Thanks for the input on this guys, it is very interesting to me. I guess there is some efficiency gained in my boat with the longer waterline, but likely some lost in the greater weight and windage as well.
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You're right as to the gain at the greater length. While this doesn't mean your nmpg will necessarily be better, it definitely means your most economical cruising speed will be higher. I don't know the waterline lengths of either of your boats. But just for purposes of comparison and lets say we're talking 36' versus 30'. Then hull speed would be 7.8 knots vs. 7.1 knots. Doesn't sound like much but it's about 10%. There are so many factors with hull design being a huge one.
The ideal economical setting however, as you related, isn't always the most fuel efficient. Unless or until we're crossing an ocean or otherwise must extend range to it's maximum we'll never run for long at hull speed. I've seen a lot of boats recently use terms like "Range at Cruise" and "Range at Economic Cruise". Honestly, I think most of us have several different cruises. They're based on some combination of comfort, conditions, and fuel usage. That's where design really comes into play and match to engine.
I looked at a
63' Riva (using a non trawler just to show the difference) usage chart that has three distinctive ranges. From 11 knots to 15 knots is loses 20% of it's range. But from 15 knots to 28 knots nothing changes. The nmpg is flat. Then from 28 knots to 39 knots it loses nearly 20% again. So three good levels to run for combination of speed and efficiency. So 11 knots and 28 knots are the cruising speeds.
Meanwhile I just looked at tests on a
Nordic Tug 39 which has a 37' waterline length so a hull speed of around 7 knots. Yet it's design gives an odd chart. At 4 knots it gets 10 nmpg. At 6 knots, it drops to 7. At 7.5 knots it drops to 5. By 8.1 knots it's down to 3.1 nmpg. By 9 knots it's 2 nmpg. 11 knots is 1.2 nmpg. Then between 11 knots and 15 knots the drop is less dramatic to 1.0 nmpg. And from 15 knots to 17.5 knots then it only drops to 0.9 nmpg. So from 11 knots down to 4 knots you save huge amounts of fuel with every drop you make. But then from 11 knots to WOT of 17.5 knots the change in nmpg is relatively small. It seems that it's a single engine that really plows up to 11 knots or so and then it starts rising just enough to do better from there up. I guess I'd then say that boat has idling speed of 4 knots, cruising of 6, cruising of 7, cruising of 8, cruising of 9, cruising of 11. Significant differences with minor changes but 6 knots to 11 knots is an increase from 1000 to 2250 rpm. So it's obvious that the efficiency is dropping. From 11 to 17.5 only increases to 3050 rpm so it takes much less to accelerate through that range.
I did find a
41' Mainship Expedition with a 380 hp Yanmar. At 4.8 knots 6.4 nmpg. From there to 8.9 knots, rapid drops. 5.7 knots is 4.5 nmpg, 6.5 knots is 3.2 nmpg, 7.3 knots is 2.3 nmpg, 8 knots is 1.4, and 8.9 knots is .9 nmpg.
But then a strange thing happens. From 8.9 knots at 0.9, at 16.7 knots is still gets 0.9 nmpg. In fact around 10-11 knots it is worse at 0.7. So just looking at fuel on that boat, you lose all the way up to 8.9 knots but once there might as well it any speed you want up to WOT. Much like the Tug, hard to get it going but once you do it has little change.
I recommend metering to find out your real flow and learn those ranges for your specific boat, not just one like it. You may be propped differently or have some other difference. And as you can see from just two examples it's not a linear drop as you get faster. Most boats have areas over their operating range of rapid drops and areas where the fuel usage doesn't change much.
I'll bet there are very few Mainship 41 owners who realize how little things change from 8.9 to 16.7 knots. The value of really knowing the flow you're experiencing.
Congrats!
