Reducing the number of through-hulls

[Zanshin]

I might soon be in a position where I would be getting a new boat and where I can make a number of decisions regarding commissioning.
My last two boats have, once out of the water, had so many through-hulls below the water line that they look like Swiss Cheese.

I would like to have one large through hull leading to a small below-waterline distribution tank or device that would then feed water-hungry devices such as engine and genset and shaft, watermaker, heads, kitchen and air conditioning instead of having separate through-hulls for each of those system intakes. I imagine that one master through-hull shutoff and each device hose from the collector with a shutoff should be functionally equivalent to the usual method.

Does a manufacturer (or name) exist for a such a tank? I can't be the first person to dislike all those hard-to-get-at-and-check through-hulls so perhaps there is a good reason for continuing to have a dedicated through-hull for each system (I can't think of one right now).

[FarCry]

The only negative I can see to your one big thru hull would be a potential siphoning problem. Think about your genst running with your engine off. What prevents the genset from sucking the raw water out of your engine and potentially sucking air instead of trying to pull the water up from below the boat? Or vice versa. I don't know if it could actually happen but it seems possible. I suppose back flow preventer could be installed on every line but that leads to it's own set of potential problems.

It would also seem problematic on a larger vessel to have just one centrally located thru hull for everything because of sheer distance. You don't want your raw water too far from engine or genset. That would be a long run for forward head. Speaking of heads. Are you intending to have a second large thru hull for discharge all heads, sinks and shower drains etc?

You also have thru hulls for knot meters, some transduces etc.

Off topic-----Do you need a set of dishes to go with either vessel? Next time you are in the area I would make you a really good deal.

[Jeff_H]

You are describing a 'sea chest' and it is used all the time on big ships. The are a number of constraints to a sea chest:

-They can only be used for intake water.
-They need to have a clean-out.
-Ideally the water enters the sea chest through a water strainer.
-The intake needs to be sized big enough for all of the loads simultaneously requiring one very big hole in the boat
-You need some way to know when something goes wrong since anything that needs intake water will be out of commission when the sea chest is blocked (for example you can't run your genset or your main engine if you need to recharge the batteries.)

There are sometimes redundant through-hull/ seacocks supplying the sea chest.

Jeff

[olson34]

Please do think that I am being a wise__s, but have you considered just specifying that some of those thru hulls be above the normal waterline? And some might be above the sailing waterline.
I realize that many builders consider obvious thruhulls a visual/advertising photo distraction... but this may have been over-thought by the marketing folks.

Our design has all of the cockpit drain and bilge pump outlets above the DWL by about a foot. Even under sail they are mostly clear. I kind of prefer this "form follows function" design philosophy, FWIW.

I would note that lots of powerboats have up to ten thruhulls right at the waterline, and that's kind a worrisome too.

Congrats on getting a new boat, BTW.


I tried to add a small picture of our transom, but the site software will not allow an attachment today. ??

L

[Jeff_H]

I am a big fan of cockpit drains through the transom and above the waterline if the cockpit sole is high enough. I also agree about getting discharges above the waterline, especially items like bilge pumps (which I currently pipe into my cockpit drain).

Jeff

[mitiempo]

Bilge pump outlets should always be above the waterline, ideally centred on the transom so they do not submerge when heeling. Cockpit drains should be above the waterlline if possible, but this can't always be done on smaller boats. Intakes have to be below the waterline at all times and placing them close to their function (head intake etc) makes sense. I would make sure they are all proper through bolted seacocks with fiberglass backing plates (not wood) and maintain them by exercising them once a month. I would also use high quality hoses and double clamp each connection.
What you are describing is also called a standpipe and is seen on aluminum & steel yachts as well as commercial vessels.

[sailingdog]

You need to make a distinction between INTAKE through-hulls, and DISCHARGE through-hulls. The intake through-hulls can often be ganged together using a manifold of a sort. Jeff_H's description of a manifold/seachest is excellent, and covers the requirements of a well-designed one. The discharge ones usually can not be ganged.

[oceanscapt]

Most, if not all, of the powerboats I work on have sea chests. They do a good job of reducing the intake thru hulls. However there are concerns.

The sea chest and thru hull must be properly sized to exceed the demands on the system and that generally means worse case (i.e. everything running whether it does or not). That can mean a large thru hull, strainer, valving and chest.

Most effective sea chests have a clear top so that you can "see" the sea water flow or look for constrictions.

If the raw water strainer needs cleaning, you either have to shut down the entire system if you have one intake, or change valving if you're port and starboard.

Then there's the problem of running hoses containing sea water from the chest to the parts of the boat (head and sink for example). A leaking hose can do considerable damage to the vessel as well as put it in harms way.

Most of the power boats I'm familiar with have the sea chest in the engine room and since 99% of the raw water's used there, long runs and leaks are generally confined to that area. We have port and starboard as well as upper and lower intakes and valving. Since the boats use fresh water for toilets and don't have salt water available at the sinks, the needs are somewhat diminished.

I know of a few sailboats (mostly home made) that get by with port and starboard lower intakes, but they have dual filters and valving.

If you were to go with a single raw water thru hull, then you might save a couple thru hulls and shutoff valves. I don't know how that would affect the failure analysis numbers compared to adding salt water filled hoses to the head(s) and sink(s).

It might be worth considering a somewhat larger intake with, a "Y" connection. One part of the "Y" would go to the engine and the other maybe the sink as the galley tends to be close to the engine room. My only concern with that is if the engine draws a lot of water, you may find that the sink pump doesn't provide enough water or pull or worse, the sink takes too much water away from the engine.

Finally, the best designed sea chests have an access above the water line. This means we can keep the systems running, remove the top, and still get things done. That can be a tough problem on sailboats where space is a premium and heel considerably more than most powerboats.

[gtod25]

Sea Chest on a Whitby 42

[sailingdog]

GTod—

I don't think you have enough wingnuts on that.