Nimbus 280 Fuel Efficiency II

[sir jasper]

Thanks to all those who replied to my last post “Nimbus 280 Fuel Efficiency”; following the forum consensus I subsequently increased my cruising speed from 9 to 14 knots and approximately halved my fuel consumption.

My next question relates to use of the trim tabs. At 3400rpm with the tabs raised the resultant speed is 12 knots as I lower the tabs and the bow lowers the speed increases to 14 knots. As the speed increases and the bow lowers, it presses down onto the bow wave displacing quite a lot of water. At this angle (pressed down into the bow wave), is more fuel being consumed due to the greater resistance?

 

[Deleted User YDKXO]

Thanks to all those who replied to my last post “Nimbus 280 Fuel Efficiency”; following the forum consensus I subsequently increased my cruising speed from 9 to 14 knots and approximately halved my fuel consumption.

My next question relates to use of the trim tabs. At 3400rpm with the tabs raised the resultant speed is 12 knots as I lower the tabs and the bow lowers the speed increases to 14 knots. As the speed increases and the bow lowers, it presses down onto the bow wave displacing quite a lot of water. At this angle (pressed down into the bow wave), is more fuel being consumed due to the greater resistance?

If the speed increases and you haven't moved the throttles, then you will be using less fuel in terms of mpg. Many boats, particularly semi displacement boats, go faster with the trim tabs down. You should experiment by driving the boat at your usual cruising rpm and adjust the trim tabs in small increments to find the optimum speed. Don't forget that if you find yourself in a big following sea, the trim tabs should be raised as there is the potential risk of a broach whereby the bow stuffs itself into the wave in front and the wave behind pushes the stern around. Again, different boats behave differently so experiment to find a comfortable setting for the boat and tabs in these conditions

 

[sir jasper]

So, is the fuel flow constant what ever the load?

Or, does the engine management system use the rpm as a set-point, regulating the fuel to maintain the rpm, so as load increases so does the flow of fuel?

 

[Deleted User YDKXO]

So, is the fuel flow constant what ever the load?

Or, does the engine management system use the rpm as a set-point, regulating the fuel to maintain the rpm, so as load increases so does the flow of fuel?

I'm no expert on this and it is made more complex by the issue of prop demand. For any given boat/prop/engine combination, the prop can only absorb a specific amount of power at a given rpm. My boat, which is a planing boat, does exhibit the same characteristic as yours in that I can increase the boat speed over a certain range of rpm by trimming the bow down. When this happens, the engine rpm remains constant but the boat speed increases. What's happening when the tabs are trimmed down is that the stern of the boat is being lifted and the running angle of the boat is being reduced which, for our boats, decreases the drag between the hull and the water. For some boats like yours and mine, this results in an increase in speed, for other boats it may result in a decrease in speed because lifting the stern immerses the bow causing extra drag or an increase in speed at lower speeds and a decrease in speed at higher speed. It is a complex issue and there are no hard and fast rules which is why I say that you should experiment at various engine rpm to find what position of trim tab offers the optimum speed and, hence, efficiency. I think you can say one thing for sure and that is for any given throttle position, if you can find a way of increasing the speed of the boat, the fuel efficiency will improve

 

[sir jasper]

Thanks Mike, from now on I’ll trim assuming the higher the speed the greater the efficiency. I suppose the only way to be sure would be to install a fuel flow meter.