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Old 28-01-2013, 09:08   #1
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Props for Dummies...

I have searched the forum and have not found a situation like mine. It does appear that once a boat has had a certain number of owners for a certain number of years....no matter how well maintained the boat....there are parts that just don't match up!

My question has to do with the prop. My boat came stocked with a Universal M-15 (5411, 11hp), but has been upgraded to a M-18 (14hp). The previous owner told me that he did not change the prop when he upgraded the engine and after research it appears the manufacturer calls for the same size prop for both engines. With that said, I need a new prop and would like to go to a 3 blade as it now has a 2 blade. As we do a bit of gunk-holing I have found that having that extra under power is more beneficial than what I loose under sail.

The 'kicker' is, the current blade is stamped: 2X12RH (maybe a '1' at the beginning has been worn off??) and the manufacturer suggests a 13X10. I am not a 'prop guy' by any means....so what am I to make of this?

Now, I could use the MFG specs and calculate from there, but there seems to be so many 'generics' out there about how to size from a 2 blade to a 3 blade. How would I make the conversion so that I can maximize my speed under power?

You know my engine. Here is the boat: ERICSON 28+ sailboat specifications and details on sailboatdata.com

Any help would be greatly appreciated!
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Old 28-01-2013, 09:22   #2
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Re: Props for dummies...

this is about the best one to use for your application,you also need to be specific about the gear box ratio to make the calculation as well as displacement and lenth,rpm etc

Vicprop - Propeller Calculator
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Old 28-01-2013, 10:19   #3
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Re: Props for dummies...

Vicprop is useful and so is the prop calculator on the boatdiesel site. But propping is an art and no calculator is going to get it right all of the time. Find a good prop shop close to you that will figure what you need and repitch it at no cost (other than the diver or quick haaul costs).

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Old 28-01-2013, 10:22   #4
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Re: Props for dummies...

13X10

The 13 means prop Diameter
The 10 mean prop pitch

Pitch is the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood." For example, a 10-pitch propeller would move forward 10 inches in one revolution
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Old 28-01-2013, 10:44   #5
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Re: Props for dummies...

Also don't forget the 20% distance factor. Some say 15%, but either way.......


>>> http://mysite.verizon.net/resqp86c/p...lers/id29.html <<<

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Old 29-01-2013, 21:01   #6
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Re: Props for dummies...

OK
Here's the big picture as i know it. Diameter is set by having at least 15% hull to blade tip clearance. Pitch is set by calculations using hull speed engine power etc. You want the engine to be able to reach 90% power and cruise at 70-80%. That power is Shaft HP, the power is what you have after the gearbox and things like generators. The number of blades you need is based on having enough blade area to apply the engine power without cavitating. You can do those calcs as well. A wider blade or larger diameter blade has more area and can mean that two blades are sufficient. The bible for understanding all this and making these calculations is Dave Gerr's Propeller Handbook. I had to read the relevant chapters a couple of times but it was worth it. Usually the prop manufacturer's will do the calcs. I'd recommend going to Michigan Wheel. They'll do the calcs on thier computer. That works for normal props but not something like a Campbell cupped prop. You can use Gerr's method to second check. All this is trying to calculate something that's not precise, so going to owners who have a similar engine and finding out what's working for them is just as good. remember you can have the prop re-pitched an inch or so to fine tune it.
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Old 31-01-2013, 06:41   #7
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Thanks to all for the info. I have a much better understanding of what I am looking for when I haul. Just in my initial assessment with this info, it would appear the two blade I have is under sized. I prob won't have to go to a three blade.......that is good!

Thanks again all!
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Old 03-02-2013, 17:36   #8
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Re: Props for dummies...

Waltthesalt has the deal.
BUT you left out the most important info-
How does the current prop work?
If you get nearly full RPM at full throttle and are going hull speed then you are correctly propped. If not full RPM, then less pitch is needed and if you make less than hull speed at full throttle, then more pitch.
If possible, maximize diameter, although it is uncommon to see a prop too small a diameter installed.
In my experience of one case, if the 2 blade is right, use the same diameter and pitch for a three blade.
search craigslist for used props, I use adhuntr.com for a national search
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Old 03-02-2013, 20:03   #9
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Re: Props for dummies...

Quote:
Originally Posted by sanibel sailor View Post
Waltthesalt has the deal.
BUT you left out the most important info-
How does the current prop work?
If you get nearly full RPM at full throttle and are going hull speed then you are correctly propped. If not full RPM, then less pitch is needed and if you make less than hull speed at full throttle, then more pitch.
If possible, maximize diameter, although it is uncommon to see a prop too small a diameter installed.
In my experience of one case, if the 2 blade is right, use the same diameter and pitch for a three blade.
search craigslist for used props, I use adhuntr.com for a national search
Everything I have read about the engine says that a 13 diameter is needed, like my original post stated the blade is stamped with what appears to be a 12, when I pull it out I will verify before I reorder. When I pulled last spring I already made the commitment to replace the prop, the picture shows why. And according to all the info, it does appear that I have the room for the bigger prop.

And no, I don't think this boat has ever gotten up to hull speed under power, unless you take into account that one time I was moving with the current AND the wind!
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Old 03-02-2013, 21:59   #10
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Re: Props for dummies...

Quote:
Originally Posted by sanibel sailor View Post
If you get nearly full RPM at full throttle and are going hull speed then you are correctly propped. If not full RPM, then less pitch is needed and if you make less than hull speed at full throttle, then more pitch.
That is basically correct, but perhaps not definitive as reaching hull speed is problematic. Some boats cannot reach hull speed under power:

Calm conditions, set control to full speed (power/fuel). Wait for boat speed to stabilize. Engine should reach the RPM specified by the manufacturer. Often the RPM of maximum HP. If full RPM cannot be attained then the boat is "over propped." This is harmful to the engine and the engines full power may not be available.

Tied in the slip, or at zero speed, again set full speed, engine should not reach that same maximum RPM but be several hundred RPM less. The engine may smoke a bit as well because it is lugging. so don't do this for any more time than needed for reading the RPM. If the same maximum RPM is attained then the boat is "under propped." The prop should also sound like it is unhappy. Perhaps rough and cavitating. An under propped boat is okay for the engine, but may not be able to give the best power in tough conditions, e.g. wind, waves, towing, aground.
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Old 04-02-2013, 20:56   #11
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I just ordered a 3 blade prop to replace my 2 blade folding prop the previous owner used. I went with he Campbell Sailer from West by North. You fill out a lot of data for them to size it right, you should check it with them and see what they recommend.
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Old 04-02-2013, 23:02   #12
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Re: Props for Dummies...

It doesn't cost a thing to consult with a prop shop. They'll need to know the engine, the tranny gear ratio, the shaft size, the boat and how you use it. Do you ever race? Motorsail? What's your usual routine in light air? They will probably also want to hear what happens with the current setup at WOT (Wide Open Throttle.)
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Old 11-02-2013, 11:29   #13
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Re: Props for Dummies...

I have a Vetus m4.14, a 4 cylinder 1415cc ( Mitsubishi K4E )
rated at 33hp max rpm 3000rpm, max torque 78Nm (57.5ft/lb) at 2100rpm, Hurth trans 1.89:1
fuel consumption is listed at 260 g/Kw-h (gph?) at 2000rpm.
My boat is 30' w/25' lwl 15,000lbs(hull speed 6.75kt)
1000rpm=4kt, 2000rpm=6.5kt, 2200rpm=7kt (stern squatted down)
I have a 16"x9"" three bladed prop, because I have room for such a big prop could I replace it with a 2 bladed prop with more pitch and achieve the same thing...I could line up the prop with the keel while under sail and cut down drag).
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Old 11-02-2013, 11:43   #14
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Re: Props for Dummies...

If the 2 blade operates properly as sized, I would ask the prop shop what pitch to use in a 3 blade to have the same result. You have actual info on hand, all the prop sizing programs are approximate and often will not be as accurate as your empirical data. If I remember right a 3 blade will be 1 less pitch compared with the two... dont take that to the bank though! You will like the reverse/stopping power of the 3 blade a lot better than the two.
Larger siameter is always more efficient, but 12 or 13 not a big deal.
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Old 11-02-2013, 12:52   #15
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Re: Props for Dummies...

I've found that if you achieve fair performance with the 2 blades, close to hull speed at higher rpm then going to 3 blades at the same diameter you might want to drop a notch in pitch. On my Norsea 27, Yanmar 2GM20 with 2.6/1 tranny I went from 2 bladed 14 X 12 to a 3 blade 14 X 11. I got much better low speed maneuvering and better fuel efficiency. I went be the recommendations of the Prop Shop.
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