Bigfoot 50 (Props & Economy)

[cuisto]

I know this was been covered a bit and have read some archive posts but would like some further info

Would merc 50 bigfot owners please post their prop size and pitch and their top speeds.

I would be happy to compile the data collected and make a proper chart for consumption on the performance pages.

Thanks all,
Scott

[Graham Carr]

My boat is a 2002 26X with a 2003 Mercury EFI Bigfoot 50. Two years ago I installed the smart craft monitoring system. One of the functions is fuel flow rate. I did run a few tests.

Test conditions;
Sea level, full ballast, 4 adults, 1 small dog, mast up and all my gear on board, 18 gallons of fuel, 14 D x 13 P. bucking a ≈2.5 mph current and 10 mph wind.

16 mph, 3.5 GPH, RPM = 4200
10 mph, 2.0 GPH, RPM was around 3000
5 mph, 0.8 GPH, RPM was around 2,000 (corrected)

I did run a test before I installed the “smart craft system monitor”
Test conditions;
Sea level, no ballast, 2 adults, 2 teenagers, mast up and all my gear on board, 18 gallons of fuel, 14 D x 13 P. bucking a small current but smooth water with no wind. I ran at 17 MPH nonstop for 37.08 miles or 32.25 NM before one tank ran dry (9 gal tank but only 8 gal in that tank). The run was from (Washington state) Shilshole to Port Susan.

All Speeds were checked with GPS

I have two props; 14D x 10P and a 14D x 13P. I use the 10 for Colorado. I did a test with both props at Steamboat Lake - Elevation: 8,000 feet. With the 13P I could only hit 13mph WOT. With the 10, I hit 19 MPH. The lake was like glass. So choosing the prop size clearly makes a difference. Changing props (per Mercury) will give a 150-350 RPM between pitches sizes.

I hope this helps you

Graham

[James V]

I have posted this before - I have a 14 x 10 SS prop.
NO wind near glass conditions 17.5 mph at 4 gph lightly loaded, motor had less than 100 hrs on it. I did get up to 18.5 mph with a 10 mph tail wind. I did 90 miles that trip, all on one heading.

Loaded, towing a dinghy, about 15.5 in near glass conditions. 3 to 4 gph depending on waves and wind.

When I was motoring in 15+ mph head winds in the ICW, loaded and towing a dinghy - 7 mph at 6 to 5 gph.

At 5 mph, now it depends on wind and waves. Motor off and 15 mph winds, going down wind, I go 2.7 mph with no sails.

I think that I can get 12 to 15 mpg at 5 mph, per my smart craft system. But if the winds are blowing the right direction I will sail.

[Frank C]

... 16 mph, 4200 RPM, 3.5 GPH
10 mph, 2 GPH, RPM was around 3000
5 mph, 8 GPH, RPM was around 2,000

Graham,

I'm certain you must have misplaced a decimal point. Should be as below??

5 mph, 0.8 GPH, RPM was around 2,000

[Sloop John B]

Gees, get an 11 inch pitch like everybody else and relax, put away the calculator, a grab a beer.

[Graham Carr]

A few too many beers I’d say! Thanks Frank, .8 is correct.

[Frank C]

Thanks, Graham!

After many years of reading here, I've observed that the majority of members with 4-stroke outboards report about 4 to 5 MPG (miles per gallon), REGARDLESS of that outboard's size!. By taking the liberty to "push" Graham's "Smart Craft" data results a bit (just to ease the math) I believe his data illustrate well a pretty reliable rule-of-thumb for estimating a Mac's 4-stroke fuel economy.

Code: Select all

First two columns show Graham's data (pushed a bit). 
Last two columns show distance traveled in one hour, w/fuel economy. 

Graham's_Data       Miles 
  MPH     GPH       Traveled       MPG
                    (in 1 hour)
    15      3         15            5
    10      2         10            5
     5      1          5            5

It's worth the time to illustrate this gross reality. We collectively ascribe too much hocus-pocus to the question of our boating fuel economy. Paraphrasing the Clinton Admin's catch-phrase:
"It's 5 miles per gallon, Stupid~!"

BUT ... shouldn't fuel economy improve as the boat begins planing??
Yes, once the boat gets on top of the water, it should maintain speed while reducing the throttle setting. However, absent the 90+ horses to really get up on top (say ~25 mph):

• The boat is still pushing the same weight of water, whether going 5 mph or 15 mph;
• Pushing that weight of water demands a certain number of horses;
• And, that number of horses will demand the same amount of nourishment.

This ALSO illustrates the fallacy in the idea that a smaller outboard, say 15 to 25 hp, will reduce the fuel burn ...
A SMALLER OUTBOARD CANNOT SAVE SIGNIFICANT AMOUNTS OF FUEL~!

These boats displace a certain amount of water (assume a ballasted cruising wt of 4500 lbs). You must PUSH THAT WATER in order to move the boat. Pushing a given weight of water requres the same amount of horsepower (maybe 3 or 4 horses?) regardless of the size of outboard. And those 3 horses eat the same quantities, regardless of the number of other horses resting in the stable. Compared with the weight of water we must push, the extra effort of spinning 40, or 70, or 100 surplus horses ?? .... that effort (rotational friction plus some extra pounds on the transom) is lost in the 'noise.'

Finally, this also illustrates the ONLY 2 WAYs to save fuel in our powersailers. Use the wind, or motor with an empty the ballast tank. You can clearly save fuel burned by dumping the 1400 lbs of ballast, but that REQUIRES a minimum 40 horsepower. Except for their initial purchase cost, mounting a smaller outboards on the Macgregor powersailer amounts to false economy, while it sacrifices the preponderence of the design's versatility.

[beene]

Very good and useful info, tks Frank and Graham.

G

[Moe]

When you don't manipulate his numbers to try to disprove the laws of physics, you see some significant differences in Graham's data, on the order of 25% more fuel used to go 10 mph than to go 5 mph over the same distance.

Code: Select all

MPH / GPH = MPG
 16 / 3.5 = 4.57
 10 / 2.0 = 5.00
  5 / 0.8 = 6.25

And you're looking at another 12% increase in fuel used to go 16 mph vs 10 mph over the same distance.

Assuming Graham's motor would still get 6.25 mpg using 1.2 gph at the 7-1/2 mph (6-1/2 knot) hull speed, compare that to the 6HP that would do the same speed using 0.5-0.6 gph (12.5-15 mpg). I'm not saying that's all the motor you'd want on a Mac powersailor, with no reserve for pushing all that freeboard into a head wind or driving into a head sea, but it illustrates the inefficiency of using a 50-60HP motor to provide 6HP worth of speed, for those for whom that speed is fast enough.

[James V]

I get a little better gph at displacement speeds. Maby because of the 14 x 10 prop and that it is SS. MY Smart Craft System reads 0.4 with the GPS reading 5 mph.

My 2 hp dinghy motor gets 20 gph.

[Mikebe]

It makes sense you would get better fuel economy at displacement speeds than you would halfway up on a plane, and that on a full plane it would be better than at half plane. Some hull designs might get better fuel economy on a plane than at displacement speeds, but I doubt that is true for a Mac. This is due to the varying resistance of the elements (air, gravity, and water) encountered at different speeds. As the opposing force of resistance increases with speed, it requires increasingly more energy from the motor to maintain a constant velocity or to accelerate further. There is the gravitational component the motor must overcome in order to lift the boat out of the water and into a plane, as well as increasing air resistance. At some velocity, air resistance will become the primary factor, (assuming you can get on a plane) and the degree to which the hull is streamlined will become the primary design feature affecting fuel economy. Even in a vacuum, there is a point at which no amount of energy will move you any faster...

[Duane Dunn, Allegro]

Ahh, this should get a rise out of some people.

To me this data proves what I have felt all along, a Mac with a 50hp motor never really planes. Planing speed with our hulls is 20+ mph and you just can't get there with a 50. Now I'm sure many will argue otherwise as they have in the past, stuff about leaving your bow wave behind, anything faster than hull speed must be planing etc. But from a pure fuel consumption point of view you never get to that sweet spot that a planing boat does. The Mac with a 50 never fully gets out of the hole, flattens it's running attitude, and experiences the release in both speed and lower fuel consumption that a boat up on a plane does.

I think if you were to do the same tests on a boat with a 90 hp you would see a noticable drop in fuel consumption at speeds over 20 mph and a increase in MPG, not the constant decrease these numbers show. Most likely a boat with a 70 can get to that point as well but you are near WOT at that point and the effects could be harder to see.

All right all you "It's gotta be planing" folks, bring it on.

[Frank C]

Duane ... that's exactly (one of) the points that I was trying to make. In all the years we've both been reading here, my guesstimate is that, all averaged out, the Mac26 gets about 5 mpg (w/4-strokes). Some maybe more, some probably less, differences due to weather, wind and water conditions as much as to choice of the engine size.

Granting Moe's valid criticism of my "poetic license" , I still predict that any 4-stroke capable of pushing the Mac up to 10 miles per hour (say a 25hp?) would incur the same 25% consumption penalty as the cited Bigfoot 50, perhaps even worse. But even though a smaller engine, surely no better economy at that same speed. The fallacy of assuming that small engines always give greater fuel economy ... is failure to consider that weights and speeds are never equalized. Stated conversely, a Mac26 with either a 15hp and a 50hp outboard will deliver about the same fuel economy at hull speed, fully ballasted.

By contrast, automotive fuel economy has innumerable variables. Preeminent among them, we drive vehicles of vastly different weights over differing terrain at differing speeds (most of our boating occurs over identical "terrain"). Adding to the complexity of predicting auto fuel economy are various aerodynamics, tires and pressures, transmissions, gearing, etc.

But regardless of those differences if we take two autos (similar aero), up a very long and very steep hill, at 30 mph, we will get predictably identical fuel consumption per pound. Engine size may vary (simply due to the weight of the vehicle) but not the effective fuel consumption per pound moved up that hill.

We agree on that one foundation point, that none of the 4-stroke 50s (even the 70s) ever really have the ability to climb up on top of the water, reducing throttle, and gaining speed and mileage. Therefore, we are all consigned "to pushing" the same two+ tons of water, in a similarly tight range of fuel economy.

Pushing that water without gaining the liberty of planing on top ... that's like climbing a very long, very steep hill ... at moderate speed ... constantly, never-endingly, pushing uphill~! And, the extra cost of a big engine is simply another 200 lbs (one person) plus the inate rotational & pumping losses in the larger engine. Engine weight and frictions are deminimus versus the primary 2+ tons of load. Further, and more importantly, I was observing that;

• Larger outboard engines enable us to UNLOAD THE BALLAST TANK.
• Dumping 1,400 pounds of ballast is the single,
  most fuel-efficient alteration possible in a Mac 26!
• BAR NONE~!
• Thereby permitting the 50hp owner to go faster;
• or GREATLY improve his fuel economy;
• or perhaps BOTH.

Earlier this year I stumbled across a webpage that described the actual components of boating economy. From memory, they included hull displacement, defined # of horsepower to move it, and BTUs per gallon of fuel. (IIRC, each gallon of diesel has about 110 percent of the power in a gallon of gasoline.) I haven't been able to find that page again, but the essense of that wisdom was quite logical:

• pushing a given quantity of weight (effectively, displaced water) takes a defined # of horsepower;
• (assumption, non-planing displacement speeds);
• throttle setting at a given speed defines (equates to) a given number of horsepower;
• surplus horsepower not employed (in the stable) incur only pumping losses;
• each "horse" employed consumes a predictable volume of fuel;
• fuel economy is predictably related to the number of horses employed.

In conclusion, moving a given weight, at a given speed, employs a defined number of horsepower, and consumes a predictable number of BTUs. All Mac owners are moving the same weight, from 4,000 to 5,000 pounds, up the same hill, at less than planing speeds. Our higher (semi-displacement) speeds, still in the cruising speeds for these engines, don't generate any real planing efficiencies. We're still just moving water with horses. The only significant impact we can induce is by dumping ballast we can significantly reduce the amount of weight --
a bit like swapping from the Suburban down to the Taurus.

[Moe]

I agree with Duane in that I don't believe a Mac powersailor can achieve planing speed with 50HP, unless perhaps totally stripped and unloaded.

Here's a boat at hull speed, riding between the first and second bow wave, the latter also called the stern wave.




Here's a planing hull as it exceeds hull speed. The stern wave separates from the transom, and the stern falls/bow rises, as the boat begins to climb its bow wave. As the speed increases above hull speed, fuel mileage drops because the engine is working hard, constantly trying to push the boat uphill on its bow wave. The outboard has to be trimmed down, usually all the way down, to keep the bow down. Some call this planing (and there's debate even in the boat design community), defined as exceeding hull speed and achieving stern wave separation, but it's a range of speeds most powerboaters I know refer to as plowing. And it's the way the Macs perform with 50HP.




Finally, here is a planing hull boat getting on plane, aka out of the hole. The characteristic is that the bow drops. Having the outboard trimmed all the way down usually puts too much of the hull in the water and isn't as fast as the boat can be when the outboard is trimmed up. The load on the outboard drops considerably and fuel mileage increases as the boat gets out of the hole, plus more of the outboard's power can be used for forward propulsion, rather than for lifting the stern trimmed all the way down. It isn't unusual for fuel mileage just above this real planing speed to be as good as it is at hull speed.




Here's a sample of what Duane is talking about. This is a graph I made for a friend with a Boston Whaler, but you can find many like it at Yamaha's website for different boat/motor combinations. I'm not implying these are the speeds at which a Mac plows. Note the dip in fuel mileage in the plowing range, with I previously labeled as Dimitri's Planing since he believes anything above hull speed is.



I've estimated before the planing speed of a 26X is roughly 21-23 statute mph, and IIRC one of our big engine owners wrote that was about the point at which he experienced the bow dropping. The 26M's deeper V won't generate quite as much lift, so that MIGHT be a little higher. Anyway, that's my position.

[Catigale]

(Pulls pin)

Mac with a 50hp motor never really planes

(Drops grenade)

Even if its painted blue underneath??

(Runs...........................................)