Boat hull formula

[duncan webster 1Participant @duncanwebster1]

as long as you have one of those drawings showing lots of sections of the hull at regular spacing you can get a better estimate of hull weight and displacement as follows:

measure the periphery of each section, one of those little wheel things would be good, or use string.
Multiply by the distance between sections. This gives you the area of the hull less deck. Multiply by weight per unit area of copper sheeting and you have the mass of the hull.

Now make templates from cereal packets of each section up to the waterline. Weigh them (all together). You'll need a fairly sensitive balance. Cut out a square foot of cereal packet and weigh that. You can now work out the total area of your sections. (mass of all / mass of a square foot). Multiply by the distance between sections and you have the volume up to the waterline.

This doesn't get the fore/aft position of the cg, that gets a bit more involved

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Again thanks for the replies, I can see the differences balsa wood and 16g copper weight and the displacement has been noted with the imput here. I am going to rough make the hull out of cardboard cover with plastic and put the weight of the copper and parts distributed and see what happens.

Bob

[Ramon WilsonParticipant @ramonwilson3]

Just been looking for a fibre glass tug hull – Kingston Mouldings who did several ceased trading in 2016 but you may find something on eBay.

What I did find however is this article on 'Making ship models in steel'. Steel? yes tinplate as already mentioned. Key to construction believe it or not is lightness! Well worth a read.

For some reason the discussion seems fixated on copper sheeting and 16swg at that – the use of which is more akin to a small boiler than being a suitable material for a hull.

If copper sheet of a suitable thickness were to be used it would need to be very thin. Being ductile of course this would allow some nice curvatures to be formed but conversely would easily be distressed by the slightest knock once built.

Depending on design, building a boat hull is not as difficult as it seems to many. There's plenty of information freely available to support it. There are also several options from which material to use point of view but I don't see copper as one of them – Sorry!

Regards – Tug

[JasonBModerator @jasonb]

Ramon, I think the reason the thread is fixated on 16g copper is that Bob said he already had some so wanted to use it, no other reason.

[Ron LadenParticipant @ronladen17547]

You could alway make the hull from strips of brown packing paper and pva glue it works great, I've done it..

[BOB BLACKSHAW 1Participant @bobblackshaw1]

I have a load of packing carboard from a fridge that was delivered, I am to make the hull out of that and see how much weight it will take for the the correct water line.

Bob

[roy entwistleParticipant @royentwistle24699]

Casting my mind back some 70 years could calculus be used for this ? I could never find a use for it so it didn't sink in ( pardon the unintended pun )

Roy

[SillyOldDufferModerator @sillyoldduffer]

I hope no-one thinks I'm proposing Copper is a good choice for a hull, only that the numbers suggest it's feasible.

Advantages:

• Bob has a sheet of it!
• Waterproof, rot-proof and fire-proof
• Monocoque construction possible
• High joint integrity, soldered, riveted, welded.

Disadvantages:

• Cost
• Weight
• Need for Copper-smithing skills

In Bob's case, only excessive weight is a potential show-stopper, but provided the total weight of the model is smaller than the displacement, it will float. I think it's feasible to use Copper. Actually, hulls have been made from all sorts, some highly unlikely! Woods, leather, bark, fibre-glass, plastic, concrete, iron, steel, aluminium, carbon-fibre, and exotics like newspaper, duct tape, and Pykrete (sawdust and ice). Wooden ships were often 'copper-bottomed'.

The material used to make a hull is a wide engineering choice. Leather is good for a primitive agricultural community with no trees, because they had few alternatives. Not so smart today. Concrete is normally considered too heavy for the same reason Tug and others rightly object to Copper, but concrete being cheaper than steel has been much used for static floating objects, lighters, and – most famously – the Mulberry Harbours. Rarely used for mobile vessels because fuel is wasted moving a heavy concrete hull rather than profitable cargo.

Balsa is much favoured by model makers for the reasons explained by Andy, but Balsa's high Specific Strength doesn't scale up. Balsa isn't used much in shipbuilding or aero-space. In common with all woods the size of individual members is limited. Above a certain size wood has to be jointed and joints are weak. The Kon-Tiki raft is about as big a vessel as can be made from balsa, and it's performance is easily exceeded by steel oil-drums! But Balsa's scale problem doesn't apply to a 30" long model boats, where most woods are suitable. Balsa is a better choice than copper, as are all the other model boat builder alternatives.

One advantage of copper, perhaps, is it allows monocoque construction. Hulls built from components like strips of wood supported on frames have multiple points of weakness. Considered as girders, wooden ships are weaklings. Hogging and sagging sank many vessels and it's all but impossible to build a wooden ship above about 1800 tons displacement, or beyond about 6000 tons with help from metals. Bob's hull, I think, will be self supporting with no need for a keel or frames, Unfortunately 16 gauge is heavier than needed, but hey, that's what he's got. Monocoque is difficult to do in non ductile metals like steel, but easy in fibre-glass.

I don't think there's any disagreement in this, we're looking at the same problem from different angles, If I was building my first model tug, and had to buy all the materials in, I'd follow the time-assured advice of experienced boat-builders. But Bob's interest in Copper sheet seems reasonable to me! If nothing else, it'll give the Antiques Roadshow something to talk about in a hundred years!

Dave

[SillyOldDufferModerator @sillyoldduffer]

Posted by BOB BLACKSHAW on 08/12/2020 10:45:50:

I have a load of packing carboard from a fridge that was delivered, I am to make the hull out of that and see how much weight it will take for the the correct water line.

Bob

Good idea!

[Andy_GParticipant @andy_g]

Posted by SillyOldDuffer on 08/12/2020 10:52:52

provided the total weight of the model is smaller than the displacement, it will float.

It won’t float for long unless the centre of gravity is also below the centre of buoyancy (lateral and transverse)

I’d be surprised if the displacement didn’t come out between 7 & 8 kg (15-18 lbs).

Anyone remember the ‘Krispie’? 36” long sailing boat built from strips of cereal packet (I built one once…).

[Ramon WilsonParticipant @ramonwilson3]

Dave – You raise some good points but you'll not convince me on copper!

I'm not disputing the numbers but the question was – is the material to hand suitable before I go ahead. I am not 'objecting' to the use of copper – but am saying that its use is eminently unsuitable for the task in hand. Just because 'you have a sheet of it' does not make it right to use.

I did point out, on my first post, that something will float providing its weight does not exceed the water it displaces.

If the material (16swg copper) were to be used however think of the heat required for any soldering – even soft soldering. If it's rivited then that requires over laps or butt plates – more material even more weight.

You can't compare the making a model boat to a full size ship – either up or down. The principle may be the same but scaling the material down would give a very thin sheet to make the hull indeed. 'Copper bottoms' were not to help the boat float either – it was to prevent growth of organisms. As far as I have been aware it was just nailed in place and certainly not caulked so hardly water tight. Beside it's protective qualities then all it did then was to add weight to the hull.

Concrete was indeed used to make boats even small cruising yachts as well as 'Mulberry harbours' but for some reason has gone out of favour probably no doubt due to the increase in strength of resins.

Balsa is eminently feasable for a model to a considerable size – if skinned inside and out with glass and epoxy resin – the strength comes from the resin and cloth – the balsa is merely the form. The more layers of glass cloth the stronger it becomes but the more resin required can then make it too heavy – for some purposes. A tug hull wouldn't be that affected but the hull for a model racing yacht would.

So in all I come back to what I've been continuing to say all along – the lighter it is the more you can place inside it – for that given water line.

Great discussion BTW

Tug

[BufferParticipant @buffer]

Posted by Andy Gray 3 on 08/12/2020 13:02:46

It won’t float for long unless the centre of gravity is also below the centre of buoyancy (lateral and transverse)

Are you sure about that?

[BufferParticipant @buffer]

[Andy_GParticipant @andy_g]

Posted by Buffer on 08/12/2020 15:40:22:

Are you sure about that?

Fair cop. Didn’t want to get into righting moments, but simplified it a step too far

Point should have been that stability is a consideration as well as just displacement which requires consideration of the C of G.

Must try harder!

[Tim Rowe 1Participant @timrowe1]

Buffer has it correct. A vessel has to have a positive Metacentric Height to be stable. The cg can be above the cb.
His diagram shows that the cg stays fixed but the centre of buoyancy moves outwards creating a righting lever. All the time you have a righting lever the vessel will return to upright (wave motion excepted) but once a certain angle of heel reached, you are going to get wet.

If a ship is carrying a very dense cargo such as iron ore, the cargo is built up into a pyramid to raise the cg. Otherwise the ship could be too stable and be over-stressed by the more violent motion this would produce.

The is a high speed superyacht that was built with an alloy hull to save overall weight and a steel superstructure to make it less twitchy and more comfortable. It is true however that the other way around is the norm.

Tim

[Ady1Participant @ady1]

We used G and M a lot at sea

The weights on the ship were put into average distance boxes from the centreline and keel if memory serves, it was a long time ago…

So the engine room was a fixed weight with a low centre of gravity on the centreline

The accommodation had a high centre of gravity

There was a lot of guesswork, but it was well informed average guesswork

Then it all got checked manually via the fore and aft drafts to check there wasn't anything badly amiss

Only material weights were relevant, cargo placement, large tanks of fuel or water, even the 25ton swimming pool on one 5000t ship since it was only just below the bridge deck so a long way above the centre of gravity

You will have to do the same sort of thing, then trim your creation with "ballast" as required to get the best result once it has been launched

Model sailing yachts are simple since you put a big fat bit of shaped wood on the surface then hang a stonking bit of lead at the bottom of the keel, change the depth of the keel, change the righting moment lever

Edited By Ady1 on 09/12/2020 01:20:32

[SillyOldDufferModerator @sillyoldduffer]

Posted by Ady1 on 09/12/2020 01:07:47:

We used G and M a lot at sea

…

Then it all got checked manually via the fore and aft drafts to check there wasn't anything badly amiss

Only material weights were relevant, cargo placement, large tanks of fuel or water, even the 25ton swimming pool on one 5000t ship since it was only just below the bridge deck so a long way above the centre of gravity

You will have to do the same sort of thing, then trim your creation with "ballast" as required to get the best result once it has been launched

Model sailing yachts are simple since you put a big fat bit of shaped wood on the surface then hang a stonking bit of lead at the bottom of the keel, change the depth of the keel, change the righting moment lever

…

That's exactly as I understand loadings – vital to put weight in the right places! What I don't understand is how the stability of container ships are managed:

There's a lot of weight stacked high on the deck. Presumably heavy containers full of Chinese lathes are loaded first deep in the holds, and only light containers full of bubble wrap go on the top layer. Although the pictured ship is carrying a full load of containers, it appears to be running light – the red bottom and bow bulb are both above the water line. I don't recall any container ship capsizes so it must be safe. Unlike badly loaded aircraft, and roll-on, roll-off, roll-over car ferries!

Dave

[JasonBModerator @jasonb]

Following my post about using the Trace function in Alibre someone on MEM queried it and I went looking for an answer. I mentioned using CAD to model the hull early on and if you look here at the third row down, left hand video "Trace images in 3D" it starts with how a boat hull can be drawn from the profiles on the plans

[Ady1Participant @ady1]

Container weights are declared in advance for the loading plan

Containers are pulled by a tug onto a weighbridge before loading, weight is checked, then tug moves under crane

Light ones go up top

Can be a pain if they are last off at the last port of call, nowadays I would reckon the loading plan is 95% computerised with a few PITA containers in a dedicated couple of slots which are faster to load/unload

edit: I always avoided maersk. Lovely ships, awful wages.

Edited By Ady1 on 09/12/2020 10:30:23

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Great replies thanks all. The Maersk ship showing the load line you can just see the top of the bow bulge, how much of the ship hull is in the water in depth, say fifteen feet for example. Looks like a good storm would roll the ship over, as stated light loads on top.

Bob

[Ramon WilsonParticipant @ramonwilson3]

Bob, I intended to say this on my last post but for some reason it got ommitted!

Your idea to mock the hull up and test is a good one. Hope it provides the answer you are looking for and allows you to go ahead.

Good luck with your build whatever way you choose.

Regards – Tug

[Neil WyattModerator @neilwyatt]

Posted by Ady1 on 09/12/2020 10:28:44:

Container weights are declared in advance for the loading plan

Containers are pulled by a tug onto a weighbridge before loading, weight is checked, then tug moves under crane

Light ones go up top

Can be a pain if they are last off at the last port of call, nowadays I would reckon the loading plan is 95% computerised with a few PITA containers in a dedicated couple of slots which are faster to load/unload

edit: I always avoided maersk. Lovely ships, awful wages.

Edited By Ady1 on 09/12/2020 10:30:23

Reminds me of my steplad explaining how he calculated the loading of mail planes for DHL. Basically you start with the heavy ones at the middle and work outwards balancing as best you can.

He got a worksheet and had to work it out with a pen or pencil, multiplying each cage's weight with the distance of its loading bay from the CofG IIRC.

The pilot then had to check his workings before takeoff.

Neil

[BufferParticipant @buffer]

With regards to the container ship loading. Anyone who has been in a rowing boat ( or kayak) will have first hand knowledge of just how high the c of g can be with what seems like very little under water and still be perfectly stable. But as we all know stand up in either and it's a different matter.

[Dave HalfordParticipant @davehalford22513]

Posted by Buffer on 11/12/2020 08:19:16:

With regards to the container ship loading. Anyone who has been in a rowing boat ( or kayak) will have first hand knowledge of just how high the c of g can be with what seems like very little under water and still be perfectly stable. But as we all know stand up in either and it's a different matter.

Yep, it changes the name to paddle board

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