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Old 22-06-2024, 08:24   #1
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replace failed water pump - what size?

I have to replace a failed fresh water pump on a new-to-me boat. I could replace with similar but I am curious as to how one calculates the optimum capacity pump for this...? It's a 34 foot boat with galley and lav taps and a 6 gallon hot water tank. Thx.
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Old 22-06-2024, 08:35   #2
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Re: replace failed water pump - what size?

Not really an "optimum" size IMO other than "it keeps up" or doesn't. If you what a pump that just does what it needs and no more you could consider a variable flow. If you weren't having any past problems make life easy and replace with the same for fit consideration.
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Old 22-06-2024, 09:11   #3
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Re: replace failed water pump - what size?

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Originally Posted by sailorboy1 View Post
Not really an "optimum" size IMO other than "it keeps up" or doesn't. If you what a pump that just does what it needs and no more you could consider a variable flow. If you weren't having any past problems make life easy and replace with the same for fit consideration.
I have no idea how it functioned previously...it was U/S when I bought the boat.
Most likely the 4GPH / 40 PSI exact replacement pump is probably the right solution.
But still wondering how you actually figure this out...
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Old 22-06-2024, 09:12   #4
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Re: replace failed water pump - what size?

What is the capacity of the old pump? Was it weak flow, okay, too much? If it was okay at the old rate just go with that. I cant think of any calculation really. Either the old flow rate was good enough, or it wasn't. Adjust accordingly and dont over think it.

Seeing your above post- 4-5 GPH is pretty typical for average size boats.
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Old 22-06-2024, 09:31   #5
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Re: replace failed water pump - what size?

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Originally Posted by Rol1964 View Post
I have no idea how it functioned previously...it was U/S when I bought the boat.
Most likely the 4GPH / 40 PSI exact replacement pump is probably the right solution.
But still wondering how you actually figure this out...
I had a 3 gpM pump on my 41' boat and it never had a problem
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Old 22-06-2024, 10:13   #6
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Re: replace failed water pump - what size?

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Originally Posted by Rol1964 View Post
I have no idea how it functioned previously...it was U/S when I bought the boat.
Most likely the 4GPH / 40 PSI exact replacement pump is probably the right solution.
But still wondering how you actually figure this out...
Normally the value used is 1.5 to 2 GPM (3 to 4 l/min) for each tap in use. For most boats of ‘reasonable” size, you assume you might need to supply two taps at once, no matter how many are installed. If you are a solo sailor, obviously a single tap would be in use at a time. It is not an exact science.
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Old 22-06-2024, 10:30   #7
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Re: replace failed water pump - what size?

Since people want hard numbers, I looked up US housing code in various states, whatever randomly came up. Looks like 1.5-3.0 GPM is about par for the course. So for 1 or 2 fixtures running water at the same time a 5 GPM pump should be fine.
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Old 22-06-2024, 10:41   #8
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Re: replace failed water pump - what size?

you're not talking about an engine water pump.
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Old 22-06-2024, 14:06   #9
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Re: replace failed water pump - what size?

Quote:
Originally Posted by CrispyCringle View Post
Since people want hard numbers, I looked up US housing code in various states, whatever randomly came up. Looks like 1.5-3.0 GPM is about par for the course. So for 1 or 2 fixtures running water at the same time a 5 GPM pump should be fine.
Sure would be nice if we could edit our own posts…..

What I should have said was “I was looking up housing codes for minimum flow rates for sinks and showers….etc….
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Old 22-06-2024, 15:24   #10
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Re: replace failed water pump - what size?

As has been noted, it is normally based on how many fixtures you have and might be used at the same time. If you don't have an accumulator, consider one of the pumps with a high flow bypass. It reduces the flow under low demand situations and greatly reduces the sudden on/off vibration and noise.
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