When two boats share the same hull type — in this case both the Riviera Yachts 47 Open Flybridge SII 2008 and the Riviera Yachts 51 Enclosed Flybridge SII 2009 are modified vee designs with fiberglass construction — the buying decision usually comes down to a handful of practical questions: how many people are you putting on the water, how far do you trailer, and what does your tow vehicle weigh?

Size is the most obvious dividing line here. The Riviera Yachts 51 Enclosed Flybridge SII 2009 measures 54,0 feet overall (2009), giving it roughly 5,0 additional feet of deck space compared to the Riviera Yachts 47 Open Flybridge SII 2008 at 49,0 feet (2008). Weight tells a clearer story for trailering families: the Riviera Yachts 51 Enclosed Flybridge SII 2009 tips the scales at 4 806 lbs — 4 374 lbs less than the Riviera Yachts 47 Open Flybridge SII 2008 at 432 lbs. That difference is meaningful if you're working within a half-ton or three-quarter-ton truck's tow rating, especially once you factor in a motor, gear, and fuel.

The power gap is worth calling out. Rated to 1 430 hp, the Riviera Yachts 47 Open Flybridge SII 2008 has a 605-hp advantage over the Riviera Yachts 51 Enclosed Flybridge SII 2009's 825-hp ceiling — enough to notice on acceleration and at cruising speed, particularly with a full passenger load. Both carry nearly identical fuel loads — 977 gal and 977 gal — so range won't be a tiebreaker here.

For family outings this is probably the sharpest distinction between the two. The Riviera Yachts 51 Enclosed Flybridge SII 2009 is rated for 16 passengers, while the Riviera Yachts 47 Open Flybridge SII 2008 caps at 15. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Riviera Yachts 51 Enclosed Flybridge SII 2009 could be the deciding factor.