Boat hull formula

Specific strength (strength to weight ratio) - Higher is better:

Balsa is 521 (kNm/kg)

Copper is 24.7 (kNm/kg)

For reference, carbon / epoxy composite 785 (kNm/kg)

(Ref)

So if everything was built to the same strength, a copper hull would be ~21 times heavier than balsa wood, a carbon fibre / epoxy one would be 50% lighter.

Regarding the business about calculating volume - the block coefficient gives the ratio between the volume (and displacement).of a hull as a fraction of the rectangular bounding box of the underwater dimensions. There are many references on line where one can find typical block coefficient values for different hull forms which can be applied to the model dimensions to give a idea of displacement - as already pointed out, it's best to have something in reserve for trimming & stability.

Sod and Bob

working out the displacement to the planned waterline is fairly easy if Bob has a drawing of the frames or stations along the length of the hull. But I thought Bob would have a very hard time working out if the boat would float at the waterline in trim ans stable before he builds it.

From memory of about 30 years ago, Bob if you dont have CAD you need to measure the area of each station or frame for the boat that is below the planned waterline. The more stations along the length of the boat the more accuracy you will get. If you have a lines plan of the hull then you could draw your own stations as many as you like although I believe you need an odd number for simpsons rule Once you have the areas of the stations beneath the waterline use simpsons rule and you get your volume or displacement.

If you have a lirnes plan you can work out all sorts of things like the centre of bouyancy but unless you know where you centre of gravity is going to be you wont be able to work out how stable it will be.

OK, done some sums.

Above I suggested a rectangular box 30" x 7.6" x 6" would support about 47lbs (21.6kg)

The box bottom has an area of 30 x 7.6" = 228 sq inches

A box end has an area of 7.6 x 6" = 45.6 sq in

A box side has an area of 30 x 6" = 180 sq in

Total area, bottom, two sides and two ends = 679.2 sq in

16swg is 0.064"

So volume of copper is 679.2 x 0.064 = 43.47 cubic inches

As Copper weighs 0.332lb per cubic inch

Hull weighs 14.94lbs, (6.43kg) leaving 33lbs available for the rest of the boat.

But as the full weight would have the deck level with the water, 33lbs is too much. I'd reduce it by at least 25%, leaving about 24lbs for machinery and superstructure.

Doesn't seem unreasonable, but the box shape provides maximum floatation. To be sure the actual volume of copper in the hull should be compared with the hull's actual displacement. That depends on the shape of the hull.

The triangular hull, capable of supporting 10.9kg (24lbs) is less well placed. It weighs 8.38lbs leaving less than 16lbs for the rest of the boat (only 12lbs with a 25% safety factor.)

Dave

Dave - I don't dispute your figures but having built the odd boat or two I would venture to suggest that a model of 30" LOA with a 7" beam and probably around 3-4" draft would be rather well down in the water if made from any material that weighed 15lbs before anything else is added to make up another 33lbs - 48lbs in total - not that far short of half a hwt.

I suggested balsa as being as far from copper purely as an example of two extremes but what ever material is chosen it should be suitable for the task.

Lightness of hull is key no matter what it is made from - the lighter that is the more you can add as internal structure, superstructure and power plant. Strength is a different matter however - it has to be strong enough to deal with the forces involved. Time was very resilient hulls were made from several layers of brown gummed paper tape laid over a form. Once cured and reinforced with suitable scantlings however a very strong hull indeed could be achieved. Now a layer of very thin glass fibre over a balsa shell is a common method among others.

To labour a point (sorry) - the heavier the hull the less you have for the rest - for a desired waterline. That was what the original question sought to find

Model boats of all forms have been made for many many years - to my knowledge none have been made with metal with anything heavier than tinplate or shim brass - 16swg copper is simply not the material to use unless the hull is proprtionately larger of course and that indeed would be a large model!

Sory to be so 'violently' different

Regards - Tug

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

For what its worth. I am not really a model boat afficionado, however I like to look and admire the work and on an irregular basis used to buy the magazine. I am trying to remember but I think about 3 years ago in Model Boat magazine there was a long quite protracted series from a uni lecturer (?) about building a warship entirely from metals. The plan was to have some pretty sohisticated propulsion equipment which failed in the early instances.

The whole project seemed to be fraught with difficulty at many levels. Perhaps it would, I respectfully suggest, that digging that out from the archives would be of some help.

Regards.

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

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

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.

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

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

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

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

Good idea!

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...).

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

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!

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