Lightweight building with wood?

[T Sailor]

I'm thinking about building a catamaran and am thinking about how to improve on the stiffness of the hulls and thought I would run a couple of ideas off here for comment:

The original plans call for 4mm Okoume skinned with 4oz glass on the outside and 10oz glass on the inside. This has proven to make some light fast boats, but compared to the current production boats, they are lacking. So, I was thinking of ways to build the hulls and increase the stiffness. Vacuum bagging is doable, but I'm thinking I don't want to have to build a plug and make a mold. I'm also considering these methods for another smaller design.

Idea 1: strip plank the hull using 9mm Durakore strips and use 10oz glass on the outside and 4oz on the inside. The problem with this is that the durakore is about 50% heavier than 1/4" ply. don't think it can be built down to weight with this.

Idea 2: Build the hulls using 3mm Okoume and then laminate 1/4 balsa core to the plywood. Then glass inside with 10 oz and outside with 4 or 6 oz. I think this is close to the original panel weight (maybe 5-10% heavier)

Idea 3: Build the hulls using 3/16 cedar strip and then laminate 1/4 balsa core to the cedar. Then glass inside with 10oz and outside with 4 or 6 oz. This is probably slightly heavier than idea 2.

Idea 4: Rip 3/8" endgrain balsa into strips and router with bead and cove profile, then strip the hull. Glass outside with 15oz and inside with 10oz.

One of the problems i'm having with estimating weights is how to account for the epoxy. Is there any formula that can be used for determining this?? Thanks!!

Fire Away....

T

[FranklinRatliff]

That just seems like a lot of work for perhaps little or no gain.

Are there any proven performance advantages to stiffer hulls?

[gonzo]

Are you sure Durakore is heavier than plywood? In sandwich construction the core is mainly stressed in compression and then in shear. Balsa planking, not durakore but plain lumber, would be stiffer because you can increase the thickness for the same weight.

[SeaSpark]

From http://www.europedinghy.org:

HULL STIFFNESS

The hull is constantly being exposed to the forces of the waves, mast and sailor. To allow the forces to be beneficial to the speed, the hull should be as stiff as possible, to limit the effects of both outside pressures and twisting of the hull.

A hull that is too soft will make a negative difference in heavy wind while going to windward and downwind in choppy short waves because there is alot of force that is being lost into the soft hull. In addition, a soft hull has an increased chance of getting cracks and stress fractures permanently in the hull. A stiff hull will definitively last much longer than a soft hull.

In my own words: The flexing of a hull wastes energy that otherwise could have been used to make it go faster.


T Sailor, i like your idea's a lot, #2 the most.

[dougfrolich]

To optimize your hull shell plating you also need to think about the internal structure that supports the plating. As designed what is the long and short span of your hull shell panel? How long are the hulls? What is the displacement? What is the design speed? and what type of service is the craft used for?

[T Sailor]

In small boat performance, hull stiffness is a significant contributor to the speed of the boat. The hulls in question are 20' and 16'. As far as internal structure goes, there are ply bulkheads about every 4' to distribute load.

I'm really curious if anyone has any experience laminating a balsa core to a thinner ply surface. I'm trying to determine how much weight would get added for the resin required to bond the two. I remember something in Gougeons book about adding a honeycomb core to ply but don't think they discussed it too much.

Essentially the goal is to come up with a hull panel that will be significantly stiffer than ply sheathed in glass but still the same weight.

Durakore seemed ideal for this purpose, but the published weight is about .65lbs/Sqft. while the 4mm Okoume is at .42 lbs/Sqft. Now, if they made it with a lighter wood for the skins and a lighter core, it would be the ideal material for homebuilders to make highperformance boats.

Thanks again for the input...

T

[Tactic]

sheath both sides of the ply with carbon fibre cloth.
using the same cloth weights as for fiberglass you will acheive a stiffer hull for the same weight.

[BOATMIK]

The original layup of the hull seems ill-conceived. Maybe it is - maybe it isn't

Having such a thin "core" as 4mm between glass indicates quite a high weight for the degree of stiffness. And having 10oz of glass on the inside just seems totally out of proportion to the rest of the structure.

I think we need to know more about the construction method and the type of boat before making much comment - maybe there is justification for the original layup?!?

If it is an "off the beach" cat I don't know what the glass is doing there at all. - the 4mm with some internal stiffening would probably be adequate. Maybe a bit externally along the keel line to take grounding. 4mm would be a typical skin thickness total for such a boat.

If it is not a round bilged hull I would make the ply a lot thicker and get rid of most of the glass for a net increase in stiffness.

The strategy for increasing stiffness and reducing weight depends a great deal on the boat type and the designers intended construction method.

[frosh]

DougFrolich is onto part of the solution questioning, re internal reinforcement spacings. Plywood is much stronger in one direction than the other, and this is why the 10 oz. glass was specified for the inside to counteract this property of the ply. You said that bulkheads are spaced at 4 ft. intervals. This is too widely spaced in my opinion for a 20 ft. hull, and 2 ft spacing would be a lot better to set the hull shape correctly, and stiffen the ply panels. You havent said whether it is round bilged or flat panelled, but if flat, you can substitute light timber battens (instead of extra bulkheads), around 1 inch square section running across panels equi-distant between bulkheads, and glued to longitudinal timbers such as chines, and inner gunwhales.
The use of glass inside and out is optional and if you want it, then 4 oz on each side should suffice. You can for lightness just epoxy coat the inner and outer surfaces.
The other options are in my opinion a lot of extra work that is not really necessary to achieve the desired level of stiffness as 4 mm Okume sounds about right for this size catamaran.
BTW balsa absorbs quite a lot of resin probably equivalent to the weight of the balsa when coated on both sides.