just making sure I've got it right

Okay, this is a very newbie wanting to make sure I've got my calcs right....
I was looking at some pics of the Sea mule tug/barge I'm wanting to purchase. Two seperate laterally attached hulls with an I.D. tag in each both showing 38,000 lbs. The seller listed it in the ad as weighing 38,000 lbs total, but then I got to thinking perhaps that it's 38k for EACH hull. The water line is visible on the pics so it occured to me I could estimate weight by figuring out the displacement. I ended up calculating 1344 cubic ft displacement, so that gives me about 80,000 lbs weight...just about what the total should be if it is 38,000 for each hull. Now all of a sudden I am realizing this boat is a 40 tonner, not 20! Wow. How could basically a 1/4" hull with some reinforcing i beam end up weighing 40 ton? It's a shoe box--square bottom...38 long, 13' beam and 7 high. Two motors weighing about 1 ton each, but other than that it's totally empty, not even fuel tanks.
Perhaps you are thinking what a no-brainer this question is, but I just want to make sure of this because this changes everything in terms of my purchasing this.
thanks,
~martin

 

You did'nt say what material, but ide guess steel?

Its not going to weight 40tons but if you've got a pic and can post it.

We could have fun having a guess.

 

It will take 40 tons to TOTALLY submerge the pontoons. Something you don't want. You need freeboard. 20 tons will submerge them 1/2 of their height, giving the remaining height above the water (freeboard).
There is always a safety factor (usually 20%+) figured into displacements.

 

clarifying the question

Yes, it is a steel hull. WHat I want a definitive answer to is the total hull 20 tons or 40 tons?

This thing is basically a 1/4" steel shoe box. Flat bottom, blunt nose, vertical chines.

The volume of the hull BELOW the waterline (obvious markings on the side of the hull...I assume I can go by that) is 1344 ft cubed. Now, taking 60 lbs per I get 40 tons.

The hull is actually 2 narrow hulls bolted together. Each has an id tag inside that says 38,000 lbs. The question is if that's 38,000 for the total joined hull, or 38,000 each hull half. The seller assumed 38k was the TOTAL weight. Seems my simple displacement calcs confirm that it is 38,000 each half for a total of 76,000 since I get 40 ton in my displacement calcs.

Right?

 

Frosty,
Here's a picture of the ugly thing. BTW, when doing the actual displaced volume calcs, I did take into account both the missing area back by the props and I leveled out the waterline from it's diagonal markings.

There's 3.4' of hull under the waterline. The boat is 13.5' beam and 38' long. SO that's 3.4 x 13.5 x 38 for 1744 ftcubed. Then the missing prop area is 3.4 x 13.5 x 8.5 for 400 ft cubed. And 1744-400 for a total displaced volume of 1344. Then that times 60 lbs per cubic foot of water gives me 80,000 lbs.
SO, it's gotta be 40 tons, right?

If this helps, what's inside is 2 Chrystler straight 8's and trannies--one set in each hull. That's it. No fuel tanks or anything else.

So, is the boat 40,000 or 80,000?

 

Measure the WHOLE hull! L X W X H = Total cubic ft.
cubic feet X weight of water (62lbs - fresh ; 64lbs - salt) = water lbs
to
sink hull just below waterline. THAT is the displacement of that hull.
To figure draft.... divide the H (height) in inches into the displacement, that tells you how many pounds it takes to sink that hull 1"
.
I'm betting the 38,000 is the displacement of EACH hull
Again... 40 tons (both) to sink to waterline, 20 tons (both) to sink 1/2 way.
read this;
http://www.tedbrewer.com/yachtdesign.html

 

I assume there is a deck on it, even so I cant see that weighing 40 tons.

If its 2 hulls joined together then the centre will be 1/2 inch steel?

A lot of metal there but even so!!

Ok ille go out on a limb here and say 12 tons.

I have no explanation to the reference of 38,000 stampings. I am trying to imagine that in 1/4 plate with a 1/2 inch centre and a deck on it with 2 motors.

 

Okay, Ted, now I'm really confused...Let's make this thing real simple so my brain can grab it. Let's take a one foot square open top metal container and float it in a pond. Let's say it floated with a water line half way up the side, so we know there's a half a cubic foot under the water line. What would this metal container weigh? Walk me thru those steps, please.

I've been thinking that it would be this: 1/2ft cubed times 62 lbs for a cubic ft of water, so the container must weigh 31 lbs because it displaces that much water.

thanks for some teachin'
~martin

 

Yes, that's exactly how it's done.

 

:idea:
No moss is growing on you!