I would not cut the mast in two as after doing that the mast is weaker and you may have a lot of preseen and also unseen problems. Do not forget the rigging
has a much harder use on a catamaran
as on a monohull
. On my 40foot catamaran
the standing rigging
was with 12mm dia (around 7/16) 1x19 stainless steel
. Coming to Australia
after 11/2 circumnavigations, arriving from Fireland, I had to replace the shrouds as on one shroud
4 strands of 19 were broken where coming of a "fitting".
As told by other users your mast is not 21 m long, as in technical specification you see the mast height over water. So you can easily deduct about 3 m.
Having experience with mast stowed on deck
I would recommend that you stowe the mast with the mastfoot forward and this not forward of the bows. So you have only to watch to the mast head
who will be aft of the stern of your catamaran. Make the masthead visible with a orange piece of fabric
and during the night with a white light e.g. with the stern light.
I was sailing with the mast stowed on deck down the river Rhone and also the River Rhine and was happy so far. Both rivers are comparable in view of commercial
traffic with the Danube.