As said before voltage drop over such a long length will undoubtedly be a problem. Would not one of the £30 ish jump start type boxes kept in the locker be a better bet? I've got one that has no problem at all starting my Ford XLD 4 cylinder, and was 29.99 in the local cheapie outlet...
ToolCrew sell long jump leads up to 7.0m and road start packs. They're not cheap but they don't cost as much as a wind generator. http://www.toolcrew.co.uk/catalogue/ then select Recovery and towing. I've never dealt with them so i can't say what their service is like.
HTH
BG
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Welding cable properly made and terminated could probably do 20 -30ft.
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Thats what they use at my marina (about 30`ish) and the clips were from a motor factor as i remember, but you may get the clips from a welding supplies merchant as well.
AAbout 25 yaers ago I made up a 25 foot pair of jump leads from some 4mm cables I happened to come across free. They're about half the thickness of proper jump leads, but I've used them sucessfully to start both cars and boats. There is voltage drop, and they get a bit warm, but if you leave the engine on the charging end running at fast tickover you have about 14 volts plus to pour down the wire - and after five or ten minutes the flat battery usually gets enough voltage to start. What you can't do is put enough current through them to start an engine instantly, as you can with proper short, thick jump leads.
The 'starter packs' you get now do seem to work very well - I borrowed one to start a 3 litre Merc diesel last year, and I was amazed it worked. How long their little batteries will last is dubious however, particularly on the cheaper ones.
John has the answer, run the engine for 10 minutes . get some of the biggest cable you can (cooker cable even) but dont turn the starter for too long otherwise the wire will get hot. Disconnect jumpers as soon as the engine starts. Those portable jump start gadgets are good .. but if it lets you down then you'll still be looking for jumpers.