Props - how do they do that?

[jw.]

Originally Posted by Matt

jw. I've been thinking about how to demonstrate this in yer bathtub. I'll probably need some help to refine it, but like you I can't find a definitive article that explains it (all articles simply refer to cup and rake as statements of fact)....

Yeh, they do. Cup is easy cos I reckon it's equivalent to putting the flaps down a bit.

On a graph of force this would be x & y axis covered.
We want to look at the z-axis as well.

Making sense so far?

Yup.

[jw.]

Originally Posted by Matt

Not necessarily. Props are always very empirical.

That certainly seems to be the case for the lay folk but I find it hard to believe that, for instance, my duoprops were arrived at by bashing a piece of steel until they worked.

IMVHO, Blueprinting actually means is that all the blades follow exactly the same shape. Otherwise the slightly different shape of each blade means they'll all be counteracting each other (like a car with too much toe in), reducing efficiency.

Ok. That wasn't my understanding of blue printing, that was just part of the process.

[Matt]

I recall sitting in a lecture with the lecturer clearly stating that props are more art than science and that the large majority of data is empirically gathered.

The basic principals are understood pretty well, but since the environment they run in is almost impossible to model, the sums only get in the ballpark for prop choice.

They even got it wrong on HMS Ark Royal - she can't run at 100% power.

[Matt]

On the subject of your duoprops.

On Marc's V24, his fastest set of props have cavitation burns on the trailing prop, and I recall some discussion about the trailing prop being significantly cut down.

By minimising the power transmitted through the trailing prop, the tip losses are minimised.

It's an ongoing theory of "ours" that a V24 with no trailing prop at all should have a faster max speed - although handling would probably be compromised, so dunno whether it would be driveable or not.

[Jon Fuller]

Just out of interest, I'll take this opportunity to post a couple of pics from a new photographer on the block, the first showing a nice pair of V24 props, and the second, just a nice one for Scottyboy

[verytricky]

Originally Posted by Matt

On the subject of your duoprops.

On Marc's V24, his fastest set of props have cavitation burns on the trailing prop, and I recall some discussion about the trailing prop being significantly cut down.

By minimising the power transmitted through the trailing prop, the tip losses are minimised.

It's an ongoing theory of "ours" that a V24 with no trailing prop at all should have a faster max speed - although handling would probably be compromised, so dunno whether it would be driveable or not.

We have moved on slightly with this, and we do have a cut down trailing prop, but it 'scavenges' more than the previous sized prop, cutting down the tip loss but making up for it by moving more water away from the centre. There is also the matter of where you mount the duoprops, this being significant ( around 2% ). This is really the big issue with the duoprop - there is nothing like it elsewhere, and the lessons learned with a single prop can not be successfully applied to the duoprop.

For example - the calculations for pitch are out by 40% in the 'standard' setup.

Also, by playing with the props, you can get them going through the water faster than Volvo say you can. We currently exceed the max speed for the props by 20%, something Volvo said you could not do with the Duoprop. They were aware of a design limitation IRO speed when they built the system.

The stuff we are doing now goes against the view of our propellor guys, who say we are doing it wrong, but we are gaining speed all the time. The engine remains constant, as does the boat weight and drive system. But we have moved the flat out speed from our initial 74mph to 80mph over two years by following empirical data, not the scientific calculations. The quoted speeds were my first record attempt and my last record attempt - as such they are the accurately measured average speed over a set course, and are more accuratethan a brief look at the GPS. Max GPS readings at any one instant have been as high as 88mph. The engine revs were within 200 rpm at both attempts, so the increase is due mostly to prop design changes.

[Matt]

To understand rake:
Hold a thin sheet of plastic vertically.
Rotate it about its vertical axis to generate some pitch.
Rotate it about its horizontal axis to generate rake.
Now move it sideways through the a body of water.
Not only does it generate force forward as a reaction, but the rotation about the horizontal axis also means it also creates a downwards force too.

[Matt]

I'm pretty sceptical of stated pitch values since the cup of the prop has a pretty big impact on the measured pitch. So if you've been adding lots of cup, the core blade angle could be the same but the measured in water pitch could be loads higher.

Originally Posted by verytricky

For example - the calculations for pitch are out by 40% in the 'standard' setup.

[verytricky]

Cup screws up the performance quite a bit. We have been aiming to have a compound pitch, to create the most laminar flow off the blade trailing edge, to the point of removing more than an inch of pitch. Then the scavenging blade has a chance to work in cleaner water.

A biggie with the duaoprop is how you load the props onto the spindal - it is irrelevant on a single prop, it is important on a duoprop.

[Matt]

Have you looked at fore & aft positions of the props as well? (ie different thrust washer thickness or somethinglike that?)

[jw.]

Originally Posted by Matt

I'm pretty sceptical of stated pitch values since the cup of the prop has a pretty big impact on the measured pitch. So if you've been adding lots of cup, the core blade angle could be the same but the measured in water pitch could be loads higher.

Unless you consider the water flow on the back of the blade to be turbulent and the laminar flow to kinda bridge the turbulence, then I don't think cupping adds any pitch to the blade. It is here I think there is a confusion of how the prop is working in the water. This isn't helped by explanations often given using a fan as an analogy for prop function. I feel it's a kinda back to front way of explanation and leads to confusion. A prop isn't a fan, it only behaves similarly when the boat is stationary or at low speed.

A fan is stationary and moves a fluid through it. That's the last thing you want a prop to do; you want the fluid to be stationary and the blade to advance through it. Seems sorta similar doesn't it but let me give you a wee example of how I feel it sets faulty thinking.

Bear with me here. Lets say you've taken up canoeing and you're explaining to someone, who doesn't know anything about it, how you go about it. You reach the part about how you make the canoe move and you might say something like this, "You sit in the canoe and reach forward with one paddle blade. You dip it into the water and pull it back past your body then lift it out."

Does that sound reasonable? (Shit, I hope you answer yes to that bit. )

Now, try this as an alternative explanation, " You sit in the canoe and reach forward with one paddle blade. You dip it into the water and you pull the canoe past it."

A subtle difference but an entirely different notion.

I started this thread partly because I've been interested in this stuff for a while but it was brought to the fore by a few things recently. One was a conversation with a Volvo prop guru regarding the suitability of my E series props for the application I'm using them; the second was the results I found from modifying a set of E series props and thirdly a pal of mine bought the Poole Belle, (The Poole trip boat.) and it had odd props on it. He has tried a couple of combinations of alternatives but finally paid the money and got a company to design and make a pair of props. This they did but they never saw the boat and it was all done by calculation using the boat data. Well the result was spectacular for this vessel. A top speed in excess of the calculated design speed. Bow lift and vague steering, so they now have to fill a bow tank with many tons of water. All the sort of things we talk about with out boats so I reckoned someone has the knowledge. I guess I was wanting a quick fix rather than having to swither it all out but I'm beginning to see the influences the various feature may have. Of course, this way I'm just fkkn guestimating and could be entirely wrong.

A biggie with the duaoprop is how you load the props onto the spindal - it is irrelevant on a single prop, it is important on a duoprop.

Marc, would you care to clarify that, please?

[Matt]

The difference is only in the point of reference, but yeh, I take your point.

Originally Posted by jw.

A subtle difference bit an entirely different notion.

[jw.]

I guess what I'm getting at is that if you consider a prop to be a water mover, you might start to imagine such things as water bouncing off the blade at the reflected angle and all sorts of other misconceptions could enter the process.

[Matt]

In a perfect fluid (and therefore for calc purposes) the concept of mover is irrelevant, it's all about forces.

I guess we could go into aerodynamic theory as to how the forces are created etc which does involve motion of the fluid over the surface, but I reckon it complicates it more - at least as far as understanding cup & rake goes. I don't know much about aerodynamic theory at that level, mebbe blufin could chime in. I do reckon the tip profile could be in interesting development area (like winglets on modern aircraft for example).

What about forward swept blades to improve efficiency?

If you know the propshaft torque and the pitch of the prop, could you calculate the theoretical thrust off the prop?
Obviously there would need to be some assumptions (assume perfect fluid, 100% efficiency, etc) but i reckon it wouldn't be that difficult.

[Jon Fuller]

Originally Posted by Matt

If you know the propshaft torque and the pitch of the prop, could you calculate the theoretical thrust off the prop?
Obviously there would need to be some assumptions (assume perfect fluid, 100% efficiency, etc) but i reckon it wouldn't be that difficult.

I believe so, as I happen to know that Steve Baker phoned Bill Maloney a week or so ago to discuss this very subject, after he and I had been discussing my scrapheap challenger. It all sounded very complex to me.

[jw.]

Originally Posted by Matt

...I guess we could go into aerodynamic theory as to how the forces are created etc which does involve motion of the fluid over the surface, but I reckon it complicates it more - at least as far as understanding cup & rake goes.

Matt, we'll need to disagree a wee bit there. Let's take the case you mention of the effect of cupping. I find it easy enough to envisage the aerofoil (blade) flying in the water and then add the cup to the trailing edge. This increases the distance the water has to flow over the top surface compared to the lower surface. In turn, this will speed up the flow on the top surface and so reduce the pressure. The result is more lift without an increase in pitch. The centre of pressure will move rearwards which may or may not be significant in a particular instance.

I feel I'm beginning to get a handle on how various prop characteristics may come about. For example, if, in the situation above, the cupping was more pronounced towards the outer part of the blade only that area would have the increased lift and, referring to your explanation of rake and it's forces, it is easy to see that this will apply some of the extra lift as a downward force on the boat, further away from the pivot about which the boat rotates. If we accept that bow lift is actually downward pressure on the prop's lower blade then increasing the cup towards the outer edge would be more effective at producing bow lift than increasing it at only the inner part.

Because of centrifugal force and the friction of the water against the blade, I feel the actual flow of water (Now I'm doing it. ) over the blade will not be parallel to the propshaft but angled outward and rearward. Consequently, the aerofoil shape which we need to consider will be along the motion of the water. With regard to bow lift, if cupping is added around the blade tip it is effectively going to increase the lift of the aerofoil as previously mentioned but this time the centre of pressure moving back towards the tip will be significant because it's further out and can provide more leverage.

It's dangerous getting me thinkin'.

I do reckon the tip profile could be in interesting development area (like winglets on modern aircraft for example).

Yep, wing tips are always a problem and, if my reasoning is correct, especially so on a prop because of this centrifugal component. What you imagine as being the tip may actually be more of a trailing edge and the real tip is moved forward. My brain fekkin hurts.

What about forward swept blades to improve efficiency?

Dunno but you could have real stern lift.

If you know the propshaft torque and the pitch of the prop, could you calculate the theoretical thrust off the prop?

But there's physical strength of the blade to consider and the aerofoil section will need to take that into consideration. Also, I'd guess that sectional shape will have a bearing on propwalk and of course all the things we're talking about or the props could turn out to produce the thrust but be dogs to handle.

[Matt]

I don't think we're disagreeing at all - when you started talking about flow bounce and laminar flow etc you hit the limit of my understanding. I did some work on the effects of boundary layers and so forth, but mostly with effect to the surface finish versus friction. I understand that aerofoils work and what kinda flow shape there is over them etc, but in terms of the details as to why, I freely admit I dunno.

Also agree with you about the tip and being able to add effective rake by tweaking. I do think the terms cup & rake are misused a lot and that they are very linked.

Not completely convinced about the cenrifugal force of water flowing over the prop though - simply because the water's stationary and in theory remains reasonably so (compared to the prop it does, anyway). I'm sure there's some effect, but my feeling is that it's a minor factor. This is why I made a linear rake example up, to remove things I reckoned potentially misleading and certainly confusing.

[scottyboy]

Originally Posted by Jon Fuller

Just out of interest, I'll take this opportunity to post a couple of pics from a new photographer on the block, the first showing a nice pair of V24 props, and the second, just a nice one for Scottyboy

I have that very photo on my wall now jon! Lovely shot! I forgot to ask for his web address! Do you have it at all?

[scottyboy]

Just found it on the gallery section!

[robbie]

Struthe Thats a lot of stuff!!!!!!!!

I always thought it was to do with the compression of fluids.

As the prop blade moves down it is attempting to compress all the water below the boat. While the blade moving up is compressing the less dense water between it and the surface above. As the upwards motion of the prop blade meets far less resistance than the blade moving downwards the tendancy is to lift the stern. This force would increase with any degree of spooning the blades, and introduces the paddle wheel effect. To fit a smaller conta rotating prop in front of the main one helps to minimise these forces as the effect of compression now is working on each side of the prop at the same time, helping to cancel the stern lift and paddle wheel forces.

Robbies theory of energy transfer and adverse forces. (could be bolox version of something really clever though)