G'day Lee,
That is a very good question.
There has been considerable debate in various forums about this. I have been a fairly staunch advocate of the spacer method. Matt recommended it and that was good enough for me.
There are pretty solid arguments for both methods.
As I understnd things, the spacer method eliminates the likelyhood of a wood on wood join. The spacers also make for easier work in aligning panels that are not quite timmed perfectly.
Using the no spacer method requires more care in cutting and trimming the panels and their is more trouble in getting the corners of the ply panels to sit exactly on the corresponding corner.
I have experimented with the no spacer method for my wifes boat, partly because I am really trying to keep it as light as I can. The no spacer method requires lees wood flour, hence less weight.
The other reason is that Chuck has done it this way and I wanted to see if I could too.
Now that I have tried both methods, I think that I like the no spacer method better but it not something I would agonise over.
First build? I would recommend you build the way the instructions say. Further down the track, have a play with other ideas and then do what suits you best.
