Attaching scoop thru hull to hull

Hello all,

I am not finding a previous thread covering this when I search, so looking for someone who has done it. I am installing a Spectra water maker, and the manufacturer calls for a scoop (facing forward) thru hull for the unit. I have installed many mushroom thru hulls, but wondered about the four screws that mount the strainer to the hull.

Do they go all the way through the hull, part way into the hull, or what? Also, I am assuming they are stainless screws as I find no bronze screws available anywhere in an appropriate size. I am also assuming removing bottom paint to gel coat under the scoop, and generous 4200 or 5200 as in my previous installs.

Looking for the poop from someone who has done it!

Thanks

You should be able to get bronze from Jamestown

In the absence of oxygen, any stainless corrodes. And stainless screws in a bronze fitting sets electrolysis.

Or McMaster Carr.

Hi AKDoug,

A scoop on a watermaker intake? Assuming the intake is placed appropriately below the waterline and well aft of beam, not downstream from other hull "protrusions" that might cause ventilation or prolonged water starvation of the watermaker intake, and certainly well away from sewage outlets, I fail to see the need for a scoop. A simple mushroom seacock and ball valve works just fine. But an internal sea strainer is never a bad idea.

My watermaker(s), on multiple boats, have worked just fine underway in all sea states that us normal recreational boaters operate in. I presume the manufacturer is trying to CYA on starvation issues to the HP pump. Seems like a red herring to me, particularly if a primary boost pump is used. While no expert, I doubt HP piston pumps will instantly grenade even if the flow is momentarily interrupted. Mine never did.

Regards,

Pete

Hi Pete,

Yes, I was aware of all of the scoop controversies... However, the manufacturer calls for a dedicated scoop strainer, facing forward, as well as an internal strainer between thru hull and pump. I think it would probably do fine with a mushroom thru hull, but since I have toilet and salt water wash down on the one available, I will add one more hole in the hull.

The manufacturer warns the warranty will be voided if not installed per their directions, and the engine raw water intake came with a scoop strainer mounted and I have had no issues with it. I see no reason not to meet their requirements, since I am going to add a thru hull dedicated to the R/O unit. We really don't get marine growth on our hulls here, just a little slime over the season, those of us who keep our boats moving anyway

I knew stainless screws weren't the best solution, but haven't found silicon bronze yet locally. I hate to buy a 100 pack when I need just four screws... I had kind of assumed I would be putting bottom pain over the screws before launching and it would eliminate the electrolysis issue from dissimilar metals.

Trying to line my ducks up before I head to the boatyard and start projecting, it's still too cold to actually do any work here as of yet :-(

Still no input as to whether the screw should be through the hull of just tapped into it... I suppose copious use of 4200 or 5200 makes that a non issue as well...

Thanks to all who have responded!

Groco and Perko both make thru hulls with scoops integral. That might be the easy way to go. I would not use S/S screws or bolts below the waterline. And painting the S/S screws may promote crevice corrosion. Jamestown Distributors sells bronze stuff by the box. McMaster Carr may sell by a smaller quantity.

The integral scoop models still require four screws to mount to the hull, plus either the nut or a flange fitting. I was planning to use the flange fitting with a backing plate with the integral scoop, the mushroom thru hull and a cover cost more than the integral unit. I did consider just not putting the screws into the hull since the flanged valve would hold it in place anyway... It seemed like a sub par idea, using the screws just seems like the right way to do it.

I would use something like a G10 backing plate. Use the integral scoop thru hull. Then use bronze bolts and thru bolt the scoop thru the backing plate. That way it is good and will be strong. But I tend to go overboard on things. I just ran 8 gauge wire to my new big bilge pump and 10 gauge to the small pump. But they will get good voltage that way...

I’m a big fan of the Groco APHS scoop strainers. You can clean inside the thru-hull without removing the strainer from the hull. If you use an APHS you may never need to clean your internal sea strainer.

I just used short stainless self tapping screws because I didn’t want to put five more holes through the hull. I never had a problem but I may have just gotten lucky. I used 5200 as the bedding.

Bronze machine screws shouldn’t be hard to find. Any good marine store should have them.

https://www.groco.net/products/raw-water-strainers/hull-strainer/aphs-series

I used a Marlon material scoop on the thru hull for the AC cooling water pickup. It's fastened with #8 or #10 1/2" self tapping stainless screws. Our boat isn't particularly thick regarding the fiberglass layup so it was not without some apprehension that I drilled into the bottom but I was careful and didn't come through on the inside of the boat with the pilot hole or the screws. I'm not sure how much confidence I would have with the screws holding a larger, heavier bronze strainer, but the smaller strainer sized for a 3/4" thru hull fitting has been secure going on it's 4th season now in salt water,too.

Most scoops I have dealt with just had used short, stainless screws as Parks described. As long as the bottom was thick enough and not cored.


The type of sealant helps.... as most still require a lot of prying force to get them off after the screws are removed.