Location of thru-hulls for drains and exhaust

[JonnyBoats]

On most boats ?above the water? thru-hulls for such things as exhaust, bilge pump discharge, drains etc. are generally found close to the boot topping or just slightly above the water line. From a safety point of view, would it not be better to locate these thru-hills a foot or two above the water line? Is there any negative (other than esthetics) in doing this?

[Tim]

I think the counter is one of the best locations for these fittings, depending on the boat's design. This is not always feasible, of course, particularly for boats that have no counter to speak of (i.e. double-enders, etc.) The transom itself is a decent choice on many boats. Again, this isn't always an option, particularly since transoms don't always even exist (double-enders).

Exhaust outlets shouldn't be too high, since they will stain the hull beneath, but all things require functional compromises too. On each boat, there is some happy medium, a "perfect" location that takes all factors into account as best as possible.

The same goes for bilge pump outlets, which is why they tend to be close to the waterline (and also because it makes the most direct--and easiest--run for the production builder). I don't think they belong at the waterline, but placing them higher in the topsides, particularly amidships, can create back-siphoning problems when the boat is heeled. This also applies to midships-located fittings at the waterline, too. Another reason you find bilge pump fittings close to the waterline is because the pumps are so pathetically anemic that they can't handle much of a discharge head. The lower the outlet, the more the pump can pump, at least when we're talking about standard centrifugal bilge pumps. Raising the outlet typically requires a larger pump, and also can cause a lot of backwash into the bilge--all that water that remains in the hose after the pump stops. Check valves work for about two weeks before they die, so there are few good solutions to this. Using a different sort of bilge pump is probably the best answer.

My preference, if the counter isn't an option, is for these fittings to be as far aft as practicable, and quite high. A fitting with a lip might help prevent the staining beneath. The engine exhaust on a sea boat should have a shutoff valve to prevent water ingress in terrible following seas; for a coastal boat, this is overkill.

[dasein668]

A fitting with a lip might help prevent the staining beneath.

Something like this is nice. I plan to use these for both exhaust and bilge. They will be located high on the counter.

[JonnyBoats]

The engine exhaust on a sea boat should have a shutoff valve to prevent water ingress in terrible following seas; for a coastal boat, this is overkill.

I was planning on this. In fact I am planning on proper seacocks for all thruhulls below the level of the main deck (e.g. everything going through the side of the boat. For reference, it is a double ender with a transom hung rudder that I am working on, so everything comes out the side.

Is this overkill for an offshore boat?

[Tim]

I suppose it makes sense to have shutoff valves on all through hulls, regardless of location, since most are, at one time or another, below the waterline while sailing offshore. It's certainly less critical on those above-waterline fittings, but if they're close to the waterline, why not install the seacocks as you suggest? It's a pretty small cost in the scheme of things, and if it feels right, then by all means.

At a minimum, I would avoid all plastic for those above-waterline fittings. (This does not include Marelon, which would be fine, and is plastic in the same sense as our fiberglass boats are plastic.) Stay away from those cheap plastic/nylon bilge pump outlets and the like, which are pretty chintzy at best.