Prop Primer

[hanleyclifford]

http://www.miwheel.com/propellers/resources/terms/ Since this subject comes up so often I thought we ought to have this in our "library".

[Marian Claire]

Good info from HC: Stuff I have found. From: http://www.olds.com.au/marine/propeller_faq.html
Q. What Is propeller "slip"?

A. Slip refers to apparent slip and is a non-dimensional figure expressed in percentage. It is the difference between theoretical mph and actual mph divided by theoretical mph. Theoretical mph is calculated by multiplying propeller pitch and propeller rpm and dividing by 1056. As an example, a boat that goes 20 mph measured speed, is driven by a 12" pitch propeller turning 2600 rpm. Theoretical mph is 12 x 2600 divided by 1056 equals 29.6 total mph. Subtracting 20 from 29.6 equals 9.6 which divided by 29.6 equals 32.5%.
Slip is not to be confused with propeller efficiency.

Q. What are normal slip percentages for various craft?

A. With propellers correctly selected for the operating conditions, the slip percentages would be as follows:
racing hulls 10 to 15 percent
planing runabouts 15 to 25
planing cruisers 25 to 35
displacement cruisers 30 to 40
sailing auxiliaries 35 to 40
work boats 40 to 80.
Dan S/V Marian Claire

[Marian Claire]

More: Specific for A-4 from Robert Hess. From: http://www.cncphotoalbum.com/atomic4/atomic4specs.htm
Propeller Size
Note: propeller fitted must allow the engine to reach at least 1,800 rpm during hull speed / full throttle testing 2 Blade
Direct drive: 11 x 7, 12 x 6 (most common size), 12 x 7, 13 x 5
2:1 Reduction Drive: 12 x 11, 12 x 12, 13 x 10, 13 x 12, 15 x 8, 15 x 10

3 Blade (recommended for areas with severe currents, winds, tides)
Direct drive: 10 x 6, 10 x 7
2:1 Reduction Drive: 12 x 9, 12 x 10, 13 x 8, 13 x 9
Dan S/V Marian Claire

[prstack7]

Proper prop for application

How does one know the installed prop is the correct one for his application? As I throttle up my stern drops about 12". I don't get much increase in hull speed from 1/2 to full throttle.

[Marian Claire]

Well that is the 1000$ question. You can base your choice of prop on many factors and all are important. Ability to achieve the recommended RPM for the A-4. It is different for the direct drive and the 2 to 1 reduction. The load you are placing on the A-4 as indicated by a vacuum gauge. Overall performance or just what feels right to you. Some like to run a little over-propped. I am happy slightly under-propped. You have to have good data, RPM, vac #s, theoretical hull speed, and consider your intended use and the conditions you may encounter. What speed you would like to hit at the recommended cruising RPM for your type of transmission or the RPM you are happy running? Also prop clearances and hull shape need to be allowed for.
I can say that with the help of others here, LOTS of searching and reading, a good prop shop and some dumb luck my re-propping was a success. My saga is here. But please take note of my early bad data. http://www.moyermarine.com//forums/s...ead.php?t=6046
Need more info. But it sounds as if you are over-propped.
Dan S/V Marian Claire

[Marian Claire]

MMI's comments on prop selection.

http://www.moyermarine.com/faq/12.1.html

Dan S/V Marian Claire

[romantic comedy]

For most of our applications the prop is a serious compromise. There is no real correct prop. They are highly loaded, and run in that gray area.

If we had a usual planning hull, we would just find the correct prop from a chart.

We have issues with aperture size, tip clearance, drag when sailing,and many other factors. The good news is that we have a lot of other boats to compare to.

[Marian Claire]

What size propeller should I use?

The boat must be fitted with a properly balanced propeller of a pitch and diameter that will allow the engine to reach specified full rpm during full throttle, top speed testing as per the engine specifications (diesel engine specifications usually state the engine must be able to reach very close to maximum governor engine speed, direct drive Atomic Four engines must be able to reach a minimum of 1,800 rpm, and Atomic Four engines fitted with a reduction gear set in a reduction gearbox or V-drive should reach over 3,000 rpm). If the engine cannot reach minimum rated rpm the propeller is too large and must be replaced with one with a smaller pitch and/or diameter. The use of a propeller that restricts engine rpm to below specification will create a lugging condition that will cause engine overheating and damage. 3 blade propellers (we like the Campbell Sailor 3 blade propeller) are usually recommended for cruising boats in areas with strong currents and tides. 2 blade propellers are useful because they can be "parked" in a vertical position which will allow the upper blade to be shielded by the shaft strut, allowing slightly more speed under sail. Folding and feathering propellers are usually fitted to boats used for racing, however when fitting them type keep in mind that it is quite common for engines to be damaged when an excessively large used folding/feathering propeller from a larger boat is installed for racing.
This is from Robert Hess: http://www.cncphotoalbum.com/atomic4/faq/atomic4faq.htm
Dan S/V Marian Claire

[Skywalker]

Thanks for re posting Don's comments. I've read them many times, but as I sit here, I still fel conflicted.

Like most things in life, there is no "right" answer.

The comforting thought is in all likelihood, I'm doing no damage regardless of choice.

Chris