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Guest Ketchup

Dunno if that was meant to be directed at me or not.

Far be it from me to stifle any ideas, I come from development classes as well as one designs.

I was half serious with my foil idea. Granted, probably not much use on KMM's boat, but could actually work on a long range cruiser. Think of the extra drag you could create by angling said foil downward, say 15deg. Could help slow the boat quite a bit in a big following sea. Like an inbuilt drogue of sorts. Could also dampen pitching when beating into nasty stuff.

Probably work pretty well on a foiling R or something

no definitely not you. You are a free thinker. Just a couple of people that think they know better based on theory.
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"What has KM got to lose in building a twin rudder design of the same weight (or less) with less wetted surface area and drag (one blade up in some /most situations) and with the benefit of maybe better control and less drag."

 

His money.  You think it's a great idea - have at it. Seriously, give it a shot. Show us how it's done, with your money

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Guest Ketchup

I understand your thoughts but I don't think KM is worried about money and having some bleedng edge Techno bits on board. (Could be wrong?).

 

If I was in his position I would go for it and just see.

 

It would not be hard to unbolt the twin, canting, whatever... set up and bolt a single rudder back on later.

 

In fact, if he is inclined, I would build both and compare. After all, it is just a couple of extra gudgeons/pintals? On the boat.... and money (which could be spent on other non boating stuff)

 

As for twin rudders on lighter wider boats.. sure that is where these things have been perfected, but any boat that "leans" can benefit from a rudder/rudders that cut through the water better and offer better control.

 

Just saying. KM is a big boy and can work it out for himself. After all this is just a forum offering ideas, and there are no guns being held to ones head.

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Guest Ketchup

T Sauce, The only assumption here is your assumption that no one has any experience...

Experince with a 930 with twin rudders?

 

No assumptions on peoples experience booboo. I don't think any one has done twins rudders on many old NZ boats...including the 930

 

 

So no one has experience.... that's a fact!

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I'd be wiling to put money on it that a 2 rudder setup on a 930 would be slower than a modern single rudder, but of course that is my own opinion and funnily enough repeating it over and over again doesn't necessarily make it right.

 

For what it's worth I have put new rudders on both of the boats I currently own one 26ft and one 40ft both improved control vastly over the old barn doors they came with.

 

I also have invested a lot of time and money to get them going better in other ways and my time and money is finite so you have to decide where best to use it. Weight reduction, sails, foils and simple reliable control systems are the best way to go in my opinion.

 

Of course getting out there and sailing the thing better is cheap and effective.

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Okay, so putting the stuff I studied at Uni to some practical use for the first time in ages... Does a 930 pitch much beating in the Gulf? 

 

Reason I ask is pretty simple, when the boat is heeling, every little pitch drags the rudder blade sideways through the water, and the reverse is also true, ie pull the tiller and the stern lifts, push it away and it sinks.

 

That should make a vertical foil, which is only slicing like a knife as the boat pitches, create less drag.

 

In addition, any weather helm that is generated in a flat water pinch mode, would be purely sideways, not wasting energy trying to lift the stern out of the water.

 

Canting or twin? Canting with fine adjustment would be best for my way of thinking, because downhill you don't really want the rudder sticking out sideways, or again you end up with the lift/sink thing happening.

 

I do wonder, tho, about tiller movement when canted. Would there need to be some form of linkage so the tiller still travels level with the hull? I've only sailed on a 930 once, about 20 years back, and can't remember much about the cockpit. 

 

My studies were sports coaching, specifically sailing, so I had to look at buoyancy, resistance, air and fluid dynamics, plus all the usual sports psych, nutrition, processes, training etc.

 

Any boat designers feel free to correct any statements I've made above.

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The canting is going to be tricky on many levels right thru to fitting auto pilots. As tricky is what I'm trying to avoid canting is losing it's shine.

 

I'm getting conflicting ideas and advice on twins. Making them is easy, fitting them is easy, cost isn't a concern. I both like and dislike assorted bits with the twins so still pondering that.

 

Visually, not to fussed. A Mack truck doesn't look even 1/100th of the sexiness a DB9 does but the truck is the more efficient of the 2.

 

A single is easy to do. Clean simple but not super effective when heeled a lot, something I'm letting her do less. Also the current blade bends more then Beckham so that can't help.... nor does it help the nerves when in lumpy stuff as it looks like it's mm's from busting... but it hasn't and doesn't show any signs which was good.

 

I do have to build everything as all I have is a naked transom...... and the old blade that's delaminating. I like the building process, in fact I'm having a ball doing all this, even the twisted upside down in a back corner sanding bits .....sort of.

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I don't have enough design knowledge to give any advice on how many or design of rudder etc etc, but I do have enough knowledge to suggest that a few points are being over looked here. And I appologise that I may not describe this accurately with the correct terminology and I will stating some rather obvious to all in my endeavor to get to what I am trying to describe.
Firstly, when a Boat is going down wind, the Hull is flat to the water and a Rudder simply steers by swinging the Stern left or right. But when a Hull is "sailing" into wind and on it's side, there are several different forces being applied to make the boat move forward. The Hull has the particular shape that it does, so as it acts partly as a wing. The Keel is also acting as a wing and both are literally "lifting" the Boat, but because it cannot actually lift out of the water and because of the angle, all the forces combined causes the boat to move forward. Some may have noticed that that a Boat Hull does not travel in a straight attitude toward it's destination, but it is actually sliding slightly sideways.
OK, so the Rudder is doing something similar. Or I should perhaps say, that because a rudder is also a wing, its action is also similar. In fact, the Rudder generates 10% of the lift. The Hulls that have twin rudders are designed to be a lot more up right, because of the wide shallow Hull.They go better down wind.They do not perform well in upwind conditions. So going to a twin rudder on a more traditional Hull shape like KM's, would actually be detrimental, because the Hull will lose some of the lift that all of those various points generate. 
That is most likely the issue that BooBoo was finding on their Boat.
I assume ( I didn't read the starting comment for this thread) that the issue is the Rudder becoming less effective at steering as the Boat heels over. But really the best way to solve that is a longer rudder, or more aggressive profile. That comes at a cost of course (leaving aside any possible rule violations) of drag. But that it always going to be an issue anyway. The more of anything in the water, the greater the effect, be it controlling or whatever, but the greater the drag. It's always a case of finding the 50/50 point of affect/advantage vs loss/disadvantage.

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Guest Ketchup

Nice thoughts there wheels but I must correct a few things...

 

1..rudders create no liftn(upwards) as do most verticle surfaces on a yacht. They only create drag. If you put horizontal wings on then you would create lift (depending on their set up)

 

2. Their is "lift" generated "to windward" by the keel shapes interaction with the water (and the horsepower of the sails) and there is upward lift generated by the keels bottom shape and wings (if they have any) as long as they are orientated for lift versus stability. But the rudder offers no upwards lift and very minimal "to windward" Lift.

 

3. Single rudders suffer from many problems. Namely being in turbulance of the keel/prop... and they lose bite/control when heeled.

 

4. Twin rudders typically are angled to allow for maximum control weather the boats is flat or heeled and they are in cleaner water (especially the leeward one. They also offer better control for the same surface area, less draft, are less prone to loss of steerage and Give a benefit of two blades to break rather than one.

 

5. Twin rudders, by virtue of their smaller size actually have less drag/friction than a single fatter/stronger blade. That's because they have less overal'load on each and can be made lighter and narrower (chord) and thinner.

 

The fact that most desigener of fast boats are now well converted to twin blades proves the point. And as I show abovethey are now on narrow boats as well as wide boats.

 

But the most import and thing to remember is this...

 

With a single rudder every degree you heel the rudder heels a degree to weather and loose bite with the water. At 20 degrees that could mean the rudder is 20 degrees out of verticle which mean it has lost alot of "bite" and is losing control.

 

With twin angled rudders the more you heel the better the leeward rudder works and the better overall cotrol. (It just a matter of getting the angles to suit the boats usual heel angle uowind.

 

Bruce farrs twin rudder design above is narrow and fast! And I think he would know more about boats than most people put together.

 

Twin rudders of the same drag offer better control/ performance and reliability than a single jobby.

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IMO that is not entirely correct Ketchup.

A rudder DOES or at least can provide some lift when heeled. Depending on it's angle of attack on the water flow, it can work like any other foil, including the keel, providing lift - both up and to weather. When vertical, this lift is mostly to weather. When heeled at 45 deg, it is 1/2 to weather and 1/2 up. If the angle of attack is 0 - ie the water flow is laminar and the rudder is perfectly aligned, then there is no lift, but most boats carry a little weather helm, and then there IS and angle of attack, and lift. Also drag of course.

The twin rudder design gives the leeward rudder more lift to weather, as it is more vertical.

My training was in mechanical engineering, not naval architecture, but that is my understanding. Happy to be educated by those more knowledgeable than me, and Farr would certainly be one of those :-)

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You are pretty darn near spot on IT.
Ketchup, lift does not mean lifting up vertically. The rudder is not trying to lift the boat up out of the water as such. But it is indeed lifting the boat forward. Or forward when in relation to and opposing the other forces trying to lift the bow. The Rudder generates as much as 10% of the overall lift. But of course, that will depend on design. You have to always think about angles of force and how they all relate to a final motion. You have Sail shape, Hull shape, Keel shape and Rudder shape and all of them are wings that are generating a force in some particular direction. Acting against that force are two things that also aided in creating the forces, and that is both the wind and the water and to some point, gravity. Because gravity is stopping the boat from jumping up out of the water. The result is that the boat moves forward and the faster the boat moves forward the greater the forces these wing shapes create.
The amount of shape a rudder has, determines how well it will "hold" for want of a better word, before the laminar flow breaks from the surface and the Rudder stalls. A flat plate will stand very little before it stalls a lot of foil shape will be more efficient at slow speeds, but it will also produce drag. So a compromise between effectiveness and speed is always a factor in any design.

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Guest Ketchup

Technically/theoreticlly you are partially right. but the lift is not productive in terms of the rudders control of the boat or lift to weather and it is counter productive to keeping fore and aft trim level and the rudder in the water.

 

If you do a loadings vector diagram/calculation the final force and direction would be more up than to widward with no resulting gain in speed or climbing to weather ability.

 

But even more critical is the fact that all the sails force is transferred into forward momentum (at its most effecient) when the keel is verticle (the rudder as well) so any percieved lift from the rudder being heeled is well and truly negated by the keel being heeled and less effective.

 

Remember a heeled boat has less draft and lateral resistance than a boat sailed flat ( unless it has wings)

 

After all... every degree of heel reduces the lateral resistance of the keel and the forward momentum it generates. but It also heels the rudder and creates less lateral resistance/steerage and more helm loadings.

 

Also, when heeled, the sails are are not at right angles to the wind (travelling along the surface at +/-5 degrees max) so maximum power is not gained (through spillage).

 

Even with 0 angle of attack and/or weather helm there is only lift to weather if the foil shape is creating lift this would be very little unless you put a trim tab on it.

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Guest Ketchup

You are pretty darn near spot on IT.

Ketchup, lift does not mean lifting up vertically. The rudder is not trying to lift the boat up out of the water as such. But it is indeed lifting the boat forward. Or forward when in relation to and opposing the other forces trying to lift the bow. The Rudder generates as much as 10% of the overall lift. But of course, that will depend on design. You have to always think about angles of force and how they all relate to a final motion. You have Sail shape, Hull shape, Keel shape and Rudder shape and all of them are wings that are generating a force in some particular direction. Acting against that force are two things that also aided in creating the forces, and that is both the wind and the water and to some point, gravity. Because gravity is stopping the boat from jumping up out of the water. The result is that the boat moves forward and the faster the boat moves forward the greater the forces these wing shapes create.

The amount of shape a rudder has, determines how well it will "hold" for want of a better word, before the laminar flow breaks from the surface and the Rudder stalls. A flat plate will stand very little before it stalls a lot of foil shape will be more efficient at slow speeds, but it will also produce drag. So a compromise between effectiveness and speed is always a factor in any design.

Wheels I did not just mention lift upwards. I mention the two types of lift generated of foils... upwards (or downwards) depending on horizontal foiled surfaces such as wings on keels , and lift a foil gives to weather ( and forward) by opposing the forces of the sails and generating forward and windward forces.

 

Rudder do not give very much lift to windward and forward.

 

To explain this look at a plane! The main wings give the Lift up and the drive forward (as A keel does). flaps/flaperons, aerilons etc provide this and adjust to do this across a wide speed range. But the verrticle rudder on the back does not provide any lift up, or left or right, or forward Lift. The same applies to a rudder on a boat.

 

now look at what the old AC boats did to gain more lift from their keels.... they put a trim tab on (a wing flap for a yacht) to provied even more lift to windward.

 

wheels, there is a small amount of lift generated from a rudder foil when its held in a perfect Smooth laminar flow situation. but this is near impossible to achive when sailing. Remember the foil shape of the rudder is designed to reduce stall from constant changes of attack (tiller movement) and not really designed for lift (to weather and forward) that is what the keel and tab is for.

 

Thin rudders are fast when they are held in neqr static angles of attack at low angles. But if you move your rudder at high angles then a fatter (low stall) shape is the go. And these provide low lift

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