Rudders

[HHParticipant @hh]

[HHParticipant @hh]

Having scratch built several models, all of which sailed OK, I’ve never been sure that the rudder used was the best for the job.  In most cases it has been a simple flat plate, cut from metal sheet, stuck to a shaft so as to give about 25-35% of its area ahead of the shaft (ie semi-balanced).  Most of the models have been single screw designs so the rudder was placed directly behind the propeller.

Now comes the questions,

a) Is the rudder area critical?

b) Does the shape matter more than the area?

c) Would a more rearwards pivot point have any advantages?

d) Is a streamlined section better than a flat plate?

e) Should the rudder be as close to the propeller as possible?

Yes, lots of questions but I have never seen them clearly answered in the magazines or on any websites.

Do any members of this forum have any experience with these points?

Hal

[ashley needhamParticipant @ashleyneedham69188]

Hmmmm..having clicked on “rudders” to see what its about,,,,,I find there `aint nowt there ????    Ashley

[neil hpParticipant @neilhp]

 Ithought i was going doolallee, ashley.

[Colin BishopModerator @colinbishop34627]

Another member who seems to have been made invisible before my arrival! You should be able to see Hal’s post now.

 

Colin

[ashley needhamParticipant @ashleyneedham69188]

Colin, thank you. 

 

Hal.  My answers, not definitive, just off the top of me` `ed, however if your setup was satisfactory to you, then whatever you had fitted was sound practice…

answers, A) Yes, too small an area is useless, too much may give rise to too much drag, and/or too much turning moment.

B) No idea, but the two must reach a balance, as 9 Sq cm of rudder area could be a long thin 1×9 size , or a 3×3 size …   I wonder if, out of the propeller wash, it would actually make any difference??? A 3×3 is obviously more practical.

c) The pivot point on a balanced rudder is chosen to make the turning effort less on the steering gear , having a bit of area forward of the pivot lessens the effort needed to turn the rudder. On RN ships of “warrior” 1860 vintage , balanced rudders were thought not to give such good sailing when under sail as a non-balanced rudder…..what do model sailors use?? (no sailing vessels in the garage). On a model, it dont make much odds, a servo being beefy enough, unless you have one of Pauls 10 foot creations….

d) Yes , as its a sticky-out bit n the water, it theoretically needs to be streamlined . Practically on a model boat, I cant see that it would make any difference,.

e) No idea. Rudders do seem to be jammed up against the props normally, and I tend to do the same, leaving enough room to unscrew the prop in case of trouble. Again, having the rudder IN the prop wash magnifies is turning effort as it diverts some of the water stream. In air props, a better turning effort is obtained by having the rudder OFF centre, as evinced by the SRN4. They must have known what they were doing, but on a water setup one cant help feeling that the streering one way would be better than the other ??    Once moving, my HMS Courageous, with a central rudder and Twin props turns ok, and steering STARTS sooner than one might expect…so….what?

 My answers.   Gosh, I can hear those keyboards tapping as I type!!!!    Ashley

 

 

[Colin BishopModerator @colinbishop34627]

Hal, this is a complex subject and there are no easy answers although most of us get by on empirical experience.

 

If you are building a scale model or one from model boat plans then the shape and size of the rudder will be indicated on the drawing so the best bet is to just copy that. If it was OK for the full size vessel or the model prototype then it should work for you.

 

If you are building something like a freelance cabin cruiser where the rudder just sticks out underneath then a simple flat plate balanced type should do the job, as Ashley says, no need to streamline unless you want scale appearance. As a rule of thumb don’t put more than 30% or so of the area ahead of the rudder post. As far as size is concerned, make it a little bigger rather than smaller.

 

Leaving the effect of propellors out of it, it isn’t the rudder itself that turns a ship. Moving it creates a hydrodynamic imbalance in the waterflow down the sides of the ship and it is this which exerts the turning moment. That is why the old sailing ironclads were felt to steer better under sail with an unbalanced rudder.

 

However many model boats these days have balanced rudder(s) located immediately behind the prop(s) which, as Ashley points out, allows the propstream to impact directly on the rudder to push the stern round bodily. If you have the rudder directly behind the prop then (as per Ashley) do leave room to screw the prop on and off, or alternatively, as is seen in twin screw full size vessels, mount the rudders slightly off centre so that you can not only get the prop on and off but also withdraw the shaft as well without having to remove the rudder.

 

Rudder throw should not be beyond 30 degrees or so each way otherwise the rudder just acts as a brake.

 

Colin

[Len OchiltreeParticipant @lenochiltree67043]

Hi Hal,

 

You will not go far wrong if you make something like the Rudder below,with one motor and one Rudder your boat will spin 360 degrees in its own length.

 

 

Len ..in a spin

[Paul TParticipant @pault84577]

Hello Hal

 

Have a look at this site http://www.solarnavigator.net/rudders.htm

 

Paul

[Glynn GuestParticipant @glynnguest59245]

Hal,

Your post jogged my memory and I remembered a short item published in the May-June 1987 issue of RC Boat Modeller.  It covers some of the points you raised.

Using four different models, the effect of rudder angle and turning performance was investigated.  All the rudders were virtual “flat plate” and mounted close behind the propellers.  One model had twin screws, the others were single screw.

 

Using a range of known rudder angles, each models turning circle diameter and the speed whilst turning were measured.  The motors were run at full speed throughout the trials.

The conclusions were as follows,

1) A rudder angle of 30 degrees is optimum for general sailing.  Greater angle s produce little benefit.

2) The minimum time to complete a turn occurs when the circle diameter is about twice the models length.

3) For any given rudder angle the turning circle diameter was directly proportional to the models length.

I hope this might go some way to helping you.

Glynn Guest

 

[HHParticipant @hh]

Thanks for the contributions.  Having failed to find any experiemental work that might answer my questions, I guess I was hoping for too much.  At least it is nice to know that a maximum rudder angle of 30 degrees has been shown to be correct by actual tests.

Perhaps this means that will have to follow in the steps of Galileo, hopefull without the problems he encountered!

Hal

[Glynn GuestParticipant @glynnguest59245]

Hal,

After my last post I remembered a small RC model (published in Nov 1990) in which I tried to see if a thick (about 30 % T/C ratio) rudder would work at large angles.

Up to an angle of 30 degrees it worked normally.  Going up to 60/70 degrees and it still worked but the model rotated about a point just aft of the bows!

My conclusion was that with thick but streamlined sections, even the turbulant water flow in the propwash will remain attached to a rudder at large angles.

Hope this helps, Glynn Guest

[ArneSParticipant @arnes]

Do yachts have deep but thin rudders for the same reason that gliders have high aspect ratio wings?  That is to produce maximum lift, or in a boat a sideways turn inducing force, but with minimum drag.

ArneS

[Colin BishopModerator @colinbishop34627]

ArneS,

 

It is the hull/keel design combination that gives hydrodynamic stability to a yacht hull. This is particularly noticeable with some bilge keelers which have two keels to allow them to dry out. You will find that the keels are of assymetric section to make the one which is most deeply immersed  “bite” on the waterflow better so as to to reduce leeway (sliding sideways through the water opposite to the wind direction).

 

Rudders on yachts have to be deep because when the boat heels right over they tend to come out of the water and lose their grip. That is why a lot of modern yachts have twin rudders mounted at an angle to the hull so that whichever tack the boat is on, one of them will be deeply immersed and working properly.

 

Colin

[Glynn GuestParticipant @glynnguest59245]

I’m sure Colin is right about the reason for deep rudders on yachts, but there is another reason to use them on models.

Many years ago I read about a modeller who was having problems turning his side wheel paddle boat using a scale rudder.  Increasing the rudders area by extending rearwards gave little improvement but extending downwards, below the bottom of the hull, gave perfect control.  His conclusion was that the original rudder was operating in the sluggish boundary layer of water around the hull.  Extending rearwards would do little to improve things but extending downwards, into the faster flowing water stream, did.

I’ve not experimented directly with this effect but, if a rudder needs enlarging I try to go downwards so at least some of it is below the hull.  It is tempting to knock up a test hull and measure the effect of this changes, much better than accepting as modelling mythology.

GlynnGuest

[Paul TParticipant @pault84577]

Hello ArneS

 

This might help explain:

The rudder is a simple but important part of the model yacht. Because of its simplicity, the importance of rudder design can easily be overlooked. The question is, what makes the rudder so important? Well, the obvious answer is the rudder controls the direction the model yacht sails in. But there is more to it than just that. This article will explain the other important facts and how they relate to the model yacht.

Yachts have an intentional imbalance between the center of effort of the sails and the center of lift of the keel. For most model yachts the sail’s center of effort is aft the keel’s center of lift. This gives the yacht a weather helm and is an asset in sailing efficiently. Without a counteracting force, the yacht’s stern will swing to leeward as weather helm turns the boat into the wind. To stop this from happening, the rudder must generate enough lift to hold the stern to windward and keep the yacht moving on a straight course. The key phase here is lift that is generated by the rudder. If a rudder creates too little lift, the yacht will not perform well. It is not the size of the rudder that influences performance, it is the shape that makes the difference.

On most model yachts, the rudder is placed well aft of the keel. The hydrodynamic behavior of the rudder as it travels through water is similar to that of the keel. Because the rudder’s behavior is like the keel’s, there is an added twist. The angle of the rudder to the water can at times get large. Normally, the inflow of water ranges from 3 to 10 degrees while the keel is between 2 to 8 degrees. These higher angles change the parameters of design and influence the foil section and aspect ratio.

The rudder sits in the outflow of the keel and is called upon to provide lift at very small angles of attack and not stall when required to prevent a broach at high angles of attack. Because of these demands of the rudder’s foil section needs to produce a large lift force and function at high angle of attack.

The rudder for the most part is a foil. And because it is a foil shape, lift is created. The secret for a rudder to have good lift is the foil thickness. If a rudder foil shape is too thin or too thick, the lift is effected, (see Example 1 above). In Example 1 (published in Abbott and Von Doenhoff’s Theory of Wing Sections), a foil thickness between 11 and 14 percent produces higher lift coefficient at sailing speeds. So if a rudder is too thin and drag is less, so is the lift. On the other hand if a rudder is too thick, lift is less, but drag is increased causing a slower speed. For a good idea of a foil shape see Example 2.

For all model yachts, the rudders are fully immersed types. Because of this, a fully immersed rudder enjoys high lift values at the top of the rudder because of the “end plate” effect, (see Example 3). Another reason for the high lift at the top is because that is where most of the foil shape is located. Where as at the tip, the foil is thinner and the water vortexes off the tip, and lift is lost.

That is about it on the design of model yacht rudders. It all comes down to using the right foil thickness. Too thin and out of shape and the rudder will not create enough lift. Too thick and the rudder creates too much drag and lowers the amount of lift it generates. I hope this article has explained what makes a difference in rudder design to produce a winning model yacht.

Paul

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[Paul TParticipant @pault84577]

The link to the website for the above article didnt copy.

[HHParticipant @hh]

In view of the apparent lack, save the item mentioned in RCBM, of any experimental work done on “model” boat rudders, I’m going to try to carry out some tests to get quantitative results.

I do not feel that work on fullsized vessels is truely applicable, yes I know all about Reynold’s Numbers and scaling effects. But our models work in the top few centimetres of the water and we often look for different things compared with the fullsized world.

The effect of area might be the easiest thing to try first.

Hal

[Glynn GuestParticipant @glynnguest59245]

Good idea Hal,

                        I intended to do  more work after the rudder angle experiements but “life” seemed to get in the way.  How about you investigating the effect of rudder area and I’ll try something else?  The effect of rudder position, relative to the prop, or single, twin even triple screws and turning circles?

Working in a colaboration ought to get more reliable results quicker.

GlynnGuest

[PierreParticipant @pierre]

Very interesting topic – I tend to agree with Hal – full size rules don’t necessarily apply to models.  

 Just as a matter of interest, for Glynn, years ago I worked on two twin screw conventional tugs. The one had twin rudders, each  one directly behind the props, the second one had a single rudder, situated between the props. The difference in handling was quite marked – the second one was far less manoeuverable, turning circle was far larger and it was almost impossible to “crab” the tug sideways by using opposing propellers and opposing rudder.

I’ll watch this space with interest!

Cheers – Pierre

[ashley needhamParticipant @ashleyneedham69188]

I was in a bit of a quandry for a rudder on the Sunderland, as the drive was actually NOT in the water, . I went through 4 rudders before settling on a rather long and deep  design, and its not balanced either. I didnt want to go deep, as it seemed a shame to build in a weed catcher when in theory the thing would power across weedy areas with no trouble but in order to get the manouverability I wanted, the rudder was getting ridiculously long. For the German flying boat, the rudder is balanced and nowhere near as deep, but set about 2/3 back. The hull is a gradual curve on this one, and I think the main difference is that the props are fairly central on the HBA, but spread out on the Sunderland. Ashley

[ashley needhamParticipant @ashleyneedham69188]

I will mention it before anyone else does, but with differential prop control I would not have needed a rudder at all…but the complication, expense and weight meant that this idea got the chop early on. Ashley

[HHParticipant @hh]

Glynn, Would like to colaborate on trhis subject.  Rather than fill this board up with details only relevant to you and I, will send you a PM.

By the way, if I ever had to spend more than £100 on a model, I would find another hobby (perhaps buy an Ferrari instead)

Hal

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