Calculating interior volume... roughly

[dragsterhund]

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?)

[colemj]

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.

Mark

[Cowboy Sailer]

length x beam x height/2.5
boats are kind of horizontal cones!

[goat]

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.


goat

[hamburking]

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.

[Truemettle]

What do I do with all the water after I am done :-)

[Steadman Uhlich]

Originally Posted by dragsterhund

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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[goat]

Originally Posted by Truemettle

What do I do with all the water after I am done :-)

Drill a hole in the hull to let the water out.

[Skuzzlebutt]

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

[Scout 30]

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.

[Hoohaa]

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

[estarzinger]

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.

[newhaul]

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

[Fiddlerman]

I had a good laugh reading all the suggestions and replies.
Obviously it's the average length times average width times average height, but that is way more complicated than is sounds.
The average beam is possibly half as much as the max beam when taking into consideration that the bow is pointy and slightly less beamy by your transom.
The height is at times only high enough to allow a person to lay in a berth. So average height is very hard to figure out with all the lockers and storages. Not to mention that some of those spaces don't need to be counted if you are calculating for HVAC.
I'd say that you could just make a wild guess. IMAO you probably need at least 12,000. Better to have too much then too little but too much will cycle on and off too quickly to keep your boat dry.
It also depends on where you live and how sensitive you are.

[Tillsbury]

12000 on a 1977 Dufour 31? Blimey that will be generous. Boats of that vintage have a very small interior volume.

I recently put a 10000 on my 50’ boat, serving only the master cabin and saloon. The master cabin on this boat is easily the volume of a 70s 31-footer, and it can get that room icy cold however hot it is outside. It can get the saloon pleasantly cool, given that our external temperatures are rarely much over 30C. I’d imagine that in the tropics it would struggle to get the entire boat cold, but I can divert to only one room if need be.

It depends, as stated above, what temperature you want. And, most importantly, how you intend to power it.