Any Reason Not To Use PEX On A Boat?

[Boatguy30]

I wouldn't use any of the fittings below the waterline with raw water. No builders I'm aware of use it for that.

[SSgtPitt]

As a General Contractor, building and working on well over several thousand homes and now specializing in remodeling PEX is what we use. Fantastic product and if it freezes it expands and doesn't burst. Sharkbites on the otherhand are used only for temporary or exposed repairs. We don't bury them in walls or floors. They're fantastic on a limited basis and we buy them by the sack full but even our plumbers will not consider them as an in-wall product.
Even after all my experience, repiping my boat in Pex never occured to me Ahhh, more projects. Satan hates busy hands.

[Arthur Garfield]

I too sing the praises of PEX. One thing missing is the anti-microbial action of copper piping.Important to keep in mind.


All the Best

[HughMcB]

Originally Posted by seahag

Also, I already have a 4 GPM pump and would like to locate it near the thru hull, which may be about 5-6 feet lower than the outlet; any worries about pumping ability?

Don't suppose you happen to have a little diagram of the proposed system set up? Or even a little more written detail?

The pump needs to overcome the head loss of the pipe system. That needs to be calculated (or roughly calculated anyway). You could have a 100gpm pump that won't do anything if it can't overcome the friction losses.

[sailorchic34]

Originally Posted by Boatguy30

I wouldn't use any of the fittings below the waterline with raw water. No builders I'm aware of use it for that.

A very good point. I missed that it was for raw water washdown first time through. Brass pex fittings would tend to dezinc in salt water. Oddly for fresh water only it would be OK.

[seahag]

Wow so much great info-thanks everyone! Yeah same here, I had been a general contractor for years, and then a remodeler and have a lot of experience in home plumbing but never thought of pex for the boat until today; been looking at the PO's crazy plumbing for so long now that it never seemed to occur to me I guess.

As for the proposed new system, I have some options. One would be to take a new line right off the thru hull by teeing the existing plastic stuff, and starting the pex right after placing the pump right there, then running laterally about six feet and going straight up through a locker to the deck. Option two would be to put this pump, which would be the third pump under the galley sink, which is about a ten foot run with some bends in after the thru hull, and then about the same situation afterward as plan A-six feet laterally then six feet up. This might be easier to route, but having an even bigger cluster of plumbing under the sink than is already there.

Should I just use 1/2" pex (which I have the most supplies on hand for) to supply the pump and wash down hose? Seems adequate for 4 gpm to me.

Thanks again for the advice.


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[seahag]

Oh, just noticed the posts about not using below waterline with raw water. That may throw off the plan if it's that serious. I would rather not use Fresh water for this application but it isn't out of the question as it's a 100 gallon tank and fresh water is pretty easy to come by here, though on extended anchorages I would have to bring it in on the dingy with jerry cans or catch rain water when available--typically twice a season these trips happen. It would still be below waterline but with fresh water in that case.


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[jackheape]

I have used PEX on several homes and just re-plumbed my Hunter 34 with it. Its great stuff. Bought the crimping tool online at Amazon and the PEX and fittings at Lowes. Worked great. Also it doesn't burst when it freezes, which is a big plus.

[IRG]

What's wrong with PEX below the waterline? As long as none of the fittings are likely to be immersed in salt water I can't see a problem.

Originally Posted by seahag

Oh, just noticed the posts about not using below waterline with raw water. That may throw off the plan if it's that serious. I would rather not use Fresh water for this application but it isn't out of the question as it's a 100 gallon tank and fresh water is pretty easy to come by here, though on extended anchorages I would have to bring it in on the dingy with jerry cans or catch rain water when available--typically twice a season these trips happen. It would still be below waterline but with fresh water in that case.


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[sailorchic34]

Originally Posted by IRG

What's wrong with PEX below the waterline? As long as none of the fittings are likely to be immersed in salt water I can't see a problem.

He's talking about a wash down system that uses seawater to rinse the chain as it comes up. PEX itself is fine. The brass fittings in seawater not so much. They are high zinc content and would suffer from dezincing over a year or three.

[s/v michaela]

Use "Whale" lines and the fittings are available at the local marine chandler...Home depot isn't the place to shop for marine equipment.

[HughMcB]

Originally Posted by seahag

One would be to take a new line right off the thru hull by teeing the existing plastic stuff, and starting the pex right after placing the pump right there, then running laterally about six feet and going straight up through a locker to the deck. Option two would be to put this pump, which would be the third pump under the galley sink, which is about a ten foot run with some bends in after the thru hull, and then about the same situation afterward as plan A-six feet laterally then six feet up. This might be easier to route, but having an even bigger cluster of plumbing under the sink than is already there. Should I just use 1/2" pex (which I have the most supplies on hand for) to supply the pump and wash down hose? Seems adequate for 4 gpm to me.

4 gpm through 1/2" PEX will give you a velocity of about 6ft/min.

I think recommended for PEX piping is somewhere around 10ft/min so you should be good there.

The next is working out the head pressure required for the pump.

A simplified way to calculate this is;
Head = Vertical Lift + Friction loss of total length of pipe (+ Fixture Losses if not already included in the friction loss)

Vertical lift is simply difference in inlet to outlet height.

Friction loss needs to be looked up on a table (as it's a function of GPM, velocity and pipe material). In this case it's about 0.014 ft per foot of pipe.

Fixture losses can just be assumed to be a percentage of the total pipe length, assume 20% for PEX.

You can use this online tool;
Total Dynamic Head Calculator (TDH)

Option A is about 10.5 ft (6ft lift, 12ft length)
Option B is about 12.7 ft (6ft lift, 18ft length)

I don't know if this tool allows for fixture losses, so you could add 20% to be safe.

Option A -> 12.6ft
Option B -> 15.2ft

So you need a pump capable of pumping 4 gpm @ 12.6 (or 15.2) ft of head.

Btw, "feet" in this case is actually a measurement a pressure (thanks imperial system!) and 1ft = 0.433 PSI

So if you need PSI it's 4 gpm @ 5.5 (or 6.6) PSI

Believe it or not this is actually the "simplified" approach.

Lookup the model number of your pump and you should find what head or PSI it is capable of producing.

[HughMcB]

This guide also goes through the steps, but I can see some gaps in the explanation. Still useful to read for a novice though.

[Mikado]

Originally Posted by HughMcB

4 gpm through 1/2" PEX will give you a velocity of about 6ft/min.

I think recommended for PEX piping is somewhere around 10ft/min so you should be good there.

The next is working out the head pressure required for the pump.

A simplified way to calculate this is;
Head = Vertical Lift + Friction loss of total length of pipe (+ Fixture Losses if not already included in the friction loss)

Vertical lift is simply difference in inlet to outlet height.

Friction loss needs to be looked up on a table (as it's a function of GPM, velocity and pipe material). In this case it's about 0.014 ft per foot of pipe.

Fixture losses can just be assumed to be a percentage of the total pipe length, assume 20% for PEX.

You can use this online tool;
Total Dynamic Head Calculator (TDH)

Option A is about 10.5 ft (6ft lift, 12ft length)
Option B is about 12.7 ft (6ft lift, 18ft length)

I don't know if this tool allows for fixture losses, so you could add 20% to be safe.

Option A -> 12.6ft
Option B -> 15.2ft

So you need a pump capable of pumping 4 gpm @ 12.6 (or 15.2) ft of head.

Btw, "feet" in this case is actually a measurement a pressure (thanks imperial system!) and 1ft = 0.433 PSI

So if you need PSI it's 4 gpm @ 5.5 (or 6.6) PSI

Believe it or not this is actually the "simplified" approach.

Lookup the model number of your pump and you should find what head or PSI it is capable of producing.

I'm not this smart, so I just experimented a little before I chose what size pex to go with. I started with 1/2" and a 20' run to the shower. I wasn't really happy with the pressure, so I tried 3/8". Much higher pressure and the flow rate seemed acceptable, so I went with 3/8". My first properly routed run was to the galley sink. After running the sink (cold water only) I was very happy with the pressure and volume so final decision was to go with 3/8".

[HughMcB]

Originally Posted by Mikado

I'm not this smart, so I just experimented a little before I chose what size pex to go with. I started with 1/2" and a 20' run to the shower. I wasn't really happy with the pressure, so I tried 3/8". Much higher pressure and the flow rate seemed acceptable, so I went with 3/8". My first properly routed run was to the galley sink. After running the sink (cold water only) I was very happy with the pressure and volume so final decision was to go with 3/8".

Well the trial and error method will always yield good results, provided you don't mind paying for those trials.