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Old 07-08-2015, 14:37   #1
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Calculating interior volume... roughly

I tried doing a search on this, but I didn't find anything. Apologies if this has been asked in the past, but I'm trying to figure out the interior volume of my 77 Dufour 31. I know that interior volume scales geometrically, and I could figure it out with a lot of measuring, but is there a basic formula for figuring this out? Doesn't have to be exact, rough estimates are fine. I'm trying to figure out how large of a HVAC system to get.

Length = 31"
Beam = 10.5"
Draft = 5.66"
Displacement = 8,400 lbs

Would it be something like length x beam x (draft - keel) / (some constant? 2?)
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Old 08-08-2015, 05:51   #2
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Re: Calculating interior volume... roughly

Draft and displacement wouldn't have anything to do with interior volume. You could get a rough overestimate by length x beam x height (headroom inside). If you are not near your boat now, you can probably just guesstimate headroom.

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Old 08-08-2015, 07:32   #3
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Re: Calculating interior volume... roughly

length x beam x height/2.5
boats are kind of horizontal cones!
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Old 08-08-2015, 07:47   #4
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Re: Calculating interior volume... roughly

I'm assuming part of your interior volume is eaten up by the cockpit n'est ce pas? So you can't use overall length.


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Old 08-08-2015, 10:39   #5
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Re: Calculating interior volume... roughly

Fill the boat with water, then measure how much water it took. For example, if you have a bucket that holds 1 cubic foot of water (about 62 pounds), then just count the buckets till the boat is full.
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Old 08-08-2015, 13:44   #6
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Re: Calculating interior volume... roughly

What do I do with all the water after I am done :-)
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Old 08-08-2015, 14:13   #7
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Re: Calculating interior volume... roughly

Quote:
Originally Posted by dragsterhund View Post
I tried doing a search on this, but I didn't find anything. Apologies if this has been asked in the past, but I'm trying to figure out the interior volume of my 77 Dufour 31. I know that interior volume scales geometrically, and I could figure it out with a lot of measuring, but is there a basic formula for figuring this out? Doesn't have to be exact, rough estimates are fine. I'm trying to figure out how large of a HVAC system to get.

Length = 31"
Beam = 10.5"
Draft = 5.66"
Displacement = 8,400 lbs

Would it be something like length x beam x (draft - keel) / (some constant? 2?)
I have wondered this myself (the volume issue). But I quickly got over it.

Regarding a Air Conditioner for your boat?
There have been multiple threads that discussed air conditioners (of various types) for sailboats.

Here is a quick Google Custom Search on the key words "Air Conditioner."

Go to that link and look down the page of links to see that someone else with a 31 foot boat has asked similar questions related to air-conditioning a boat. Etc. etc.

I hope you find this helpful.

Good luck!

air conditioner - Google Search

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Old 08-08-2015, 14:29   #8
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Re: Calculating interior volume... roughly

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What do I do with all the water after I am done :-)
Drill a hole in the hull to let the water out.
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Old 08-08-2015, 18:30   #9
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Re: Calculating interior volume... roughly

You need that hole in the hull to drain the bilge, anyway, while you're underway. Also good if you have a dog that pee's in the bilge
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Old 08-08-2015, 21:54   #10
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Re: Calculating interior volume... roughly

Length times width times height. These would be interior measurements which means headroom, not draft. The width is the hard part. You need to try to square it so that means trying to average the beam which is typically widest at the top & amidships.
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Old 08-08-2015, 22:14   #11
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Re: Calculating interior volume... roughly

Here's my attempt.
(((BW+SW)/3)*L+((WLBW+WLSW)/3)*WLL/3)*Height

Where
BW= beam width
SW=stern width
L=length
WLL=waterline length
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Old 08-08-2015, 22:34   #12
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Re: Calculating interior volume... roughly

Rough volume = pi x L x B x H / 6

Or simplified further = L x B x H / 2 = 31 x 10 x 6 / 2= 930 cu ft.

It is essentially half a cone (split lengthwise) and the volume of a cone is pi r^2 h/3. There is more volume in the bow than a half cone, but less in the stern with the cockpit than a half cone, so the half cone overall is a decent approximation.
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Old 09-08-2015, 10:25   #13
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Re: Calculating interior volume... roughly

Use the USCG formula for figuring the documented tonnages and reduce by approx 30% to allow for cockpit losses better to calculate a lil high than any to low for sizing a heating system.
http://www.uscg.mil/hq/cg5/msc/docs/CG-5397.pdf
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