Thanks also to Kurt for the link to Dan's machine, you have 50% less magnet area than Dan and I already think Dan's design was a bit marginal.

I also think your prop must be comparatively slow for your performance to be as bad as it is. 250 rpm cut in is high for that size of prop and would certainly bring your cut in wind speed well up, but you should have reached over 300W at 25mph. Maybe your wind speeds are way off or your prop is really slow.

You would need at least 150 turns to bring the cut in down to something reasonable for that size prop, but if you did that then you would need thinner wire or a thicker stator and either way you would have trouble loading that prop in a reasonable wind.

Your alternator would better suit 6ft dia with prop tsr 6 to 7. That would bring your cut in down to sensible wind speeds and provide a reasonable match.

Single rotor makes very poor use of the magnets and if your stator is more than 3/8" thick you will have little flux reaching the backs of the coils. ( I guess your stator is nearer 1/2" thick)

At this stage you could try adding a crude laminated core to the back of the stator to increase the flux, it will never be the ideal solution but you may end up with a higher general output from a poor 7'5" rotor compared with a good 6ft as the power gained from the larger diameter is considerable and may offset the iron loss. I still think you may start ok.

For the core you could try a coil of well annealed crate banding steel strip. Alternatively you could make a coil of mig wire about 3/16" thick and with inner and outer diameters to suit your magnet rotor. It may help somewhat if it was rusty scrap wire with most of the copper plating gone. Soaking it in polyester resin or polyurethane varnish would probably help. Stick it to the back of your stator.