Mittens
Active Member
- Mar 5, 2017
- 610
- Boat Info
- 2002 340 Sundancer, All Cherry Cabin, Black Canvas.
- Engines
- 8.1 Mercs, V Drives, 4.5kw Westerbeke.
Interesting stuff. The cocktail cruise comparison is probably the one most open to interpretation. Cocktail cruises for us are literally rides to nowhere (around the bay/lake with no destination except get home) so, speed is irrelevant, and the 6.2 would be slightly more efficient. That being said, these numbers are so close it's almost a wash. The appeal of the big blocks for me is being able to run them at lower RPM and theoretically less wear but who knows if that's even true.
What's hull speed in a 340? I'm guessing the waterline length is like 30' so about 7.5 mph?
Yes if you are going nowhere...... and just driving for a time period AT THE SAME RPM, then a pair of 6.2s burn exactly 1 gallon LESS per hour then a pair of 8.1s. (and take you 1.9 miles shorter of a distance)
That being said, if you have both boats and where going no where, then you are probably going to stay around the same speed of 7-8mph... and if thats the case you using a speed for a constant, not the RPM... and the 8.1 boat would be at like 1100-1200 rpm, not 1500 like the 6.2.. which would put it burning less fuel, as well.. for the same hull speed.
6.2 is at 3GPH@1500 X 2 === 6GPH = 7.5mph (using cliffs real word numbers)
8.1 at 1000= 2.3GPH X 2 === 4.6GPH = 6.8 mph (using my real world numbers)
so they are 500 rpms off, and only 0.7mph off... so lets say you barley raise the RPM of the 8.1s... to match the speed/cocktail speed of the hull. for the same ride around the bay for nothing but a fun. and your still burning less fuel...
if we think 1100 rpms is enough for 0.7mph thats a 10% raise in RPM, 4.6gph X 10% = 5.06GPH... still less then the 6.2s 6GPH
if we think 1200 rpms is enough for 0.7 mph thats a 20% raise in RPM, 4.6GPH X 20% = 5.52GPH.. still less then the 6.2s 6GPH
if we think 1300 rpms is enough for 0.7 mph thats a 30% raise in RPM, 4.6GPH X 20% = 5.98GPH.. still less then the 6.2s 6GPH
if we think 1400 rpms is enough for 0.7 mph thats a 40% raise in RPM, 4.6GPH X 20% = 6.44GPH.. which is more the 6.2s 6GPH.. but at this point we are way past the 7.5 mph matching speed which was the goal of a similar cruise to no point.
1000rpms=6.8mph
1500rpms=9.4mph
Differecne of 2.4mph
2.4/500 rpms = .48mph per 100 RPMS....so
7.5mph is somewhere between 1100 and 1200 RPMs on a 8.1 boat if we use 1150 (1.5X.48mph = 0.72mph) for it then:
8.1 is 5.29GPH@1150RPMs = 7.5 mph.....which still is less then 7.5mph at 6GPH on the 6.2.
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