Thistle rudders

[eagle419]

Hey Benn,
I noticed Tidahapah's thistle rudder.
Is it to your design, and what difference do you reckon it makes? Even though Flemingo has a thruster (I've never used one) maybe it will be sufficient, perhaps I should also explore fitting a thistle, or an articulated rudder.

 

[Tidahapah]

Yes it is my own design.
When I was building Tidahapah I was at sea with Ampol as a Chief Engineer on their tankers and allso ashore as Superintendant Engineer for the fleet.
We had a schilling rudder on one of our product tankers and this was a minor adaption of their design. I had one of their engineers have a look at it whilst he was in Aus dioing an inspection for me on the tanker. His reccomendation was the under and over flow plates with the fishtale and it does give me better steerage when berthing etc. I have also increase my rudder angle to 40 deg port & stbd so this also helps.

If you have a stern, bow or both thruster all is good.
Me, I am single engine and no thrusters, I'm still getting better at it.
Articulated rudders are very good but also bring in another form of mechanics under water.
Cheers
Benn

 

[eagle419]

Yes it is my own design.
When I was building Tidahapah I was at sea with Ampol as a Chief Engineer on their tankers and allso ashore as Superintendant Engineer for the fleet.
We had a schilling rudder on one of our product tankers and this was a minor adaption of their design. I had one of their engineers have a look at it whilst he was in Aus dioing an inspection for me on the tanker. His reccomendation was the under and over flow plates with the fishtale and it does give me better steerage when berthing etc. I have also increase my rudder angle to 40 deg port & stbd so this also helps.

If you have a stern, bow or both thruster all is good.
Me, I am single engine and no thrusters, I'm still getting better at it.
Articulated rudders are very good but also bring in another form of mechanics under water.
Cheers
Benn

Thanks Benn,
I'll be spending a week on the boat with the Previous Owner learning about her (very for me) different characteristics.
I also noticed you use Propspeed. I've always used it, but my guys reckoned that Flemingo's lower revs would stop the coating action working properly - obviously not.

 

[Tidahapah]

No , best coating I have ever used usually get a couple of years out of a coating. never have to dive the boat before leaving for the north as the prop is usually clean.
Hauling Tidahapah at Lawries Marina Tuesday week. Should be out for a couple of days as I am just going to antifoul her and a couple of small touch ups on the gloss. International Copper Coat extra. been using this for tyhe last 5/6 years and so far has proved to be the best anti foul paint I have come accross for a timber hulled displacement cruiser.
Big gloss paint job comming up next year.

 

[Phil Fill]

I do not understand the advantage of Thistle rudder has over a standard rudder that is a long oval in shape? Most fiber glassed rudder have an oval shape, so is the Thistle just a name for steel oval rudders?

The advantage of the articulating and fish tail rudder is increasing the response and being able to thrust the stern more the side with less forward motion. How ever the concern is the rudder would be too responsive, so maintaining a straight course would be difficult requiring hunting or small corrections. The Eagle when under way, 6 to 9 knots, very small helm corrects are required. When on auto pilot the pump is constantly, every 15 to 30 seconds making small 1 to 2 second corrections just enough to bring the bow around.


The prop walk in reverse is one thing I would like to reduce so the boat can back up straight, and make close maneuvering easier to the prop walk side. The Eagle prop walk is to port so turning to port is difficult, and that is where the bow thruster is mostly used. Using the prop walk and forward thrusting the stern to port the Eagle can be turned 360 degree going into forward and reverse. I read that a articulating and fish tail rudder reduce the prop walk, plus being able to thrust the stern to the side reducing the need for a bow and/or stern thruster.

FF mentioned that some of the Navy launches had a cone on the rudder, which countered the reverse prop walk and I think fish tailed. If I modify the rudder it would be a fish tail on the back trailing edged?

 

[eagle419]

Hello Phil, The advantage of the thistle (a UK design term for the shape as viewed from above), together with ‘fences’ at top and bottom is, apparently, that the rudder is much more efficient at slow speeds. When combined with increasing rudder angle, that allows the prop flow to be diverted to almost past right angle to the keel. That, together with the always predictable prop-walk, that makes the boat far more controllable.
Reports I’ve read say that there is minimal effect on handling at cruising speeds, and variations on this theme are in use everywhere in working boats, as well as Benn’s Tidahapa.
As for prop-walk, I’m totally new to trawlers, but I’m used to relying on propwalk while manoeuvring alongside on my previous boat, a 46ft timber sloop, and couldn’t get by without it.
I plan to try Benn’s suggestion and adjust rudder stops to allow for 40 degrees of rudder and see how I go before thinking about a Thistle, or even an articulated rudder.
All will be revealed for me soon - I’ve got two week’s familiarisation with the new boat before passage back to Sydney.

 

[Tidahapah]

John,
You may find that with a fish tale type rudder that your prop walk effect may be minimised. So one can back up in a fairly straight line.
The underflow and overflow plates are very important aspect of the rudder as they direct the water flow accross the rudder surface .

You may find that the existing set up on Flemingo will be to your liking and not require any modification.

Well it looks like my haul out on Tuesday is off the boil as I have been callled over to Port Headland to my new job on the tugs.
Will have to haul out early Sept now .
Cheers
Benn

 

[Phil Fill]

The prop walk can be an advantage, but it can also be a disadvantage, depending on which way you are maneuvering/turning. It would be nice to counter the prop walk when needed. Not fun backing down a long narrow water way. The best way I have found so far is to use the bow thruster to thrust the bow to port so boat back to stern at an angle to starboard to counter the prop walk to port. The Y or fish tail on the trailing edge would reduce the prop walk and to thrust the stern to the side reducing the forward motion.

However, the top and bottom plates might be an advantage of funneling the water? I have not read/heard much about the advantage of top and bottom plates?

 

[Scary]

Top and bottom plates act as damns

The top and bottom plates keep water from escaping vertically off the rudder. On sail boats that use them they are called damns. On my daughters single I find I can back fairly straight by placing the rudder in full lock position opposite the direction of prop walk. In her case to port to counter her prop walk to starboard.

 

[Nomad Willy]

Yea Scary,
I do that w Willy but need 2 or 3 knots of sternway to make it work.
Re the 40 degree swing I use 45 and don't have any thoughts of needing to modify my rudder. No tricks needed.
Top and bottom plates also add strength.

 

[dpaws]

Schilling vs Thistle High Lift rudder profiles

An old thread, my apologies, but a useful update hopefully.

I've been chewing over both and trying to understand the difference. Came across the following paper which may be of interest:

It considers various chord ratios and trailing edge thicknesses, concluding that a medium chord ratio with a thin training edge provides the best hydrodynamic performance (high lift/low drag).

The Thistle profile would be approximate to Fishtail 11 in the paper. The results therein demonstrate that thinner foil with a small fishtail has the maximum lift to drag ratio performance as a rudder.

I have attached my own sketch showing the two. (Thistle profile in yellow) Following the advice in the paper for my 65ft narrowboat I will follow the modified HSVA MP73; the fishtail profile has been obtained by simply mirroring the final 10% of the chord.

 

[dpaws]

Various google results on the topic refer to an article by Gerr in Professional Boatbuilder magazine. Here is the link to the electronic back issue that is presently available to read free online:

https://pbbackissues.advanced-pub.com/?issueID=102&pageID=63

The reason for my choosing the fatter Schilling profile over the Thistle is an increase in effective rudder angle, from 40° typical for a Thistle to 70° for the bulbous nose Schilling, the penalty being increased drag. My application is for a narrowboat with a steering tiller; hanging off the stern with arm extended is common practice on the UK's narrow canals with the challenging sharp turns into junctions.

https://youtu.be/ziNRSQFHNJI

Thistle rudder profiles seem to be a more common choice for sailing craft for better maneouvering within marinas.

Hopefully someone will find these thouights to be of use. Happy sailing!

 

[dpaws]

Schilling rudder for a 65ft narrowboat

An afternoon spent doodling with Rhino3D

 

[boatpoker]

Our boat was extremely difficult to turn a docking speeds. The designer specified a balanced rudder but the builder didn't build it that way.

We installed ss plate, 15 degree fishtails and the difference is amazing, very
pleased with them.

We tried the fishtails first because of simplicity,ease of installation (1/2hr. install) and no welding required as opposed to adding balance which would have been more time consuming and expensive

 

[Kit_L]

I am reposting here, to add my take on a pseudo-Schilling rudder modification that has transformed Seabiscuit's slow-speed handling. I show two versions.

There is perspective distortion in the image, but the 50mm (2") pieces of stainless angle attached to the rudder are perpendicular to the propellor (Seabiscuit's a sharpie, so shallow draft of 1.2m, so engine is installed at a considerable angle).

Drilling the holes in the rudder was the hardest part. Many small steps in drill size up to 10mm worked the best.

The first two images show Version I, a triangular closed bar, 50mm in depth, attached to the trailing edge of the rudder. This made turning to port easier, but did not help directionality astern.

I then wanted to see if it were possible to make a version II that not only improved slow speed turning ahead, but also astern. I do need seaway astern to be able to use port rudder to offset prop wash, but it does work better than version I in this regard.

Seabiscuit's rudder deflection angle is only about 30°, so the version II mod. is a vast improvement over no mod. On the new boat, I have increased the rudder deflection as the first step; those images can be seen in the thread, "The adventures of Anika J"