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Prop Knocking


DShK

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Hi, me again with more questions! I tried to figure this one out but haven't been able to, so seeking the your collective wisdom once again. I've noticed that as I get up to cruising speed, I get a knocking sound coming from below my feet. (See video link below). I've had a friend helm and got below the deck and it's definitely coming from below the weed hatch. It's in time with how fast the prop is spinning, and at lower speeds its much less noticeable. It's generally loud enough that it makes me reluctant to get up to a good speed. I don't think it's as loud/problematic all the time though.

 

I've had a few theories as to what's going on:

 

1. My counter plate was sitting basically on the water when the bow water tank is full (see picture) - I wondered if this was causing ventilation or something. However, while it might make a bit of difference, experimentation makes me think this isn't the issue.

 

2. Prop/rudder fouled - this was of course my first port of call. I can't feel anything on either.

 

3. Bent prop - my prop is bent - this was highlighted in my survey - "distortions to the profile of a couple of the blades but nothing which should impair performance" I didn't question it at the time, but I don't take the surveyor's word as gospel.

 

4. Canal profile - I feel it's worse at times than others, so have wondered if sometimes it's the canal and not my boat. However it is always extremely pronounced when I knock the engine up for winding turns.

 

5. Stern gear/shaft issue - It's definitely not the gearbox as my gearbox is mounted next to my mid-engine. I have yet to see a single drop of water fall from my stern gear, my understanding is there should be the occasional drop? I don't have a bilge pump here, only a "catch pot", perhaps I should empty this and see if it remains dry.

 

Pertinen info:

 

Trad stern

Gardner 2LW

PRM 260 gearbox

Cardan shaft with UJs at each end and thrust bearing (no idea what this means but seems relevant!)

 

 

 

VIDEO

https://drive.google.com/file/d/1T_5SMVeWUURCrqzbUY7axnWuSAxF_A8w/view?usp=sharing

 

 

Any help is appreciated! Thanks!

photo_2022-06-13_11-19-38.jpg

msedge_rilKmGrHoI.png

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57 minutes ago, OldGoat said:

Methinks that's not 'knocking' more like a dry bearing somewhere in the chain??

 

48 minutes ago, Alan de Enfield said:

An out of balance (bent) prop will certainly put strange variable loadings onto the shaft and bearings, maybe it a bearing that is going / gone ?

 

Yeah I did wonder if maybe a bearing or something when considering a problem with the stern gear. Is there any way I can verify this? I'm assuming that if it's this, I should avoid moving the boat where possible? (need to move it about 6 hours back to the marina though).

 

Would the stern tube bearing be the most likely culprit? I assume that's an out of water job?

 

I assume also then it's probably a good idea to have the prop repaired sometime in the future? (it'd be best to time this with blacking I suppose? - which I'll do in a year)

 

42 minutes ago, Tracy D'arth said:

Any side or vertical play in the shaft bearing when you get hold of the prop?

 

Both the rudder and prop felt pretty secure, although I think I will have another root around in the weed hatch later just to double check everything.

Edited by DShK
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Look for fore and aft play at the prop, as well as port/starboard and up/down.

 

Check the Universal Joint on each end of the cardan shaft for play between the central spider and the two yokes.  There are usually needle roller bearings in the yokes and these may be expiring.  

 

Check the thrust bearing for play up/down and sideways.  Ball type thrust bearings do  ot last forever, especially if they are not lubricated.

 

N

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1 hour ago, DShK said:

when I knock the engine up for winding turns.

As an aside, why do you feel this is necessary? Unless winding against a strong wind you can do it all at or just over tickover. Speed (lack of) is the key.

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49 minutes ago, BEngo said:

Look for fore and aft play at the prop, as well as port/starboard and up/down.

 

Check the Universal Joint on each end of the cardan shaft for play between the central spider and the two yokes.  There are usually needle roller bearings in the yokes and these may be expiring.  

 

Check the thrust bearing for play up/down and sideways.  Ball type thrust bearings do  ot last forever, especially if they are not lubricated.

 

N

 

Thanks for the advice! This'll definitely help diagnose. A lot of new terms there for me so it will take a bit of figuring out on my end but should be no problem.

 

43 minutes ago, jpcdriver said:

As an aside, why do you feel this is necessary? Unless winding against a strong wind you can do it all at or just over tickover. Speed (lack of) is the key.

 

Hmm, well my method/understanding was to get a bit of backward momentum after the initital turn, and then use this with the rudder over quite far and the engine knocked up a bit - with the rudder over the forward vector is fairly low, counteracted by the rearward momentum - so really having the engine at high revs is just to make the most of the rearward movement. I haven't done a helmsman course or anything, just picking it up as I go - so I'm happy to hear better ways of doing things :)

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Ummmm, I sometimes think I might have this knocking, yet a new prop shaft was fitted three years ago, and I don't do very much mileage, I kinda thought it was related to depth of water. I haven't checked the weed hatch.

I also have singing prop at lowish speeds, there is a big notch in prop, I tried rasping it, to no effect. It's not a problem.

Edited by LadyG
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10 minutes ago, DShK said:

 

Thanks for the advice! This'll definitely help diagnose. A lot of new terms there for me so it will take a bit of figuring out on my end but should be no problem.

 

 

Hmm, well my method/understanding was to get a bit of backward momentum after the initital turn, and then use this with the rudder over quite far and the engine knocked up a bit - with the rudder over the forward vector is fairly low, counteracted by the rearward momentum - so really having the engine at high revs is just to make the most of the rearward movement. I haven't done a helmsman course or anything, just picking it up as I go - so I'm happy to hear better ways of doing things :)

The most effective way of winding a boat I have found is:

Bring the boat to a complete stop, three quarter rudder, a burst of power, full rudder, then tickover speed.

The boat should turn in it's own length,assuming no significant wind or anybody  watching.😃

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  • 3 weeks later...

An update on this because I am just as confused as I was before, hoping someone could make some recommendations on what to do next.

 

I've spoken to a few people on the cut about this. I've had people sure it's cavitation (they mean ventilation). I've also been told the area (GU between market harborough and welford) I'm in has been quite low on water and has been pretty silted up (after some reading, this explains my perceived lack of power even at higher revs). I was told it was unlikely to be the (cutlass bearing?) on the outside as I would have water gushing through the stern tube (I don't).

 

However, I've ballasted the rear of the boat with weights and tubs of water. Bow water tank about half full. It was firmly a couple of inches under the water. Noise remained. I tried revving in a filled lock, and although I didn't get much of a chance to do this, it definitely sounded like it remained.

 

To make it clear, the noise sounds like a combination of metal banging AND a watery glub glub glub sound. Always in time with the revs of the engine.

 

At this point I am wondering if my bent prop is actually causing cavitation?

 

It kind of puts me off cruising as it definitely sounds like something is wrong, and I worry in the back of your mind that it's causing secondary issues (wearing aware bearings, low fuel efficiency etc). I might just have to see if the nearby wharf can assist....

Edited by DShK
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How much does the boat draw, and what size is the prop? How much clearance is there between the blade tip (not the bent one) and the counter plate above, and between the blade tip and skeg?

 

If you are a deep drafted boat on a shallow canal, and feel that you are not proceeding at the expected speed, then ballasting the stern down will only make matters worse. Increasing the revs also makes matters worse, as it simply sucks the boat down towards the bottom, where the drag is more. Your best solution may be to keep the revs at little over tickover. Your speed over the ground will actually be quicker, you will produce less wash and damage to the banks, the engine will be quieter, produce less smoke and use less fuel. What's not to like? (A lesson I learned in a deep drafted boat on the Ashby Canal).

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Do you have an anti cavitation plate attached to the weed hatch? i.e is there a plate flush with the uxter plate above the prop? If so is it still firmly attached to the weed hatch?

Edited by Eeyore
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5 hours ago, David Mack said:

How much does the boat draw, and what size is the prop? How much clearance is there between the blade tip (not the bent one) and the counter plate above, and between the blade tip and skeg?

 

If you are a deep drafted boat on a shallow canal, and feel that you are not proceeding at the expected speed, then ballasting the stern down will only make matters worse. Increasing the revs also makes matters worse, as it simply sucks the boat down towards the bottom, where the drag is more. Your best solution may be to keep the revs at little over tickover. Your speed over the ground will actually be quicker, you will produce less wash and damage to the banks, the engine will be quieter, produce less smoke and use less fuel. What's not to like? (A lesson I learned in a deep drafted boat on the Ashby Canal).

 

I'm not sure on these, I will take a look. Good advice though, thank you. I think it needs a little ballasting though, as otherwise with the bow tank full, the counter plate is basically out of the water.

 

5 hours ago, Eeyore said:

Do you have an anti cavitation plate attached to the weed hatch? i.e is there a plate flush with the uxter plate above the prop? If so is it still firmly attached to the weed hatch?

 

I only have the one plate, which sits on the counter plate above the prop. I recently replaced the tape on it as it was letting in a fair amount of water.

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Prop cavitation is NOT ventilation .

 

Cavitation is localised boiling of the water making bubbles on the ends of the prop. This causes loss of thrust which will produce a knock if there is any fore and aft play in the prop shaft.

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15 minutes ago, Eeyore said:

Couple of generic images for you. The lower plate sits in the hole above the prop, preferably a snug and flush fit.

C09A9223-DEB9-44C5-B31B-25874F5FCA70.jpeg

717F0A63-67D0-4038-B8A0-1E7577335639.jpeg

 

I have the bottom plate that sits below the water, but not the top. It's held in place by a bar across the top which screws down onto a rod on the plate, pushing it down. Not sure why I don't have the top one, but also not sure why the top one is needed! Extra security I guess.

 

Just now, Tracy D'arth said:

Prop cavitation is NOT ventilation .

 

Cavitation is localised boiling of the water making bubbles on the ends of the prop. This causes loss of thrust which will produce a knock if there is any fore and aft play in the prop shaft.

Indeed. Incidentally Sarah at Norton Canes (it's a norton canes boat) reckons it's the bent prop, having straightened the very same prop previously. (Thanks Sarah!) Would this then actually be cavitation?

 

Guess I'll have the prop replaced, maybe when I next have it blacked, we'll see.

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1 hour ago, DShK said:

I'm not sure on these, I will take a look. Good advice though, thank you. I think it needs a little ballasting though, as otherwise with the bow tank full, the counter plate is basically out of the water.

Which is exactly how most ex-working boats are ballasted. At rest the counter plate is about an inch above the water. as soon as there is power it sucks down to just below the surface.

1 minute ago, DShK said:

Guess I'll have the prop replaced, maybe when I next have it blacked, we'll see.

Damaged props can be repaired for much less than the cost of replacement.

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Just now, David Mack said:

Which is exactly how most ex-working boats are ballasted. At rest the counter plate is about an inch above the water. as soon as there is power it sucks down to just below the surface.

Oh that is very interesting! I got the impression it could lead to air being sucked in. You certainly have to wade through propagated false info I feel. I wonder how that plays with corrosion, it's quite large surface to be continually exposed to both water and air. I believe my plate is quite thick though, so not particularly worried. If nothing else though it's a tad noisy in the wind with it out of the water!

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5 hours ago, Tracy D'arth said:

Prop cavitation is NOT ventilation .

 

Cavitation is localised boiling of the water making bubbles on the ends of the prop. This causes loss of thrust which will produce a knock if there is any fore and aft play in the prop shaft.

Interesting, so if the depth of water is minimal, as it is at the moment, does this produce cavitation at what would be normal running speeds in deeper water.

I know when there is more water in the canal my boat goes faster and more smoothly, I was thinking this was due to what I call hydraulics, ie the passage of the boat through the water in a restricted channel.

 

 

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7 hours ago, LadyG said:

Interesting, so if the depth of water is minimal, as it is at the moment, does this produce cavitation at what would be normal running speeds in deeper water.

I know when there is more water in the canal my boat goes faster and more smoothly, I was thinking this was due to what I call hydraulics, ie the passage of the boat through the water in a restricted channel.

 

 

Cavitation, in a canal boat is almost never going to happen.  There is not enough power, the blade loads are too low, and the water is too shallow.

 

Ventilation is common.

 

Swimming better in deep water is hydrodynamics at work.  In a canal most  water usually has to flow either side of the boat.  In deeper water it can take the easier route and go under the boat.  That is why sea going craft are shaped to help the water underneath , and canal craft are shaped to force it round the sides,  worsened by the need to build a rectangular box for (cargo/living) space reasons.

 

N

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