Design Innovation: Positive Caster Rudder

Sunday, 13 June 2010 21:03 | Written by  Johan van Blerk
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The PCR Positive Caster Rudder - Note the shaft angle! The PCR Positive Caster Rudder - Note the shaft angle!

[Editor: Paddlers around the world build custom rudders of all shapes and sizes.  But Johan van Blerk has taken rudder innovation to a new level – by altering the angle of his ski’s rudder shaft to give it “positive caster”.   Will all skis be built this way in future?]

For three years now I’ve been wondering if positive caster would improve the performance of a surfski on waves. Thinking it through while sliding sideways on waves I couldn’t come up with any reason why it shouldn’t. Paddling on flat water I thought, might be its only drawback but in practice this hasn’t been the case. PCR has no advantage on flat water.

I recently damaged the tail of my Fenn Millenium and decided to build in my PCR (positive caster rudder) idea. I chose a fairly aggressive rudder post angle of 23 degrees as I wanted a definite change and also think the chosen angle is the best bang for buck. Less PC will give less downward force and more PC will give more of downward force but less turning force. The implementation was simple enough and achieved simply by moving the lower rudder post hole 55mm further forward and leaving the top one in the deck well as stock. A block of epoxy glass was bonded between the hull and deck after being suitably drilled. The rudder runs directly in this block.

Positive Caster Rudder

New shaft angle: 23 degrees

Modified Stock Rudder

The stock elliptical rudder was simply modified by grinding away and exposing more of the rudder shaft to penetrate the now longer rudder tube and the resultant gap between the rudder and hull was built up with epoxy/glass. The current installation limits the steering angle in one direction to 33 degrees and this has proved adequate on the water.

Visualising PCR

Compare the sets of photos below – it’s easy to see how the PCR creates a positive incidence that pulls the tail of the ski down.

PCR-197

Positive Caster Rudder

Positive Caster Rudder

The Positive Caster Rudder

Conventional Rudder

Conventional Rudder

Conventional Rudder

Conventional Rudder

Testing the PCR Rudder

I’ve only been able to test PCR on ferry wakes for now but from the moment I got onto the first wake I knew I was onto something. The wakes I ride are regulars and I’m familiar with what it takes to stay on them without spinning out (broaching?). They’re pretty easy to ride if you’re reasonably fit and don’t lose concentration because the spin out point of no return is a fine line between running parallel with the vessel and therefore proceeding in the same direction as the vessel and maintaining an acute angle to the wave. With PCR the fine line is gone, replaced with a broad range of control and feel. Maintaining speed with less paddle effort also seems to be the case with PCR and is possibly due to the fact that previously a large (high drag) rudder angle had to be used to maintain direction while PCR requires far less rudder angle to be effective.

To date I’ve not been able to do a decent downwind but have no doubt that the stunning performance on wake waves will translate into vastly improved downwind control and speed especially on technical down winds where waves have to be ridden at an angle where increased speed and control make a major difference.

How good is it?

How good is PCR? I’ll stick my neck out and say that soon, all open water racing ski’s will incorporate PCR and Molokai will be won with PCR, possibly by next year.

PCR v Conventional

And side by side...

Comments

(We lost the comments when we upgraded...  But by the magic of SQL, here they are... as requested!)

Date Name Comments
2010-06-13 17:11:41 Shaun Rasmussen I would be curious to see someone compare these positive caster rudders to winged rudders. Seems like the ideas are similar, and that each would have advantages and disadvantages.
2010-06-13 21:48:56 Darryl Duck I'm just wondering if the benefit you perceive is just better feedback because of the caster. Does the rudder make the steering a little heavier?
2010-06-14 01:45:19 Chris Morgan Sorry Johan i dont get what your talking about. the photos look the same :'( not trying to dis. you at all. my evo rudder does some funny things at times in the flat and surf so i am interested in any mods possible.
2010-06-14 03:18:52 Rob Mousley Ok, have a look at the top photo that shows the angle of the rudder shaft. Conventional rudder shafts run vertically. This one runs at a 23 degree angle. Look at the bottom photo - the PCR is tilted whereas the conventional rudder is not. That tilt is what pulls the tail of the ski down, whereas the conventional rudder tends to push the tail up. The theory is that this improves the handling of the ski when going down waves.
2010-06-14 05:29:40 Nico Redelinghuys Well done Johan, it makes a lot of sense to me! I assume you had to adjust the top end of the shaft similarly to achieve a straight line pull from your cables? Could you see if there is any difference in the depth your ski's nose dives (the waterline) on a swell? The Downward pull resistance at the rear I would imagine is that bit of assistance one needs when the ski turns through that critical broaching line.
2010-06-14 05:47:57 Jamii Hamlin Johan, having experienment with adding a 2nd steering box 300m forward on my old millemium to improve dwd handling and tracking, I feel your innovation would be of great value to paddlers alike. Unfortunately great concepts are often negelected by manufacturer as they are difficult to be incoroprated into the manufacturing process unless there is enourmous comsumer demand. So its going to be intersting to see if the likes of Fenn, Epic, Green 7 and alike consider to adopt such new improvements or look to provide similar alternatives. My only concern with an angle shaft is the pintle linkage that would be required to turn on an incline/decline angle that might cause problems with clearance. -It might reveal which manufacturer have a fool proof pintle and steering yoke systems.- Would your concept still work if the steering shaft remain straight, but instead the steering pin were reversed to angle forward in the rudder layup to replicate your 'positive' castor angle?
2010-06-14 09:44:43 Johann van Blerck Chris, I think Rob has pointed out the difference in the bottom photo. Visualize the water flowing from left to right and your view is of two rudders from below, PCR being the one above the other. Nico and Jamii, the cables themselves are unchanged. The top of the rudder shaft is in the standard location and looks normal at first glance. Only when turning left for instance, the left arm (crank) touches down on the well deck as it moves back and is simply bent up away from the deck to provide clearance. The end result is a wide "U" shape. Need more clearance? add more "U". The arm moving forward of course has no problem because it rises above the deck. It's not a real problem because the arms are only moving through a 23 deg off horizontal plane. Jamii, it's all in the angle of the shaft through the hull. Changing the angle within the rudder layup will do nothing. Nico, If I paddle fast on flat water and turn hard the bow rises up significantly as the ski turns.It's not a big rise but it's there. This rise will increase with speed. Shaun, I've no experience with wings but I'm sure they may be effective on their own. Wings cannot be used with PCR as they will cancel the down force generated and produce a less efficient rudder (more drag) when going straight ahead. One paddler who contributes to this site has already ordered a new ski and specified PCR at 23 degrees, promising to come looking for me if it doesn't work! He may chime in here. An easy way to see the effect is to straighten your arm with the palm flat at your side. This is a standard rudder with water flowing toward you and twisting your straight arm left and right, replicates a stock rudder and the left right forces. Now swing your arm forward 45 degrees keeping your palm flat. Now twist your arm left or right as if it were a rudder shaft and the turning force with the new down force component can be visualized There has to be lot's more to come from PCR. Other angles may provide more benefit and the rudder shape itself will have to evolve some to take advantage of PCR. I'm on a wonky dial up but will add a description of how I did my Millenium and how I would do it to a ski with minimal surgery.
2010-06-14 11:10:58 Johann van Blerck I had to remove the tail end deck on my Millenium to do other repairs so adding PCR was easy. I don’t know how current ski rudder posts are incorporated but I can describe how I would add PCR to a sound Millenium, proceeding as follows. Remove and dump the deck well cover and then cut back the ledges formed by the deck to expose the entire well. It’s important not to disturb the forward part of the well where the cables and their sleeves penetrate as the existing sleeves can be a real pain to reinsert. Cut a minimum 75mm x 75mm hole in the well deck including the entire existing rudder hole in the cut. The old rudder tube should break out quite easily with the 75 x 75 square as it’s a simple block of wood and a stainless tube. One block of wood is bonded to the underside of the well deck and another is bonded to the hull bottom with the stainless tube connecting the two. I removed both blocks easily and discarded the tube. I fabricated a glass epoxy block (roughly 75 x 75 x 25) which I shaped to fit between the well deck and ski hull. I pre drilled the new shaft tube directly into the epoxy block and the rudder shaft runs directly in the epoxy block. The block has to be tapered a bit as the deck well and hull bottom are not parallel. The hole at the bottom of the block is 55mm ahead of the top hole which matches up with the existing hole in the well deck. I lightened the block by drilling it being careful not to penetrate the drilled rudder tube. Set it all up in an epoxy bog replacing the removed well deck, lining up the holes in the well deck and finally bond the well deck back in place. One advantage of using a block is that it supports the hull where a rudder strike usually causes the rudder to bend and penetrate the hull. I’ve added poor quality pictures of my crude block and a shot of the underside of the deck well to www.surfskimod.blogspot.com I modified my existing rudder by exposing more of the shaft to penetrate the now longer rudder tube but this has left very little within the rudder itself and I expect it may break off when heavily loaded. I need to build a complete new rudder.
2010-06-14 14:50:50 anton erasmus Well done - I am sure that there is some benefit provided the right angle is found, the right rudder shape found and the best actual positioning of the rudder on the ski is sought. I do have a concern that the rudder will increase the drag factor and might change the nose/tail balance of the ski. That said - I am interested enough to do some experiments.
2010-06-14 15:15:01 Johann van Blerck Darryl, well spotted, the rudder peddle feel is heavy as a result of the geometry but IMO it provides very good feedback. Theres no way the feel of this rudder can be fooling me into thinking I'm doing something I couldn't do before.
2010-06-14 23:57:27 Chris Morgan ;D yep ok guys tx...i can visualise it now..interesting concept. Any thoughts on why my evo gives a shudder when completing a turn back to straight ahead (even foot peddles) I think its something to do with the space between the reed deflector and the rudder ??
2010-06-16 02:05:55 Hiro Colombani Sorry but I don't get it. I understand what you've done and how the rudder act as a wing in the water to create a downward force, but I can't understand why you want a downward force. I feel like I'd rather angle the shaft the other way for the rudder to create an upward force, lifting the stern instead. What am I missing ?
2010-06-16 05:43:02 Johann van Blerck Hiro, Your'e right the downward force, in theory, is useless for everything except riding a wave at an angle. On open water, riding waves at an angle is important and maintaining control when doing so is also important so that should be enough. On larger waves with the standard ski running down slope and pacing the wave the rudder can induce your suggested vertical lift at the tail and reduces control considerably. Inducing lift with negative caster would make wave riding a real headache. Lift vertically may well help with turns around a mark on flat water but that’s where its benefit ends IMO

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