Welcome to the forum

Land yachts are a little different from most other wheeled vehicles as they have to cope with side force from the sail so it is important that a line drawn through the pivot of the steering meets the point where the tyre touches the ground
Side force then does not affect the steering
The now common 30 degree angle is good because as the wheel turns it lays over and the side forces are not scuffing the tyre off the rim as much
A bicycle does this by leaning into the turn Landyachts are often leaning the other way away from the turn
Hope this helps

When the line of the steering axis coincides with the centre of the tyre contact patch the steering will be neutral.

If the line is in front of the patch the steering will have "trail" like the wheels on a shopping trolley. The steering will self centre and the yacht may be heavy to steer.

If the line is behind the patch the steering will have "lead" and if released will try to invert itself towards trail. This is why all cars have "trail" and will self centre when the steering wheel is let go.

A small amount of trail is not a bad thing but in the case of a land yacht, it will be able to sail itself.

25 metres of trail Vic? Geez, that must be the reason for those spectacular shots of aus 230 standing on her front wheel.

Seriously though I think you have it dead right. With my blokart, if the wind drops off and I try to get back home by standing on the rear axle and scootering with one leg, it will not hold a straight line and I have to lean forward to grab the steering bar which makes the scootering difficult.

My LLF Mini is the same but at least I can build a new front fork to correct it.

that would exxplain why i had to fight it back into a straight line

thanks everyone, made a lot of sense and the shopping cart example is a good way to explain it. I read some articles about the alignment but i never really got trough the whole thing, mainly the technical english is sometimes a problem and google translate isnt always the solution.

What do you guys think about the wheel thickness and diameter? I know it depends on the wishes you have with speed, resistance, availability, dimensions but lets say i want to ride from the beach through the shallow water. I would like to feel the water splashing up on a sunny windy day :) What do you guys the minimum thickness of the tire should be? A normal bicycle tire is about 4cm wide. Could twice that be enough or do you need more? You probably could calculate it but there is no fun in that. but lets say the land yacht has the weight of a blokart + me = 110 kg

Love to hear your thoughts, expertise and experience about this.

PS.
It good to see the familiar atmosphere in this forum.

On the hard sand 4.8/4.0-8 (note the difference in the way the tyre sits on different width rims)
if you go narrower increase the diameter eg. 24x2

or somethink radical for hard smooth surfaces a 7wheeler

yes (bit noisy and solid ride) but the wind was all over the place to get a good idea the couple of times I tried them and it was just one more unknown while I was sorting the rig, possibly dust them off an have another play in the future but have been considering another build lately(was very keen about a year ago but had trouble getting the information I needed, I think Lachlan and Gizmo had the same problem)

There is an issue with width of tyres with the LLM system. we have found that when the wheel gets "fat " compared to the diameter it gets a speed wobble up . I think that that fat wheel that iand posted would come into that category , but somebody would need to try it

the rowlock bit is because the downhaul part of the rig is pulling the front of the boom back, and working against the rear sheeting that is pushing forward.
it appears to be only a 2:1 system too

The basic physics of wheels are...
large diameter = less rolling resistance,
small diameter = more rolling resistance
BUT large wheels need to be strong to withstand the side forces of land yachts which normally = more weight = more force to get rolling.

Over the years for mini class size yachts the "wheel barrow" size wheel 4 inch wide on an 8 inch Diameter rim have seemed to be the most reliable and practical.
[In Australia the best seems to be made by a company called "Fallshaw" wheels]
Yes some people have used bike wheels of various types but they have tended to have a limited lifespan.