Fuel Mileage

[scrit9mm]

I know, I know 1/3 to 1/3 back and 1/3 reserve. What I want to know is how many miles (approx) going in a straight line at cruising speed (35-40mph) my 1999 Chris Craft 200 Bowrider is able to go. I am planning a trip and the longest distance between marinias is 88 nautical miles. I have a 40 gal tank, the engine is running properly, 19 pitch prop, and I will have 10 gallons in cans.

 

[shrew]

Re: Fuel Mileage

There is not enough information provided to accurately answer the question. The size of the motor, whether it is properly tuned, wind, current, sea state and tides can all impact. I understand that not all variables such as tide may come into play here, but it's worth mentioning.

Boat performance is typically measured in Gallons Per Hour (GPH), not Miles Per Gallon (MPG) because the variables will effect burn rate. Running against wind and current will yield a much different MPG than running with wind and current, for example.

They key would be to determine the AVERAGE burn rate in gallons per hour. so if you know that you burn (FOR EXAMPLE) 8 GPH at 3200 RPM's, then you know exacly how much fuel your burning. If you then look at the average speed you're maintaining at that RPM and the distance to your desitination, then you can calculate how many hours you can run at that burn rate and how far you will get. Keeping in mind that the MPH cna vary even though the same RPM and GPH burn rate is being maintained.

They key is to look at how much fuel is being burned over time at your typical cruisuing RPM. This is why fuel flow calculators and guages are so popular. If there were a flat MPG, GPH for all RPM's and seastates, it would be a single value and easy to publish.

 

[cribber]

Re: Fuel Mileage

Need one more piece of information... what size engine? http://boattest.com/ has boat reviews that also list fuel consumption for various makes and models.

My Volvo Penta 3.0 in my Glastron GT-185 gets 5 mpg at 25-30 mph cruise speed. My buddy's Velocity 502 gets maybe 2 mpg with a strong tailwind on a good day at the same cruise speed.

 

[UncleWillie]

Re: Fuel Mileage

As other have said ... not enough info.
The old rule of thumb is 1 gal per hour per 10 hp.
So 250 hp will need about 25 gal per hour.
Your best cruising speed is NOT wide open.
You wouldn't floor your car and leave it there for an hour, so don't do it to your boat either.
You will get significantly better fuel consumption at 1/2 to 3/4 power.
Speed is proportional to the cube root of the power used.
It take 8 time as much power (fuel) to go twice as fast.
and twice the power will get you a 25% speed gain.
If your boat cruises at 40 mph with a 250 hp engine, 88 miles with 50 gallons on board is going to be close but doable.
You will not be running at full throttle, but if you calculate it assuming full throttle consumption, your reserve will be built into the estimate.
You will likely make 100-110 miles or so on the 50 gallons plus vapors. Your mileage WILL vary!
Your most economical speed is at hull speed (~5-6 mph).
Your next best speed is just above planning speed (~20-25 mph).
The slower you go the farther you will go.

 

[smokeonthewater]

Re: Fuel Mileage

Also need to figure where you'll be boating... if in a river with a 5 mph current that will make a big difference....

 

[QC]

Re: Fuel Mileage

Just because I love to muck up these discussions . . . if you have GPH, and you also have speed, and "you then look at the average speed you're maintaining at that RPM and the distance to your desitination, then you can calculate how many hours you can run at that burn rate and how far you will get." that is MPG

What I will add to this is that 20 footers get between 2.5 and 4.0 MPG depending on cruise speed, technology etc. Almost all planing hulls get their best MPG at idle, but that's silly, so at planing speeds you will find that they usually do their best between 20 and 35 MPH. How do I know this? Scan the tests at BoatTest.com. Be careful though, all of them are overstated by at least 10% as is top speed. New boats are never loaded like a typical owners boat. Also, be VERY careful about comparing different technologies. MPI vs. Carbed, vs, OB, vs I/O, and the worst in fuel are direct drive inboards.

My guess, with that boat, is you will easily get 3 MPG at a reasonable cruise speed, in decent weather. But I don't have to make the trip. You do. As they say "your mileage may vary"

 

[scrit9mm]

Re: Fuel Mileage

most important info... 5.0 V8. I will be traveling down river, not sure of the current speed but it will not be very strong. The boat tops out at around 50, I cruise around 35-40. Worst case I was figuring 2 mpg. As of now the engine is in tip top shape, same with the drive and prop.

 

[JDA1975]

Re: Fuel Mileage

planning the trip upriver, because you'll have to go that way to come back...unless you are planning to have someone with the trailer meet you for trip back?

 

[scrit9mm]

Re: Fuel Mileage

yeah, its a one way trip. trailering back

 

[JDA1975]

Re: Fuel Mileage

with a 40 gallon tank i wouldn't worry too much then...maybe bring along a 5 gallon reserve just for peace of mind and enjoy your trip!

 

[H20Rat]

Re: Fuel Mileage

It take 8 time as much power (fuel) to go twice as fast.
and twice the power will get you a 25% speed gain.
Your most economical speed is at hull speed (~5-6 mph).
Your next best speed is just above planning speed (~20-25 mph).
The slower you go the farther you will go.

Not always... As a general statement they might be true, but there are lots of other variables. On many boats, best MPG's is well above just past plane, check out boattest for some of the numbers. In some cases its surprisingly fast. On my boat, planing is about 20-22 or so, but best cruise for MPG's is around 35 mph.

Also, slow (hull speed) isn't always better. Some boats are REALLY inefficient off plane, and do as well at best cruise. Also, how good are your mpg's if you are moving at 5 miles per hour, in a river that is pushing you at 5 mph? If you have any current at all, your best cruise automatically goes up. The less time you spend traveling against the current, the shorter the distance will be you need to cover. In most boats, you are going to be far less efficient at even 10 mph vs a decent on plane speed, just because of the effective travel distance.


(and of course, going downstream is the opposite, with the best MPG's obtained by turning the engine off, opening the cooler, and grabbing a beer!)

 

[scrit9mm]

Re: Fuel Mileage

I am sure the last part will happen more than a few times during the trip<smokingcrater!

 

[shrew]

Re: Fuel Mileage

Just because I love to muck up these discussions . . . if you have GPH, and you also have speed, and "you then look at the average speed you're maintaining at that RPM and the distance to your desitination, then you can calculate how many hours you can run at that burn rate and how far you will get." that is MPG

You are correct, howeveer the MPG in that case is variable, which was the point I was trying to make. If for example, you are burning 8 GPH at 3200 RPM's, and you happen to be travelling at 25MPH, you can't assume this is a constant and you will ALWAYS burn 3.125 MPG. Let's say you happened to be travelling with a 4 MPH current. If you opposed the current, you drop to 21MPH at the same 3200RPM's and the same 8GPH. However, now you're burning 2.625 MPG instead of the previously recorded 3.125 MPG. such a miscalculation would potentially leave you thinking you had suffucient fuel when you didn't. If instead you increase RPM's to say 3500RPM's and now your speed is back up to 25 MPH, now your GPH burn rate will be different as well.

 

[QC]

Re: Fuel Mileage

Agreed.

I have two pet peeves:

1) All you need is torque. When in fact you need torque and RPM, and the result is horsepower . . .

2) All you need is GPH. When in fact you ALWAYS need GPH and Speed, which results in MPG . . .

Hmmmmm, just noticed my pet peeves are consistent. That's a very new thing for me, consistency

 

[tpenfield]

Re: Fuel Mileage

I know, I know 1/3 to 1/3 back and 1/3 reserve. What I want to know is how many miles (approx) going in a straight line at cruising speed (35-40mph) my 1999 Chris Craft 200 Bowrider is able to go. I am planning a trip and the longest distance between marinias is 88 nautical miles. I have a 40 gal tank, the engine is running properly, 19 pitch prop, and I will have 10 gallons in cans.

88 nautical miles = 101 statute miles. Hopefully, you will get about 3-4 mpg in that boat. I get about 2 mpg in my 24 foot boat. so your range is going to be close. Stay near shore and have your SeaTow membership current.

It would be nice if you can assess your 'burn rate' during the first few legs of the journey before you take the 100 mile leap.

 

[scrit9mm]

Re: Fuel Mileage

there will be at least one other boat on the voyage, is the water in the wake from another boat more condusive for better "fuel mileage"?

 

[lkbum]

Re: Fuel Mileage

there will be at least one other boat on the voyage, is the water in the wake from another boat more condusive for better "fuel mileage"?

No, this is a mistake boaters make when cruising together. As smoke pointed out, a river current can make a big difference if you are headed into it. If you are directly behind a boat, the water is moving from his prop, you are essentially headed into a current. Of course the distance behind the boat etc makes a difference. It's best to be side by side (in most cases). Unless of course the lead boat is plowing down waves and making smooth water for you.

As pointed out in a lot of posts, your mpg is a funtion of gph and speed. I think most planing boats stay within 10% of mpg just after planing to 3/4 speed. As was also pointed out, you only really start sucking fuel near WOT.

 

[MH Hawker]

Re: Fuel Mileage

If you have a GPS with a trip log, fill the tank and give it a hours run at WOT then refill and you will know exactly what it is.

 

[QC]

Re: Fuel Mileage

give it a hours run at WOT then refill and you will know exactly what it is.

Exactly what it is at WOT . . . How does that help him?

 

[cyclops2]

Re: Fuel Mileage

My 19' long & 8' wide Chaparral with a 5.0L Mercruiser loves ANYTHING on plane. I can run ALL day on a tank of fuel & STILL have 1 hour as a reserve at 3200 rpms to make it home.

Spreading out a load over a wide boat makes a HUGE difference..............Off plane ??? There is NOT enough gas in the boat for 1/2 a day of running. TOTALLY like a barge. Huge whitewater wake all the time wasting fuel. Tugs have better wakes than I do OFF PLANE.