This will be heavily glassed and painted on both sides, not worried about for checking

which is better for stiffness and strength as a floor core material?

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*fir checking

The fiberglass on the inside will contribute greatly to the resistance to bending. Dependent upon the weave, the type of resin and the number of layers of reinforcing glass you can dial in the stiffness and strength. I used 18 or was it 17 ounce triaxial, that is one set of strands running at 90 degrees to the initial layer with a third layer running at 45 degrees to the initial layer became so stiff that I had to use a couple of hundred pounds of weight plus numerous ratchet straps to bend the 1/2" Meranti to conform to the shape of my boat. The 17 or 18 Oz number refers to the weight of a linear yard of fiberglass fabric.

After doubling the number of screws to fasten down the plywood the structure was very stiff. To reduce damage to my boat from abrasion I added an additional layer of the same fabric to the exterior of the floor, I also mixed graphite into the last couple layers of resin. I have hit many rocks on the bottom of the boat with no significant damage to the Meranti.

The problem that is the most common with drift boats is the chine section where even a careful boat-person will hit obstacles. That's where most of us hit things. Having sacrificial outer chine caps makes the repairs easier.

Do a search for Brad Dimocks' posts and look into the Resin Research products that he tested. They have a resin that retains a great deal of flexibility and a high degree of strength. Dependent upon your eventual layup schedule you can build a very strong boat bottom.

Since you are building a composite structure where the eventual strength is more dependent upon the distance between the outer fabrics and resins than what the material is between the layers. Boats built with foam can be stronger than wood floors. However, the cost of the materials and the additional weight can make these floor too heavy and too expensive.  

The Gougeon Brothers (West System) have an excellent, free publication that explains these concepts in a much better way than I can. Here is the link to that book: https://www.westsystem.com/instruction-manuals/.

I realize I may not have directly answered your question, I, however, hope that I have provided some real-world information that may be helpful to you. There are numerous discussions on layup schedules, material selection and more already contained within this website. I suggest using sentences to select the best quality of information from the search function box at the upper right location on these pages. I have read almost every and contributed many words to that information over the last seven years I have been hanging out here. Everyone runs into the same issues when building a drift boat and often ask similar questions. Almost all the answers have already been provided. 

Rick Newman

Rick, I sincerely appreciate the detailed response.

Actually, I was just searching and my apologies, the question has been asked and answered with a few variations before:
http://woodenboatpeople.org/forum/topics/hydrotek-1?commentId=13122...

http://woodenboatpeople.org/forum/topics/materials-questions?commen...

http://woodenboatpeople.org/forum/topics/boatbuilding-styles-tradit...

I think I'll go with 6mm Hydrotek hull sides and 3/8" fir bottom.

By way of introduction: I'm new to drift boats and dories, but have been whitewater rafting for 15 years, and built wooden sea kayaks back in the late 90's (so not new to river abuse or S&G and strip-built composite glass/wood boats).  My brother worked for Jason Cajune back in the early 2000's. 
I *gasp* don't fish.  This will strictly be a whitewater/multiday camping boat.  I'm building from Andy Hutchinson's stitch and glue plans.  Briggs/Litton style decked dory, 17" x 48".


Did you glass your floor inside and out, or was it that stiff with just the inner cloth?
I had also considered laying it up with two layers of 1/4"..which may require vacuum bagging to make sure there are no big voids between the bottom sheets.  I know (2)x 1/4" would be pretty easy to work.

Actually thinking 2x 17oz biax or triax on both inside and out...and maybe 6oz woven over that for easier finishing.  I'm not afraid of having a boat that is slightly heavy...I just won't make the mistake of making it resin-heavy.  Extra glass is stronger.  Extra epoxy is just heavier!!

I'm likely going to use US Composites Epoxy only because I already have it on hand.  I had a huge project and needed a huge volume and have a lot left over.  It laminates fairly well; I just don't like that it blushes...so I have to religiously wash/sand before varnish.  If I were buying, I'd go back to Raka or use Resin Research.


I appreciate the suggestion to use a sacrificial chine cap.  That confirms what I have heard from others.  In spite of my other experience, I have never rowed a hard boat down a river, so I'm sure that I'll be finding rocks that I never worried about in my raft or plastic kayaks.  It's a sure bet I'll have dings and need repairs!  Any thoughts on rubber/neoprene chine caps?

Thanks also for the advice to use sentences in the search box.  It's good to know the tricks as not all search engines function similarly.

Cheers Rick!

Shawn

I used 17 ounce triaxial on both the inside and outside. On the inside I laminated it and then fastened down the floor. After I got the floor fastened down and screwed it on then I glassed it. I am only familiar with wooden chine caps such as found on the original 16' x 48" plywood drift boats. I am glad I could help you out.

Rick

Brad dimock put rubber chine caps on one of his dories. I remember seeing it on his blog.

https://fretwaterlines.blogspot.com/2017/03/if-it-isnt-one-thing.ht... I think he goes into more detail elsewhere

I had seen it mentioned elsewhere, but hadn't seen that post--or how to pull-cut the strips on a table saw.  Thank you!

Shawn, I would strongly recommend that you use 1/2" Fir rather than 3/8" for your bottom. You'll be happy you did, you really don't want to go light when it comes to the bottom and 3/8" could be a little problematic when it come to impact......just my 2 cents

Absolutely agree, the greater the distance between the laminations the greater the strength of the structure by the distance squared. In other words twice as thick is four times as strong.

Rick N

Jason and Rick,

Now you're talking my language.

I do plan on a heavy glass schedule, but as you noted Rick, the farther apart the face laminations, the stronger/stiffer the laminate.

Weight difference from 3/8" to 1/2" will only be 10-12# per sheet, and maybe 20# total on the overall weight of the boat.

So...that all said, should I consider (2) x 1/4" plywood for the floor?  Seems it would be easier to laminate two sheets to get a curved floor than to try to force a single 1/2" sheet to conform to the sides as Rick noted.  I will glass the floor after gluing it to the sides--and have the inner floor glass come up the inner chine.

I wouldn't laminate the two sheets of 1/4", you would be adding a lot of resin to do so costing you much more money that just buying the 1/2" ply and possibly having a delamination issue later on down the road. Thats alot of surface area to glue up with the risk of air pockets between the two sheets or a lot of resin squeeze out  causing a poor glue line and possible failure of the glue up. 1/2" fir ply shouldn't be a problem to get ahold of

Thanks Jason.

No problem getting common materials--I can get both Hydrotek Meranti BS1088 and Roseburg doug fir marine ply ordered in common thicknesses.


Not worried about the cost of epoxy, I have about 8 gallons more than I need..but air bubbles and delamination are a concern.  I was thinking with the curvature of the rocker that the ply would be curved in only one plane, and that I could get it reasonably tight between the two plies..except the flat in the middle of the floor.  My biggest reasoning was less stress in the floor.

One trick to bending the 1/2" bottom is to deliberately leave extra length at the stern. Start fastening midships and then forward to the bow. Now you can use clamps or weights on the overhang you've left at the stern to bring it tight to the chines. 

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