Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Tutorials & "How To’s" ... => Topic started by: GG on July 26, 2019, 02:43:06 pm
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This note was prompted when a newcomer to the hobby asked what angle a rudder should turn through. With no hesitation, nor any further information given or asked for, a reply of "30 degrees each way of course!" was fired back.
Now this "30 degrees" appears to have become embedded in model boating lore to the extent that some regard it as a RULE which can be dangerous. Rules inevitably have limitations, restrictions and exceptions that need to be understood if that are to be safely used. I once read a quotation (but damned if I can remember by who) to the effect that "Rules are intended for the guidance of the wise and the blind obedience of fools", sadly something repeatedly confirmed in my professional life. In the case mentioned, without knowing the type of model the original enquirer was thinking about and what he needed, just saying 30 degrees was unwise. A fast model using this maximum rudder angle might be uncontrollable whereas a sedate one could be so unresponsive as to be unpleasant to sail.
Never being happy with Rules that have no results to back them up, I carried out a series of simple tests some years ago to measure how different rudder angles affected the turning performance of scale type models. Four displacement type hulls were used in the length range of 30-42 inches. Three had a single propeller and rudder and one twin props and rudders.
Using a combination of rudder trim and stick positions, several predetermined rudder angles, up to 40 degrees, could be applied. The size of the turning circle was measured by marking the position on the bank-side when the models were sailing directly towards or away from me. Not, I admit, a highly accurate method but, by letting the model make several circles, it gave surprisingly consistent measurements.
The first graph shows the typical results for all the models. For up to 10 degrees, the rudders had a powerful effect. Between 10 and 30 degrees the effect of the rudder progressively diminishes and beyond 30 degrees it's effect is much less. This fits in with the experience that rudders can be very sensitive at small deflections and unless you apply the transmitter controls with restraint, your model is going to be all over the place.
The second graph shows how the time to make a complete circle changes with the rudder angle. As you might expect, a greater rudder angle means a tighter circle with less distance to travel and should mean a shorter time to complete the circle. In practice models slow down as the circle becomes tighter (the model traveling at an angle to the water flow and encounters greater resistance. The net result being that the turning time falls up to around 20 degrees, after that there was little further reduction and something of the proverbial "brick wall" was encountered above 30 degrees.
Playing with the results and it was found that how the models length affected the turning circle, graph 3. For any given rudder angle, the turning circle diameter was found to be directly proportional to the models length. This might be handy for the designers of scale steering courses. The actual obstacles can be as "tight" as your conscience permits but, after considering the maximum length of model expected, allow sufficient space between obstacles. This should enable models to sail smoothly between them rather than spending time shunting forwards and backwards.
These experiments have served me well over the years and can be summarized as follows;
1) Rudder angles up to 30 degrees are suitable for the general sailing of most scale models
2) For a given rudder angle the diameter of the turning circle is proportional to the models length
But, this only applies to models where the rudder is behind the propeller where the rudder blade intercepts all of the propwash. One model of a destroyer had twin props and a single rudder between them. With 30 degrees of rudder throw, it's turning circle was too large for comfort and had to be increased to 40 degrees before I was happy.
Also, for low speed maneuvering, larger rudder angles can be of benefit. I often use angles of 40/45 degrees for this reason but employ restraint in the application of rudder commands when traveling at speed.
So, 30 degree of rudder is a good staring point for many models but it isn't gospel, sometimes more or less is better, don't be afraid to experiment.
Glynn Guest
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Interesting :-)
My 30” lobster boat is the first model I’ve made with single prop and rudder I find that in forwards 30 degrees is pretty much fine and responsive but going astern is completely unpredictable %%
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I’ve never been on a ship with conventional rudder that moves more than 38° each way. I’ve sailed on supply boats with twin screw/ twin rudder where rudders controlled independently. When working close in both rudders turned inwards (Port, hard starboard, starboard hard Port. Left in this position and just use the sticks to manoeuvre.
Used this method on Azziz model. Really simple.
Jerry.
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I know that with my Cervia that engine speed has a very marked effect on turning circle. Sometimes to get out of trouble it is better to speed up to tighten the turn.
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This makes interesting reading but there is another aspect of rudder movement that hasn't been looked at here - ability to control the model when going astern.
In some of the smaller models I have built, even though control going forward is fine I find that there isn't enough range of movement on the rudder when going astern to overcome the torque effects of the propeller. I t doesn't help if the ESC has quite a high minimum speed.
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Fast boats becoming uncontrollable at high speeds- that is basically why proportional control is so useful. All those stick positions before you get to full deflection. Having a control system that does exactly as you tell it can be embarrassing. This doesn't just apply to boats - tall vehicles, assuming that the tyres maintain grip, will flip if full lock is applied at high speed, probably the main reason why so few are seen overturned is because it does take time to misapply full lock.
On powered boats, the rudder, to work well, deflects the prop wash. Out of the prop wash, its just deflecting water flowing at boat speed, so either doesn't do much, or, as on a sailboat, needs more rudder. On a boat maneuvering slowly, a blip of throttle will give a burst of fast flowing water to start the hull rotating. The hire boats on my local lake steer by rotating their outboard motor - the vectored thrust starts the turn, being short and dumpy, they need a bit of reverse steering to stop the rotation.
When reversing, any play in the rudder linkage will allow the rudder to realign itself to one side or the other, applying unwanted steering, the natural castor action that applies with unbalanced or partially balanced rudders when water flows one way is reversed when it flows the other way.
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Interesting comments and opinions.
Allnightin - steering astern is a different topic but maybe worth starting a new thread..?
JerryC - Full size practice can be a handy place to start but it has always seemed worthwhile to experiment (which ought to include measuring the results) in our models which only operate in the top inch or two of the water and have much different speed of response to rudder and throttle commands.
Experimenting with independent control of multiple props and rudders ought to be easier with these "computer" style transmitters, well worth trying.
Malcolmfrary - True but we have all seen the characters for whom proportional control is wasted, their rudder stick having only three positions, full right-center-full left.
Not sure I can agree with your astern sailing comments unless the rudder linkage is really sloppy?
BrianB6 - All the test results were obtained at full power. Investigating the rudder's turning effect at lower powers would be a worthy project. But, in my experience full rudder at lower power will generally produce a tighter turning circle, something I'll try to recheck in my next sailing session.
Glynn Guest
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Hello!
Excellent study which conforts me in my sailing.
The longest is the boat and thought it will be to turn, spending all the energy in the curb.
A 30" boat with a rudder close to the propeller will turn in an hankershief.
Thanks to share.
GRIMLIN de KBIO (http://modelismenavalvapeur.forumactif.com/t79-grimlin-de-kbio#81)
We see that the turn cannot be sharp.
La JugonNaise de KBIO.(sauce Seekadett) (http://modelismenavalvapeur.forumactif.com/t432-la-jugonnaise-de-kbio-sauce-seekadett)
A suggestion; do not use 4 blades propeller unless it is for a tug or work boat. For the speed , favor the paper size propeller.
Thanks for your interest.
Regards.
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hi there
just to add another point into the conversation about going astern - a thing called propwalk has a lot to do with it - this is where the rotation of the prop acts like a wheel and tries to move the vessel sideways, as going astern - a while ago whilst in a steering competition - I watched a gent called Stan who had a tug called 'Growler' & this was a single prop tug - he could get this model to do virtually any movement on the water when going astern - his trick was to blip the prop astern, keeping the rudder straight ahead and then when the prop had stopped turning and the model drifting, operate the model's rudder in the direction that you want to go (astern) what he was actually using was the flow of water over the rudder without the prop causing any sidewheel effect (being stationary). The other thing I noticed on the model - he had slightly increased the keel depth just in front of the prop and rudder. This allowed the model to run pretty true and in a straight line.
Food for thought. John
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I had some thoughts on going astern and some 30+ years ago when we used to have steering competitions I modified a a 28 inch launch (Graupner Condor from plans, scratch built).
I had to refurbish due to age and prop tube bearing wear, I fitted a fixed Kort nozzle and normal rudder but also 2 rudders in front of the prop which all worked together. I am afraid the reverse steering I hoped for did not materialise.
I had overlooked the fact that normal travel has the prop at the back but reverse has the prop at the front. Quite different conditions. I think a Kort nozzle as a rudder would have been better.
Still have the boat must be over 50 years old now.
regards
Roy
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Hmm, I never heard of a 30deg rule before* but reminds me of lab experiment at uni where, normally, rudders act like wings and there is a "stall" point at which it becomes more of a sea anchor. I probably have that old lab report buried somewhere in the loft**. I'm reminded also, and can't remember any of the maths, but I seem to remember lecturer saying that a rudder "generally" only contributes to about 10% of the force required to turn a ship, which fits (roughly) with a quick 'tinternet check. I guess its different for such small models as we do because everything is out of scale and not run at representative reynolds / froude numbers on the pond. Windage on models must be huge also. R/C models are R/C models so experiments on the pond are all valid.
*could that not just be spookily close to a servo throw for allot of folk?**ever since, I've always tried to make my model "spade" rudders foil shaped rather than flat blades, apart from the fact they look better for little effort
I know some are talking about prop walk and going astern (boat not ship). When I was younger I had a licence to sail a replica packet steamboat for tourists -one half open, one half cabin aft and a single cyl Lister engine and a few of us could get her to spin on a sixpence, forwards or back in almost any direction if you set her up for the wind correctly (didn't always work when the clutch played up!)....
http://www.lucs.org.uk/linlithgow-canal-centre/our-fleet/victoria-boat/
I fitted a fixed Kort nozzle and normal rudder but also 2 rudders in front of the prop which all worked together
..."flanking rudders" like they use in towboats?
I’ve sailed on supply boats with twin screw/ twin rudder where rudders controlled independently. When working close in both rudders turned inwards (Port, hard starboard, starboard hard Port. Left in this position and just use the sticks to manoeuvre.
Thats quite interesting and makes sense when working close-in where you want specific thrust in a direction without "travelling". I always watched them from a platform or on an adjacent DSV, never been on one save for a survey of the steering gear when I was young. I guess they weren't DP 2 but thats maybe a different story?
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RST, ref. My comment on independent rudders, when things start going pear shaped close in to a rig or platform the engines respond instantly but waiting for the rudders to swing from one side to the other can seem like a lifetime. Grey hair time. With them fixed inboard response is more or less instant.
Jerry.
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RST, ref. My comment on independent rudders, when things start going pear shaped close in to a rig or platform the engines respond instantly but waiting for the rudders to swing from one side to the other can seem like a lifetime. Grey hair time. With them fixed inboard response is more or less instant.
Jerry.
...Absolutely. Same reason on DP some thrusters run fixed speed and some run fixed pitch, some azimuths turn to a defualt direction. Luckily never experienced a DP failure on station apart from on FMEA during sea trials -we called that a "brown trouser moment" -and you just wouldn't believe how that started!
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It has always been a complicated subject in model form.
If you look at the old sailing ships such as Victory or the latter clipper ships such as Cutty Sark you will be struck by how small the rudder appears to be. The reason is that it was all that was needed was to alter the hydrodynamic balance of the hull which did the actual heavy lifting of turning the ship. Essentially it caused the hull to become slightly banana shaped and this acted like an aerofoil to turn the ship.
The big difference with screw propelled vessels is that the water stream from the propeller going ahead impacted on the rudder and directed it to one side or the other which pushed the stern around. This effect was most pronounced on single screw ships and less so if there was a single rudder and twin screws so the rudder was not so far into the propwash. Many modern twin screw ships have twin rudders to maximise the turning forces.
The modelling convention of 30 degrees maximum is simply that it has been found in the past that up to 30 degrees alters the direction of the boat but more than that tends to act as a drag and reduces speed.
But having said that, all boats are different and their handling characteristics depend on the waterflow along the hull and around the stern. Sometimes it is abrupt, sometimes smooth and the shape of the underwater underbody influences rudder response when going ahead and to some extent when going astern.
Propwalk in most case is not very significant when going ahead but when in reverse the direction to which the prop is throwing the water stream astern can have a big effect on steering so you find that the model will steer one way when going astern and not the other.
This was very well understood back in the 70s/80s when steering competitions were popular.
Colin
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Actually from memory turning the ship turns the waterplane into an aerofoil of sorts which creates "lift"and tends to make the turning circle bigger and not circular (I have calcs for that somewhere also). There's other mechanics. But you're right, it's the instigation of the turning moment, then the imbalance of resistances etc.
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tends to make the turning circle bigger and not circular
Bigger than what? In a screw ship more power would tend to tighten the turn as the stern is thrust around. With a windship it presumably would depend on the speed?
Twin screw ships can tighten turns by slowing or reversing the inner shaft(s) as we do with our mixers.
Colin
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Single screw Colin. They don't turn in a circle from memory.
....actually, above is rubbish. Hull wants to drift outwards, whether it's single or multiple screw. What happens with throttles on shafts etc is different and will pull it back -if it can override the momentum.
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interesting.as a sub guy with some surface targets..i was always told anything past say 15-20 degres a rudder/hydroplane starts to become less effective due to unlike air water is non compressible .and anything past that 15-20 will be no more effective that the standard 15 .I have no basis in fact to prove this,just what I have heard .
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I wouldn't expect them to.
I had a sailing yacht with an outboard. Basically it pivoted round the rearmost third of the hull.
As with many vessels, the stern swings right out when turning which can cause problems in close confines.
Colin
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It's the nautical equivalent of a 4 wheel skid.(drift) {:-{
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It has always been a complicated subject in model form.
If you look at the old sailing ships such as Victory or the latter clipper ships such as Cutty Sark you will be struck by how small the rudder appears to be. The reason is that it was all that was needed was to alter the hydrodynamic balance of the hull which did the actual heavy lifting of turning the ship. Essentially it caused the hull to become slightly banana shaped and this acted like an aerofoil to turn the ship.
Colin
In these sailing ships the rudder could be better looked at as a trim tab to make small changes in direction. Much of the steering was done by the sails and the sailors job was to constantly adjust the sail trim. Tacking could not be done without the use of the forward sails to push the bow through the wind.
Jim
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I have to admit that I have zero knowledge of hydrodynamics, if that is the correct term, but I have built a fair few models. When I'm giving a new model it's first trial run, I always set the rudder throw to a little more than common sense tells me is enough then set up the rudder stick with the dual rate setting so I can just try it and see what works best for that model. Once I'm happy I re set the throw on the servo arm and never worry about it again.
I've had a few surprises over the years especially with twin screw single rudder models but 5 minutes on the water twiddling the rates works every time.
Chas
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I'm confused.
I've always built models with plenty of rudder throw - you never really know how the boat will function until it's on the water. I work on the assumption that I'd rather have way too much and not need it compared to never having enough. Full rudder acting as brake might be useful occasionally....
From looking at the threads, it reads as though throttles can only be 0 or 100% and rudder can only be central or full left or right. I've never driven a boat like that. Is that normal?
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From looking at the threads, it reads as though throttles can only be 0 or 100% and rudder can only be central or full left or right. I've never driven a boat like that. Is that normal?
Not "normal", but unfortunately all too common.
DaveM
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Not "normal"
Unless it's a Club 500 race with no esc {-)
Ned
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Well Id say depends how big your model is and the forces that are going to be acting on the rudder. Just dont over drive your servo and make sure it has sufficient torque for those forces.
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When say “a normal” vessel moves astern the point about which the vessel pivots moves right aft. This has the effect of removing any moment caused by the rudder. It is virtually impossible to steer such a vessel going astern. The “Royal Daffodil and Royal Sovereign” used to turn round and proceed upstream on the Thames. They both were equipped with bow rudders. HMS Worcester chartered one or the other every year to bring our parents down from Tower pier to the ship at Greenhithe for prize day. she turned upstream and came alongside as we Cadets cheered ship. On completion both parents and Cadets embarked and returned to Tower Pier and went on summer leave. I can’t remember where exactly she turned about but she made sternboard at full speed and went under Tower bridge astern. Happy times.
Jerry.
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(https://www.modelboatmayhemimages.co.uk/images/2019/10/11/8D42F53C-A425-4B4C-8E7E-9DD740E227CA.jpg) (https://www.modelboatmayhemimages.co.uk/image/JpPx5)
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I'm building a twin screw cabin cruiser (Grand Banks Classic cruiser, 820mm LOA) and intend to slightly offset the rudders from the prop shafts C/L to enable the shafts to be withdrawn from the blunt end without being obstructed by the rudders.
Question: is it better to position the prop shafts inboard of the rudders or vice versa. or would it make no difference whichever way they're aligned?
I intend to run both motors from one ESC, so they won't be individually controllable when rudder is applied.
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Hi just thinking about it, I would think the rudders have slightly more effect if they they are outboard. Anyone else think otherwise?
Roy
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Yes Roy, there was a complete evolution in the design of rudders on vessels over the last Centuries
200 ton sailing vessels anchoring mid stream & using ships boats for transportation of cargo & loading
100,000 tonne passenger cruise vessels mooring directly to wharf's without the aid of tug boats
Without going to our Australian paddle ships with Barn Door rudders, sail barges from London waters from the late 1800's used substantial stern [external] rudders to sail.....moor & berth without external assistance
On of the cargos carries is listed with Coal, Barley, Timber, Wheat, Coke ........etc and Oil Cake...........was this another name for Peat?
Derek
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Going back to the question, it doesm't seem to matter much if the rudders are inboard or outboard of the shaft line:
http://hydrocompinc.com/knowledge/rudder-offset-position
Colin
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Hi Colin, the link you posted was a good read and in their opinion if the prop turns outboard
The rudder should be set outboard and if the prop turns inboard the rudder should be inboard.
But in fact it doesn't seem to make any difference in actual use.
If you haven't got acces to remove the rudders on a boat or the room to slide the shaft forward
by removing the motors offsetting the rudders inboard or outboard would be a good idea for shaft
Removal.
John
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Yes, offsetting the rudders makes a lot of sense for easy maintenance. It seems to be common full size practice.
Colin
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Let us not forget the ages when rudders were installed on the Larboard side of a vessel O0 ....... Derek
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I'm not sure what that has to do with shaft alignment Derek!
Colin
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Because it's a rudder answer in a rudder topic :D
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Well Colin....the original message in this thread posed the question......
"This note was prompted when a newcomer to the hobby asked what angle a rudder should turn through"
So I didn't see this as a shaft alignment issue............ as my postings both referred to rudders as per the OP & not shafts
I would be happier with an explanation of Oil Cake? ...as a cargo shipped between ports in the UK in the late 1800's :D
Derek
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Will this do Derek?
https://cargohandbook.com/index.php?title=Oil_Cake
These days it seems to be called seed cake and is still a common but potentially hazardous cargo.
Colin
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Let us not forget the ages when rudders were installed on the Larboard side of a vessel O0 ....... Derek
Actually this is mounted on the starboard (or steerboard) side. Larboard is now known as the port side.
Jim
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Opps... :embarrassed:
Jim of course, you are absolutely correct...[appears they only named the other side Port, as Larboard sounded too similar to Steerboard]
However despite research of the Chronicles, it is not quite clear exactly when Steerboard became Starboard
Amazing what looking upside down from the other side of the Equator can do after a few glasses of wine {-)
Derek
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Ahah...thanks Colin.....carrying Peat in wooden sailing vessel holds, then carrying Wheat or Flour didn't sound a good mix <*< ....
In Australia we well know this grain seed waste as Meal ....to this point, our Convict stock baked Meal Bread........
As a youngster we loved Weston's Oatmeal biscuits .......WG [George] Weston made Oat biscuits during WW1
Derek
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The Dutch say steer board and back board.
Jerry.