Fuel cost

At the moment, we pay (on Curaçao) for gas $ 4.99 per gallon and for diesel $ 3.79 per gallon.

But, the 'road' and 'marina' prices are the same!

FYI, we pay $ 1.66 per 100 gallon water

Fuel Welland Canal

Fuel prices at marinas both ends of Welland Canal are:
Diesel $ 1.54 (L) = $ 5.77 U S Gallon
Gas $ 1.52 (L) = $ 5.70 U S Gallon
Last week I was paying $ 142.9 (L) = $ 5.35 U S Gallon for gasoline for my car.
Over night our gas prices will go up or down .10 cents a (L) A BIG RIP OFF

Haines, Alaska Diesel $3.90/gallon
Valdez, Alaska Diesel $3.89/gallon

Puerto Vallarta Mexico Diesel $4.53 per gallon

Contractor rate her is 1.20 per litre plus 7% tax, regularly 1.65 converts approx to 4.40 and 5.80 US per gallon. no complaints on the math pls...

Yes it will but still a relatively small percentage of costs for most boaters. Maintenance, docking, insurance, crew.

But then fuel will vary greatly depending on type boat and usage. You can get a trawler and use under 10 gallons per hour. Some as low as 3-5. We will use about 800 gallons tomorrow covering 200 nm in 10 hours. We could slow to 8 knots and cover that same 200 nm in 25 hours using 200 gallons on the same boat. Many trawlers could do it with 100 gallons.

Gallons

And be carefull, our American friends are talking about US gallons, which are 3.78 Liter per gallon, whereas you might think in Imperial gallons, which is 4,54609 Liter per gallon.

Attached a typical Britsh filling station

You were probably given the benefit of the doubt for a simple typo. That won't always happen.

Just filled the tanks at our marina on georgian bay Ontario. 1.62 per liter.
Getting expensive up here to boat after you consider dockage maintenance and the short season.

First, noticed your math error but decided to let you by one time. Plus your upper number was not out of line as many planing boats will use that or more at WOT.

No, cruising speed and hull speed are two different things. Most people don't buy Sunseekers to go hull speed. The calculation of hull speed is a factor times the square root of the waterline length. That factor has varied depending on the source, but most consider it from 1.2 to 1.3, but I've seen as low as 1.1 and as high as 1.34. Using 1.3, here would be the formula:

1.3 x sq rt (Lwl)

Every boat has a different fuel curve though depending on it's design and it's power. Some, like our Riva and like some Sunseekers, aren't even efficient at hull speed and have a very low hull speed. I found a test of a Sunseeker Manhattan 70 (2 x 1550 MAN) that at 1000 rpm goes 10 knots and only gets .44 nmpg. It probably has a hull speed based on design closer to 8 knots (750 rpm or so) or a factor closer to 1.1. At 1250 rpm, 11 knots, it drops to .26 nmpg. It has a cruising level from about 18 to 25 knots at which it gets .30 nmpg and then drops off until it reaches WOT of 35 knots and .21 nmpg. That's why I didn't yell at your math error as it uses 169 gph at WOT.

Meanwhile a Sunseeker Predator 80 (2 x 1800 MAN) only does 7.1 knots at 1000 rpm but gets 1.4 nmpg at that speed. At 1500 rpm it gets 16.9 knots (obviously a far more efficient hull) and gets .63 nmpg. At 2000 rpm it goes 26 knots at .46 nmpg and WOT gets 33 knots and .38 nmpg.

So in one builder you have two very different designs and very different fuel curves.

Some builders even give range at three speeds in their basic specifications. WOT, Cruising speed (which is typically around 80% or so) and Economy cruising speed (which is generally around 1000 rpm's and slightly above hull speed).

You need to go look at some reviews and tests of various boats to better understand consumption.

Fuel costs in Europe

Yesterday, my son refuelled his boat in a little harbour in Germany in the Baltic Sea for 7.97 US $ per US Gal with automotive diesel, brought in by a fuel truck. For his tankage of appr. 2000 US Gal, he paid roughly 16.000 US $. My impression from his phone call, he was not amused. He used a German word, which translated, would sound like "a rip off". My answer was: Proper planning prevents pure performance. Better fuel planning and choosing the optimum refuelling location could have saved a lot of money. There is always a penalty, if your arm on the throttle is to long .

Refuelling the Cessna Citation with its 600 US Gal fuel capacity, will cost me at the none commercial Jet A1 prices of 6.5 $ per US Gal some 3.900 $ but this bird will take me at 410 Kts about 1.600 NM in less than 4 hrs. And the large sailboat will take me at generator fuel consumption (plus some free wind) around the globe .

Will say, you have to accept, that fast planning boats are not the most economical means of transportation.