Buoyancy calculations

Discussion in 'Boat Design' started by Len, May 24, 2020.

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1. Joined: May 2020
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LenJunior Member

Hi everyone,

Just joined the site and can't believe the amount of information that is available. What an awesome resource!

We are in the final stages of building a small flats (15') boat and we were wondering if anyone could recommend a good source for calculating buoyancy. We would like to reduce the trial and error part of the process as much as possible so any suggestions would be greatly appreciated!

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Eric ruttanSenior Member

I guess how much water you are displacing times the weight of your water is pretty close to your buoyancy?

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DCockeySenior Member

So would you suggest that Len calculate the amount of water which is displaced?

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TANSLSenior Member

Welcome to the forum, Len. Do you have a sketch of your boat with the dimensions ?. With this it will be possible to easily calculate the displacement (buoyancy) at various drafts.

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bajansailorMarine Surveyor

Len, can you post a photo or two of what you have built so far?
And a drawing if you did a basic sketch design before hand?

Here is a link to a long thread by Aransas Flat Rat about designing and building his flats boat -
Help drawing and design Texas Scooter boat https://www.boatdesign.net/threads/help-drawing-and-design-texas-scooter-boat.63386/

As TANL mentions above, it should be fairly easy to calculate hull volume at different drafts (even if you are doing the calculations by hand).
I am assuming that the beam of your flats boat is fairly constant from bow to stern?
If it is, then if you measure the cross section area of the side of the hull at a particular draft, and multiply this by the beam, you will get a rough approximation of the volume of displacement at that draft.
If you are calculating in metric (ie dimensions in metres), then multiply by 1025 for the displacement (in kg) in sea water.
But you are more likely to be using imperial measurements.
If you are, and you measure the volume in cubic feet, then one cubic foot of seawater is approx 64 lbs.

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LenJunior Member

It's actually a flats boat/paddleboard hybrid. 15' long and 5' wide at the widest. We don't have a sketch, I can draw something up if it would be helpful. These are images of the top and bottom molds.

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bajansailorMarine Surveyor

Re how you have built hull and deck moulds, this is a lot of work (and they look like nice moulds) - are you planning on building more than one boat from these moulds?
Calculating the buoyancy will be a bit more difficult compared to a more rectangular flats boat like what Aransas Rat is building, but not too difficult.
Probably the easiest way would be to mark on the hull mould the different drafts for which you want to calculate the buoyancy / displacement.
Level the hull mould such that it is trimming typically how the finished boat will be trimming.
Then start to fill up the mould with water until you reach the mark for the first draft mark.
If pouring from a known container size, then you can calculate how many gallons of water have gone into the mould.
One US gallon weighs 8.34 lbs according to Wiki -
Gallon - Wikipedia https://en.wikipedia.org/wiki/Gallon
Add more water to reach the 'deeper' draft marks.
If it takes (say) 40 gallons of water to reach a particular draft mark on the mould, then the displacement is approx 334 lbs at that draft and trim.

Last edited: May 24, 2020
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LenJunior Member

Yes, we hope to make multiple boats from the molds and offer them for sale and rent.

I don't think we can do the test as you described because we have attached a heavy steel frame to each mold with fiberglass tabs so floating the mold isn't an option. I think we're going to have to hope we get it close to correct with our estimate of how much buoyant material that we sandwich in the middle of the top and hull

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TANSLSenior Member

It is not about putting the mold afloat but about putting water inside the mold. You put a known amount of water into the mold and point to how far the water in the mold goes. This will give the displacement of the boat to that draft. Continue adding known amounts of water and recording the height.
If you need any other information about the submerged volume, you will have no choice but to draw a picture of the boat with various measurements in order to correctly define the hull.

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bajansailorMarine Surveyor

Apologies Len for not being clear - I was not suggesting that you 'float the mould' - rather, that you fill it with water up to the various drafts that you are interested in. Measure the quantity of water that you pour into the mould - as mentioned above, if you pour 40 gallons INTO the mould, and it reaches a particular draft mark, then your displacement at that mark will be 40 gallons of water or 334 lbs.
All as per Archimedes Principle.
Archimedes' principle - Wikipedia https://en.wikipedia.org/wiki/Archimedes%27_principle

You mentioned "I think we're going to have to hope we get it close to correct with our estimate of how much buoyant material that we sandwich in the middle of the top and hull"
Be aware that you will have the same amount of buoyancy in the hull no matter if it is empty of if it is full of foam.
If you fill it with foam, then it will become a bit heavier, and sink a little bit deeper in the water.
I think it will probably be the USCG regulations which determine how much foam you put in the hull before you attach the lid (deck).
You will want to use closed cell foam - Ike has in the past suggested ping pong balls and pool noodles.

I must admit that I am impressed by your faith in the boat, where by your have built plugs and moulds, yet you still have no idea how much deadweight these boats can carry, or what their load displacement with the maximum deadweight allowable will be.
You normally work out fundamental things like these quantities fairly early on in the design of the boat, not after you have built the moulds, and you are hoping that you will have enough buoyancy / floatation.......

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LenJunior Member

Yes, we are rookies at this and the more we learn the more we find that there is to learn. We have been working on this on weekends for a couple of years and the closer we get to the finish the more questions we have. We absolutely should have done more calculations before we began but that ship has sailed. Everything to this point has been done by eye and "feel". Thank you for the education, clearly we have more to learn!

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Phil_BJunior Member

I agree that using water to find the volume of the hull is the best way to do this.

To calculate the displacement (which is for practical purposes is the weight of the vessel) make sure that you include the weight of everything that is going into or on the hull (that is, the hull itself, the deck, fittings such as hatches, cleats, the weight of the foam - which is only to keep the water out of the enclosed buoyancy volume - and the expected crew weight. From this weight, calculate how many gallons of water this represents as Bajansailor and TANSL explained.

Fill the mould with this amount of water and you can mark the inside of the mould with the waterline. Measure down from the edge of the mould to the water and transfer this measurement to the outside of the hull to find your approximate waterline at this displacement. I say "approximate" because the thickness of the hull will make the water level inside slightly higher than the true waterline, but not by much.

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TANSLSenior Member

Precisely with a female mold, this correction that you suggest is not necessary, the weight of the volume of water in the mold is exactly the displacement of the boat at each draft.

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Phil_BJunior Member

Doh! Of course! I must be going senile and was thinking that he had a moulded hull in the form.

I'm glad you spotted the deliberate mistake. Ahem! >};o)

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LenJunior Member

That is an awesome explanation. We will definitely follow that process to get the displacement. It will be a couple of weeks but I will share the results once we've completed the process.

Thank you all for your assistance with educating an amateur!

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