location of thru-hulls

[falcon]

I am in the process of replacing the thru-hulls on my boat and noticed that most were located below, or in the case of the cockpit scuppers, well below the waterline. I understand that the raw water intake for the engine needs to be well below the waterline, but from a safety standpoint, would it not be better if some of the others exited above the waterline? Is there any reason that the thru-hull for the manual and the electric bilge pump be located below the waterline, or the cockpit scuppers for that matter? I suppose thru-hulls exiting below the waterline look better esthetically.

Pete

[Case]

Regarding cockpit thru hulls, some people have moved them above the waterline and it works... for some. For other models, water comes in when heeling which is no good unless foot baths is the goal...

Most thru hulls really need to have a seacock on it and in most cases they are closed so I'm not sure if its worth the effort to move thru hulls above the waterline unless access is fantastic in the new locations.

Thru hulls for the bilge pump should be above the waterline, I think. Never saw one below the waterline...

- Case

[Tim]

On small boats, discharges often end up below the waterline because there is simply not a way to make the discharge work above the waterline--interior components and cockpit soles are often either at or just above the waterline, and this often dictates drain fitting placement below the waterline to allow for proper drainage.

If there's a good alternative for above the waterline re-routing, and you don't mind the fittings piercing your hull in a visible location, then go for it. Just be sure the drain will work in its new location first--that is, be sure to reconcile your proposed location on the inside with the waterline on the outside. You may find it's harder than you think to put those drains above the waterline and have the drain operate properly, but that doesn't mean you can't at least find out if it's an option.

Also, drains are usually plumbed in such a way that they'll drain on both tacks. If you run a sink drain close to horizontally out to the hull above the waterline, you might find it won't drain on one tack. This isn't always a major concern depending on your use of the boat, but this is one reason you find drains running more or less straight down to a fitting that is often below the waterline.

Bilge pumps should never be plumbed to below-waterline fittings. For one thing, back-siphoning is a well-known condition with centrifigual electric bilge pumps, and if the fitting is below the waterline you could end up literally pumping the ocean into your boat if this occurs. All bilge pumps should always discharge above the waterline, manual or electric. For this particular purpose, it's just good sense (apparently it wasn't common-enough sense to whomever plumbed your bilge pump that way originally...)

The long and the short of it is that there is usually a good reason why a fitting is located where it is, but now always. Just be sure you understand the various factors at hand before indiscriminately moving any drain fitting to avoid creating something that doesn't work.

Remember that while it's nice to minimize through hulls below the waterline, often attempting to do so leads to unnecessary complication or other compromises. A well-installed and properly-maintained through hull below the waterline is perfectly safe, after all.

[falcon]

Thank you all for your helpful input. All through hulls will have proper seacocks regardless of their waterline possition and I certainly appreciate the fact that changes often lead to unexpected consequences but it sounds like I should definitely move the bilge pump through hulls above the waterline

Pete

[The Froon]

Pete,

I am in the process of removing cockpit drain through hulls (originals to the boat) that did not have seacocks - they simply tubing running from the scupper to the threaded portion of the through hull and attached with a clamp. I will be coming back and drilling new holes, and installing new through hulls and proper seacocks. My cockpit drain through hulls are below the water line. As Tim stated:

On small boats, discharges often end up below the waterline because there is simply not a way to make the discharge work above the waterline

I have a Sea Sprite 23, and this certainly qualifies as a small boat. My sink through hull is directly below the sink (this one has a seacock), and is below the water line.

Check out the blog for visuals.

Regards,

Brian

[DaleH]

On many boats it is fairly common to plumb bilge pumps to drain to the self-draining cockpit. Reasoning that dirty water in the cockpit is far preferable to the risks of back-siphoning. Back-siphoning is why a "riser loop" should always be put in, well above the bilge pump outlet.

[Rachel]

On many boats it is fairly common to plumb bilge pumps to drain to the self-draining cockpit. Reasoning that dirty water in the cockpit is far preferable to the risks of back-siphoning.

Although with that arrangement I always worry about the cockpit drains becoming clogged somehow and causing the whole arrangement not to drain. Or needing all the cockpit drainage and bilge pump drainage I could get, at the same time...

Not saying there's never a time to use this arrangement - just that an independent overboard exit above the (dynamic if possible) waterline would make me happier if it were practical to do so.

Rachel

[Oscar]

Bilge pumps are a convenience under way (saves having to use a cup) and a somewhat backup device when unattended. If you spring a LEAK underway the average bilge pump setup will NOT save you unless you stop the leak FAST....unless you have one of these:



Having said that, most leaks stem from problems with below waterline through hulls.......

Thusly, a bilge pump with a below waterline thru hull could actually pose more of a risk than mitigate any danger of flooding. Hence the arrangement to have a through hull less or above the waterline discharge is a good idea.