Peter Manning wrote:

Grasham, the idea of storing a batten or battens in the boom is very interesting. My boom is 75mm diameter and my battens 50mm pultruded fibreglass so storing one in the boom would be possible if I modify the end caps which at present fit into the ends of the boom.

You could also store one or two battens outside the boom, lashing them together.  Either way, it seems to me that the boom is the ideal place to store spare battens.  There is nowhere else on my short boat that I can store a full-length batten, and for serious cruising I think it is essential to carry a couple of spares.  I also think a spare yard makes sense for remote cruising, hence my decision to make my new boom to the same specs, though I doubt if I will ever break my current yard, 100x3.3mm alloy by 4m is pretty rugged!  When I did break the last one though it was pretty ugly!  The new one shows no sign of bending even when hammering along.

PJR states that the boom on a junk sail only needs to be slightly bigger than the battens and most junk rigged yachts, including Arion, have followed this advice, but I am reconsidering.  Firstly, because I'd like to experiment with really vanging down the boom to assist in gybing (in conjunction with Arne's FUP). If you can stop the boom lifting, and the FUP stops the battens lifting, you've got fan ups and accidental gybes licked.  You could even use a Waller style running boom vang (boom brake) if the boom was strong enough. Secondly, it occurs to me that if the boom was the same specification as the yard, you could use it as a replacement yard if you ever damaged or broke the yard.  Having broken mine once, I know only too well that if you don't have a yard you don't have a workable junk sail.  My yard is 100x3.3mm alloy and if the boom was the same size I could store a couple of spare full length battens inside it - another bonus.  I cannot see any disadvantage to having weight in the boom.