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Old 24-10-2020, 05:29   #1
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What is a boats max pitch angle relative to waterflow.

And don't say how long is a piece of string. 🤪 I know that is probably the right answer.

For sailing in conditions you would call only just acceptable. How many degrees of angle to the waterflow at the rudder do you think your reaching.
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Old 24-10-2020, 06:09   #2
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Re: What is a boats max pitch angle relative to waterflow.

From a physics standpoint, after 30 degrees of heel everything goes down hill in a hurry. The cosine of 30 degrees is about 0.85, so you're at 85% vertical component for things like your rudder moving you side to side rather than up and down, your sail having 85% "lift," and so forth. At 45 degrees it's dropped to cosine=0.52, so my bet is that after you pass 30 degrees changes to improve heel are in order.
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Old 24-10-2020, 10:18   #3
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Re: What is a boats max pitch angle relative to waterflow.

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Originally Posted by tkeithlu View Post
From a physics standpoint, after 30 degrees of heel everything goes down hill in a hurry. The cosine of 30 degrees is about 0.85, so you're at 85% vertical component for things like your rudder moving you side to side rather than up and down, your sail having 85% "lift," and so forth. At 45 degrees it's dropped to cosine=0.52, so my bet is that after you pass 30 degrees changes to improve heel are in order.
I hadn't really considered the loss of rudder effect from heel, that's a monohull thing �� but interesting, explains why some have two rudders at angles to upright at rest, I'll have to have a better look next time. I assume now they are assymetrical profiles so one can give a righting moment and the other steerage. but I'm after pitch of a boat.... the hobby horseing through the swell.

I'm looking at NACA foil profiles for T rudders and working out what would be a suitable stall angle to look for. My problem is if the boat is at 10 pitch up the rudders might be before and/or after the well of the swell or if the boat is pitching down the rudders can be either side of the crest. Does the correction of waterflow up and down from the rise of the swell have a significant effect on the relative angle of the foil in the waterflow.
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Old 24-10-2020, 11:12   #4
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Re: What is a boats max pitch angle relative to waterflow.

As to pitch affecting the steering capability of a spade or skeg mounted rudder, I don't see that there would be much if any difference of the horizontal force capabilities if the pitching rudder was momentarily skewed forward or swept aft, because the torque is on the shaft as applied to the bearings linked to the hull. Heeling is an entirely different issue.

By a T rudder are you implying a horizontal foil on the bottom of the rudder. A pitch of the horizontal foil will invoke either a lifting or a downward force inducing drag depending on its attack angle through the water. Not sure why you would desire such extra force on the extremely important device on a vessel. Seems like the last place I would desire to invoke extra stresses from a safety perspective, or to invoke an entanglement prone crossing.

Also, I can imagine a lot of seaweed catching on a T-shaped appendage. Please share an image or photo to clarify your intentions as to a seemingly unusual rudder configuration.

All the best.
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Old 24-10-2020, 17:08   #5
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Re: What is a boats max pitch angle relative to waterflow.

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Originally Posted by Montanan View Post
As to pitch affecting the steering capability of a spade or skeg mounted rudder, I don't see that there would be much if any difference of the horizontal force capabilities if the pitching rudder was momentarily skewed forward or swept aft, because the torque is on the shaft as applied to the bearings linked to the hull. Heeling is an entirely different issue.

By a T rudder are you implying a horizontal foil on the bottom of the rudder. A pitch of the horizontal foil will invoke either a lifting or a downward force inducing drag depending on its attack angle through the water. Not sure why you would desire such extra force on the extremely important device on a vessel. Seems like the last place I would desire to invoke extra stresses from a safety perspective, or to invoke an entanglement prone crossing.

Also, I can imagine a lot of seaweed catching on a T-shaped appendage. Please share an image or photo to clarify your intentions as to a seemingly unusual rudder configuration.

All the best.
We are off topic so I'll try to pull it back by explaining my purpose of why I'm asking about waterflow in waves.

It's a catamaran and I'm considering a T wing on the tip of the existing rudder for two reasons
  1. The rudder is already there, if I make the wing its own vertical member to support it 1m below the waterline I have produced more drag
  2. It will benefit the rudder by improving flow over the rudder tip

The objective of the wing on the rudder is to
  • increase speed at all points of sail
  • Improve tacking angles and speed upwind
  • Reduce pitch (hobby horseing)

To achieve this I am considering attaching a foil parallel to the waterflow. It may or may not happen but I am designing it now and if it looks good I'll make and fit them. It will have 0 Angle Of Attack (AOA) that is adjustable while sailing by a few degrees and be able to be locked in position or released.

In motion a boat will pitch. As the bow rises it will increase the AOA of the wing nd create a lifting force at the rudder this will counter the lift at the bow and flatten the boats pitching.

As the bow drops the the rudder will provide negative lift and create a countering load and again flatten the boats pitching.

Less pitching will increase the boats performance similar in some ways to a longer boat, provided the foils do not produce more drag than the pitching and sail efficiency losses. Done correctly this should provide all the target objectives, done poorly it will go slower. To be upfront, most attempts on cruising sized boats have failed, it seems to me because of drag, trying to incorporate lift with assymetrical profiles or build in a static AOA. I believe the losses in plowing deep into waves and lifting bows while altering AWA relative to the sails and shaking the wind out are expensive in regards to boat speed so I think there are potential gains to be made.

Therefore it is critically important to choose the wing profile with the least amount of drag while at the same time the lower drag profiles will stall at lower AOA and defeat the purpose.

It is relatively easy to see and measure the boat angle when pitching but what is not as obvious is water speed in a wave which will have an effect on the direction of waterflow across the foil (Apparent Water Angle as it would be) and therefore impact on which foil profile is the optimal choice.



P.S. entanglement is an issue, the rudders swing up so not a big deal to clear but still a pain and many boats have a lot more under the water to catch up than I do. It seems most Seaweed breaks off at around 10kn. If the rudder is engineered for the loads it is no more or less susceptible to damage than any other rudder, perhaps better equipped.
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Old 24-10-2020, 22:43   #6
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Re: What is a boats max pitch angle relative to waterflow.

If you wish to provide for a lifting foil on your stern, suggest you just place such on your hull directly. This is a well researched and commercially available attribute of ships and dinghies and outboard motorized vehicles. To put such significant effort onto your directional mechanism is an inappropriate force addition to your steering device. Separate that which is critical from that which is fanciful at the routine operating speed of a Condomoran cruiser.

If you are seeking a lifting foil configuration like the America's Cup vessels that is totally different from anything that the CruisersForum has association with. The rudder will be the least of the hull lifting surfaces that one would design towards.
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Old 24-10-2020, 23:24   #7
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Re: What is a boats max pitch angle relative to waterflow.

Quote:
Originally Posted by Montanan View Post
If you wish to provide for a lifting foil on your stern, suggest you just place such on your hull directly. This is a well researched and commercially available attribute of ships and dinghies and outboard motorized vehicles. To put such significant effort onto your directional mechanism is an inappropriate force addition to your steering device. Separate that which is critical from that which is fanciful at the routine operating speed of a Condomoran cruiser.

If you are seeking a lifting foil configuration like the America's Cup vessels that is totally different from anything that the CruisersForum has association with. The rudder will be the least of the hull lifting surfaces that one would design towards.
Sorry, I've explained as best I can what I am doing, I can't explain it better. I don't think you quite follow what I am doing. I thank you for your concern but it's not an idea I want to defend or ask for opinions on either. I'm happy to discuss the concept it if people are interested.

I posted in the hope someone understood water velocities and angles of rotation in swell and could shine some light on dynamic waterflow. I can find basic information but not documented data through testing.
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Old 25-10-2020, 08:30   #8
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Re: What is a boats max pitch angle relative to waterflow.

I do not think that a horizontal foil on the rudder will make much difference in most aspects. There is an enormous amount of force involved in pitching, Longitudinal GM is very large. If you want any effect, the foil would have to be quite large. This would put much strains on your rudder bearings in a direction they are not designed for.

If you just want a small foil to create a kind of wingtip effect, improving flow, the vertical force on the bearings also applies, but to a smaller degree. The foils will create extra drag, so speed will decrease, pointing ability will not be affected, but rudder efficiency will increase. It is more efficient just to make the rudder a little longer. For more speed, you can go to a thinner rudder profile, but that stalls out earlier.

It is the same for winged keels. You get more drag and less lift compared to a longer keel, but if there are draft restrictions, they are better than a short version.

For the same reason, winglets are used on planes. Airport fees are according to size categories, mainly wingspan. Therefore winglets. Longer wings are more efficient, but would make for higher fees.
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Old 25-10-2020, 09:00   #9
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Re: What is a boats max pitch angle relative to waterflow.

Quote:
Originally Posted by Dave_S View Post
We are off topic so I'll try to pull it back by explaining my purpose of why I'm asking about waterflow in waves.

It's a catamaran and I'm considering a T wing on the tip of the existing rudder for two reasons
  1. The rudder is already there, if I make the wing its own vertical member to support it 1m below the waterline I have produced more drag
  2. It will benefit the rudder by improving flow over the rudder tip

The objective of the wing on the rudder is to
  • increase speed at all points of sail
  • Improve tacking angles and speed upwind
  • Reduce pitch (hobby horseing)

To achieve this I am considering attaching a foil parallel to the waterflow. It may or may not happen but I am designing it now and if it looks good I'll make and fit them. It will have 0 Angle Of Attack (AOA) that is adjustable while sailing by a few degrees and be able to be locked in position or released.

In motion a boat will pitch. As the bow rises it will increase the AOA of the wing nd create a lifting force at the rudder this will counter the lift at the bow and flatten the boats pitching.

As the bow drops the the rudder will provide negative lift and create a countering load and again flatten the boats pitching.

Less pitching will increase the boats performance similar in some ways to a longer boat, provided the foils do not produce more drag than the pitching and sail efficiency losses. Done correctly this should provide all the target objectives, done poorly it will go slower. To be upfront, most attempts on cruising sized boats have failed, it seems to me because of drag, trying to incorporate lift with assymetrical profiles or build in a static AOA. I believe the losses in plowing deep into waves and lifting bows while altering AWA relative to the sails and shaking the wind out are expensive in regards to boat speed so I think there are potential gains to be made.

Therefore it is critically important to choose the wing profile with the least amount of drag while at the same time the lower drag profiles will stall at lower AOA and defeat the purpose.

It is relatively easy to see and measure the boat angle when pitching but what is not as obvious is water speed in a wave which will have an effect on the direction of waterflow across the foil (Apparent Water Angle as it would be) and therefore impact on which foil profile is the optimal choice.



P.S. entanglement is an issue, the rudders swing up so not a big deal to clear but still a pain and many boats have a lot more under the water to catch up than I do. It seems most Seaweed breaks off at around 10kn. If the rudder is engineered for the loads it is no more or less susceptible to damage than any other rudder, perhaps better equipped.
Sounds to me that what you are advocating is an end plate which is normally used to counteract end wise flow on aircraft aerofoils to improve lift and reduce drag(winglets are more advanced versions of the same thing.) I doubt there is much span wise flow on a boats rudder except at large angles of heel or heavy weather helm with a significant amount of rudder angle which is drag inducing. The ideal is to not need much rudder at all to keep the boat sailing straight in an state of equilibrium but of course waves will knock the boat off course and require momentary correction, hence the reason for constant small rudder corrections. If you are constantly holding a lot of rudder correction on (weather helm), you need to adjust your sail plan to reduce it to a minimum so only needing 1 or less of rudder correction in steady state conditions. This is significantly less than the 5~7 pitch an aircraft might have to achieve lift. Whether an end plate would make a rudder more effective I doubt and as some one else said the added likelihood of fouling would be a big consideration.
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Old 25-10-2020, 09:16   #11
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Re: What is a boats max pitch angle relative to waterflow.

Hull vane.

https://www.hullvane.com/papers/
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Old 25-10-2020, 09:45   #12
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Re: What is a boats max pitch angle relative to waterflow.

Dave_S:

Please explain in terms of hydrodynamics/aerodynamics why you think a tip-plate on a ruder would improve boat performance and attain the goals you set out here:

increase speed at all points of sail
Improve tacking angles and speed upwind
Reduce pitch (hobby horseing)

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Old 25-10-2020, 11:26   #13
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Re: What is a boats max pitch angle relative to waterflow.

The OP suggests 2 purposes for adding a 'T' to the bottom of his rudder. 1st is to increase efficiency like winglets on an airplane. 2nd is to reduce pitching. This is for an upper 40ft Schionning performance cruising cat.

1st, winglets on an airplane are not at a zero angle of attack. They work because the vortex is always rotating from below the wing toward the top creating a cross flow from the tip toward the fuselage on the upper surface. Using this cross flow, the winglet is angled (towed out) to create lift in the forward direction to increase wing efficiency. It is NOT there to 'stop' the vortex and create a higher pressure difference at the tip of the wing! Since a rudder is creating lift on either side of the surface, this winglet design is not possible. Additionally, your rudder should be creating very little lift when sailing in a straight line meaning there is Normally No Vortex and a horizontal foil would just add extra drag. T foils are very draggy at the root of the 3 foils. As stated before, making it a longer more slender foil (higher aspect) is a much better way to get a more efficient rudder. A T will add drag and slow you down at all points of sail.

While it is possible that a horizontal wing added to the bottom of your rudders may reduce pitching, the horizontal wing would have to be very large on your Schionning (about equal size to the rudders themselves I'd guess) and need to create hundreds of pounds of lift to have any noticeable affect. As said before, your existing rudder, post, bearings, etc. are not designed for vertical loading. You will need to redesign the shaft, bearings and rudder in their entirety. Note, this may make the boat feel more sea kindly, but will most certainly slow the boat down.

Since you know the designer of your boat, why not run the question by Schionning?
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Old 25-10-2020, 11:36   #14
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Re: What is a boats max pitch angle relative to waterflow.

About this:
I hadn't really considered the loss of rudder effect from heel, that's a monohull thing �� but interesting, explains why some have two rudders at angles to upright at rest, I'll have to have a better look next time. I assume now they are assymetrical profiles so one can give a righting moment and the other steerage.
Actually, that isnt it. The reason some monohulls have 2 rudders is because their transoms are so wide that when the boat is heeled over, the bow points down into the water & a normal, centrally mounted rudder would be lifted so far out of the water that it loses grip & the helmsman loses control of the boat as it rounds up into wind. So now they have a rudder each side of the hull so at least one is deep enough in the water at all times. The logical consequence of this is to have 2 wheels so the helm can see where he is going. This is all fundamentally unseaworthy compared to an older design boat which has a balanced hull, but seaworthiness is not the primary design objective here, which would instead be increased speed on a run or more accomodation below.
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Old 25-10-2020, 11:59   #15
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Re: What is a boats max pitch angle relative to waterflow.

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Originally Posted by Dave_S View Post
Sorry, I've explained as best I can what I am doing, I can't explain it better. I don't think you quite follow what I am doing. I thank you for your concern but it's not an idea I want to defend or ask for opinions on either. I'm happy to discuss the concept it if people are interested.

I posted in the hope someone understood water velocities and angles of rotation in swell and could shine some light on dynamic waterflow. I can find basic information but not documented data through testing.
The thing I'd be much more concerned about than some transient damping effect - which would require sizable horiz. end-plate foil area and attendant rudder stresses - would be the accentuated lifting effect they would give in surfing down a high, steep swell where you remain pretty well parallel to the surface before reaching the bottom. Performance cats such as yours will be at higher risk for pitch-polling without this added stern lift - which quadruples for a doubling of speed that can easily happen.

Running before a gale once in 12-14' seas under Solent jib only in a 37' cruising cat, we would frequently hit 18kts down the face, and slow to 10 climbing the next...
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