Last year - yes it really was last year I had some beer issues.
The issues being the Waeco fridge would keep John and my beer cold during the mini heatwave we endured taking Percy to be repainted on the Stourbridge canal - whule the engine was running. However in the late PM it would retire disgracefully allowing much needed beer to get warm.
I have concluded this is an issue with voltage - or voltage drop. The charging batteries would cope with the challenge of too thin cable when under charge but give a few hours of battery use the compressor start up would require more than the cable would allow resulting in a fan running but the compressor not kicking in.
So based on some studious internetting I have concluded that the voltage drop on a 6mm2 cable on a 12v circuit with a 4 amp load at a generous 8 meters (it is actually 40watts/12v = 3.33 amps) is 0.18v or 1.5%. I understand an acceptable drop of up to 3% is the norm.
Cable is very expensive compared to other things so whilst tempted to go for 10mm2 (which gives a 0.11 drop or 0.92% ) I'll risk the 6mm2 plus it will be easier to install/replace the current *ouch*
Only two things I cannot define is the present cable size, I think about 1.5mm so getting an unhealthy and risky .75v drop, plus a definitive compressor start up voltage for the Waeco.
I think I'll also follow Halfie's venting approach by drilling some holes in the floor to draw up cool air from the bilges, giving the added benefit of some bilge ventilation. I recently treated myself to some nice sharp flat bits to do the work.
Watch this space but if you think I have anything calculated wrong please let me know !!