Re-installing the skin fitting for the sink drain....tips for a first timer

Those instructions [EDIT: The ones in the OP] look reasonable although they show a recess in the hull which is relatively uncommon - normally the outer flange will sit on the surface.

It's often suggested to do the fitting up not quite all the way, allow a few days for the sealant to set, and then do it up fully to compress the now-hard sealant like a rubber gasket. Seems plausible, but note that the professionals rarely or never do this due to the time involved, and I don't think their skin fittings end up leaking.

Skin fittings are made long enough to pass through the thickest hulls which, especially for small sizes, can leave the valve perched on top of quite a tall pipe. I prefer to dry-fit everything, measure how much thread is exposed between the internal nut and the valve (or the backnut pressed against the valve, if using one), and then trim that much (minus a few mm) off the end of the fitting with a hacksaw. Now the whole assembly will be as short as it can be which looks neater, presents less leverage to potentially damage the thing, and takes up less space.

Pete
 
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Nigelb said:
http://www.pbase.com/mainecruising/installing_a_thruhull

Always good reference site and installed mine with backing plates.
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Note though that he advocates using "proper" seacocks, which bolt to the hull as well as having the skin fitting screw up into them. There's no question that this is an excellent system, superior to the skin fitting, nut, and ball valve we're more used to, but the parts for it are not widely available in the UK and are very expensive.

Just don't want the OP to get confused by the two different systems.

Pete
 
I'm about to cut the hole in the hull for the 32 mm raw water intake for the engine.

My yacht is Airex cored but solid under-water (probably 18+ mm) On my intake there are holes around the strainer for either through hull bolts or self tappers. I'm thinking of ignoring them but building up the core around the thru-hull fitting with CSM/WR (on the inside of the hull), then I will cut the thru-hull pipe to a suitable length.

View attachment 57333

On another matter I've had a lot of trouble finding a suitably sized hose barb. I've ruled out SS304 because of corrosion and brass because of corrosion(?) or galvanic corrosion with the SS316 skin fitting even though it is separated by a nylon(?) ball valve. Eventually I went for a plastic 25mm irrigation fitting. I didn't want to go down from 32mm to 25mm but figure as it is one meter below the WL there will be pressure there to squirt the water in anyway.
 
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coopec said:
On another matter I've had a lot of trouble finding a suitably sized hose barb. I've ruled out SS304 because of corrosion and brass because of corrosion(?) or galvanic corrosion with the SS316 skin fitting even though it is separated by a nylon(?) ball valve. Eventually I went for a plastic 25mm irrigation fitting. I didn't want to go down from 32mm to 25mm but figure as it is one meter below the WL there will be pressure there to squirt the water in anyway.
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32mm (aka 1-1/4") does seem to be an awkward size - my sink drains are 32mm and I've had some difficulty finding a decent quality Y-fitting to combine them together. Loads of fittings at 1" and 1.5" though - I don't know anything about your particular engine but I would have thought 1" (or 25mm) would be sufficient?

Incidentally, some people don't like those scoop shaped fittings because although the cover stops some things being sucked in, if anything does end up inside you can't poke it out again. No experience with them myself.

I don't know much about stainless for skin fittings, they're not widely used here. You're right that plain brass is not really suitable, it dezincifies over time and becomes very weak, although the rate at which it happens seems to vary a great deal. Here we have DZR, specially formulated DeZincification Resistant brass, which is what most people use, but I don't know if that's available in Australia.

"plastic 25mm irrigation fitting" sounds concerning as far as strength is concerned, but hard to tell without seeing the thing. Some "plastic" is perfectly strong - we make out boats out of it after all :)

Pete
 
prv said:
32mm (aka 1-1/4") does seem to be an awkward size - my sink drains are 32mm and I've had some difficulty finding a decent quality Y-fitting to combine them together. Loads of fittings at 1" and 1.5" though - I don't know anything about your particular engine but I would have thought 1" (or 25mm) would be sufficient?

Incidentally, some people don't like those scoop shaped fittings because although the cover stops some things being sucked in, if anything does end up inside you can't poke it out again. No experience with them myself.

I don't know much about stainless for skin fittings, they're not widely used here. You're right that plain brass is not really suitable, it dezincifies over time and becomes very weak, although the rate at which it happens seems to vary a great deal. Here we have DZR, specially formulated DeZincification Resistant brass, which is what most people use, but I don't know if that's available in Australia.

"plastic 25mm irrigation fitting" sounds concerning as far as strength is concerned, but hard to tell without seeing the thing. Some "plastic" is perfectly strong - we make out boats out of it after all :)

Pete
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I agree with you 100% that 32mm is an awkward size (28mm even more so). When I build my next yacht I will go for 25mm! I went for 32mm because that is the size of the inlet/outlet of the water strainer.

I was against using an irrigation fitting but that's what all the boat chandlers sell here. Then I reasoned if the plastic (?) hose barb could handle the tremendous forces of irrigation then it could do the job as a hose barb on a boat. But I hope it doesn't become brittle with age or something.
 
coopec said:
Eventually I went for a plastic 25mm irrigation fitting. I didn't want to go down from 32mm to 25mm but figure as it is one meter below the WL there will be pressure there to squirt the water in anyway.
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I agree with prv - I would be very nervous about using a plastic hose tail below the waterline - in fact I shudder at the thought. If your local chandler doesn't have the appropriate DZR size in stock, there are no end of suppliers on fleabay.

Incidentally - check carefully which way round you install that water scoop. I'm just installing a new (Vetus) diesel on our yacht and I see from the instructions that the recommendation is to have the scoop facing aft. This is so that when sailing, there is no chance of the inlet pipe becoming pressurised and forcing water past the pump impeller, filling the exhaust water-lock and flooding the engine. I think Beta make the same recommendation in their manuals. Somehow it looks wrong, but the logic is sound. Like most MOB's, I installed it, then read the manual. I then re-installed it according to the instructions!
 
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Arcady said:
Incidentally - check carefully which way round you install that water scoop. I'm just installing a new (Vetus) diesel on our yacht and I see from the instructions that the recommendation is to have the scoop facing aft. This is so that when sailing, there is no chance of the inlet pipe becoming pressurised and forcing water past the pump impeller, filling the exhaust water-lock and flooding the engine. I think Beta make the same recommendation in their manuals. Somehow it looks wrong, but the logic is sound. Like most MOB's, I installed it, then read the manual. I then re-installed it according to the instructions!
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The above is correct, both for the reason stated, and also the fitting offers less hydrodynamic resistance with the scoop facing aft. On a walk round a typical boat yard you will invariably see some of these fittings the wrong way round, ours was when we bought the boat. This is also the reason why pear shaped anodes should be fitted with the big end forward.
 
Arcady

When I say "plastic" I mean that as a generic term. Apparently they are PVC.

HOSE BARB
Products | Pipe Fittings. PVC-Pipe-Fitting.jpg ... Hose tail and male or female BSP threads. ... Rated to 200 Psi. • Push in options available to 290 Psi

RADIATOR HOSE
Jun 10, 2011 - The hose typically has a rated working pressure of 100 PSI, with a burst ....

I thought I knew the correct way to install the scoop but I will double check now that you have mentioned it
 
If I may throw some of my experience with my skin fittings in the pot.

Coopec

I fitted those inlet skin fittings you posted and found lots of little critters hiding inside (no blockage) and difficult to remove so I removed the bars from the face and now cleaning is easy.

On all my skin fittings below the water line I did the same as Pete and either cut the fitting down or fitted a socket in the skin fitting thread so the fitting screwed onto the skin fitting was a face to face seal and I files the face of the fitting to get the correct orientation. Yes a back nut could be used but then you have a thread seal on parallel threads.

The fitting I screwed directly othe the skin fitting is a tee with the upper leg blocked off to allow rodding of the skin fitting from inside if need be.

The ball valve was then screwed into the horizontal leg of the tee to keep the profile as low as possible.

If the outlet of the valve was to be plastic pipe I started using nylon insertion fittings the screwed into the valve. There were 40mm,32mm or 25mm. The latter were OK as the pipe was also that size but the 38mm i/d pipe was difficult to get onto the 40mm insertion fitting so I started to use 316 stainless steel swage fittings, which are very common here.

http://www.nde.co.za/index.php/products/bsp-fittings/itemlist/category/181-swage-hose-nipples

Coopec have you tried these people

http://www.prochem.com.au/piping-products/bsp-fittings/

Two of my sink outlets drained very slowly when piped to a skin fitting just above the water line so I changed then to drain straight down through a standpipe with a valve fitted above the waterline whit a tee between the top of the standpipe and the valve to allow rodding is necessary.



This is my inlet system with 2 strainers and 2 main inlet seacocks with separate isolation valves for each raw water draw off.

A bit OTT but gives me great flexibility and only 4 seacock below the waterline 2 heads outlets and duel inlets.
 
Yesterday I drove 20km to pick up the hose barb that they had put aside for me (I had quoted the catalog no) and when I found it was the wrong size I caused a scene in the chandlers. They were very apologetic but pointed out it was exactly the same as you could get from the hardware chains.

I bought one from Bunnings Hardware for $3 and the chandler was wanting $7.75 (if they had one)

Wherever I go for a 28mm hose I'm told "A very unusual size. we have 25mm and 32mm but not 28mm"'
I'm buying quite a bit of hose and other items from the UK these days mainly because I buy on eBay but I'm sure if I looked around I would find it locally.
 
coopec said:
Rated to 200 Psi. • Push in options available to 290 Ps[/B]i
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The pressure is no problem at all - at 1m down you'll have about 1.5psi in it, plus a bit more for dynamic pressure as you crash into a wave. Pressure in any yacht seawater plumbing is basically nil (except toilet outlets when the seacock is closed and you have a burly infantryman leaning on the pump, been there :) ).

The problem is more one of mechanical strength when something heavy falls against the fitting, or you're wrestling with the other end of the pipe, or you unwittingly put your foot against it while reaching for something else, or your shoulder catches the hose as you lean past it to reach the oil filter, or you're shoving the dinghy down into the locker forgetting about the plumbing at the bottom of it. How much of a potential problem this might be depends very much on where the fitting will be and what else is around it.

At least your hose barb is after the valve, so you can shut it off if you do manage to crack it. And again, some plastic fittings are strong enough, especially anything in glass-filled nylon or acetal. Just not convinced about PVC irrigation kit, but then I guess irrigation is a more heavy-duty activity in parts of Oz :)

Pete
 
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Kwik Decision said:
I would say yes, for both power and sailing boats. I'm curious to know why you think they should be different.
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Because the powerboat will only be moving forwards when the engine is running and you want water coming in. Possibly also at high speed a backwards-facing scoop will create a suction that opposes the pump, not a problem for a yacht motoring at 5 or 6 knots.

Personally I don't see any benefit to the scoop/grille fittings and have always just had a straightforward hole. On Kindred Spirit I had a T with blanking plug and a pokey stick stowed nearby, similar idea to Roger though a less elaborate system overall. Never yet actually had a cooling system blockage though, despite being on drying berths and (in KS at least) bumping around in very shallow water.

Pete
 
So whilst I appreciate these latest threads that are related to skin fitting and Seacock installation. I have already purchased the appropriate skin fitting and would still like some tips on the best way to fit it to the already existing hole. What is the best sealant, do i need to create a wooden backing plate etc etc

Regards ?
 
thesaintlyone said:
So whilst I appreciate these latest threads that are related to skin fitting and Seacock installation. I have already purchased the appropriate skin fitting and would still like some tips on the best way to fit it to the already existing hole. What is the best sealant, do i need to create a wooden backing plate etc etc

Regards ?
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I think the procedure for installing the skin fitting may depend on which method for thread sealing that you intend to use.
Often you want the handle of the ball valve to end up in a certain position when everything is finished. This can be difficult to calculate (unless you use a backing nut to position the valve relative to the skin fitting).
How I just tackled the same task: Hole prepared, skin fitting in hole, washer and nut in place, but loose, free to rotate. Screw on ball valve with Loctite 572 on threads. Suitable ring spanner dropped through the assembly stops the skin fitting from rotating when doing up. Leave for a few days to cure.
Loosen nut so that the skin fitting sticks out 20mm of the hull. Apply liberal amounts of Sika 291i under the flange and on threads. Go inside, place handle in desired position, do up the nut. Screw in hose tail with Loctite on threads.
Personally I do not bother with backing blocks, but I guess that will depend on the hull thickness and position of the fitting.
 
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