Fuel economy in your power boat
We all know that as a general rule of thumb, the faster you go, the more fuel you burn - it's basic physics. However, it's not quite that simple with a boat on the ocean, because the dynamics affecting the boat are never constant. Each power boat has an optimum power setting for every set of conditions, and quite often it means that to get better economy, you actually have to go faster.
SPECIFIC RANGE
Except during those times when we're actually deliberately operating at a set speed (such as trolling with lures, idling along with live baits, or with skip baits out. . .), what we're generally after when going from A to B, is best specific range. Specific Range is the term used to describe fuel burned per distance travelled... but the biggest question is... what distance are we talking about? There are also actually two types of specific range in play in boating - fuel burned per distance travelled through water, or fuel burned per distance travelled across the earth's surface. . . the two can be dramatically different. Specific Range is unit of fuel burned per unit of distance covered, and in this discussion, should not be confused with specific fuel consumption or brake specific fuel consumption, which is unit of fuel burned per unit of power produced - not what we're talking about here.
GROUNDSPEED versus WATERSPEED
You can obviously be travelling through the water at 20 knots and travelling over the earth's surface at 20 knots if you're moving through completely still water, but if you're steaming into a 3 knot current, your speed over the ground will obviously only be 17 knots, or 23 knots if you're going down current.
Similarly, if you're travelling exactly sideways through a strong current, you may think you're travelling with the same groundspeed as waterspeed, but you'll actually be travelling across the ground much slower because of the need to crab into the current and cover more water distance just to offset the sideways push of the current.
VARIABLES AFFECTING SPECIFIC RANGE
As mentioned, what we want to do every time we're travelling to or from the shelf or transiting from one destination to another is arrive having burned the least amount of fuel - unless of course we're in a hell of a hurry, or just feel like having a bit of a blatt turning hydrocarbons into noise.
The rub is that the power setting you will use to achieve the best specific range outcome on one day under one set of conditions may be significantly different than another day.
Factors affecting the optimum specific range power setting include. . . total weight being carried (fuel, water, crew, etc), which has a big effect, as does current (against, with, abeam . . ?), wind (against, with, abeam. . .?), boat trim, and multiple other small variables.
GETTING THE BEST SGR
While you can always work out your daily SGR after the fact by taking your total track distance covered and factoring it against the total fuel burned, that's one of those "after the fact" calculations that doesn't really help you at the time. Besides, it will only tell you an overall average if you're out on a typical fishing day, and won't tell you exactly what you want to know about the transit (to and from the fishing grounds, as distinct from operations once you get there) SGR, which is where you can really save fuel.
The only way to understand precisely the optimum power setting for best SGR is to be able to read an instantaneous fuel flow and an instantaneous groundspeed from a GPS. You can do mental arithmetic if you like, but the best solution is to have a fuel flow computer that takes an input of instantaneous fuel flow, and computes that against accurate groundspeed input from a GPS.
Once you have such a system, you basically get on the water and adjust power settings to suit the conditions as determined by the fuel computer from those two inputs.
For example, if your boat is lightly loaded, the current is behind you, and the wind is in your favour, best SGR may come at a relatively low power setting.
Counterintuitively however, a heavily loaded boat pushing into a strong current and headwind will actually need to travel at quite a high power setting to achieve best SGR under such conditions, and will burn more fuel at what you might at first think is a more economical lower power setting . Then, if the wind changes, the current changes, or even if your crew all go and sit down the back of the boat, you'll find that you'll be changing power settings to re-set the boat speed for best SGR.
Often, the same trip on different days will use a different power setting to set the boat running at max SGR, so from one day to the next, it's not simply a case of going out and setting the power that gave you the best economy on the last trip, because unless today's conditions are absolutely identical, the best SGR power setting will always change - sometimes a lot, sometimes very little.
THERE IS A WAY. . .
There used to be several manufacturers of aftermarket fuel computing systems, and these days many new electronic engine instrumentation packages do this already as standard or at least as an option.
About the only really good aftermarket fuel flow computer systems on the market now are those from the Seattle company, Floscan. There are others out there, but these guys are the specialists and I have one of these units on my boat.
The Floscan company provides a full installation kit with very clear and comprehensive instructions for installation, setup, calibration and use, when you buy one of their fuel flow systems, and while installation is not easy, most laymen with some basic engine and electrical skills should be able to get it done, although the standard of work required is fairly precise.
Other than the gear that comes with the kit, you'll need a few extras like heat activated dual-walled shrinkwrap and cable ties and other consumables, but the main extra you may need is a new fuel filter element for your filters. The latter may be required because the system does not work well with any of the finer grade of fuel filter element - in fact, they don't recommend using filters finer than 30 microns if you want maximum accuracy.
The system has highly sensitive electronic flow sensors that have to be plumbed into the fuel lines, multiple electrical loom connections, and a GPS feed from your plotter or instrumentation (NMEA) databus on your boat.
Once installed, the fuel computer system gives instantaneous readouts of total fuel flow, SGR, fuel used, RPM, and total engine running hours since installation.
So as well as providing a means for you to refine your power setting to give you the best economy on every trip, you also can see very accurate instantaneous fuel flows, and the resettable fuel used totaliser will always tell you exactly how much fuel to expect to add each time you pull up at the fuel dock to fill the tanks.
Since installing the system a little under two years ago, I've reduced my fuel burn for a 10-hour day of game fishing out beyond the edge of the continental shelf by an average of about 10 litres per trip, which is about 9% of my daily consumption. Most of the saving comes from the 45 minute average each-way transit to the edge of the shelf. The biggest saving was achieved during a transit from Coffs down to Port Macquarie, where I was able to set a power using the fuel computer which got me there sooner, and saved 15% of the fuel I had planned for the trip.
By extension, it's pretty easy to work out the payback on a system like this. Even if you dismiss the convenience of knowing exactly what you've got left in the tank at any given time, and therefore exactly what you need to plan for a refuel, you can do some conservative calculations based on say an average 5% fuel saving. Work out your monthly average fuel bill for a year, look at a 5% cost of that, factor it against the cost of the system for your boat/engine installation and then work out how many months of operations it will take to recover the cost of the new system.
FLOSCAN
The company has a great product, it works as advertised, and their customer service - even when dealing with a Floscan user in another hemisphere on the other side of the world - is excellent. Technical support and warranty coverage is great, and advice is excellent and timely.
The product is extremely well engineered, and I spent three days installing the unit on my single engined diesel boat, which I thought was pretty reasonable - no doubt a professional could do it in less.
And, they didn't pay me a cent for boosting this great piece of kit. . . I bought my unit at full retail price in case you were wondering, and I suspect they don't even know I wrote this article.
MINIMUM SAFE FUEL LOADING FOR TRIPS
A lot of people ask what sort of fuel load to plan for a long trip or transit offshore. The simple answer is that while it costs fuel to carry excess fuel, that empty air in your fuel tank is not something you can burn when you get into trouble, so always depart on an open sea transit with a full tank unless you have a good reason not to.
That said, if you want to plan a really economical fuel load for a trip, or are planning to stretch your boat's range as much as possible, then here is the best way to plan a safe minimum fuel load for any given transit.
Start with your boat's best economical boat speed through still water
Adjust that speed by the average current you expect to encounter plus any head or tailwind effect to determine your average groundspeed for the trip
Take the total distance of the trip, divide by the anticipated groundspeed reached in step 2 to reach a total trip time
Take the fuel burn per hour at the best speed you decided on in step 1, multiply that by your total trip time to determine a total planned fuel burn for the trip
Add 20% of that total as a variable to account for unforeseen current/wind variations
Add one hour of average burn
That will be the absolute minimum fuel load you should leave your departure point with if you don't want to or simply can't take full tanks.
In other words, total distance divided by average groundspeed to derive total time in transit in hours, then multiply that by hourly fuel burn, then add 20% of the total and then add a further hour of fuel burn. Remember that all this will go out the window if you subsequently drive the boat at a different speed from that you'd planned, or if you change the route, so you have to stick to the plan for your fuel load to work.
SPECIFIC RANGE
Except during those times when we're actually deliberately operating at a set speed (such as trolling with lures, idling along with live baits, or with skip baits out. . .), what we're generally after when going from A to B, is best specific range. Specific Range is the term used to describe fuel burned per distance travelled... but the biggest question is... what distance are we talking about? There are also actually two types of specific range in play in boating - fuel burned per distance travelled through water, or fuel burned per distance travelled across the earth's surface. . . the two can be dramatically different. Specific Range is unit of fuel burned per unit of distance covered, and in this discussion, should not be confused with specific fuel consumption or brake specific fuel consumption, which is unit of fuel burned per unit of power produced - not what we're talking about here.
GROUNDSPEED versus WATERSPEED
You can obviously be travelling through the water at 20 knots and travelling over the earth's surface at 20 knots if you're moving through completely still water, but if you're steaming into a 3 knot current, your speed over the ground will obviously only be 17 knots, or 23 knots if you're going down current.
Similarly, if you're travelling exactly sideways through a strong current, you may think you're travelling with the same groundspeed as waterspeed, but you'll actually be travelling across the ground much slower because of the need to crab into the current and cover more water distance just to offset the sideways push of the current.
VARIABLES AFFECTING SPECIFIC RANGE
As mentioned, what we want to do every time we're travelling to or from the shelf or transiting from one destination to another is arrive having burned the least amount of fuel - unless of course we're in a hell of a hurry, or just feel like having a bit of a blatt turning hydrocarbons into noise.
The rub is that the power setting you will use to achieve the best specific range outcome on one day under one set of conditions may be significantly different than another day.
Factors affecting the optimum specific range power setting include. . . total weight being carried (fuel, water, crew, etc), which has a big effect, as does current (against, with, abeam . . ?), wind (against, with, abeam. . .?), boat trim, and multiple other small variables.
GETTING THE BEST SGR
While you can always work out your daily SGR after the fact by taking your total track distance covered and factoring it against the total fuel burned, that's one of those "after the fact" calculations that doesn't really help you at the time. Besides, it will only tell you an overall average if you're out on a typical fishing day, and won't tell you exactly what you want to know about the transit (to and from the fishing grounds, as distinct from operations once you get there) SGR, which is where you can really save fuel.
The only way to understand precisely the optimum power setting for best SGR is to be able to read an instantaneous fuel flow and an instantaneous groundspeed from a GPS. You can do mental arithmetic if you like, but the best solution is to have a fuel flow computer that takes an input of instantaneous fuel flow, and computes that against accurate groundspeed input from a GPS.
Once you have such a system, you basically get on the water and adjust power settings to suit the conditions as determined by the fuel computer from those two inputs.
For example, if your boat is lightly loaded, the current is behind you, and the wind is in your favour, best SGR may come at a relatively low power setting.
Counterintuitively however, a heavily loaded boat pushing into a strong current and headwind will actually need to travel at quite a high power setting to achieve best SGR under such conditions, and will burn more fuel at what you might at first think is a more economical lower power setting . Then, if the wind changes, the current changes, or even if your crew all go and sit down the back of the boat, you'll find that you'll be changing power settings to re-set the boat speed for best SGR.
Often, the same trip on different days will use a different power setting to set the boat running at max SGR, so from one day to the next, it's not simply a case of going out and setting the power that gave you the best economy on the last trip, because unless today's conditions are absolutely identical, the best SGR power setting will always change - sometimes a lot, sometimes very little.
THERE IS A WAY. . .
There used to be several manufacturers of aftermarket fuel computing systems, and these days many new electronic engine instrumentation packages do this already as standard or at least as an option.
About the only really good aftermarket fuel flow computer systems on the market now are those from the Seattle company, Floscan. There are others out there, but these guys are the specialists and I have one of these units on my boat.
The Floscan company provides a full installation kit with very clear and comprehensive instructions for installation, setup, calibration and use, when you buy one of their fuel flow systems, and while installation is not easy, most laymen with some basic engine and electrical skills should be able to get it done, although the standard of work required is fairly precise.
Other than the gear that comes with the kit, you'll need a few extras like heat activated dual-walled shrinkwrap and cable ties and other consumables, but the main extra you may need is a new fuel filter element for your filters. The latter may be required because the system does not work well with any of the finer grade of fuel filter element - in fact, they don't recommend using filters finer than 30 microns if you want maximum accuracy.
The system has highly sensitive electronic flow sensors that have to be plumbed into the fuel lines, multiple electrical loom connections, and a GPS feed from your plotter or instrumentation (NMEA) databus on your boat.
Once installed, the fuel computer system gives instantaneous readouts of total fuel flow, SGR, fuel used, RPM, and total engine running hours since installation.
So as well as providing a means for you to refine your power setting to give you the best economy on every trip, you also can see very accurate instantaneous fuel flows, and the resettable fuel used totaliser will always tell you exactly how much fuel to expect to add each time you pull up at the fuel dock to fill the tanks.
Since installing the system a little under two years ago, I've reduced my fuel burn for a 10-hour day of game fishing out beyond the edge of the continental shelf by an average of about 10 litres per trip, which is about 9% of my daily consumption. Most of the saving comes from the 45 minute average each-way transit to the edge of the shelf. The biggest saving was achieved during a transit from Coffs down to Port Macquarie, where I was able to set a power using the fuel computer which got me there sooner, and saved 15% of the fuel I had planned for the trip.
By extension, it's pretty easy to work out the payback on a system like this. Even if you dismiss the convenience of knowing exactly what you've got left in the tank at any given time, and therefore exactly what you need to plan for a refuel, you can do some conservative calculations based on say an average 5% fuel saving. Work out your monthly average fuel bill for a year, look at a 5% cost of that, factor it against the cost of the system for your boat/engine installation and then work out how many months of operations it will take to recover the cost of the new system.
FLOSCAN
The company has a great product, it works as advertised, and their customer service - even when dealing with a Floscan user in another hemisphere on the other side of the world - is excellent. Technical support and warranty coverage is great, and advice is excellent and timely.
The product is extremely well engineered, and I spent three days installing the unit on my single engined diesel boat, which I thought was pretty reasonable - no doubt a professional could do it in less.
And, they didn't pay me a cent for boosting this great piece of kit. . . I bought my unit at full retail price in case you were wondering, and I suspect they don't even know I wrote this article.
MINIMUM SAFE FUEL LOADING FOR TRIPS
A lot of people ask what sort of fuel load to plan for a long trip or transit offshore. The simple answer is that while it costs fuel to carry excess fuel, that empty air in your fuel tank is not something you can burn when you get into trouble, so always depart on an open sea transit with a full tank unless you have a good reason not to.
That said, if you want to plan a really economical fuel load for a trip, or are planning to stretch your boat's range as much as possible, then here is the best way to plan a safe minimum fuel load for any given transit.
Start with your boat's best economical boat speed through still water
Adjust that speed by the average current you expect to encounter plus any head or tailwind effect to determine your average groundspeed for the trip
Take the total distance of the trip, divide by the anticipated groundspeed reached in step 2 to reach a total trip time
Take the fuel burn per hour at the best speed you decided on in step 1, multiply that by your total trip time to determine a total planned fuel burn for the trip
Add 20% of that total as a variable to account for unforeseen current/wind variations
Add one hour of average burn
That will be the absolute minimum fuel load you should leave your departure point with if you don't want to or simply can't take full tanks.
In other words, total distance divided by average groundspeed to derive total time in transit in hours, then multiply that by hourly fuel burn, then add 20% of the total and then add a further hour of fuel burn. Remember that all this will go out the window if you subsequently drive the boat at a different speed from that you'd planned, or if you change the route, so you have to stick to the plan for your fuel load to work.