Propellor tip clearance

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Lopez Mike
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Propellor tip clearance

Post by Lopez Mike » Mon May 27, 2019 7:05 am

I'm approaching the point where I must decide where to locate the propellor vertically on my fantail launch..

The upper limit is set by how close I dare let the blades get to the underside of the stern. The lower limit is set by how much extra draft I wish to put up with. There is a rudder support strap that passes under the propellor from the bottom of the keel to the lower rudder shaft bearing. It has a curve in it to clear the propellor. I haven't made the strap yet. It will be 2" wide by 1/4" thick lying flat. The rudder doesn't really need the support so much as the propellor needs protection as I beach the boat a fair bit.

The propellor is 18" in diameter with three rather wide blades and has a pitch of 24". The hull above the propellor is fairly flat. Not an extension of the keel or anything like that.

I'm assuming that the problem of getting too close to the hull would be vibration. I have a sailboat that suffers greatly from resonances due this problem.

Advice and thoughts gratefully received.

Here's the link to Steve's blog. Same lines as this.

http://steamlaunchiona.blogspot.com/201 ... esign.html
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DetroiTug
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Re: Propellor tip clearance

Post by DetroiTug » Tue May 28, 2019 12:39 pm

As simply a testimonial rather than suggestion, mine is very close to the hull and it hasn't caused any problems. The RPM we typically run and the amount of thrust we're working with, we can get away with a lot.

-Ron
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Re: Propellor tip clearance

Post by fredrosse » Fri May 31, 2019 2:16 pm

I agree with Ron, the rules for prop clearance are generally based on power levels that are far beyond what we steamboaters have, so violating the 15% clearance minimum rule is most probably OK.
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Re: Propellor tip clearance

Post by RGSP » Sat Jun 01, 2019 8:07 am

I've been out and measured mine as it sits on a trailer by the barn. Clearance is just under 3", top and bottom above and below a 16" screw. I have no doubt the clearance is more than necessary, but the hull is a standard "Nell" from Blossom's boatyard near Oxford, and that's the way it is. Maximum draft is right aft, and is 2'. The boat on its trailer is relatively heavy at 1900 kg, and I have to say is a brute to manoeuvre and launch from a normal non-ideal slipway. How much of that difficulty is the 2' draft, I'm not sure, but I'm sure it doesn't help, but once in the water I've never found it a problem.
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Re: Propellor tip clearance

Post by Desperado » Fri Jun 14, 2019 2:52 am

@ Lopez Mike: The thumb-rule of adding a tip clearance between propeller and the hull of 15 percent of the propeller diameter applies also to steam boats, irrespectively of the steam engine power. You made that experience already with your sail boat, which I assume is also relatively low powered as the engine serves as a auxiliary drive only. Classic steam launches had often less clearance, but I have no practical experience in regard to their vibrations. Perhaps someone else here? You might take a bit a risk and push the clearance to 12 percent, but I would not go for less. Increasing the number of propeller blades increases the amplitude and actually helps to reduce the propeller tip clearance. In his propeller handbook, naval architect David Gerr provides good and practical guidance. It is worth buying.
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Lopez Mike
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Re: Propellor tip clearance

Post by Lopez Mike » Fri Jun 14, 2019 3:49 am

Lots of thought and good information from the troops. A big reason why I stay on here!

The power loading is around ten times as much on my sailboat as on my launch. 18" launch prop v.s 15" sailboat prop. 20 shaft h.p. v.s. 3-4 on the launch. 400 r.p.m. on the launch v.s. 2000 on the sailboat. The sailboat is really thrashing.

I am NOT a hydrodynamicist but my thought has been that the higher the blade loading, the more turbulent the flow at the blade tips. My assumption then being that the vibration would be caused by the span wise flow at the tips being altered as the tip passes by the hull. Or any other obstruction for that matter including the rudder support. Whether that span wise flow is directly proportional to the blade loading I have no idea. Might be a square law thing.

Thanks for all. I'll do some more careful measuring. I realize as I'm typing that I haven't measured the tip clearance on the old hull which had no problems.

Oh well. The art of the possible.
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Re: Propellor tip clearance

Post by Desperado » Fri Jun 14, 2019 4:31 am

@ Lopez Mike: Measuring and taking it from there makes sense :-) Rudders and rudderstocks (and shaft brackets) do usually not contribute significantly to vibrations, providing they are well aligned, reasonably well shaped and do not sit extremely close to the prop. The latter is normally never the case as long as the prop is far enough away from these appendixes so that you can take the prop off without disassembling anything else. Another rule of thumb! But perhaps I start to become confusing here.
It might well be that your engine power is so little that any vibrations wouldn’t be noticeable at all. I can’t judge this, in my job I am rather dealing with shaft arrangements of a couple of thousands kW and more. My own little steam powered boat has not passed the construction stage yet ;-)
The book I mentioned is really great guidance, does not cost much, is an easy and interesting read and can be easily ordered by the author‘s own website (I believe) or amazon and co.
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Re: Propellor tip clearance

Post by Desperado » Fri Jun 14, 2019 4:38 am

PS
Is not the engine of your sailboat running at 2000 rpm-1 and fitted out with a reduction gear? :?
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Lopez Mike
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Re: Propellor tip clearance

Post by Lopez Mike » Fri Jun 14, 2019 5:29 am

The engine is governed at 3200 r.p.m. (Yanmar 3HM35 rated at 30 h.p continuous) and the reduction is around 1.8-1. So yeah, the shaft r.p.m is probably more like 1400 at 2500 cruise. This on a 7.5 ton 36' sloop. The days of the little one lunger auxiliary that would barely push the boat to hull speed are long gone.

My launch prop is an 18 x 24. I have an 18 x 20 and I'm sorely temped to go at it with a band saw, a big assed hammer and a grinder and make it into an 18 x 24 with narrow squared off blades like the literature sometimes calls for. Weston Farmer claims it is hard to make a really bad prop. I'd probably prove him wrong.
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