mikef
regular
Reged: 16/05/2001
Posts: 9304
Loc: chesham, bucks and palma,major...
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Spot the difference between these
and these
Nope, I can't see any difference either and they're supposed to be exactly the same pitch/dia except that the second set make my boat go 2.5kts faster than the first set
The manufacturer quotes a top speed of 28kts for my boat with the engines that are fitted but, until now, my boat has only managed 25kts and revved to 150rpm short of the rated max engine rpm. Over the 3 years I've owned the boat, I've tried everything to find those missing kts. I've had the engines and throttle linkages checked, props polished, removed weight from the boat by leaving the bimini and tender on the dock and run with near empty tanks but all to little avail. Ferretti kindly supply a spare set of props with the boat which are stored in the lazarette. I've had them examined by a yard and have been told that they're exactly the same as the set fitted to the shafts. This year when the boat was lifted, I thought what the hell, I might as well swap them over and, hey presto, the engines now run to their rated max rpm and the boat does 27kts which, given the extra equipment and cruising gear on my boat, is acceptably close to the manufacturer's own figures
So, why the difference? The props are supposedly the same pitch/dia and are in equally good condition. Does the performance of props degrade over the years?
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RogerRat
regular
Reged: 30/11/2005
Posts: 1929
Loc: Camberley / Hayling
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Good question and I wish I had an answer. The fact is that there is a difference hydrodynamically and I think you would need to have the right test equipment to find out what that would be.
It could be so many things from pitch of a blade or two or even balance. The first two may look the same but may not even be a pair. I think the only way to find out is to take all four to a prop specialist / builder.
It also just confirms what a 'black art' props can be. 
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Starting to get worried... I rubbed my eyes this morning and saw nothing but Bloo dots. Does this mean something? 
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Magnum
regular
Reged: 14/01/2004
Posts: 1821
Loc: Switzerland
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What's that projection in-between the trim tabs?
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omega2
regular
Reged: 17/10/2002
Posts: 1141
Loc: Essex Bradwell UK
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Cannot answe the question either but, clements marine took our old props which only allowed us 2000 revs and transformed them to give 2500 which for our old fords is MAX, as and when we were able to afford a pair of brand new ones, they made us a set and these will allow 2450, but deliver 2 extra knots, we too are mesmerized
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J HAMER
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DAKA
regular
Reged: 07/01/2005
Posts: 3661
Loc: Nomadic
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My boat reved to 4000 and the governor would cut in.
I took my props to Hamble props for a repitch, I wanted an extra inch.
The props had been brand new and were clearly marked as 27 inch
I requested an upgrade to 28 inch
Hamble props tested them to be
25.89, 25.49
They added uprated them both to 27 inch.
they said they often found props made abroad were wrong.
I got an extra 1.5 to 2 knts for free 
I suspect your yard will have looked at the stamp on the props and assumed the stamp is correct.
Or they may have just rolled them over the floor to give a rough guide.
(my quoted figures are from memory but serve to demonstrate an accurate overview of my situation)
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Take a tour of Felix
Lazy Git's search engine
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mikef
regular
Reged: 16/05/2001
Posts: 9304
Loc: chesham, bucks and palma,major...
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Mark, it's a fixed trim plate which seems to support the rudder stocks as well. A number of Ferrettis I've seen have this arrangement. I guess it's because the engines on some Ferretti models (mine included) are located well aft and send the drive forward thru V drive gearboxes. I think the trim plates are there to provide additional lift to counteract the weight of the engines
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jfm
regular
Reged: 16/05/2001
Posts: 7818
Loc: London/Antibes
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I've heard similar before mike. i think it is down to dimensional variations during manufacturing processes. These days some (v expensive!) props are made by CNC milling out of a solid block and the dimensional accuracy is perfect, though the grain pattern of the metal isn't. But afaik most are made (more cheaply) by casting then hand finishing and polishing, without any 3D computer check on the geometry and dimensions. Thus, there is bound to be some heat distortion of the blade shapes etc, which doesn't get corrected. Frexample people talk of "pitch" as if it were a constant value like you get on a nut or a bolt but of course each of the 5 blades has a different pitch caused by manufacturing inaccuracy and indeed each blade has a differnet pitch at each different point on the blade surface.
With all that scope for dimensional/geometrical error in the manufacturing it is perhaps not surprising that you get a 2kt or so variation.
That Sq58 I nearly bought in 2004 (1 yr old 2nd hand) that turned out to have hit rocks had the same issue. It had new props with the same nominal dimension props as the originals but they were made by someone other than teignbridge and that boat has always been 3kts down on top speed as a result.
Separately, how do your tillers work? Is there a tiller at the top of each rudder shaft, encased inside that triangular block thing at the top of the rudder shaft, ie is that triangle thing hollow? Or what?
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Poweryacht
regular
Reged: 21/05/2007
Posts: 1363
Loc: Malta - Med Sea
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it could be as JFM said
tough I have to admire the design of the Rolla Props, they cost a fortune but look at those things...;
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http://poweryacht.blogspot.com for News and www.youtube.com/PowerYacht for Videos
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Nick_H
regular
Reged: 20/04/2004
Posts: 3001
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I suppose the best way would be to machine them from oversize castings, then they would have the dimensional accuracy of the machined from solid, but at much lower cost, assuming that enough are made to amortise the cost of the casting tool. I guess there just isn't the volume in any one boat to specifically design a machined prop that absolutely maximises its performance. I agree with the above poster that it does seem to be a black art.
Mikef, i'm sure you're right with your different prop theory, but if the boat was out the water I guess you also had it antifouled and all the stern gear cleaned up, could the 2 knots be due to a different type of a/f?
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mikef
regular
Reged: 16/05/2001
Posts: 9304
Loc: chesham, bucks and palma,major...
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Quote:
I wanted an extra inch
So do most of us 
Yup, both of my sets of props were stamped the same
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