Dynamic balance for a folding prop?

[GILow]

Ben, how many rpm does your prop spin at?


Sent from my iPhone using Cruisers Sailing Forum

[Snowpetrel]

Originally Posted by GILow

Ben, how many rpm does your prop spin at?


Sent from my iPhone using Cruisers Sailing Forum

Short answer is I don't know, no tachometer yet... But engine revs out to 3600 max. I figure I'm getting somewhere around 3500 max, so my cruising RPM must be about 2500 at a guess. 2:1 (or thereabouts) box so it ok at 1200 RPM and gets a slight wobble somewhere above that. Got to get me a handheld tachometer.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[ggray]

Originally Posted by Snowpetrel

So a proper balancing has got to be my first step. And I can do that without even pulling the prop off. I'll just take off the blades and carefully weigh them. If they are out I can grind a bit of the heavy blade. Until they have the same weight and centre of balance.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

Easy enough to equalize the weight, but you rightfully mention COG, which is the important determinant, and harder to measure.

I've balanced wheels but not props, and while the theory is the same, it's a lot easier to spin a wheel to find the heavy side than it would be a prop.

But at the risk of insulting those who understand dynamic balancing (maybe that's all of you, so sorry!) I'll describe what it is. A prop can be in perfect static balance, but dynamically unbalanced if the COG of the blades are not exactly opposite each other along the axis of rotation. Centrifugal force will the equal, but not exactly opposite each other

So if there is not much depth to the prop along the axis of rotation, there is not as much likelihood there is a difference between static and dynamic imbalance. It was not until automobile wheels got wider than the old skinny wheels that dynamic balancing became important.

So with an ungeared folding prop, you really don't know where the COGs of each prop will be, along to the axis of rotation. A little difference in shape can cause one to be slightly forward of the other. I've not asked a prop shop, but I'd think it would take some very sophisticated equipment to ensure dynamic balance of a folding prop, like rotating the prop under load in the water?

One blade can have more "lift" than the other which would cause an off center force, thus vibration. I'll bet that's what it is, if your prop is truly statically balanced. You could try evening out the weights and shapes and see what happens.

I've got geared folding props, but the gears are a bit sloppy. And there is a bit of vibration at a certain RPM. But it's not too bad.

[Jim Cate]

Originally Posted by ggray

Easy enough to equalize the weight, but you rightfully mention COG, which is the important determinant, and harder to measure.

I've balanced wheels but not props, and while the theory is the same, it's a lot easier to spin a wheel to find the heavy side than it would be a prop.

But at the risk of insulting those who understand dynamic balancing (maybe that's all of you, so sorry!) I'll describe what it is. A prop can be in perfect static balance, but dynamically unbalanced if the COG of the blades are not exactly opposite each other along the axis of rotation. Centrifugal force will the equal, but not exactly opposite each other

So if there is not much depth to the prop along the axis of rotation, there is not as much likelihood there is a difference between static and dynamic imbalance. It was not until automobile wheels got wider than the old skinny wheels that dynamic balancing became important.

So with an ungeared folding prop, you really don't know where the COGs of each prop will be, along to the axis of rotation. A little difference in shape can cause one to be slightly forward of the other. I've not asked a prop shop, but I'd think it would take some very sophisticated equipment to ensure dynamic balance of a folding prop, like rotating the prop under load in the water?

One blade can have more "lift" than the other which would cause an off center force, thus vibration. I'll bet that's what it is, if your prop is truly statically balanced. You could try evening out the weights and shapes and see what happens.

I've got geared folding props, but the gears are a bit sloppy. And there is a bit of vibration at a certain RPM. But it's not too bad.

Good explanation, GG.

I dunno about Ben's prop, but our F-O-F has nylon or other hardish plastic pads that define the final position of the blades as they open. If there is any differential wear or damage to those pads, the blades would not be exactil co-planar, and that would upset the dynamic balance... just as you describe!

This seems a more likely source for the vibration than differences in the center of pressure from variance in the blade profile... although he did mention some enthusiastic cleaning by the PO!

Jim

[shakey doug]

Tasmanian Dynamic Balancing Hope St Launceston.
If the Prop has been made by a local eastern shore part time Prop maker, no 2 blades will be the same.

[Snowpetrel]

Thanks GGray, and shakey Doug, great posts that are really going to help me out.

I'm not sure what sort of ballancing goes into these props, they are fabricated locally by a couple of very clever chaps. But they are aimed at the racing market, and a little bit of vibration might be inherent in them. This one's old and has been hacked and fudged over the years, though it's done a few thousand odd hours on a nice catamaran, so can't have been too bad.

A friend has the same type of prop, with no vibration isues, though his old yanmar is quite noisy and lumpy, so it might be masking any small prop vibrations.

My 4 cylinder 30 hp nanni is very smooth, and the empty alloy hull might magnify any vibrations. And it's not really noticeable at 6 knots cruising RPM. Only when I crank up the revs to max for 7.5 knots does it become noticeable, but minor.

Anyway, Ggray, you've given me a good idea to test the blades shape and pitch. I'll clamp a reference plate onto the strut, and carefully measure both blades against it, before I remove them and weigh and ballance them myself. If they are out slightly I can get it as good as I can, and then try it again. If it is as perfect as I can get and I've still got vibration I'll give the Launceston place a buzz. Thanks for the heads up on that Shaky Doug.

By the way the old martec prop was way worse, so I'm half way there.

I'm hoping it's not just the vibration of the two bladed prop passing through the wake of the strut, or the pressure wave hitting the hull. Is there an easy way to tell what the vibration frequency is? In theory if it's an unbalanced prop or shaft issue it should vibrate at the prop rotation frequency? If it's the blades it should be twice the rotation frequency give the blades pass the hull/strut twice for each rotation.

I don't think it's resonance, but could it be over a larger rev range? Normally I thought resonance is only at or close to specific revs?

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[Snowpetrel]

Just tested the vibration frequency with an app for my android. I got around 3400 for the engine and around 1700ish around the prop area. So about half the engine RPM and with a 2:1 box that makes the vibration frequency at once per prop shaft revolution.

So if I believe the free app on a cheap phone then it points to the vibration being caused by one blade being heavy or off shape, rather than the blades passing the hull or skeg.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[Snowpetrel]

Originally Posted by shakey doug

Tasmanian Dynamic Balancing Hope St Launceston.
If the Prop has been made by a local eastern shore part time Prop maker, no 2 blades will be the same.

Just rung them, very helpful, and not too expensive depending on how long it takes them to fine tune the blades. About $150 plus Labour. They apparently do a lot of props, and can do the whole shaft as well. Thanks for the heads up.


Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[GILow]

Hmmm, at that price I will give them a call and see if they can do my old three bladed monster.

[Snowpetrel]

Maybe a simmiar outfit in Adelaide? Freights going to be the killer. Sounds like they do airplane props and all sorts of other things. So I would assume you have a similar place in your industrial estates?

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[GILow]

We do have Eagle Marine, and I have heard they are good from people I trust, but I just double checked and his quote was, reasonably enough, much more ambiguous. He made the valid point that it really depended on the nature/cause of the problem and therefore did not want to get into talking prices.

Fair enough I suppose.

Anyway, the simple answer to the freight problem for me is to sail to Tassie, using my newly (in my dreams) fitted Kiwiprop, and drop the old prop in by hand. But then I wouldn't really need to worry about the balance of the current prop because it would be my spare prop....

chicken/egg/chicken/egg/chicken/egg....

Matt

[Snowpetrel]

Good plan, then we can catch up for a beer, and swing by and give RC a hand with his scraping...

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[GILow]

Originally Posted by Snowpetrel

.... and give RC a hand with his scraping...

Oh dear, sounds like trouble? I might have missed that thread.

[GILow]

Oooh, nasty. I just found the thread.

[Snowpetrel]

Hopefully his boat will be nicely painted by the time you get here down this way, and he can relax and swap salty sea stories with us!

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app