Hi
Thanks for a great forum with plenty of construction tips. Now I know why my previous half-baked projects has failed so..
The title "Nordic combination" does not refer to the obscure Olympic sport combining cross-country skiing and ski jumping but that I want to use the yacht both for land sailing (Danish beaches) and ice (Swedish lakes). I?ve seen posts where others have tried this before, e.g.. with Mini Skeeters, but this is my version of it.

I have used very much of Paul Days drawing for LLM, but since I?m a sucker for steel space-frames I came up with a "Ducati & Super-seven" inspired design, which I modelled in straw.

But after some weight calculations I realized that this would be too heavy, so I reduced some sections, ending up in this (diagonal struts missing):



Another source for inspiration is my friends DN-yacht (ice) from where I?ve checked body position and will copy how the steel runners are constructed and fastened to the hull.

To verify that geometry fits I?m gluing together a 1:1 scale in 20 mm plastic tubing. I hope to advance and get some welding done during X-mas break.

Scale 1:1 done and geometry seems OK for my body.


Next step - welding the real thing....

Twisting forces of landyachts are something to keep in mind when considering spaceframe designs The single large diameter tube is hard to beat weight wise in this respect.

Yup, Hiko
My greatest concern is that the whole thing will just twist to shreds at the first gust.
Half-way welding chassis now.

Select to expand quote

Hiko said..
Twisting forces of landyachts are something to keep in mind when considering spaceframe designs The single large diameter tube is hard to beat weight wise in this respect.

When you lift a wheel the twist will be at its greatest. The energy used will be partially stored in the frame, that is why the spine tube works so well.

Hi again
with crappy weather in Sweden (no ice - no snow) there?s been plenty of time for welding and thinking..
With the mast foot in place I could do some torsional strength tests. I put a mast in the socket and asked my son to tilt the boat while I was in the seat. Lousy results, just as forum experts has warned me about
Now I need to reinforce with a spine pipe - which kind of makes the whole space frame idea pointless - except as a "seat".

Since I plan to use flexible rear axle (skis or pre-bent marine plywood) I?appreciate suggestions (threads?) on how to fasten those to a 50x50 tube spine.

The spines I have used are made from 2 inch or 51mm exhaust tube 2mm thick I think they are. To mount ski axles to these I have used 75X50 box section 3mm thick in the wall and with one side cut out to form a channel section. This is welded to the rear of the spine and goes right across the seat

The ski axles are doubled up face to face and are mounted into the channel with two 10mm studs welded to the channel on each side and fitted with flange nuts to secure each axle.

Round tubes are stronger in torsion weight for weight than square section

I have used a similar mounting system to Hiko with very good results. Have a look at my old yachts in my picture gallery. My latest yacht I have engineered the skis to fit 60x40 box section. I did initially try laminating axles from 3mm ply, they looked good but did not survive the beach.

I think Bynorthseas method of attaching ski axles is better than mine.
I suggest you follow that.

Hi
The spine tube I welded into that chassis is handling the twisting forces well!
Tnx for that. Next up is steering and rear axis.

You've a lot of steel there iceman. One thing I have learnt is that too easy to over engineer, look how little and how thin the sections are in Blowkarts. All the steel in front of the mast step does not seem to do anything, the arrangement you have for steering in your model will not give you the geometry you need. The tube spine and frame are now doubling up what you really need?. Your mast step tube looks a large diameter, is it a fixed rake?

After a long pause I have finally been able to get back to my combo-project, and done some successful (!) beta-tests on a nearby beach. Lot of stuff is still makeshift; steering, rear axle and rigging (using a small dinghy sail on pic). But I am very excited and happy that the fundamentals work such as my seating position, steering geometry and the sailing is fun, although too slow with such a small sail.
Now I?m working on a conversion of a Neil Pryde wind-surf sail using KAONAONAs tutorial: www.seabreeze.com.au/forums/Land-Yacht-Sailing/Sail-making/Sail-conversion?page=1#13

I?ve got some questions/feedback on this project and I?ll try to explain:
1. Why the space frame? A: OK, this does add a some metal-weight and I could have done it smarter but since I?ll use this on ice going 60 mph I want some caging around me. So mostly a psychological thing...
2. Metal frame in front and weird steering? A: Again this is needed since I want to change steering mechanism during winter (to skids) without having to re-weld.
3. General mistakes and over-engineering: A: Well, to me that?s half the fun :-) But without the fundamental drawings and expertise of this forum I would never have made a working craft.

Just get out and use it! Remember things to improve upon, however focus on what goes well. Once you get out on the beach sailing is too much fun to worry about much else.

Chassis looks fine. But i would be upgrading the optimist rig.