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Author Topic:   Speed and Fuel Consumption
ConB posted 09-13-2003 08:04 PM ET (US)   Profile for ConB   Send Email to ConB  
What would be a ball park figure for cruising speed and fuel consumption for a 1990 yamaha 150 on an Outrage 18?
Also, what would a competent mechanic check for during a condition survey for the above engine?
Thanks, Con B
jimh posted 09-14-2003 10:34 AM ET (US)     Profile for jimh  Send Email to jimh     
Fuel consumption is not directly related to boat speed, but to horsepower being used.

If you operate the OUTRAGE-18 at a speed which allows it to hydroplane, the specifications suggest you will be using a minimum of 75-horsepower.

A good rule of thumb is that a 2-stroke engine will burn about 0.5 pounds of fuel per hour per horsepower.

This implies that one hour of operation at hydroplane of an OUTRAGE-18 will require:

75 X 0.5 = 37.5 POUNDS OF GASOLINE

Gasoline weighs about 6-pounds/gallon, so this implies you would burn

37.5/6 = 6.25 GALLONS/HOUR

Depending on your boat, its hull condition, the sea state, the winds, the load being carried, you will be able to travel at a particular speed when maintaining hydroplane with your OUTRAGE-18.

Let us assume that you will, with a light load, a clean hull, the proper propeller, etc., be able to maintain a speed of 22-MPH when using 75-HP from your engine. (We also assume your engine is in good tune, your fuel is fresh, there are no leaks, etc.)

This implies in one hour you could go 22 Miles.

Thus your fuel economy will be

22/6.25 = 3.5 MILES/GALLON

If you carry a heavy load, travel against head seas, are faced with head winds, have bottom paint, have canvas or other frames, carry large amounts of fuel, water, passengers, etc., all these factors will tend to increase the horsepower needed to maintain the OUTRAGE-18 on plane.

If your engine is not operating efficiently, if your propeller is damaged or has a poorly chosen pitch or diameter or blade design, then all of these factors will tend to increase the amount of horsepower you need to maintain the OUTRAGE-18 on plane.

If you travel faster you will likely use more horsepower to maintain the OUTRAGE-18 on plane. Fuel consumption will increase.

You may be able to estimate horsepower being used by asking the maker of your engine to furnish you with a chart that shows horsepower plotted as a function of crankshaft speed.

The figure cited above is probably about the best case situation. I would estimate that fuel economy will likely be less in most cases.

A good figure to use is probably more like 2.5 MILES/GALLON.

As far as I know, the fundamental assumptions of thermodynamics (the 0.5 POUNDS/HP/HOUR) are not mutated by a particular engine size or by a particular engine maker. Thus a 1990 YAMAHA 150-HP engine tends to obey these fundamental rules just like any other engine.

When you ask for a mechanic to check an engine, they will likely perform a close visual inspection. They will remove the spark plugs and check the compression of the individual cylinders. Generally no further disassembly of the engine power head is done. Were I the seller of an engine I would not permit any further disassembly by a buyer's agent. It would be very unusual for a seller to consent to any further disassembly of the engine for inspection contingent to sale. The mechanic may open the lower unit oil sump and remove a small amount of the oil to inspect the lubrication for water contamination.

I would first clear all this with the seller.

ConB posted 09-14-2003 02:31 PM ET (US)     Profile for ConB  Send Email to ConB     
Thanks for the reply and this forum,jimh.
As to the compession of an engine, if memory serves, a more than 10% variation in psi is not good, but what psi ranges could I expect to find on an engine in good condition?
Thanks, Con B
Sal DiMercurio posted 09-14-2003 04:53 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Jim, I must dissagree with your calculations.
I don't think i'v ever seen a 150 ....2 stroke get better then 2.5 mpg on any Whaler larger then 17 ft, unless it was run under 3,500 rpms but over 3,000 rpms.
Those 150s are not noted for fuel economy & 3.5 mpg is being overly generous by about 1 mpg.
The very best that engine can do is 2.5 mpg on a lightly loaded 18 ft Outrage in 1 ft chop.
If he runs at 4,000 rpms he will be lucky to see 2.5 mpg.
Any 2 stroke "V" 6 is a gas hog with the exception of DFI & EFI engines.
Peter posted 09-14-2003 09:53 PM ET (US)     Profile for Peter  Send Email to Peter     
You should be able to get nearly 4 mpg out of a carburated 150 on an 18 Outrage at optimal cruising speed somehere between 25 and 30 mph. On the 18 Outrage, a 150 spinning at 3500 rpm, depending on the propeller, will produce speeds of about 30 mph while burning something on the order of 7.5 gallons per hour. That works out to 4 mpg.
jimh posted 09-14-2003 11:06 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is a link to a test report on the Yamaha website. The report shows a 150-HP Yamaha getting 3.5 MPG on a much heavier (3,100#) 21-foot boat. This was at 4000 RPM.


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