QUESTION FOR OWNERS OF FULL KEEL, "BARN DOOR" RUDDER VESSELS

[VChild]

Thanks Benz. Sounds like 35 degrees might be that magic number.

[Benz]

Originally Posted by VChild

Thanks psk125. I do understand the physics and the forces involved. But as you state, there is a point of diminishing returns and that is what I am seeking. What is the ideal rudder angle beyond which it is hurting more than helping. That is all I am asking. So far I’ve heard another double ender has 35 degrees. Mine has less than 30 degrees and that is most likely due to an arbitrary placement of the hydraulic ram by a previous owner. I’m just trying to find that magic number. It seems like it would be more than 30 degrees. If I can at least get mine up to the ideal angle, I could potentially avoid more expensive options and more distant marinas. Moving the ram attachment farther out on the support beam would be quite easy but I wouldn't want to go beyond the ideal angle. I would like to hear from more double ender owners to see if they have 35 degrees or even more. I have a balanced rudder with the axis of the rudder post well behind the leading edge of the rudder. So, when I turn the rudder hard to port, for instance, the leading edge of the rudder actually crosses midline of the boat well onto the starboard side which, in theory, would help capture even more prop wash and direct it to the port side. I’m not sure how much effect that has due to the blunt taper of the leading edge.

Just to split hairs a little, a "barn door" rudder is usually taken to mean one that pivots on its leading edge, as opposed to a "balanced" rudder, which is what you seem to have. I don't know the physics of either, only know the different terms.
Also, it doesn't make any difference to maneuvering whether the boat is a "double ender" or has a canoe stern or a transom. What matters is what's underwater: the shape of the keel , the rudder attachment, and the propeller cutout. What happens above the waterline is simply a matter of aesthetics.
Your post might get more and better results if you asked it of more people than just the barn door crowd.

[bletso]

Barn door, did someone say barn door? Here's a real barn door.. Out of the water weighs a lot, but to pilot easy.

[hpeer]

Vchild,

Yes, we have had similar issues except we were on a canal that usually had a lot of current. So I’d be spinning the boat in her own length while being swept past the docks, then I’d have to motor back to dock. Low bridge and wires gave the situation some spice.

We “solved” the problem by retiring to the Caribbean and living in the hook. It’s a great solution, completely satisfied and had to make no modifications to the boat.

But seriously, if you are spinning in near to her own length I think that is about as good as You are going to get. The gains you get, if any, will be negligible.

Might be wiser to buy the Marina Manager a case of good scotch and negotiate an new slip location.

In any case good luck.

[VChild]

Good points Benz. When I purchased my boat, comments were made about the "barn door" rudder. I asked why it was called that and was told its because it is as big as a barn door. When I learned my rudder was balanced, I just assumed it was still a type of barn door rudder.
What you're saying makes more sense. Had I known that subtle difference, I might have worded the title differently to include other large rudder-full keel-aperture arrangements.
I believe I read somewhere that the difference in the physics between a regular rudder and a balanced rudder is that a balanced rudder assists with the steering effort like power steering. The leading edge, as it crosses slightly to the opposite side when the rudder is turned, feels the pressure of the water flow due to the boat's movement through the water. This pressure acts upon it to exert a force that would tend to increase its deflection which partially counteracts the much larger force on the large trailing part of the rudder which is deflected on the opposite side.
My boat definitely has effortless steering. But again I don't know if that rudder shape makes any difference during "back and fill."

[VChild]

[QUOTE=hpeer;3058609]Vchild,

"We “solved” the problem by retiring to the Caribbean and living in the hook. It’s a great solution, completely satisfied and had to make no modifications to the boat."


Best answer I've heard yet!
Now I just have to figure out the retiring part.

[Bill O]

Originally Posted by VChild

But again I don't know if that rudder shape makes any difference during "back and fill."

Rudder shape and lift considerations are important factors in rudder design. Did quite a bit of research before building our new rudder and a modified rudder shape may help in your situation.
Here's a link to a useful article by Dave Gerr. He basically created a high lift rudder w/a fishtail or Schilling type rudder design he called a "Thistle> design. See attached Bingham article for more info on a high lift Schilling rudder.

Don't know what type of foil shape is on your boat, but if its a NACA type foil you may be able to add on the fishtail to give you the extra lift.
Also a general article on ship rudder design that is useful to see the various rudder shapes.


Bill O.

[Boatyarddog]

Originally Posted by VChild

This picture may also help. Notice the window between the rudder and the aft end of the keel where the prop sits. Because of the closeness of the prop to the rudder, the rudder "feels" a dramatic effect from the prop wash, unlike boats where the prop is much farther from the rudder.

Prop aperture, acts a bit like a funnel.
Acts on the flow like a moderate push on the rudder when turned.
I've got plenty of control in reverse as long as I'm moving with enough speed to overcome prop walk.
SV Cloud Duster

[VChild]

Originally Posted by Bill O

Rudder shape and lift considerations are important factors in rudder design. Did quite a bit of research before building our new rudder and a modified rudder shape may help in your situation.
Here's a link to a useful article by Dave Gerr. He basically created a high lift rudder w/a fishtail or Schilling type rudder design he called a "Thistle> design. See attached Bingham article for more info on a high lift Schilling rudder.

Don't know what type of foil shape is on your boat, but if its a NACA type foil you may be able to add on the fishtail to give you the extra lift.
Also a general article on ship rudder design that is useful to see the various rudder shapes.


Bill O.

Wow! Truly fascinating articles. Thanks.
I will definitely be looking into a rudder modification like this.
Amazed at the extreme rudder angles which essentially make the system act like a very powerful stern thruster. Exactly what I want to accomplish.
Hopefully my current rudder can be modified to that design. It seems to be somewhat NACA like. Not sure where in the chord the stock is, percentage-wise. Otherwise may have to have one built from scratch.
Definitely going to research it. Thanks.

[boat driver]

Vchild-
Rudder angle max consensus is your preference. But as mentioned when [passing a certain angle the stall becomes more apparent. Becker rudders, designed shape rudders, trim tab style rudders are all attempts to do what you desire with various expense, complexity , and improvement.
The noted concern is the setting of your stops- Rudders have a higher risk of jam/malfunction and extreme overload if systems are taking the load rather than a fixed stop. This is very apparent when moving astern and your mention of slamming over. The simple fix is set the stop at the extreme of the auto pilot ability- how it is designed (chain, hardware,etc. ) is your choice.
To be clear- walking sideways utilizing the rudder hard over and forward washfrom the prop is not going to improve on a flat rudder from the present 30 degree setting.

[svfinlandia]

I find that other than the barn door rudder, the biggest help in the “back and fill” maneuver is a large prop and a big motor.
My Nauticat NC 36 does not back very well but does the back and fill maneuver well with her large prop and 85 hp Perkins.
Maybe you need a bigger engine. ��

Al, S/V Finlandia

[Cadence]

I would guess your barn door rudder has significant leading edge to allow good steerage
forward and under sail with a full keel boat. Every thing has it's trade offs.

[Mike Banks]

One might imagine the maximum angle of rudder would be 45 degrees, but in practice it is much less than this. I think what you have is about right. The idea of a rudder is to deflect the stern laterally, not to act as a brake. The bigger the rudder angle, the more it slows the vessel, and the slower the vessel, the less the effect of the rudder regardless of angle.

So--I would leave it alone, if it seems to be working OK. By a Barn Door rudder I take it to mean one hung abaft the keel itself with a cutaway in the keel heel--and maybe the rudder face too, to allow for a propeller.

[VChild]

Originally Posted by svfinlandia

I find that other than the barn door rudder, the biggest help in the “back and fill” maneuver is a large prop and a big motor.
My Nauticat NC 36 does not back very well but does the back and fill maneuver well with her large prop and 85 hp Perkins.
Maybe you need a bigger engine. ��

Al, S/V Finlandia

Thanks for your input. I don’t think generation of enough thrust is my issue, though. I have a brand new, powerful engine and a 24” diameter 4 blade MaxProp. It brings the vessel up to hull speed very quickly. It’s really about directing the thrust at a better angle. Because of poor planning during the installation by a previous owner, my autopilot currently cannot take full advantage of its useful range. I get less than 30 degrees max rudder angle on each side. The consensus seems to be 35 degrees before stalling for standard rudder types and much higher rudder angles for Schilling type rudders. In fact, in one of the articles posted above by Bill O about Schilling rudders, it states they can achieve angles as high as 75 degrees and produce a 90 degree thrust at the stern with 70% of main engine power. Pretty amazing!
So, although my boat can actually turn within its own boat length, given enough forward and reverse thrust cycles, I really need to do it faster, with fewer cycles because of the prevailing wind which is quickly blowing me into obstacles in my extremely tight marina. I need to whip the stern around with more lateral thrust (and less forward thrust) with each burst of thrust. I need to take full advantage of my autopilot’s achievable max rudder angle. If that achievable angle happens to be greater than 35 degrees, I would be very interested in looking into getting a Schilling type rudder.
Interestingly, a Schilling rudder in combination with a bow thruster would actually allow you to move directly sideways up to a dock, an amazing feat for a full keel vessel.

[fish53]

30 degrees is fine but 35 is better, over that the rudder stalls and actually has less effect.