What's the Poop on the scoop strainer

[TFlight]

My 2007 Hunter 41 is on the hard for this winter and as I was inspecting the thru-hulls I realized the scoop sea strainer I thought was for the engine pickup was actually for the air conditioning pickup pump.

I was considering adding a Groco Bronze Thru-Hull Scoop Strainers on the engine Thru-Hall facing forward.
https://www.jamestowndistributors.c...lyName=Groco+Bronze+Thru-Hull+Scoop+Strainers

I was hoping it might help keep grass out of the sea strainer and keep my engine cooler at high rpm' (>2,750 rpm, 3,000=WOT)
P.S. I have other things to check for the high rpm heating issue.

I am a little concerned about running the risk of forcing raw - water into my cooling system and flooding my engine if on a fast point of sail or surfing down a wave with the engine off.

Is this a reasonable concern?

Are there different types of scoop / strainers for sailboats?

Or is it just not necessary?

 

[thinwater]

There is not nearly that much force. This is not a problem (I've had these on cats near 20 knots). The bigger problem is keeping the crap cleaned out of them.

 

[RoyS]

I recommend a raw water filter after every raw water intake on your boat. I have had our manual toilet pump fail due to ingestion of eel grass. Last year I installed a raw water filter on that intake and when putting the boat away discovered several strands of eel grass trapped safely inside the filter. My engine intake has a filter and this also collects the grass. Both of these raw water intakes have the strainer that you describe as well but the grass slips through those. Grocco makes a great raw water filter.

 

[Ron20324]

Remember the guy who's head didn't work? He found a fish in the intake hose!

 

[thinwater]

^^ Yup, had that happen to my AC once.

AC seems to be the more persistent problem, because it runs continuously and the boat is not moving.

 

[kloudie1]

Slots should be facing aft.. Many are installed facing the wrong direction.. The scoop, unless restricted/partially plugged, is probably not a cause of engine overheating..

 

[seadaddler]

I have the vented scoop for my AC also and never had a problem even though they are harder to keep clean and had mine get loaded inside with barnacales.
nick

 

[Johnb]

I am a little concerned about running the risk of forcing raw - water into my cooling system and flooding my engine if on a fast point of sail or surfing down a wave with the engine off.

Is this a reasonable concern?

I don't see water getting through the raw water pump impellers - anyone else have a opinion on this?

 

[seadaddler]

Agree water will not go through the pump and impeller and I
really don’t like the scoop with the screen fins because they are harder to clean and harder to keep clean and maybe obstruct the flow of water into the pump.
Nick

 

[kloudie1]

My slotted scoop type intake is on backwards from PO, but I have never had a problem with it, even occasionally surfing down wave fronts at 12 kts (GPS) or so.. Eventually, I plan to turn it around, but it is way down the list.

 

[thinwater]

Slots should be facing aft.. Many are installed facing the wrong direction.. The scoop, unless restricted/partially plugged, is probably not a cause of engine overheating..

Not to be rude, but could you post a link showing this in the instructions? Not a forum post, but something from a manufacturer.

I just read both Groco and Apolo instructions, and they show the slots on the forward side. In fact, I know that some boats at high speed will have trouble priming if the slots face aft (suction). First, I'm pretty sure that it makes no difference at all at sailboat speeds. Second, I doubt this could have been the actual cause of hydrolock, since EVERY powerboat strainer faces forward. In any case, the hydrodynamics just don't make sense; the water pressure, through the screen at sailing speeds, can't be more than a few inches of water column. If the intake is that sensitive to water pressure, the boat will probably hydrolock when they take on heavy stores.

This seems to accepted lore, but so are many things that just ain't so. If there is a solid reason, let's spell it out. I'd like to know.

 

[seadaddler]

I have 1 scoop thru hull from the factory for my AC and all of my other 6 thru hulls are
a round open thru hull from the factory.
Nick

 

[chuckwayne]

I agree with thinwater - simply not enough pressure to force water past a positive displacement pump's impeller vanes. After a bad eelgrass/mud episode, I installed the intake strainers - facing forward - and have never had a problem

 

[SG]

I don't think you should not have a scoop on engine intake, especially a forward facing one. If your boat is moving fast, you might regret the hydro lock.

There is a school of thought that:
The AC intakes are fine and might be aided by a coward facing scoop because it could help avoid loosing or correcting a loss of "prime" to the AC pumps.

 

[thinwater]

I don't think you should not have a scoop on engine intake, especially a forward facing one. If your boat is moving fast, you might regret the hydro lock....

Then why is forward recommended by the manufactures for powerboats that go 4 times faster and have 16 times the pressure?
No, that does not make sense without some backup.

The reason AC pumps loose prime has nothing to do with speed for two reasons: first, few people operate AC underway (no generator); second, the actual reason they lose prime is always trapped air due to poor plumbing--more pressure, within these limits, just packs in more air.

 

[Maine Sail]

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.

 

[Maine Sail]

Then why is forward recommended by the manufactures for powerboats that go 4 times faster and have 16 times the pressure?
No, that does not make sense without some backup.

It all has to do with the location of the raw water circuit in relation to the Ocean/Lake. Engines on most power boats, even displacement hulls, rarely sit below the waterline thus they don't need or use "water lift" type exhaust systems which is where the hydrolocking occurs from. Displacement cruising trawlers are approx 30% of my business, commercial fishing and planing boats about 15-18% and none of them uses a waterlift exhaust system. If a little extra water gets past the impeller when doing 10 - 40 knots, it is no big deal, it just goes right out the exhaust and does not need to be "lifted" out of the boat like it does on a sailboat which is where the risk of a hydrolock comes from because the high point in the "lift" is always higher than the exhaust manifold. Unlike sailboats, power boats also don't move without the impeller spinning or engine exhaust pressure to "push out" any excess water in a water-lift, for the few power boats that do have them.

 

[thinwater]

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.

Thank you. That is what I was looking for. The section of the Groco manual I read said the opposite, but was obviously referring to power boats, without making the distinction clear.

I have witnessed the suction/vacuum problem on high speed boats, typically over 20 knots.

And I wish they were all hinged. I also wish they were not needed, but in my home marina, they are. In certain seasons there are large schools of minnows, and twice I sucked up half the school. After adding the strainer, no problems. Yes, I have to swim twice a year, but the water is mostly warm here (and I have a dry suit for when it is not--I'm quite comfortable diving in winter water and sometimes even use the dry suit as deckwear in nasty weather). The fittings are not so far below the water (catamaran) that I need to duck my head under water. Heck, most of the time I just clean it while I'm swimming for recreation. Unclogging from the inside would be more work for me. But I do see the need.

So horses for courses.

 

[SG]

Thinwater:

We've been a bit south of you in the Chesapeake. Every other year we haul the boat, the little slots in the filter seem to help with the nettles (though they still get sucked-up into the strainer occasionally. I wouldn't fixate on it.

 

[thinwater]

Thinwater:

We've been a bit south of you in the Chesapeake. Every other year we haul the boat, the little slots in the filter seem to help with the nettles (though they still get sucked-up into the strainer occasionally. I wouldn't fixate on it.

That to. Without the external strainer they can foul you in seconds, with it they tend to slide by.