Prop Sizing

[Derek Wenn]

Last year we purchased on '05 Mac26 and installed a Murcury 60 Bigfoot outboard. We live in Utah at 4500" altitude. It took us 3 tries with different props to finally end up with a 11* pitch that maximized out our outboard at 6000 RPM. I still don''t know if it gives us the best speed for skiing. But the matter today is that we are planning a trip to San Diego this summer and want suggestions on what prop pitch we should change to for the altitude change and salt water?
Please share your wisdom.
Thanks, The new guy!

[Moe]

Was that 6,000 rpm heavily loaded? Ballasted or unballasted? What's more important is that the wide-open throttle rpm isn't lower than 5,500 rpm in your worst case heavy loading.

All other things being equal, you will need more pitch at sea level, where your engine is making more power and in saltwater where your boat is more bouyant with the same load.

[Divecoz]

If your entering some sort of race by all means make the changes. Or if your still in posession of those other two props bring-em. But if your not wanting to spend the extra $$$.$$ you really have no need to . .

[baldbaby2000]

I changed my prop to get more rpm. That part worked but I lost speed. The dealer said I'd use less gas with the higher rpm prop.

[Graham Carr]

I use a 13P when I am running at sea level and when I am running at home ( Colorado ≈ 5000 feet) I use a 10P. I also copied some info from two of my earlier post, this might help; that information is below the chart.

Graham

I scanned this table from my service manual
SPECIFICATIONS


-
Mercury/Mariner 60 EFI (4-Stroke) 2.3:1 Bigfoot
Special soft rubber hub propellers designed to reduce clutch rattle
Wide Open Throttle RPM: 5500-6000
Recommended Transom Heights: 20", 25"
Right Hand Rotation Standard
Gear Reduction: 2.31:1
IMPORTANT: These specially designed rubber hub propellers are rated for 60 horsepower MAXIMUM.

13-3/4 D x 15 P, 2000-2500 boat wgt., 16-18 boat length, speed range 25-32
14 D x 13 P, 2300-3200 boat wgt., 17-20 boat length, speed range 23-27
14 D x 11 P, 3000-4300 boat wgt., pontoon, speed range 17-21
14 D x 10 P, 3500 + boat wgt., pontoon/work, speed range 14-19
14 D x 9 P, 5500+ boat wgt., House boat/work, speed range 1-16



1st post
Do you have a tachometer on board? If so, run your boat at W.O.T to see what your RPM is. You want a prop that will allow the engine to run in the top part of its recommended full throttle range, W.O.T. Mercury specs for the 50 HP Bigfoot is 5500-6000 W.O.T. Each engine manufacture and engine will have a different spec. It would be wise to check your owners manual if you have one or contact a dealer and give them the serial number. They will be able to tell you for sure. Perfect prop size is a bit allusive because it can change depending on conditions. Sea conditions, boat weight, engine condition and the condition of the prop will have an effect on performance. Weather also plays a role in Horsepower. The air density; air temp, barometric pressure and humidity can rob up to 14% of the HP. The hotter and more humid the less power you will have at the prop, hence changing props between Colorado, Washington or going to Florida. So a dry spring day will give more power over a hot humid August day.

Outboard engine companies use the I.S.O. (International Standards Organizations) for engine testing to obtain HP. The test values are produced at sea level, 30% relative humidity, air temp of 77F and a barometric pressure of 29.1 .

Changing props will give a 150-350 RPM difference between pitches sizes. If the RPM is to low you would change to a smaller pitch. For example when I am running at sea leave I use a 13P and when I am running at home ≈ 5000 feet I use a 10P. This change brings my R.P.M into spec and increases boat speed. At 8000 feet the 13P runs at 3500RPM/13 mph and the 10P is 4500RPM/19 mph. The 10 is still under spec and I could probably go with a 9, but I hardly run at 8000 feet.


2nd post

I have 2002 26X with a 2003 50 EFI Bigfoot.

I just installed the smart craft system monitor a few weeks ago. One of the functions is fuel flow rate. I did some test but I left the info in the boat. I am working out in Washington in the San Juan Island area for the summer. I had to come home for a week. I can give you a rough idea now and then in about a week; I can give you the actual test data.

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, 4200 RPM, 3.5 GPH
10 mph, ≈ 2 GPH, I think the RPM was around 3000
5 mph, ≈ .8 GPH, I think the RPM was around 2,000

Pre-smart craft system monitor I did a test run. 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 Shilshole to Port Susan.

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.
All Speeds were checked with GPS

I also found that when cursing less than 7 MPH, if I trimmed out with out the prop breaking the water and still running the cooling water, greatly reduced the steering effort. Tail wagging was greatly reduced. It was like turning on a power steering pump and steering felt balanced. When I drop some keel I tracked straight.

As Moe points out, sea conditions will also effect speed and fuel consumption. Another factor that affects the horsepower is the weather. The air density; air temp, barometric pressure and humidity can rob up to 14% of the HP. The hotter and more humid the less power you will have at the prop, hence changing props between Colorado, Washington or going to Florida.
Outboard engine companies use the I.S.O. (International Standards Organizations) for engine testing to obtain HP. The test values are produced at sea level, 30% relative humidity, air temp of 77F and a barometric pressure of 29.1 .


I do all of my own engine work so I purchased the factory service manual for the 40/50/60, 4 Cyl, FourStroke EFI engines, years 2002 & newer, serial # OT409000 and above.

I always put my boat on the truck scales to check the weight before I had cross country.
On this last trip; the boat, trailer and gear weighed 3560 LBS. So take out the trailer weight of 710# and add the ballast of 1400# and 700# for crew of 4 adults puts my boat at ≈ 4950 #. So based on the chart running a 13 P prop is wrong for my set-up and the 11 would be better. So when I get back to Washington (next summer, 06) I am going to test the 10 out. Ill let you know how it goes.

Another issue is the engine should be installed so the anti-ventilation plate is in-line or within 1 (25mm) below the bottom of the boat. If it is not, they do make an extension kit that mounts between the drive shaft housing (under the bottom cowl) and the gear housing.

[Richard O'Brien]

Thanks Graham. I also have a 14" -10, and a 14"-13 pitch. I've never tried the 13 pitch, and it sounds like it's going to be only a back-up now.

[Graham Carr]

Hey Richard, are you going to The Denver Boat Show? Its takes place at the Colorado Convention Center on January 19 -22. If so, stop by the The Anchorages booth. I am helping them on SatSun 2122. I will be there all day on the 26M answering questions. Just head for the coast and enjoy your 13" pitch prop !!!!!!
Graham

[Derek Wenn]

Thanks everyone for the great input.
I am planning on trying out props in the 13P to 15P range. I plan on installing a Raymarine DS400 fishfinder/Knot meter/Depth sounder this spring and will check to see which prop gives me the highest speed at 4500' and at sea level this summer.
One concern I have is, lugging the engine at a lower RPM. I am trying to get the highest speed possible to allow my kids to ski & tube behind the boat, without lugging the engine and causing problems. Then on top of that, I need to do this at 2 extreem different altitudes!!!

[baldbaby2000]

One concern I have is, lugging the engine at a lower RPM.

Does it hurt an engine to run with WOT at too low an rpm? I have a Tohatsu 40hp carburated 2-stroke. I think it runs at about 4200 rpm at WOT at 8000 feet. I believe the red line is at around 5600. As I mentioned I can get the rpm to go up with a smaller prop but I loose about 1 knot in speed.

[Richard O'Brien]

Hey Richard, are you going to The Denver Boat Show?

Yea! I'll look for you Graham, Richard

The one thing I've always wondered about the Steve(Anchorage) at the boat show and this board's advice is "Bigger is better". I've often wondered if I'd gone with a 25 hp bigfoot I'd be just as happy, and have a lighter stern? I know Baldbaby's must be much lighter. I wonder if Roger designed these boats with 250lbs. or more on the stern. What would happen with a 150 lb. motor? Would it respond better to tacks?

[They Theirs]

Richard O'Brien

You will not have to search far for an answer:
Any Sailboat designer or textbook will back up the fact that weight on the extremities and aloft should be avoided.
Installing a big engine in a Pinto may make it faster with very little for handling.
I believe your thoughts are respectable! Keep then coming.

[waternwaves]

what about us larger deckhands, did Rodger design for that. Fact is, we have a sloppy draggy transom that sucks................ (literally) If you look at the balance of the boat, weights & moment arms..., change in wetted surface area, change in bow angle (around CB), change in Centerboard angle., change in Centerof pressure on the hull

we find.........

that adding 105 more pounds to the back of the boat is not particularly significant, when motoring , and about as critical as having a 200lb sit in back with you when you are sailing.


Might be fun to format an Excel spreadsheet for weight and balance triming of the mac (similar to how an airplane is staticly calculated for trim)

But for those of us sailing single handed..seems not to be much of a problem. more likely to notice the difference when you are trying to carry an extra 400 lbs of crew..........when you want the extra hp anyway.

adieu

|>

[Divecoz]

Hey Richard, are you going to The Denver Boat Show?

Yea! I'll look for you Graham, Richard

The one thing I've always wondered about the Steve(Anchorage) at the boat show and this board's advice is "Bigger is better". I've often wondered if I'd gone with a 25 hp bigfoot I'd be just as happy, and have a lighter stern? I know Baldbaby's must be much lighter. I wonder if Roger designed these boats with 250lbs. or more on the stern. What would happen with a 150 lb. motor? Would it respond better to tacks?

I too wonder if I should or could have settled for a motor half the size and the cost of what I got . In my case lack of experience was the problem. . . I too fell victim the BIG Motor syndrome .

[baldbaby2000]

The one thing I've always wondered about the Steve(Anchorage) at the boat show and this board's advice is "Bigger is better". I've often wondered if I'd gone with a 25 hp bigfoot I'd be just as happy, and have a lighter stern?

From my Scow sailing days the theory was to keep the weight (crew) close together so that the boat would pitch with the waves instead of trying to fight it as it would if the crew were widely spaced (moment of inertia thing). This may apply to the Macgreggors too. Thus, the motor weight seems to be in a "bad" spot.

Also, my Macgreggor manual says that performance can be improved by moving weight forward to get the transom further out of the water and reduce that "giant sucking sound." We did some experiments with this during our Lake of the Woods race and it seemed to be true. Again it seems like the lighter motor is better. I have the carburated version so to compare apples to apples I guess you'd have to concede that my mpg is 30% worse than the TLDI, but with my two 12 gal tanks full I'm still a little better for weight as the TLDI for a the same range. It just gets better as I use gas. Although my motor was one of Tohatsu's best sellers, I think it's been discontinued.

Richard, we'll probably be at the boat show Saturday. Do you know when you'll be there?

[Richard O'Brien]

Richard, we'll probably be at the boat show Saturday. Do you know when you'll be there?

I'm going to try and get there Sat PM. I have a sister flying down from Juneau so I'm not sure about the time. I'll try to find you and Graham Sunday is going to be pretty dead after 1:00 P.M. with the AFC game. This Saturday during the game the streets looked like one of those sci-fi films after the aliens vaporized the populace.