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Old 28-11-2006, 11:11   #1
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Proper Prop Size

Just in the last month we've been discussing problems that have been traced back to improper propeller size.

Engine overheating and transmission overheating are two serious consequences. Engine not reaching proper rpm for the best and most efficient power curve is another symptom.

The formula I used came from Skene's and seems to be pretty close. You can also call Michigan Wheel or any local reputable prop shop and they'll give you information.

When comparing your prop to other sail/power boats please remember that there are many factors in what size best fits your boat. Engine hp, transmission gear ratio, waterline length and weight of vessel all have something to do with the calculation and might be different on the very same design.

Many times a boat owner was told that their Perkins 4-108 is a 50 hp engine or thier Atomic 4 is a 40 hp engine and so they make up their minds that the engine can handle a much larger prop than they now have and give them more speed and economy. What really happens is they install a larger propeller and bad things happen in the engine compartment. It really is advantageous for an owner to study the engine manufacturers power curves and their transmission specs to ensure they are working with the correct information when repropping.

Just thought this post might save some folks some problems when repropping in the future.

Kind Regards,
JohnL
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Old 02-12-2006, 10:28   #2
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I have copied this from a post made by Delmarrey. It shows all the minimum measurments. Hope this is helpful.
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And, I have copied it from Wheels' post. I think it belongs here.

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Old 02-12-2006, 21:08   #3
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This is a great guide for over all fit, but if I am understanding the question correctly, the dia and pitch calcs are what is being asked. Max diameter is easy to determine using this diagram, but pitch is a function of horsepower, RPM, boat weight (fully loaded), and waterline. If anyone has that formula, I would really like to know what it is. I also need the variables to calculate proper pitch and diameter for a multi hull. Never Monday provided me with some basic info when I bought my engine, but I was mistaken about the prop diameter. I have searched online, but have only found places to submit the specs and wait for a reply. Anyone know the math?
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Old 03-12-2006, 11:01   #4
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Aloha Kai,
I've lent my copy of Skenes out and it has a graph and a couple calculations. When I get it back I'll let you know about a formula. Have you talked with Michigan Wheel?
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JohnL
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Old 03-12-2006, 11:52   #5
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Try this link:

http://www.bomon.com/jprop/techinfo.htm
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Old 03-12-2006, 15:17   #6
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Aloha CSY,
I tried the forumula at the link you sent and it seems to be just about spot on. As long as people know their actual engine hp output the diagram for diameter will work too.
As soon as I can figure out how to scan and include a couple of tables from Skene's Elements of Yacht Design I will include them in this thread.
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Old 03-12-2006, 15:42   #7
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BTW

The picture above is from http://www.yanmarhelp.com/
so as not to create a copywrite infringement.

http://www.yanmarhelp.com/images/propaperture.gif
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Old 03-12-2006, 16:33   #8
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Propeller Diameter (Three Blade)

I hope this comes out readable. From Skene's.
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Old 03-12-2006, 16:51   #9
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Propeller Pitch Chart

Need your magnifyer again. From Skene's.
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Old 03-12-2006, 19:24   #10
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Skiprjohn and CSY, thanks for the great info. What appears missing for my needs is any variables relating to multihulls. Anyone know if I would use the standard hull speed formula? I have also been asked, every time I have purchased a prop, what the loaded weight of the vessel is. I did not see (may have missed) this info in the link. Any ideas?
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Old 03-12-2006, 20:53   #11
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Multihulls are just the same. The loaded weight should not be an issue for the prop. The loaded weight comes into the equation for Hp.
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Old 03-12-2006, 22:36   #12
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Aloha Kai,
The Graphs I sent from Skene's is Skene's short method for determining prop diameter and pitch. The longer and more accurate method is following the two graphs with four more pages of formuli and graphs. If you truly want to do the math then it would be good to pick up a copy of Skene's Elements of Yacht Design.
I'm sorry that I can't help you with multihulls but from the graphs one of the things factored into the formula for pitch is speed which would be considerably different for multis.
I truly believe that loaded weight has little to do with pitch or diameter of the prop.
Good luck.
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Old 03-12-2006, 22:39   #13
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Propellor Calculations 101

So if my understanding is correct the horsepower and the revolutions control the diameter (of the propellor).
The desired speed controls the pitch (or vice versa).
I found some pitch calculation calculators at:-
http://www.rbbi.com/folders/prop/propcalc.htm
and
http://www.boatramp.com/prop_applet/...erAppletG.html
and
http://continuouswave.com/cgi-bin/propcalc.pl
Gerr's Propellor handbook notes that 45% is an appropriate slip angle for large, heavy slow craft (ie my boat).
To some extent it would seem that pitch is a matter of taste. ie A pitch suitable for high speed motor sailing would not suit someone who likes to motor into a headwind/chop.
My propellor would seem to have a tip/hull gap of much less than 15% of diameter and so far I have notice any ill effects.
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Old 03-12-2006, 22:48   #14
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Aloha Chris,
It's not desired speed but hull speed that should be factored into the formula for pitch.
Percent slip is different for slower moving shafts. So if your trans is a 3 to 1 ratio it would have less percentage of slip than a 1.5 to 1 ratio.
The point of proper diameter and pitch is to make your engine work more efficiently and at its maximum power for engine speed.
Don't know if that helps but I don't think it is a matter of preference but a matter of finding the correct diameter and pitch for your engine, transmission and boat's potential hull speed.
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JohnL
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Old 18-09-2007, 20:01   #15
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Not sure why this subject popped up on my search for "New Posts" as the last reply was almost a year old..?

My take on this is easy:

Get 100% RPM underway and at full throttle ya are about right.

Get 90% RPM static at full throttle. (Tied up to a dock or something.)

Ya get less than the above and ya are Over-Propped.

Get more than that and you are under-propped, or your governor is not working and you will soon blow up the engine..

As you check out the various configurations and learn the boat and the engine ya can always have the prop tweaked at the local prop shop.

More or less pitch will usually solve the problems unless ya are grossly OFF due to an ignorant previous owner...
(1000 RPM seem to be way OFF..Start from scratch.
200 RPM is more like an adjustment problem.)
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