Water heater sizing for 35 ft coastal cruiser

[JSSailem]

I'm the new owner of a 1974 Cal35Cruiser. I've spent the summer working on engine repair to long past non maintenance issues and now looking at the setup for hot water. I have a 1974 water heater that is beginning to drip. (Not observed on the survey). It is a 10 gal unit. I'm looking at replacing the unit, but unsure if I want to go with the same size or a smaller unit. System is plumbed to 110 AC or Heat exchange with engine coolant system. I plan to repeat this setup.

Question is: How big of a tank is needed?

For now we plan to cruise marinas and couple of day stops in remote bays. We have a 5 gal Solar shower to hang from the mast or boom and no problem plumbing in a sprayer to shower in the cockpit. While there is a shower plumbed in the head can not see fighting a wrap around shower curtain while spraying steamy water inside the boat.

Currently looking at units with 6 to 8 gallons of water. 110 AC 1200 watts / stainless heat exchanger / stainless tank.

I'm likely missing something so help me with your insight.

[Orion Jim]

It's up to you. Only you know how many people you'll have on board and what their habits are. I take a shower with two to three gallons but everyone is different. I would take into consideration what you can easily fit in the existing space and go from there.

[autumnbreeze27]

We just replaced a 18 gallon seaward that the tank failed With an isotherm 6.4 gallon. They aren't cheap, but I searched here and found they are engineered very well. Stainless tank instead of aluminum, a mixer so you can mix cold with hot to stretch it farther; it keeps water hot much longer; we r very happy with it.


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[roverhi]

Really depends on you, the Admiral where and when/where you'll be cruising. If you just wet down, wash up and rinse, a 5 gallon tank will do. If you are into luxuriating under hot water in the shower, 80 gallons should do you up fine. Have always done just fine with 5 gallons but haven't cruised higher latitudes where an actual shower might be nice. In warm weather, just wash down with salt water in the cockpit and rinse with a half gallon or less of water in a bucket.

Also your water capacity measured against your use will be a factor. Large tankage and/or a water maker and are willing to expend the carbon to make it work could make for much higher need for hot water. Know a cruiser who equipped his boat around his need to take constant showers. Large capacity water maker and a generator to power it was his biggest concern.

[Xthewater]

Originally Posted by JSSailem

I'm the new owner of a 1974 Cal35Cruiser. I've spent the summer working on engine repair to long past non maintenance issues and now looking at the setup for hot water. I have a 1974 water heater that is beginning to drip. (Not observed on the survey). It is a 10 gal unit. I'm looking at replacing the unit, but unsure if I want to go with the same size or a smaller unit. System is plumbed to 110 AC or Heat exchange with engine coolant system. I plan to repeat this setup.

Question is: How big of a tank is needed?

For now we plan to cruise marinas and couple of day stops in remote bays. We have a 5 gal Solar shower to hang from the mast or boom and no problem plumbing in a sprayer to shower in the cockpit. While there is a shower plumbed in the head can not see fighting a wrap around shower curtain while spraying steamy water inside the boat.

Currently looking at units with 6 to 8 gallons of water. 110 AC 1200 watts / stainless heat exchanger / stainless tank.

I'm likely missing something so help me with your insight.

Have/had a similar issue with our Raritan 6 gal. leaking so took it home and disassembled it. Found the tank to be sound, the sacrificial anode completely exhausted and a fitting corroded through. Very poorly insulated. There was quite a bit of scaling on the element and heat exchanger. It all has cleaned up very well and now looking like a new one. Replaced the relief valve and reinsulated it. Cost is about $100 for all materials, much better than the $1250+ for a new one. Haven't posted yet on our blog as I'm reinstalling next couple days.

Merely suggesting here to take yours apart and evaluate before running out for a new one.

[darylat8750]

I also would look into the cause of the leak. The overpressure relief valve started leaking on mine and it was an easy fix with a trip to the Lowes marine dept.

One thing I've noticed is that 8 gallons was plenty in the tropics because the bulk cold water was usually 80 degrees plus. When in the great lakes and the ambient water was much colder the second shower had to be short. I would be inclined to put in as will fit the space.

[roland stockham]

Also worth noting that the bigger the tank the longer it stays hot for. Unless it will really give you more useful space fit the biggest tank you can.

[Tayana42]

The bigger the tank the more energy it takes to heat it. And remember, in most set ups, if you have an 8 gallon tank and use 4 gallons of hot water you don't have 4 gallons of hot water left in the tank. You have 8 gallons of half hot and half cold water until you put more heat energy into the tank.

We have a 6 gallon tank and a sun shower and that works fine for us most of the time. At anchor for longer periods, not underway, we recharge our batteries with a Honda 2000i and heat water at the same time.


S/V B'Shert

[wrwakefield]

Originally Posted by JSSailem

I'm the new owner of a 1974 Cal35Cruiser. I've spent the summer working on engine repair to long past non maintenance issues and now looking at the setup for hot water. I have a 1974 water heater that is beginning to drip. (Not observed on the survey). It is a 10 gal unit. I'm looking at replacing the unit, but unsure if I want to go with the same size or a smaller unit. System is plumbed to 110 AC or Heat exchange with engine coolant system. I plan to repeat this setup.

Question is: How big of a tank is needed?

For now we plan to cruise marinas and couple of day stops in remote bays. We have a 5 gal Solar shower to hang from the mast or boom and no problem plumbing in a sprayer to shower in the cockpit. While there is a shower plumbed in the head can not see fighting a wrap around shower curtain while spraying steamy water inside the boat.

Currently looking at units with 6 to 8 gallons of water. 110 AC 1200 watts / stainless heat exchanger / stainless tank.

I'm likely missing something so help me with your insight.

JS- Congratulations on your new vessel.

Here is the link to some posts I consolidated on this topic on my blog.

My blog post has the content of those discussions, and contains links to the original posts on this forum.

It describes our reasoning for installing an IsoTemp Spa model, and the favorable results.

Have fun prepping for your journeys.

Cheers!

-Bill

[jreiter190]

You haven't missed anything. The six gal. Makes much better sense. No offense intended

[HopCar]

Take a look at the SPA series of water heaters by Isotherm. They have the same guts as the stainless Isotherms but with a plastic housing. The plastic housing keeps the price within reason. They also have a mixing valve. The valve allows you to set the temperature high, say 150 degrees, but the mixing valve adds cold water as it comes out of the tank and drops it down to a safe level to avoid scalding. This allows a small water heater to deliver more usable hot water than you would think from it's size.

Link: Isotherm SPA Water Heaters

[ReMetau]

I would think 6 gallon would be adequate, but you could also consider going with a propane instant heater and then use the space you save for something else. Where our old water heater was we now have 4 trojan batteries.

[Lepke]

An option that has worked for me, I have a house size water heater because I occasionally have several people on board. But I also have a 10 gallon near the galley sink where most of the hot water is used. It also can supply the showers if you want to wait for it. When I'm alone, I shut down the big one. Both are on a circulation system and can be heated by my diesel stove.

[JSSailem]

Sounds to me like the smaller size such as an 8 gal will work well on our boat. I like the idea shared about a mix valve. I will be looking for that feature. The engine heat exchange would produce water temps in the 175 range, well above the preferred 120 degree normal use temp. I am learning that cruising in the Puget Sound is a mix of sail and motor time. The little bit of prevention is a good idea.

Thanks all.

[AlwaysFORSAIL]

I have lived aboard in sw florida since 2005 with 3 gal heater plumbed as your boat is and have rarely run out of hot water. on those really cold days, less than 65 degrees, it doesn't take long for the eng to heat things up.