Through hulls/ scoops

[jerry f]

I was planning the install of a scoop through hull for a a/c pump. All manufucturers seem to call for a scooped inlet but I dont like the idea of the exta small screws/bolts into the hull for the scoop. Can anyone give me real world advice on what the correct install procedure is for a scooped through hull regarding the added fasteners. Is the main nut and proper bedding with sealant sufficient ?? Thanks, Jerry.

[exposure]

Are you planning to use this while you are underway? I not, the scoop is not needed. I just installed a 16k btu unit on my boat and it gets all the water it needs through a 1/2" through hull. I do plan to upgrade to a 3/4" at the next haul out.

Woody

[jerry f]

Thanks Woody. I believe i read that even if you only use at rest, you can get air in the line and lose your prime if you dont use a scoop for the positive pressure it creates underway. In other words, underway you get cavitation at through hull (without scoop) then when you pull into dock you have may have lost your prime.

[Alan Wheeler]

If it is a rubber impellor pump, they are self priming, so losing prime is not a problem.
Catvitation is not air, but a vacum-nothing- once the pressure has normalised again, the water fills the void again. You don't get air from nothing.
A scoop will only give positive pressure when the boat is in motion. At rest, it does nothing. Infact, it is possibly detrimental if anything. Even at speed, it would be debatable how much positive pressure it could add. If your skin fitting is a few feet down below water line, the water pressure alone would be far greater than that which a small scoop could develop.

[jerry f]

Most a/c units use a non-self priming pump such as the "march 500" or "cal" pump. Thus my concern. I know you must re-prime most pumps after haul out but are most intall's done without the scooped through hull??? Any feedback pro or con for the scooped through hull is appreciated. As far as using a rubber impellor pump for a/c that is not the norm from what i know. but i dont know much so i'm willing to listen....

[Alan Wheeler]

Well I am also no AC expert. Infact, no experiance at all, so I am ready to step aside when the experts step in.
But I do know pumps. I am presuming you must be useing a centrifugal pump. They are high volume pumps and can be high pressure. Just depends on the design need. However, they will lose pumping ability as soon as air is introduced and if cavitation is seriuose enough. They rely on the inertia of the water flow to maintain lift. It seems a little strange that this type of pump is mounted above waterline. It must be a real hassle, as the slightest little air leak into the system (often around the shaft gland) would mean priming again. I would have suggested that the pump is below water line. But as I said, I know nothing about the AC stuff, so I guess there must be a reason why the pump is not.
As for scoops, when staionary, no the scoop will not aid the pump. As I said before, if anything, it can be detrimental. Any restriction to water flow and ingress could cause cavitation. The bigger the inlet the better is the idea, but of course on a boat, that isn't praticle and the the biggest skin fitting you can get away with is what will be required. The less restriction, via either inlet diameter and/or length, the better the flow of water into the pump, thus the bigger volume of water out of the pump.
A scoop may help when under way, but it needs to be big to aid giving the inlet any positive pressure. Most off shelf scoops I have seen are simply not such. The only scoop I would consider is the AC manufcaturers recomended one, if one is made and is practicable to fit.

[jerry f]

Alan, the pump install is below waterline but they still need priming. Usually just after launch and basically i'm hoping that the expertise i get tells me to forget about using the scooped through hull. However, almost all "install kits" from manufacturer include the scooped through hull. Maybe because then the are covered if the boat has a gen and air is used underway. Also i'm going to start a new thread on keel cooling which i know even less about but i'm hoping to get a job installing twin units in a new 42' sail. I think i'm going with standard seawater cooled but i want to know about the keel cooled before i modify not after.

[GordMay]

I wouldn’t endorse any advice the suggests inlet “scoops” for anything beyond seawater “straining” - and straining might best be performed inside the hull, where cleaning can be a simple part of routine maintenance & operation.

I don’t believe that adding an exterior inlet appendage (strainer-scoop) can do anything to reduce inlet turbulence, nor improve circulation. Remember, scoops are usually installed in the aft-facing orientation.
I’d guess that a clean laminar water flow might best be accomplished with a “flush” inlet.
FWIW,
Gord May