Reward for lost Kraken!
Join Date: Apr 2006
Thanked 141 Times in 138 Posts
Rep Power: 11
Many of the LN alternators are LARGE FRAME and simply won't fit in a marine engine bay.
As to speeds, any alternator can run at any speed regardless of engine speed--if the pulley sizes are matched up. A custom pulley will run about $100 from a machine shop, half that in the boonies, half more in the city.
As Balamr surely must have said, somewhere around 100A you start to get a lot of slippage on any single V-belt. If you can't change to a ribbed belt, at least make sure the belt size your engine is set up for, can handle that 100A load.