Why the difference between 2 sets of props?

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Deleted User YDKXO

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Spot the difference between these

Infinito022-2.jpg


and these

Infinito019-1.jpg


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?
 
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. /forums/images/graemlins/wink.gif
 
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
 
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 /forums/images/graemlins/cool.gif

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)
 
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
 
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?
 
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.

Deleted User, 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?
 
[ QUOTE ]
I wanted an extra inch

[/ QUOTE ]

So do most of us /forums/images/graemlins/smile.gif

Yup, both of my sets of props were stamped the same
 
Good point about each blade having a different pitch, jfm. I suppose it's possible that one or both of the first props were dinged at some stage and not accurately repaired
No, that triangular trim plate thingy is not accessible. There is a separate lazarette under the bathing platform and the rudder linkage mechanism is accessible from the back of that
 
Yes, I deliberately wasn't specific about whether the solid block CNC milling method used a cuboid solid ingot or an oversize casting. That's a fine point of weighing the casting cost against the wastage from milling the solid cuboid. Of course all the milling waste/swarf is recycled so the lost metal cost aint that high.
 
I meant how are the tillers attached to the rubber shafts, and how do the tillers go into the boat? I was referring to the two little triangle block shape things, one at top of each rudder shaft. Are the tillers attached in there, so are those two triangle block things hollow, with a hole cut thru the trnasom (behind the "footprint" of those triangle thingies) to allow the tillers to pass thru the trnasom?
 
[ QUOTE ]
Deleted User, 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?

[/ QUOTE ]

No, houghn, I'm comparing the performance directly after my 2007 lift out with the performance directly after this year's lift out so, on both occasions, condition of hull and sterngear was the same. Type of antifoul paint used was same as well
 
Sorry, I misunderstood. Yes those triangular bits are hollow and there's a hole through the transom. The triangular bits are triangular to allow the left/right movement of the rudder linkage or tillers as you refer
Btw, I forgot to mention, jfm. When I lifted the boat this year, the fouling on the hull was definitely less than what It was when I lifted the boat last year indicating that, at least in Majorca, water temps were probably lower last year than previous year. I took some pics with my phone but I can't work out how to download them! The boatyard did'nt even want to repaint the antifoul because they thought it would last another year but I insisted they slapped one coat of paint on the hull, mainly so that I could carry out a like for like test on the different props. Because it was just one coat, I decided to use the same antifoul paint (Titan) as last time rather than the Micron self eroding type which I wanted to try following your experience but I would have needed 2/3 coats for that which would have been a lot more expensive
 
Thnaks

So it sounds like lots of boats (well, yours+mine+houghn's) had much less fouling than usual. Yup, must be some macro effect like water temp or something. Let's compare again next year - mine, houghn's and Magnum's all have M66 so at least we'll be able to compare that with your Titan. A friend of me+ Magnum has a new hattan 70 in Antibes (it unloaded from ship in Genoa this morning and is running to its new home berth in Antibes as I type) and I'll find out from him what s/seeker painted on it, so we can include that in the comparison - he is parked in TCM's old berth, aka Barnacle Central
 
Hmmm.... I don't disagree with other comments re.multiple variables affecting props performance, but!
Achieving such a proportional increase in rpm and speed can hardly be due just to different "quality" imho, considering that the two sets appear to be made by the same producer (are they?).
Unless of course the previous props had something wrong, but to an extent that should be visible somehow, or at least perceivable in terms of vibrations.
What you're describing reminds me of a boat where a throttle leverage was badly regulated, thus not allowing one engine to deliver its max power. A few millimiters in cables excursions were responsible for a 4kts difference.
I understand you didn't try both sets under exactly the same conditions, but just set1 last year and set2 now.
Are you sure nothing happened elsewhere in between?
It would be interesting to swap all those props and re-test the boat, trying all the four combinations.
A nice way to kill the time and make some exercise during those boring hours at anchor in hot weather, either...
 
No difference in vibrations, Mario. Both sets of props feel quite smooth. I've had the throttle mechanism checked and I've also checked it myself for both engines. The mechanisms open fully
Swapping props is not cheap. I'm not a diver and it would cost me hundreds of euros to hire a diver for an exercise like this! Anyway I'm satisfied that I made a fair comparison and there is a difference. The only problem is why /forums/images/graemlins/confused.gif
 
Agreed, hiring a diver is not worth the expense, no doubt about it.
You should consider some basic diving experience, though.
Boating and diving are a wonderful combination - aside from swapping props, I mean!... /forums/images/graemlins/smile.gif
And in med warm waters, that's less demanding that you might have heard in the UK, either.
With apologies for the off topic...
 
You're right but I'm too lazy although I do a bit of snorkelling. It's not the diving itself that I mind but taking 30 mins to squeeze yourself into a silly rubber suit and having to manhandle lumps of metal with air in them. Anyway there is nothing to see in the Med. There is no marine wildlife at all except jellyfish
 
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