Rudder Question

As some might recall, I was planning on making a kick-up rudder for my little Kite Dinghy.

I acquired some Meranti marine ply and epoxied together with some 4oz between.

I have been thinking on shape and researched rudder shapes...check Phil's Foils and some other sites.

Frankly, the rudder shapes are really all over the place, not the fore aft shape but the general shape from side.

Seems to me, a 14 foot dinghy is a 14 foot dinghy. Sure, SA, center/dagger boad and aspect have a bearing, but the rudder shapes seem so different as to indicate more aesthetics is involved than design.

Is that so?

If so, why would a longer, straighter rudder not work or even be more efficient than what comes on a Finn or FJ, class rules notwithstanding?

In my case, I have a barn door rudder on the Kite, but its design requires a new head and blade to be kick-up...so I am using a Laser II head and cutting a slightly longer and wider blade but not barn door.

Current rudder and boat has virtually no weather helm in any breeze.

I intend to match that characterizing with longer and straighter, and wonder why there is not more conformity in other boats?

 

The basic differences are due to fashion, draft requirements and balancing the helm. Barn doors emerged as a way to provide lower draft, good surface area to balance the helm - at a time where speeds were lower and hunting for a 2% improvement for hulls going greater than windspeed wasn't an issue.

Older, slower displacement designs need lots of rudder to get the hulls to turn - a high aspect ratio skinny rudder from today's boats would do nothing but ventilate and stall if turned on a older boat.

Since you aren't allergic to experimenting, I'd make up a couple "rough" kick-up shapes to try in your kickup headstock and see what you can get away with - if no one with better knowledge doesn't provide you with the "right" profile. It would be a great exercise to document the process and your findings. Once you determine what works best, then you could refine the shape and finish to your liking.

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CutOnce

 

up-grade

I have replaced barn door rudders with newer designed foils on several "traditional" boats with very good results. If draft is not an issue, a modern high aspect foil will always work better, and is usually not hard to engineer as a kick-up or lift up. CE and pivot points do have to be correct and strong- a deep foil can be very powerful with deceptively little tiller force. B

 

Thanks cutonce.

Bruceb...Am using different head. Actually only head is more or less not issue, as it was simply top to rudder blade.

By, CE and pivot points I take it you refer to pintle/gudgeons, kick pivot...I am confined to both, in using the Laser head...

Pintle/gudgeon attachments to boat are higher and closer together than orig and closer, as Laser II transom is lower. Pl;an to add carbon strip to strengthen already reinforced transom center.

Kick up point should not matter.


Will take pics to clarify and post.

 

besides what is fashionable, high aspect ratio rudders will have less drag for the same amount of rudder output, but are also much more sensitive to tiller input and are easy to stall (making them draggy and ineffective). Low aspect ratio are more forgiving, more stall resistant, and less unacceptable to damage, but will have more drag.

So you see high aspect ratio dagger boards and rudders on performance sailboats (which takes more skill to use effectively), and low aspect ratio on cruisers, day sailors and work boats.

 

BB, in designing your kick up rudder you may want to have a portion(up to 19% ahead of rudder turning axis. This has no effect on weather or lee helm but does reduce the "pull" on the tiller with weather helm.
This is one of the best articles I've found that explains the difference. It's directed at cats but applies to any kick up rudder: (scroll down to "rudder tug") http://www.thebeachcats.com/news/15/about-weather-helm/
Every kick up rudder I ever made had at least 10% of the area forward of the rudder turning axis. Reduces the physical pull on the tiller w/o affecting weather helm. On a small non-high performance dinghy I wouldn't go with too high an aspect ratio because the rudder will stall easier. Rudders used in a cassette where the rudder is lifted vertically can be designed with a slight forward rake to accomplish the same thing.


Lower left picture is on an O'day mariner. You can see the notch on the leading edge of the rudder at the top bringing the leading edge close to or just forward of the rudder turning axis.

 

Doubt a high aspect ratio i-14 rudder would make a Beetle Cat happy. Just saying. Horses for courses.

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CutOnce

 

Thanks.

Yes, C, I knew that, but in my case, needed kicker and figured to play with profile a bit.

Then, when I went though the examples, it seemed some boats of a similar length with different rudders would really not realize a performance change, say between "squarish and barn-doorish," laminar flow considered, if you get my drift, so to speak.

I have idea of what the answer might be, but not sure, I am asking 'cause I do not know.

Bruceb seems to have some experience that unscientifically bears out my guess.

Doug hints at that I already figured or guess to be helpful helm-wise, though more sensitive, requiring more attention and anticipation.

I am always experimenting with everything and this project is fun, mostly because I am just going by seat of pants because the boat is so small yet hot enough to beg interest.

Years ago, I would have gone ahead, with careful thought and learned. So far, my guesses have been correct, but the WWW questions are requirement.

+1 to all for helping.

 

more rudders

To take this a little farther, I used to be a sailboat dealer, refitted and sold many small used boats with replacement rudders (rudders seem to be the most often "lost" part), and never met a boat that didn't work better with a deeper foil rudder. Doug's cat article covers the design issues very well. Sloping transoms can present some real challenges to getting a rudder to balance and not be subject to "self servo" action when turned. Some of the older boat designs used wider blades to help overcome other design challenges, but that does not mean that they are good or even work very well. I would call them "modern" foils, but even that is not really correct- many of the useful foil designs pre-date world war two. A 12 %-14 % decent aspect ratio foil will hold on long after a barn door shape is just making a larger wake. A craft like a Cat boat that uses a large shallow rudder to keep the draft at a minimum can be transformed by a good deep foil. I have done it!- Nice light helm and almost impossible to stall, plus faster. Large or small craft are no different, the same rules apply whether it is a Beetle cat or a foiler moth- and reduced drag ALWAYS works best. BB, your laser II rudder head should be fine, those are fast boats that put more load on the head than your boat will. I think you will find that as long as you have about the same area as the original, the actual blade shape is not that critical. B Post pics

 

Rudder Foil Shape

Thanks Doug and bruceb. Will post pics tomorrow. I feel better already.

 

Doug. Great piece on rudder and balance. Thanks much.

 

The technology of high aspect ratio ratio rudders and keels applies best to situations where the flow speed over the foil is adequate for the foil to develop lift. Short chord foils may not develop adequate lift in low speed operations, therefore the foil section has to be appropriate for the intended usage. This is simple physics.

There is no such thing as a one-size-fits-all best approach, and you should give the original designer credit for choosing a profile that fits his intended use. Keeping flow attached, and generating lift at the intended usage speed is pretty important.

If you are going to experiment, you should try a range of conditions, wind speeds, sea states and venues. I'm speaking out of experience here as the original rudder designed for my single hand trap boat miserably stalled in low speed/ low wind conditions. We had to change the rudder size to get it to work well in light conditions, probably compromising some drag in higher winds. Everything is a trade off in design.

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CutOnce

 

Rudder Foil

CutOnce, Thanks.

I figure to go between low or no and high aspects for very reasons you point out. Low pressure control and with enough forward edge to lighten tiller load per Doug's article and past note from Par.

Here is promised pics. Center is orig barn door. Works fine, but head etc not conducive to making good kick-up blade conversion, hence the right rudder head, orig for Laser.

Laser head will go on the rudder on left, Meranti ply with glass center top now cut to fit new head.

To sand and shape, then glass.

This boat is only 11' 7" in length, as you can see in second pic. Refurbished 73 Kite Dinghy.

 

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BB, why so much more area? Looks like more than double? Did the white rudder in the picture not work?

 

At under 12 feet, hull speed is about 5 knots, regardless of how much power is developed. A short dinghy like this is going to be stuck in it's own waves, and will probably never be able to climb out of the trough and plane.

Sure, the Laser blade could work, but it certainly would not be as positive a control as the larger barn door - because it will never get the laminar flow necessary to develop superior lift.

The cool part about having a working headstock that fits the boat is that BobBill can tweak the larger rudder's size down a little at a time to optimize the feel and performance.

Dinghies, unlike keelboats are really sensitive to the lateral area provided by rudders - and it's location in terms of overall CLR. Balanced rudders with a portion of the area in front of the pintle/gudgeon axis may change the overall CLR enough to affect handling - worsening actual weather helm, but desensitizing the effect from the person holding the tiller. Although you may not "feel" the weather helm, you will have more turbulence developed - and hence slower speed.

All the reasons for balance rudders are good - but only if they don't create other problems. Whatever you end up with should be considered in terms of the original design and real world testing. A tiller that "feels" better but causes a drop in speed isn't worthwhile.

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CutOnce