I built a Wood Duck and used butt joints with no problem, and there's quite a bit of twist/bend in that build.  I suspect they'd be fine on a Kaholo also; hopefully someone who's done a Kaholo using butt joints can chime in here. 

Here is a method of joining sheets of plywood without cutting a scarf not having a butt block and ending up with two smooth surfaces.  It has been around since the 1960 s.  You may care to call it a "fiber glass scarf.   Here is how to do it for thin, 3 mm plywood.

1.  Place the sheets of ply with the good side up butted together.

2.  Epoxy a 3 inch strip if fiberglass over the butted joint.

3.  Turn the sheets over.

4.  With 60 grit paper cut out a tapered groove about 3 inches wide, 1.5"s on each half, along the butted joint.  Cut the groove to the underside of the good face laminant.  Cut with a side grinder or orbital sander.  Finish the depth by hand.  Use the lines and colors of the laminates as a guide.

5. Cut  strips of fiberglass varying widths to fill the groove.  Maximize your epoxy to glass ratio..

6.  Lay up your laminant of epoxy and glass to fill the groove flush or a bit proud to the top.  When still wet lay down a sheet of waxed paper with a board and weights on top of the lay up. 

7. When cured sand flush.

8.  Make a practice scarf using scraps or Luan plywood.

Scarfing is not as difficult as it appears.  The accuracy of the cuts for a scarfed joint is very forgiving with thickened epoxy.  There are many fine notes in this forum on various methods for cutting scarfs and tips on building the Kaholo.

Butt blocks end up being a pain.  In  the case of the Kaholo construction they will interfere where the shear clamp/shear strake joins both the deck and sides.

Having built a Kaholo, I think a butt joint for the bottom panels is a go. The sides have a curve so you would need to avoid a "hard spot" if using a butt joint. Google "Payson joint" - that's a butt join withoiut the piece of wood (plywood) backer. Uses a strip of fiberglass on both sides and is rumored to curve better than a wood backed butt joint. I used both styles on my last build and as advertised the Payson style joint went well but the ply backed butt joint was just a wee bit cranky.

If you own a sharp plane scarf joints are not that difficullt. On the Kaholo I ignored the pattern on ply layout by CLC and scarfed two 3mm panels from which I cut the deck and both sides out. The aft bottom I centered on the 3rd panel and the front pieces on either side of the aft panel. Those have to be scarfed/joined after cut-out since the little dip curve near the bow overlaps. 

gee, while I was typing Catalina Charlie described the Payson Joint.

Thanks for info/opinions. I was thinking Payson joint even though I didn't know it had a name. Was never considering backing block.

I've scarfed before using belt sander and plane methods. 2 x Ches 17's, oneocean Cirrus, Pax20 surfski.

Problem i have is that the marine ply I buy comes out of malaysia and although stamped BS1088, does not comply. It has very thin outer plys of approx. .6mm. Scarfing is doable but difficult.

The only marine ply that I can purchase that complies is made from Hoop Pine and is 16% heavier than the Gaboon/Satinwood and far more expensive. If I can't build a craft that is comparable to a commercial product then I can't see the point.

Of course painting the hull sorts out the aesthetics but if a scarf is not needed then i won't bother. i have never considered a butt joint for a kayak but the Kaholo hull just looks so basic.

cheers

Check out this site

 http://www2.swaylocks.com/forums/wood_ogre-builds-14-ft-koa-kaholo-kook-box

Swaylocks has a real interesting method of making a finger joint and appears to be a very talented builder with lots of tricks.  His finished Kaholo is beautiful.  The entire thread was amusing,interesting, and informative.  This above link was posted on this CLC site a few weeks ago.

I scarfed really cheap luan that had an exterior ply that was so thin that you could not sand it without the glue showing through.  It is a little tricky but can be done.