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Old 12 November 2003, 09:11   #31
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Information on how a prop works..

Chapter 3 - how propellers work

Chapter 4 - propeller technology
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Old 12 November 2003, 09:23   #32
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Re: JW

Quote:
Originally posted by Steel Dev
JW

Do you have a boat or a plane?

I assume that your prop is fat and thick to go faster? This will not work. Propellers work in the water not the air. If you think a thick blade section will go faster explain why racing boats which want to go as fast as possible to win use props with very thin blade sections? In fact they buy a new propeller and then have the blades lab finished and the blade section thinned it to make it go faster!

Julie
I assume that your prop is fat and thick to go faster? No, and I did not suggest it.

Propellers work in the water not the air. Exactly he same principles apply.

In fact they buy a new propeller and then have the blades lab finished and the blade section thinned.....
Yep, I know this. And I know why.

I accept that there is a thrust component and I supect that this is, proportionally, at its greatest when the boat is still.
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Old 12 November 2003, 10:00   #33
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Do you know anything about it?
Shaun hi,

To get back to your original question yes we do know about this boat. Used to belong to a guy called John Deere. Assuming its the same one, orange with side by console, I think. Last saw your boat on a rib cruise to the IOM in 92. He towed a 6.5m Osprey through the night from Lancashire to get there after the Opsrey Falcon stuffed a wave and the electrics on the stern drive failed. At the time it was a huge rib, built like a battle ship and deffinately inspired confidence in all who sailed in her. John used to complain about stiff steering but that could be down to the twin engines and a stiff cable. She wasn' particularly quick perhaps 26 knots but would just keep going through anything.

We seemed to gain a lot of experts on this thread. Before you start playing with props you will need to know what are the max revs at wide open throttle (WOT). From this you can judge if the props are the correct size. Then start to think about other prop sizes and materials. S/S does improve performance but so does sticking a 150 Optimax on the back end. You are loosing 25 - 30% by having twin engines, offset by the safety they offer assuming separate fuel tanks etc. This has been discussed at length on the forum have a search.

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Old 12 November 2003, 10:05   #34
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Quote:
Originally posted by DJL
Information on how a prop works..

Chapter 3 - how propellers work

Chapter 4 - propeller technology
It looks to me that with a prop there are two main components of how it pushes a boat - the pitch of the blades, and the foil shape of the blade.

A prop with flat blades will work, but not very well (particularly at speed). Hence the need to refine the design with the aerofoil type cross section as mentioned by JW.

Looking at chapter 4 though, I don't understand their use of the term "angle of attack" for props (although I do understand it for an aircraft wing).

Surely the angle of attack of a prop blade is directly related to its pitch? A blade with zero angle of attack has zero pitch, and as the angle increases so does the pitch.

The pictures 4-26 and 4-27 seem to be the same to me!
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Old 12 November 2003, 10:13   #35
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Good links Daniel.

However, one link describes how the prop blades can be likened to an aeroplane wing and require an angle of attack to generate lift. The other description implies that this is not the case and that the prop works by diverting water backward.

Both these from Mercury Marine.

It is easy to see why there is confusion.
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Old 12 November 2003, 10:21   #36
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Quote:
Originally posted by John Kennett
Surely the angle of attack of a prop blade is directly related to its pitch
...and the speed of travel through the water (think about it - zero angle becomes 90 degree angle of attack once moving forward).
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Old 12 November 2003, 10:35   #37
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JK

This is the case only when the blades begin to turn and when the boat is standing still. As soon as the boat is moving forward, the angle of attack is reducing. The faster the boat goes, the more reduction in that angle.

The article describes how the wing works and you can see that, when stationary, the pitch equals the angle of attack and this is far too great for a prop blade to operate properly. This, of course, is why big pitches don't work well at low speed.

Now, this bit is only what I feel must be happening at low speeds and it is only based on deduction. The water at the front of the blade will be largely turbulent and this will be a low pressure area. The water at the back of the blade will be impinging on the blade face and be diverted back producing increased pressure on the blade so providing some thrust. The water captured within the blade area will probably be thrown outward under centrifugal force and so behave as a centrifugal fan. These forces combined are sufficient to produce forward motion. This begins to reduce the angle of attack so all these factors begin to reduce and the efficiency improves and off we go gaining speed.
If the prop has less pitch to begin with, it is a step ahead.

This is why the Torqueshift works so well and also why I am a genuine convert.


--------
Edit...JK can I ask a favour? Are you able to increase the time allowed to compose replies because I am often logged off the system before I am finished. I have to remember to cut the text and then go through a loop to repost before the next timeout.
Ta.
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Old 12 November 2003, 10:54   #38
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Sounds like you need to prepare your thesis off-line!
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Old 12 November 2003, 13:04   #39
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Prop blade thickness & Alum Vs Stainless Steel

Both the sites mentioned earlier are really good and give sound advice.

The original question pose was to give justification for blade thickness. The sites mentioned also states that a thinner blade section is more efficient. "Since there is only so much power available, blades should be as thin as practical (considering the strength of their material) because it takes more power to push a thick blade through the water than a thin blade." This is precisely why a stainless steel prop is usually more efficient than aluminium. Because the material is stronger the blade thickness can be made thinner. The only reason I have mentioned blade thickness was to put across to the average boat user that this is a consideration. A lot of customers tend to think that the reason to change to stainless steel is because it is stronger. Although this is true the main reason should really be performance advantages and when selecting a new propeller this needs to be carefully considered. A propeller is usually a one off purchase and it is best to make a good informed decision, props are not cheap and no one would want to keep buying props that are not best suited to the boat and set up.

JW.
You have certainly brought some good points forward an obviously know quite a lot about propellers but I will have to disagree with you that a propeller gives lift it gives thrust i.e. driving force. A propeller works by deflecting or accelerating water astern of it i.e. thrust not lift.
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Old 12 November 2003, 16:42   #40
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Quote:
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Sounds like you need to prepare your thesis off-line!
Perhaps they're just well considered rather then flippant, Richard. And that takes a bit of time.
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