Through-hull or engine water supply for watermaker?

[ItDepends]

Originally Posted by mvweebles

Tellie offered a very general explanation without boundaries. Had he bracketed his explanation with caveat about size of thru-hull vs demand from engine and water maker, it would have made more sense and been actionable. This is all calculable.

From a fluid dynamics perspective, there is absolutely no difference between a thru-hull/manifold and a seachest. Either can have the cross sectional area to support multiple users, though seachests are usually designed for it. Thru-hulls are usually sized for a specific user so alteration may be required. Some guidance on design and sizing would have been helpful.

But to make a sweeping statement that drawing off the engine intake is always a bad idea and cannot be made to be acceptable is incorrect. No confirmation bias (except perhaps accepting Tellies input at face value and assuming all others are wrong).

This is exactly correct. Can you do it? IT DEPENDS. In most cases, NO you can not just tee off an existing engine thru-hull because it will be too small. Most boat builders make them marginally too small for the engine by itself, let alone other users. But the blanket statement that the engine and other raw water users can NEVER share a thru-hull is flat out WRONG. Anybody familiar with the design of larger boat systems knows this (or should!).

It takes proper design, but it can be done. On our boat we have a single thru-hull that serves the engine, the generator, two air conditioners, and the water maker. It not only works on our boat, it works on more than 500 of the same or similar models by this builder.

What makes the design right? The thruhull and sea-strainer need to be sized for the maximum expected water flow. Yes, this can be done. You also need to be sure that reverse flow through any of the circuits can not happen. This can also be done.

Now, if I was a watermaker manufacturer, I would tell you that a dedicated thru-hull is REQUIRED, or your warrantee is void. Why? Because the average boat owner can’t do the engineering needed to build a proper seachest, and I don’t want to waste my technical support resources with the avoidable problems that occur through ignorance.

[SOLAR SUPPORT]

No throughull, No engine cooler line T link.

https://youtu.be/PJI1daty-vA

[nofacey]

cooling water">Engine cooling water
Watermaker
Wash down pump
Air conditioner
Genset cooling

That’s 5 thruhulls if we follow the purist regime - I installed one 1-1/2 seacock in the deepest possible hull location, coarse screen outside, beautiful big bronze strainer inside, manifolded everything off, no check valves or selector switches (did have full bore shut offs on each feed except main engine). Ran in every conceivable combination over 5+ years, zero flow issues. Yes you need to look at all flows….but a reasoned approach usually offers options.

[mcon12000]

This has been a helpful thread for me since I’m installing a water maker this summer.

What I’ve gathered is sharing an engine cooling thru hull with the water maker can create a situation where the water maker membrane is pumped dry.

My AC and Engine thru hulls are well below the waterline (next to keel) and my plan is to use the AC intake for my water maker. The thought is I never use my AC cruising it’s only used in the marina and I can easily remember to turn off/on the the correct combination for the task. Currently I’m already in the habit of closing and opening that thru hull anyway so essentially it’s (AC Intake) always off in any situation where I’d be making water. Might make it idiot proof with a three way valve rated for below the waterline use?

[SV THIRD DAY]

What sounds good in planning often results in too much of a PITA in practice.
There are ALREADY so many things to do and remember to do on a Cruising Boat, the simpler you can make your boats systems the better. Unforeseen problems also make once "good plans" look crazy. Take the one guy who shared his water maker through hull with his engine raw water cooling through hull. "Bob" we will call him to protect the innocent, was making water from his shared engine/water maker through hull while at anchor in La Pa, Mexico. Relaxing enjoying the day. As he watch a vessel approaching him with a failed steering system, leaving the channel and plowing into boats in the anchorage. He quickly put down his margarita and started his motor to get out of the runaway vessels path. He succeeded in motoring forward on his anchor but as the chaos was over he heard the sound of his engine High Temp alarm sounding. Oh Crap...his water maker running off the generator has starved the flow from his engine cooling water. His raw water impeller was shredded with the impeller blades of course getting lodged in his engine heat exchanger. Everything works....until it doesn't on a Cruising boat. Your mindset during gear installs has to be "How can this Screw me? And how can I keep it from happening?"

[mvweebles]

The design of a raw water manifold isn't overly complicated. Sum of cross sectional area of all consumers (3.14 * r^2 - a 1/2" inlet has cross section of 0.2 sq in, 3/4" inlet is 0.44 sq in, 1" is 0.8 sq in, etc). Sgould be less than the cross sectional area of the inlet (1-1/4" thru hull has area of 1.2 sq in, 1-1/2" thru-hull has area of 1.8 sq in, a 2" thru hull has area of 3.1 sq in).

But it is likely you will need a larger thru hull. I installed a 2-inch thru hull and sea strainer and have my engine intake to my Perkins 4.236 (1"), an AC inlet (3/4") and my washdown pump (also 3/4"). Sum of my consumers is around 1.7 sq in, supply is 3.1 sq in. Zero contention. I could have gotten away with a 1-1/2" thru hull, but would have been very close. Saving grace is the washdown pump is intermittent only.

In my mind nothing wrong with this. I did it not to reduce holes in my boat - I use bronze seacocks which are incredibly strong and reliable. But I wanted to centralize valves, pumps, and sea strainers in an accessible location where they can be easily serviced.

I do agree that blithely tapping into a line for raw water is a recipe for disaster but this isn't rocket science either.

[mcon12000]

Originally Posted by mcon12000

This has been a helpful thread for me since I’m installing a water maker this summer.

What I’ve gathered is sharing an engine cooling thru hull with the water maker can create a situation where the water maker membrane is pumped dry.

My AC and Engine thru hulls are well below the waterline (next to keel) and my plan is to use the AC intake for my water maker. The thought is I never use my AC cruising it’s only used in the marina and I can easily remember to turn off/on the the correct combination for the task. Currently I’m already in the habit of closing and opening that thru hull anyway so essentially it’s (AC Intake) always off in any situation where I’d be making water. Might make it idiot proof with a three way valve rated for below the waterline use?

To be clearer using a manifold on this single thru hull is the plan. They would never operate at the same time on my vessel.

[Cynara]

I would do another through hull. Then you can T off of that an add a hand/foot pump to provide salt water at your sink

[mvweebles]

Originally Posted by mcon12000

To be clearer using a manifold on this single thru hull is the plan. They would never operate at the same time on my vessel.

SV THIRD DAY point was a good one. "It will never happen" is not always a finite.

You've clearly made a decision that works for you, but for others, to my thinking, there are two acceptable ways to do this. Punch a new hole for a new thru-hull. Or an adequatly sized manifold that accounts for worst case scenario - and even this, although proper in my mind, will run risk of voiding warranties and getting flagged at survey. Everything else is a compromise.

[YANDINA]

Don't rely on the impeller of the water pump providing a seal when the engine is not running.

If both were running and you then turn off the main motor, hot water will suck back from the motor and destroy your membrane.

At one stage (25 years ago) I had tried the plastic "run dry" water pump impeller. It lasted one run and melted from the hot water flowing back after then engine stopped.

Our watermaker with dedicated through hull would pick up bubbles while underway that would accumulate in the top of the oil/water separator. I solved the problem with a bubble extractor that would automatically vent the air out whenever the watermaker was off. [URL="http://www.yandina.com/BubbleOut.htm"]

[sv Stella Maris]

Years of making desal and installing a few systems, I'm looking to T off my engine thru hull. Goal is to eliminate a thru-hull and ensure all ball valves are exercised regularly ( I make water more than I run the engine, we sail even onto anchor at times).

Sharing thru hulls can be dangerous, know your raw water demands and respect them. My engine and water maker are NEVER on at the same time so I'm safe there. And we don't make water underway; if necessary we will slow to a crawl to get the tanks to a safe level, but risk of damaging the membranes are our cruising sail speed of 8kts+ is too risky...

Good Luck and be bold !