Umm, a few more details perhaps?

To be in with a chance, we need to know at least the following –

• Engine make & model, the year is always helpful if you know it, no problems if not though.
• The type of pulley fitted to the existing alternator. It’s only going to be a single or dual V, or a serpentine of some size, just count the grooves.
• The chemistry of the batteries to be charged, for example, lead acid, AGM or LiFePO4 etc.
• The size of the battery bank.
• The banks voltage, is it 12, 24 or 48V?
• The type of sailing you do and other charge sources you may have (this isn’t essential, but it does help).

If you can send some clear photos of the existing alternators mounting brackets, both top and bottom, that’s always helpful, as is a photo of the manufacturers serial number plate.

Basically, just give us as much info as you can.