Calculating Fuel and Cruising Speed

Sounds like a lousy manual. For airplanes they tell you how much you will burn at cruise speed in the manual. I'd be upset with the ship manufacturer for giving me a lousy manual myself.

Maybe I'm dumb. Most of the planes I flew around by myself were slow. I made 'em go as fast as I could sometimes. Those slow planes burned 5 gallons an hour. 120 knots was about all you got out of a Piper Tomahawk. -Course higher, and leaner, and slower, well, might burn less, but ships do run at a constant of sea level. Not much call for turbo chargers I guess. Guess the optimum burn for the engine is the thing for a ship, and then the speed is determined by that.

It is very much the standard for engines of this type to give the specific fuel consumption in mass/kWhr.

These are the figures for the latest Sulzer RTA 96-C Marine diesel (I think still the most powerful and efficient built):

Fuel consumption at maximum power is 0.278 lbs per hp per hour (Brake Specific Fuel Consumption). Fuel consumption at maximum economy is 0.260 lbs/hp/hour. At maximum economy the engine exceeds 50% thermal efficiency. That is, more than 50% of the energy in the fuel in converted to motion.
For comparison, most automotive and small aircraft engines have BSFC figures in the 0.40-0.60 lbs/hp/hr range and 25-30% thermal efficiency range.

OP, to give you a start, you use an amount of fuel to travel a distance in a unit of time. You can not calculate the most economical speed, but you can calculate the more economical of the two given (ignore rpm).

Really?

Did you really get 120 knots from a tomahawk at level flight? if you say yes to this rhetoric question, you have never flown a Piper Tomahawk in your life...

Wangito.

Definitely it has been awhile since I've flown a Tomahawk so I may have not given the correct top speed. Tail number 2535D is what is in my logbook. The cross country from FMY to FLL shows 1.8 hours. For 2548B from Punta Gorda to Melborne I show one at 1.2 hours. Last entry is for some touch and goes in June of 1982, so hey, I could have been mistaken, and I'm not sure where the manual for that plane might be right now.

Far as my point that the manual for a plane does from my remembrance, tell you what you are likely to burn at most efficient cruise speed, is that true or not? Maybe I'm wrong about that too. Seems they do depending on altitude and air density.

I did find the contributors answer which pointed out that might be more efficient overall to burn more fuel and go a little faster in consideration of other cost factors, useful to the Original Poster.

What point would there be to lying about flying a Tomahawk? If I was a liar I'd lie about flying something like a Learjet.

Hey, Don't take it personal...

And BTW, it will fly more than 120knots with 90 degrees nose down...

And there's nothing glorious about a Lear. Want real pride? Say you flew my 2T1A-2

Just kidding.

Wangito.

The arithmetic is simple and the calculation can be carried out readily using the data supplied. Go for it.

This is what I think:

180 kW

1800 rpm

Fuel 200 gm / kw hr

= 180 x 0.200 = 36 kg / hr

Volume = mass divided ÷ relative density = 36 ÷0.8 = 45 lts / hr

Speed ÷ Fuel consumption

11 ÷ 45 = 0.244 efficiency / economy

195 kw

2100 rpm

Fuel 200 gm / kw hr

= 195 x 0.200 = 39 kg / hr

39 ÷ 0.8 = 48.75 lts / hr

11.5 ÷ 48.75 = 0.235

2100 rpm more fuel efficient.

You have to get it down to miles per dollar, too.

Stu.

Looks like homework to me, but:-

Specific fuel consumption for the engine is 200gm / kW hr

so cost is directly related to power.

and, you use 195/180= 1.083 times as much power to get 11.5/11=1.045 times as much speed.

I think I'd try sticking to the speed limit of 4 mph (3.476 knots): enjoy the scenery.

Hi Guest,

OK, here is the useless data: 195 kW and 11.5 knots at 2100 rpm. With the 200 gm per kW hr and 180 kW at 1800 rpm you can multiply to find 200X180=36 Kgms fuel burned per hour at 1800 rpm. Since you travel 11 knots (11 nautical miles per hour), this works out to 36/11 or 3.27 Kgms fuel used per nautical mile, 6000 feet. Because you don't know fuel consumption at 2100 rpm, you can not know most economical cruising speed. Here is a clue, drag increases faster than engine efficiency at increasing speeds and the best efficiency will be not much faster than idle. Since time is money if you work, there is a point just before the efficiency goes to heck and staying just below the curve break is practical for most people. Too bad that you don't have curves for hull speed and engine efficiency.

Regards,

Luther M

I don't see how you've distinguished between the two cases: you've got exactly the same amount of information about the 11.5 knots case as the 11 knots case.
(By the way a nautical mile is approximately 6076 ft: bizarrely exactly 1852 meters).

You can only determine fuel consumption as a function of power output. It will determine your endurance. The actual true speed is dependent on other factors, not only fuel consumption. The figure of 11.5 Nautical miles /hour refers only to still water no wind and standard ISA day, Almost none existent. Weight of the boat is also a consideration.

Wangito.

I am not sure if I got your units correct but here is what I calculate:

I just assumed a representative fuel price & transit distance. Based on this it seems the lower speed is marginally beneficial. However, for a complete cost-benefit analysis you also need to take into account your asset depreciation cost, salaries you pay, possibility of revenue generation if you travel faster, etc. Generally I found "economic speed" is misleading if based solely on fuel cost (unless you are on the steep part of the speed-power (fuel consumption) curve and often it is better to travel at reasonably fast speed to be more economic. Anyway this is my 2-cents worth. You probably have got feedback from somebody already.

Rgds

Indra Datta, Aker Solutions, indra.datta@akersolutions.com

Indra,

good stuff. However

" often it is better to travel at reasonably fast speed to be more economic."

is not the case here. The most economical speed to run a boat (ship) is below 'hull speed'. Depending on that designed hull speed, and the actual shape of the boat, you could find that the most economical speed may well be 4.5 - 6 kts, in which case it may be possible to cover twice the ground for a given fuel consumption.

Stu.

Lots of good comments !

Did it occur to anyone, that the manufacturer provided the 1800 RPM figures because they were the Best, Fuel Burn/speed relationship.??

crossed my mind. Don't know why I didn't fasten on it.

Dear Guest,

The Definition of " SPECIFIC FUEL CONSUMPTION " will remain theoritically, the same for entire range of operating speed of the engine.

But in practise additional fuel consumption on account of increased Frictional Losses, when SPEED IS INCREASED, is occuring, and this is very marginal.

If you are particular about Fuel Consumption in terms absolute quantity, then operate the engine at the speed recommended by the Engine Manufacturer, who designs the engine to give a maximum Thermal Efficiency for a particular speed.

Other particicipants have given the detailed calculations and I need not repeat the same.

thanks,

DHAYANANDHAN.S,

CR4 MEMBER,

INDIA.