Has anyone ever tried this? It would be pinned in to place with 2 bolts where the holes are. Slide for and aft on main spine for different sized sails. Would the bolts break?

I tried something similar, not as an adjustable mast pocket but when I was experimenting with mast position. I found it impossible to make it fixed with simple clamp bolts, if I went back to it I would build in a fixed keyway along the spine.

Weld a vertical strip under the spine with a series of holes and make you sliding mast section clamp onto the strip. for fine adjustment have two sets of holes in both strip and clamp but have different spacing on strip and clamp e.g. 1" spacing on strip and 3/4" on clamp
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Drilling holes in the spine would make a weak snap/bend point as there is a lot of downward force at the mast step

This one (lefthand side) only works with sqare spine, but works great. The rectangular mast socket can take some wedges for altering the mast angle. Its build a bit too heavy. The next one gets a circular mast socket, slide for and aft might be enough to get good balance.(says Dad, i did'nt had time to test yet)
Add two chimneys more, call it Titanic...
Cheers, Joe

Make certain that the bolts are tight or it could twist off. Other than that a great solution using the rectangular tube as mast base

What size is the square tubing and wall thickness? Any problems with twist with square tubing?

It ia rectangular not square

-Rectangular- spine is 60 x 40 mm, wallthickness probably 2mm.
(I bought some stuff with 3mm wall wich appears too heavy and has a bigger radius in the corners than original...)
As both ends are closed theres no way to see, if its reinforced on critical areas.

No problems with twist so far.
Joe

how do you know when the mast base is in the correct position?
would it better to know what sail is going to be used before a yacht is designed and built?
is there any relationship between mast rake and mast position? for example would it be better to have an upright mast with the mast step back a bit or a mast step forward with a mast on an angle?

Have a look at the "how to find balance point...."-thread above.

with upright mast its easier to start initial airflow in low winds, more rake makes it harder to start but might give more top speed.

Also depends on wheels and ground you sail on. on soft sand my 17" wheels run faster but sometimes with more drift than
big foot wheels. With the fat wheels i never thought of moving the mast step forward, it felt good. those skinny wheels are much more sensible.

www.seabreeze.com.au/forums/Land-Yacht-Sailing/Construction/How-to-find-the-balance-points-for-a-Landyacht/

thankx

Just for those, who want to know: 40x60 spine is 3mm original (says the expert, who cut it up)