Calculating interior volume... roughly

[Montanan]

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?)

Volume is just one factor. Ambient air and water temperature differential and thermal properties of the hull and openings are the other.

And season comes to play, e.g., Air conditioning not an issue in Montana but maybe one week of the year during mid-day. Easier to just grab a cold one from the chilly bin to cool oneself than to install an A/C unit.

Heating requirements well that depends on how many cords of wood one can stack on deck and the size of your fireplace. 24 to 36 inch length split timbers are easy to haul and burn well in most fireplaces.

With the water temperature at say 34.5 degrees Fahrenheit one should add weight to the boat to increase displacement, [i.e., load several more cords of wood] when the air temperature is colder than 34.5 so as to settle the hull deeper in order to obtain greater heat transfer from the comparatively warmer water so as to warm more of the hull and convect into the interior; whereas, if the air temperature is above 34.5 degrees F, then that is shirt sleeve weather and you can simply open the companionway and hatches to cross ventilate the interior to take advantage of the balmy breeze.

[SantaFeTrailer]

Come on guys, does the newb really have to explain this to you? Obviously you fill it with ping pong balls, then count them and multiply by the spherical volume, plus you'll need to figure out the volume of the interstices. That's just simple math. Then, you will have positive flotation.

Originally Posted by 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.

[Spot]

Taking an elliptical volume, waterline x average of beam & headroom
=23' x 8.375'
=1.46e6in^3
/1728
=845 cubic feet.

[Sailmonkey]

We’ve got a 32’ boat on the Texas gulf coast, our installed AC is 5200 btu.

While not able to sustain meat locker like temperatures during the day it does keep the boat cooler and dry. At night it’s plenty to cycle and keep the boat comfortable.

Added bonus is that it’s not taking up huge amounts of room.

[Adelie]

Easier would be to measure the surface area of the boat during the next haul out.
Starting at the bow and moving aft every 31” measure down from the gunwale to the centerline of the boat or the turn of the bilge following the hull. The bow is station 0 and the stern is station 12.
Average each pair of station heights and multiply by 31” and again by 2 to get area of both sides
For example
S0-0
S1-24”
S2-31”
S3-43”
....
A1=[0”+24”]*31”/2*2=744si
A2=[24+31]*31=1705
A3=(31+43)*31=2294

Do the same for the beam of the boat.
Add the cabin sides and the stern. Measuring everything in inches

Add all the result together and divide by 144si/sf to get total square feet.

Don’t worry about measuring down the keel.

That will get you square footage expose to the elements which works better for figuring out heat transfer.

[Tillsbury]

Originally Posted by SantaFeTrailer

Come on guys, does the newb really have to explain this to you? Obviously you fill it with ping pong balls, then count them and multiply by the spherical volume, plus you'll need to figure out the volume of the interstices. That's just simple math. Then, you will have positive flotation.

It would be easier to use cans of expanding foam fired down an open hatch. You can work out the volume by how many tins are required to fill the boat. Bonus: it will insulate the boat as well as making it unsinkable.

[newhaul]

You guys do realise this is essentially a zombie thread right .
I was the last poster back in August of 2015

[GordMay]

Originally Posted by newhaul

You guys do realise this is essentially a zombie thread right .
I was the last poster back in August of 2015

And yet, we continue to return to, and read the thread, indicating there may be ongoing interest.
For instance, I liked Spot’s (Yesterday #18) ellipsoid calculation - something that hadn't occurred to me.
More ➥ https://www.web-formulas.com/Math_Fo...Ellipsoid.aspx

[Spot]

PS: Volume of ellipsoid:
V = 4/3 × π× a× b× c
where a,b, and c are the radii (semi-axis each way) of the ellipse. In the above example the radii are 11.5', 4.1875' and 4.1875' as b and c are the same.
https://planetcalc.com/149/


PPS: I cannot believe I did maths for a zombie thread!

[Adelie]

Originally Posted by newhaul

You guys do realise this is essentially a zombie thread right .
I was the last poster back in August of 2015

I prefer to think of this as more of a Lazarus thread than a Zombie thread.

[newhaul]

Originally Posted by Adelie

I prefer to think of this as more of a Lazarus thread than a Zombie thread.

true it is more like that

[Outlaw7]

You could go here, they have some good info, check out not only this page but the installation do's and don'ts as well.
How to Size Your Boat?

[Cheechako]

A ballpark is fine. The reality is if you try to cool the ends of the boat, you will be freezing in the main cabin. For the first time in my 70 years I have a small AC in my house the last couple of years. Never even considered it in the past here in the PNW. My little house is 1120 sq ft. A 8000 btu does fine when it gets too hot. On a 31 ft boat I would get the smallest available.

[rsn48]

From West Marine, link below:

"Sizing your air conditioning system
Air conditioning systems are rated in Btu, or British Thermal Units, a universal measure of heating and cooling. To calculate the number of Btu needed to cool or heat your space, follow the steps below, then select the unit with the capacity you need.

Step 1 - Required Capacity
Divide your boat into three basic load areas:

Below deck: cabins where the hull slopes inward toward the keel and there are minimal port lights and hatches.
Mid-deck: areas on the main deck with small or shaded windows.
Above deck: areas with large glass surfaces and direct sunlight.
Measure the length and width of each room to be conditioned to determine the square footage. It is assumed that your boat has average headroom of about 6 1/2' and you have an average amount of furniture. If one end of the compartment is narrower than the other, take your measurement in the middle. Multiply the length by width to get the area of each.

Determine which load factors to use from the table below and multiply the area of each cabin by the load factor (Btu per sq. ft. per hour) to determine the required air conditioner capacity:

Below Deck Mid-Deck Above-Deck
Climate Moderate Tropical Moderate Tropical Moderate Tropical
Factor 60 80 90 120 120 150
Engine room and galley air duct diagram for air conditioning system
Select Moderate Factors where the maximum air temperature is 95°F, maximum water temperature is 85°F, and moderate humidity exists.

Select Tropical Factors where maximum air temperature is 105°F, water temperature is 95°F, and high humidity exists.

Step 2 - Number of Units
The number of air conditioners and their locations will be determined by the size and layout of your boat, and the space limitations for ducting and plumbing. The typical maximum ducting run is 15', but if there are many bends then the overall length must be reduced to ensure good airflow. Also consider which areas require independent thermostat control, and which cabins will be served by ducting or a secondary air handler (where the only temperature control is by reducing airflow with an adjustable grille or fan-speed control).

Step 3 - Location
As in step 2, the location of the units will be determined by your boat layout. Check the appropriate specification sheet for unit sizes, and make sure that there is sufficient space to service the unit and remove if necessary.

The self-contained unit or air handler must have an open return-air path, and be located such that the discharge ducting can be routed to a high point in the cabin. The return-air grille does not need to be directly in front of the unit, in fact, the system will be less noisy if there is an indirect path for the return air to follow. Most units have blowers that rotate to allow routing the discharge duct in the most direct path to reduce restrictions.

Seawater cooling system diagram
Step 4 - Seawater Components
The seawater cooling system consists of an inlet thru-hull fitting, seacock (water valve), strainer, pump, and overboard discharge fitting, all connected by hose or piping. If multiple air conditioning units are served by a single seawater pump, then a pump relay and water manifold are required. Cruisair recommends a centrifugal seawater pump for efficient, quiet operation and long life. Centrifugal pumps are not self-priming and must be mounted below the waterline.

It is important that the seawater plumbing be “self-draining”, meaning that if the boat is lifted, all water in the piping will drain out. An air conditioning system plumbed this way will have no air locks that could disrupt the flow of seawater. For shallow-draft boats where it is impossible to mount the pump below the waterline, a self-priming pump must be used.

It is normally recommended that you use one seawater circulation pump of adequate capacity for all of the air conditioning systems on board. The rule of thumb is to have 180 gallons per hour (3gpm) of water per ton of air conditioning (one ton is 12,000 Btu/hr). If more than one Marine Air System shares a common pump, you will also need a pump relay and manifold.

The table below shows recommended seawater flow rates and minimum inlet (through-hull) for a system of a given capacity. Our Product Advisors can work with our suppliers to give you more guidance.

System Capacity (BTU/hr) Seawater Flow (Rate gph) Inlet Size
5-12,000 180 1/2"
16-24,000 360 3/4"
30-48,000 720 1"
Step 5 - Duct and Grille Sizing
See the table below for recommended duct and grille sizing. Our Product Advisors can help with duct transition boxes and sizing of branch ducting, as well as with the large selection of grilles available, some of which include aluminum, plastic, and many species of wood.

Capacity (BTU/hr) Duct Size Return Air Grille Discharge Grille
5,000 4" 60 sq. in. 30 sq. in.
7,000 5" 80 sq. in. 45 sq. in.
10,000 6" 100 sq. in. 60 sq. in.
12,000 6" 130 sq. in. 70 sq. in.
16,000 7" 160 sq. in. 80 sq. in.
18,000 7" 200 sq. in. 100 sq. in.
Climate Control 4 part system
Other Required Elements: A complete air conditioning system requires the control/switch assembly, the seawater cooling system, and the air distribution system. The Seawater Cooling System consists of an inlet through-hull fitting, seacock (water valve), strainer, pump, and overboard discharge fitting, all connected by hose or piping. If multiple air conditioning units are served by a single seawater pump, then a pump relay and water manifold are required. Cruisair recommends a centrifugal seawater pump for efficient, quiet operation and long life. Centrifugal pumps are not self-priming and must be mounted below the waterline.

It is important that the seawater plumbing be "self-draining", meaning that if the boat is lifted, all water in the piping will drain out. An air conditioning system plumbed this way will have no air locks which could disrupt the flow of seawater. For shallow-draft boats where it is impossible to mount the pump below the waterline, a self-priming pump must be used.

Air distribution systems: Cabin air is drawn into the self-contained unit or air handler through a return air grille. It is then cooled or warmed and blown back into the cabin through a ducting system. The air should be discharged high in the cabin and away from the return air grille to ensure good circulation. Plenums (transition boxes) can be installed in the ducting to allow multiple discharge grilles, in one or more cabins. An insulated duct is recommended to prevent secondary condensation. An air filter, located on the cooling unit or on the return air grille, must be cleaned regularly."

https://www.westmarine.com/WestAdvis...-for-Your-Boat

[Pete17C]

I have worked in the HVAC business for years (airside), a couple of cubic feet off shouldn't kill your calculations. Typically you design for an outside temp less than maximum (otherwise you overcool or short cycle on the less than average days (you cool, but the unit doesn't run long enough to dehumidify). So if you're a bit off you may only be able to maintain 76F instead of 72F on the hottest days.

just take interior cabin length X height X avg width and you'll be close enough

from there it's 6-9 air changes an hour (maybe up to 12 in very hot/humid climates or not so well sealed cabin), then do the math to get CFM, then it's 12KBTU needed (1ton) per 400CFM

/rules of thumb
//or you can pay an engineer
///to do the math