Plank cored double check

Building a 7-8 meter trailable cat using a reputable designer. I want to use core plank with glass skins. Using "Gougeon Brothers on Boat Construction" and Dave Gerrs "The Elements of Boat Strength" I have come up with a 6mm core with 1200gm outer and 600 inner skin biaxle for both, of course using epoxy.

OK, so far so good!

I have available for free lots, and I mean "LOTS", of 25x40mm Norwegion white fir, the lengths after cutting out the knots will be from 1-2 meters. I am thinking I can make up a laminating table and scarf these peices into longer lengths ie 3 meters before ripping down into the 6mm stock. so I would end up with 25x6 mm planks to lay as the core.

Does this sound OK?

Bob

 

lot of work, but sounds doable.

How about making a small sample first, and destroy it, to see if it behaves like you expect it to behave?

 

I have a test batch in the basement curing as we speak!

 

Sounds excessive on the glass

I would go thicker core and less glass
For example, I am currently doing a 15m cat, 16mm Kiri core with 600db in and out
My last 10m cat was 10mm cedar core with 400db in and out
38 ft cats have been done with 12mm divinycell and 350 gsm in and out

I would also think white fir would be stronger than Kiri, cedar and foam.

 

Sabahcat

Thanks for the reply.

What you describe is a strip planked boat with glass sheathing. I am looking to build a cored glass boat with glass skins and fir core. Does this make sense. I got this from Gerrs.

I am most concerned that the fir material is not the finest wood. I will cut the knots out and end up with 1-2 meter long planks. I plan on scarfing them into 3 meter long boards and then ripping them to the 6mm thickness. Then I will have 25mm x 6mm planks to use.

Bob

Edit: OK, can I use this wood as a standard strip plank with sheathing?

 

No, both sound the same thing to me
My boat is strip planked with glass sheathing
It it a cored glass boat with glass skins and kiri core

What does your "reputable designer" call for?

If he is reputable, he would be more knowledgeable as to the specifics of your design than your interpretation of Gerrs.

What does your "reputable designer" say?
Who is it?

I would think, off the top of my head, that as your Norwegion white fir is heavier than Western red cedar and Kiri, that it would be stiffer than both and would require less glass.
As shown below, 12 meter cats have been built using 8mm cedar (weaker than fir) and 220gsm making what you propose seem excessive in the extreme.

You do realize that weight is the enemy for a multi?

Pictured below is Split Enz, a 12.19m Givens cat
(from memory) 8mm cedar core, 220gsm unidirectional inside and out
Timber strip provides strength for and aft as (unidirectional) grain runs length of hull, unidirectional glass provides strength across the grain

 

I agree in that the skin schedule seems overly robust, though this would be typical of Geer's "scantlings rule" builds and by Geer's own admission. Such a thin core doesn't take much advantage of this build technique. Core thickness is key, the thicker, the stiffer and less "goo factor" too.

Discuss your build method modifications with your designer and see what they have to say. Also, it's fairly easy to make core comparisons with their physical attributes at hand. If working with a stiffer, heavier, etc. core, make adjustments to the skin schedule as necessary to insure you come in on your LWL per plan.

Lastly, you don't have to scarf your strips (Geer's book covers this too). A simple butt joint will do, especially with a substantial skin schedule like yours. Stagger your joints and you're good to go.

 

Thanks all for the replies and info! And yes I know intellectually that weight is a problem in multi's but coming from the mono world, this is my first cat since owning (and destroying) a Hobie 14 35 years ago, I am having a hard time with the lighter scantlings.

Ok, so I will try some test planks with a few different thicknesses and glass schedules to see where my "comfort" zone resides. I think that after testing I'll probably be in the ballpark of your remarks.

Again, thanks for the replies!

Bob

 

Ok, I will go with foam core! Where can I get it here in Europe? I live close to Munich!

Bob

 

I would be interested in your reasoning

What sort of foam, there are many brands.

 

Well, it looks like I can save some weight while getting a stiffer hull with a foam core. As to the type of foam...
I am still trying to get pricing info on foam coring here in Europe. Seems like I am trying to buy an "illegal substance". Why can't I find recommended cost for these products on the web? Is the price "fixed". How will I know if I am getting a good deal or if quotes are fair market?

I am open to using Corecell, Divinicell, balsa ect... I need to know the price of material before I can make a decision. I can do the calculations to meet the current design scantling and with the manufacturing data I have all the numbers.

All of these products have been in the market long enough to accept them as a coring material, all have their pro's and con's and can be used interchangeably with caveats.

Bob

 

Search the fabricators websites for distributors near you. Time-out for instance would be able to get you foam cores, R-G distributes Airex, and there probably will be a lot more.