Hiko said..
Surprising you cannot find 20mm bearings they are plain vanilla world wide Even the USA I would have thought
If you must 3/4 inch is very close maybe you can get those to match wheelbarrow wheels in your area

It's because to fit my wheels I need a 35mm OD. I can find 20mm ID bearings but not with an OD that small.

I found 3/4" ID bearings. they were expensive though. My wheels aren't metric so I couldn't get metric bearings for them.

It seems you have a problem with off the shelf match ups You might be forced to do some custom work for your axles
!7mm high tensile axles should be OK if you use metal tube spacers between the bearings and an end nut is done up tight
You could drill and tap the ends of the axles to take washers and setscrews or studs and nuts instead of threading the end of the axles
I have that setup on my minis Works fine

Most dont bother with brakes
If you feel where you sail you need them then probably the easiest for your yacht would be to fit levers operated from your seating position with the outer ends
fitted with a pad that rubs on the rear tyres One each side A few people use this system
The only time I tried brakes was on the front wheel and it proved next to useless due to the little weight on the front wheel
Another system used on beaches is a drag lever that ploughs up the beach mounted in the centre of the yacht toward the rear
You might come up with your own design that you can share with the rest of us that is worth the trouble

Thanks! I'm not sure whether I need an active brake or not. I'll probably try to mount something.

Looking at the simplicity of that system, I think i'll definitely have to make a couple brakes for mine. I don't have lake beds to go sailing in. for me it's mostly just parking lots so brakes are a good idea. If don't want to run into something and cause property damage. My thought on seatbelts is that dissipating momentum separate from the yacht after a flip over is just as safe as being strapped in and being dragged by the yacht upside down as you dissipate energy together. 3 people have accidentally flipped my landsailer and none of them were injured tumbling out and stopping themselves well the yacht flew a couple feet without them. With that being said, those were all relatively low speed crashes as I've only got up to 24 mph so far (working on making it faster) at higher race competitive speed its possible that staying in the yacht would be beneficial.

JA staying in the yacht is very beneficial The mast will protect you in a capsize and as long as all your body parts are inboard you should be fine
There is a great temptation to put your hand out to fend off the ground but resisting that as above is the best in my opinion A seat belt and something to lock your feet into is a must for me Before brakes even

A snug lap belt is enough on the blokart. You want it tight enough to keep you off the ground when you're hanging from it after a flip.

To give you an idea of how much of a non-event a capsize is, here's a video of one of my flips:
(don't put your hand out like I did!)
The flip is around 25 seconds into the clip...

getting a seatbelt is on the list of things to do. probably a sturdier seat also. I like the box method for measuring wheels and tires! it seems like every type of tire has a different sizing system to go with it, makes it a little confusing. an absolute measurement is a great idea! Thanks for the tip on finding 20mm bearings. I'll keep that in mind whenever I make my next wheel/axle/bearing upgrade. what I have is working adequately at the moment. I live in Kansas which is a long way from any landsailing races, but i'll try to make it to one someday.