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yosquire 05-02-2011 06:40 PM

MPGuino calibration on a boat
I'm looking at adding MPGuino to a ski boat. I know this is outside the common cause for this forum and a ski boats have very little to do with efficiency. However this seems to be the best place to find MPGuino experts. I've often wondered the optimal fuel efficiency point for my ski boat. It seems that near top speed (~35mph) would be the most efficient cruise speed since the amount of surface area on the water is lowest. However, its hard to believe that when the engine is turning 4200rpm. So I tend to cruise slower ~25mph. My gut says slower is more efficient, but logic says faster is. I'm also curious about the impact of weight balance, both Port vs Starboard and Stern vs Bow. Enter MPGuinio...

The boat I'm working with has a paddle wheel speed sensor that is accurate down to 1/5 of a mph. I'm quite certain it is a pulsed signal similar to VSS. This is a fuel injected V8 so injector signal is a matter of running a wire.

Where I'm not sure is calibrating the MPG. There is no odometer. From the research I've done, calibration appears to be a manual feedback function of miles traveled verses fuel added to the tank. I suppose I could use my Arduino Uno and build a program to replicate an odometer-(I think). I'm not looking for something super accurate, just a semi-accurate ratio of speed / fuel burn.

I appreciate any thoughts.

P.S. I tried searching the forums, but I picked up hundreds of results for "Metro Boat Tail" :)

Ryland 05-02-2011 07:09 PM

your "MPG" figure doesn't have to be accurate to the mile or even to the gallon as long as it is consistent because you want to know if going twice as fast uses twice as much fuel so your figures could be in metric for all you care as long as you get a distance/fuel figure out of it, right?

yosquire 05-02-2011 07:16 PM

Correct. It would be "nice to have" accuracy. But not required to accomplish goal -- 'improve fuel efficiency by use of speed/throttle position and weight balancing.'

dcb 05-02-2011 07:24 PM

I think this is one case where I would seriously consider a gps, perhaps connected to a duino to simulate a speed signal just for "simplicities sake". On a car it is a non-issue (unless you spin the tires constantly), but a boat is of course another matter.

There might be some other methods, i.e. I assume boat speed is largely a function of engine rpm plus some fudge for weight/acceleration. If you have a duino monitoring rpm, and you map out the slippage at different rpms and the slippage under various acceleration points, you could get a pretty accurate distance perhaps, and wish you spent the $60 on a gps module :)

dcb 05-02-2011 07:42 PM

edit, shoot gps modules are getting cheaper, this looks like $25

and one of the values it spits out is speed.

yosquire 05-02-2011 07:55 PM

I think the GPS is a good idea. To add a layer of complication is the 'ground speed' verses 'indicated speed' issue. Similar to aviation, 100mph indicated is equal to 90mph ground speed with a 10mph head wind.

The river I navigate has a current speed of 2-5mph. So if I avoided the paddle wheel for VSS and went straight to GPS, then I would have more accurate MPG readings (since fuel burn is greater and efficiency is reduced traveling upstream.) This would also allow for some interesting trip MPG --(motor up river, float back down to the boat launch.) Though, while pulling a skier indicated MPG would be slightly more useful since it would remain constant upriver vs down river.

Thanks for the input so far.

msc 05-02-2011 10:16 PM

The paddle wheel speed sensor is of interest for establishing the most efficient speed over water. This would give you the best speed to use when there is no current and is probably the most useful bit of information.

For calibrating the MPGuino speed you just need to play with the pulse per mile value until the MPGuino speed reading matches the boats speedometer.

GPS data would of course give speed made good and would be useful for determining how much to speed up or slow down for best efficiency in the presence of a current.

A boat lends it's self well for steady sate cruising so it would be easy to collect data to generate a performance curve that relates fuel consumption to speed. From there it should be straight forward to calculate the most efficient speed for different currents. I think it will be a variation on the calculations for determining the best speed to fly gliders in a head wind.

Having both speed over water and GPS speed will allow you to map out the currents in the river so you can try to travel in the weaker currents heading upstream and stronger currents heading downstream.


dcb 05-02-2011 10:42 PM

the boat already has a speedometer? What sort of signal is it? That might be the simplest solution.

While of course you *could* compare indicated to made-good, nobody is asking for it :) and you would have to get programming to make sense out of it, for an individual user.

I think if there is a "digital" speed signal on the boat, see if we can use that, else gps up a speed signal is the pragmatic approach. gps would give a reading more in-tune with what us lubbers would consider mpg.

bestclimb 05-02-2011 11:05 PM

a speed through the water would be best to figure out the most efficient hull speed. Otherwise you are getting or loosing a little distance from the current. A calm lake early in the morning without much wind would be a good test for figureing out the most efficient speed. In my experience, the least throttle just after the boat breaks over on step is the most efficient. Just as you can feel it surge ahead.

If you know instantaneous fuel consumption, you can figure out MPG just by looking at your speedo. Get an E6B (whizwheel) to do the quick calculations.

Currents can be felt, and often change so mapping them out may not be a workable solution.

When flying I find that a decent technique is to throttle back with a tail wind, to take advantage of an airspeed with lower fuel consumption, then speed up with a head wind to limit the amount of time I am bucking the wind. On the river this would equate to minimum throttle to remain fully on step down river. Then some amount faster than that for the trip up.

dcb 05-02-2011 11:31 PM

I'm not so sure, I would use the water speed signal if it is already there just for convenience, but distance made good is distance made good.

Case and point, kill the engine and float back to the ramp and the whizweel will indicate 0mph and 0 mpg the whole way back, and the gps based speed signal will give you the actual infinity mpg and actual speed on the way back, just like EOC.

I mean who really cares how fast the current is going for efficiency? Your destination is still x actual miles away, and you still want to find the best way to get to that actual point with the least gas and it has to be in relation to actual miles, not water flow miles.

I don't see the fascination with water speed, at least not enough to buy a water speed sensor over a gps module, or why it is of any use except as an approximation of actual speed.

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