Some Novice Questions (Fuel Consumption, long distance cruises, etc)

That's exactly what I thought you were thinking about, and why I broached it as I did. It's rare that you're wrong about anything. But that's entirely different than what this is about.
That's about how far you can safely run on a tank of fuel, because time at the fuel depot is time not making money. Same as we consider when transporting. That's different than prudent seamanship for someone bringing a small boat home or cruising their recreational boat.
Check the cites that I added to post #19. I could have posted another 30 cites.

From post #1:
Please forgive the novice questions. I'm looking at getting a "]55ft boat from Italy and bringing it to Canada for doing small charters, and weekend cruises. I'm trying to get a gauge on what it would cost to drive over here (captain aside) in terms of fuel consumption, and if the tanks on board are big enough to support it.

It has 2 - 1050 hp MAN Diesel engines, with 2600L fuel capacity.

Ok, guys. The range safety factor rises again. I think first, the ocean range talk on this boat the OP posted is totally irrelevant as the fact is the boat has no ocean crossing range in any way. It has day cruiser range.

Now, I think as to the safety margin for other use, the reason we see such varying answers is that it's highly dependent on the boat, the use planned, the knowledge of that specific boat's range and the experience of the Captain and crew. Reality is that there can't be a universal percentage to apply to all situations. Kiwi's numbers are quite reasonable for the boat and situation he describes. For the boat and experience of the OP on an ocean crossing there is no percentage high enough.

You get into issues such as how big the contingencies you need to plan for are. If I'm somewhere that I know if I run low I'm always close to fuel, then it's one thing. If it's a boat I've used in similar situations and have a good fuel history it's another factor. It starts even with the stated distance. If you take "as the crow flies" and the shortest distance then you're not considering storm systems and the possibility of having to take a different route. That is more of an issue on a recreational boat than perhaps a commercial vessel. In some cases a combination of percentage and absolute allowance may make sense. Perhaps in certain circumstances one might want to say 10% for general factors like wind and current but then I always want to have a safety factor of 500 gallons.

Other factors such as the ability to adjust speed to make up for over consumption play a role. This isn't generally a factor for a commercial vessel. However, take a recreational vessel that has a range of 2,500 miles at 12 knots and it may, depending on the vessel, have a range of 30% more if slowed down to 6-8 knots. On the other hand it may need to be run occasionally each day at 15-18 knots with certain engine combinations. All those factors come into play. I was just playing with numbers as an exercise on a boat quoted which gets .33 nmpg at 12 knots. I found out that at 11 knots it gets .38 nmpg. At 8 knots however it gets .7 nmpg. So, if one is consuming more than anticipated at 12 knots the option to slow further is there. However, there's one factor against that. The manufacturer recommends on it running faster every 12 hours or so for a few minutes. Well at 15 knots it only gets .25 nmpg and at 20 knots it only gets .15 nmpg. Suddenly a boat with a quoted range of 3,000 miles at 12 knots with a 10% allowance has one of 3,100 nm to 3,500 at weighted averages. But in an urgent situation if you had to run faster it becomes much less. Even taking a 30% factor however, it's range is 2,300 - 2,500 nm. Now would one trust it for a Bermuda to Azores of around 2,000 nm. I would think so. But even then I'd want to see those consumption numbers proved in actual experience under the heavy load you'd have for ocean crossing, not just on a manufacturers test data.

Guess all I'm saying is that there are many factors and in certain circumstances the numbers Kiwi uses are more than adequate. But in other circumstances 20% or 30% or even more is called for.

As an aside, almost all tests I see run are at 10% and I think we'd all agree that isn't enough for the average recreational boater. On the other hand, the sites quoting 30% or 33% are probably being exceptionally conservative in thinking of the inexperienced boater who may not have a firm grasp on usage. I had a 30 foot runabout on a lake. I have no idea what it's consumption was. I just kept an eye on the fuel gauge.

That's not adequate for ocean or even coastal. I have accumulated tons of data on both of my boats now and will on my future purchases. I have a 63' Riva that between 15 and 30 knots has very little difference in consumption per mile. That's from 1250 to 1750 rpm. Even dropping to 1000 rpm only increases range 20-25%. Increasing to 2300 rpm decreases range about 20%. Sea and wind conditions actually decreased range around 12% on a recent trip. Fact is I know to be very conservative on this boat, not to exceed 1750 rpm if I'm at all cutting things close and that I have very little ability to increase my range. Dropping to an idle does some but tracking in seas at idle isn't very good. It has a 51' waterline so even 1000 rpm exceeds it's displacement speed as 1000 rpm gives 11 knots. All these numbers would love very different in another vessel.

As to the OP there is not allowance that works. An unknown boat, inexperienced operator, planing hull on sea conditions, even the 30% would be questionable and it doesn't matter as even 0% doesn't give adequate range. As to Kiwi's commercial vessels, I have no doubt that his percentages are time and experience proven and adequate.

Oh and my Riva which is not dissimilar to the boat he's talking about but has larger engines and a larger fuel tank, the maximum distance I feel safe with is 220 nm if watching my speed, 200 nm if not doing so.

Hi,

I was not referring to commercial vessels in my post, it will be a private yacht and will operate the same way many other private and commercial yachts operate.

The shipyards will give a contracted speed and range before they start building, once you take delivery if you need to go further just slow down a bit , the savings might be quite surprising.

We are looking for 6000nm at 16kts but might have to moderate this a bit after tank testing results as we don't want to have to make the boat bigger just to carry more fuel.

While not looking at a commercial vessel then, you're still looking at factors of knowledge and experience and testing. That's the key aspect. I'd trust your calculations after testing and with knowledge. I wouldn't trust the OP's. You also hit on a key element and that is that most boats are still not quoted at their absolutely lowest consumption. It's something one needs to look at very carefully in comparing too. On the same length vessel, I've seen one manufacturer quote conservatively at 12 knots and another at 9 knots. So the one at 9 gives a picture of better range when, in fact, the one quoted at 12 would have greater range at 9.

Do you have a trip in your thoughts or plans that will actually require the 6000 nm range? I am curious as in looking at our future dream trips the longest range we've required has been around 3000 nm.

My advice would be to forget buying a boat in Europe.. sounds too complicated with taxes, transport costs, unfamiliar builder [over here anyway], parts, servicing engines etc.Stick with a well known builder and it will be easier from a resale perspective.

Would also involve a trip there if you actually wanted to see what you were buying, arranging for the survey, and getting representation there to coordinate the shipping. If you're buying a $2 million boat, then adding $50,000 to the cost is no big deal. If you're buying a $150,000 boat you've increased your investment 33%. One more complication you're adding is exchange rate and determining what currency is controlling.

At one time I liked to buy that which no one else had. But getting parts can become an issue later and cost of them if you can get them. Perhaps my favorite car I owned when younger was one they imported very few of and stopped importing after that year. The cover of the right rear tail light got cracked. Six weeks and $200. Turned out cover wasn't available so entire assembly had to be replaced. That said, we own three European cars today. Yes parts and service are outrageous, but they are cars that are well known. Each of them is less than a year old and worth $40,000 each less today than what we paid.