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Old 05-08-2018, 09:33   #196
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Re: Multihull storm tactics?

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Originally Posted by mark_morwood View Post
... so with a drogue off the bows, even a long Jordan Series Drogue, you may be going 3 or 4 knots in reverse in storm seas, which is unlikely to end well.
But still significantly better case than without the drogue at all, going 10+ knots, or even 15...20 knots depending on the boat.

Please notice I never claimed it to better than a parachute from the bows.

And having any wave impact on the front of the boat is still much better than at the aft end on most (but not all) boats.
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Old 05-08-2018, 11:13   #197
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Re: Multihull storm tactics?

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The rudders are designed for a certain load, not for a certain speed. The rudders can develop more load when going forwards than backwards (if at the same speed), because the max lift coefficient is significantly higher when going forwards, just like most airfoils or hydrofoils. Therefore they can tolerate a little faster speed going backwards than going forwards (in order to have the same load). Since everything is symmetric structurally, the direction of drag force or lift force relative to the boat makes no difference whatsoever.

As long as we are not talking about storm or survival conditions, you might claim that you can limit lift coefficient to something less than max when going forwards, reducing the loads when going forwards, but that would no matter be possible when conditions really become tough, resulting different but still maximum lift coefficient for both directions at some point in time anyway.
I'm confused by this post.

You jumped right into hydrofoil lift, bypassing the much greater/sudden/dangerous forces that will bend/jam/snap-off a rudder going in REVERSE. You missed the simple fact that rudders have a forward pivot. Even "balanced" rudders are biased forward, with the rudder post within 10%-18% aft of the leading edge- certainly forward of the typical 25% CE. Among naval architects, any rudder angle beyond 35 degrees is even considered unsafe due to the possibility that even following seas could damage the rudder, trying to slam it beyond its stop and bending/breaking the post.

Reverse at significant speeds (backwards down a wave) is really really bad. The failure mode can involve a sudden enormous force, many times that of any hydrofoil effects. Imagine your car's hood/bonnet opening at highway speeds.
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Old 06-08-2018, 08:31   #198
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Re: Multihull storm tactics?

Nope, I canít see a yacht going 10 to 20 kts backwards as is suggested above.
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Old 06-08-2018, 08:46   #199
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Re: Multihull storm tactics?

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Nope, I canít see a yacht going 10 to 20 kts backwards as is suggested above.
Agree.
In simple terms:
A rudder is #2 most critical, after #1 hull integrity.
Rudders are designed to go forward.
Rudders WILL fail going backwards at speed.
You are risking your life believing otherwise.
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Old 06-08-2018, 09:47   #200
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Re: Multihull storm tactics?

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I'm confused by this post.

You jumped right into hydrofoil lift, bypassing the much greater/sudden/dangerous forces that will bend/jam/snap-off a rudder going in REVERSE. You missed the simple fact that rudders have a forward pivot. Even "balanced" rudders are biased forward, with the rudder post within 10%-18% aft of the leading edge- certainly forward of the typical 25% CE. Among naval architects, any rudder angle beyond 35 degrees is even considered unsafe due to the possibility that even following seas could damage the rudder, trying to slam it beyond its stop and bending/breaking the post.

Reverse at significant speeds (backwards down a wave) is really really bad. The failure mode can involve a sudden enormous force, many times that of any hydrofoil effects. Imagine your car's hood/bonnet opening at highway speeds.
There are only 2 significant external forces acting on the rudder. One caused by hydrodynamic effects, and the other by the bearings and steering system.
The hydrodynamic part can always, regardless of direction of motion, be separated into 2 components, lift taken perpendicular the motion relative to water, and drag in parallel direction. There is no additional
Quote:
greater/sudden/dangerous forces
as you put it.
The 2 significant external forces acting on the rudder can of course be great or sudden or even dangerous or some combination of those.

The leverage of lift and drag forces causing torque to the ruddershaft does change already when going forwards, 50% chord at stall angles. That is not any better or worse than the maximum torque while moving backwards. The center of effort is at the same location in both cases at 50% chord in maximum load case for torque, involving stalling.
The case when the flow is not stalling is not relevant, because it is not the maximum load case for torque, and thus does not determine the maximum speed the rudder can safely take.
In bending the longitudinal location of center of effort is irrelevant, the vertical or spanwise location determines the leverage, and the maximum bending moment happens just before stalling angle of attack while going in forwards. Going backwards it is less than that at the same speed, because the force is less at the same leverage.

The maximum combination of torque and bending is thus achieved when going forwards, not backwards, if speed is the same in both directions. the additional shear load, due to magnitude of the hydrodynamic force and cross sectional size of the rudder shaft, is such a low magnitude, that it makes no real difference to that conclusion, but it too is higher load while going forwards.
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Old 06-08-2018, 09:57   #201
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Re: Multihull storm tactics?

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Originally Posted by cyan View Post
Agree.
In simple terms:
A rudder is #2 most critical, after #1 hull integrity.
Agreed.
Quote:
Originally Posted by cyan View Post
Rudders are designed to go forward.
Rudders are hydrodynamically designed to go forward, not structurally.
Typically ruddershafts use circular section, having equal strength in all directions. The bearings are also symmetric.
Quote:
Originally Posted by cyan View Post
Rudders WILL fail going backwards at speed.
You are risking your life believing otherwise.
And you can break the ruddershaft at the same speed by going forward.
Rudders will be capable of taking far higher load than ruddershaft in most boats. Thus structural strength of the rudder blade is irrelevant in this context.
It does matter when you hit something solid with the blade. The easier it suffers the less load it puts into the shaft in such case.
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Old 06-08-2018, 10:04   #202
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Re: Multihull storm tactics?

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Originally Posted by DavefromNZ View Post
Nope, I canít see a yacht going 10 to 20 kts backwards as is suggested above.
That's not what I suggested.
What I do claim is that drogue can reduce speed by 80% or more.
20kts is only a mathematical result of that fact and what mark_morwood claimed for backward speeds when using a drogue.

20kts is therefore a result of claims by 2 different persons, not by any single individual.
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Old 06-08-2018, 10:18   #203
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Re: Multihull storm tactics?

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The failure mode can involve a sudden enormous force, many times that of any hydrofoil effects. Imagine your car's hood/bonnet opening at highway speeds.
Only when failure mode involves some part attached to the ruddershaft slamming against a stop, designed to limit rudder angle with no soft bushing used to absorb the energy.
That can happen while going forwards in real storm conditions.

There are no other sudden enormous forces than that, just the hydrodynamic force with lift and drag components. The rudder is already in the flow of water, there exists no equivalent case for car's hood/bonnet opening at highway speeds to consider. The large changes of angle of attack involving stalling will happen in storm seas no matter which direction the boat might be going. You are risking your life believing otherwise. It is unavoidable even with the parachute holding the boat in place from the bows using a proper bridle and long rode. Only in that case the speed of the water flow is much less and so is the load.
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Old 06-08-2018, 10:33   #204
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Re: Multihull storm tactics?

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Originally Posted by cyan View Post
Among naval architects, any rudder angle beyond 35 degrees is even considered unsafe due to the possibility that even following seas could damage the rudder, trying to slam it beyond its stop and bending/breaking the post.
Such stops are dangerous, not large rudder angles.
Please notice that angle of attack of the rudder blade is not the same thing as rudder angle. the difference can be very large even 45 degrees or more. Thus limiting the latter does not limit the former to prevent stalling or high lift coefficients just before stalling. The direction of water flow near the rudder shaft location is not just fore & aft, it has very significant lateral component due to sway and yawing motions in case of sailing multihulls in stormy seas. That is not the same for the big ships naval architects first consider due to significant increase in moment of inertia with size. Ships never experience rapid yawing, and typically swaying is also much less.
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Old 06-08-2018, 13:16   #205
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Re: Multihull storm tactics?

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Originally Posted by Just Another Sa View Post
Rudders are hydrodynamically designed to go forward, not structurally.
Typically ruddershafts use circular section, having equal strength in all directions. The bearings are also symmetric.
Um, no.
When you place the rudder post on the leading edge, the rudder system is STRUCTURALLY designed to go forward.
Please see attached image of a Lagoon 410 rudder.

Quote:
Originally Posted by Just Another Sa View Post
Only when failure mode involves some part attached to the ruddershaft slamming against a stop, designed to limit rudder angle with no soft bushing used to absorb the energy.
That's actually the point.

High forward boat speed will force the rudder to straight ahead. Zero degrees. High reverse boat speed will force the rudder onto a either port or starboard hard stop.
Please see the attached image showing steering quadrant stops for a Lagoon 400. (borrowed from arsenelupiga)

You don't need to do fancy lift/drag calculations to estimate those shaft-bending forces. What you DO need to know is the Young's modulus of your rudder post, the size and elasticity of those tiny rubber bumpers, and the equation for shock load on a single ended beam.

Hint:
The math doesn't end well for rudder posts which are designed to break before ripping up a hull.
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Old 07-08-2018, 09:06   #206
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Re: Multihull storm tactics?

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Originally Posted by cyan View Post
Um, no.
When you place the rudder post on the leading edge, the rudder system is STRUCTURALLY designed to go forward.
Please see attached image of a Lagoon 410 rudder.


That's actually the point.

High forward boat speed will force the rudder to straight ahead. Zero degrees. High reverse boat speed will force the rudder onto a either port or starboard hard stop.
Please see the attached image showing steering quadrant stops for a Lagoon 400. (borrowed from arsenelupiga)

You don't need to do fancy lift/drag calculations to estimate those shaft-bending forces. What you DO need to know is the Young's modulus of your rudder post, the size and elasticity of those tiny rubber bumpers, and the equation for shock load on a single ended beam.

Hint:
The math doesn't end well for rudder posts which are designed to break before ripping up a hull.
You still do not differentiate between rudder angle and angle of attack of the rudder blade.
High forward boat speed will try to force the rudder blade into zero angle of attack, which can result hitting the rudderstops hard if the boat has such.
There are steering moments to consider, which tend to prevent zero angle of attack from happening quite often. And there might also be bending which tend to partially jam the shaft in addition of those steering forces. Together they can result of angle of attack of more than 45 degrees in sea states related to storms. Even with rudderstops in place and moving forward with high rate of yawing.

You are correct that maximum bending moment on the shaft is typically much larger than torque. But both are still needed to determine max loading for the safe case with no stops involved. there is nothing fancy about lift & drag calculations and they are necessary to determine max bending loading for the ruddershaft.
There are carbon shafts designed and built successfully to break before ripping up a hull. I'm not aware of any metal ones doing the same, and doubt that they exist due the large plastic elongation of metals.

Your attached images do not show any structural unsymmetry for that shaft. Care to explain what you intended to show with them?
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Old 07-08-2018, 10:15   #207
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Re: Multihull storm tactics?

A simple test is to try motoring forward then at the same speed in reverse. With any type of balanced rudder you need to be holding onto the wheel tightly in reverse if done at reasonable speed.

Damage due to reverse flow is not unique to yachts. I always found a stall turn in a glider one of the scarier aerobatic manoeuvres to try and teach. If the rudder was kicked in too late and the glider started sliding backwards the control surfaces could be damaged by the reverse airflow, and this was in air, not water.
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Old 07-08-2018, 11:20   #208
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Re: Multihull storm tactics?

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You still do not differentiate between rudder angle and angle of attack of the rudder blade.
....
Your attached images do not show any structural unsymmetry for that shaft. Care to explain what you intended to show with them?
I intended to show that the pointy bit of the rudder sticks out away from the shaft. This is important.
Pointing this pointy bit into the direction of water flow is bad.

Have you ever finished snorkeling at the beach, removed your fins, and then casually pointed them into the direction of the incoming waves? They won't point that way for long before they are ripped to the opposite (stable) direction... in the direction of water flow.

I do understand that the angle of attack of a rudder combines the lee angle through a fluid and the actual rudder angle. I do understand that this lee angle can be very large in storm conditions. I do (intimately) understand that Daniel Bernoulli has some serious things to say about the fluid flow at its forces on foil surfaces.

NONE of that changes the fact that the type of rudder in the picture will slam to a violent stop when its boat slips in REVERSE down a large wave. Your suggestion that a rudder in reverse can be be stronger than the rudder going forward fails to account for this.

I really can't see why this is so difficult to understand.
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Old 07-08-2018, 15:30   #209
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Re: Multihull storm tactics?

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Originally Posted by cyan View Post
I intended to show that the pointy bit of the rudder sticks out away from the shaft. This is important.
Pointing this pointy bit into the direction of water flow is bad.

Have you ever finished snorkeling at the beach, removed your fins, and then casually pointed them into the direction of the incoming waves? They won't point that way for long before they are ripped to the opposite (stable) direction... in the direction of water flow.

I do understand that the angle of attack of a rudder combines the lee angle through a fluid and the actual rudder angle. I do understand that this lee angle can be very large in storm conditions. I do (intimately) understand that Daniel Bernoulli has some serious things to say about the fluid flow at its forces on foil surfaces.

NONE of that changes the fact that the type of rudder in the picture will slam to a violent stop when its boat slips in REVERSE down a large wave. Your suggestion that a rudder in reverse can be be stronger than the rudder going forward fails to account for this.

I really can't see why this is so difficult to understand.
There is nothing difficult about that at all. I don't know what gave you that idea. A rudder in reverse can be stronger than the rudder going forward at the same speed, provided there is no slamming against a stop, simply because there is no stop in every boat. The maximum coefficient of lift is higher for forward flow than in reverse, thus the forces experienced caused by the flow are also higher. It really is that simple as long as there is no impact load against a mechanical stop.
If the rudder system of your boat is so badly designed by having a hard stop causing such hard impacts, it of course changes everything. But it's not impossible to fix to be far less dangerous.
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Old 07-08-2018, 15:38   #210
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Re: Multihull storm tactics?

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A simple test is to try motoring forward then at the same speed in reverse. With any type of balanced rudder you need to be holding onto the wheel tightly in reverse if done at reasonable speed.
I don't think the rudder blade being balanced makes it any worse. If anything it means less holding force is needed.
You need to hold on the wheel with much higher force to keep it steady when going forwards with 45 degrees angle of attack on the blade. Thus the simple test you suggest has nothing to do with survival conditions, which this thread is about. It does not indicate how well your rudder system is capable of surviving those.
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