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Old 23-03-2021, 14:48   #16
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Re: Propellor interference noise

Matt, do you have a more detailed photo of the prop and aperture?

A I correct in assuming the new Kiwi prop envelope takes up considerably more volume of the aperture than the prop the aperture was originally designed for? If so, it would be reasonable to expect some degree of degradation of prop operation. Whether that shows up as noise or vibration or cavitation or ... I am not qualified to say.

Depending on the observed changes, one has to either live with it or increase the size of the aperture. Again I could not say what the new size would be but my approach would to take the easier approach first. Make bigger what you can do without too much effort and suck it and see.
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Old 24-03-2021, 00:12   #17
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Re: Propellor interference noise

Quote:
Originally Posted by Wotname View Post
Matt, do you have a more detailed photo of the prop and aperture?

A I correct in assuming the new Kiwi prop envelope takes up considerably more volume of the aperture than the prop the aperture was originally designed for? If so, it would be reasonable to expect some degree of degradation of prop operation. Whether that shows up as noise or vibration or cavitation or ... I am not qualified to say.

Depending on the observed changes, one has to either live with it or increase the size of the aperture. Again I could not say what the new size would be but my approach would to take the easier approach first. Make bigger what you can do without too much effort and suck it and see.


The following sequence of photos give some idea of the situation, but you need to understand how the Kiwiprop works before panicking. The pictures show the blades in the feathered position, a state they are in only while sailing with the engine off.

When in gear and spinning, the blades rotate on their hubs and move well forward. So much so that the blade tips are much closer to the upper face of the hull aperture than the leading edge of the rudder.

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Old 24-03-2021, 00:35   #18
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Re: Propellor interference noise

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Originally Posted by GILow View Post
The following sequence of photos give some idea of the situation, but you need to understand how the Kiwiprop works before panicking. The pictures show the blades in the feathered position, a state they are in only while sailing with the engine off.

When in gear and spinning, the blades rotate on their hubs and move well forward. So much so that the blade tips are much closer to the upper face of the hull aperture than the leading edge of the rudder.

Attachment 235135Attachment 235136Attachment 235137
Disclaimer - I have zero qualifications to make the following suggestion but hey, I do have an opinion (mostly uninformed opinion).

Me, I would grind back the edges of the keel section of the aperture in order to remove the square face and make it more "round". Then I would enlarge the rudder side of the aperture by as much as practical say ~1.5 inches and also round off the edges.

But reread the disclaimer...
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Old 24-03-2021, 02:15   #19
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Re: Propellor interference noise

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Originally Posted by Wotname View Post
Disclaimer - I have zero qualifications to make the following suggestion but hey, I do have an opinion (mostly uninformed opinion).



Me, I would grind back the edges of the keel section of the aperture in order to remove the square face and make it more "round". Then I would enlarge the rudder side of the aperture by as much as practical say ~1.5 inches and also round off the edges.



But reread the disclaimer...


Well, grinding back the keel section would have the minor inconvenient side effect of causing the boat to sink.

Canít think of any other issues.
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Old 24-03-2021, 02:17   #20
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Re: Propellor interference noise

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Well, grinding back the keel section would have the minor inconvenient side effect of causing the boat to sink.

Canít think of any other issues.


Edit: removing more of the rudder may also have the minor side effect of causing it snap off. On the upside, if the boat has already sunk then the lack of steering is unlikely to be noticed.
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Old 24-03-2021, 02:36   #21
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Re: Propellor interference noise

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Well, grinding back the keel section would have the minor inconvenient side effect of causing the boat to sink.

Canít think of any other issues.
READ THE DISCLAIMER ()

However surely that section of the hull would be solid glass, no?

Grind it and see
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Old 24-03-2021, 04:29   #22
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Re: Propellor interference noise

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Well, grinding back the keel section would have the minor inconvenient side effect of causing the boat to sink.

Canít think of any other issues.
I'm confused... which for me is a normal state of affairs on the internet.
  • You don't want to change props,
  • You seem convinced that you are hurting something,
  • You don't want to modify the hull.

How exactly can we help?

It appears that your prop is painted. If you actually are having cavitation damage (which I doubt) it will be pretty clear on the blade ends, which will be cleanly stripped of paint on the forward side. Cavitation can't happen on the BACK side (high pressure) of the prop.
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Old 24-03-2021, 05:20   #23
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Re: Propellor interference noise

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Originally Posted by BillKny View Post
I'm confused... which for me is a normal state of affairs on the internet.
  • You don't want to change props,
  • You seem convinced that you are hurting something,
  • You don't want to modify the hull.

How exactly can we help?

It appears that your prop is painted. If you actually are having cavitation damage (which I doubt) it will be pretty clear on the blade ends, which will be cleanly stripped of paint on the forward side. Cavitation can't happen on the BACK side (high pressure) of the prop.

Maaate I think his prop blades are composite, hence the black colour but I can see your other points.
Sometimes ya just need to have a bitch about it.
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Old 24-03-2021, 05:43   #24
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Re: Propellor interference noise

What is the diameter of the propeller and what is the gear reduction ratio?
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Old 24-03-2021, 05:48   #25
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Re: Propellor interference noise

Bletso that is NOT a barn door ,if thatís on a 38fter itís probably less than 600mils cord should work verry well ,a 200ft square rigger had a full length rudder less than 5ft in cord check out the tp52s rudders a bit extreme to me .⛵️⚓️
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Old 24-03-2021, 06:12   #26
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Re: Propellor interference noise

Instead of grinding back (or smoothing) the keel to provide cleaner water flow, would it not be possible to build up a more tapered or rounded edge onto the back of the keel, so as to induce smoother water flow to the prop ???

My limited experience with these problems is that the sound is generally associated with cavitation, or the entrapment of bubbles on the prop blade...

If a prop is located behind a wide square edge, to my mind, this would induce a swirl of water, right behind the keel....drag a 6" wide plank thru' the water....face forward...and you will see what I mean....drag it fast enough, and you will see a void appear behind the blank...and air space of you will....a prop turning in this mess will not be a happy camper...

I think the solution lies in providing a cleaner water flow to the prop. You could form something up out of closed cell foam or similar material and glass or affix it to the rear of the keel at the prop aperture...
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Old 24-03-2021, 06:17   #27
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Re: Propellor interference noise

my english is terrible....sorry.....was trying to describe the turbulent water seen behind a a plank moved thru' the water..the prop has to grab ahold of this " turbulent" water to move forward, not good.....clean water flow is the happy medium.
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Old 24-03-2021, 07:56   #28
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Re: Propellor interference noise

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Originally Posted by MicHughV View Post
my english is terrible....sorry.....was trying to describe the turbulent water seen behind a a plank moved thru' the water..the prop has to grab ahold of this " turbulent" water to move forward, not good.....clean water flow is the happy medium.

Yes I think you are on the right track, that wide square face in front of the prop is guaranteed to produce turbulence AFAIK. Put it this way it doesnt much resemble the leading or trailing edges of the foils on an AC75.
But his problem is that he lacks room to make a taper it seems as the blades move a lot.
Guess strength was the desired design characteristic rather than low drag in the designers mind.
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Old 24-03-2021, 08:42   #29
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Re: Propellor interference noise

I did remember once seeing a guy building up a " fairing" in front of his prop behind the keel...it's something that has to be "shaped"...I seem to recall closed cell foam was used and it was shaped with a rasp...prior to the foam, some small holes were drilled into that flat surface behind the keel and some screws or bolts were epoxied into the holes...this gave the foam some " grip" to the back of the keel, the foam was also glued here....WEST maybe, I don't recall.....after shaping it was glassed....but I think a coat of WEST epoxy would likely also work.
My recollection is that it made a world of difference in performance and noise.
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Old 24-03-2021, 11:06   #30
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Re: Propellor interference noise

Quote:
Originally Posted by BillKny View Post
I'm confused... which for me is a normal state of affairs on the internet.


  • You don't want to change props,
  • You seem convinced that you are hurting something,
  • You don't want to modify the hull.



How exactly can we help?



It appears that your prop is painted. If you actually are having cavitation damage (which I doubt) it will be pretty clear on the blade ends, which will be cleanly stripped of paint on the forward side. Cavitation can't happen on the BACK side (high pressure) of the prop.


You are confused because, yet again, you are so busy criticising that you didnít actually read the question. You keep doing this when I post a question.

I was NOT convinced I was hurting anything. I ASKED if I was hurting anything so that I could decide how hard I should try to fix the issue.

Aside from boat pokerís very relevant post there is no reason to believe I am damaging anything. And, Iím not foolish enough to assume cavitation is the only issue, I also asked about drivetrain damage.

So, please, read the question I posted, donít answer the question I never asked.
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