Dinghy scantlings

I think a nice light fiberglass dinghy could be built using Nida or Hexa-Core honeycomb to replace the plywood.....

 

Agreed a cored composite will be light, but cost is a consideration. How much to make a 4'x8' Nida or Hexa-Core panel, as stiff and strong as say a 1/2" sheet of Joubert BS-1088 Okoume?

 

Anything that keeps your feet not going throw the bottom and oarlocks in place is adequate, or actually 'overkill' as scantlings IMHO

 

I'm still doing research on Spectra fabrics, but I can picture a simple hollow half tube, stringer system, supporting a thin Spectra skin, as a super light and incredibly strong structure. The weights I'm getting are very light (unbelievably so), strength, stiffness, penetration and abrasion resistance and cost are also exceptional. Currently there isn't a lot of information available about these fabrics for use in a laminate online, but this will change and I predict it will revolutionize high end weight and stiffness sensitive designs, pretty quickly. With the ballistic uses currently satisfying the manufactures in sales, it'll be slow for a while, but as with all things, this too will change. I suspect the use of these materials in such proprietary products, is keeping it's secrets under wraps presently.

 

Plywood is nice and stiff.

GRP is usually strong enough but a single skin takes a lot of thickness to become stiff.This is heavy.

Foam cored is the answer , but adds lots of cost and complexity to the build.

Perhaps a duplicate of your dink , but with heavier reinforcement might be simplest.

 

Over the years I have had to beef up my current dinghy quite a bit to take the wear and tear. I don't think most people realize how much abuse a dinghy takes that is used 7 days a week for years on end. Until this winter and only then just for the holidays, my boat has only been in a slip for 2 nights in over 4 years. My dinghy is in constant daily use, rain or shine, no matter what the weather is. I don't think that a super light weight dinghy would hold up under the hard use. I certainly wouldn't risk it.
The plans for my dinghy spec'd 6 oz cloth on the outside and just coat the inside with epoxy. The didn't hold up very well. The first place to fail was the edge of the transom. Water at speed must be pretty abrasive because it quickly wore through the glass cloth on the trailing edge of the transom allowing water to soak the end grain of the plywood before I discovered the problem.
If I were to built my dinghy again with plywood stitch and glue I would layup 1708 cloth set in epoxy on both the inside and the outside with probably an extra layer on the trailing edge of the transom and additional stiffening. The weight of the dinghy would probably go up to around 120 lbs. And I would still have plywood in the middle with the potential to rot.
This is why I am looking into making the dinghy with just glass and epoxy. I don't foresee a big increase in weight. But what do I know. So I am asking the about scantlings.
Left to my thoughts, this is how I would layup a cloth and epoxy dinghy.
The hull would be 3 layers of 1708 cloth. Between the 2nd and 3rd layers I would glass in half round stiffeners either made from PVC pipe or water proof insulation foam (if it didn't melt in epoxy). I would use 2 stiffeners on each side of the bottom equally spaced between the chine and the center. I would place a stiffener on each side half way between the sheer and the chine. Finally I would place a PVC pipe split lengthwise over the gunnel with one or two layers of 1708 over it to stiffen the gunnel. I would stiffen the transom with plywood totally encapsulated with epoxy. I would like to use starboard to stiffen the transom but epoxy doesn't stick to starboard and it is ridiculously heavy. I would have 2, maybe 3, seats that could moved to suit the use.
Would this be inadequate, overkill, or just right. I don't know.
Thanks for all your thoughts and suggestions.
Chuck

 

I dont own a car, I live on a boat , use my tender as a truck and with care I get 7 years out of it Its a riib.

When the Rib reaches the end of its life cycle, its not the tubes that are shot, its the fiberglass bottom that is chafed away along the keel and transom corner and the fiberglass floor that has gone soft and broken by crew jumping into the tender from docks .

I would imagine that a glass dingy would wear the same way.

Nothing lightweight and easy to use will last forever.

If I were to build a "5 year" hard dingy it would be ply, 12oz biax skins inside and out, with uni direction fiber tape laid on top of the 12oz biax at keel and transom for chafe protection . Unidirectional withstands chafe better because the fibers run for and aft...the direction you haul a tender over the beach rocks ,launch ramp, dock edge , .

Also....A few years ago I inherited some Copper Coat epoxy antifoul paint . This is watersoluable two component epoxy , with copper powder added as you mix.

Its incredible !!

No fouling, very hard surface and no antifoul colour on your clothing or deck when you manhandle the tender. If you can get some , use it for your new tender. They say ten year life span per application .

 

Some good thoughts with a couple reservations. Glass is not very abrasion resistant and the kayak crowd has some much better ideas. They use strips of Kevlar or Kevar paste on critical areas like chines and keels. Tough stuff and takes banging into or dragging over rocks with no damage. I use Xynole polyester in the same way on the bottoms of many boats. Have also used the copperpoxy over Xynole that survives beaching on oyster shells, rocks and suchlike without even a scratch. For paint, I use Devoe 4508 industrial epoxy as the toughest paint going. Chalks but just repaint every few years.

 

Good question....out here 1/2" Joubert is over $5.00 a square foot......

This price list http://www.nida-core.com/pdfs/pricelists/nidacore-h8pp-retail-listc.pdf Claims 1/2" Nida-Core is about $1.60 sq. ft., I'm guessing with glass and epoxy the composite boat could be cheaper than the plywood one.....

 

Ouch, retail 1/2" Joubert is $3.60 a square foot here and I'm getting it (bulk) at $2.70, unless I buy a pallet, where I get another 10% - 20% off (depending on number of pallets). After fabric and goo, what do you think a Nida panel, of similar physical attributes would work out per foot?

 

I once tired Kevlar cloth tape as a keel chafe strip. It worked, but as the resin eroded , chafed out of the Kevlar fabric , the chafe strip became a fuzzy blotch of algae and barnacles. When repair time came it was so tedious to sand and repair the kevlar that I scraped and ground the whole thing off and used glass.

Kevlar is better for boats that don't spend a long time in the water.. like canoes or trailer skiffs. .

Ive not used Xynole. Both kevlar and xynole are hard to find locally. Eglass is in every boat shop.

 

Well, as you've stated above one person might pay twice what another pays for the same thing, depending on location and the amount you buy.

But first by "physical attributes" I assume you mean mechanical properties? They will be nothing like the same between a plywood boat (heavy and dense) and a honeycomb cored fiberglass boat, light and fragile.....This goes back to the SOR, what do you want? Some folks like to abuse their dinghy (Me), while others will treat them with kid gloves. If you go on the assumption that small dinghies are massively overbuilt in 1/2" ply (which I believe to be true)......then you can look at lighter materials.

The honeycomb has far lower sheer strength and puncture resistance than plywood.

3/8" Meranti ply will about 15oz sq/ft, the three layers of 1708 mentioned above will be over 20oz sq/ft with epoxy, a foam or honeycomb core with 6oz glass inside and out will be less than 8 oz sq/ft.

Russel Brown had a really nice carbon dinghy on his proa a few years ago, it was in the 40-50 pound range. One person could easily sling it around, but it was incredibly noisy, every move echoed all around the bay.....

Regional differences in cost can be huge, I can buy 3/16" 5083 aluminum here for $6.25 a square foot, and there are aluminum boats builders on every street corner.....so an aluminum dinghy makes more sense than ply or glass.....except that it will be far heavier(2.6 lbs/sq. ft.), and thus last forever, or at least a lot longer than 5-10 years.

My longboat is 20', made of 3/8" Joubert sheathed inside and out with 2oz glass set in epoxy. Used every day for 11 years, received a few coats of paint, I expect to use her for the rest of my life.....

 

I too am hard on dinghies, expecting them to tolerate all sorts of indignities. Most of mine will get "treated" to some sort of toughening up, like a Xynole sheath or at least some sacrificial runners.

You're getting some great aluminum pricing out there, I'd pay twice that easily.

Agreed, it's difficult to compare a cored panel to plywood, but I've worked up scantlings both ways and the cored stuff usually works out more, with labor costs worked in, even more.

In the next ten years, I expect this issue to be resolved some what, with aramid and polyurethane derivatives becoming more available and cost friendly. With plasma and other surface treatments coming on, well see huge changes in cored and composite structures.

 

Out of curiosity I dug out my Roberts dinghy plans, the fiberglass version built in a female mold, layup calls for gel coat and 2 layers of 2 oz mat, doubled in the chine area, no stiffeners . Cheap and simple

 

That would be for a small dingy ? 4oz mat is about 2mm thick