Capping off a barbed thru hull

[Adodero]

We have a thru-hull in the transom with a hose barb that is permanently attached (it's part of the fitting). The thru hull is currently not in use and I'd like to cap it off, any suggestions?

[Richard5]

Depending upon your future needs you may want to delete it. Remove the void in your hull. One likely scenario is a capped off, unused through hull escapes your attention until some time later it announces it's presence in a fairly dramatic way.

[Adodero]

It will be used in the future, I just want to avoid a situation where the transom sinks below the waterline and all of a sudden it becomes a below the waterline fitting

I'd remove it, but we will need it later.

[Cheechako]

Well, of course there is the short hose with another barb fitting and cap on that. Many installed barbed mushroom fittings have enough thread between the barb and the nut to hacksaw the barb off and put a pipe cap on. The threads are not an exact fit but usually work. Either of these options assume it's above the waterline. Otherwise replace it with an appropriate fitting.
Put a second bilge pump in and attach the hose there!

[Richard5]

That you want to keep the through hull I agree with the Alaska newcomer...replace with appropriate fittings. Don't be a slave to the problematic hardware, replace it to your liking.

[Ambler]

What is your boat made out of? If it is GRP it is easy to remove and patch. I have patched many.
I have also a 1 1/2" outlet that I may use again one day so I have capped it by removing the valve from the skin fitting then I screwed on an end cap inside the boat with plumbers tape. Consult the plumbers supply for one.

[Andrew Troup]

The best method of all (I think) would require machining from a solid length of a suitable resilient material in round bar form (ideally, a urethane elastomer) a blind-ended hollow cylindrical form, with a reverse of the hose barb machined into the internal bore, and then force that axially over the barb, and clamp it.

Lacking access to a lathe and a rather exotic material, you might consider trying to find a suitable close-fitting solid internal plug to fit the bore, and bond that in place (having cleaned out the bore of the thru-hull if necessary) and also check with hydraulic suppliers for a suitably sized moulded plastic port protector fitting - for some reason, these are often red (similar to the first idea, but not custom made, and thinner/ not intended to be leakproof)
and clamp this onto the barb as above, as a second line of defence, helping to prevent the plug from being forced through to the inside.

At a pinch, the second purpose could be served by two (or more) rectangular strips of strong material (even heavy sailcloth would work) laid like a '+' over the 'o' of the thruhull, then folded down the barbed portion, contacting it, full-length, in four places, to be held by the hoseclamps.

[Ambler]

Just read the other replies that have come in. Sorry I miss read you have a barb as part of the fitting. You should Not ever use plastic skin fittings above or below the WL, nor should you have any skin fittings without a valve. Therefore remove and replace it with a SS or Bronze threaded fitting with standard size threads to take a valve, then follow my previous post.

[colemj]

less than $5

Mark

[Panope]

If an expanding plug is used, I would also add a hose clamp (or two) on the outside just to keep the plastic from spitting from the outward pressure of the plug. Probably very unlikely to happen but would be cheap insurance.

The hose clamps could also be used to secure some sort of safety strap like the one Andrew described.

Steve

[Jim Cate]

I think that I would use a short length of hose, clamped over the barb with another barb, fitted with a pipe cap clamped into the other end. This seems equally secure as any other barb/hose/pump (or whatever) as used elsewhere in the boat.

Don't see the need to overthink this problem.

Jim

[Andrew Troup]

Originally Posted by Jim Cate

I think that I would use a short length of hose, clamped over the barb with another barb, fitted with a pipe cap clamped into the other end. This seems equally secure as any other barb/hose/pump (or whatever) as used elsewhere in the boat.

Don't see the need to overthink this problem.

Jim

A fitting blanked off as Jim suggests would (I think) be fine in a compartment where no shifting cargo might come into contact after getting a bit of a run up.

Because there was no such detail supplied, my suggestion erred towards the worst case scenario where such a situation might arise.

As an analogy for the respective security of the two cases quoted above, consider a riverbank with a tree on the south (muddy) bank which has fallen down to the horizontal across the river, roots still embedded in the bank.

In one case the tree has been shortened (before or after falling) so it projects only a short distance across the river, cantilevering out above the water.

In the other case, the trunk remains long enough to reach the muddy bank on the far side.

A heavy barge comes bumping along the south shallows of the river.

Which tree is more likely to arrest it without being pulled out?

(Bear in mind that this is a tidal river, and if the barge is not arrested, it will keep travelling past the tree with every turn of the tide.)

[Terra Nova]

Originally Posted by Cheechako

...hacksaw the barb off and put a pipe cap on. The threads are not an exact fit but usually work...

Sorry, but this is a bad idea. Pipe fittings have tapered pipe threads. Thru-hull fittings have straight threads. And, though you can sometimes get a half turn started, they do not mate and should never be used this way.

Originally Posted by Jim Cate

...I would use a short length of hose, clamped over the barb with another barb, fitted with a pipe cap clamped into the other end...

This should be plenty safe. A hose plug is the preferred termination.