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Drilling holes through the rudder stock


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26 minutes ago, blackrose said:

Yes but you need to keep it perfectly square on or the hole may end up oval.  Wouldn't cost much to take rudder and stock and bolt to a machine shop, they would machine ream it accurately.

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10 minutes ago, bizzard said:

Yes but you need to keep it perfectly square on or the hole may end up oval.  Wouldn't cost much to take rudder and stock and bolt to a machine shop, they would machine ream it accurately.

I see, because the rudder is hollow you wouldn't want to drill more holes through it. Perhaps drill through and bolt through on one hole as accurately as you can, assembly on the ground keepin drill and or reamer as square and upright as possible, and retain pinch bolt as security on the other.

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3 hours ago, bizzard said:

Yes but you need to keep it perfectly square on or the hole may end up oval.  Wouldn't cost much to take rudder and stock and bolt to a machine shop, they would machine ream it accurately.

 

It might not cost much but it's a bit difficult to organise while I'm in a dry dock without a car. 

 

Perhaps I'll plan to get the car down there and cycle back to get the boat. 

Edited by blackrose
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On 19/06/2022 at 18:37, blackrose said:

Any tips on how to drill 2 x 12mm holes squarely through a 35mm rudder stock?

 

My boat is coming out of the water and as part of the work I'm going to be taking the rudder off and epoxying the rudder stock tube.

 

I'm going to put this new rudder on because I've had it laying about for the past 15 years and I think I may as well use it. It might be interesting to see if the profile works any better than the flat plate rudder that's on there now. 

 

The two bolts in the picture are pinch bolts that don't go all the way through. The idea was that I drill two recesses into the rudder stock for the bolts to clamp into, but it doesn't seem very secure to me so I want to go all the way through with A4 stainless bolts and use the welded on nuts + A4 nyloc nuts on the other side.

 

That's all fine but I'm not sure how to drill the holes through the rudder stock squarely to make sure they meet the holes in the rudder on the other side. I could use one of the pinch bolts to hold it in place and drill through the rudder stock and the other side of the rudder together but that would risk stripping the threads in the welded on M12 nuts. I suppose I could go through carefully with a narrower drill bit but just wondered if there was a better way?

 

 

IMG_20220619_181915.jpg

 

 

It will be better than a flat plate rudder, but it's a pretty thin section (looks like NACA005?) so really optimised for low drag at higher speeds not low-speed manoeuvrability. The pivot point also looks rather close to the front (small balance area), a profiled rudder like this needs more balance than a flat plate because the extra lift moves the centre of pressure backwards.

 

To really help with low-speed turning (trawlers, tugs, narrowboats) you need a much thicker profile, typically around 20% (maximum thickness divided by chord) -- this is the (partly-finished) Schilling rudder being built for my boat, 100mm thick with 500mm chord and 25% balance...

 

 

schilling.jpg

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In all honesty, unless your through bolts will crush the rudder onto the stock to clamp the two firmly together I'd stick to the pinch bolts which you know have worked fine since to bought your boat. Any loose fit anywhere on the through bolts, either in the stock or the holes either side of your rudder will introduce slop that even if it's tiny to begin with will get worse on an accelerating scale. Doing this job (through bolt) by hand, well, I think I'm pretty good but I wouldn't attempt it, it needs clamping and a pillar drill to drill through all parts in one go. If you try and use say those nuts as a guide you WILL end up with them being a loose fit.

Do you know how much clearance there is between your stock and the tube through the new rudder? Unless it's a very tight fit, which I doubt or you'd never get it off again, IMO the through bolt idea is more trouble than it's worth.

 

Love the new rudder BTW, I'd want to fit it too but it's a shame the stop part of it wasn't built in when it was fabricated.

 

I wonder if this would be a good time to re-think the whole Liverpool boats style steering. I believe you're having to go to all this trouble due to the impossibility of changing the top bearing without first removing the rudder? Madness! And all to save LB machining a couple of tapers. Talk about passing the buck to the customer...

 

This job is a piece of cake on mine. The rudder is welded to the stock, the stock has a taper on the top and a simple bolt clamping the tiller onto it.

upside down in cloud: replacing the rudder stock bearing on a narrowboat

Edited by Slow and Steady
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Other question about the new rudder would be - were the holes for the pinch bolts welded internally to the tube before the nuts were added? It would seem the logical thing to do to prevent the rudder filling with water at least up to the bolt level. If they were and you drill through the other side without that welding, you'll be letting water in. Apologies for the cruddy drawing.

image.png.e115277d5043074eafbff6ef8065fc48.png

Edited by Slow and Steady
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3 hours ago, IanD said:

 

It will be better than a flat plate rudder, but it's a pretty thin section (looks like NACA005?) so really optimised for low drag at higher speeds not low-speed manoeuvrability. The pivot point also looks rather close to the front (small balance area), a profiled rudder like this needs more balance than a flat plate because the extra lift moves the centre of pressure backwards.

 

To really help with low-speed turning (trawlers, tugs, narrowboats) you need a much thicker profile, typically around 20% (maximum thickness divided by chord) -- this is the (partly-finished) Schilling rudder being built for my boat, 100mm thick with 500mm chord and 25% balance...

 

 

schilling.jpg

 

Thanks. Actually I really have no problem with low speed maneuverability. I can turn my boat almost within its own length. This was without using the bow thruster. 

 

 

Edited by blackrose
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1 hour ago, Slow and Steady said:

In all honesty, unless your through bolts will crush the rudder onto the stock to clamp the two firmly together I'd stick to the pinch bolts which you know have worked fine since to bought your boat. Any loose fit anywhere on the through bolts, either in the stock or the holes either side of your rudder will introduce slop that even if it's tiny to begin with will get worse on an accelerating scale. Doing this job (through bolt) by hand, well, I think I'm pretty good but I wouldn't attempt it, it needs clamping and a pillar drill to drill through all parts in one go. If you try and use say those nuts as a guide you WILL end up with them being a loose fit.

Do you know how much clearance there is between your stock and the tube through the new rudder? Unless it's a very tight fit, which I doubt or you'd never get it off again, IMO the through bolt idea is more trouble than it's worth.

 

Love the new rudder BTW, I'd want to fit it too but it's a shame the stop part of it wasn't built in when it was fabricated.

 

I wonder if this would be a good time to re-think the whole Liverpool boats style steering. I believe you're having to go to all this trouble due to the impossibility of changing the top bearing without first removing the rudder? Madness! And all to save LB machining a couple of tapers. Talk about passing the buck to the customer...

 

This job is a piece of cake on mine. The rudder is welded to the stock, the stock has a taper on the top and a simple bolt clamping the tiller onto it.

upside down in cloud: replacing the rudder stock bearing on a narrowboat

 

Thanks for this, but I'm not going to all the trouble of removing the rudder stock just to change the bearing, I'm removing it primarily so that I can blast and epoxy the rudder stock tube. I think you'd have to do the same thing if you wanted to paint yours? I'm changing the bearing (and the rudder) because it seems like a good time to do it. 

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9 minutes ago, blackrose said:

 

Thanks. Actually I really have no problem with low speed maneuverability. I can turn my boat almost within its own length. This was without using the bow thruster. 

 

This

 

Not suggesting you had a problem, but there's no harm in making things better still... 😉

 

(a lot of the boats I've been on certainly couldn't turn like that, but some were better than others...)

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1 hour ago, Slow and Steady said:

Other question about the new rudder would be - were the holes for the pinch bolts welded internally to the tube before the nuts were added? It would seem the logical thing to do to prevent the rudder filling with water at least up to the bolt level. If they were and you drill through the other side without that welding, you'll be letting water in. Apologies for the cruddy drawing.

image.png.e115277d5043074eafbff6ef8065fc48.png

 

Yes they were welded internally, but I was thinking about drilling a hole into the top of the rudder and either pouring epoxy inside, sloshing it around and pouring it out again or just filling the rudder full of PU sealant. Just seems counter-intuitive to have a hollow rudder underwater.

 

By the way, I'm pretty sure the current LB arrangement is two pinch bolts and one through bolt. Perhaps I should do the same with the new rudder? Keep the pinch bolts that are there and just take the lot to a machine shop and get them to fit a through bolt. I appreciate everything that's been said about making a mess of it with a hand drill. 

  • Greenie 1
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5 minutes ago, IanD said:

 

Not suggesting you had a problem, but there's no harm in making things better still... 😉

 

(a lot of the boats I've been on certainly couldn't turn like that, but some were better than others...)

 

Yes I'm just saying that the current rudder blade/balance plate combination does a pretty good job in terms of manouvring a big boat. The new rudder is a copy of those proportions. 

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14 minutes ago, blackrose said:

 

Yes I'm just saying that the current rudder blade/balance plate combination does a pretty good job in terms of manouvring a big boat. The new rudder is a copy of those proportions. 

 

In which case it should indeed be better for manoeuvring, but might be heavier to steer because the balance is the same size (small!) as the flat plate one, it should be a bit bigger to keep the same tiller weight.

 

IIRC the extra lift from the aerofoil (hydrofoil?) shape moves the centre of pressure backwards compared to a flat plate -- not as much for a thin section like yours as a thicker one like mine, but probably still noticeable.

Edited by IanD
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7 hours ago, IanD said:

 

In which case it should indeed be better for manoeuvring, but might be heavier to steer because the balance is the same size (small!) as the flat plate one, it should be a bit bigger to keep the same tiller weight.

 

 

Is tiller weight an issue once it's in the water? I don't really understand what you mean about keeping the same tiller weight?

 

The new tiller is likely to be around the same weight as the old one as it's made from steel that's half the plate thickness. It's also full of air at the moment so there would be some buoyancy. I might just drill a hole and coat the inside with epoxy and then tap the hole and put a stainless screw in with some sealant. 

Edited by blackrose
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7 minutes ago, blackrose said:

 

Is tiller weight an issue once it's in the water? I don't really understand what you mean about keeping the same tiller weight?

 

The new tiller is likely to be around the same weight as the old one as it's made from steel that's half the plate thickness. It's also full of air at the moment so there would be some buoyancy. I might just drill a hole and coat the inside with epoxy and then tap the hole and put a stainless screw in with some sealant. 

I meant the amount of effort needed to move the tiller against the force of the prop wash, which I thought was called "weight" just like for steering on a car?

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17 minutes ago, IanD said:

I meant the amount of effort needed to move the tiller against the force of the prop wash, which I thought was called "weight" just like for steering on a car?

 

Ok I see, but I still don't understand why the new rudder should be any heavier to steer than the flat plate rudder? 

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Also there are many different types of rudder and I'm not sure that my balance plate is especially small as you say. If the balance plate is too close to the prop I imagine that can result in vibration and cavitation. 

 

I guess mine is a semi-balanced rudder. 

Types of Rudder

Balanced rudder

  • When 20% to 37% of the area is forward of the turning axis there is no torque on the rudder stock at certain angles.
  • At some angle of rudder, it is balanced. i.e., torque is zero, to keep rudder at that angle.
  • Axis of rotation lies between 0.2 L and 0.37 L.

Semi-balanced rudder

  • A rudder with a small part of its area, less than 20%, forward of the turning axis.
  • At no angle rudder is balanced.
  • Axis of rotation lies less than 0.2 L.

 

Anyway, all I know is that it turns the boat very easily at low engine revs. 

Edited by blackrose
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Have you taken into account that the plain part of any bolt is usually less than the nominal size and generally not round. A 12mm properly reamer hole will not give you a tight fit on a standard bolt. A hand drilled 12mm hole even less so.

Also unless any bolt is large enough to squeeze the tube in very tightly then fretting is going to take place.

A better option is to use tapered pins ( commonly used industrially) with a retaining thread on the small end or a retaining cap. Search 12mm din taper pins and reamers for options.

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1 hour ago, blackrose said:

 

Ok I see, but I still don't understand why the new rudder should be any heavier to steer than the flat plate rudder? 

 

I tried explaining this, let's have another go... 😉
 

The force needed to move the rudder when the tiller is pushed over depends on where the force of the prop wash flowing over it acts, and there is a point called the CoP (Centre of Pressure) where you can take all the force as acting -- and where this is depends on how the pressure is distributed along the length of the rudder, which depends on its shape. With higher lift rudders (e.g. NACA aerofoil shapes) the CoP tends to be further back than with flat plate rudders, and even more so with Schilling rudders -- and higher lift tends to also means more force is needed to turn the rudder.

 

So if you take a flat plate rudder of a given size which feels good to steer with and replace it with a shaped one the same size with the pivot in the same place (your plan) the result is likely to be heavier to steer (more force on the tiller needed) because the CoP will be further behind the pivot.

 

A fully balanced rudder (which minimises the force needed) is not a good idea with tiller steering since the CoP also moves with rudder angle, and this can lead to the rudder "flipping over" (away from straight ahead) at either small or large angles depending on how the CoP moves.

Edited by IanD
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3 hours ago, adrianh said:

Have you taken into account that the plain part of any bolt is usually less than the nominal size and generally not round. A 12mm properly reamer hole will not give you a tight fit on a standard bolt. A hand drilled 12mm hole even less so.

Also unless any bolt is large enough to squeeze the tube in very tightly then fretting is going to take place.

A better option is to use tapered pins ( commonly used industrially) with a retaining thread on the small end or a retaining cap. Search 12mm din taper pins and reamers for options.

 

Wow, I really don't know how my rudder has stayed on for the last 17 years? 

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2 hours ago, IanD said:

 

I tried explaining this, let's have another go... 😉
 

The force needed to move the rudder when the tiller is pushed over depends on where the force of the prop wash flowing over it acts, and there is a point called the CoP (Centre of Pressure) where you can take all the force as acting -- and where this is depends on how the pressure is distributed along the length of the rudder, which depends on its shape. With higher lift rudders (e.g. NACA aerofoil shapes) the CoP tends to be further back than with flat plate rudders, and even more so with Schilling rudders -- and higher lift tends to also means more force is needed to turn the rudder.

 

So if you take a flat plate rudder of a given size which feels good to steer with and replace it with a shaped one the same size with the pivot in the same place (your plan) the result is likely to be heavier to steer (more force on the tiller needed) because the CoP will be further behind the pivot.

 

A fully balanced rudder (which minimises the force needed) is not a good idea with tiller steering since the CoP also moves with rudder angle, and this can lead to the rudder "flipping over" (away from straight ahead) at either small or large angles depending on how the CoP moves.

 

Thanks. I read that there was actually no such thing as a fully balanced rudder. It will only ever be balanced at particular angles. Anyway, it will be interesting to see how the theory works in practice. My feeling is that at a maximum speed of 5 knots the effects that you're talking about won't be significant or perhaps even noticable on my boat. I'll report back once it's done. 

Edited by blackrose
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Air trapped inside hollow objects like Mikes rudder has nothng whatsoever to do with it's or giving it more buoyancy. It is purely it's displacement size and weight that determins it's level of buoyancy. If it's air was sucked out and it was put into a vacuum inside it wouldn't make any difference either as long as it retained the same overall size and weight.

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7 hours ago, blackrose said:

 

Thanks. I read that there was actually no such thing as a fully balanced rudder. It will only ever be balanced at particular angles. Anyway, it will be interesting to see how the theory works in practice. My feeling is that at a maximum speed of 5 knots the effects that you're talking about won't be significant or perhaps even noticable on my boat. I'll report back once it's done. 

 

That's what I meant when I said "the CoP moves with rudder angle" -- and also with prop rpm.

 

The difference is nothing to do with speed, it will happen when manoeuvring or cruising slowly at a few mph -- but given that your rudder has a relatively fine profile (5%?) the difference won't be as big as with mine (20%).

 

If you want to actually measure the before-and-after difference you need three spring balances, one attached to a rope holding the stern back to something solid (thrust), one attached to a rope running sideways (steering), and one to pull the tiller sideways (steering effort) -- this is how rudders are measured. With the engine rpm constant, measure the three forces (thrust, steering, tiller) vs. rudder angle. You should find the new one gives more sideways force (steering thrust) at a given angle than the flat plate, and carries on working to bigger angles, but might also be heavier to steer -- or maybe the tiller force needed for a given steering effect is the same but you don't have to push it over so far.

 

This would give you some actual measurements about how much better it works, as opposed to subjective "it feels better" which is always prone to expectation bias (result being what you expected).

 

I'm hoping to try this with Ricky when my boat is built, if there's another similar hybrid boat with a flat plate rudder in the water at the same time...

 

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Thanks, but I'm definitely not going to all the trouble of taking measurements by attaching spring balances to various ropes, I just don't have time for that, so it will have to be my subjective view. But I have no expectation bias. I'm quite prepared for it to be worse or to make no difference at all - anything is possible.

 

The best I can do objectively is get someone at the marina on the bank to take a video of the turning circle of my boat in each direction before and then after the rudder change. 

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  • 1 month later...

Just took the old rudder off this morning and offered up the new one. The new one doesn't have the slight angle where the rudder stock runs through it so consequently it hangs down a bit at the trailing edge but I don't think that would be a problem. 

 

The new rudder is 30mm longer than the old one. Most of that is a 20mm increase in the length of leading balance part which is now only 10mm from the end of the propellor nut. 

 

The old rudder is made from 10mm plate and I think the new one is 3mm on both sides so it's lighter and I guess not as strong.

 

So which would you put back on? The old one had to come off anyway as I'm going to blast and epoxy the tube that goes through my fuel tank.

 

The yard owner said if he was fitting the new rudder he wouldn't drill right through the rudder stock as it's already got 3 holes in it. He'd just countersink it and use 2 pinch bolts plus a bit of weld at the top. I could take the rudder and stock to a steel fabricator nearby if I want accurate holes drilled right through it, but since it's likely that bolts alone would become sloppy over time and it would still need a bit of weld around the top is there any point having through bolts? 

 

IMG_20220730_120355.jpg

 

IMG_20220730_120413.jpg

 

IMG_20220730_122032.jpg

Edited by blackrose
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