Rudder Design

Good day to all of you. I am new to this forum and I hope I will be able to get the help that I need.

I have a 57ft wooden fishing trawler with LWL 0f 52ft under construction. I have got a point that I need to design and build a rudder which conforms to best practices in the business. The chord length is estimated to be 2.6ft. I have got three designs in mind. The NACA-0012 foil, Thistle rudder, or combination of the two. That is adopting the trailing end of the Thistle to the NACA foil.
I have read that Dave Geer wrote articles on rules and formula/calculations to determine the size and shape of rudders. I have checked the archives of proboat.com but that information is no longer available. In my part of the world sometimes it is very difficult to get technical/engineering information.

I would appreciate if someone out there can PM me with information on rudder design by Dave Geer.

Thank you for reading this post.

Edington

 

Hi Edington,
I warmly suggest you to buy "Boat Mechanical Systems Handbook" by D. Gerr. You can find it at a bargain price at Amazon.co.uk:
http://www.amazon.co.uk/gp/offer-li...5096&sr=8-1&keywords=dave gerr&condition=used
It covers a wide range of topics which you will encounter during the construction of your boat and I consider it an excellent investment.

Apart that book, you can find a lots of good info in this paper written by Eric Sponberg: http://www.sponbergyachtdesign.com/Keel and Rudder Design.pdf
Though it was written mostly for sailboats, the general principles are valid for motor boats too.

Regarding the hydraulics, a very good review of hydraulic steering systems - their choice, installation and maintainance - can be found in this manual bt Kobelt: http://www.kobelt.com/pdf/Manual - Steering Gear Selection and Installation.pdf

Cheers

 

Welcome to the forum, Edington.

Your new rudder should be application specific, meaning sized and shaped for the expectations of your vessel.

If really interested in Geer's work his books "Elements of Boat Strength" and to a lesser degree "The Nature of Boats" should be read. Most of the information you'll need is within. This said, you'll still need some understanding of what your vessel needs for the decision making process to select an appropriate set of sections and plan form.

Simply put, the Thistle rudder wouldn't be a reasonable choice on a "57ft wooden fishing trawler". If it's under construction, why aren't the plans being adhered to? What about the current rudder are you having difficulty with?

 

Hi daiquiri,

Thank you for replying to my query. I have looked up the book you suggested. For 16GBP is not bad. I will get a copy. I thank you for the other information as well.

Edington

 

Name is Dave Gerr I think.
A fishtail shaped rudder (Schilling, Thistle, etc) is a good idea.

 

Oops, Dave don't be pissed. It's Gerr, pronounced like Bear.

Optimize the rudder for your needs.

 

What is the rudder aspect ratio? How is it to be hung? is it to be balanced? Maybe post a sketch of the profile, prop location relative to the rudder, prop diameter and RPM and then you can be given some help.

A lot of ideal rudder theory for calm water and ideal flow that suit other types of craft just don't translate to HD fishing boats and you can spend a lot of expense and effort building a foil that will have no real benefit over a semi balanced flat plate type with a trailing wedge .

 

What is a trailing wedge on a flat plate type rudder?

 

Agreed. I've done a lot of rudder research this summer, though not for a boat like this and eventually went back to what I'd originally drawn up, which was a wedge style (high speed boat).

Without a fair bit more information about this particular project, there's a lot of guessing that can get tossed around, without any real usefulness.

What's the make, model and year of this vessel, it's compliment and general targets/hopes a new rudder might offer?



A classic wedge style of rudder.

 

definitely not the kind of rudder you would have on a heavy, displacement hull, boat

 

I am no pro but for a fish boat I would think a flat section with some forward balance supported above and below and as much area as you can get in your aperture. Don't forget a hole and plug to pull the shaft.

 

The advantage of a bulkier rudder profile is that it can be lighter and it can displace some of it's weight If a plate rudder was allowed for in the weights then the form in pic shown will be as good as anything.

Ideal flow foil theory is something of a distraction in real world applications like this where a NACA foil can perform just as well when used backwards or forwards !

Importantly the rudder recovers rotational energy from the prop stream as as well as directing it. Flow over the rudder varies considerable and even reverses along the span. In real world tests flat plate rudders like this can be indistinguishable from any theoretical ideal foil in terms of L-D in a turn and can have a lower inline drag.

 

A Hot Rod ! And you can see the photographer mirrored in the prop, lovely pic.

 

So true. Too much emphasis on correct NACA numbers and CFD etc..waste of time.

I have always used:
"The performance of High-speed rudders in a cavitating environment" by D.L. Gregory & G.F. Dobay, SNAME 1973.

I've used this paper and its charts on endless different types of boats...works very well. It is simple too for a dumby like me

 

Mike's last image shows a classic "wedged" rudder with end plates. Surprisingly effective at certain speeds, which I think this trawler will be moving at, though we still need some verification of what it might be.