Whats the best number of blades on a prop?

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Gludy

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I am told that five blade props reduce prop noise a lot and they seem to be the latest must have development but are they really suited to cruising power boats? What is prop noise as such?

What are the advantages of the 3 blade or 4 blade props?

I do not know the answers - it s a subject I am currently involved in as my props are having to be tuned in and Trader are carrying out experiments with three bladers on my boat now whilst the five blade props are in Cornwall being de-pitched a bit. I frankly have my doubts over the 5 bladers ever giving me the performance I am after.

So what are your views? Is there a 'best' number of blades?
 
Loads and loads of blades is good, particularly in the various stages of a jet engine, you do have jet engines don't you?

Seriously, a Squadron 55 in Chichester had them fitted and he thinks he gained 2-3kts which is quite decent, of course it depends how much they cost to start with!
 
MBM or MBY did a test about 5to6 yrs ago or was it 10 !!! putting 3 blades against 4 they found 3 was more efficient altho. I think the 3 wasnt as quiet.Prob. someone can remember.
 
If something is emitting noise it is expending energy in doing so, so less noise in theory equals a more efficient propeller.
I cannot imagine noise being a practical problem on a normal boat, an aquadrive thrust bearing would be far more use in reducing noise. The majority of noise presumebly comes from the engine and gearbox. Perhaps an aquadrive, a 5 bladed prop and a nicely sound insulated aft section of the hull would be the ultimate in noiseless mobo'ing.
I did some googling but there doesn't seem to be anything of use on the web.
 
Ok Here we go:
The less blades and the bigger blades and the slower the blade rotates the more efficient, hence big old lumps of super tanker who have one prop masive blades which turn at slow RPM. The problem with this is that you dont have much of a speed range. Hence cross channel ferries and alike have a proppellor which turns at constant revolutions and to vary the speed the blades are feathered ie variable pitch.
Your everyday run of the mill power boat builder has to produce a product that is equally at home on the rivers poodling about at 6 knots, but also can do 30 knots out in the oggin. So, a ballance is found, smallish multiple blade propellors which can handle a range of RPM from 350 - 3000 (depending on engine rpm and transmition ratio) .
I use an example if i may about 8 years ago building 2 identical boats same engines same layout same everything same underwater gear by same manufacturer same antifouling (same colour aswell) same (more or less) weight on completion 1 st boat, 3 bladed props, 31knots @ 2600rpm
boat number 2, 4 bladed props ,29.5 knots @2,600rpm
both props were matched
at slower speeds boat no 2's proppellors (4 bladed props) were considerably smoother that the one with 3 blades.

As for noise ie cavitation at high speeds most power boat blades cavitate to some extent you just have to go into the lazzerrette of aft cabin and you will hear it!
Simplified:
Basicaly the speed and angle at which the prop attacks the water makes a lower pressure at the back of the proppellor causing the water to actualy boil, this makes bubbles of air which you can hear! It sounds like ping pong balls bouncing on the underside of the hull, and if you look closely at a badly propped boat you can see evidence, pitting, on the blades or proppellor boss.

Submarines and Modern war ships and high performance craft have different shapes and multiple blades to compensate for this in fwd motion but these proppelors still cavitate in reverse direction!!
 
No- It\'s one!

Theoretically the most effecient is a one blade prop. However as is obvious, a tad impractible.

The development of that, is a 2 bladed surface drive prop eg racing power boats, where one blade is always out of the water, thus reducing drag in the water.
The big problem is always blade loading, ie the pressure at the blade tips as a result of the thrust.

For normal cruisers, the most effecient is a 3 bladed prop, where lossess due to friction are balanced by an acceptable loading on the blade tips. However losses are greatly increased as the blades begin to slip due to over-rotation, ie during acceleration. Can also result in cavitation, where negative pressure on the trailing surface, can erode the tips.

So to the 4 & 5 bladed props. These have greater thrust capability due to the fact that the blade loading is proportionally reduced, thus give greater thrust. Acceleration is higher, but so are the losses - due to friction. Vibration & ultimately prop 'noise' is reduced.

To summarise:-

3 blades for economic crusing

4 or more blades for quietness and acceleration

If you go to 2 x 3 bade counter rotating (Duoprop), then that is yet a different ball game
 
Re: No- It\'s one!

[ QUOTE ]

So to the 4 & 5 bladed props. These have greater thrust capability due to the fact that the blade loading is proportionally reduced, thus give greater thrust. Acceleration is higher, but so are the losses - due to friction. Vibration & ultimately prop 'noise' is reduced.


[/ QUOTE ]

So the 5 blade prop has the greatest thrust - do you mean the greatest capacity for thrust? In other words it needs more power to drive it?

What you have stated agrees with what others have told me - I am just trying to get my head around it.
 
[ QUOTE ]
If something is emitting noise it is expending energy in doing so, so less noise in theory equals a more efficient propeller.

[/ QUOTE ]

That is what i would have thought but the five blade prop seems to be less efficient not more.

[ QUOTE ]
"Perhaps an aquadrive, a 5 bladed prop and a nicely sound insulated aft section of the hull would be the ultimate in noiseless mobo'ing. "

[/ QUOTE ]

That is what I have - the trouble is that the three blade prop is doing over 24 knots and could be tweaked to probably give over 25 knots. The five blader before being de-pitched is doing 20 knots and only using about 80% of the power and is now away being tweaked. An advisor friend has told me it will never get to the 25 knots and he is expecting 22 knots.

So the five blade props seems to cost a lot more and seems to produce less!! I will know next week what the five blader does.
 
The way i look at it is that more blades= more working area to push water therefore capable of transmitting more thrust (similar to using long hub props on a diesel) BUT you loose some of this advantage because the blades then operate in disturbed water due to prop-slip (difference between prop pitch and actual distance travelled) and can also overload engines coz you dont get as much slip when accelerating (analogous to allowing the clutch to slip when accelerating a car from standstill in a high gear)
therefore fewr blades are theoretically more efficient but may not be able to deliver sufficient thrust without slipping.
Did lots of experiments when selecting the best setup when fitting diesels.
Call me for more discussions
 
Re: No- It\'s one!

Generaly 4 or 5 bladed props will have greater thrust for the same input, by virtue of the greater area in contact with the water. However, as I have already pointed out, the frictional losses are that much higher. Also, to achieve the same hull speed, the pitch of say a 5 blade, will be significantly less. ie a 27 inch pitch may well drop down to 24. The greater 'bite' the blades have in the water, will mean less power is wasted during acceleration. Frictional losses will be greater when cruising, compared with the 3 blade.
I went thro the same excercise some years ago with an ozzie firm called Austroprop. My props are 21 x 31 pitch (3 blade). Their estimation for a 4 blade was 21 x 27. The contours and balance of the props were supposedly so good, that there would have been little vibration.

So in the end, I decided the cost (£600/prop) of going to a 4 blade really wasn't worth it, just for the sake of inreased acceleration. A 5 blade was horrendously expensive.

A well balanced, well contoured 3 blade prop, at cruising speed, with no slippage or cavitation, is generally accepted as the best compromise.

Hope all this helps.

Stu
 
Re: No- It\'s one!

I am finding all the contributions helpful thank you all - and I think I am beginning to understand.

The five blade props cost a lot more than the three blade version - acceleration does not worry me - its a cruising boat ....... from what you are all saying the top speed will be down on the 5 blade prop and basically I am gaining reduced prop noise and for that paying a lot more plus losing out of efficiency.

I will see what happens with the trails next week and frankly would be amazed if the five blade prop gets over 24 knots. Apparantly it also has cupped blades and is made by 'Poseidon'. They are (762D x 880P x 5 @ 105% BAR.

She goes faster with the three blade 3-blade props (838D x 851P x 3 @
80% BAR)
 
Re: No- It\'s one!

I have just heard that they are not going to change the existing props but design and make new ones instead. So at least they are now making more sense.

I am going to get an agreed performance criteria spec before these are made. ... otherwise we can be in a time wasting phase.
 
Seven is considered a lucky number in many countries, but I would always go for an even number as it means the blades can wear evenly. Four is a square and best avoided, but six is a good number, being the product of two primes. It will also match the number of flats on the prop nut, in all probability (although I am unsure of the goodness of this).
 
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