Size might matter

Can I increase my rudder size. Add 3 inches to trailing edge, without hurting steering system.

There is a whole science to rudder design, and some art, too. Adding 3" to the trailing edge will increase the force needed to turn the wheel, and may have other unanticipated impacts as well.

On a 38 ft'er we had a while back, I complained to the builder that the boat was sluggish to respond to the helm at slow speeds. He basically doubled the area of the rudders by welding on a reverse "L" on to the existing rudders. it eliminated the slow speed response issue. It did not have ant effect at higher speeds both the helm and autopilot operated normally...

I'd just add length.

But consider that the boat in question probably was designed for a given speed range and if your rudder is too small you're probably going too slow. Most trawlers are semi-displacement (SD) designs and have small rudders optimized for their higher speed capabilities.

But if you never go faster than displacement speeds a much larger rudder may work very well. If you have mechanical steering a bigger rudder may increase helm forces to undesirable levels. Every aspect of structural integrity should be considered.

The Shilling rudder and all other flap like things attached to the trailing edge can be very effective at low speeds but being in the prop wash must increase drag significantly. I've read otherwise but think extending the length of the rudder is preferable. At any rate welding something onto the TE can be easily cut off if it dos'nt work well. It's simple enough that trying both may be the best option.

Also if you add or otherwise change the design of your boat you should change it back when you sell the boat. Or at least tell the new owner all about the changes and that not being a NA there may be unforseen problems associated w the change or changes that you are not aware of. And whether you have or have not consulted a NA. Ideally this should be in writing.

Displacement-speed boats should have "barn door" sized rudders.

One is good... are two better?

While reading a Nordhavn owner's blog a while back I came across an interview they did with a fellow boat owner who owns a 77' Northern Marine. The following is what the Northern Marine owner had to say about his then new boat:

Starr is a 77ft long, 100T, single-screw boat. We use about 150HP from our 400HP Cummins 855. The boat is interesting, in that when we bought her we noticed that the rudder activity was just not right. I used to operate tugboats as a young man and I knew she didn’t feel normal. I talked to some naval architects and the guys we were talking to recommend that we go to twin rudders on the single screw. Starr has (2)-12sq ft rudders that are placed 34inches apart on a 50inch propeller. She now tracks like a slot car!

I'm curious if any TF members have experience with twin rudders on a single screw 'trawler' and if so what do you think about them?

A bigger rudder will mostly make up for a too small rudder that makes the boat hard to handle at docking manuvers.

Underway it will make little difference

We have an oversized rudder that was added to our boat by the prior owner. It is extended on both leading and trailing edges to keep it balanced. It does not seem to impact steering effort, but that is with hydraulic steering. We turn great at low speeds.

Ka_sea_ta said:

On a 38 ft'er we had a while back, I complained to the builder that the boat was sluggish to respond to the helm at slow speeds. He basically doubled the area of the rudders by welding on a reverse "L" on to the existing rudders. it eliminated the slow speed response issue. It did not have ant effect at higher speeds both the helm and autopilot operated normally...

Click to expand...

By reverse L, do you mean that, when applied to both sides of the rudder, it formed a wedge at the aft edge?

markpierce said:

Displacement-speed boats should have "barn door" sized rudders.

Click to expand...

A rudder should have a NACA profile. Your picture shows a sea brake.

markpierce said:

Displacement-speed boats should have "barn door" sized rudders.

Click to expand...

Not when they're designed correctly

Lost Horizons said:

A rudder should have a NACA profile. Your picture shows a sea brake.

Click to expand...

Similar to these posted above
Not sure if NACA, but some foil is better than no foil.

"A rudder should have a NACA profile. Your picture shows a sea brake."

Fine for a sailboat wher even a 1% increase in induced drag can loose a race.

BUT a flat plate has good lifting properties , even on aircraft flat wings are possible , tho less efficient at most angle of attack.

For a motor cruiser flat with a hole for shaft removal works just fine.

FF said:

"A rudder should have a NACA profile. Your picture shows a sea brake."

Fine for a sailboat wher even a 1% increase in induced drag can loose a race.

BUT a flat plate has good lifting properties , even on aircraft flat wings are possible , tho less efficient at most angle of attack.

For a motor cruiser flat with a hole for shaft removal works just fine.

Click to expand...

You are absolutely right. One can tow a bucket behind a power boat (which would provide additional resistance similar to a slabsided rudder) and compensate added resistance with a throttle. Can't do that with a sailboat.

Inclination Effects on Lift and Drag - Nasa

https://www.grc.nasa.gov/www/k-12/airplane/kiteincl.html


For any object, the lift and drag depend on the lift coefficient, Cl, and the drag ... For a thin flat plate at a low angle of attack the lift coefficient Clo is equal to 2.0 ...

Naca or flat...

FF said:

"A rudder should have a NACA profile. Your picture shows a sea brake."
................
................BUT a flat plate has good lifting properties , even on aircraft flat wings are possible , tho less efficient at most angle of attack.

For a motor cruiser flat with a hole for shaft removal works just fine.

Click to expand...

As a pilot for the past 37 years and a life long boater your comment, a flat rudder works just fine, is like saying it works okay so don't change a thing. If airplane designers and naval architects took this approach we'd still be in the Orville and Wilbur Wright days of airplane design and we'd be floating around in reed boats.

From what I understand, through my reading and research, is most naval architects would agree that a 'naca' style rudder is what works best. Considering all the time and money invested in designing and building boats I can't see why someone would use a flat rudder unless it was pure economics as in the case of Mark's rudder. Controlling a boat, especially at slow speeds, is critical if you think about it. Why wouldn't you want as much control as possible? Maybe it's just me but I would want as close to 100% control of my boat as I could get while docking next to some guys million dollar yacht and if a 'naca' style rudder gives me that additional 10% more control then, for me, it's the only way to go. Just Sayin'

FlyWright said:

By reverse L, do you mean that, when applied to both sides of the rudder, it formed a wedge at the aft edge?

Click to expand...

The "L" or Reverse "L" was due to a perspective I had on the rudder orientation in my mind as I tried to explain it. The rudder was supported only by the rudder post, no shoe. So the builder simply welded an "L" shaped piece on the trailing edge and bottom of the existing rudder. effectively doubling the size of the rudders ( it was a twin engine ) hope that clarifies it some...

" the 'naca' style rudder gives me that additional 10% more control then, for me, it's the only way to go. Just Sayin'"


At low speed manuvering there will be no difference in a flat or a fancy rudder , only the drag underway at different angles of helm .

There are far better rudders for low speed work , they are articulated , complex , expensive and require maint.

Aviation is driven by Profit , as any business should be.

Each new batch of aluminum overcast is required to be at least 15% better at fuel burn and lower maint costs.

Most rec. motor boats could not see the difference between a $500 rudder and a $2500 rudder in terms of lower drag ,better fuel burn in a century.

Articulating Rudder on a Mainship 34T - YouTube

▶ 1:22

Mar 11, 2012 - Uploaded by Dave Anderer
Bayview Engineering articulating rudder on a 2006 Mainship 34 Trawler. ... Bavaria yacht rudder bearing and ...


Articulating rudder 101 - Trawler Forum

www.trawlerforum.com › Trawler Forum › Maintenance and Systems › Power Systems




Sep 18, 2012 - 14 posts - ‎12 authors
I need a lesson on articulating rudders! I saw a boat for sale where the owner claims he spent 7k on it and it works better then a stern thruster??



Articulated Rudder

www.georgebuehler.com/Articulated%20Rudders.html




Ross can cut donuts with the boat, bringing it into very tight situations without problems. Of course he also knows what he's doing but just the same, this rudder ...



A low-tech option in the joystick era | Soundings Online

www.soundingsonline.com/component/.../222462-a-low-tech-option-in-the-joystick-e...




May 6, 2008 - The articulated rudder eases the challenge of maneuvering a trawler in ... The stern moves toward the low-pressure side, and the boat turns.

Size Matters and can be calculated, this example is for displacement yachts.

The formula is:

A=K x √ [ { D : (0,1 x L)³ } : {1400 x (v :√ g L) } ]

Where:

A=rudder area in m²
D=displacement in metric ton
L=length waterline in meters
g=acceleration of gravity 9.81 m/s²
v=maximum speed in m/s
K=correctionfactor taken from the grahics

Hi OP Arthur, this thread is getting interesting with lots of calculations and technical know-how. I'm an ex-aerospace engineer but let's just look at this from a practical point of view. So you have a steel flat plat rudder and you need better low speed maneuverability.

I would recommend three things that you can do shooting off-the-hip, and like Nomad said, if you don't like it then just bring out the cutting torch and cut it off, so consider:

1. weld the additional plate to the trailing edge, but if it gets too large then add a bit ahead to balance the rudder;
2. Weld some flat horizontal end plates on the top and bottom, which helps to diect the slip stream;
3. Weld a piece of angle at 45 degrees to the trailing edge, thus creating a fish tail.

Anyway, just my input for what it's worth.

Decisions to make the Rudder for M/Y "Molecule" wih the fore mentioned Area calculations.

The maximum Rudder height is limited to 610mm.

The ratio between the average height of a Rudder and the average length is called aspect ratio. A tall and narrow rudder is more effective than a low and long rudder. In general, it appears that because of the limited draft of yachts an aspect ratio of 1.29 to a maximum of 2 the best choice.

For a bit aspect ratio of the rudder, I opted for a length of 420mm. The ratio is then 610 : 488 = 1.25

Balance
The balance portion may not exceed 20% of the rudder surface, the pressure then remains positive.

The area of the rudder is (598 x 610)/100 = = 3648cm² == 0,36m²
Rudderarea is (488x610)/100 = 2977cm² = 0.2977 m²
The balance part is (110x610)/100 = 671cm² = 0.0671 m²
The balance portion is 19%

The client is very happy with his new rudder.