MacGregorSailors.com

Do you think that would work? I was looking into Webasto AC/Heat units. Looks like a great idea, but they require a sea water pump to operate. Normally you take water from under the hull, and discharge back out. I was thinking of installing intake and return in two opposite corners of the largest (elevated) part of water ballast tank under the v-berth (where the plug is) to avoid two through hull connections. Vacuum should not be an issue as the same amount of water would circulate in and out. Water getting hot inside the tank should not be a problem as well since the tank is large, and should work as a heat exchanger exchanging heat outside through the hull bottom. What do you think?

It willing also exchange heat with the interior of the boat.

Not trying to talk you out of it, but check carefully before you commit.

5,000 btu/hr, at 1 gpm flow will be a 10 deg temp rise in the heat exchanger. If I counted right, the 1400 lb ballast would increase by at least 3.6 deg per hour - discarding hull/cabin heat transfer. However, you are correct, you will lose some heat through the hull to the lake, river, or ocean you're sailing in - as long as the ballast is hotter than the body of water. Unfortunately, you'll also be putting heat back into the cabin as the ballast tank warms, and starts heating the floor.

Bottom line, you'll be losing some capacity due to heating up the ballast tank, which heats the floor and then the cabin. If the water you're sailing in is cold enough to allow you to transfer heat out through the hull faster than than you're putting it back into the cabin you're trying to cool, you might get some benefit. Too late at night to do the numbers tonight, but interesting idea. It would be a fairly interesting heat transfer problem to balance.

Make sure you post if you try it.

Basically if you are on a cold enough water place to get any effective cooling, you don't need AC.

This has been hashed over many times here, and the HVAC numbers don't work.

Get a gen set and a small 5000 BTU AC and fire it up if you are in the hot/muggy zones.

I assume you are attempting this to avoid putting a thru hole in the bottom. However, it would seem to be vastly more difficult to put 2 thru ballast tank holes because you don't have access to the other side. Theoretically, if you had a leak in the ballast line you limit the flooding to the contents of your ballast.

Seems to me a proper sea cock and thru hole are the way to go. Outlet way above the water line.

---Russ

Ran some rough numbers. Agree with Catigale. If you do it, you won't like it. Roughly 20-25% of the ballast heat rejected is going to come back into the cabin. Additionally, depending on the hull thickness, and how cool the water is, the ballast tank is going to have to be quite hot to generate the heat transfer you need - greatly reducing the efficiency and capacity of the A/C unit. Like Russ says, if you're going to install water-cooled A/C, thru hull is the way to go. If you don't want to run seawater in your A/C, or want to limit flooding if your coolant lines break, you can install a keel cooler. That would probably be more trouble (and expense) than it's worth, though. Good Luck!

Thank you very much for your responses! I am convinced now that what I had in mind is a bad idea. I think you are right, it will be heating floor, and with fiberglass being very poor heat conductor the water in the tank could get way too hot for AC unit to work.

The reason I am looking at something like Webasto is that they are AC/Heat units (not I just AC). Here in Ohio I could use both heat and AC depending on weather and time of year.

It looks like through hulls for intake/discharge are unavoidable. Discharge is not a problem as it can be way above the water line. As for intake I could probably install through hull slightly above the water line next to the sink drain for example and plumb it about a foot under the water. Then install self-priming pump with a check-valve within a foot or so from the intake through hull under the rear v-berth. It would only be about 3 feet of hose for self-priming pump to fill, then check valve would hold water in. This way I could still avoid making a hole under the water line. Do you see any problems with that? Really appreciate your input!

Catigale wrote:Get a gen set and a small 5000 BTU AC and fire it up if you are in the hot/muggy zones.

We have a Honda 1000 watt generator and run our 5000 BTU AC When it's hot, also a 700 watt electric heater for cold weather.

Marc

I agree, water cooled is superior in several ways.

I'd just install a proper thru hull and quality hose. Double clamp everything.
http://www.boatus.com/boattech/casey/in ... eacock.asp
I'm confused about check valves and pumps. How does water come in if it's ABOVE the water line? What does the install manual recommend?

Exhaust, maybe tap anchor locker drain or put it next to it up high. Seems bow is the place I see many A/C exhaust.

RussMT wrote: I'm confused about check valves and pumps. How does water come in if it's ABOVE the water line? What does the install manual

I thought through hull would be just above the water (similar to sink drain), then a 90, and about a foot of pipe with a strainer submerged in the water. Then inside the boat shut off valve first, then finer strainer, check valve, and self-priming pump. Then hose ran to the AC unit under the v-berth, and discharge through hull at the bow. Self-priming pump should be able to pull water in easily, and the check valve would prevent system from draining and re-priming when AC is not in use.

Manual recommends using regular pump (not self-priming) installed at least a foot below water line, which is not possible on our boats. But I think self-priming pipe should work for the above setup. Really appreciate your thoughts, help me think this through.

What's your concern about a thru-hull below the water line? It's done every day. Use a proper ball valve with easy access, and you're good to go. Plenty of how-to out there on installing below-the-waterline thru-hulls.

I've considered doing this for a refrigerator. The compelling reason vs a thru-hull, is that it would work while the boat is beached.

Also, the thermal conductivity of solid fiberglass is pretty high. Heat would be lost through the hull through conduction, which is a lot more efficient than the convection into the cabin.

I understand another hesitation to install a through-hull (I share it): they are a point of failure, although, of course, they can't always be avoided.

The logic is something like: a boat can't leak (and swamp, or sink) from a through-hull breach if it doesn't have a through-hull fitting in the first place. And (older, mostly) boats do sink from through-hull, or its associated plumbing, failure. They are areas that need close inspection before buying a used (or even new, for that matter) boat; it's sometimes hard to know whether it's been installed properly.

You need to be there to close a ball, or other, valve (and it needs to be regularly exercised to be sure it works when you need it) but it does lower the risk, and I wouldn't want to be without one.

BTW - in my eyes an outboard motor on any boat offers an advantage over other drive methods in his regard.

Tomfoolery wrote:What's your concern about a thru-hull below the water line? It's done every day. Use a proper ball valve with easy access, and you're good to go. Plenty of how-to out there on installing below-the-waterline thru-hulls.

Recommended pump (not self-priming) is about 4" high. Manual also recommends installing it at least one foot below the water line which is not possible on our boats. Then most self priming pumps offer lift between 15 - 25 feet. They are just more expensive and it is hard to find small one (around recommended 300 GPH). With self-priming pump you don't really need below water line through hull. I am just trying to find the most safe and reliable way for sea water system.

It wasn't a bad idea... In fact, thought your original idea had some great points. I particularly like no thru-hulls below the waterline aspect. In fact, with a steel hull and an insulated cabin floor, you might have had a winner. Just might not work too well with a Mac.

The self-priming pump route would probably work if you pick the pump very carefully. There are numerous 12v 'self-priming' pumps available, but not all of them will meet your requirements - and getting solid performance information on cheap internet pumps may be tricky. Also, be careful putting too many strainers on the suction side of the pump. It might be better to use a pump that could digest smaller particulate rather than putting fine strainers in the suction.

Certainly understand the need for heat, but you might want to look at other ways to do it than with electricity - which either means a marina or generator (or a Tesla battery pack).

Just an opinion - it's your money and your boat.