Design of dagger rudder for trimaran

Hi. I'm new to this list and have enjoyed some excellent posts.

I am re-building/modifying my 28' Hortsman trimaran. I plan to use a transom hung dagger rudder so I can get good steerage in the shallow tidal river I am moored in, with the rudder partly raised.

I've done some research and found it is quite straight forward to design and build a rectangular planform NACA 00XX board. The problem I have is how to get other planforms such as elliptical or a tapered trailing edge.

I'll probably use a NACA 0015 section to start. The leading edge will be vertical (straight). If I were to just taper the rudder by angling the trailing edge, then since the thickest part of the board is always at 30% of the cord from the leading edge, the thickest part would move closer to the leading edge - not buildable and doesn't seem correct.

What do I do, start off with tapering leading and trailing edges and then just tilt the whole thing so the leading edge is vertical and so is the thickest portion of the board? The horizontal sections would no longer be NACA 0015 and I don't think it would slide through the case.

Any help would be greatly appreciated.

Thanks,
jmckerrow

 

Actually, you should taper the leading edge back to a chord length of maybe 50% or 60% of the max chord length. A tiny bit of tapering on the trailing edge is OK, but don't do too much. You really don't have to worry about maintaining section shape at the tip. In fact, section shape plays secondary importance to planform shape. I think too many "boat designers" and people who play in the field of boat design spend too much time analyzing section shape. A lot of shapes work really well, but they are just the 2-D part of something in 3-D flow. The planform and tip shapes are much more important than section shape. So taper the leading edge the most, and fair out the tip so that it looks right, smooth, and fair.

Eric

 

Thanks for the reply Eric, very informative.

Being an engineer myself, I'm going to ask some more questions.

I've attached an appoximate profile of a dagger rudder I've seen on a recent trimaran design. It apears easy to achieve except I can't figure out how to get a smooth NACA section on the tapered section of this profile.

The dagger rudders I've seen all seem to have vertical leading edges. The trailing edges are tapered for the lower 1/3 to 1/2 of the board. The upper portion is straight for good support in the case.

I am hoping to build this board in a female mold so I'm not keen on the idea of just fairing in the tapered portion by eye. The method I've seen described on a couple of web sites involves the use of female templates and a coated shhet of plywood forced to conform to the shape by fastening to the forms. The leading edge radius form is usually made from a separate piece of wood routered to the correct radius. This is dead easy for a rectangular profile but a lot more complicated for a tapered rudder.

Looking forward to some good discussion on design and building methods.

Jamie

 

Jamie,

The planform that you show in your .dxf file should be turned around. Your longer leading edge should be the trailing edge and vice versa. The drag of this shape will be higher if you keep it the way you have it, and it will be lower (smaller tip vortex) if you flip it the way I suggest.

To taper the section shape, you have to taper thickness along with the chord. And as I said earlier, you do not have to be absolutely perfect in foil shape at the tip--just about any kind of streamlined section will work fine. You can scale thickness in direct proportion to chord length, so as the chord tapers, the section gets thinner. You can do this in AutoCad easily enough. The very tip should be rounded off from port to starboard.

Eric

 

Daggerboard rudder design and fabrication

Thanks Erik for the further clarification.

One thing about that profile I attached to the last post; the casette design was for a kick-up feature that also allowed the rudder to be tilted to give some balance (rudder tip could be moved forward of the pivot point). Might that have been one of the reasons for having a straight leading edge?

OK, might as well be direct and say the rudder design I am referencing is for the Farrier F-33 trimaran.

Also, I have read that inclining a vertical hydrofoil forward can relieve ventilation problems for surface piercing foils.

Does anyone have opinions/experience building a tapered foil using a female mold built from sheet ply fastened to female form frames?

Jamie

 

Rudder Planform

On a dagger type rudder that you want to be able to use partially retracted it seems to me that perhaps a rectangular planform might be the best compromise.
If this is a transom mounted rudder trunk the center of lift of the foil is already going to be substantially aft of the pivot line of the rudder assembly. By tapering the leading edge it seems that the center of lift would move further aft imparting even more force to the tiller/steering system as compared ,say, to a balanced dinghy rudder.
Also, if you taper either edge you will have problems holding the rudder blade securely in the trunk as it is raised. Tapering either edge will result in there being some degree of slop in the trunk in a fore and aft direction as well as side to side as the board is pulled up.
I built a dagger rudder assembly for my 16' hydrofoil last year and had to pay very careful attention to this because if the rudder blade tilted aft at all it would cause the t-foil on the bottom of the rudder to pull down a lot.Since the t-foil pulls down and pushs up at different times I had to make sure that it couldn't move forward either.
In this case w/o a t-foil it's not critical but any design that allows slop when the rudder is being used in a partially retracted mode might need to be looked at more carefully....

 

The straight vertical leading edge would have nothing to do with the pivot point. It is the result of someone thinking (In this case, I guess, Ian Farrier) that some taper is beneficial, so why not do it off the trailing edge instead of the leading edge.

Tapering the foil is for bleeding off the tip vortex which is the flow element that creates induced drag. There is not way to get rid of induced drag entirely, you can only make it bigger or smaller to a certain minimum. Tapering is a compromise to get an untwisted planform into a quasi-elliptical shape for elliptical lift distribution, the most efficient kind (smallest tip vortex and therefore induced drag). The best way to taper a more or less rectangular planform is to taper the leading edge back toward the trailing edge, not the other way around.

As for Lorsail's comments, you only need to taper a relatively small region of the tip of the blade, say a length up equal to the chord length. When the blade is retracted to the point where the taper starts, you don't have very much blade left in the water to do any good steering, so you don't have to worry about his comment re: the blade not sitting well in its holder.

As for sweeping the blade forward to counteract ventilation, yes perhaps a bit of foward sweep will do that, but you also run a strong risk of making the rudder unstable in flow. Planes have wings swept aft for stability of flow. When planes have wings swept forward, they are very unstable and hard to control. Forward sweep on a sailboat does not work very well. Don't do it.

Eric

 

Rudder force/tip

I agree 100% in angling the foil tip to reduce drag-good idea. Just don't taper the foil above the angled portion....
If the pivot axis of the rudder is vertical and the leading edge is vertical then on a dagger type rudder the center of lift of the rudder will be further aft in comparison to most balanced kick up rudders.
This means more of the force acting on the rudder will be transmitted to steering system.If the foil is relatively high aspect it won't make a whole lot of difference... But the "feel" is not as lite as a balanced rudder.....
If the foil was raked forward with the pivot axis of the rudder assembly still vertical then it might be posssible to balance the rudder. You'd have to look at it closely and make sure the area of the rudder forward of the pivot axis was limited to 15-19% of the total foil area.I don't think it is necessary or desirable in this case.
The new Ilett foiler Moths have the lower third to half of the daggerboard angled substantially forward to improve the loading of the main foil and increase the separation from the rudder t-foil. And Rutan has used swept forward wings successfully(among others).
So it can be done and will work but the improvement in this application is just not worth it ,in my opinion.

 

Ok, I'm going to post something which will be rather controversial...

I shall take Eric's recent comment to mean the planform has no effect on the pivot point. Well, perhaps no effect on where it has to be, but it certainly does have an effect on how much torque is transmitted through it. As we all know, torque is force times distance, therefore, the same force at a greater distance from the pivot (Hydrodynamic centre moves) means a 'heavier' helm.

Moreover, I do not think that Yacht designers use enough design theory,and I think that there is only now an increase in the appreciation of the problems we are faced with in terms of induced drag and loading. and with the advent of more powerful computers, and with CFD and tank testing, we will be able to develop far better foils and hulls. I do not think that anyone can criticise others for investigating the complex design side, and I would suggest that it shows a lack of enthusiasm (if nothing more) to consider improvement. One must remember, that one can fiddle with planform until you are blue in the face, but you are only playing with induced drag and torques (assuming the structural element is irrelevant).

The real difference, certainly in racing yachts, comes in improving the section used, and manipulating the section to delay cavitation and improve the L/D ratio. There are some good bits of analysis code on the web, the foremost being Mark Drela's X-Foil. I suggest that you have a play with it, read the various aerodynamics books, and work out why the section is so critical.

While I am at it, I hope this forum can be a place for encouragement in all aspects of design, but also a useful resource for those who would like to broaden thier knowledge. This can only occur if we offer the chance to be open about all aspects of design and building, and not be dismissive. I openly admit that I don't know all the answers, but I hope I can help, or refer the question to someone who does.

Tim B

 

In reply to Tim B.

I never said that planform has no effect on the pivot point. Of course planform has effect on the pivot point, depending on how the planform is distributed in relation to the turning axis of the rudder. My comments were directed at a specific question regarding a specific rudder design, which you incorrectly took out of context and assumed to generalize toward all appendage design.

Also, I do not begrudge other designers for not using available technology in the design of boats. We do what we can with the technology available to us. As I have said before, the America's Cup and Grand Prix racing boats are the areas where most of the technology is developed, because the owner's and syndicate heads have the budgets to pay for it.

Oh that we could all be Bruce Farr, who sits in a most enviable position to design such a cadre of boats that the fleets of his designs become veritable "test tanks"of sorts in which to test the nuances of design, even within a single campaign. Bruce and company are really good at what they do, and they can command the clientele and design fees to support it. More power to them. I have a lot of respect for Bruce Farr and his work.

But the rest of us in the field, the vast majority of us, do not have that level of client and we cannot charge the fees necessary to pay for research on every new design--the clients just don't have the money. I had a marvelous opportunity to test one of my rig designs at the wind tunnel at the Wolfson Unit under the direction of C.A. Marchaj back in 1986. In all my years of yacht design, that is the only instance where the client was willing to pay for a wind tunnel test program. I would love to go back and do more wind tunnel testing on my rig designs, but I can't afford to pay for it myself, and my clients can't afford it either. Most of our clients just want a boat designed, thank you very much, and they cannot afford to pay us for the research necessary to develop the latest and greatest in aerodynamic and hydrodynamic design. Sometimes, they do not have enough money even for a whole design, and all they pay for is the hull shape, never mind the structure that goes along with it ("Oh, I'll just figure that out on my own." Good Luck, and don't come crying to me when your boat breaks.")

It is up to us "other" designers to be aware of the technological advances and to apply what little amount of useful data is available into our designs. We cannot afford to pay for the research, so we must rely upon the generosity of others in the field to publish useful information that we all can use. Sadly, designers are much more often protective of their own technology that trickle-down rarely happens.

I tell these stories from the benefit of over 25 years in the business of designing boats. I, for one, am a frequent participant in this forum and am more than willing to divulge design information and guidance that I think is useful. Hence my contributions on this very question of section shape vs. planform. I am not dismissive of others. I welcome the contributions of others, yours included. I am only telling it like it is from my perspective.

Eric

 

Hi there,

Willing to build a boat from scratch based on sketches if found and I redrawed in solidworks to get full 3D moddeling and weight and surface calculations. Looking for all kind of parts I noticed all thos things cost a xxxxx fold of what they should cost? My idea. Even an ordinary rudder cost apprx. 600USD (Belgium) I think I could make a more robust one with waterproof/tight bearings, stainlessteel, alu, etc and having it machined by a machining company for less the half off that price?

But wat forces could I possibly expect on that rudder? Engine 150-160HP prop I dont know yet, maybe some kind of surface drive thing... any hints?

Thx,
Daniel Peeters

 

Dagger rudder design

Interesting exchange on design approaches Eric and Tim.

I deal with similar issues in my job all the time - I'd like to have the time and money for basic research and lab work but I have to fall back on the old empirical approximations most of the .

In the case of this rudder of - my main constraint (self imposed) is that I want to build it easily and accurately using a female mold without creating a faired plug first. The build method is one used by Kurt Hughes for daggerboards. A plywood tileboard is forced into the basic shape of the surface by fastening it to female form frames. The tight leading edge radius is made with a separate piece of molded wood. Obviously, this limits the number of shapes possible.

I think I'm just going to model the mold using a sheet metal CAD program and see what is possible to develop in a simple surface.

As to planform - Malcolm Tennant seems to find rectangular profiiles just fine for his very fast boats, the new TomCat 32 cataramaran out of Toronto appears to have elliptical trailing edges (kick-ups) and many beachcats appear to have semi-elliptical profiles with the leading edge straighter than the trailing edge. Guess I'll just have to see what I can reasonably make in a simple mold - try for the taper but if it's too much hassle, just go with the rectangular profile.

Jamie

 

Tim, I'm going to take issue with that (in a small way). I feel strongly that the "computer world" testing will never get close enough to "real world" conditions to be as useful as we would all like it to be. I was involved with one project where a hoghly-designed (waaaaayyy too many hours in the puter) foil was found to be less than useless in real life. The L/D was brilliant, total drag was far below any current foil, but when faced with a boat that pitched, heaved, yawed, etc, it basically (close your eyes if easily offended) sucked.

No amount of computer time will equal the action of a small boat in waves adequately to allow fine-tuning foils to extract that n-th of a percent of lift. In calmer water, or in bigger boats (AC sized is probably the minimum) then it can be done - for everything else, you are going ot be just as well off using a known good section, and making sure it is as fair as possible - which automatically ensures that it is not exactly the correct section anyway

I love researching stuff, and I love finding little ways to improve things, but I can never get paid for it. Believe me, I've tried.

Steve

 

Jamie,

You do not have to restrict yourself to tortured plywood or sheet metal for your female mold. You can use the plywood for the constant section, and use strip-planking + filling and fairing for the tip. And what is wrong with starting with a male plug? A male shape is always easier for the eye to fair than a female shape. The cost of the plug is not that much, and in fact may be cheaper and more accurate than going directly with a female mold.

There are at least two factors that might be influencing Malcolm Tennet's rudder designs: 1) He may always have done it that way, and he sees no reason to change. In fact, going back an updating designs is more drudgery and less interesting than doing new designs. Once it is designed, why change it, for relatively little effect? 2) He may have designed the shape that way when that shape was popular, but now he may not think that way. Which refers back to 1). If it works OK, don't "fix" it. Just incorporate the new thinking on the next design.

Finally, you can see by your own thinking that you are compromising: "Guess I'll just have to see what I can reasonably make in a simple mold - try for the taper but if it's too much hassle, just go with the rectangular profile." That means that cost and time are factors in your construction. 'Twas ever thus in boat design and construction. In the end, the ultimate refinements in design might just not be worth it for the current job at hand.

Eric

 

I think Steve hit on a useful point to note there... that doing it all on computer may not be a good way to approach the problem, but I was talking about using the combined knowledge to better design... anyway, now for something a little less controversial...

Jamie, I am currently considering a similar project, and the method I am considering is, in some ways, similar to yours. I am not, at present too concerned about the hydrodynamic design, though, as my intention is to consider the viability of the method which is as follows...

The same idea of a female mould is used, but it is CNC milled. This removes most of the carving/fairing problems, and allows any planform to be used. The foil is then laid up on one side using glass and carbon woven cloth and epoxy, then a Carbon spar is laid in (whilst lay-up is still wet), then the other side is laid onto the other mould-half, and the two halves are brought together. Having clamped the mould tightly, the inside is then filled with expanding foam to make the core of the foil, because of the expansion, ensure proper bonding to the layers of glass/carbon and the core.

That is the basic outline of the process, and I admit that the CNC machining is generally impractical unless there are facilities available, but the rest of the method should be of some use.

incidentally, you may like to look at the 'research' section of my website.

Best of luck,

Tim B.