prop clearance

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farmerden
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prop clearance

Post by farmerden » Wed Feb 16, 2011 2:06 am

Still haven't found the prop of my dreams but I have seen several references to 2" clearance between the prop and the hull and skeg . With the slow RPM's we are turning is that measurement that critical? Den :?:
Chris W
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Re: prop clearance

Post by Chris W » Wed Feb 16, 2011 4:25 am

I've always used 20% of the diameter as the tip clearance rule of thumb. Seems to work.
farmerden
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Re: prop clearance

Post by farmerden » Thu Feb 17, 2011 2:24 am

Hummmm :D I have 1 1/2 in clearance to the skeg now with a 19in prop. I've got 37% slippage and I want to get that figure down to around 25% so I thought it's go big or go home and that puts me into the clearance issues. What happens if it's too close? OK so we stay at 19 in diameter and go to a four bladed prop. Now I have a 19x28 three bladed prop .The extra blade should slow the engine down [that's OK] and I should go to a lower pitch [that should help the slippage] Maybe :lol: If I had a ton of props I could experiment a bit but I'm afraid to ask the wife for that big a bag ofmoney!! :? Any thoughts? Den
Chris W
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Re: prop clearance

Post by Chris W » Thu Feb 17, 2011 5:31 am

Too little clearance can cause cavitation and can cause erosion on the blade tips but with the generally low number of hours of operation these boats see it may not make a real difference. Another rule of thumb is that a an inch of diameter is worth 2" of pitch. I'd say that this rule is a bit more "thumbish" than the other rule but, again, I've had good luck with it.
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Dhutch
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Re: prop clearance

Post by Dhutch » Mon Oct 10, 2011 11:04 am

Chris W wrote:I've always used 20% of the diameter as the tip clearance rule of thumb. Seems to work.
Sounds nice.

We have around 4inchs clearance to the skeg on a 26*32inch blade, more like 2.5 to the hull which doesnt appear to give problems, running a shaft speed of around 150-350rpm (typically crusing at 200-250).

I know at one point Geoff had Suilven running with a cutout in the skeg to get the prop in as it was that close it overlapped without, it did give some vibration at certain revs, but it shows how close you can get it you really want.


Daniel
Last edited by Dhutch on Mon Jun 25, 2012 10:17 am, edited 1 time in total.
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artemis
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Re: prop clearance

Post by artemis » Wed Oct 12, 2011 6:30 pm

From Dave Gerr's Propeller Handbook, page 83:

"Once you know the maximum distance from the shaft centerline to the hull (and down to the skeg below, in an aperture), you can determine the largest acceptable propeller diameter. Generally, there should be a tip clearance of at least 15 percent of the overalll propeller diameter between the blade tips and the hull. The ideal tip clearance is 20 percent or more..."

"Actually the slower the shaft RPM and the lower the boat speed, the lower the minimum tip clearance may be. Minimum Tip Clearance Table 7-1 below gives minimum tip clearance at varying RPM."

From the Table:
RPM: 200-500 -- SL Ratio: under 1.2 ----- Minimum Tip Clearance: 8%
RPM: 300-1800 - SL Ratio: from 1.2-2.5 - Minimum Tip Clearance: 10%

"The clearances in Table 7-1 represent the absolute minimum, so you should always strive to do better. Tugs and trawlers frequently accept the additional vibration of propellers with only 8 to 10 percent tip clearance to gain additional thrust at low speed from increased propeller diameter. On propellers in an aperture or with a protective skeg below, the tip clearance should be at least 12 percent of the diameter. Most other vessels should use 15 percent or greater if possible... Tip clearance should never be leass than 2 inches (50mm) on any vessel"

There's also a lot of info about clearances between the blade tip and the deadwood as well as between the blade tip and the rudder. Too small a clearance between the deadwood and the blade tip can result in a "thumping" sound/vibration.

For those who are not aware, Mr. Gerr is a Director and instructor at the Westlawn Institute of Marine Technology http://www.westlawn.edu/ and has designed and is interested in moderate sized "hobby steamboats". Any of his books is well worth the price.
Ron Fossum
Steamboating Magazine Editor
http://www.steamboating.org
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