Kick-Up Rudder

I agree that hydrodynamic forces are not going to keep the rudder down. If it was efficiently possible, we would see a lot of unbalanced rigs with very long spade rudders running about per the aforesaid discussion. . No such thing as a free ride.

I think that what SP has done is clean up his blade hydrodynamicly so the drag is less conducive to lifting it. He has also moved it's center of buoyancy forward, relative to it's pivot, by allowing it to drop to vertical. This has also reduced the lifting forces dramatically, or at least aligned the CoB with the rudder pivot point to negate any moment developing around it. It would be conceivable to have a lifting rudder, that once the COB pivots forward of the pivot point, it would try to pivot forward instead of aftward.

 

It would be possible for the rudder to produce a depressing moment about the kick-up axis, but it would take a lot of leeway to do it, so it's not probable. With leeway, there would be a component of the lift that is directed in the forward direction relative to the boat reference system.

In the early days of aviation, some designers didn't realize this and built wings that only had diagonal drag wires in one direction between the wing spars to take the shear due to drag loads. At high angles of attack, the forward component of the lift caused the wings to collapse due to there being a forward force in the planes of the wings.

 

All of your comments have been validated.

The boat was sailing in 10-15 knot gusts, the rudder was full DOWN, and I untethered the kick-up. At first the rudder stayed down, but after a few tacks it crept up to 10 - 15 - 20 degrees of sweep. I was afraid that I would lose all steering if it was to pop up to 90 degrees.

The experiment was repeated 2 or 3 times and in each case, the rudder eventually dragged back.

Initially, there was either enough lift to keep the rudder locked forward, or there was so much friction between the cheeks of the rudderhead that the blade couldn't kick back. There was never so much lift that the blade could drive itself back down to vertical.

What if there was a horizontal foil (inverted-T, or anti-cavitation fence) somewhere on the rudder that would steer the rudder blade down if it started to kick up?

 

That could work, at least in theory. But it would have to be placed way aft of the pivot axis, in order to give it a sufficient pull-down lever arm. And it would add some drag, if that counts much for you.
Without a good lever arm, the foil would probably just prevent the rudder from tilting up above a certain angle, and would work at a high angle of attack. Hence even more drag.
In my opinion, the simplest solution is to mechanically block the rudder motion without impeding the possibility of automatic kicking-up in case of grounding. The best way to do that will depend on the current rudder assembly. Can you post a couple of pics?

 

It would not just steer the rudder blade down but the stern of the boat too!.

T foils can be made to give lift or sink and generally are securely fixed with a pivot mechanism such as fore/aft pin or rotating gantry. Also consider if the blade came up a bit, what angle would the T foil work at then?. Look at the cassette and fixed configurations already used by Moths and Cherubs.

One other consideration is the position of the pivot pin to the rudder blade and stock. In order to keep the stock from splitting apart with high loads more recent foils have the pivot well back where the bolt helps keep the cheeks together. My personal take on lifting blades on dinghies is I prefer a wind up clamp pin (with end thread peened over) and possibly a plastic or glass fibre locking pin. Metal pins which bend are brutes to remove if the blade hits something. Some cord systems are sort of OK, elastic as a downhaul a no no. Important to have a fixed stop so the blade is at the correct angle every time it is lowered fully. Mostly a vertical start to the leading edge of the rudder gives good results for control and reasonably low drag.

 

The T-foil should not pull the stern down if it has a non-cambered airfoil section and is set up at zero angle of attack when the rudder is fully down. But the latter thing is in fact not easy to achieve on a small boat with an uncertain position of the CoG.

A plastic locking clamp (not pin) is the best option, imo. It keeps rudder down when underway, yet yields softly enough to allow an easy rudder retraction when necessary.

 

It was my understanding that the OP had asked what happened when the T foil rudder Quote 'Started to kick up' Unquote?. Hence my previous one liner.

Yes, the plastic locking clamps, if you mean the sliding type locating into groove on head of rudder blade are OK. I have known too many of them have too much play for my liking, though so must be a good solid fit for heavy weather sailing especially. After all 1 or 2 mm at the head translates to a lot of movement (ratio dependent) at the tip.

 

Curious about your thoughts on this. What are the concerns? Recoil? Stretch?

 

I've designed and built many kickup systems and most have been refined down to an over center (pivot) down haul, on an auto release cleat (small craft) or a weighted blade, that finds it's own way in a strike (pops up, then falls back down, when clear). The weighted blade often has an up haul, but no down haul. I've never had issue with flow pushing them around, particularly the down haul equipped units, as they're fixed, until the auto release cleat is popped. Some of the weighted ones have been more neural buoyant, than weighted and these have moved underway, but the addition of a down haul and/or weight usually kept them in place. I've found the pivot location has little impact on this trend and I often place it dead in the middle of the chord, to save space and make a uniform circle of contact patch with the rudderhead/cheeks, which I find particularly important when the blade is up and being used as a sweep.

Attached is a kickup rudder assembly and typical of my efforts. You can see the pivot is dead bang in the middle and the up haul (marked lanyard) holds the blade in the down position. It runs out of the leading edge of the rudderhead, up, behind the pintles, through the tiller and back to an auto release cleat.

The other rudder drawing shows an earlier design with an off center pivot, which I needed to get enough lift to clear the bottom of the boat. The auto release cleat is shown on the tiller and the hoisting lanyard also runs up the leading edge of the rudderhead (in a shallow groove), through (behind) the tangs on the pintles, through the tiller and back to the cleat. My instructions suggest the lanyard could be shock cord, though I recommend 1/4" line. This is a fairly small boat and cord will do, though it's not my preference. Larger boats need a stouter arrangement, but this system is simple, has few moving parts and doesn't often fail or screw up. The lanyard should be replaced every few years of course. On larger rudders, I've placed the lanyard inside the rudderhead assembly, but it still does the same thing.

 

There are a few potential problems with elastic (shock cord) type downhauls, but one system can be OK if maintained. Worst system is where the shock cord is pinned to the rudder blade with brass or S/S pin, maybe screwed through a small loop. After a year or two and the cord has rotted, you simply have no pressure and it is often ignored by the owner. Then under pressure, say whilst planing the blade lifts slightly and weights the helm and a degree of solid control is lost. Also a lot of these systems use undersized shock cord, and a bad run so you need too much stretch to allow the required blade movement in the rudder assembly. Sometimes the shock cord is hidden inside the cheeks so you are not aware of the condition of it until it fails.

Better is a lanyard from blade fed through to a shock cord loop, which is at tiller level. Again this is often allowed to go soft and not provide enough tension in heavy weather. However it is more visible and much easier to replace If a rack hooking solution is used on the tiller, decent pressure can be maintained over a reasonable period of time. The elastic simply looses the ability to stretch and return over a relatively short time say 1 to 2 years.

PAR has illustrated a good solution which uses a longer life lanyard (not shock cord) and is much more solid, yet can allow 'give' if you hit something large. Nice work, keeping the lanyard 'run' simple, and accessible.

There is less problem with pure displacement small boats because they do not generate enough speed to up the pressure on the leading edge. However on boats that do plane at say 10-15Kn and up, let us say a Finn or 470 up to an 18' skiff the foil can take reasonable pressure. Most Finns use fixed rudders, probably the best solution. Most stuff like skiffs have elected for cassette systems to keep the foil in line vertically and prevent the helm weight incurred from pivoting. Not to mention the potential load sideways that the rudder cheeks might have to bear. The AR also comes into play with those too, and any T foil arrangement.

It is very rare for a dinghy to actually do monumental damage from a fixed rudder. I have seen just one transom taken out, from a well built wooden boat in around 50 years (down to UFO). I see more rudder problems with lifting/kick up rudders, including professionally made and marketed ones, than almost any other boat part!. Most of the newer ones are a lot better than older designs, but some still leave room for improvement. At least this side of the Pond. Quite a bit is down to poor maintenance.
Fixed blades have almost nothing to go wrong and are direct in feel and control. Hence clamping the side cheeks on a lifting blade restores this solidity and confidence for control.

One design of lifting rudder assemby I hate, is the old (gravity die?) cast aluminium spider cheek type. Having witnessed fingers trying to undo the retaining blade down metal pin being severed as the blade hit the beach.....

Sometimes I wonder why lifting blades are actually needed, especially on small craft. When I ask the boat owners, the usual answer is 'for shallow water'. Well as a lot of small dinghies rudders are only knee or thigh depth when fully down and almost no racing is done in this. Even launching off a lee shore requires the rudder to be in place and down, so what benefit for the lifting blade?. I have yet to witness some one sail through surf off a lee shore from 6" draft water... Maybe a lot of day sailers try and sail across shallow bars as part of the fun?, or expore very shallow lagoons and flats?.

The c/board will, if adjusted to, give you warning of shallow draft so you have some time to react. I fully accept that for day boats and similar, rocky uncharted waters, shifting sands in estuaries, a kick up rudder may be a useful item. For these vessels, a well thought out solution which minimises the drawbacks but retains solidity is the best course. Owners of this type of boat should understand the system and ensure it is maintained and will not let them down.

 

My waters are shoal and I use a kickup all the time. Rather then have my big, heavy centerboard act as the depth sounder, I'll lift it higher then the rudder and let the rudder do the work. It'll bounce along, establishing the draft in the area, until I'm clear, at which point it's pulled back down. Admittedly, no racing in these waters, but in my neck of the woods, you can be in knee deep water hundreds of feet off a shore line, so having this ability is handy. I use the rudder on my 23' boat, because, the centerboard is 500 pounds and not nearly as easy to repair in a hard strike, as the rudder. In smaller boats, I do use the centerboard as a sounder, but there have been several occasions I was running well off the wind, with the centerboard mostly retracted, just to have the rudder hit first.

The damage aspect I've found is in line with Suki's comments and usually related to stupid hardware or cheek arrangements. Pot metal parts, weak cheeks, foolishly designed pin and bushing arrangements, etc. It's not hard to develop a solid kickup assembly, but you do need to break a few to find out what works. I don't clip or eye strap hoisting lanyards, but prefer to glue them in place. Simply drill a hole, coil the line inside it and fill with epoxy. Sand flush when cured and it's married to the blade, until it breaks. If it does break, just drill out the hole again, insert a replacement line and repeat the process.

 

Don't worry PAR, I've bumped the fixed keel of a few boats on the wonderful sands of the UK East Coast.....

 

One boat I had was equipped with a lead shoe on bottom of rudder.

The rudder would still float if lost overboard, but floated vertical and not very high.

I liked the nicks and scratches in the lead shoe, as mementos of groundings avoided.
Comparable to notching your 6gun. Survived!

 

The latest centerboard I've made has a lead leading edge, that extends around the bottom of the board. This places the weight as low as practical, when the board is hoisted and the most vulnerable edges are easy to repair and accept damage readily, with little harm.

 

SS,

Thanks for elaborating.

Paul,

Thanks as always for your contributions and willingness to share your designs.