Attaching Clamshell

[jviss]

This is from MaineSails Marine How to site:

HYDROLOCK! Cause = Improper Orientation of Scoop Strainer
This owner was the lucky recipient of an engine hydrolock event while under sail. When he went to start the engine a cylinder was filled with water, and water is not-compressible. He though something was wrong with the starter motor, so kept trying, and he eventually burned it up. A dead starter motor on top of a hydrolock.. Cha-ching $$$$$….
“But RC it worked for 6 years with no issues.”
Another way to look at that statement:
“You got lucky for 6 years.”
Yes, a “professional” boatyard did this installation… Apparently no one explained to them that when sailing a scoop-strainer can potentially force water up and over the siphon break, into the water lift muffler, and can eventually back fill a cylinder and cause a hydrolock..
Please let me be very clear on this point:
Raw water pump impellers are not shut off valves!
Impellers do and can leak water past them. Facing a scoop-strainer forward is simply an incorrect installation/orientation for a sailboat. Please, if your scoop strainer faces forward consider;
1- Removing it entirely
2- Installing a non-directional sailboat strainer
3- Reversing it
If you’ve not filled your engine with sea-water, and experienced a hydrolock, consider yourself lucky.

Thanks, but this isn't a scoop strainer.

Edit:
I can see how you might think it is, or it could act as one. I would face the slotted end aft.

 

[Richard19068]

Thanks, but this isn't a scoop strainer.

Edit:
I can see how you might think it is, or it could act as one. I would face the slotted end aft.

Right. Just replying to nwsearle. Not telling you what to do.

 

[Apex]

@jviss: entertaining thread so far. My 2cents? yes, short bronze screws. And bottom-tap the holes in FRP is a good idea. Otherwise the risk is cracking the gel coat. A little adhesive on the threads and as gasketing on the edge, with slots facing backward. I have much like others, I have this version:

you're boat, you're option...LOL

 

[Dalliance]

I'll give you a quick lesson. My sailing buddy and I on different boats transiting the Fox River, WI locks, (when you could) had to wait for the lock to open. On one particular lock he got an engine overheating alarm. Shut down his engine and in the meantime was slowly being swept to the lock overflow, not a great position to be in. He found fresh water snails in his engine intake and was able to clear in time to not get pulled into the overflow. Lesson learned. We both installed the strainer you've shown at our next winter haul out. That was 30 years ago.
Now it gets fuzzy, do you face the strainer into the prevailing flow of water or away from? I've seen and read many accounts, no one unequivocally says one way or the other. I choose away, thinking the water pressure would assure water flow into the engine water inlet and nothing would have a tendency to stay on the filter long enough to embed. I also filled the screw openings with screws, I've seen some that don't. Here it is many years later, I've never had a problem with my intake water. Hope this helps.

I have the same integral slotted strainer thru-hull and love it for protection from debris in the water, especially in the river, but I can say from experience it does not protect against all fresh water mollusks. Zebra Mussels get in behind the screen when they’re small, attach themselves, grow in size and multiply until you have a problem. This is one of the reasons MaineSail hates exterior strainers. Depending on your waters, periodic diving between haulouts may be required to clean out behind the screen.
Our invasive Zebra Mussel population has been way down here the last few years. They so depleted the algae supply that they starved themselves. In the past, after winter on the hard, I‘ve used a knife blade between the slots to break out dead snail shells that were too large to have fit through the screen, but had not yet caused any noticeable problem.

 

[jviss]

@jviss: entertaining thread so far. My 2cents? yes, short bronze screws. And bottom-tap the holes in FRP is a good idea. Otherwise the risk is cracking the gel coat. A little adhesive on the threads and as gasketing on the edge, with slots facing backward. I have much like others, I have this version:
View attachment 202629

you're boat, you're option...LOL

I like that version, but I'm just too lazy lately to replace the entire through-hull. Maybe I should consider it.

I'm not crazy about how the through-hulls (TH) are done on my boat. The bilge is very shallow, and there's an elbow immediately after the TH and then a ball cock. It's like a regular plumbing ball cock, not sure if it's brass or bronze. So, not sure I want to disturb this at this point.

 

[PaulK]

The clamshell filter on our NTU (new to us) boat mounts on a bronze gasket or plate with threaded holes for the bolts that hold the clamshell on it. The gasket is embedded into the hull around the intake. When we hauled in October the clamshell was almost completely clogged with barnacles. We're not putting it back on next season, so I get to remove the bonze mounting gasket.