Foam Core Shiplap

I'm building a trimaran using a foam core sandwiched between two CF skins. This method is championed by John and Ian Lindahl, where the port and starboard sides of the hulls are partially built separately. For example, foam slats are laid in a female strongback to form the shape of the port half of the boat (see jpg below). The inside of the hull is glassed. then the process is repeated for the starboard side. The two halves are then glued together, faired, and then the outside of the hull is glassed.

There no way to shiplap, or otherwise reinforce, the inside joint after the halves are glued together. Can anyone offer a recommendation for reinforcing the joint on the inside before the hulls are joined, or offer any other advice about this joint? Is a butt joint sufficient?

Many thanks.

 

There is no need for any "reinforcement". The core gets glued with a butt joint, just like all the other foam to foam joints you have to make. The fiberglass is laid up as specified, inside and out. If some areas need to be stronger, more fiberglass is laid locally in the required orientation, and the edges tapered. If needed some areas can be decored, and a solid layup substituted in its place.
It all depends what you are building, if you can get inside to laminate over the joint, or if you can use long handle extensions for your rollers, or if you have to prelaminate the inside tape onto one side.

 

My best option is to prelaminate a shiplap. This is my first boat, so I've never done it before. I believe I will be able to hold very tight tolerances, so the half-hulls will be almost mirror images, i.e., The two sides will be almost perfectly aligned. Would you recommend glassing both cores, trimming the mating butt joint, and then adding an inside tape to one side? If so, what should I recommend for the tape?

Many thanks!

 

I can't tell you what and how much tape to use, that's the designers job.
What are you building? Can you get inside and laminate over the joint? If not you can always look at kayak builders, they have the same problems of joining the deck to the hull with no space to crawl in.
If you want to prelaminate the interior tape the best way is to plan for it from the start, loft the female jigs past the parting line and use a dummy core to form the mating surface for a perfectly shaped rebate. Final glueing is by thickened epoxy.

 

Many thanks for your help.

 

You would make a mating flange. But it is really only a wide open glue surface. A mating flange might be built with something like 2-3 layers of 1708 4" tapes and you are really only trying to backup the bond so you get good glue. Certain thicknesses of foam cores are more difficult to get a good bond, thicker is easier.. And movement of the joint during bonding can force out the glues. So a mating flange would be designed not to get a precise fit, but to aid in making the bond easy without forcing out the bog, or rather keeping it in.

As Rumars mentions; the designer needs to spec..

 

I am the designer. Thanks for the reference to 1708 tape. That helps a lot since this all new to me.

 

To be honest, the tapes would be a bit wrong as I described them because they would create a hard point. You would need to taper them at least a little bit I'd say. Even just say 1/4" would be good, so a mating flange might be say 3 pieces of tape made on the table 3.5,3.75.4" wide or less. Any allowance for curvature would need to be considered.

It may be possible to make a smaller mating flange in mould. You would, for example, apply a 1708 tape to the part as a mating flange and make it very minimal. The tape might stand up 6 mm or you could use a dummy core as Rumars suggests and build a one inch flange and remove the core section. Or you could ise the dummy and make the mating flange from your hull laminate. Then you use and oscillating tool to remove the unneeded core.

Also, as Rumars mentions familiarize yourself with rebates and reliefs. The outside would be best served with a relief for tabbing. If you pre-relieved all the core sections, then when removed, you could sand out the glues and clean up the relief for tabbing.

I setup an elecric planer to cut reliefs. But I technically did it wrong. (Mine were flat with a radius) The reliefs need to be on an angle and allow for the bonding tapes. Bonding with 1708 or 1208 tapes is clean, but the weight penalty and xtra resin cost for the csm is really steep and the reliefs need to be pretty thick. 1708 is 600/225 in Europe.

I have a video if you don't understand the rebates, but because it is technically wrong, I would only send you a link privately. Send me a private message if interested.

 

Also, your mould is probably going to need ribbands or battens running the long way. It would reduce the number of stations and allow for screws if you needed them temporarily while glues are curing before glasswork. Or, walking in, for a larger mould. We have no scale here, mind you.

 

My main hull is 19 feet long. I expect your method would be appropriate for a much larger boat. But I'm here to learn, so I'd appreciate viewing your video. I'm new to the forum. How do I send you a private message? You can sent it to myfirstname(at)splinedesings.com

 

Hi Doran, If you click on a member's name, this comes up :

 

Please don't take this wrong, but as a designer you should be totally able to solve such a simple problem, after all you were able to specify the scantlings in the first place. 1708 on carbon fibre skins is unappropiate, it defeats the purpose of using carbon in the first place.

 

As a matter of semantics, I was only pointing out A method of making A mating flange.

I was not providing THE specification. 1708 and carbon are indeed a mismatch, but he is also asking about rebates, so the knowledge level seems to be missing. I didn't even care what he was using as a layup. There are plenty of ways to make it. It would need to be stiff enough to not break or fall into the seam is all.

 

As I see it, there are two aspects to his question. One is the practical aspect, regarding the simplest and easiest way to do things. It's always good to ask for such info, after all somebody may have thought of a better way then oneself.
The second aspect is theoretical, how much and what type of reinforcemet to use.
To provide any meaningful concrete answer we need more info. What is the layup, does he need a specific fibre orientation, how is access into the hulls, etc. The generic answers would be "restore strength over the discontinuity" or "use the same fabric overlap as elsewhere on the hull", but those don't help him much.

 

Using the same as the existing hull is also important to think through. The dummy core has to be well placed or the remaining laminate would hit the laminate on the other side..not simple

I'd be curious to see a detail.