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Old 05-08-2016, 17:58   #1
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Question about ventilation

I've only been on 2 or 3 sailboats. Once was just a quick look at a small boat that a guy had for sale (I'm guessing this thing was about 15 to 18 feet). He showed me the cabin and we sat there for less than 10 minutes.

That little boat was hot as hell and it's something that has stuck in the back of my mind whenever I think about buying a boat. This one didn't seem to have any type of insulation from the heat but I don't know anything about sailboat construction. It was in the 90s that day in Tennessee.

Do larger sailboats get that hot and do they have better ventilation? I don't love the heat so any insight into this is appreciated.
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Old 05-08-2016, 18:10   #2
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Re: Question about ventilation

Small boats are generally not ventilated while bigger boats have things called dorades which do a great job. My boat has 6 dorados. If you ever buy a small boat invest in a Nicro solar vent which is much smaller than a dorade a pretty good trade off.
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Old 06-08-2016, 04:33   #3
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Re: Question about ventilation

Boat insulation is an issue mostly when you're moored. boom tents help a little, hatch chutes can direct more wind into the interior but lack of wind can be an issue itself. Small fans help but use up valuable energy. We have a small 800w reverse cycle AC but that wont cope when the heat cranks up and must be connected to shore power. It seems to be less of an issue when you're sailing or anchored in a bay. If it's hot and you're tied up, then maybe it's time to visit a nicely air conditioned mall, hire a car with AC and go for a drive, go swimming, or whatever to find a cool spot.We pack up and go home when the temperature starts getting above 30C.
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Old 06-08-2016, 06:45   #4
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Re: Question about ventilation

It varies, hugely, with plenty of factors.

Like how hot your zone is, how much wind you get, how the boat was built (materials, techniques, etc), how much deck awnings you deploy, etc.

A big boat needs many deck hatches or opening side windows to get a good draft. A small one often just the forepeak hatch and the companionway wide open will do the magic.

In a small boat you may prefer sandwich deck and cabin and docking it in the line of the wind (I find cockpit to the wind works best for us). We use awnings thru the whole boat stem to stern.

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Old 06-08-2016, 07:13   #5
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Re: Question about ventilation

Modern cruising boats have excellent ventilation. Older designs and race boats do not.

My boat, a 39 foot Beneteau has 13 opening hatches!! And they all open forward to scoop in the air. As i write this i have only 3 hatches open: both heads and one saloon. The whole boat is cool and a draught of wind is coming through.
Even in the tropics its very rare to need all the hatches open.

Dorades work in a different way and use suction to drag cockpit air against the wind through the companionway. Not only is it less efficient the size of the sucking hole is tiny compared to an open hatch.

Its really important, when looking for a boat, to sit in it for a time to give you a better understanding. Your example of 10 minutes is great! By then you realised something was wrong. An hour would be better.

Realisation of what is important can be difficult: years ago at the Boat Show my GF and i were comparing 2 different designs. I was snapping photos. She was a non-sailor but when got home and i reviewed the photos i saw she was sitting primly on one boat but being stupid on the other., lying all over the beds, playing hide and seek in the Heads, just being a stupid girl.

I realised that was the boat!! For one reason or another one boat gave her heaps of enjoyment. I still have the photos...
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Old 06-08-2016, 07:37   #6
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Re: Question about ventilation

Disagree about older boats don't have good ventilation. Lots ofOlder Cruising boats from 80s have great ventilation, but are designed to be at anchor, not a dock queen. Our 41' 1981 ketch has two large hatches and three opening ports in V Berth alone. ttotal of three large hatches, 15 opening ports and two large dorados and a butterfly hatch in salon.
Friends with Whitby 42 from 80s similar. Amels, HRs...lots of good cruising boats. Also many have some form of insulation between deck/cabin top and head liner. A lot of modern production boats have little or no opening ports and smaller flush mounted hatches.
We find when anchored and put up or sun shade (main to mizzen) that the tendancy to rest bow into wind and reduced solar load on deck/cabin make below decks comfortable with a good breeze through the whole boat.
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Old 06-08-2016, 07:44   #7
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Re: Question about ventilation

Quote:
Originally Posted by MarkJ View Post

Realisation of what is important can be difficult: years ago at the Boat Show my GF and i were comparing 2 different designs. I was snapping photos. She was a non-sailor but when got home and i reviewed the photos i saw she was sitting primly on one boat but being stupid on the other., lying all over the beds, playing hide and seek in the Heads, just being a stupid girl.

I realised that was the boat!! For one reason or another one boat gave her heaps of enjoyment. I still have the photos...
Great story!

Between now and then I plan on getting into as many boats as I can. In the meantime I plan to ask as many newb questions as I can here on CF without wearing out my welcome, just to get an idea of what to look for and how.

I've only been looking into sailboats for about 1 year now, but really seem to like those Beneteaus.

Thanks for the advice.
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Old 06-08-2016, 07:57   #8
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Re: Question about ventilation

A lot depends on where the hatches are located. Blue's forecabin hatch is directly above my head. At 0400, when the outside air temp. is still 81F, I need a sheet. Good cowl vents, facing forward, work well when the Dorade boxes have baffles and drains.
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Old 06-08-2016, 09:10   #9
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Re: Question about ventilation

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Originally Posted by MarkJ View Post
Modern cruising boats have excellent ventilation. Older designs and race boats do not.

I respectfully disagree. My 1981 vintage has 4 overhead hatches, 9 opening ports, 2 dorades, as well as a companionway that can remain open under most weather conditions. As long as there is a hint of a breeze we are okay. When the breeze stops, we have 4 small fans to stir the air inside. Every boat differs, and every boat likely could benefit from improved ventilation.

Dorades work in a different way and use suction to drag cockpit air against the wind through the companionway. Not only is it less efficient the size of the sucking hole is tiny compared to an open hatch.
...
For a dorade to function as you describe, it would require the cowl facing aft in such strong winds that a negative pressure is developed inside the mouth of the cowl, relative to the passing wind.

The modern dorade, typically credited to Rod Stephens of the American design firm of Sparkman and Stephens (see also classic sailing yacht "Dorade"), primarly functions as a wind scoop via its associated cowl. The serpentine air travel through the dorade box allows water entrained in the air to fall out of the air stream and then drain from the dorade box via gravity through drain hole(s).

In addition to providing or supplying outside air, it will also function as an exhaust to vent interior air (as long as there is another supply source). A forward mounted deck hatch, along with an aft-mounted rear-facing dorade mounted cowl vent, will ensure ventilation throughout the entire length of the boat. In this case it is not "sucking" out the air, it is instead relieving the overpressure of the boat's interior due to the incoming air at the forward hatch.

Oftentimes you can encourage more air to enter the boat when you improve the means by which the air can exit the boat. HVAC 101.

As for how much air a dorade vent can supply, when the breeze is up, it supplies a lot. So much so that we keep a couple of socks handy to stuff inside the interior opening for the dorades when we need to keep that air out. Not very elegant, but functional.

In addition to supply air, attention should also be given to exhaust. Heads, galleys, engine compartments, etc. On my boat I also exhaust the locker which contains the refrigeration compressor. Frequently boats can benefit from ventilating the bilge and below floor areas. Have mildew problems? Ventilate. Odors? Ventilate. Lazarette lockers always damp? Ventilate. And that includes bad weather ventilation, and that brings us back to the dorade!
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