Replacing Ply stringer with plastic honeycomb material

[xiphas]

Hi All

Im working on my little 4 metre vickers easyrider. Im about to replace the stringers and floor/sole. The boat has a single centre stringer is 25mm ply with 4 horizontal bulkheads. Im trying to find any information on the mechanical propeties of the honeycomb materials like nidacore, nidaplast, plascore etc, to see what would provide equivalent properties as the plywood. Ive seen other projects which seem succesful over time and use and can just copy materials and lamination schedule used there but just looking for anything a bit firmer than that.

If anyone can point me in the way of any info or resources or provide any advice it would be much appreciated.

thanks

Matt

 

[TANSL]

It is unlikely that you will get the same resistance with honeycomb materials than with plywood. I think you should think about another solution. In any case, to give a definitive answer, it would be nice to have an image with data of the current situation.

 

[PAR]

Welcoem to the forum.

The reasons plywood and solid wood are employed in these applications is simply cost and ease of use. When you count up the time to cut and fit foam or honeycomb, then skin it with an appropriate schedule of fabric, then smooth it out, to an acceptable finish, the labor alone makes you run to the lumber store. Then there's the cost. A sheet of plywood is far cheaper than a sheet of structural foam or honeycomb and this doesn't count the fabrics or goo yet.

If you're bent on one of these materials, the structural properties and the fabric schedule needs to be worked out for the anticipated loads, plus a safety margin you can live with. The math portion of this, isn't particularly difficult, though you do need some familiarity with the material properties involved. As to the material properties, these are available through many sources, typically engineering data sites if not the old fashioned way of opening a book or two. There are some online "calculators" for working up laminate schedules, though these usually produce some heavy schedules.

 

[TANSL]

Not only about knowing the mechanical properties of a material but properly analyze the loads to be supported by the material and how it is intended to work the material. The properties of heterogeneous materials change much in the direction of the solicitation. For example, the cement is very good for working in compression, as a pillar, but very bad for bending, working as a beam.
A stringer has to withstand bending and material noneycomb, as I suppose you want to place it, has little resistance to bending. It is similar to cement. It can be very good to absorb certain solicitation and very bad in other cases.

 

[SukiSolo]

Personally, I'd be very tempted just to use Q/S solid timber with the grain orientated to take the max load. Anything local with reasonable properties ie stiffness and durability will do. Better than ply and it should be pre sealed prior to bonding in. As long as any through holes are epoxy/microfibre filled and re drilled it should stay sealed and give a very long life. Trying to find long honeycomb is a pain, it's better suited to sheet or gentle curve(d) applications.

A foam core is doable but as PAR says, it can be a pain getting it to layup really smooth and sound with no air trapped. It can be done but having done such before, I'd use a bit of Doug Fir..... I'd guess you just need to find a local timber that is somewhere between say Brazilian mahgany (maybe Khaya) and Ash (European) or, Douglas Fir. It will be cheaper and quicker and much more durable then the ply which I would hazard a guess at is/was not proper marine grade anyway, as well as not properly sealed.

 

[PAR]

Agreed, there's a bit more to it than just slapping in some lumber, though for a repair, it's usually just more practical to use similar materials, with similar properties of similar dimensions, if only to save the engineering. Also agree that solid wood is much preferred over plywood, though at times, the ability to make a 24" deep web or partition or something that's simply not passable with solid, without huge trees or a laminate is the (again) practical way to go.

 

[whitepointer23]

A vickers easyrider will be from the 1980's. So it is at least 25 yrs old . I would glass in new ply with poly.resin the same as original and it will be good for many more years. The easyriders are a low budget boat you don't want to sink too much money into. Thats not to say they aren't a good boat
Just doesn't have the resale value of the sought after hulls like haines hunters and the other top brands.

 

[PAR]

Plywood has only 2/3's the longitudinal stiffness of solid lumber of the same dimensions.