How to calculate balast ratio/ water line

[Ian Farnell]

I am fitting out a hartley figian which has never been launched. The designers calculate a displacement of 17 tonnes. She currently weighs 21 tonnes and I am expecting when fully fitted 23 tonnes plus. How can I calculate at what level she will float ie the waterline before launching?

[Skippy]

submerged volume == total displacement / density of water

density of water == 62.5 lb/ft^3 fresh or 64 lb/ft^3 salt

You just need to find the waterline with that volume below it by adding up horizontal slices of the hull volume. Is that what you're looking for?

[Ian Farnell]

Thanks Skippy, lets see if i have grasped this
total displacment at 23 tonnes taking a ton as 2,240 pounds = 51520

51520/64=805 cubic feet.
I then calculate at which point I reach 805 cubic feet as you suggest with horizontal slices.

The designer prescribes a waterline lenght of 36 feet and a draft of 5ft 9in. So I will start by calculating the volume at that point and then add or subtract layers.

Thanks again
Ian

[jehardiman]

Ahhhhhh....just a moment here. I hope there is a good reason that your weights are off by 20%.

Is that 21 tonnes weighed? By what? A crane readout? And are you sure if that long tons (2240 lbs), short tons (2000 lbs) or tonnes (1000 kg = 2204 lbs)? Because 21 short tons (a comon crane readout) is 18.75 long tons, which is a much more reasonable number for a slightly overweight hull.

You know that this could mean that the craft is dangerously unstable unless you did a complete stability analysis and from your question I take it that is not the case. You should see if you have or can get a copy of the "Displacement and Other" (normally called the D&O) curves from the designer (this will also tell you where to put the waterline). You will also may need to do an air inclining to determine the VCG. With thr D&Os and the VCG, you can quickly determine the initial stability so you won't risk rolling her over at launching.

[Ian Farnell]

I have been in touch with the designer who tells me she should weigh 17 to 18 tons before fit out. You are quite right the 21 tons was from the crane and as you say this would make her 18.75 which is close enough as she was not totally striped out when transported and I have taken at least the difference out of her before starting the refit. I would normally expect to add ballast to bring here down to her lines. The design is tried and trusted and she appears to be built to spec so this would explain the apparent discrepancy.
Thanks for your insight I had totally missed the difference in the measurements
What a great site this is. giving access to so much knowledge.
Thanks again
Regards
Ian

[jehardiman]

I'd still double check the units used by the crane. Weights are an area that can bite you hard.

[Ian Farnell]

Ok it's best to check ,I will contact the crane operator and check what units his crane uses.
Thanks again
Ian