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Building A 7 ft. 10 In. Sailing Pram

There are other methods to calculate displacement weight:
1. Put weight in the boat until water comes in. Add up the weight and subtract the hull weight.
2. A variation on that, since most of us don't have 1000 lb. of weights laying around, is the bucket method. Use a bucket that is marked with the number of gallons it holds. Fill the bucket and pour it in the boat. Do this until the water is at the same level as the outside. In other words, water flows in and out freely through the centerboard trunk. Count the number of buckets and multiply them times the gallons per bucket. Multiply that times 8 lb. per gallon (approximate weight of water is 8 lb.) That is the displacement weight.
Caution: Do Not Do This With The Boat Out Of The Water. You Will break the boat!
Finding Maximum Capacities:
Maximum weight capacity = 3/10 of Displacement weight = 0.30 X 1291.792 = 387.5376 lb
Persons Capacity in pounds = 9/10 of Maximum Weight Capacity = 387.5736 X 9/10 = 348. 78
Persons = Maximum Weight Capacity + 32 divided by 141 = 348.78 + 32 / 141 = 2.7
I have done this using four decimal places but this is not necessary. You could round the displacement weight to 1292 and the Maximum weight capacity to 387. The numbers would still be correct.
I then down rated the maximum persons pounds to 300. This allows me a capacity of 2.4, or in this case 2 people. If you used the full number, 2.7 then it can be rounded to 3, but I feel 3 adults is too many for this little boat.
Calculating Flotation
To determine the required amount of flotation you need to break down the weights into three categories.
Hull Weight
Persons Weight
Machinery Weight
This boat has no machinery weight.
Hull Weight = 60 lb. (weigh the boat)
Persons weight = 300 lb.
The Hull is wood so you multiply the hull weight by a buoyancy factor (see Table of Factors)
The factor for plywood = 0.81 ( the minus indicates the material is buoyant so adds buoyancy to the boat
Wb = boat weight in water.
Fb = the amount of flotation to float the boat weight.
B = buoyancy of the flotation material = 60.4
Wb = 60 lb X 0.81 = 48.6
Fb = 48.6 / 60.4 = 0.80 CU FT Since this is negative you do not need any additional flotation to float the boat.
PC = persons Capacity = 300 lb.
Fp = 1/2 X PC / b = 1/2 X 300 / 60.4 = 150 /60.4 = 1.44 Cu. ft.
You then add together the Flotation for the boat Fb, and the flotation for the persons Fp.
Ft = total flotation = 1.44 Cu. Ft. + (0.80) cu. ft. = 0.64 cu. ft.
The bow seat is an air chamber of 0.58 cu. ft. and the rear seat is 0.88 cu. ft.
Total = 1.46 Cu. ft. So we have over twice the required flotation.
If this were to be built without the centerboard trunk and sold as a rowing dinghy then the requirements would be greater.
Capacity For rowing dinghy:
The point where water comes in is now about three to four inches higher in the boat, so the boat has a larger displacement weight and can hold more weight. (the following are estimated from the measurements of the sailing dinghy)
Maximum Weight Capacity = 498 lbs
Maximum persons Capacity = 448 lbs
Maximum Person = 3
So by not having a centerboard we could add one more person safely. However, from personal experience, I would be reluctant to put more than 2 persons in this boat.
Additionally, the Flotation requirements would be slightly greater but we still have more than required so no change.
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Revised 10/12/2017 © newboatbuilders.com 2010 All rights reserved.Photos Updated 07/18/2017