Powerdiodes in alternator need to be fast?

I recently installed a new alternator (Iskra AAK 70A, datasheet) in my father's boat. I also installed an external regulator witch compensates for voltage and temp. at the battery (Ladac 1204, link ). All functioned well for a week or so, but when I went on a trip with my friends the charging voltage disappeared and I found the alternators diode bridge was fried.

I currently checking if I can get a new org. diode bridge, but in case I can't get one, I am planning on changing the power diodes. The inscription on them hasn’t given me enough data to find out what part they are. (3 pcs.: 1923 417 cathode at casing, 3pcs: 1961 418 anode at casing).

I have found some diodes that will fit as replacements (IR 8AF, Datasheet ) but I don't know if they can be used at the switching speeds that are required. The alternator is spec'd at max 18000rpm and the polarity in the rotor changes 6 times per revelation so maximum frequency should be 18000 * 6 / 60 = 1800Hz. Will the suggested diode be able to cope with this frequency?

(I have bought a new alternator (the same Iskra) and are currently not using the external regulator witch of course needs to be checked. I don't know that caused the malfunction. I would like to use the external regulator because there are 2 batteries in the boat that are separated with a diode battery isolator with causes .6-.7 voltage drop from alternator side to battery side.)