Just a different method of retaining the props. Duoprops are held on by a pair of large nylon stop nuts. The condition of the nylon should be monitored and the nuts replaced if/when the nylon wears to the point where the nuts don't tighten securely. Ideally, the two props should both be torqued with a torque wrench, but few people do it. The most common Duoprop wrench has no provision to attach a torque wrench:
If you want to "do it right," you'll need a 1/2" drive torque wrench, a 30mm socket for the rear prop, and one of these for the front prop (part #3855876-3):
http://cgi.ebay.com/ebaymotors/volvo-pe ... enameZWDVW
Proper torque values are:
Front: 45 Ft-lbs.
Rear: 50 Ft-lbs.
But as I said, very few people do anything other than get them "real tight." However you do it, make certain to grease the prop shaft before installing the props!! Volvo does sell a specific prop shaft grease, for around $20 for a large tub. Just about any grease will work, but I like this grease because it seems to be non-toxic.