"Blade power (watts) = 1/2 x air-density x swept-area x wind-speed cubed"

Yes that formula gives you the power in the wind. You need to add an extra factor to give you the power into the alternator and that is Cp, the coefficient of performance ( effectively prop efficiency).

According to Betz the maximum power you can extract from the wind is .593 of the power in the wind. Others have worked out figures based on other than simple momentum theory but as yet the Betz figure has not been equalled so let's assume it means 100% prop efficiency. You will not reach that with any real prop and if you manage 50% of the ideal you are doing ok, so a realistic value for Cp for a small prop is about .35.

That brings your available power from the wind down to nearer .3 of the equation you gave.

Now for electrical output we need to include another factor and that is the efficiency of the alternator, rectifier and transmission. At near cut in you may see nearly 80% for a 24v system but with a conventionally loaded battery charging scheme if you attempt to hold electrical efficiency high you will pull the prop off the peak of its power curve and reduce Cp to a very low figure. By compromising electrical efficiency you can hold the prop speed up and gain more from a higher Cp than you loose on the electrical side.

This effectively means that you drop to 50% electrical efficiency but 3 times cut in wind speed. At this point Cp will likely be below .3 so you reach a state where your initial power in the wind now looks rather less impressive.

You will inly see about 15% of the lovely figure that you first calculated.

Now to your practical points. Voltage is no issue, forget it, the battery will hold the volts down ( that is a cause of the low overall efficiency), it doesn't come into the furling issue.

Current is limited by the wire in the stator and the way you can dissipate the heat. You will need to furl probably to limit current long term to about 10A for a 24v machine, short peaks over this will not hurt.

This in real life for a 4ft machine comes down to the fact that you will probably not be furling until near 30 mph and so you will only be limiting power output on a few rare exceptionally windy days and it will have negligible effect on your total power capture.

Unless you have some form of embedded temperature detector you will have no direct way to know your winding temperature. If you make the furling to Hugh's plans and you don't let it exceed 250W for long periods you should be ok ( I suspect the original version with stator on the front is well cooled).

Fuse is only to protect your cables from meltdown in the event of a fault. It had better not be chosen for any other reason. It will not give long term protection to the windings and a blown fuse will result in a runaway machine that is better avoided. a 20A fuse should be fine here.

Prop speed will be determined by how far into stall you drag the prop. It is a relatively slow machine and when reasonably loaded I doubt the speed will ever go over 600 rpm ( unless you blow that fuse and then it may reach 3 times that).

You will probably err on loading it too hard and keeping the speed too low for decent prop output so with your very short line you may need to add a bit of resistance in the line or move the magnets away from the stator to raise the cut in and lower the alternator slope ( probably a better option in a high wind area, resistance may be better in a poor wind area ).

Now you have all the benefits of Hugh's book and all his design work that went into the machine and you have just got a free book from me. Feel free to ask any more questions.