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Old 08-03-2015, 11:16   #1
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Shared Thru Hull and Strainer

Can the auxiliary engine and generator happily share a single thru hull and water strainer?
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Old 08-03-2015, 11:28   #2
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Re: Shared Thru Hull and Strainer

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Originally Posted by TxCoastSailor View Post
Can the auxiliary engine and generator happily share a single thru hull and water strainer?
This topic has been well covered before. The conclusion in general is 'not a great idea' IF: It is possible to starve the engine cooling system if you run the generator or other users under way. You also may want to isolate the engine from the other boat systems while you work on the engine. You may want to isolate the boat systems from the engine while you work on them. You would not want to have to shut down the engine to repair a leak in a boat utility.

Those concepts can be handled by installing a very large single intake and a valved, branching system. This however, increases the length of the intake and adds restrictions to the engine.
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Old 08-03-2015, 11:49   #3
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Re: Shared Thru Hull and Strainer

**No.**
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Old 08-03-2015, 12:03   #4
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Re: Shared Thru Hull and Strainer

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Those concepts can be handled by installing a very large single intake and a valved, branching system. This however, increases the length of the intake and adds restrictions to the engine.
AKA sea chest.

But do you ever run the main engine and genset at the same time? I don't. I have separate through-hulls and strainers, as the genset was installed in the original build, but I don't see a thing wrong with using a single through hull for both so long as you never run them at the same time. I literally never do, because I have a 2.5kW alternator on the main engine plus inverter, so as long as the main engine is running, I have abundant power and there would be no reason to start up the genset.

This would be otherwise if you have one of those boats where there's a hydraulic pump on a PTO on the generator, used for very large bowthrusters, windlasses, etc., but my boat doesn't have that.
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Old 08-03-2015, 21:02   #5
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Re: Shared Thru Hull and Strainer

OK, let's put some other options on the table.

I want to add a water maker and don't want to punch another hole in the hull. So three thru hulls and four thirsty machines ... engine, generator, air conditioner water pump and water maker. Who should share an inlet?
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Old 08-03-2015, 21:11   #6
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Re: Shared Thru Hull and Strainer

None.
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Old 08-03-2015, 21:14   #7
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Re: Shared Thru Hull and Strainer

We have a Sea-Chest, just like most ships. Advantages are one seacock for the whole boat. Engine, Generator, Domestic Seawater system and fire hydrant pump. There is a strainer, a transparent lid, and every system has it's own valve from the chest.
Disatvantage is that if that breaks off there is a 4" hole in the hull, but there is a 4" bung nearby.
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Old 08-03-2015, 22:02   #8
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Re: Shared Thru Hull and Strainer

It is all very well for Terra Nova to say nope, never, nix, nada etc, but the caviat is how big is the thru-hull and what are the actual volume demands. My two inch thru-hull supplies all my needs for generator, main, watermakers and heads/galley. On my last boat I did the same for 30/ years and never had an issue even with its tiny 1 1/4" valve. It is possible to end up with an inadvertent venturi effect where flow is not always directed to where you want it, but given a large enough opening it is just straight forward math. I agree that in certain installations you could get caught out with everything down in a plugged sea strainer scenario, but I suck my seawater from either side of my keel so the likelihood of that happening are slim indeed.
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Old 08-03-2015, 22:09   #9
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Re: Shared Thru Hull and Strainer

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Originally Posted by TxCoastSailor View Post
OK, let's put some other options on the table.

I want to add a water maker and don't want to punch another hole in the hull. So three thru hulls and four thirsty machines ... engine, generator, air conditioner water pump and water maker. Who should share an inlet?
Dissenting viewport.

I don't consider the generator, AC water pump and water maker to be critical systems I.e. it shouldn't be an issue if any or all of them is down for an hour or two. Therefore let them share a single seacock; just make sure it is a decently sized one.

I often see people state that every item should have their own inlet and then say it's OK to just to have one single big one (sea chest). Yep, I know isn't quite apples and oranges but the concept is the same
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Old 08-03-2015, 22:11   #10
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Re: Shared Thru Hull and Strainer

Mirar types faster
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Old 08-03-2015, 22:39   #11
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Re: Shared Thru Hull and Strainer

It mostly depends on where the manifold is and the back pressure (suction). If the manifold is well below the water line it can work so long as the inlet and strainer are large enough. We have 4 water circuits fed by one big through hull and strainer.

One thing to watch out for is air conditioner and fridge cooling circuits. If there is too much negative pressure at the manifold because the through hull is small or strainer is restricted it may allow air to back flow into the system. This will greatly reduce the cooling effectiveness and can lead to some major engine damage.
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Old 08-03-2015, 23:46   #12
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Re: Shared Thru Hull and Strainer

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...I don't consider the generator, AC water pump and water maker to be critical systems...
That's no reason to try to kill them prematurely or reduce their efficiency unnecessarily.

And the idea that fewer larger thru-hulls is safer than more smaller thru-hulls doesn't pencil out. If a thru-hull were to fail and allow the sea to enter, would you rather it be a large one or a small one?

A true sea chest, well conceived and put together, is another story.
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Old 09-03-2015, 18:03   #13
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Re: Shared Thru Hull and Strainer

Thanks to all for your comments and insights.

I am leaning toward having the AC and the water maker share a thru hull. Don't think I will have to run them at the same time.

An alternative is to use the sea water inlet for the head (which we converted to fresh water) for one or the other but I'm not sure I can get a hose from the head inlet to the AC pump or the water maker.

Again thanks for your replies.

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Old 09-03-2015, 18:27   #14
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Re: Shared Thru Hull and Strainer

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...Don't think I will have to run them at the same time...
Don't be fooled thinking you'll be safe by running one at a time. Some equipment will suck the prime out of a shared load, for example.
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Old 09-03-2015, 21:04   #15
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Re: Shared Thru Hull and Strainer

TN,

Thanks for the reminder. I believe the AC pump is positive displacement type and should be self priming. Don't have the water maker yet but I will check into that. The thru hull used by the AC pump is quite far below the water line and that should help mitigate this issue.

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