Manufacturers quoting range

[BandB]

Wifey B: Earlier this week experienced an interesting discussion. Looking at performance charts saw at cruise with all three engines a range of 611 nm. Yet the builder only quotes 500 nm. His reasoning was that you must accelerate to that speed and performance charts have you at that speed and running constant plus you have generators running. He laughed and said, "You know us crazy Italians, if I said 600 nm, I'd have one who wouldn't even look at his guages or anything else and would be cursing me out when he ran out of fuel at 590 nm." He said that you'd be amazed with all the marinas in the med how many people ran out of fuel on a summer weekend, in all brands and types of boats. He said with fuel prices what they are today, seemed like everyone wanted to run their tanks down to nothing. :lol:

He also never mentions "Slow range" or the other terms others use for perhaps cruising at 1250 RPM instead of 2450. His reasoning? Buyers of his boats have no intention of going those speeds. Actually the max range in all the charts is one engine, 8 knots, 3803 nm. Yeah, right.

Now obviously this is a planing boat, but I looked back at many semi displacement/semi planing and even displacement boats. I found that universally builders quote directly off charts. I found on displacement boats they quote often the "perfect" range, that is typically at a knot or so below hull speed. Now, I doubt anyone achieves that range as hitting that speed exactly all the time is a challenge and you have winds and currents and generators.

There's just a difference between maximum range at various speeds and practical range. I've seen people who ran out and said, "I was only going 200 miles and my range is 220 miles and my first thought was "Clearly it isn't." I think like a lot of terms, we toss range around a bit too freely. Perhaps we have no better number than to use the charts but the 10% in the charts is likely not enough for all situations. I've always been ultra conservative in range, but this just made me think as we all quote the ranges of our boats but then what are we really saying and what is the practical range.

 

[Moonfish]

We always tell people if we had "a full tank of gas" (840 gals diesel), we have a range of 2,300 miles. I think that's being practical. But please double check my math:

We cruise 7 kts at 1,400rpm (FL120s) with each engine burning .9gph for a total of 1.8gph. We always round up to 2gph. I take the 840 gals, subtract 40 for gennie use, and use 800 gals for calculating range. At 2gph that halves the fuel to 400 gals. Multiply that by 7 and we get 2,800nm range. We then lower by 500 that to a conservative 2,300 miles.

Am I right, or am I off?

 

[Irish Rambler]

As an aside, many people cruising the Med despite their fancy boats are cringing at the price of fuel. Gas oil (diesel) is currently Euro 1,40 cents a LITRE, check out that in dollars ! Red,or, agricultural diesel is not allowed and boats using the sea have to pay a tax to support the lighthouses/navigation marks.
For guys like me with a trusty tolerant Perkins there are ways of reducing the fuel costs by 25%, but that's another story.

 

[rslifkin]

I quote my range based on a slightly conservative estimate of fuel consumption and a healthy reserve (25% and assume the last 10 gal in each tank is unusable). And then round the final number down slightly.

That puts me at 160nm range at 17 - 18 kts and about 360nm at 6.5 - 7 kts with 300 gallons of usable fuel (out of 420 total capacity).

 

[BandB]

We always tell people if we had "a full tank of gas" (840 gals diesel), we have a range of 2,300 miles. I think that's being practical. But please double check my math:

We cruise 7 kts at 1,400rpm (FL120s) with each engine burning .9gph for a total of 1.8gph. We always round up to 2gph. I take the 840 gals, subtract 40 for gennie use, and use 800 gals for calculating range. At 2gph that halves the fuel to 400 gals. Multiply that by 7 and we get 2,800nm range. We then lower by 500 that to a conservative 2,300 miles.

Am I right, or am I off?

Wifey B: You get to a reasonable number but simpler to just say at 7 knots I use 2 gph so 3.5 nmpg x 800 gallons = 2800 nm less 20% = 2240.

However, what do you get at 6 knots or at 8 knots and as to the generator, how many hours would you use it while putting on 2300 nm? If it takes you a year and you use it 1600 hours then there goes a significant amount of fuel and numbers assume equal current both ways but if you find yourself doing a lot of boating into a 4 knot current it all changes. Just a lot of variables.

Now, if I was thinking of crossing an ocean in your boat, I'd only be comfy up to about 2000 nm until I'd really proven it all.

 

[BandB]

As an aside, many people cruising the Med despite their fancy boats are cringing at the price of fuel. Gas oil (diesel) is currently Euro 1,40 cents a LITRE, check out that in dollars ! Red,or, agricultural diesel is not allowed and boats using the sea have to pay a tax to support the lighthouses/navigation marks.
For guys like me with a trusty tolerant Perkins there are ways of reducing the fuel costs by 25%, but that's another story.

Wifey B: 1.4 x 3.8 = 5.32 euros = $5.82 holy @%$#%@$.

Unfortunately, we are well aware as we intend to boat a year or two in Europe before shipping the boat home. Glad the builder has to turn it over to us with a full tank. That's nearly $25,000.

Most of the boats are going 50 to 100 nm and anchoring and then a week later another 50 to 100. We've seen a lot of large, high speed boats for sale that have averaged 100 hours per year, just a few as high as 200.

Most high speed boats use MTU engines, a subsidiary of Rolls-Royce Marine and this shows why they're introducing hybrids.

https://www.rolls-royce.com/media/p...-mtu-hybrid-propulsion-systems-for-ships.aspx

We saw them in MTU's Solution Guide as E-Drive Systems.

 

[Moonfish]

Wifey B: You get to a reasonable number but simpler to just say at 7 knots I use 2 gph so 3.5 nmpg x 800 gallons = 2800 nm less 20% = 2240.

However, what do you get at 6 knots or at 8 knots and as to the generator, how many hours would you use it while putting on 2300 nm? If it takes you a year and you use it 1600 hours then there goes a significant amount of fuel and numbers assume equal current both ways but if you find yourself doing a lot of boating into a 4 knot current it all changes. Just a lot of variables.

Now, if I was thinking of crossing an ocean in your boat, I'd only be comfy up to about 2000 nm until I'd really proven it all.

Thank you, I like your simpler approach. As to genny use, we run our 3500Kw NextGen an hour/day only when we are on the hook and not moving (we have Firefly batts so they charge super fast). So we'll use about a gallon every three days. And up here in the PNW we do deal with currents. Sometimes we do 5 kts, sometimes we do 10. But the average works out to 7. So I still feel good about the 2,300 (2,240nm) range.

And yes, were we to try and cross any significant body of water like an ocean I'd be estimating 2,000 or less!