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Old 10-08-2008, 15:40   #1
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Prop Size: 17" vs 18"

Ok, I've got the pitch thing figured out. Does anyone know what diference
a 17 x 13 prop compared to a 18 x 12. I have the room for the bigger prop
(a least 4 inches) from hull only need 0.5 in more. Currently I'm using a 3 blade 17 x 13 but it's old/unbalanced, etc. The new one only comes with 17 x 12 ,18 x 12 or 18 x 13. The rpm's seem close to spec as far as max rpm's for engine etc. But has a little room for more push. Anyone have real world experience changing this rather than pitch?
Thank
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Old 10-08-2008, 15:50   #2
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From what I have experienced, going up in diameter by one inch and going down in pitch by one inch gives you close to the same thrust per revolution, given everything else remains the same. You will want to confirm this with a propeller shop for your specific situation. If they get it wrong the first time, they can give a refund for the correct sized propeller.

I bought this book years ago when I re-engined my work boat. I doubled the horsepower. The book nailed the correct size propeller for the boat. There are some formulas you have to calculate but its kind of fun.

The Propeller Handbook

If you go on the internet to do a calculation, don't use the same tables for a planing boat as you would use for a full displacement boat. Its probably best not to use the propeller size calculators on the internet at all in my opinion. They do not ask for all the necessary parameters to make an accurate calculation.
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Old 12-08-2008, 20:06   #3
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I found a prop calculator on boatdiesel.com that seems to work well.
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Old 12-08-2008, 21:44   #4
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Thanks to all replies, I guess I left out one important detail. The old 17 x 13 prop I'm replacing is a max-prop which has no cupping. Just straight blades (less effeicient).
I'm told that the volvo 3 blade folding 17 x 12 will be simialar but no refund if wrong.
The other side of my brain says go to the 18 x 12, the engines is new and can handle it.(D2-40) Max rpm is 3200 and with present prop I just get there.

Btw
I found this calculator to be usefull and doesn't require a membership. Pitch Calculation
I ran many different figures thur it using both my old and new engine/drive spec and come out with 17 x 13 as best choice, in the real world it's 17 x12 or 18 x12. I'm stuck with volvo as I already have the hub, just need to decide on blades.
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Old 15-08-2008, 03:25   #5
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Engine power does not really dictate prop diameter. The diameter is dictated by the tip clearance to hull. Apart from a little less slip and a little more power it takes from the engine to turn it, the difference of 17 or 18" will not give you any more speed. Speed is pitch.
Cupped blades are usually considered to be 1" of pitch up on flat blades. But this is not always so cut and dry. It depends on the shape. A prop is a rotary wing and the blades are, or should be, shaped like a wing. A prop actually produces more "suck" through the water than thrust believe it or not.
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Old 15-08-2008, 04:48   #6
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I decided to attack this problem from a different angle. I called the good people at PYI - the Max-prop dealer and discussed this. Fred instantly recommended going with the 18 in if I had clearance. Max prop has more blade area than most 3 blades to compensate for not having cupping. So I have ordered the 18 x 12 and will let you know how it turns out in about 2-3 weeks. Thanks eveyone.
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Old 15-08-2008, 14:20   #7
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Blade area has nothing to do with pitch. Cupping of a blade does. The angle of attack of a cupped blade is higher than a flat blade, although the blade angle itself is on the same plain as that of a flat blade. ie, 12" flat blade, 12" cupped blade both on a 12"plain, but cupped blade leading edge more on line of 13" pitch.
Ensure the distance between blade tip and hull is enough or you will create a loud hammering noise and even to the point of the shock damaging the hull eventually.
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Old 19-08-2008, 10:18   #8
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Pitch vs. diameter

If you in a big displacement hull, the more blade in the water the more bite. You need at least, one full inch of clearance from the hull or in an apeture, a little more is better.
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Old 19-08-2008, 14:21   #9
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In the Study Hall for this engine forum, there is a diagram that shows all the important clearances. All clearances are expressed in percentages. 1" may not be enough of a gap in many installations.
Diameter should not be expressed as "bite". Although I understand what you meant. Diameter usually expresses efficiency in regards to slip. But please understand that surface area is the more important aspect of slip efficiency. So a wider 17"blade maybe just as efficient as a narrow 18" blade. Of course one other aspect comes into play. Surface friction. The more area of blade, either diameter or blade size, the more power loss due to friction.
The more important aspect to worry about is tip speed. The bigger the diameter, the faster the tip travels through the water. There is a maximum tip speed (in Sailboat props, not referring to powered race boat props which is a different design completely) that you do not want to exceed, or cavitation at the tip results. So you will see that the diameter of a prop also works in conjunction with gearing of the drive box. The larger the prop, normally the higher the gearing and the slower the prop rotation.
So to sum up, it is not a simple case of just putting on a bigger diameter prop. A well designed propulsion system takes into account all the details so as you get maximum efficiency. Often an account over looked in past installations, but I think now becoming important in relation to todays fuel prices and the change in people wanting to be just that little more "green" to the planet. Certainly a major calculation if electrical drives systems are proposed.
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Old 19-08-2008, 16:08   #10
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Quote:
Originally Posted by snbrooker View Post
If you in a big displacement hull, the more blade in the water the more bite. You need at least, one full inch of clearance from the hull or in an apeture, a little more is better.
Is that right?

I was told 75mm as a minimum and have been freaking out a littlebit that I allowed ONLY 75 mm in my build after seeing clearance on other powercats.

Pic of the other powercats

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Old 19-08-2008, 19:02   #11
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re: the free prop calculator posted above, I got a much different pitch recommedation from this calculator, than what the boat diesel one suggested (approx 2" more pitch).
I know my boat is overpropped anyway, as I get up near hull speed at around 2000rpm, and get serious white smoke after 2200rpms, on a Yanmar 2gm with a cruising rpm of approx 3200rpms.
The baotdiesel calc suggested a 14x8.6, while this free calc suggested apprx a 13.78x11.80 . I have a 15x10 on the boat, and will be having it depitched, when the boat comes out for the season, in a couple months. Would like to do it now, but can't justify the cost of a haulout, or the hassle of getting a diver (local diver doesn't want to do it at my dock, due to very muddy bottom, so woudl be a hassel to go somewhere else, and wait until prop is repitched, before reinstalling!)
The boat only went in the water about a month ago, so the bottom and prop were clean during testing.

Just wondered if anyone else noticed a difference between the two prop calc sites??
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Old 20-08-2008, 00:27   #12
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Did you factor the gear ratio into the equation??
Also, don't measure the under/over pitch by boat speed. Some hulls will make hull speed easily, some will not. The important part is getting within 10% of maximum continuous RPM. If Max Cont. RPM is 3200, then you need to reach that or at the very least 2900. Don't worry about white smoke. That will most likely be steam. It is black smoke you need to worry about. That is the sure sign of over propping.
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Old 20-08-2008, 05:25   #13
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Allan- thanks for the reply. I didn't mean to steal the thread - just wondered if others had noticed a difference.

Yes, I did factor in the trannmission 2.62:1, as well as LWL 25ft, disp 9000, max HP 15, etc.

I thought that white smoke was a sign of unburned fuel, and therefore the engine trying to work too hard!! (as well as possible steam with head gasket trouble, etc)
I guess I never really tried to push her past 2500rpms, as I get alot of white smoke, and it doesn't look / feel good. Since I am already at about 6kts by that time (near my top motoring speed, not hull speed, as I mentioned lazily!), I didn't figure it would go much faster, if I tried pushing through the white smoke, to see how high she would go!!

I though that mine sounded like a classic case of overpropped!!

A couple of notes:
- engine is 25 year old raw water cooled 2GM
- engine works great / starts immediately when cold, etc, even after sitting months, therefore i assume I have decent compression.
- 15x10 prop has likely been on there for many, many years, whether correct sized, or overproped!!
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Old 20-08-2008, 14:09   #14
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I doubt it is actually smoke. Does it dissipate as it moves away from the boat? If it is smoke, it will not dissipate, it will stay in the air. Steam is normal and will not indicate that you have a problem. It moist certainly will not suggest you have any head gasket issue. Instead, if the engine starts and runs sweet, then all is well.
The difference in the calculator results could be a factor of information and thus accuracy. You can have a simple calculator based on pitch = distance traveled forward/1 revolution x time. A better calculator will allow you to add efficiency(slip) and a more complex calculator will factor in additional information like Displacement. Whatever the calculator is, the end result is still a "suck and see" type answer and the final judgement is actually fitting the prop and see how it goes. That can be difficult and expensive for many, so gaining the expertise of the manufacturers experience can be helpful.
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Old 21-08-2008, 02:13   #15
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I thought the diagram was in the Study Hall but it wasn't. Now it is. Thanks Gord. The Diagram shows that the minimum clearance from blade tip to hull is 15% of the Blade diameter.
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