Marine Designs
We have boat plans (both yacht designs, catamaran designs and power designs) for all sizes. From the 12ft Mushulu Dinghy stitch & glue yacht tender to the Oceansky 57 catamaran. We design timber boats, plywood boats, strip plank boats, fibreglass composite boats, steel and aluminum boats to very modern styling for both monohull designs or multihull designs, powerboat or sailboat and in round bilge, multi-chine or radius chine hull forms.
G'day Everyone,
Just thought I'd post this thread about how to weigh your boat without going down to the local weighbridge.
As usual when building, your Mate's/ friends or visitors tend to drop around and the most common question asked is:
"What does she weigh"?
"Hmmm, Dunno, good question" you may think. "Probably around xx kg I reckon"
You may remember a previous article ( about the discrepancies in the weight of the plywood and how it can effect the final weight of your boat), well here's a real simple way of keeping track of the weight of your boat.
I'll do an example with a length of timber to show just how simple it is first, then we'll apply it to the Mushulu 14.
Here, I've grabbed my household scales and a lump of hardwood timber which is 9ft long. Standing it upright and throwing it on the scales, you can see it weighs 24 kg (52.8 lbs).
That was easy enough, but what if the timber had have been longer, heavier or both. Crikey, what a struggle!
Well, there is an easier way and its commonly used in the big boat industry.
Find a level spot and lift one end up and support it on a block of wood, right at the very end. At the other end, lift up the timber and place it on a set of household scales. Use a small block of wood/ rubber etc between the scales and the timber. Note the weight. Here we can see that the weight for one end is 12.125kg (26.73 lbs)
Now dismantle and place the scales at the other end and repeat. Here too we can see that the weight is 12.125kg (26.73 lbs)
After weighing, take the timber off and in this case I then weighed the rubber block. It weighed .125 kg.
Subtract this rubber block weight of the weights you took for each end and you end up with 12kg
(12.125kg - .125kg = 12kg)
Now, by adding both the final weights together, we arrive at the total weight of the timber. 12kg + 12kg = 24kg
Too easy huh?
Now you don't have to place the supports right at the very end. You can come in at odd distances from the ends, but once you marked those spots you cannot change them other wise it will give wrong results.
Let's now do the same with the Mushulu 14 and compared it with my weights and moments calculations that I compiled when designing the boat in the beginning.
Following the same procedure as above, grabbing the scales, we lifted the back end of the boat and weighed it.
We then went to the bow and repeated the same here
You'll notice in the picture that I used a drum of Acetone. The weight of the Acetone drum came to 1.5kg ( 3.3 lbs)
I'll now use the bow as our reference point for our calculations.
Transom weight = 50 kg (110.25 lbs) less 1.5 kg = 48.5 kg (106.94 lbs)
Bow weight = 31 kg (68.35 lbs) less 1.5 kg = 29.5 kg (65 lbs)
Weight 106.94 lbs
65 lbs
171.9 lbs total weight to this point in the build process
(171.9 x .454 = 78 kg)
Now referring to my weights and moments spreadsheet and comparing weights of the build so far, the calculated weight should be approx 165 lbs ( 75 kg)
So with the weight difference in plywood weights, we're 3 kg over weight (6.6 lbs)
That's a large bottle of Coke! I can live with that !!!
Have fun
regards
Mark Bowdidge (MRINA)
Bowdidge Marine Designs
