May be a silly question but !
Why the rudder is always positioned at the stern ?
What will happen if positioned at the bow ?
Claudio
It’s not a silly question Claudio. Over the years forward rudders have been used on full sized boats, but mostly I think in conjunction with a conventional aft rudder. I can’t remember which boat - but there was a 12 meter at the Americas Cup summer of 1986/7 in Fremantle, Australia, that had a forward rudder.
Of course many of the new canting keel boats have a forward canard.
As for a forward rudder, as a replacement for an aft rudder - two things come to mind:
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Wouldn’t the flow of water of the keel would be affected by a large steering rudder forward?
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Even in our RC boats (and certainly in big boats) the bow spends some time out of the water if the waves are up (which may explain why, during my ocean racing days as a helmsman, I was nicknamed “Thumper”
). And aft rudder stays in the water (mostly) and this is key to control of the boat.
Muzza,
I agree on what you says.
I was thinking that, turning the rudder it would produce a lifting action when the boat is heeling.
The rudder could be equipped with small winglets producing extra lift during running in order to reduce the probability to put the nose under water.
Probably is only a fantasy of my mind.
Claudio
Hi Claudio
Exactly, and this is one of the strong attractions of the forward rudder.
But it is a control surface in a place that makes it difficult to maintain smooth, damped control of the whole boat. You may remember the configuration of the original Wright “Flyer” – it had its elevators forward of the wings. Subsequently, almost all planes placed their elevators aft of the wings. Only with the recent advent of computer control can elevators be placed forward again, and this is done with fighter aircraft for extra manoeuverability. The reason is to imagine what happens as the plane begins to respond to the action of the elevator.
If the elevator is forward, and the nose begins to lift, for example, the elevator gains an increased angle of attack and lifts the nose even more and very quickly the plane has somersaulted due to runaway positive feedback.
If the elevator is aft of the wing, as the nose lifts the angle of attack of the elevator decreases, giving negative feedback and a nicely progressive action of the elevator, while the plane remains under control.
I think the same effects apply to a forward rudder. It is the same reason, for example, that the fins on bombs are at the end of the bomb, not at the front.
I think there might be another disadvantage to the forward-mounted rudder, although possibly not large enough to offset the advantages.
Typically while beating the rudder must be applying yawing moments to leeward to counter the boat’s tendency to round up into the wind. When mounted aft, this amounts to pushing the stern to windward, thus adding to the windward forces on the fin from the boat’s angle of attack to the water. A forward-mounted rudder would need to shove the bow to leeward, thus adding to the forces the fin (and hull) must counter, implying a greater angle of attack, which means more leeway. The greater angle of attack also means higher drag. So one effect of a forward-mounted rudder, while possibly small enough not to make much difference, seems to be to make the boat go to leeward more and to go slower.
The angle of attack “problem” with the fin might be countered by increasing the area of the fin, but this also would increase the drag.
Mike Biggs
Actually, in the full size “canters” the forward canard assists them in retaining control during overpowered/wind up situations. A forward rudder, in conjunction with a conventional rudder, can help prevent a boat from broaching.
thank you all , you used very good arguments - forget rudder in the front
It was a silly idea !
Claudio
I don’t think it was a silly idea at all! It may or may not be advantageous, in those classes that allow it, but it isn’t a silly idea.
By the way, Claudio, I really like your sail-making gadget. I’m still working out issues, such as curvature along the luff, that have nothing to do with the basic shape instilled by use of your gadget. Soon as I get the sail looking as pretty as it should, I’ll see about taking a picture or two.
Mike Biggs
I am actually building a Footy that is designed around a forward rudder so we might find something out.
First, Cladio, forget the ‘lifting the bow’ theory. Putting it very simply, the hull is a rigid beam. If it is heeled and you are tryng to turn it away grom the wind, ‘down’ at the bow is on the leeward side no matter which end you apply the force at. It took me a long time to work that one out.
As to the fin operating in the turbulent wake of the rudder is concerned, we can reverse the argument. With a conventional layout the rudder is operating in the downwash of the fin. Since common sense suggests that the rudder should also be generating sideforce, it is 6 of one and half a dozen of the other.
I also disagree with Muzza. If he has sailed typical wide-beam IOR/IMS boats, he will be all to well aware that loss of control down-wind happens as often as not because the wide stern lifts an increasing amount of he rudder out of the water as the boat heels - so the helm angle has to increase until the thing stalls - then ‘whooosh’. That’s why you steer into an incipient broach.
With a bow rudder, the same tendency causes the rudder to be more deeply immersed - so that problem goes away. I am very new to these little boats but I have yet to see a picture of a Footy with its forefoot out of the water, so I think the upwind probalem is probaly imaginary.
The other advantage as I see it is that we can move the rig (and fin) further aft. Moving the fin aft does something to counter the downwash problem. It also increases the lever arm between rudder and fin. However, it should also counteract submarining. Obviously it has no effect on the horozontal tripping foce: rogid beams and all that. However, at any angle of heel (and probaby anyway) there is some spanwise flow over the rig (consider how telltales line up as the boat heels). Since every reaction has an equal and opposite reaction, this implies a downforce on the rig (i.e. the mast). The further aft we can move this, the less it will tend to depress the bow.
A.
I’m not sure that you disagree with me at all Angus. I did caveat my comment with the rider “mostly” after my comment.
I think your point addresses a slightly different, but just as relevent, issue, being the effect of heel on the amount of rudder left working in the water. My point had to do with the bow exiting the water when sailing through waves.
However with regard to loss of control through heel pulling the rudder out of the water in a wide-sterned design, it is not loss of working rudder in the water that causes the problem, but the rapid increase in weather helm brought about by the sudden increase in force and heel. The forces involved create a couple that wants to turn the boat to windward. Broaches occur in gusts that are not anticipated by the crew. Design has a part in it - for example, the mid-70s IOR designs in the style of Holland and Peterson had a deep forefoot and straight rocker, and tended to wind out a little earlier than the shallow forefoot and rounded rocker designs. Britton Chance’s design, Resolute Salmon, which won the One Ton Cup in '76 is a good example of this. She was a cunning design under the rule, and a centreboarder, but she had some control issues when hard reaching.
If the helmsman has to apply excessive rudder to retain control - it’s too late. As you’ve correctly noted, the result is first, excessive drag, and secondly, stalling and complete loss of control. The idea of a deeper rudder to keep more of it in the water is of course valid, but at the cost of increased drag when the boat is upright.
Sadly, I’ve wiped out on IOR and IMS boats more often than I care to remember :rolleyes: When racing, I do not steer into an incipient broach other than when sailing to windward (anyone that has raced a high aspect ratio masthead rigged boat with big overlapping genoas will know what I mean). Even then it is a temporary maneuver. Likewise however I do not pull the helm up the weather more than a few degrees. Most wipe outs in the big boats occur when hard reaching on the edge of control. My standard sequence of events in such circumstances - as the puff hits and before the boat starts to round up - is 1. blow the vang, 2. ease the spinnaker sheet (if flying). 3. dump the main down the traveller (if there is anything left) and 4. further ease sheets of mainsail and spinnaker. All this happens more quickly than you can read the words, and hopefully serves the dual purposes of keeping the boat upright (thus keeping the rudder in the water) and keeping the CoE forward of the CLR. We have a term for it on the big boats I’ve raced over the years - “flow and go”. We also get the crew as far outboard as possible - if they are not already.
Of course in RC boats we can’t do these things - with 2 channels all we can do is ease main and headsail together - preferable to using a lot of rudder. For these reasons, amongst others, I have a preference for so-called “balanced” hulls in RC sailboats. We had another thread on this subject very recently.
With a bow rudder, the problem (i.e. a sudden increase in weather helm) does not go away, it just means you have more rudder in the water to counter the problem. The effect of both rudder in the water, and turning that rudder against the water flow, is still drag - even of control is retained. Its so much better to anticipate and control the boat via the sails - even if a luffing sail also creates drag. Of course in reality a combination of the too is necessary.
It is interesting to note that, for many years, one of the leading RC boat suppliers, Sails Etc, used a “standard” IOM rudder which has a relatively low aspect ratio and is about 170mm deep. This is considered at the short end for the class, with others at 200mm, 225mm and deeper.
Recently, Sails Etc have introduced a deeper rudder for the IOMs, and I believe the reason given was that Mr Bantock wanted to address control issues in more boisterous conditions. It’s an example of the constant comprimise necessary in design.
The other approach to retaining control with smaller rudders and wide hulls is two rudders at the stern. Although we see this on the Open 60s etc, it is not a new idea. The scows sailing on North American lakes have used two rudders for decades. There are twin-rudder scows in Europe too.
If you ever did any sailing around the Solent in the late 1980s and early 1990s you may remember Bob Fisher’s Barracuda - 45 feet long, and with twin rudders.
I’m not sure that I follow your comment with regard to the lifting effect of a forward rudder. I think a forward rudder, which in order to turn the boat to leeward is also angled in the direction of the water flow (because the boat makes leeway), will operate at a reduced, and often negative, angle of attack cf a rudder at the stern - which must turn again against the water flow? We may be saying the same thing - just using different words. When the rudder is operating at a negative angle of attack (the water flow striking the rudder on the windward side), it would generate lift to leeward and (if heeled) downwards. This is why in the canters, a forward canard is useful in retaining control and avoiding windouts to weather.
I’m curious about the idea that a stern rudder will opearate in as much turbulence from the keel and the keel will operate in turbulence from a forward rudder. A keel is fixed in its fore and aft direction. Thus the significant turbulence created by the keel (other than off the base - which is below the rudder anyway) comes about when its angle of attack to the water flow is other than zero. Of course in most conditions and points of sail, the angle of attack with be a few degrees, being the extent of leeway the boat is making. But again in most cases, with a well chosen foil profile, the fin will not be turbulent and will create lift. There will be drag too of course, but not such that it would alter the behaviour of the rudder if and when that water flow reaches the rudder. A rudder however is moving through a range of angles and will on occaisions will have a more muddled, sometimes turbulent, wash when operating at high angles of attack. If placed forward of the keel, I’m wondering whether this impacts negatively on the keels performance. Thinking about aircraft for a moment, an analogy may be those designs where a T Tail design evolved to avoid the turbulence from wings destroying the effectiveness of the elevators at high angles of attack. I guess the question is how broad the path of the wash is when it comes off the keel/rudder and in what direction does it flow.
Moving the rig and the keel aft together, if other aspects of the design are left as is, will reduce the tendancy to bury the bow - particularly offwind in a breeze. There may however be some downside. Moving CoE anf CLR away from other centres in the hull (floatation/bouyancy) may result in detrimental pitch and yaw effects at different angles of heel. Bringing these centres aft to better align with the rig and keel results in finer forward sections, which in turn makes the boat easier to nosedive. So again the need to comrpomise kicks in.
An aft rig can perform well to windward but in, lighter breezes (when nose diving is not a problem) they do not seem to perform as well off the wind.
You note a benefit (with a forward rudder) as a result of lengthing the turning arm by moving the rig and keel aft. I’m sure you are right, as the opposite has been shown when moving them closer together with an aft rudder. Moving the keel and rudder close to each other fore and aft will make the boat “twitchy”. This is one of the reasons why, in the days of the RORC rule in ocean racing, some of the early and mid 1960’s designs which were evolving towards smaller keels, but still had the rudder fixed to the aft edge of the keel, were modified after launching to hang the rudder on a skeg further after to solve comtrol problems. I can think of two S&S designs that underwent this modification during the '60s. Bringing the rudder and keel closer together shortens the lever being used to turn against the resitance of the hull. On the upside you get a more instant response. On the downside you tend to use more rudder, thus resulting in more stalling - especially when tacking. This approach works on very fast skiffs - like 18 footers - and fast cats. These are not tactical boats - but stright line machines. In racing RC classes like the IOM, I want maneuvrability and expect to tack many times on a windward leg. Thus I think you would see the benefit you’ve identified.
Well, if you want to get the CLR and the rig aft, why not try Gus Lassel’s old “finless fin keel” approach. I know, a real weed-catcher, but they worked, especially on the run. (In this pic the sliding rig is clearly way forward).
Cheers,
Earl
Than you for your very full response.
I too was brought up in some boats that are now very old hat - the first thing I can remember sailing was an Illingworth Tiger V (Wista) type. Then progressed through a North Sea 24 (big Holman Twister) to a Holman Hustler 30, S&S 34, Carter 3/4 Ton, Golden Shamrock, First generation Farr 1 ton, home brewed 1/2 tonners & mini tonners …
The easy things Rudder lever arm is well demonstrated. I sailed Clarion of Wight (S&S) briefly before and after she had her rudder moved from the back of the keel to somewhere more sensible. She went from being a total bitch to being quite a nice boat to sail.
I agree with you about the problem of luffing into a broach with a masthead rig and a ribbon mainsail. The whole thing opens and shuts like a barn door. However, with a fractional rig and something doing somethng sensible about dumping heel it is a useful technique - at least in extremis.
I also agree that the behaviour of the turbulent wake of a rudder on a fin will be greater than the other way round. However, I do not believe that it is a case of all or nothing as (I think) you originally implied.
As to the lift on the bow problem, I used to think the way you do until I and a friend who makes his living writing textbooks about ship handling thought about it very carefully. One of us has missed a minus sign (or else we are talking about slightly different things). Ok - the effect of a constant angle of attack such as to offset consistent weather helm is to produce a constant sideforce to leeward. Does this mean that a forward rudder boat should be trimmed to carry slight lee rather than weather helm, I wonder.
However,the effect is for all practical purposes exacty the same once we are in a loss of control situation no matter where the rudder is. Until things start to stall, we are going up to windward - far more than we want. The end at which control is applied has to be pretty irrelevant from the leeway point of view - particluarly if the alternative is to lay the boat over on her ear so that the keel ceases to generate any useful side force.
In the Footy application, I agree with almost everything you say. I am indeed moving CG and CF well aft. The aim is to fine the bow down for better wave penetration without the brute submarining. The result is, indeed, a short steep run. However, a Footy is intrinsically quite heavy for its length - displacement/length ratio somewhere in the 300s and is unlikely to leap onto a screaming plane very often (and I’ve never seen one doing so in a photo). Given a maximum likely speed a little on the high side of 0.5 m/s, I think that it should be fairly difficult to induce major separation. I do, however, look at the half-built hull regularly and shudder: the brute looks like nothing more than a foraging armadillo. However, we shall see.
Incidentally, do you have any good ideas on building non-chine Footies quickly? Bombur is strip-planked, to be covered by carbon tissue inside and out. I have been peruaded by an exchange with TomaHawk that this is grossly over strength and I think I might use the boat as the plug for a pure carbon shell as well. If this works, what about bread and butter construction by waterlines using thin high-ish density foam to make a plug?
A
In my considerable ignorance of model yachts I didn’t know about that one. I do have a sketch that looks rather like it (but with a forward rudder) but ruled it out as looking too much like the Starship Enterprise to be a good thing. Hmmmm!
Angus,
I can see you will be a welcome addition to these forums.
Ah - Clarion of Wight. A great example! One of the two I was thinking of was a direct development of Clarion - Cotton Blossom (IV I think - there were several over the years), a New Zealand built S&S launched after Clarion.
I’ll have to give some thought to the bow lift theory.
Which 1st generation Farr 1 Tonner did you sail on?
I’m not into Footies at this stage I’m afraid, so can’t help with class specific input. There are various posts that talk about making a quick male mould using foam between shadows.
Prospect of Puffin - one of the three plastic Prospect of Ponsonby copies built in UK. The other two were Solent Saracen and Plane Crazy. Coming from the North West we were the poor relations but we still beat 45 South and Plane Crazy in the 1977 Fastnet!
Incidentally it was very early on in Plastic Puffin’s career that I learned about lifting rudders out of the water. We had no very clear idea how to sail her (owner’s previous boat was a Scampi), came on the wind grossly overcanvassed rounding the leeward mark (small Welsh island - the East Mouse) and nearly joined our namesakes who were happily nesting on it!
A.
I used to race occasionally on one of the New Zealand fibreglass production versions, of which many were built.
I think that Plastic Puffin was the nicest boat I have ever sailed on despite being very badly built and leaking like a sieve through every deck fitting!
At least she was until they changed the rule and Rob Humphries put a microballoon pot belly on her to up the rated displacement. That was when the IOR started to go very badly wrong.
A.
Hi to every body.
At the time I raised the question I was thinking about the “Nose diving” that the class M suffers and apparently is not alone. I therefore developped two different concepts, what I call “all in the front” bringing more volume ahead and positioning the max draft around Section 4 - the CB is avanced to across the section 5. This form, was dictated by the reading of Bethwaite books when discussing about planning characteristics.
The second is instead " all in the back" based upon the fact that putting the CE at a longer the distance from the bow, the “noise diving” could reduced, just the time to manouver. All centers are positioned forward the stern, the CB, the CLR and CE.
the max draft is on section 6. Is there, when I started to think about the front rudder. You have said almost all, here is one of my last design with multiple ballast choice as function of weather and consequently with variable LWL lenght. Wet surface was a driving point to. The fine bow is a sort of wave piercing type in order to reduce the drag effect that generally contribute to keep the nose under water among the succion effect when is too large and flat.
The “all in the front” is under testing and appears rather fast specially if the wind is above 3-4 m/s, the “all in the back” will go under construction after september once defined the max width and dept. I want to go narrover …
Any comments ?
Wow - that looks very smart Claudio! Well done.
Just a thought - given the very fine sections foward, what if you were to flare the topsides above the waterline forward to provide some reserve bouyancy - in the style of the Crazy Tube M, or the Gadget/Triple Crown family of IOMs? In might take away some of the good looks, but help prevent the bow from going under. If the prototype has no nosedive problems, the you could remove the flare in future versions.
Very interesting. Our logic seems to be very similar so fas as the ‘all at the back’ boat is concerned. The problem in a Footy is worse. Cube/square relationships mean that they are low on stability and hence much wider than a marblehead - so we need to fine the entry angle down to get good wave pentration and I do not think that the ‘all in front’ is really an option.
Muzza. Nice - but. With a short, fat, rather heavy little brute (on the building frame it looks like a cross between an armadillo and a Swan 37) there is a lot to be said for a lot of tumblehome. The transition from that to exaggearated falir in the upper topsides is more than even my debased aesthetic sensibilities will allow!
Angus