optimum speed for fuel economy m26

[Boblee]

To compensate for headwind going upstream against the current etc, you would need to raise the rpm up above idle somewhat to get maximum range.

Yes you can raise the rpm to compensate for wind without paying as much penalty but current is different if you raise your rpm you are effectively raising your hull speed (in the water) and will pay a higher penalty by increasing drag disproportionately.
My normal method of determining best speed is by checking the bow wave, at optimum there is none but at 1500 rpm it would rock a boat nearby at 2000 and above probably get a reaction from the occupants.

[BOAT]

In the hour long video that Roger made about the M boat he said that the X boat accelerates quicker off the line under power and that the M takes longer to get on plane. He also adds that when both boats are on plane the M is faster than the X under power. That tells me that the X is much more efficent at lower speeds than the M when under power. The fact that the M may be faster at WOT than the X does not mean it's more effecient on plane at high speeds either. It just means the M probably has less drag somwhere at that speed where drag would make a difference (Like a ceneterboard well?).

I suspect the M will probably get realy lousy MPG but since I'm hauling 24 gallons I guess I have plenty of room to explore that. I think mastreb has the best elctonics and monitoring setup for the ETEC and I'm guessing the M is not going to do well in the mid range (as well as the X) so in the end I think I will need to contact mastrb for advice on setting up the proper nema cables and sensors for monitoring feul usage.

there are no gas stations between Oceanside and Avalon and that's about 45 miles so a flow monitor might not be a bad idea.

[Divecoz]

I beg to differ with the below statement..

Planing hull— Achieving high speeds on the water requires a hull that easily transitions into the planing mode. Characteristics of a planing hull include flat bottom surfaces from amidships aft (from the middle to the back of the bottom) and flat transom (back of the hull). The transom must meet the bottom at a sharp angle. At certain speeds that vary with each bottom, the boat is supported by the same dynamic forces that keep a water skier on top of the water. Put simply, it planes.
The sharp angle (or "break") between the transom and the bottom is critical to a planing hull. It allows water flowing beneath the boat to come free of the hull without climbing up the transom and where it would create speed-robbing suction. The smooth, rounded shape of a displacement hull does not allow the water to break free in this manner.

let's be clear

A has a 5 degree V-slope - more of a powerboat - planes faster and better.

A has a 15 degree V-slope - better sailing - except for blue hull drag factors.

[Divecoz]

I dont " think ...believe " you will find substantial differences between the two boats..As Has been proven here time and time again.. Be it under power or under sail.. The Operator makes more of a difference than the particular boat.. Just Like Race Cars or Race Boats or Race Planes Its The Driver and his abilities, and his tactical decisions that determine the winner.. Yes some new world class race boats now have rotating mast and now have adjustable dagger boards and even multiple canted daggers and even adjustable canted rudders.... But They Are Not The Only Boats Winning Races..
There is an older fellow ( I know him only as Tony ) who lives on St Croix who has won possibly more sanctioned races in this hemisphere than anyone else and he has none of those things going for him.. He has skill knowledge and guts and is a master tactician.

In the hour long video that Roger made about the M boat he said that the X boat accelerates quicker off the line under power and that the M takes longer to get on plane. He also adds that when both boats are on plane the M is faster than the X under power. That tells me that the X is much more efficent at lower speeds than the M when under power. The fact that the M may be faster at WOT than the X does not mean it's more effecient on plane at high speeds either. It just means the M probably has less drag somwhere at that speed where drag would make a difference (Like a ceneterboard well?).

I suspect the M will probably get realy lousy MPG but since I'm hauling 24 gallons I guess I have plenty of room to explore that. I think mastreb has the best elctonics and monitoring setup for the ETEC and I'm guessing the M is not going to do well in the mid range (as well as the X) so in the end I think I will need to contact mastrb for advice on setting up the proper nema cables and sensors for monitoring feul usage.

there are no gas stations between Oceanside and Avalon and that's about 45 miles so a flow monitor might not be a bad idea.

[yukonbob]

Can't remember the actual speed, but getting up on plane then throttling back, you can go quite slow while on plane, I want to say six knts, but can't really remember now. Never really payed too much attention. You can tell the boat moves through the water easier on plane, when you drop off you can feel the water you're now pushing. Wonder if the 35% thicker hull on the M would have more to do with the slower speed? But then again, the whole reason for that was to take a larger engine…So is it really slower then?

[seahouse]

Some good points above.

Seahouse--

To compensate for headwind going upstream against the current etc, you would need to raise the rpm up above idle somewhat to get maximum range.

Note I like to use the less ambiguous term “range”... going the maximum amount of distance from a given quantity of fuel.

Boblee--

Yes you can raise the rpm to compensate for wind without paying as much penalty but current is different if you raise your rpm you are effectively raising your hull speed (in the water) and will pay a higher penalty by increasing drag disproportionately.

Even though your drag in the water will be increased, failing to increase the throttle sufficiently to compensate for moving against the current can have a huge negative influence on your range- in the extreme your range can actually go into the negative numbers!

Consider moving against a 5kt current for example. If you're moving forward relative to the water at anything less than 5 kts, you're getting negative miles per gallon. So your speed relative to the water for best range might be something closer to 9 knots (depending on ~?), because even though you're burning more fuel per unit of time, you will get to your destination sooner, ultimately, (and maybe counter-intuitively) ending up with better range, having used less fuel than if you'd gone more slowly.

Similar factors are well-known and quantifiable in the aviation industry.

Yeah BOAT– another beauty of the E-tec is that sensors are in place, operating data is already collected and monitored, recorded internally by the engine computer, just waiting for an NMEA2000 device to retrieve, read, and display it. More and more NMEA2000 compatible devices are coming on the market now to choose from too.

[Tomfoolery]

Also note that when I was using the term “powerboat” above, I was not referring to our (Mac) boats. While I can't speak for an “X”, and suspect it's not much different, I seriously doubt that the “M” hull has a throttle setting on plane where the fuel consumption rate (that would therefore give higher mpg) is better than that of any displacement speed. The “pseudo-plane” of our boats still has too much drag to arrive at that point.

Are there any boats where this is true? Not trying to be funny - as I've said earlier, I'm new to powerboating (as it were), or at least planing or semi-planing hulls, and really don't know.

It would seem to me that nothing is more efficient than slow, not counting engine properties. The slower through the water, the less energy is needed to move a certain distance. Heavy mule-drawn canal barges and human powered gondolas in Venice will attest to that. Fluids being what they are, I would think power required would increase exponentially with speed, even below hull speed, though still not a big gain as long as you stay well below hull speed. Exceeding hull speed, essentially plowing, I would think takes progressively more power (and energy per unit distance) but on a different and much steeper exponential curve, then laying out into a shallower (and still exponential) curve once on plane.

Or looked at another way, distance travelled per unit energy would be essentially infinite at infinitesimal speed, and decrease as the speed increases, then drop like a rock when hull speed is approached and exceeded, and perhaps go back up a bit once on plane (plowing is a giant energy waster), and dropping off again as speed increases from there. That's theoretical, and of course, the engine has it's own characteristics, where you obviously don't get infinite mpg at infinitesimal speed, so there's going to be some point where the engine curve crosses the boat curve and it's actually more efficient (in mpg) to go faster. A flat out idle is probably pretty bad as far as fuel mileage goes, for instance, but that would depend on the engine I'm sure.

This is just navel examination on my part, of course. I know something about fluids, and something about engines, but never gave any thought to power boats and how it all works together, especially with three distinct operating regions. I'd love to see some actual (informal) test results with one of the Mac power sailers.

[Boblee]

Consider moving against a 5kt current for example. If you're moving forward relative to the water at anything less than 5 kts, you're getting negative miles per gallon. So your speed relative to the water for best range might be something closer to 9 knots (depending on ~?), because even though you're burning more fuel per unit of time, you will get to your destination sooner, ultimately, (and maybe counter-intuitively) ending up with better range, having used less fuel than if you'd gone more slowly.

If the current is a constant 5 knots ie river flow yes you need to increase your speed to above hull speed and pay the penalty but if it is a tidal current and you are trying to preserve fuel waiting is the best option and then the tide will be with you.
We had this situation in the Kimberlies fighting against a 7m tide but I wanted to play with the sails so spent a pleasant but arduous 4-5 hr's with a good side wind and a head on tide, at the end of it I would have been better to either anchor or pay a huge fuel penalty and fight the tide, I think I probably made at most 5k's progress between tides but enjoyed it anyway and never had the fuel to waste.

[BOAT]

"Yes some new world class race boats now have rotating mast and now have adjustable dagger boards and "

This brings up a good point for another post I have been thinking about. Now that I see how easy it is to remove the daggerboard I am already thinging about building a NEW daggerboard with a trim tab on it. for racing that I can control from the cokpit

[drams_1999]

It sounds like alot of people have danced around it here....but when I was in the Coasy Guard 20 years ago we were taught that when fuel is limited the best thing to do is "clutch speed" or "bare steerageway", regardless of the boat being a planing or displacement hull.

Obviously there are considerations such as tide, etc, but the point made to us at the time was that when fuel was limited, proceeding as slow as possible as needed to maintain a decreasing distance to the target would yield the greatest distance on a set (limited) amount of fuel. I can see how there is a point on a planing hull where it could possibly be more efficient to get on plane, but I suspect the currents would have to be very high for that to be the case. Then again, this is all theoreteical and more data would be needed to determine if that's the case.

Clutch speed (or steerageway) can vary from vessel to vessel, but the idea makes sense.

I was wondering if anyone has some real world experience about this concept of steerageway or clutch speed?

I realize that at some (slow speed) point the engine is using fuel just to keep running, and will run many more hours at these speeds than it would otherwise need to be run at higher speeds, but this is what we were taught back in the day.

Great thread!

[kurz]

well I made a test some time ago:
at 720 1.75kn 20.4 liter/100km
at 1400 4.75kn 22 liter/100km
at 2000 5kn 24 liter/100km
at 2400 6kn 28l/100km
...
...

at 3500 8.1kn 47 liter /100km
at 5100 15kn 65liter/100km

Ballast tank empty...

Well, I think can vary much, but as slower the less fuel of course. A quite a good will be at 2000 so you get around 5 knots and take 24 for 100km...

[seahouse]

What I would expect to see with our boats. Thanks for posting that, Kurz.

Are there any boats where this is true?

Tom - yes, this is true for planing power boats (in the length I have encountered< 24', and maybe bigger ones too) that have sufficient power to plane. Yukonbob is describing that in his throttle-back after powering up onto plane, but still maintaining plane. More power is needed to get up onto plane than is needed to keep it there, so you can throttle back.

The point I make is that if you're after maximum range, you aren't going to find it up on plane, on any boat. But if you are powering at near hull speed with a powerboat, your throttle setting (and corresponding fuel burn) may very well be the same as it would be if you were up on plane. So it would not make sense to use fuel at the same rate as you would if you were on plane, when you could actually be on plane, moving at a higher speed.

[BOAT]

Sounds like it's better to just sail.

[seahouse]

Yeah – good point BOAT- when you are sailing you are getting infinite mile per gallon (mpg)*. That's a big reason why we're all here – because we like that! And your fuel range is really good, just like drifting downstream in a powerboat with the engine off. Yay!

I see automotive journalists sometimes get this concept a little off. In an attempt to dissuade readers from idling more than necessary, they say that when your car is idling (and not moving) you are getting zero mpg.

Hmmm? Then exactly what you are getting when the engine is not running? Clearly – you are getting zero mpg when the engine isn't running; but you are actually getting negative mpg when your engine is idling, even worse than what they are usually trying to convey.

But I digress, and maybe I split hairs on this point... (So that's where they all went)!

*for the sticklers - assuming your fuel vent is closed, fuel tank sealed.

[mastreb]

I think I will need to contact mastrb for advice on setting up the proper nema cables and sensors for monitoring feul usage.

Just google "Lowrance Evinrude Engine Cable"--you'll find plenty of sources. I see one on Ebay now for $20.

Complete parts kit:

2 NMEA 2K bus terminators
3 NMEA 2K T adapters
1 NMEA 2K 15' cable
1 NMEA 2K Power connector cable.

You can get most of these parts in the "Lowrance NMEA 2000 Starter Kit"--it has everything except a 3rd T.

Connect the three Ts together in a line, put the two terminators on both ends. Connect the Engine cable to one T, the power connector to the second T, and the 15' cable to the 3rd T. Take the 15' cable to your chart plotter. Connect the Engine cable to your motor.

I put the NMEA network in the battery box under the stairs, running the cable under the aft berth and out the engine cableway. It cable ties right along with everything else. Chart-plotter cable routes the same way, but goes up into the pedestal and out a clamshell on the top of the pedestal where my chart-plotter is mounted.

Easy peasy lemon breezy. To add additional NMEA 2000 devices, just insert a T connector and run the NMEA cables.

When you fill your gas tanks, just set the gas level on your chartplotter, and it will keep track of your usage and let you know how much fuel you have remaining. Seems to be pretty accurate, and eliminates the need for a tank sensor.

Matt

P.S. Thanks bryon for the NEMA/NMEA correction--I'm not sure why I thinkoed that, but at least I was consistent!