Size of boat equals = thickness of Hull

[Toby knight]

Could anybody please explain to me,

that the size of the boat limits the thickness of steel on the hull.

I've been enquiring about having a 30 foot boat for weekend use and been told that the 30 foot boat can only be made of 8mm thickness, is this true?

Many thanks .

[Alan de Enfield]

Rubbish

 

A company called 'Springer Engineering' used to build narrowboats with whatever steel they could get hold of - some boats were built with 3.75mm (1/8"), most with 6mm.

The only possible justification for saying 8mm minimum is that their design is so poor that its needs the additional strength to give it structural integrity.

 

Many, many se-going steel boats are built using 5mm steel.

[Toby knight]

Thank you for your reply, out of curiosity could I ask for the boat to be made in 14 mm steel

Only thinking in 20 years time, hopefully with a regular blacking, it would not need to be re-plated or am I being over cautious ?

[Alan de Enfield]

If you are having it built for you I would suggest you can have it any thickness you want - even gold plated.

It may cost you a fair bit more as they may already have 8mm plate in stock and will have to do a special 'small' order for 14mm, and, they may get a surcharge from their supplier.

 

My last NB had a 13mm thick baseplate.

[Joe the plumber]

If you mean for the sides of the hull, remember that whoever builds the boat has to be able to actually manipulate the steel. Bending 14mm plate would take some seriously heavy duty equipment.

 

There's no reason why the (flat) base plate couldn't be made of it though.

[Paul C]

Could anybody please explain to me,

that the size of the boat limits the thickness of steel on the hull.

I've been enquiring about having a 30 foot boat for weekend use and been told that the 30 foot boat can only be made of 8mm thickness, is this true?

Many thanks .

 

How many builders did you enquire with, and did they all say it can only be made of 8mm?

[Kendorr]

Halcyon was built C1989. The base plate is 6mm, probably 1/4 inch. Last survey in 2012 the minimum measured thickness was 5.7mm. So, lost a maximum of 0.5mm in 25 years. I'm sure 8mm plate will last your lifetime.

[Stewart Kirby]

Could anybody please explain to me,

that the size of the boat limits the thickness of steel on the hull.

I've been enquiring about having a 30 foot boat for weekend use and been told that the 30 foot boat can only be made of 8mm thickness, is this true?

Many thanks .

You can have whatever you like. In recent years 10x6x4 (bottom plate, sides up to gunnels, cabin and roof) has become very common. If cared for correctly theres no reason why a boat made of that spec cannot last for 25 years or more without needing remedial work. My last boat was that spec and the hull was still in excellent condition at 25 years old. I think going any other way is just adding to the cost for no good reason.

[Iain_S]

14mm steel is going to weigh a lot more than 8mm or 10/6/4 or similar, and the effect will be a lot more pronounced on a 30ft boat.

[Graham Davis]

Could anybody please explain to me,

that the size of the boat limits the thickness of steel on the hull.

I've been enquiring about having a 30 foot boat for weekend use and been told that the 30 foot boat can only be made of 8mm thickness, is this true?

Many thanks .

 

Take that to it's logical conclusion; a supertanker some 900 ft long would need a hull over 240mm thick!

Whoever told you that was VERY misinformed.

[Geo]

Rubbish

 

A company called 'Springer Engineering' used to build narrowboats with whatever steel they could get hold of - some boats were built with 3.75mm (1/8"), most with 6mm.

The only possible justification for saying 8mm minimum is that their design is so poor that its needs the additional strength to give it structural integrity.

 

Many, many se-going steel boats are built using 5mm steel.

He did not say minimum just "can only be made of 8mm thickness, is this true? "

 

The answer could be true. depends on the total weight of the boat. If it is too heavy it could either be too deep in the water or worst case sink, unlikely.

 

If I was you I would ask another builder

[system 4-50]

Are you thinking too much?

 

If you are asking for a boat to be built, the best way is to go to a good builder and tell him your requirements for what the boat is to do, not how the boat is to be made. He will tell you how it can be done with his standard equipment, standard materials, and his skills. If it can't be done he will tell you and, if he is willing, tell you how much extra it will cost you for extra equipment, non-standard

materials, and extra skills training and it will be a lot. This could be much more than the cost of the problem you are trying to avoid.

He also will tell you the downsides of what you are proposing.

 

Alternatively you can decide to do the design bit yourself and then you will need to learn about all the skills required to work all the materials and all the equipment possible - not to do it but so that you know what can be done, then design the thing, then finally find somebody who has or can get the set that you have decided upon to actually do it.

What you "can't" do is decide a few of the how to make the boat features in isolation. Well you can, but the result is likely to be bad.

I decided that my boat was to be built straight not banana shaped. One result of this is that my hatch drainage is not ideal. I specified a small part in isolation from the thing as a whole. This is a minor flaw but shows the risk.

 

Was your builder just saying "if I build it its got to be 8mm because that is what I do?"

 

How wide is your boat going to be?

Edited September 12, 2016 by system 4-50

[Athy]

He did not say minimum just "can only be made of 8mm thickness, is this true? "

 

 

I noticed that, too. It could be that Toby's statement means that 8mm is the maximum thickness which that builder would use for a 30 foot boat. Liverpool Boats used to use 8mm baseplates for shorter boats and 10mm for longer ones. I don't know at what point the change occurred; ours was a 40-footer and had an 8mm bottom.

[Toby knight]

The builder said anything more than 8mm would be to heavy for a 30ft boat

[dmr]

If we take this argument to its illogical conclusion then the builder just might be right. Imagine a boat only one foot long in 10mm steel, I reckon that would sink like a stone. Its all down to that Archimedes chap.

 

A seventy foot boat is said to go down by an inch for every ton that it weighs. A 35 foot boat, allowing for the swims, should go down by a bit more than 2 inches per ton, so engines, people and "stuff" will have more influence on a shorter boat. However a boat still needs ballast to make it stable so its not clear cut here.

 

...................Dave

[Geo]

The builder said anything more than 8mm would be to heavy for a 30ft boat

 

That makes good sense.

 

Think about a standard Trad boat. Suspect that if you look at them no matter what the length, plus or minus a bit, the bow section and the swim/utxer plate plate are of very similar size and do not change in length in proportion to the over all length of the boat.

 

No being near a boat at present I cannot do any measuring, but take the bow as being say 6 foot long and the swim/utxer similar a 30 foot boat would have a box between them of 18 feet x 6 foot 10 inches. That box is the main thing that displaces the water and thus carry the weight of the boat in the water.

 

If you the take a 50 foot boat the central box is 38 feet x 6 foot 10 inches over twice the carrying ability by weight for the same draft.

 

If you assume a 24 inch draft for both you can work out the weight permissible for that boat subtract from it what is going to be put into the boat by way of engines, fittings etc plus the overall weight of the steel used, baseplate, sides, bow, swim/utxer plate, rudder etc.

 

No knowing off the top of my head the weight of 8mm steel plate it would not surprise me if 8mm baseplate was all a 30 footer could carry.

[Mike Tee]

My last boat was 40' with a RN engine (heavy!) - base plate was 12.5mm (I'm guessing 1/2" plate) and floated and handled just fine so I don't think 8mm is the limit for a 30' boat. However, if looked after, it would outlast most people's expectations.

[Geo]

My last boat was 40' with a RN engine (heavy!) - base plate was 12.5mm (I'm guessing 1/2" plate) and floated and handled just fine so I don't think 8mm is the limit for a 30' boat. However, if looked after, it would outlast most people's expectations.

 

That extra 10 foot would give your boat assuming a 24 inch draft the capacity to carry an extra 3.66 tonnes. If the draft was 28 inches that would carry an extra 4.27 tonnes.

[Geo]

If you are having it built for you I would suggest you can have it any thickness you want - even gold plated.

It may cost you a fair bit more as they may already have 8mm plate in stock and will have to do a special 'small' order for 14mm, and, they may get a surcharge from their supplier.

 

My last NB had a 13mm thick baseplate.

 

How long was your last narrowboat?

[Alan de Enfield]

 

How long was your last narrowboat?

 

45 foot, and it still had 'tons' of concrete slabs under the floor as ballast.

[Captain Pegg]

The difference between an 8mm and a 10mm baseplate on a 30' boat will be in the order of 0.25 tonnes.

 

I believe Mike Tee's old boat sat relatively low in the water as does mine which is 35' and has two lots of 6mm plate on the bottom.

 

It sounds to me as though the builder is saying that for the type of boat they build he would use an 8mm thick baseplate. I am guessing that this builder probably builds to a style rather than builds bespoke boats. Or perhaps the OP wants a fairly conventional boat?

 

JP

Edited September 13, 2016 by Captain Pegg

[Geo]

 

45 foot, and it still had 'tons' of concrete slabs under the floor as ballast.

 

Yep and that 15 foot gives at a 24 inch draft an extra 5.5 tonnes of carrying capacity. Now a funny thing I think you will find that the stuff added to the basic steel of a boat cookers beds etc come out with a very similar weight regardless of the length of the boat. Yep you will get some boats that have two fixed berths but the weight of those compared to that of the steel is not great. Of course 30 footers tend no to have a fixed berth.

 

At a guess your draft was greater than 24 inches and the ballast was 3 or 4 tonnes.

 

In small boats it all comes down to the weight of the steel not just the base plate. On your 45 footer you had a box section approximately 30 foot long which had a capacity with a 24 inch draft to carry 11 tonnes compare with a 30 footer the box section would carry with a 24 inch draft 6.6 tonnes

[Geo]

According to my tables the base plate for a 30 footer would weight approx

 

8 mm steel plate 3014 kg 920 kg

10mm steel plate 3768 kg 1150.5 kg

12.5mm steel plate 4708 kg 1437.8 kg

 

ED wait I used feet not metres need to redo

 

corrected

Edited September 13, 2016 by Geo

[RLWP]

750kg, getting on for 2" difference in displacement for the 10mm plate

 

Interesting!

 

Richard

Edited September 13, 2016 by RLWP

[Geo]

changed mind

Edited September 13, 2016 by Geo