The following describes my experience of building a Gemini RIB. Gemini supply the tubes and the plans to construct a hull to match the tubes. Having paid for the tubes the plans arrived over the Internet with a list of materials. After advice from Jack at Gemini and looking at several web sites on boat building I decided to construct the hull of exterior quality ply (WBP Ply). This is significantly cheaper than Marine ply and as long as you build and paint the hull correctly it is as good.
There are several methods you can use for bonding the wood together. The traditional method is to use special boat building resin glues such as cascamite. I dismissed them straight away because it requires accurate joining for full strength of bond and it does not fill any inaccuracies or voids in the joint. For an amateur like me that would be a problem.
There are two new methods of bonding wood. The first is to use the new polyurethane glue, which foams on contact with air. It sets quickly and has quite a strong bond within half an hour. The foaming action fills voids and inaccuracies in the joint (which was ideal for me!) it is fully waterproof and recommended for boat building.
The other method was to use what is called composite construction. This is where the bonding is done with epoxy resin. The joints are filleted with paste (which is resin mixed with a powder) then taped with fiber glass tape. I saw this method of construction at a local boat show and I was impressed with the strength and finish. The resin comes in a thin liquid form, which you mix with various quantities of the powder to make different constituencies of paste for gluing, filleting and coating. As the strength of the bond is not dependent on the quality of the joint it was also ideal for my use. The bond is actually stronger if the joint is not too close. There are many web sites devoted to this type of construction so I will not go into detail here. Both suppliers I used sent the resins with comprehensive instruction sheets.
Because I was not using marine ply I decided to use the composite method of construction. The downside to it is the material is more expensive than glue and you have to be careful in its use. The strength and quality of finish however makes the extra cost worth it.
Do not be tempted to use the cheaper polyester resins. They are porous and moisture will work its way through the resin and cause the wood to get damp with the result being a failed joint and the coating lifting from the ply base.
To hold everything in place while the resin sets you need to either use screws or wire. I decided to use screws. The choice of screw types was dependent on who you spoke to. The various choices were:
Plated steel dry wall type screws. These quote ‘Grip like a rabid dog’ and as long as they are carefully filled and painted over should not corrode. I did not want to take the chance, there is always the risk that water would get to them and they would start to rust. Once rust does start it is difficult to stop it. It would be a pity if after a short period of time all one’s hard work was marred by nasty rust marks, lifting paint and filler.
Brass wood screws, these I dismissed quite quickly because they are expensive and weak. They have a tendency of twisting their heads off before they are fully driven home. I also wanted to use an electric screwdriver and slot head screws do not lend themselves well to this type of driver.
I settled on using stainless self taping screws firstly because they were readily available at my local hardware shop, they had Phillips counter sunk heads which is ideal for use with an electric screwdriver and they are strong enough not to break while being screwed home. The only disadvantage was they were designed for use with metal so the threads were small and not ideal for gripping wood. Because of the method of construction I planned to use I did not think that that was an issue.
Whichever method of construction you use the building sequence and methods will be the same.