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Old 30-01-2015, 08:02   #1
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Galley/Head sink drain hose reduction before thru-hull?

My boat's galley and head sink drains both reduce to a smaller hose and thru-hull than the drain itself. I am replacing all of the thru-hulls and seackocks, and am kind of stumped on what size to install for these drains.

The galley sink reduces from a 1-1/2" drain to something smaller (3/4" to 1", not sure which) hose and thru-hull. Similar for the head sink, reduces from a 1" drain to a 1/2" or 3/4" hose and thru-hull.

Is this done for a particularly good reason? Wouldn't it make sense to replace the drain thru-hulls and seacocks with the same size as the drains?
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Old 30-01-2015, 08:56   #2
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Re: Galley/Head sink drain hose reduction before thru-hull?

My two cents. My galley and head drain using a 3/4 hose to a 3/4 seacock. I haven't once seen an issue suggesting that this is an inadequate amount of flow.

A 3/4 seacock is, what, half the price of a 1 1/2? That's probably the reason for what you are seeing; saving money.

If it isn't causing problems, spend the money elsewhere. Changing the seacock is no joke.


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Old 30-01-2015, 08:58   #3
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Re: Galley/Head sink drain hose reduction before thru-hull?

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Originally Posted by fallingeggs View Post
My two cents. My galley and head drain using a 3/4 hose to a 3/4 seacock. I haven't once seen an issue suggesting that this is an inadequate amount of flow.

A 3/4 seacock is, what, half the price of a 1 1/2? That's probably the reason for what you are seeing; saving money.

If it isn't causing problems, spend the money elsewhere. Changing the seacock is no joke.
The original seacocks and thru hulls are out and pretty much destroyed (see: Removing Corroded Seacocks and Thru-Hulls)

, so now the only issue is which size to replace it with. I'm not worried about the cost, I'm just curious if the reduced thru-hull size is for a good reason (safety?), or if I should open it up to the drain size for better drainage.
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Old 30-01-2015, 09:10   #4
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Re: Galley/Head sink drain hose reduction before thru-hull?

It won't make much difference as long as the bottom of the sinks are high enough above the waterline.

Galley Sink drain assembly with plastic elbow
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Old 30-01-2015, 09:51   #5
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Re: Galley/Head sink drain hose reduction before thru-hull?

I'd replace the sea cock with the larger size.
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Old 30-01-2015, 11:59   #6
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Re: Galley/Head sink drain hose reduction before thru-hull?

Quote:
Originally Posted by Beersmith View Post
My boat's galley and head sink drains both reduce to a smaller hose and thru-hull than the drain itself. I am replacing all of the thru-hulls and seackocks, and am kind of stumped on what size to install for these drains.

The galley sink reduces from a 1-1/2" drain to something smaller (3/4" to 1", not sure which) hose and thru-hull. Similar for the head sink, reduces from a 1" drain to a 1/2" or 3/4" hose and thru-hull.

Is this done for a particularly good reason? Wouldn't it make sense to replace the drain thru-hulls and seacocks with the same size as the drains?
There's no reason except cost to reduce the size of the drain hose and thru hull. If you're replacing them anyway, upsizing would not cost much and that is what I would do. You're less likely to have a problem with clogging from food waste with a larger hose and thru hull.
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Old 31-01-2015, 09:52   #7
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Re: Galley/Head sink drain hose reduction before thru-hull?

Slightly off topic: whilst in El Cid, Mexico, a 50 odd foot Benetau came racing in to harbor, whilst pounding off the coast water sea had started blowing back out of the (forward ?) head sink(s).
To stop this they had shut off the valve to the sinks. Next thing they knew was the boat was filling with water because the pounding had blown this valve off and water was pouring in. With pumps running flat out they raced for harbour.
It raises several points about having an actual seacock on the thru hull and maybe it's better to take a few splashes in thru the sink to absorb the shock of pounding.
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Old 31-01-2015, 10:01   #8
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Re: Galley/Head sink drain hose reduction before thru-hull?

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Originally Posted by Beersmith View Post
The original seacocks and thru hulls are out and pretty much destroyed (see: Removing Corroded Seacocks and Thru-Hulls)

, so now the only issue is which size to replace it with. I'm not worried about the cost, I'm just curious if the reduced thru-hull size is for a good reason (safety?), or if I should open it up to the drain size for better drainage.
are out and pretty much destroyed

Since cost isn't an issue:

1. The smaller ones will still work

2. Bigger wouldn't hurt

Your boat, your choice.

Good luck.
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Old 31-01-2015, 10:05   #9
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Re: Galley/Head sink drain hose reduction before thru-hull?

Mine are also 3/4" with no issues on a 30 year old boat heavily used/ often lived on years at a time 46' sailboat. Replaced through-hulls once 15 years ago with same.). So I wouldn't worry about clogging/ drain speed... never had an issue. Just some things I considered that led to my decision to stay with 3:4":
- a lot less water would come into the boat if a 3/4" line failed than 1.5"
- if you/ new owner ever find themselves up north and have to winterize in-water, it's a lot easier dealing with 3/4" hose. And if you ever had a full 1.5" hose failure from freezing/ hose clamp failure, not sure bilge pump could keep up if owner not on boat
- while we should all exercise all our hull valves once a month/ ever several months, most of us/ future owners end up having regrettable lapses in this important practice. Getting a 3/4" stuck valve working again is significantly easier than a 1.5
- in rough water with pitching boat the momentum of column of water in a 3/4" hose vs 1.5" hose is significant and the 1.5 may support salt water back flow back up into the sink/ splash out. Many of us have a fwd head sink that we have to close hull valve to keep this from happening. I have forgotten a few times and soaked all the fwd carpet. I'd hate to think of the extra soaking I'd have with 1.5!

All of these, while admittedly very remote, out weighed any benefit I could really could point to of increasing to 1.5"... so I stayed with the boats original 3/4". Never regretted the decision.


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Old 31-01-2015, 10:19   #10
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Re: Galley/Head sink drain hose reduction before thru-hull?

Another reason to go with a larger thru-hull is that it is less likely to get partially or fully clogged with growth.
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Old 31-01-2015, 10:22   #11
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Re: Galley/Head sink drain hose reduction before thru-hull?

I see no need for 1.5 on sink drains. I might go 1" or so if the adaptors are readily available.... as 3/4 necks down pretty small inside a bronze barb! Probably 1/2" or so.
The other thing to watch for when trying to "improve" this situation is not all valves are the same inside diameter. I've seen ball valves that are a lot smaller thru hole in the center than others... so pay attention to what you are buying!
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