Inflatable boat construction

[odysseus]

I've been playing with the idea of making an inflatable boat. True, there are many available at a low price, but isn't it more fun to built it yourself - and get all your design ideas into it? Does anybody know of any instructions on how to make an inflatable?

I figure that one first has to make the tubes (they would be glued since a welding machine is not for the home builder). In what order and how should one assemble the pieces? Next it seems one has to inflate it, test for leaks, and attach the transom. I'm considering using a urethane resin to actually mold it in place. Finally the floor is attached, which seems to be rather straightforward. And after that all the little details, plywood floor, etc.

Instructions or experience in general, or specifically on making the tubes or attaching the transom, are most welcome...

 

[Sealion]

Making Inflatable Boats

I am not aware of any plans or instructions on the net for inflatable boats, give me some idea of the size, power and intended use for your dinghy and I will try and steer you in the right direction.

 

[odysseus]

I'm considering making a max 3 m dinghy for max 8 hp, with max 50 cm tubes. I have material over from another project. Specific questions:
1. When making the bow, what is easier, to first make the "cylinders" and then attach them to one another, or to do the along seam last?
2. Since there are several chambers, is it a good plan to make one first with V-shaped ends (i.e. the bow section), inflate it, and then glue the long sides onto the inflated one? (Of course this requires that the along-seam is made first in the side tube.)
This is strictly for the fun of it (although I try to avoid sniffing the glue) so I'm not after commercial quality, just something relatively easy to make and safe to use.

PS. Intended use = cruising along the beach looking for good snorkeling sites.

 

[Sealion]

Inflatable Construction

1. When making the bow, what is easier, to first make the "cylinders" and then attach them to one another, or to do the along seam last?
-It can be done both ways, though straight seams are easier to do neatly.

2. Since there are several chambers, is it a good plan to make one first with V-shaped ends (i.e. the bow section), inflate it, and then glue the long sides onto the inflated one?
-It is usual to make the outer "skin" first, then insert the baffles(made of lighter , more flexible fabric) afterwards. If you make each chamber separatley and then try to join non- inflated sections to inflated there is a good possibility of your boat ending up looking like a badly stuffed sausage.

I will post more information later.

 

[Sealion]

Odysseus,
Some questions for you-
What type of material do you have? PVC, Hypalon or other?
Do you intend to build using straight sections of tube (stovepipe style) joined together, or cut curved sections from the fabric?
How much fabric do you have? What is the width of the roll?

Draw a quick sketch of what you have in mind, and any other specific (or general) questions you may have I will try to answer them. Your final design may depend on what materials you can lay your hands on.

 

[odysseus]

Sketch

Here is a sketch, a simple design 3.0 x 1.6 m, 0.45 m diameter tubes. The material I have is urethane over polyester, tensile strength about 40 kN/m, fabric weight 0.78 kg/m^2, it is 1.56 m wide and I have more than enough.

I appreciate your insight!

 

[Sealion]

Pictures make it so much easier to reply!
My first suggestion (not a critisism) would be to change the shape of the bow slightly. I would get rid of the \/ from the inside of the bow and change it to \_/ with at least 3" across the flat. this would give you a little more space inboard, but more importantly it would make for easier construction as you would avoid overlapping seam tapes. To glue this little beauty together I would first join all the sections to each other, leaving the long, straight outer seam 'till last. I would join each section together using a butted joint with a seam tape on the inside to start. The outer seam tape would be glued on after the craft was inflated.

more to come in a minute

 

[Sealion]

You realise none of these transverse seams are straight? They will look something like this. To join curved edges together, I place a suitable sized football (soccer ball please) on the workbench before laying a 2" wide glued seam tape on the top of the ball and then place the two edges together onto the tape, having drawn aligning marks along each edge as a certain amount of tension and stretching is required to stick the two sides neatly to the centreline of the tape. This job can be practised without glue using 2" masking tape or similar. The idea is to get the two edges hard up against each other without gaps or overlaps.
More to come in a minute

 

[odysseus]

One can be even more explicit and add that the fabric must be cut in a sinus curve, why it is not hard to create the exact shape with a computer.

Regarding butt seams, it sounds reasonably achievable the way you do it. Let me just ask you, as I have seen some manufacturer boosting that they overlap the panels and then put seam tape on, effectively creating not 3 but 4 layers in the seam. What could the pros and cons be with that method?

 

[Sealion]

You got it, I apologize if I'm teaching you to suck eggs, but I don't know how knowlegable you are. Are you familiar with the gluing process (how many coats, drying times etc)?
As for the differences between butted and overlapping seams, butt seams are entirely adequate as they are already stronger then the surrounding fabric and are much easier than overlaps when joining sections of tube with a large mitre angle as there is less distortion (stretching and bunching up) of the tube fabric. When joining sections with a big angle between them I would cut the joining tape on the bias i.e 45 degrees from the roll which gives a much more flexible tape. Overlapping joints are good for straight sections and places where you dont want or need an outside seam tape like joining tail cones on to the tubes. 4 layers of fabric is probably overkill and makes the joint bulkier and visually more obtrusive.

 

[Sealion]

Joining straight seams.

Another advantage to a butted seam is that you can fix the two pieces of fabric to be joined together on the back with 2" masking tape and make sure you have it all aligned properly before gluing the seam tape down on top.

 

[odysseus]

Selecting tube diameter, pressure, and fabric

This is how I figure a suitable diameter of the tubes. Please correct me if I'm wrong.

To create a large, sea-worthy and fast boat one wants large diameter tubes with high inflation pressure. This air pressure is exactly counteracted by the stress in the fabric along the perimeter of the tube (assuming an infinitely long tube).

Let P be air pressure in pascal (Pa = N/m^2) [100 kPa = 1 bar, ca 1 atm, ca 15 psi]
Let F be force in newton (N = kg*m/s^2)
Let A be surface area of the tube in square metres (m^2)
Let Ø be diameter in metres
Let L be the length of a tube section in metres

then we have:
A = Ø * pi * L
F = P * A

The tensile strength of the fabric must be greater than this force. For instance, 0.45 m tubes made of a fabric with tensile strength 40 kN/m would have a breaking pressure of just over 28 kPa (4.1 psi). How much margin would be adviceable to keep to that value (in terms of installing a pressure relief valve)?

BTW, the equation means that for a given fabric the tube diameter and air pressure are inversely proportional.

I guess in most fabrics the tensile strength is some 10% to 20% higher in the warp than in the weft (fill). It therefore seems that the best strategy would be to use the warp around the tubes. Is there any disadvantage with that (except that the length of each tube segment is limited by the width of the fabric)?

 

[odysseus]

Are you familiar with the gluing process (how many coats, drying times etc)?

I have tested a bunch of glues on overlapping seams, and found that Weld-On 66 works for most tasks, is strong and can be used outdoors in our climate.

However, I realize that I need a seam tape for this purpose (at least on the outside; I guess I could cut strips of the fabric for use on the inside). Since I have no experience with that, feel free to pass on some advice.

 

[Sealion]

Before joining up the long seam, stick 3"or 4" circular patches as doublers on the inside of the tube where the inflation and pressure relief valves are to be positioned. Cut the holes for the valves but do not fit them as they are likely to get in the way while you are gluing the main seam. Also mark and prepare the positions where the baffles between the compartments are to be situated. On your sized boat 3 separate sections should be enough (2 baffles). I'll give you my opinions on making and fitting baffles in a later post.
As I said in my previous post, these long straight seams can be taped into position using masking tape first. It will probably easier to do this in 4 sections, from the centreline of the bow to past all the joins both sides and then the two straight sections. You will be doing the inside tapes first so you end up with an inside-out tube with no tail cones. Leave a section 450mm-500mm unglued in the middle of each of these last two long seams to enable you to fit the baffles and end cones. (glue the seam tape down all the way along the seam but leave the glue off one side of the fabric.

The world cup has just started, I'll Be back later.

 

[E Hanson]

Sealion,

You sound like a builder...Any trade secretes you need to get off your chest?

Odysseus,

A appaud your ambition. For a long time I have felt that inflatables and RHIB's are under-represented in the world of home builts.

Here are my two bits.

Make a test section of your collar to determine the fabric's stretch factor. It will differ allong the warp and the weft. Draw a square aligned with the fabrics core. 10 cm x 10 cm or so. Inflate to operating pressure and let the sample sit for 24 hours. while inflated, measure the square. Use this to work out your stretch % allong the warp and weft.

Rhino is a great program to develope and flatten collar patterns. Work out the collar shape to the final size you want. Develope the panels. reduce the panel size by the streatch %. If you are using overlap seams, add the overlap to the perimeter of the pattern. If you choose butt seams, your patterns are done.

Overlap seams can be done without seam tape in most hypalon/neoprene fabrics. Urethane fabrics often requre seam tape. The problem is that some fabrics "wick" the air through the core. My not knowing what your fabic is, you may need to test this. Butt seams do not wick air.

Good luck,

EWH