A Photo & Discussion Forum for Wharram Design Enthusiasts
Interesting comment >"bad quality epoxy and high import cost got me to build a mold and go plastic"
Are you not going to use any epoxy resin or wood ?
Certainly I think that using a vinyl ester resin along with glass fibre and suitable core material is fine -- provided their quality is no worse than the epoxy you speak of -- but surely you will be using wood in some or other part of the structure, and what adhesive will you then use?
Agreed, a glass composite bottom is hard and will stand up to possible rough usage. It will also be easily repairable if damaged by rocks, as you say.
We have many 40 ft paddling waka here with similar laminate to what you specify, and they do not even have benefit of thicker skin area in the V bottom. Buy what does concern me is the large flat hull area on a Wharram which is best stiffened by stringers, frames or bulkheads. I would prefer more stringers bonded into the hull than the one foam one you mention.
I have one of the old GRP Tiki 26's built by Imagine Multihulls. The deckhead and cabin sole appear to be a GRP/foam/GRP sandwich, not coremat. I don't know about the bottom panels because I've not cut into them so can't be sure what is inside, I could be wrong but I believe the hull topsides are solid and the bottom below the chine has a core.
There are only three bulkheads. One forms the forward buoyancy chamber, then there's an anchor locker with another bulkhead separating that from the cabin. The other bulkhead separates the cabin from the aft buoyancy compartment. All three bulkheads are plywood, I think from memory about 9mm thick. When I look at pictures of normal Tiki's it is noticeable that they have a lot more bulkheads.
GRP is strong but it has a poor stiffness to weight ratio. The stiffness has to be created by shape, such as chines, stringers, bulkheads, top-hat sections, etc. Foam sandwich is also very good for adding stiffness without too much weight. It creates a structure which works the same as an I-beam, moving the strong material away from the neutral bending axis.
Stiffness along the hull comes from the bunk tops, which are a single GRP/foam/GRP unit, and from the fake plank line that forms a chine. There is also a solid GRP panel in the bottom of the anchor locker, just above the waterline. The GRP skins on the sandwich used in the deck and the bunk tops/cabin floor unit are very thin.
You are right to think carefully about stiffness with a GRP hull, but from the description you've given I think you will need to watch the weight as well. The small Tiki's are beautiful streamlined designs. They perform very well, but are not good at carrying lots of extra weight, or the performance suffers and they become very wet to sail. Think of a modern dinghy rather than a heavy traditional yacht. When I look at how mine was built it's obvious that a lot of thought went into keeping the weight low.
If there's anything you would like me to measure or take pictures of just let me know.
I would love to see photos of your boat and how it was built.
As soon as weight becomes a priority you then eliminate gelcoat and fibreglass mat (except for the outer skin beneath paint on the hull outside only, which can be reduced to 225gms). Stiffness is achieved with a PVC core as Rob says, but then you go down the same road you are trying to avoid with epoxy and plywood............quality and price becomes an issue of similar magnitude. Also, you need to use vacuum and find the equipment; which you are aware of and are hoping to avoid. So this brings us full circle back to the original Wharram's apparent solution of plywood being both light and stiff enough.
Having said that, I have been in a similar place and situation to your own where polyester resin and chop mat was the only available material to build a canoe (this was on the wharf of the island of St Helena). Results were satisfactory after building a masonite female mould to get the basic shape layed up with bucket and brush. Then stiffening members were bonded in and laminated over to achieve required overall stiffness. You can even make light and stiff stringers by bonding aluminium tubing into such a hull, but the print through on the hull outside is not going to be easy to fix when it comes to cosmetics.........unless you get over the aversion to core mat, or possibly use your imagination to help disguise print-through of internal structure in other ways.Necessity can be a good mother of invention.
Thanks for your message. You asked about the dimensions of my front aluminium beam. I've measured it and the diameter is 100mm, with a wall thickness of about 3mm. Just to be clear for anyone else reading this who might not be familiar with the GRP Tiki's, we are talking about the beam 1.25m from the bows, not the beam carrying the mast...
I'll take some more pics of my boat and upload them soon (bit busy at the moment). The inside is stripped out while I'm doing a refit of the accommodation, so for now the way the mouldings fit together can be seen better than usual.
Thank you very much !
Just an update .
Hull no 1 is molded , I will take it out the mold next week and ...weigth it !
I hope to be below 200 kg !