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Old 05-10-2020, 01:08   #1
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How many BTUs of heating for 37' sloop?

Hello! My wife & I will live aboard our Dufour 382 in Annapolis for the second half of October and then sail south to FL in Nov/Dec. We will definitely need some heating, especially while in the northern latitudes.

It's been recommended to us to buy one of those plug-in, oil-filled heaters. (They look like small radiators.) The question is, "How many BTUs to buy?" I see 700W units for $50 and 1500W units for around $90. Those seem to be the primary choices.

The boat's got a very large interior for a 37-footer, and we will sleep in the forward V-berth. There's no insulation.

Is 700W enough?
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Old 09-10-2020, 06:30   #2
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Re: How many BTUs of heating for 37' sloop?

Probably not. Maybe stronger than "probably," more like "I sincerely doubt it."

Let's back up. Will you be anchoring or in marinas? If in marinas, you can use electrical heating, which will allow inexpensive add-ons as needed. If anchoring, and have a genset, you can very inefficiently do the same as a stop-gap.

How far south how soon? Key West is a different story than St. Augustine, and December is not November.

My guess is that the v-berth needs 1,000 watts as a starting point.

Given this very changeable situation, you might consider either a system with bunches of reserve capacity or a system that allows add-on as needed. A honking big diesel or LPG heater feeding into the main cabin, plus fans to distribute the heat, might be the former. Electricity is really the only option for the latter, because everything else has a high unit cost. A resistance coil backed by a fan is cheap.

If you are going to south Florida, and staying there, or going elsewhere in Florida and not living aboard all year, it's really only getting there that counts. Here in Northwest Florida, a small LPG fireplace (Newport type) and two 1,000 watt space heaters got us through three winters with minimal discomfort. Fans helped. January was the only really nasty month.

Given your plans, I'd probably go with an add-on system rather than investing in something big. Live aboard all year, I'd install something permanent.
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Old 09-10-2020, 07:16   #3
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Re: How many BTUs of heating for 37' sloop?

I'm installing a 14kw diesel hydronic heater in my boat. I calculated heat loss at 32,000 btu's per hour with an outside temp of 25*F.

1500w is about 5,100 btu/hr.

Back of the envelop heat loss for your 37' is ~20,000 btu/hr with proper distribution.
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Old 09-10-2020, 09:09   #4
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Re: How many BTUs of heating for 37' sloop?

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Is 700W enough?

Depends what you want.


It will not be warm enough to make the boat, warm like a house.


On the other hand it will be enough to take the worst of the chill off and keep condensation from forming on everything. I would expect that you will find that it raises the inside temperature by around 10 degrees F.
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Old 09-10-2020, 13:22   #5
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Re: How many BTUs of heating for 37' sloop?

It depends a bit on how the boat is insulated (solid hull is worse) but you definitely need more power. If going electric, you need three three electric heaters x 1,500W each. If using a diesel heater, you need one 5kW unit or 2x 2KW units.

I had a 2KW diesel heater on a 31 ft boat and found it insufficient on colder nights.
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Old 09-10-2020, 17:15   #6
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Re: How many BTUs of heating for 37' sloop?

Besides not providing enough heat in a cold climate, with an electric heater you're not changing any air in the boat. All the steam from showers, cooking and moisture in your breath stays in the boat unless you vent it out. That means water forming on cooler surfaces. Your clothes and bedding will be damp or wet.
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Old 09-10-2020, 20:27   #7
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Re: How many BTUs of heating for 37' sloop?

Your 38 ft Dufour is probably not super well insulated. In below freezing weather you will need about 10,000 BTU to be toasty. You'll need less in warmer weather and less if you are OK with cold toes and heavy parkas.

So that means about two of your 1500watt electric heaters, which you cannot run on normal 30 amp shore power. Obviously you can't even do that at the dock, let alone at anchor. At anchor you'd have no heat.

I hate to say it, with the short time you plan to be in Annapolis, but a good diesel heater is the ticket. Propane and alcohol don't have enough BTU. Get something that burns Diesel and has a smoke stack.

With this you can be warm at the dock and at anchor and the smoke stack will mean that the air inside the boat will be dry (Less condensation)
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Old 12-10-2020, 13:54   #8
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Re: How many BTUs of heating for 37' sloop?

This is our heater. Been using it for 15 years now. As you can see it is gas & vented. Heats a 32ft yacht easy. Full output is 6000 BTU, we run it at half heat. Keeps the whole boat dry and warm in a New Zealand winter 41 degrees South Click image for larger version

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Old 12-10-2020, 20:46   #9
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Re: How many BTUs of heating for 37' sloop?

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This is our heater. Been using it for 15 years now. As you can see it is gas & vented. Heats a 32ft yacht easy. Full output is 6000 BTU, we run it at half heat. Keeps the whole boat dry and warm in a New Zealand winter 41 degrees South Attachment 225076
Excellent, This is the type of heater which works well but your 32 Lidgard is has a much smaller volume than the OP's 38 Dufour and is probably wood, which is an excellent insulation.

If your location is Nelson I think that below freezing weather and snow is rare. The OP will be living in Annapolis where the low temperatures in January, the coldest month, average in the low to mid 20s.

They need a heater like yours but MORE.
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Old 12-10-2020, 20:56   #10
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Re: How many BTUs of heating for 37' sloop?

Yes it is timber. The temperature routinely drops below zero in the winter months. No snow at sea level tho. Friends with a 43 ft Lidgard have the larger version of this heater. Built by Dickinson & are really happy with it.
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Excellent, This is the type of heater which works well but your 32 Lidgard is has a much smaller volume than the OP's 38 Dufour and is probably wood, which is an excellent insulation.

If your location is Nelson I think that below freezing weather and snow is rare. The OP will be living in Annapolis where the low temperatures in January, the coldest month, average in the low to mid 20s.

They need a heater like yours but MORE.
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Old 13-10-2020, 18:32   #11
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Re: How many BTUs of heating for 37' sloop?

Yep, a diesel air heater would be best. But we need just (a) 2 weeks to get south to warmer climes, and (b) enough heating to make it bearable. We don't need to be toasty-warm. We just don't want to freeze.

We have sleeping bags rated to 20F. The biggest problem, really, is a crazy level of condensation. (The boat is not insulated.)

We've talked to other "locals" who have made the passage we're making. They all use the propane "Mr Heater" / "Buddy" units that utilize the Coleman 1lb cylinders. They run them for 2 hrs before bed, get the place toasty-warm, get into bed, then turn them off. Sleep for 5-6 hrs. Get up and turn-on the "Buddy" again. Run it for an hour or two to get yourself ready for the day (read: dress warmly). Do this for a week, until you're in the Carolina's.

Anyway ... with the time we have, that's what we're planning. Picked-up the "Buddy" (9000 Btu) and the Colman cylinders today. Will give it a try in-port, toward the end of the month, even when we've got shore power, just to see how it goes.

Many thx for the input! If we find ourselves cruising back north at some point, we'll DEFINITELY install the diesel unit.
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Old 13-10-2020, 18:39   #12
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Re: How many BTUs of heating for 37' sloop?

Non vented propane creates lots of moisture.
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Old 14-10-2020, 04:49   #13
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Re: How many BTUs of heating for 37' sloop?

Indeed, Robert. But the LPG fireplace/stove (Newport type) solves two problems. The air used comes from the outside to the flames and then exits out concentric pipes, so the heater does not use air from the cabin, nor does it release carbon monoxide into the cabin. It's a sealed system.
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Old 14-10-2020, 06:45   #14
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Re: How many BTUs of heating for 37' sloop?

FIRST: I would be very concerned about CO poisioning with a Mr Buddy heater.

https://www.doityourselfrv.com/propa...r-safety-tips/

Our 32’ boat has a kerosene bulkhead heater. It is nice for a cool evening. But when it gets rainy and cold and miserable then we kick on the Espar D-4 diesel heater. 15,000 BTU IIRC. That will heat that boat nicely. It is a steel boat with sprayed foam insulation. The water will be near freezing with night temps in low 50’s. High humidity, cold rain is killer for me. I have sailed through ice berg fields with this thing running and providing a warm cabin.

Our 44’ steel boat, rigid insulation, also has an Espar D-4 heater and we have over wintered in Delaware a few years. Down into the teens at times. I would close off the forepeak and aft cabin and just heat the saloon. By reducing volume the D-4 would keep us warm enough, if chilly during the worst conditions. We wojld always sleep in the aft cabin just leave the door open over night to take off the worst of the cold. Good sleeping bags, don't argue! This boat has a bulkhead heater in the aft cabin which can heat that small volume well.

There are now cheaper alternatives to Espar, about half the cost.

If you can mount the heater somewhere so that it will blow down the central saloon area you really don't need much ducting. Keep the installation as simple as possible.

The GREAT thing about these small diesel heaters is you can run then while underway. One of you can warm up and dry out while the other is on watch.

I have heard good things about these Planar heaters. A lot cheaper than Espar/Webasto. No personal experience.

https://planarheaters.com/
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Old 14-10-2020, 14:17   #15
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Re: How many BTUs of heating for 37' sloop?

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Indeed, Robert. But the LPG fireplace/stove (Newport type) solves two problems. The air used comes from the outside to the flames and then exits out concentric pipes, so the heater does not use air from the cabin, nor does it release carbon monoxide into the cabin. It's a sealed system.
Using air from INSIDE the cabin dries the boat. The inside air is quite humid from your body's moisture, cooking, etc. Drawing that moist air into the heater and combusting it is excellent. The products of combustion then go out through a smoke stack, and fresh, cold, (DRY) air from outside is pulled into the boat and replaces it.

Diesel is vastly better the LPG because it has many more BTU per volume.
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