Universlal's stance here's Yanmar's:


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."

Yanmar also want's it facing aft if using a "scoop" type strainer.

[MV-Romnya]

Thanks Main Sail, excellent information on how to install a seacock without drilling through the hull - I have a wooden boat - can I use hardwood instead of fibreglass as the backing plate ?? - Karl R

[delmarrey]

Originally Posted by MV-Romnya

I have a wooden boat - can I use hardwood instead of fibreglass as the backing plate ?? - Karl R

What I have found that works really well is composite decking, like what's used for homes. It's PVC impregnated with wood fiber and feels, acts and looks like real wood but will not rot. It will crack if one tries to bend it too far but once it's epoxied down and covered with a coat of epoxy it works excellent. Screws go into it just like wood.

I used the TimberTech TF with excellent results. Check out my blog (Sealing the Hull to Deck Joint).


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