Mercury "Bigfoot" ??

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dennisdl
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Mercury "Bigfoot" ??

Post by dennisdl » Sun Nov 07, 2004 8:31 pm

I am looking at buying a Mac26X with a Merc 50 - standard(?) - not a bigfoot like I notice that a lot of Mac's have. Is the "bigfoot" really necessary ?? From the Mercury website http://www.mercurymarine.com/mercury_bigfoot_fourstroke
it says "The hull of a traditional boat slices through the water, allowing the engine to work in fairly undisturbed water. But the twin tubes of a pontoon force turbulent water toward the engine. So BigFoots larger gear case and propeller offer better handling, maneuverability and performance"
Pontoon boats, or up here in the Pacific northwest 'Log Pushers" seems to be a good fit, but a "stretch" when comparing those to a Mac 26 or am I missing something??
If I AM missing "something" can the "bigfoot" be retro-fitted??
And if so, is it really worth "it" ???

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Duane Dunn, Allegro
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Post by Duane Dunn, Allegro » Sun Nov 07, 2004 11:18 pm

For the size and weight of the X with a 50hp on it the load is just like a big poor performing pontoon boat. That's why we all have to use very low pitch props to get any reasonable performance out of the boat. You pick the same prop they sell for houseboats, typically 7" to 11".

An X hulls sailboat properties force you to move it at powerboat speeds by brute force. The X is not a slicing through the water cleanly powerboat.

The real advantage of the bigfoot is the bigger 14" prop. This gives you better control around the docks at low speed, and better reverse preformance.

Any way you cut it, the X is a load for a 50.

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norbert
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Post by norbert » Mon Nov 08, 2004 3:05 am

i have the standard - not "high thrust" - yamaha 50hp which is almost the same engine as the mercury. although the larger gear box and propeller would eventually be better i do not have any problem with the standard version. may be the top speed could be a half knot higher... but who cares? l like sailing!

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Roy B. Highland
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Big Foot

Post by Roy B. Highland » Mon Nov 08, 2004 11:30 am

Duane hit it on the head. The boat we love to sail ( and motor) is a tub. I have just turned in my 50 HP Merc 2-stroke for a new 60hp Merc 4-stroke BIG FOOT. We tried different props but settled on the 14X9 for the best of everthing.
Yes, the BIG FOOTs were designed for pontoon boats but by the time we rig ours, gas em up, and bring coolers and stuff aboard, we have more wallow than a pontoon boat. Believe me, you need all the help in a motor that you can get.
"Salty Dog" is a lot of things, but svelt she is not!

J Dower
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Post by J Dower » Mon Nov 08, 2004 5:32 pm

Norbert - I am getting a Yamaha 50 for an M so I was glad to see your post. What prop do you use, and what speed at cruisng RPM ?

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TampaMac
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Post by TampaMac » Mon Nov 08, 2004 7:03 pm

Is the bigfoot faster?

You have the same horsepower and more drag through the water. I don't see how it could be faster, it should be slower.

At lower speeds you might get more bite and hence more low speed handling, but as far as going fast - I don't see how.

50 hp = 50 hp - the bigger prop will only drag more.

Any of you engineers care to weigh in? I may be wrong.

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Dimitri-2000X-Tampa
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Post by Dimitri-2000X-Tampa » Tue Nov 09, 2004 4:43 am

I suppose theoretically, a bigfoot would be just a hair slower at high speed due to the extra width of the foot...however, this is likely to be pretty negligible seeing as the leading edge of the foot and hub is pointed. In other words, you are going to have pretty good flow around the foot with only a teeny tiny bit more drag. However, if a BF has less slippage at high speed, it could actually be faster.

I would say that the benefits are pretty obvious at slow speed though and I would get the bigfoot unless it was seriously more expensive. I happen to singlehand around docks all the time and I can do this with both rudders up which is something that other folks have said they have a problem with.

Talking about top speed.... I have the 50HP BF and on Sunday, I hit my fastest speed ever with it. A sustained speed of 19.1 mph with a momentary GPS reading of 19.7. This was on flat water with a slight headwind component and bottom paint..although I think I had a 0.5 mph tail current due to the outgoing tide. :wink: Regardless, mine does not seem to be any slower than any speeds posted for small foots. It is faster now that I took off the hydrofoils that the P.O. had put on. Oh yea...and my speed record was accomplished with 3 month old gas too!

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Post by Moe » Tue Nov 09, 2004 9:41 am

There are many factors to consider when comparing the BigFoot and small foot motors, and the issues are complex. I'm certainly not going to try to give an engineering analysis here, but just want to point out a few of them.

The larger diameter prop will tend to increase rotational drag. However, for a given engine rpm, the rotational prop speed of the BigFoot (2.33:1) is only about 79% of that of the small foot (1.83:1), and when it comes to drag, speed increases it exponentially, while area increases it linearly. Without knowing all the details, one might surmise that the rotational drag could even be less with the BigFoot. Of course, it could also be more, or the same. I don't have enough info to know for sure.

The larger gearcase and prop will also increase drag in the forward direction, but the amount of drag due to the motor and prop is a relatively small portion of the total drag of the whole boat. This is MUCH more true for a 3,000 lb total load 26' MacGregor, which if it does plane, rides on about 2/3 of its bottom, than it would be for a 1,000 lb total load 15' fishing boat, which rides on 1/3 or less of its bottom when planing. While 50HP may struggle to push the former only to 20 mph, it will easily push the latter to 35 mph, in spite of the much higher speed's 3 times greater effect on drag. In laymen's terms, the MacGregor is a a barge when compared to a small fishing boat.

Yes, 50 prop shaft horsepower at 6,000 engine rpm is 50 prop shaft horsepower at 6,000 engine rpm, but it's delivered differently with the BigFoot vs small foot. The BigFoot is delivering that 50 prop shaft HP at 6,000 engine rpm as 102 ft-lbs of torque at 2575 prop rpm, while the small foot is delivering it as 80 ft-lbs at 3280 prop rpm. All other things being equal (and they aren't), to translate these rpms into the same boat speed, the faster spinning, lower torque small foot would have to turn a 7" pitch prop to equal the slower turning, higher torque BigFoot with a 9" pitch prop. In laymen's terms, think low-rpm/high-torque (like a truck's diesel engine) when pulling/pushing a huge load slower, vs high-rpm/low-torque (like a rice rocket engine) when pulling/pushing a tiny load faster.

The 7" pitch is "flatter" than the 9" pitch, and as forward speed increases, it presents more drag, to movement in the forward direction, than a prop with greater pitch. The lower drag of the greater pitch can offset the greater drag of a larger diameter. Without knowing all the details, one might surmise that the forward drag of the BigFoot's 14" diameter by 9" pitch prop could be the same or less than the small foot's 10.5" diameter by 7" prop. Of course, it could also be greater. I don't have enough info to know for sure.

If a prop's pitch is like vehicle tire diameter, in that it affects drivetrain rpm, vehicle speed, and torque, then the prop's diameter, and hence blade surface area, are like the tires' contact patches on the road surface. However, pushing a boat through water with a prop isn't like pushing a vehicle with tires, on dry pavement. The boat in water is more like the vehicle on an icy uphill grade. As with tires on ice, a prop in water has slip. In both examples, the greater the load, the greater the slip. Using a larger diameter prop increases the driving surface, and is like using four wheel drive versus two wheel drive. While it might not be required for a light load, it is certainly of benefit to mitigate the higher slip of a heavy load.

In the end, drag isn't the only issue, nor should its disadvantages only be attributed to the BigFoot. In the end, top speed isn't the only factor in performance. Acceleration and fuel-efficiency also fall under the "performance" umbrella.

And even just looking at top speeds, about the only comparisons we can make are from ancedotal reports from owners here. What I've gathered from that is that there are too many differences in boats and owners to reliably determine which "foot" is faster, and that it appears that there's little, if any, difference. That's probably due to any increase in drag being offset by reduction in slip.

What's important is that as speed decreases from maximum, so does drag (exponentially), so that of a larger prop quickly becomes less of an issue. However, the opposite happens with slip. As speed slows and more of the boat settles into the water, it quickly becomes more of an issue. This is the range of operation where the larger diameter prop comes into play for both acceleration and fuel-efficiency.

That's how I see it.
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Moe

[on edit]Grammar correction
Last edited by Moe on Tue Nov 09, 2004 11:39 am, edited 2 times in total.

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Roy B. Highland
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Post by Roy B. Highland » Tue Nov 09, 2004 10:08 am

Jeeese Moe, that should end discussion for a while.

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Post by Moe » Tue Nov 09, 2004 10:56 am

Roy, if you could find time to organize and publish the results of your BigFoot prop testing, some of us here would be very grateful.

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watermwaves
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Post by watermwaves » Tue Nov 09, 2004 12:51 pm

Moe.

a 3000 lb mac....... hmmm, seems a little light for most of us....

boat, sails+mast+boom, bottom paint, ballast less than 15 gals (estimate),
bimini+dodger+doors+connectors+windows
2250+60+40+130+21 =2501

36 gal fuel =223
35 gal water =291
210 lb operator =210
Electronics/cabling =44
cushions, =65
Head/plumbing/tank/pumps = 115
2 additional 85 AH house batteries = 125
safety gear/docking =71
anchors/chain/rode =104
luggage, gear, books/charts. tools =200
stove/galley gear/fuel/cooler/ food = 81
Dinghy/paddles/lines =84
Outboard/lines/controls =217
Total 4331

This number is decreased 60 lbs from my earlier estimate which did not correctly subract the weight of mast and boom from the 12o lbs someone else had weighed with sails, mast and boom together. Realisticly tho, as I would guess most macs are outfitted, I think a tank empty weight of about 3600 seems a little closer for most., most seem to go out with two batteries, stoves, portapotty, additional water, a couple of fuel tanks etc.


As far as bigfoot /littlefoot,

The bigfoot delivers its horspower in approximately twice the area of the little foot, (subtracting the hub size differences for the respective gearcases). the slip/shear and drag of the blade through the water is not an exponentional change W.R.T to RPM. Reynolds numbers for flow around the tips indicates that at low speed the bite of the bigger prop is
still fast enough to generate considerable slip. The increase in perceived power and manuervability is due to the decrease in blade loading (per unit area) while still having an increased angle of attack (prop pitch) resulting in the higher thrust.

The small prop blade loading at slow speeds translates to more of the water thrown outward and around the prop rather than behind., as indicated on the lower torque numbers for the littlefoot.

Along why many housboat operators put tubes around there props to increase efficiency and decrease the amount of wter pushed out to the sides with high speed smaller power heads.

Torque alone is not a perfect indicator if efficiency at the low speeds. I suppose if one wanted to improve the low speed efficincy of a small foot, a slip on tube which could be added to the lower unit prior to docking in adverse conditions might be a good 3 lbs to carry, albeit a little difficult to add and remove in a pitching sea, and test results show a marked increase in thrust in that configuration. (With some props on motors in the 30-40 hp range) measured thrust increased as much as 18 % at some RPMs),

OH well, if you dock in a place with high tidal currents, or high winds,, the bigfoot seems to have an advantage,

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Terry
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Prop Speak

Post by Terry » Tue Nov 09, 2004 2:23 pm

As much as some of this talk is over my head it sure is enlightening to get such a technically detailed explanation of propeller mechanics & put into layman's terms to boot. I'm always interested in how to sqeeze out that last bit of performance by tweeking the propeller selection. Curious though, I read what I thought was reference to blade load and surface area (larger diameter & hub) so I got to wondering the advantages of a 4 blade vs 3 blade. I did some internet research on it and decided to try something different and bought a 4 blade prop. I wanted to increase cruising efficiency around the 3500-4000 rpm range wich is the range I prefer for fuel efficiency. Now I read all this tech info on drag characteristics and wonder if I made the wrong decision Can any of the above principles be applied to the 3 vs 4 blade performance?

waternwaves
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prop comparisons on bigfoot vs littlefoot

Post by waternwaves » Tue Nov 09, 2004 5:19 pm

Terry,

This generalization is somewhat difficult, and is best looked at on a prop by prop basis.

For example, material becomes important when the differences are only 3 or 4%. Stainless steel being stiffer than aluminum translates to a performance change as well as going to 4 blades from 3. The change in mass has less of an effect..than blade area and and profile.

Given all things being equal....
1) for the geometricly same prop of different materials, the stiffest material will be most efficient.
2) For props of the same "Pitch", depth, blade area, and diameter, there is still enough variation in profile, (AoA angle of attack section), blade Radius, taper and thickness to preclude any hard and fast rule. A three blade can be made as fast as a 4 blade, or 5 or 7, in our power ranges the differences due to mechanical harmonics are relatively small, but most High horsepower marine applications tend to avoid 2 axis symetrical hubs for vibration and noise reasons. In general greater mechanical efficiency is obtained from an increase in the number of blades.
3) Full numerical comparison would require a very detailed solid model with an extremely accuate measurement of surfaces. A dozen years ago I contacted the Mercury and Michigan factories and enquired about how they did their prop design...... and even with the improvement of numerical analysis and NC machining, trial and error was still a necessary part of the process. They were still improving their models.

So in answer to your final question, given a large enough recirculation tank, and the ability to mount the outboard over it, and the ability to adjust water velocity in the recirculation tank, it would be relatively simple to comapre thrust vs. engine speed, vs water velocity.

Wihout that.......I would go for the stiffest prop that gives me the highest engine speed while while remaining in the power band (peak torque)of the engine.

at a reduced rpm, this same prop will maintain its efficiency over a softer prop.

Carbon fiber and graphite props are flexible. Their primary advantage is replacable blades in some cases, and a decrease of weight of up to 50 % over even aluminum. But they are not stiff, and as designed, most are less efficient .

Soon it will be titanium props that will be the rage........ but most of us, would have problem with the $2400 price.

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Post by Moe » Tue Nov 09, 2004 5:45 pm

Terry, in general, four-blade props are chosen for better "bite" to resolve problems of ventilation, particularly in tight turns, on high power, shallow running (meaning the prop is close to or partially above the surface) applications. This also includes offshore use.

In general, they're also known for good bow lift and holeshot, primarily for bassboats.

As Darren alludes, a lot of prop design and selection is still trial and error. That's why Mercury has those test sites.
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Moe

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Graham Carr
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Post by Graham Carr » Tue Nov 09, 2004 7:48 pm

Are we talking about airplane props? That went way over my head. Well Im a carpenter from Colorado. Let me put my tape on this and see how I measure up. Ok I wont quit my day job. :) My 26X is a 2002, the engine is a 2003 Big Foot 50HP EFI 4-stroke. I have two props, one for high altitude and the other for sea level. I have sailed at three different elevations. At sea level I am able to reach 18 to 20 mph. I did hit 22 mph with the mast down with just me aboard. At 5958 and 8000 with the High altitude prop I can still hit 18 mph, even at 8000. Thats with two adults and gear for the weekend. I did a test at 8000 with the sea level prop and could only get 13 mph. So even with a 50 and the big foot the wrong prop wont cut it. Let me see if I can get this correct, I was told that the pitch change compensates for the power loss from going up in elevation. It was something about the different pitchs travel a different distance per revolution. So one or the other compensates for the power loss, I think. Also if you plan on sailing at different elevations EFI is a great way to go. Sorry I dont remember the prop size. I am currently working in Vermont, I am heading home Dec 13th, Ill get the size of my two props and post the info.

Graham Carr

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