I have a total of 5 strip builds and two hybrids using ¼, 3/16 and 1/8 inch strips.  One of the hybrids was a Kaholo 12.5 and I used ¼” for that.  Although I like using 3/16” for kayaks, I would strongly recommend sticking with ¼” for a SUP deck.  Here is why:

1.       The glass/wood/glass sandwich basically acts like an I beam.  The strength (resistance to bending is a function of the cube (^3) of the thickness.  If you decrease the thickness by 25%, the strength is reduced to 42% (all other things being equal).

2.       One of the reasons that the thinner strips work in kayaks is because of the shape of the structure.  Kayak hulls and decks have a rounded shape so when a load is applied, it is shared with the neighboring area.  This is why the ancient Romans used semi-circular stone caps above doorways in stone construction.  The deck of your SUP will be perfectly flat, so the loading will be more localized and prone to bending.  This is why you can feel some give on the flat parts of a kayak stern deck but not on the rounded sections forward of the cockpit.

3.       Another difference between a kayak and SUP is point loading.  When sitting in a kayak, your weight is spread over the entire area that you are in contact with the boat (seat, legs and heals).  When standing on a SUP, all of your weight is transmitted to the boat in a much smaller area of your feet.  When you step onto the board, all of your weight is supported by a single foot.

This is not just theory and easily demonstrated in my various builds.  The boats built with ¼ have basically zero flex when I push on them, regardless of the shape of the area.  This includes my wife’s Kohola deck.  For the boats built with 3/16, there is no flex in the rounded hull areas but there is a bit in the flat areas.  There is considerable flex in the flat areas and a bit less in rounded areas for 1/8” construction.  This is why I only use 1/8” for the decks of race boats.

My concern with the use 3/16 on your Kohola is primarily with longevity.  I am sure that a 3/16 deck will support your boat, but it will flex every time you move which over time will result in delamination.  If it were mine, I would definatly go with ¼” strips. 

One other point is that you do not need tongue and groove strips for a Kohola deck because it is built on a flat table so strip alignment is no problem.  This may save you a bit of money custom ordering strips.      

Thanks for the incredible response, Mark!  Given your first-hand experience with flexing of surfaces (and your quantitative structural engineering argument), it's pretty obvious that CLC recommends 1/4"-thick strips for good reason, whic is of course not surprising.  I am definitely not interested in unwise skimping on materials critical to the strength of the deck (it'd be difficult to appreciate the marginal cost savings while watching an underbuilt deck fall apart either gradually or with one big crack).  I know that the strength is a strong function of the FG/epoxy sandwich (and wood/epoxy laminate in this case) but did not have good intuition for/technical understanding of the extent to which the thinner core/strips would affect strength.

In fact, one reason I'm exploring the hybrid option is that I've seen a few posts over the past many years about people damaging their decks (presumably 3mm okoume) by falling on them.  Makes me wonder whether they did something unwise/unfortunate to the FG/epoxy to weaken their decks.  The manual is clear about applying 4oz fiberglass cloth to both sides of the deck but implies that a single coat of epoxy to wet out the glass is adequate on the bottom of the deck (which surprised me).  Presumably the additional strength offered by additional fill coats on the underside of the deck wouldn't be enough to justify the additional weight/cost/time.  I'm tempted to add a 2nd coat there or reinforce the center of the deck in some other way (maybe with a 2nd layer of fiberglass, though I don't like the idea of complicating the deck filling/sanding with this).  I would be pretty happy if someone could talk me out of it, though!

I also appreciate your comment about square-edged strips being adequate given the lack of curvature.  Having strips milled locally is a great option given that I have lots of good lumber suppliers and a few hobbyist-friendly mills within 20-minute drives.

Hi HerbieRidesAgain, 

first i concur with everything Mark has put forward.  i don't have the experience of a kaholo but have a lot of strip built experience under my belt and Mark is spot on about the challenge with flat sections when you go with a thin core.  there are a couple points i thought i might be able to add that will be helpful.

1) with respect to glassing, as you point out, it is critical that you glass both sides.   The glass acts as the horizontal 'caps' on the I beam and the core is the vertical section.   so the key thing to maintain strength is to ensure that the caps are bonded to the core.    so on you question about additional fill coats on the glass that is on the bottom of your deck (e.g., sealed inside the boat), there is no need for fill coats here.  it does not add strength...it just fills the weave.  the purpose of fill coats is to allow for the creation of a smooth surface without compromizing the glass (e.g., sanding into it).  

2) the i beam caps fundamentally are strong in tension - not compression.  so if you want additional strength to support weight, you put additional glass on the inside, not the outside.  because when you are standing on the board, the layer opposite the core will be 'stretched' ...and if you want to minimize the stretch that will occur (e.g., make it stronger) you want to reinforce that side.  the main value of additional glass on the outside is for additional abrasion resistence....which i don't think is a big deal on a board.  and to tie this point into the prior point, you dont need to fill the weave....just put another layer of glass on the inside of the board where you think there will be the most force on the deck.   a very similar process is followed on strip built kayaks where extra layers of glass are typically added on the inside immediately behind the cockpit where somebody is likely to try to sit on the deck in a rescue.

all of these boats, fwiw, are pretty sturdy but they are not like polyethelene boats that can be thrown into the rocks, picked up and paddled away.   

the great thing about building fwiw, is if there is an accident, you will have the skills to fix it.  1/4 inch thick cedar....glass on both sides, and maybe an extra layers on the inside where you expect your feet to mostly be......avoid wearing stilletos when paddling....you should be fine :)

h

As always, good advice from hspira. 

As he said, do not fill the weave inside.  Doing so will not add strength, but it will add cost and weight.

Another way to make the deck stronger is to use S-glass on the bottom instead of the standard E-glass.  S-glass is expensive but will be lighter than two layers of E-glass.  You don't want to use S-glass on top the deck because it does not wet out quite as clear.

I use S-glass on the interior of my race boats.

Appreciate your expert insights, gents - particularly about the fiberglass providing strength under tension and S-glass being a good alternative given its superior strength but inferior appearance (immaterial on the deck bottom, of course).  I will consider using S-glass on the deck underside or at least doubling up the E-glass in a 4- or 5-foot-long section to address my concerns (whether rational or not) about deck resilience.