I would stagger the scarf joints. How do you do that? The first s Strip is lets say 8 ft. plus 8ft. Make the second 6ft plus 8ft plus 2ft. Third 4ft plus 8ft plus 4ft.

grumpy laid out the basic idea. 

i would add, however, that butt joints work just fine for every other joint except the actual sheer strip and the strip just below it that forms what typically looks like the horizontal reference line.

for the sheer strip you use the scarf joint to make a single strip so you can carefully site it and ensure its in the right position.  this is the strip that gives 'spring' to the boat and you want a single piece to do this well.  same with the strip right below it.  every other strip is going to be adjacent to these reference strips and for these strips, butt joints work just fine.

here is the sheer strip made from a single strip that was constructed by scarfing two 8 foot pieces together.

the next strip, the horizontal reference was made with similar construction:

all the other joints are easy to fabricate butt joints that work great and are easy....and as grumpy pointed out are staggered:

for me, my general rule on staggering is not two joints within between the same forms....but like grumpy, i typically go for at least a two foot offset.   

the petrel play is a relativey short boat so in most cases you can get a row done with only one joint.

h