Rudder Cavition.

[Joe]

I noticed that the rudder tends to cavitate when sailing downwind (with the spinnaker) in
strong conditions.  On other boats that I have sailed (and windsurfers) this leads to spin
out and knock downs.   The Johnson seems to behave better: even though there is some
loss of rudder control, the boat continues to track downwind.   My question, though, is
whether I am doing something wrong.  We are sailing downwind with the rudder partially
retracted (perhaps half way).  Should it be further up?    The spin is properly trimmed.
The main is close to (if not just touching) the spreaders.

Any suggestions?

Cheers,

Joe

[Jeremy Lyman]

We don't adjust the rudder at all for downwind sailing, but we do raise the centerboard depending on the wind and waves.  This makes the boat track diagonal, so the bow isn't pointed where you're going, the tiller is.  I can't speak for much for cavitations since I'm normally on the pointy end, but I think they're more likely to happen when the rudder isn't all the way down.

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[derek mckesson]

Deff remember this, need rudder all the way down, and cb up a little when reachin, makes all the diff!
 

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Date: Thu, 20 Dec 2012 10:04:18 -0500
Subject: Re: [johnson-18] Rudder Cavition.
From: jea...@gmail.com
To: johnson-18...@googlegroups.com

[Joe]

Yeah, what I meant to say is that the centerboard was halfway up.  If the rudder
slips backward, it is a major problem when the boat is in the teens.   I have already learned how to secure the
rudder  enough so that it does not  move at all....   Still wondering how far the centerboard should be
raised in strong conditions downwind.

By the way, the reason (I believe) that the Johnson can tolerate rudder cavitation downwind
(when compared to boats such as the U20 and Viper 640)  is that raising the centerboard
moves its center of resistance aft while changing its angle of attack (so it does not also
cavitate).  The boat is much less likely to pivot on a centerboard angled back then on
a keel fixed straight down.

Cheers,

Joe

On Thursday, December 20, 2012 9:04:18 AM UTC-6, Jeremy Lyman wrote:

We don't adjust the rudder at all for downwind sailing, but we do raise the centerboard depending on the wind and waves.  This makes the boat track diagonal, so the bow isn't pointed where you're going, the tiller is.  I can't speak for much for cavitations since I'm normally on the pointy end, but I think they're more likely to happen when the rudder isn't all the way down.

On Wed, Dec 19, 2012 at 10:56 PM, Joe <jepa...@gmail.com> wrote:

I noticed that the rudder tends to cavitate when sailing downwind (with the spinnaker) in
strong conditions.  On other boats that I have sailed (and windsurfers) this leads to spin
out and knock downs.   The Johnson seems to behave better: even though there is some
loss of rudder control, the boat continues to track downwind.   My question, though, is
whether I am doing something wrong.  We are sailing downwind with the rudder partially
retracted (perhaps half way).  Should it be further up?    The spin is properly trimmed.
The main is close to (if not just touching) the spreaders.

Any suggestions?

Cheers,

Joe

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[Rick Paullin]

Joe - from your description, it sounds like you're talking about sailing on a broad reach, with the main almost touching the spreaders.  I've found that the boat actually has leeward helm in those conditions (even with the centerboard all the way down), because the pole/chute is so far forward compared to other boats.  The leeward helm helps makes the boat really stable, so spin-outs and broaches are rare on a broad reach.  I've never experienced cavitation in those conditions -- the boat just tracks beautifully and surfs like crazy!

I think the board being partially raised is causing your cavitation problem.  The board's section is so narrow, that raising it part way simply moves it aft -- it hardly reduces the wetted surface area at all.  So moving the center of resistance aft would make the leeward helm worse.  Jeremy and Brian Lyman might be able to tell you what the conditions are like when they raise the board, but I seem to remember they said something about flat water the last time I asked.  I think that raising the board makes so little difference to the water resistance in heavy air, and it harms the steering so much, that I just leave it all the way down on a broad reach.  Light air might be a different story.

On a close reach, I think the boat has more weather helm, so spinning out is definitely more of a problem in heavy air.  Maybe moving the center of resistance aft in those conditions would help with additional steering control.

Let us know how it goes,

Rick

[Mga...@aol.com]

The centerboard should stay down.

 

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[Five Cubed Skipper]

I like to raise the centerboard completely in 9-13 knots, so long as the water is flat (less than 1.5 feet of chop).  Also if you have not seen my other posts on kite trim, I like to ease at least a couple feet of halyard on the kite OUT to keep the head of the kite out and away from the lee of the main as much as possible.  Jibes are tough to get used to, and the boat really "slides" through them, pivoting around a point just ahead of the rudder.  Think of "drifting" a sports car around a corner...the pivot happens gradually, and not immediately around the point where the board creates resistance in the water.  You just have to do it a bunch and plan ahead. Or you can put the board, or the tip of the boat down to create a pivot point.  If you lose flow across the rudder during the jibe, the boat will really slow down and you will probably need to put the board down to get the boat back up to speed before pulling the board all the way up again.   As Rick says, above some might view this as "harming the steering"... yes, the boat handles much differently.  You have to coax it more than steer it, making small adjustments to the helm and WAITING for the boat to react -- actually like steering a larger keel boat or another less responsive dinghy.  Plus you need to make sure that you keep the weight in the center of the boat (fore to aft). I say center, but I mean that both skipper and crew should sit forward of the traveler, which will feel forward to the skipper if you are used to sitting in the back of the boat.  It also helps to keep the weight down in the bottom of the boat, and even a slight amount of windward heel helps to keep the bow from slapping the waves so much.  If you have too much chop, the boat will not "slide" down wind, it will simply drift and hobbyhorse...too much chop will break the flow across the hull and rudder, and you will be much slower than the other boats sailing down wind with you.  Trim the sails as you would other wise, main with top two tails breaking occasionally and bottom two flying. The kite, is eased until it breaks and then trimmed again. WATCH for puffs, The skipper needs to steer down in them and trimmer must be ready for easing (just like if your board were down).  The main difference in trim and steering is that it is MUCH easier to capsize the boat over without the board down. (duh, right?) The idea in puffs is to steer the boat down so it can accelerate along it's center line as if the board were down, and using that acceleration to keep the boat tracking that same direction (down) even as you turn the bow up to keep pressure in the kite and pull it out of the lee of the main.  Like most of sailing, practice is the key to figuring out when you have to steer up and down and what is the right boat speed and angle to the wind for the conditions.         

(Now I'm sure you're asking:  "Okay genius, thanks for making this fun boat a lot less fun...it handles like a dump truck with flat tires, I have to sit in the bottom of the boat (forward of the traveler) and pay extra attention to the puffs and chop, can't jibe on a dime and I'm freaking out about turtling this thing...so what's the point of all this?!!")

The point is that you can sail much less distance to a leeward mark,  your VMG (velocity made good) is much better when "sliding" the boat down wind.  I say sliding because the centerline angle to the wind remains about the same as if you had the board down, you are actually able to to catch more wind in your kite, and keep it less blanketed by the main... essentially you can trim your sails as if you were sailing a hotter angle (like the other guys in the fleet with their board down). The boat can actually be tracking (travelling down the course) ten or sometimes more degrees lower than the centerline angle of the boat to the wind.  You do sacrifice apparent wind pressure, but the goal is to get down wind fastest, so even when sailing with the board down you're accelerating the boat and then burning that speed (apparent pressure) down course.  While you're sliding, the rudder (and tiller) will point the direction that the boat is actually sailing/tracking.  The rudder/tiller angle to centerline will show you exactly how much the tracking course differs from the centerline course the guys with their boards down are sailing.  The conditions need to be right for this.  When you get more wind, you have enough power in the sails to sail a lower course toward the mark anyway and the added chop that comes with more wind tends to stall the boat and you need more responsive steering to control the boat through the chop and pivot point to jibe. Plus, in more wind and chop you can get the boat planing and surfing waves, when done correctly is faster down wind than sliding.  In less wind, there isn't enough pressure to keep the boat sailing "forward enough" and it will simply slide to leeward or stop.  You have to keep the boat moving forward to keep proper flow across the rudder to allow you to steer the boat...other wise it's easy to completely stall the boat out and have it stop entirely.  This happens in too much chop even if the wind is 9-14 knots.

I've attached a drawing (PDF) to show you the idea... I suspect this works in the J18, probably because it is so flat, and sliding it not entirely on it's designed lines doesn't mean you're pushing huge wedge of water out of the way, like it would in a FJ or 420 for example... the stern wake is still coming off the transom, it's just off to the windward side of the transom.

-Brian (#125)

 

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[Jeremy Lyman]

Nice diagram, Bri!  I sometimes like to think of sliding as using an articulating pole.  If there is enough wind to keep the kite from getting sucked behind the main's shadow while pointing straight a the leeward mark, we don't bother with it.  But to keep the kite flying in less wind you've got to get "pole back" somehow, which on a Johnson is accomplished by articulating the entire boat.  Luckily the wind is blowing the direction you want to go, even if you can't point the boat that way. At least that's how it works from the crew perspective.  And remember, the first thing you do when the skipper starts talking about a douse is pop that board back down.  He might be able to turn the boat at the mark and load up the main, but you won't round the mark and capsizing is no fun when you're still hauling in a chute.

Stay tuned for more Lyman Boys advanced sailing tips, like the Shrimp Boat Sea Anchor© for hair pin turns in high wind.

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[Rick Paullin]

Nice write-up - now I can't wait to get out on the water to try it.  And I do like the diagram!

  - Rick