The schooner/una arguments and 1 vs 2 piece masts are all correct, and well explained.  The light air/tall rig vs strong air/low rig is a challenge and will require reefing.  Whether that includes the mast as well as the sail is a function of how much complexity you can put up with.  On a 40' cruiser, I doubt it is worth it. In fact, I don't think it is worth it on the 24m, except for racing or use in seriously windy places.   

How tall

Assuming I get the no track, no batten cars, non rotating mast rig working, the masts wil be 21m/70' tall with rectangular sails 18m/60' high x 4m/14' wide. This sail area is required if the boat is going to carry 10 tonnes of cargo.  However,  empty,  it will weigh about 3 tonnes, so should get along pretty well.  It has been a fun exercise reducing the weight and cost.  

Hey Rob.

Most 40' cats have 60' masts, right? Again, that's what google says, I know nothing.

How did you decide on the mast height on the 24m?

Same question for the ex40.

My impression is the HP's have huge righting moment, so taller masts are obvious. as a mostly unpowered freighter, I would think you would want lots of sail?

If the free labour and overheads are still on offer after the lockdown, the most expensive part is the carbon, then the foam.  Using a truss structure, the beams can be made from glass rods which are easier to infuse than carbon, cost very little and although they have to be a little bigger to compensate for the lower stiffness, should still have less windage..  

Getting away from foam requires much smaller hull panels and maybe some form of corrugated core, which I am experimenting with this week.    Otherwise, the hull construction will be solid glass with easily built f'glass I section stringers.   Fairly thick glass (max 10 layers of 400 gsm uni in various directions) is required for toughness and to stop the small cross section hulls bending excessively, so we should get away with only one stringer per panel.  Building the hulls in half moulds, the join will act as the stringer on the deck and bottom.    10 layers of infused 400 gsm weighs 6 kgs per sq m.  20mm H100 (recyclable PET or similar) foam with 1200 glass each side also weighs 6 kgs per sqm, has better panel stiffness, is less tough, requires extra laminate at the beams and is a lot more costly.

hey, i was talking up reducing the foam costs before it was trendy!

If you are looking at protruded rods anyway, have you looked at half rods,square tubes ,or flat plates to corrugate your panels?

Why not infuse the stringer in the mold?

Didn't our friend in the northern European country have F700 for very cheap per m^2? have you decided against it? Apparently they are not locked down.