Just wondering, if instead of going with larger tires, anyone has just doubled up on the wheels to give more floatation.
+ Width (drag) is your enemy..... + Diameter (less rolling resistance) is your friend.
I'm not worried much about the extra weight. My sailing will consist going straight one way down the beach a few miles and then straight back and I'm not going to be racing.
I would go with a larger diameter wheel if could find one, but I've been searching the WWW and can't seem to find anything better than what I have.
I'm probably getting ahead of myself anyway as I haven't tried my yacht on my beach yet. It will probably be fine as is.
A good indication of what is sailable on the wheels you have is, if you walk on the beach and you leave 'full' foot prints in the sand its marginal..... If you are just leaving 'heel & ball / toe' marks on the beach then its fine and you will sail well.
My beach can be very variable so I run 20 x 12 slicks on the rear with a normal wheel barrow wheel on the front, overall the effect is beneficial keeps you moving in the soft stuff, takes the bumps and hollows.
I show a dual wheel barrow version in my photo collection, have a look. The rear axles can fit either two wheels each side or one wheel with tube spacers. The idea was to be able to roll on softer sand but needs more wind as you would expect. Larger diameter balloon tyres would be better but you pay for them.
its a great waay to make a cheap balloon tyre, the woomeral yacht club did it alot. . pull the second wheel off the axle when your on the hard. make a set of stubs and try it let us know the results 
There are plenty of balloon tyres around if you do some searching, but bearings are a must..
beachwheelsaustralia.com/compare-all-polyurethane-balloon-wheels/
One of the yachts on this trip used 'Used Aero Tyres' that were quite Baloony and worked very well, a light plane airport often has a good supply of high spec used tyres that would be suitable for land yachts, due to safety factors they need to be replaced often on aircraft.
www.seabreeze.com.au/forums/Land-Yacht-Sailing/Events-Competitions/Beach-Blast/
I have a set of four wheel motorbike rear wheels that i put on my promo for rough ground/sand hills. made to fit the same axle stub as Fallshaws 
Here's another way to arrange double-wheels: in-line:
Sorry for the poor quality photo, it's a small section of a wide-angle video frame.
I don't have any better photos, but the rear wheels are slightly staggered (i.e. stub axles sticking out of opposite sides of the horizontal connecting piece. IIRC the connecting piece pivots where the normal axle would be mounted so the tires can ride over bumps.
Video(Mike is at about 1:38):
