Thanks Ian and Graham
Graham's big battery solution is probably the most robust and simple although the investment is off-putting and I like to think I might need power for more than 5 days although this is probably wishful thinking. In any case someone gave me a nice couple of 7Ah VRLA (Valve Regulated Lead Acid apparently) batteries when he bought himself a fancy Lithium 24Ah one
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I had ambitious ideas of linking two of the Yuasa 7Ah batteries in parallel and connecting them through my cheapie Ebay controller to the GPS and solar panel. But I am nervous of connecting batteries with marginal voltage differences engaging in secret and debilitating electrical Sumo wrestling with each other, the controller and the GPS. I'm not about to start messing with diodes to try and prevent complicated back currents and stuff so now I am simply connecting the batteries one at a time with good quality weather proof plugs and sockets to the controller, and hence the solar panels, for charging or connecting direct to the GPS. This means I have to physically disconnect and reconnect to swap batteries from charging to GPS supply but at least I know what is going on.
I think the best place to mount solar panels on a Bayraider is the after deck but space is limited so the maximum size I can fit is two 20W panels in parallel on either side of the mizzen. I think this makes sense because at least one panel will not be in the mizzens shade. I am using semi-flexible panels and mounting them with hefty 50mm wide velcro (£5 from Screwfix but black or £mega from B&Q in white) to the deck so they can be removed when not on tour and, like Jonno I don't like making unnecessary holes.
Maybe unwisely I've ordered a pair of "20w Semi Flexible Light Weight Sunpower Solar Panels" from Ebay that are not only the cheapest at £30 each but also claim to be the most efficient at 23%. Probably fall apart - I'll let you know.
Ian's advice on controllers is useful and chastening - I expect I'll need to replace mine but as I've already mounted it in an IP65 box I'll give it a go.
Total cost so far, given a little help from my friend with his surplus batteries is about £100 and ? happy hours faffing at my workbench.
All good fun and I'll keep my fire extinguisher handy.
Matthew
BR20 Gladys