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Old 22-03-2021, 17:45   #1
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Propellor interference noise

Hi all,

I have a traditional barn door style rudder on the back of a 3/4 keel boat. The prop aperture is a bit smaller than ideal and I get a little bit of propellor interference noise as a result.

Iím looking at my options on how to create more space, weighing up those options would be better if I knew whether interference noise is linked in any way to propellor or drive train damage.

Just to be clear, I have about a 10% blade diameter clearance at the moment, and this leads to a light ďchitteringĒ sound at anything above about 30% power from my Beta 50.

A smaller prop is not an option, so this is really about how hard I try to create more space. If there is no link with damage I am willing to put up with the noise because I very rarely motor anyway.

Matt
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Old 22-03-2021, 20:00   #2
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Re: Propellor interference noise

Quote:
Originally Posted by GILow View Post
I have a traditional barn door style rudder on the back of a 3/4 keel boat. The prop aperture is a bit smaller than ideal and I get a little bit of propellor interference noise as a result.
If you're talking about the rudder on your Swanson 42, and if that Swanson 42 were Swanson designed and Swanson built, then I suspect you are mis-understanding and mis-using the term 'barn door rudder'.

Barn door rudders are generally found on vessels designed for shoal draft and relatively slow speeds.

A barn door rudder is of quite low aspect ratio, meaning the rudder is long and does not extend deeply into the water.

Long, in the sense that the rudder is a respectable fraction of the waterline length of the hull (as in one-seventh or 1/7 in some cases). So if you had a 42 foot scow with a barn door rudder, the rudder might be 6 foot long!

Not deep, in that the rudder generally does not extend below the canoe hull of the vessel.

The design idea of a barn door rudder is that it does not compromise the shoal draft capabilities of the vessel, but provides lots of surface area. Quite a few functions for that surface area. Some scows with barn door rudders were made so the boat could be sculled by pumping the rudder. A handy way of getting through a tack with some rigs (e.g. catboats).

Take a Captain Cook at a dinkum barn door rudder: https://www.cruisersforum.com/forums...ml#post3285057
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Old 22-03-2021, 20:13   #3
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Propellor interference noise

Quote:
Originally Posted by Alan Mighty View Post
If you're talking about the rudder on your Swanson 42, and if that Swanson 42 were Swanson designed and Swanson built, then I suspect you are mis-understanding and mis-using the term 'barn door rudder'.

Barn door rudders are generally found on vessels designed for shoal draft and relatively slow speeds.

A barn door rudder is of quite low aspect ratio, meaning the rudder is long and does not extend deeply into the water.

Long, in the sense that the rudder is a respectable fraction of the waterline length of the hull (as in one-seventh or 1/7 in some cases).

Not deep, in that the rudder generally does not extend below the canoe hull of the vessel.

The design idea of a barn door rudder is that it does not compromise the shoal draft capabilities of the vessel, but provides lots of surface area.

Take a Captain Cook at a dinkum barn door rudder: https://www.cruisersforum.com/forums...ml#post3285057


Oh, right, I thought it applied to all those relatively deep chord rudders that swung from the back of a keel.

Good to know, but it doesnít really change the question.

For clarity, this is what I meant..,

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Old 22-03-2021, 20:16   #4
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Re: Propellor interference noise

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Originally Posted by GILow View Post
Hi all,
Iím looking at my options on how to create more space, weighing up those options would be better if I knew whether interference noise is linked in any way to propellor or drive train damage.

Matt
Matt,

There is no damage associate with small prop clearance. It's just noisy. Creating more space is a very complex job. Probably not worth the trouble.

In normal cruising people conversation a "barn door rudder" is a rudder hung directly on the back of the keel and extending all the way to the bottom of the keel. No matter what others might seem to think, it is not a derogatory term.
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Old 22-03-2021, 20:31   #5
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Re: Propellor interference noise

A barn door rudder is generally considered to be any rudder about twice the max cord in depth ,have seen many spade barn doors as people think that a large rudder makes the vsl easier to steer ,the opposite is actually true ,have cut the after one third off and re faired many so called barn doors ,with good results every time ,I suspect the noise apparent is due to poor fairing of the appature causing the prop ,which may not be suited to this disturbed water to vibrate hence noise ,a fixable problem ⛵️⚓️
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Old 22-03-2021, 20:37   #6
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Re: Propellor interference noise

I had a Swanson 42 many years ago, I changed from a two blade to 3 blade prop, this decreased the vibration under power a lot. The blades of the 2 blade pass through the "keel shadow" at the same time, causing the vibration. The 3 blade smoothed , but did not eliminate the vibration.
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Old 22-03-2021, 20:56   #7
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Re: Propellor interference noise

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Matt,

There is no damage associate with small prop clearance. It's just noisy.
The reason 15% prop clearance is preferred is to avoid blade tip cavitation and erosion which will thin the blade tips.
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Old 22-03-2021, 21:30   #8
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Re: Propellor interference noise

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The reason 15% prop clearance is preferred is to avoid blade tip cavitation and erosion which will thin the blade tips.

Yikes!

My clearance is a lot better than that setup, but that is a bit of a worry.
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Old 22-03-2021, 21:31   #9
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Re: Propellor interference noise

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I had a Swanson 42 many years ago, I changed from a two blade to 3 blade prop, this decreased the vibration under power a lot. The blades of the 2 blade pass through the "keel shadow" at the same time, causing the vibration. The 3 blade smoothed , but did not eliminate the vibration.

My three blade was ok, but this four blade, ironically, has a slightly greater diameter.

I donít THINK Iím getting a shadowing problem from the keel, but you are making me wonder.
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Old 22-03-2021, 21:32   #10
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Re: Propellor interference noise

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...,a fixable problem ⛵️⚓️

Really? How?
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Old 22-03-2021, 21:38   #11
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Re: Propellor interference noise

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Originally Posted by GILow View Post
My three blade was ok, but this four blade, ironically, has a slightly greater diameter.

I donít THINK Iím getting a shadowing problem from the keel, but you are making me wonder.
Matt, there must be some differences in turbulence and direction of flow in way of the keel vs elsewhere, and that just might be an issue. If, as you imply above, you have >15% tip clearance, that in itself should not be a noise source, but the alternating loading presented by the keel "shadow" just might be.

"Course, you could trade up to the five blade model and eliminate that issue!

Did you enjoy Rosemears?

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Old 22-03-2021, 22:58   #12
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Re: Propellor interference noise

Quote:
Originally Posted by GILow View Post
Hi all,

I have a traditional barn door style rudder on the back of a 3/4 keel boat. The prop aperture is a bit smaller than ideal and I get a little bit of propellor interference noise as a result.

Iím looking at my options on how to create more space, weighing up those options would be better if I knew whether interference noise is linked in any way to propellor or drive train damage.

Just to be clear, I have about a 10% blade diameter clearance at the moment, and this leads to a light ďchitteringĒ sound at anything above about 30% power from my Beta 50.

A smaller prop is not an option, so this is really about how hard I try to create more space. If there is no link with damage I am willing to put up with the noise because I very rarely motor anyway.

Matt

Just curious here. Why is a smaller prop not an option?
Have you talked to a prop specialist?. Maybe they can reduce dia slightly & increase pitch slightly. Following with interest.
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Old 22-03-2021, 23:03   #13
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Re: Propellor interference noise

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Originally Posted by Compass790 View Post
Just curious here. Why is a smaller prop not an option?
Have you talked to a prop specialist?. Maybe they can reduce dia slightly & increase pitch slightly. Following with interest.


I have a Kiwiprop. A truly excellent bit of kit, but short of cutting the blades down, I am stuck with this size to be able to transfer the power.
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Old 22-03-2021, 23:08   #14
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Re: Propellor interference noise

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Originally Posted by Jim Cate View Post
Matt, there must be some differences in turbulence and direction of flow in way of the keel vs elsewhere, and that just might be an issue. If, as you imply above, you have >15% tip clearance, that in itself should not be a noise source, but the alternating loading presented by the keel "shadow" just might be.



"Course, you could trade up to the five blade model and eliminate that issue!



Did you enjoy Rosemears?



Jim


Not sure where you got the 15%? I think clearance is closer to 10%, from the tip to the aperture.

The prop itself sits well back from the keel, much further than the previous fixed blade prop, thatís one of the challenges in fitting a Kiwiprop to this particular boat.

But Bruce makes a good point, the keel on this boat is monstrously wide and although there has been some attempt at fairing back to the aperture, it feels a bit half arsed to me.

However, at the end of the day, the nature of the noise to me makes me think it is some kind of tip interference with the aperture. No science in that, just a gut feel. Itís a very light, high pitched sound, ALMOST hinting at cavitation, but different again to that noise.

And tip noise interference would make sense when you look at how square and broad the aperture faces are. If there was more room I would certainly fill and round them. But thereís just no room to spare.
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Old 23-03-2021, 12:33   #15
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Re: Propellor interference noise

If this isn't a barn door, what is it?
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