Fuel economy

[robert644]

I have a 20' Wellcraft, a heavier V-hull, and a 90hp Merc. Right now I am planning an extended voyage with this boat, of several hundred miles, and I am trying to figure out some performance numbers on the boat.

I read in other posts that gph is 10% of hp. So, for a 90, this would be 9gph @ 90hp. but what about at slower speeds?, say 3/4 throttle.

Does anyone have a similar boat with a 90, I am not looking to run wide open, but I am wondering what kind of optimum mph/gph ratio that this type of boat would have.

The boat isn't going anywhere for at least another 6 months, so I am just toying with ideas right now.

Thanks!

 

[Silvertip]

Re: Fuel economy

I can assure you your engine will burn approximately 9 g/hr at WOT and I can also assure you it will likely burn about 7 g/hr at 3/4 throttle. If you know the difference in speed at those two throttle settings, you can calculate how much longer you would need to run (and hence the more fuel it takes to run that amount of time) at 3/4 throttle to get to your destination and you would then be able to determine whether it's even worth it to cruise at the slower speed. You will probably find there is very little difference. you see, fuel flow rate is exactly that, its the amount of fuel your engine uses each hour, minute, day or whatever period of time you select. MPG is an average and ignores the time factor. You might get 5 or 6 MPG at displacement speed (5 - 6 mph) and only 2 or 3 at WOT but it would take you forever to get anywhere at displacement speed. Run the numbers and do what you feel you need to do. If you have to figure your fuel consuption that close you need to consider 1/3 fuel load out, 1/3 fuel load back, and 1/3 for reserve. or 2/3 of the fuel load to reach your destination and 1/3 reserve. If there are plenty of fuel stops along the way this represents a whole 'nuther set of calculations.

 

[robert644]

Re: Fuel economy

Thanks Silvertip,

I read somewhere in a different forum where people were talking about getting 1 to 3 gph from engines of the same HP. I have no idea what they were talking about, and at 25-30mph speeds, I don't know why I thought a 90hp outboard would get better fuel economy than some cars sold today! I must have missed something on the thread, perhaps these folks were talking about trolling speeds.

So from the same forum I get 34mph from the 90, assuming this is wot. This gets me 34 miles at 9 gallons, 3.7 mpg. For my 1000 mile trip to Alaska, that's 270 gallons. I'll guesstimate and say fuel at the pump is $3.50, so that's approx. $950 in gasoline.

To pay the ticket on the ferry for myself is $350, and the boat is $1400 extra.

What beautiful logic and justification for a 1000 mile boat ride. (I'm going home to Alaska after the army)

Thanks.

 

[tashasdaddy]

Re: Fuel economy

there can be a lot of rough water in that distance. make sure the boat is in top notch shape, and you have some knowledge of the motor, and other systems, tools, and simple repair parts.

 

[Silvertip]

Re: Fuel economy

There is absolutely no comparison between a car and a boat when it comes to fuel consumption. A boat has no transmission and therefore is essentially operating in high gear all the time -- even when starting out. When you said you read that people were getting 1 - 3 GPH from the same size motor you need to dismiss claims like that unless you can see RPM, SPEED and actual FUEL FLOW. Without those numbers its just a "claim". I have a 75 HP Merc on a 17 ft walleye boat and at wide open throttle it burns 7.5 gph as indicated on a fuel flow monitor. That motor and yours share the same engine block. Where will this trip take you from and where will you end up.