Bent prop blade: worth fixing?

[rafiki_]

Very nice P. Yes, they do shout speed.

 

[vas]

Don't they shout SPEED?

nah, they're off a barge I recon, speed means these straight cut edges surface props P.

are they Eliche Radice?
and btw, do you have other brands of props in Italy, all mentions seem to be on Radice (apparently mine has them as well, but tbh cannot tell as I've not seen any stamps on them!)

nice work, I assume they have CNC machinery to check them right? Do they route and then fill (or the other way around) or is it a more manual thing (heat and hammer) ?

cheers

V.

 

[Portofino]

Don,t know much about props --back art or everybody's got different ideas
I note In your pics ( they do look barge like -sure they are not ) they overlap and one edge ( nearest the cam ) is curved ,rounded .

Here's a pic of mine the same edge is straight and there's no overlap of the blades --- pitch is steep
Different to yours ?

No idea what revolutions they are optimised for ? Guess a bit depends on g-box ratios engine torque and design speed (s) of the hull .

 

[BartW]

well done P, (I was in these 80% votes)
I'm a big fan of impeccable props, (after usefull experience with damaged props on smaller boats)

yours look fairly similar shape like mine,

do you know what material your props are made from ?
according MPR in NL mine were a old sort of alloy, not used anymore

 

[oceanfroggie]

Bent prop? That's minimal damage. Any decent prop shop can repair and rebalance for you. I've seen far worse, badly mangled props that were repaired by professional 'prop shops' like new.

 

[MapisM]

are they Eliche Radice?
and btw, do you have other brands of props in Italy, all mentions seem to be on Radice

Yup, Radice built them, and now just did the refurbishment.
They are only a 10 mins drive from where I live, so it was a no brainer, really.
But even if they are the bigger and more popular IT props builder, there are others.
Some others popping to my mind are Bisognani (they built the props of my old tub), Italian Propellers, Albinese... And I'm sure there are others.
It's a cottage industry after all, as we all know...!

TBH, I didnt't ask them about the working method - I just assumed that nobody could do a better job than the biulder.
They do look CNC machined for good, anyway. :encouragement:

 

[MapisM]

Here's a pic of mine the same edge is straight and there's no overlap of the blades --- pitch is steep
Different to yours ?

...

No idea what revolutions they are optimised for ?

Yeah, the "exit" part of the blade (if that's the correct wording - what I mean is the side opposite to the one that "grabs" the water first) is typically straight in props meant for high speed, surface props included.
In this repect, your props seem designed for higher speeds than mine or BartW props.
Hardly surprising, considering the respective boats...

But I must say that I have no clue about whether my props were specifically aimed at a specific utilization range.
Fwiw, the boat itself was designed for a 27kts cruising speed, and since the pitch is 780mm, you can just do the math for the correspondent rotation speed (regardless of gear ratio, torque, whatever).

 

[MapisM]

do you know what material your props are made from ?
according MPR in NL mine were a old sort of alloy, not used anymore

Afaik it's Nibral, which is indeed a Nichel/Bronze/Alu alloy (as the name suggests... )
I would have thought it's still common also nowadays, though.

 

[Deleted User YDKXO]

Don't they shout SPEED?

No they shout VIBRATION. All good Italian boats should have 5 bladed props

 

[rafiki_]

No they shout VIBRATION. All good Italian boats should have 5 bladed props

Not so Mike. My good Italian boat has 4 blades, but it is a bambina compared to yours

 

[MapisM]

No they shout VIBRATION. All good Italian boats should have 5 bladed props

LOL, you might rather not wish to go there, M...
Let's put aside my own new toy, for which you can obviously say that I'm biased.
Let's also forget rafiki's bambina as he called her - though AZ has a reputation for optimizing/engineering their boats like not many other yards.

But let's think of Amati: he was a notorious maniac of hull efficiency, and I don't doubt for a second that he would have used 5 as well as 15 blades props, if he had a chance to get a 0.1% improvement.
In spite of that, you can see above how many blades PF has on his props...

Not to mention BartW boat, built by a yard which (particularly back in BA days!) used to build pretty much with a money no object approach. In fact, when she was built, Ferretti bros probably did not even dream to become more well known than Canados, one day...

Last but not least, by sheer coincidence I just posted a few pics on the hull thread of a boat which you will easily recognize.
Now, try to tell Cesare that their boats aren't comfortable due to prop vibrations.
I'm sure he'll have something to say about that...!

The fact is, with props there are pros and cons in just about any detail.
It's no coincidence that props engineering still nowadays is defined as a sort of black art, as PF said.
To the best of my knowledge, there is some consensus that a high blade fractioning (higher number, smaller surface of each blade) can either make sense with very slow D vessels, or VERY fast ones, but not so much with "normal" P boats/speeds.
Don't ask me why, though...

 

[Portofino]

Drag
4 blade Vs 5 in a "normal " P boat with submerged props , the 4 will create less drag at speed than a 5 blader .
As MapisM mentioned its the sum of the tiny bits that add up ,not just one .
So say you want to be in the 28-42 knots cruise range ---all day ---( not a one off WOT early season test )that extra blade,s drag will slow it down .

Also adding an extra blade going from 4 to 5 ,may mean dropping down a pitch ,by a inch or two -for the same Hp , also knocking a knot or two off ,or you could increase the Dia , **which also will increase drag --so you kinda stuck in the low 30 ,s top end say 32 knots and sit all day @ 24-26 knots in your "best Italian " 5 blader propped boats .

** you are gonna need bigger engines more Hp .

Remember to knock off 4 or so knots off for Real World Speed --- due to seasonal growth and for some extra "lardy " cruising stores /tender /kit /etc --- a few more knots off !

Can,t see many sitting north of 30 knots all day .

Just to clarify multi -5-6 or more blade surface piecing props can cope with the extra blades because when running 1/2 of them are out of the water in air = less drag .

I,am talking about submerged props here .
In theory a 5blade can be easier to balance than a 4 blader submerged , that's not to say you can not balance a 4 blade ,it's just not as easy .
I read somewhere that adding an extra blade 4 to 5 also creates more bow lift ,or stern sinking -same thing .
A shallower deadrise or flatter rear section will mitigate this effect -help balance the running angle --but we are moving into another multiple variant area --running angle -if you need to put flap s down at cruising speed or any speed , then you agian --more drag or your boats just not balanced --a prop that lifting the bow more is not helping -- time to stop

Basically 4 is less draggy than a 5 @ speeds N of 30 knots ,

A 4 like Barts ,has huge pitch --does not need an extra blade --but and it's a big but the engines all 22L each iirc ?
Have enough torque to spin them -here its more about about pitch = thrust less about drag at speed .
Barts is a heavey boat now ---interesting to see if a lower pitched smaller 5blader might improve things ?
Probably not as to go faster it needs more lift --get it out of the water -less overall drag --a5 blader may knacker the running angle --needing more flap---more drag - going round in circles --zero benefit €8000 later --nah -leave it

 

[Deleted User YDKXO]

The fact is, with props there are pros and cons in just about any detail.
It's no coincidence that props engineering still nowadays is defined as a sort of black art, as PF said.
To the best of my knowledge, there is some consensus that a high blade fractioning (higher number, smaller surface of each blade) can either make sense with very slow D vessels, or VERY fast ones, but not so much with "normal" P boats/speeds.
Don't ask me why, though...

Of course. My comment was tongue in cheek

Ferretti claim they use 5 bladed props in order to reduce vibration which may or may not be true. All I can say is that all 3 of my Ferrettis have run very smoothly. But I think there is another reason they use 5 bladed props instead of 4 and that is that a 5 bladed prop allows them to reduce the shaft angle which with the compact V drive arrangements on most of their boats is probably critical

From my understanding though, the more blades a (submerged) prop has, the more drag it has and the less efficient it is so I think there is a price to pay for having 5 bladed props instead of 4 in terms of reduced speed and higher fuel consumption

 

[MapisM]

Yep, all agreed. And also my reply was a bit tongue in cheek TBH, because in principle I believe it's true that the higher the number of blades, the lower the prop-induced vibrations.
But I think there's much more than that, behind the smooth run of Ferrettis - as well as of SL, Canados et all, for that matter... :encouragement:

There's one thing I'm not sure to understand, though:

a 5 bladed prop allows them to reduce the shaft angle

I can't see the rationale of that in fact, 'cause the shaft angle essentially depends on the prop diameter, rather than anything else.
By chance, did they tell you that with 5 blades they can use smaller dia props, AOTBE?
If so, well, I take their word for it, but I still struggle with the logic...

Incidentally, I did ask a Radice engineer whether he would suggest anything else nowadays (different number or shape of blades), in the event I would decide to buy a couple of spare props.
You know, the boat is one of the last 56 they built (in 2004), but the props selection was actually made by the yard back in the late 90s, so I expected that after 20 years the prop builder would have suggested something more "modern", one way or another...
His answer was, quite simply, no.

 

[Deleted User YDKXO]

I can't see the rationale of that in fact, 'cause the shaft angle essentially depends on the prop diameter, rather than anything else.
By chance, did they tell you that with 5 blades they can use smaller dia props, AOTBE?
If so, well, I take their word for it, but I still struggle with the logic...
.

Well thats the point. Maybe I'm wrong but I always assumed that because there are a greater number of blades the prop could be smaller in dia than a prop with fewer blades and provide the same thrust

 

[Portofino]

Well thats the point. Maybe I'm wrong but I always assumed that because there are a greater number of blades the prop could be smaller in dia than a prop with fewer blades and provide the same thrust

Yup -agree makes sence .
It's all designed in from the start so they fit a shorter P bracket ,lowering the shaft angle ,knowing they can find the lost thrust of a smaller Dia prop by adding a 5 th blade -accepting the extra drag .In the mid twenty knot range that extra drag is not as important than say in the mid thirty cruise range .
From a vibtrations /refinement point of view there's also the added layer of the extra drive shaft spinning ,more UJ joints to wear loose -more spinning bits in the V drive box ,and an extra shaft to align .
So in an overall package makes sence to go for a 5 blade --last thing you need with all that extra stuff spinning away --in or slightly out of harmony is big Dia ,big pitched 4 blader --chipping in with it's 0.02 p worth of imbalance /vibration .

Theres always the cop out of semi tunnels ---tend to loose lift in that part of the hull ( well more lift lost than without a tunnel ) -stern sinks -- bow high ride ,or loads of added trim --AND are a source of noise --so run against refinement .

Cat skinning --many ways

 

[MapisM]

Well thats the point. Maybe I'm wrong but I always assumed that because there are a greater number of blades the prop could be smaller in dia than a prop with fewer blades and provide the same thrust

Far from pretending that you are wrong, let alone that Ferretti engineering are.
But while I would see the logic of that sort of trade-off between 3 and 4 blades (and a fortiori between 3 and 5, of course), I wonder whether another blade really allows a meaningful diameter reduction compared to 4, for any given thrust.
I mean, the more external part of the blades is the one that matters more as we all know, and as you can see in my previous pic above (as well as in the one posted by BartW, or the other one of the SL72), 4 blades already allow for a shape that "covers" most of the external circumference.
I'd be curious to hear what sort of % reduction of the diameter can a 5th blade bring to the party (AOTBE), because my gut feeling would have suggested that it takes decimals to measure that.
Maybe I'll ask that Radice engineer, the next time I'll get in touch for some reason.
Though hopefully I will not have any for a while...

 

[vas]

since we always seem to hover/drift call it as you wish in many threads (this not being an exception) may I ask WHAT was that white thing on the props P.?
Was it there for a season (intact!) on a planning hull, or what?

Cause if it spend a year in the Med and is still in place, I want a kilo please

Yes, getting ready for a/f the metals again and the TK spray was a waste of 20euro IMHO, so need to find something else to waste my money on!

cheers

V.

 

[MapisM]

Not a clue Vas, but in these days I am right at the yard where the boat has been in the last years, and which surely antifouled the u/w gear. So, I'll ask and report back.
I see what you mean, anyway, and it's even more impressive if you consider that the boat clocked almost 200 hours in the last season alone...!