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PostPosted: Wed Aug 08, 2018 9:28 pm 
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Technical challenge here :

Getting back into sailing an 18 after many years away and picked up an 84 Redline in great shape despite living outside its whole life in Minnesota. Has new Hobie sails, ropes and tramp, hulls have the usual nicks / scratches but no soft spots. With no salt and pretty short seasons, all the Aluminum, fasteners, hardware and wires look excellent. We did the full glass reinforcement of the hulls at the front crossbar and the reinforcement at the side stays was already there. One hull had some minor gelcoat cracks below the crossbar (we assume from flexing) and the hull seam was starting to open up. Rudders had always been outside but only needed to be sprayed with one coat of new gelcoat to look like new again. Dagger boards and hull trunks had some nicks and worn spots on trailing edges we easily touched up with a couple small fiberglass / gelcoat repairs and are tight, square and right on even front to back toe.

Getting out on the water has been great except for one major issue. When we get some power or start lifting a hull, the rudder pressure gets extreme and the boat wants to turn hard downwind about 1/2 the time (exact same both directions). If you feel it starting and resist the rudder slightly upwind you can find a very delicate balance point and get by until a wave or some rocking and it will come back suddenly. It's so bad we pretty much do not ever have skippers on the trap because when it takes off downwind, the rudder can be so strong you will loose your footing trying to hold the line because you have to push so hard. Happens mostly when the hull comes back down (nose down) or a side hit wave.

We are not veteran expert Cat owners or sailers but with several previous 18's and other cats, we have never experienced anything like this. It almost feels like a car with too much toe out or bad alignment that darts all over the road. You can feel it with light air going gently along in a 5-6 mph wind with both hulls down if you steer slightly back and forth or full power double trapped and only 1 hull / rudder and dagger board in the water. Figuring there is a veteran 18 owner out there who has experienced something like this and found the cause.

Below is a list of all the attempts to solve this issue which each made no effect:

Checked hulls for square - right on within 1/8" checking multiple spots.
Retorqued crossbar bolts, adjusted spreader bar various tensions.
Tramp very tight and fairly loose.
Replaced rudder bushings / pins and drilled out to 3/8" bolts - rudders are very tight and also tried raking under boat various amounts from right on factory spec to 1" forward at bottom of blades. Toe was tried from 1/2" in all the way to 1/2" out about an 1/8" at a time.
Mast rake moved from slight forward to 18" + back in increments - usual results.
Jib blocks all positions.
Main traveler car in / out
Dagger boards originally about 1/4" of slop at trailing edge now tight, square and straight ahead.

Ideas and comments welcome !


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PostPosted: Thu Aug 09, 2018 2:44 am 
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Location: Jersey Shore
What type of rudders are you running? I experienced some similar handling issues on my 18 a while back (though not as extreme as what you describe) when I tried using a pair of EPO II rudders. The handling became very squirrely and the boat would unexpectedly develop lee helm. I believe it was related to the rudder blades being too thin which was causing flow separation under load. The problems went away when I switched back to original EPO blades (which are thicker). I also recall similar lee helm issues on the Hobie 20. Those rudders would actually fully ventillate under load when sailing downwind.

Anyway, the only real things you can do are make sure the blades are as clean and smooth as possible. Rake the rudder tips aft slightly to increase weather helm. And as a last resort, source a better pair of rudder blades. I can tell you my 18 has nearly perfectly balanced helm on all points of sail, so it is a correctable problem.

sm


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PostPosted: Thu Aug 09, 2018 8:30 am 
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How do you have your mast rake set? Generally, sailboats are designed to always have weather helm (tending to turn the boat into the wind), as lee helm is dangerous and can result in a runaway boat. The way this is done is to make sure that the center of effort of the sail plan is aft of the center of resistance of the boat (keels/daggerboards included). If you lessen the aft rake on the mast, you will lessen the weather helm, and too far forward will result in lee helm. Raking the mast further aft will strengthen the weather helm. The other things that affects this are of course the fore/aft position of the daggerboards (not adjustable), and how the sails are trimmed. If the jib's in tight and the main is loose, you're effectively moving the center of effort of the sail plan manually.

The conventional wisdom on mast rake, from the Hobie 18 performance manual (for sale at murray's sports), is to measure the distance from the bow tangs to a point 4' up the front of the mast. you should generally come up with a measurement of 103-107". Anything less and your mast is too far forward. If you have the 10 hole shroud adjusters, the shrouds should be pinned at least 3-4 holes down from the top, and the forestay similarly should be pinned probably around 4-5 holes from the top.

The other thing to look at is the rudder rake. While it does NOT affect the weather or lee helm of the boat, it does affect how hard you need to push or pull on the tiller. Properly raked rudders will pivot about the quarter chord length of the rudders... i.e. about 1/4 of the length of the rudder is forward of the rudder pin, and 3/4 of the length of it is aft of the rudder pin. To add force on the tiller, rake the rudders further aft. To remove force on the tiller, rake them further forward. Unfortunately, if you have the old style rudder system, this is not easily adjustable, and requires re-drilling the rudder holes.

**edited to correct mast rake measurement**

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Last edited by SabresfortheCup on Thu Aug 09, 2018 2:31 pm, edited 1 time in total.

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PostPosted: Thu Aug 09, 2018 9:08 am 
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The most extreme solution I can think of is to rake the mast as far aft as possible and furl the jib. If the boat still tries to turn downwind, I'd say something else is going on, because at that point almost your entire sail plan will be aft of the center of lateral resistance!

If you let go of the tiller, the boat definitely turns downwind? Does that change if you furl up the jib or get one hull out of the water? What about if you pick up one daggerboard or one rudder? I'm assuming you have stock sails/daggers/mast & rigging?

srm wrote:
I believe it was related to the rudder blades being too thin which was causing flow separation under load. The problems went away when I switched back to original EPO blades (which are thicker). I also recall similar lee helm issues on the Hobie 20. Those rudders would actually fully ventillate under load when sailing downwind.

I don't doubt your experience with the EPO/EPO II's srm, but I don't think it's a ventilation or flow separation issue, at least in this case. That would just mean you'd have to fight harder to keep the boat from turning into the wind, or if they lose effectiveness completely the boat will "steer" itself according to your sail trim (which, if everything's tight sailing upwind, should be to turn into the wind), but rudder effectiveness issues alone wouldn't cause the boat to try to turn away from the wind for any reason that I can see. But absent any better options, changing rudders would certainly be worth trying. The upgrade to EPO's is worthwhile anyways :lol:
srm wrote:
Rake the rudder tips aft slightly to increase weather helm.

Unfortunately that should only change the tiller force, not the weather helm.

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'79 H18 standard 'Rocketman II' sail #14921


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PostPosted: Thu Aug 09, 2018 9:25 am 
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You are correct, mast rake could also be a factor. It is not a problem that is easy to immediately diagnose or correct without sailing the boat. But the OP stated that this was an intermittent problem - he can find a balance point with the helm half the time but then a wave will come along, knock the bow, and the lee helm kicks in. That sounds very similar to what I experienced with the EPO II rudders. The boat did not have a consistent “locked in helm” like it does with the original EPOs. Flow separation will make the boat squirrely. It was the same way with the 20 rudders. You would be blasting along downwind and “Bam” the flow would separate (you could actually see it) and the boat would be all over the place. Dump the mainsheet, wiggle the rudder, and it would hook back up again. Not all rudder blades are created equal....

If he had said the boat has a consistent lee helm all the time, then I would definitely say check the mast rake.

sm


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PostPosted: Thu Aug 09, 2018 9:58 am 
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Interesting problem - agree with SRM that this is an rudder flow problem.
Are there any other H18's around, and can you swap out rudders to test this issue?

Mike, for rake measurement, it is at a point 48" up the mast, then between 103" to 107" to each bow tang.

We run with 'old' EPO's, bushings, and two years ago, we switched to s/steel rudder pins.
The only time she gets wiggly is running downwind under spin in winds over 15 k....
she turns into a lee helm beast, which has more to do with the sail plan than the rudders.

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PostPosted: Thu Aug 09, 2018 2:28 pm 
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srm wrote:
But the OP stated that this was an intermittent problem - he can find a balance point with the helm half the time but then a wave will come along, knock the bow, and the lee helm kicks in. That sounds very similar to what I experienced with the EPO II rudders.

Good point, I suppose I was conveniently ignoring the intermittent nature of it. Seems odd indeed that you could intermittently have lee helm, though I can absolutely see it if you're surfing a wave (or if it strikes your bows at just the right angle) and the lateral resistance becomes a surge. Not sure what size waves we're talking about either, that could play a role! True too that it would seem to be more likely ventilation is the cause of an intermittent problem, but I'm still a little stuck on the "lee helm" bit. But it could be a combination of factors that makes it seem as though the boat has lee helm.

John Lunn wrote:
Mike, for rake measurement, it is at a point 48" up the mast, then between 103" to 107" to each bow tang.

Ahh, I actually was debating if it's 4' or 5', but 5 was stuck in my head for some reason. I think mine always comes out around 105", which is probably why I botched that number too. Thanks for the correction!


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PostPosted: Thu Aug 09, 2018 9:04 pm 
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Thanks much for all the feedback so quick all ! I really am listening to the rudder ventilation talk as that is exactly what we have been feeling and seeing. To us the cavitation felt like a reaction to the amount of push force needed to "break" the hard lee turning issue and that we were basically pushing so hard on the stick / rudder that we simply pushed the rudder out of its normal flow to the point of ventilation (and yes as SRM said, see and hear it !) - especially when only 1 rudder was in the water. The odd thing is you can feel the same thing, but not as extreme with light air and both hulls down and running pretty flat. If a wave comes (normal boat wake wave on a lake) especially from the rear and the nose goes down (rudders up) it will pull to lee and a slight push on the rudder will break it right away and all is good again.
Rudders are original fiberglass we restored. Outer gel coat was pretty thin and you could see and feel fibers in many spots but straight and no damage so we re-gel coated them. Had the boat out in 25 MPH winds on the big lake (only folks on the lake that day:-) last summer in 3' – 4’ max seas (some over cross bar at times) with no jib / jib, mast raked back about 10" from straight up and gave everything a pretty good stress test with no issues at all. Had a blast, except still same issue of constant fighting the lee turn under load.
We definitely have normal weather helm until the lee pull comes suddenly with some type of "disturbance to good flow" and under power we have no tiller pressure most of the time.

Rather than re drill the rudders we have been using stainless flat washers and long screws to test various rudder rake attempts and was planning on drilling and filling once we found the best set up. Attached is a photo of current rudder set up and rake. We had a few more washers last summer with about 1.5" at 12" down from Gudgeon, but nothing changed in handling and it caused the pin to bind so we went to current to save pins and bushings.

According to SabresfortheCup we should be about the 2" mark, rudder is about 9" wide at the 12" down point so 2 1/8" would be 1/4 blade. We have been from 0 to about 1.5" 12" down pin center line to front edge of blade with no change. toed in and out over 1/2" with no change. The original rudder holes were not too bad but bushings were gone and tons of slop and play everywhere which we thought would fix the issue. We drilled everything out to 3/8" (drill press) just to make sure the rudder is held tight as possible installed the washers for rake to suggested specs, set toe 0" - 1/8" in and had no change at all.

The part that is hard to take is where the original factory rudder rake was almost right at the edge of the rudder. Our previous 18 had no rudder rake, loose pins and bushings, toe who knows where and sailed perfectly with mast rake all over the place and floppy dagger boards.

Maybe next attempt should be polishing the rudders smooth and getting more rudder rake again to the 2" ? Mast rake was right on the mark checking the suggested way and other ways,but made no change. Problem we have is changing mast rake is a bit of a problem now because the new Hobie side stays were about 3/4" shorter than the originals and the front stays were longer so getting close to straight up or a slight rake is tough because we are in the top hole for side stays and have to remove the forestay adjuster and go direct to the furler tube to get correct slight rake adjustment. Straight up required using the old shorter forestays. We have a teflon washer under the mast base we could take out which might help.

Thanks again for your thoughts!

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PostPosted: Fri Aug 10, 2018 10:01 am 
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With regards to the shrouds and stays, Hobie changed the lengths of the shrouds (and probably forestays too) a few years back (not sure how long ago, actually) to allow for more mast rake. They also changed the shroud adjusting plates from 7 hole to 10 holes, to allow for more adjustment. I think the forestay adjusting plate remained a 10 hole plate. When I rigged my boat with new shrouds, I had to change out the anchor plates, otherwise the shrouds were too short. Check to see what shroud anchor plates you have.

Here's an article on the weather helm/lee helm with diagrams, though I think everyone here understands it pretty well: http://www.catsail.com/archives/v2-i8/feature1.htm

This Hobie Hotline article speaks more to the effects of rudder rake, how to measure it and what your target rake should be. They suggest 1" on a Hobie 18, but if you're experiencing a lot of tiller force you may want to continue to adjust the rudder rake further forward. http://w1dm.com/11%20Winter%20Hotline%20TOH.pdf

Note that the location of the "working center" in the "on the wire" article is a bit incorrect, as the center of effort of any foil is generally at the quarter chord length. The diagrams in the hobie hotline article are much more accurate.

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PostPosted: Fri Aug 10, 2018 1:57 pm 
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I think this subject has been skirted, but I wanted to hit it bluntly.

Rudders have a center of effort. Normally the center of effort lies about 1/4 of the way along the cord of the rudder (fore-aft) and doesn't move too much... (cord = centerline of rudder seen in a top view, 1/4 = closer to front than rear)

Except when....

Flow separation happens
or (rudder specific)
waterline changes

When sailing down wind the stern often lifts a little. this pulls the rudders further out of the water, impacting rudder balance. Your rudder rake might be well balanced when the rudder is fully immersed. However when it is just the tip your rudders may have too much rake, and the center of effort may move infront of the hinge line.

Ideally the center of effort falls just behind the hinge line on the rudder. This gives you a balance between 'feel' and too much effort. As the center of effort moves back the rudder gets heavy. If the center of effort moves in front of the hinge line things invert. Weatherhelm feels like leehelm and skippers get confused and tossed about.


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PostPosted: Sat Aug 18, 2018 4:30 pm 
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Try two things (one at a time):
More mast rake - maximum possible
Rake rudders back slightly. Put a small piece of carpet or rubber (3-5mm thick) on the front edge of the bottom casting.


If these don’t fix your problem then it is a rudder shape or flow issue. Are the leading edges of the rudder smooth with no join line visible? Are the trailing edges sharp? Trailing edges (the last 1”) from the factory are too thick and need sanding down to have a thin exit.

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PostPosted: Sat Nov 10, 2018 11:30 am 
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Problem solved, Sad to say what it was, as its very embarrassing considering a lifetime on the water. In the whole process of buying the boat and moving it we somewhere got the rudders on backwards (trailing edges forward) and never noticed or thought hard enough about the possibility until one day looking at the shape of the dagger pockets and boards, then noticing the gap from the rudder to the casting was huge and wondering about all the deflection under load that must be going on. Interesting thing is how the mounting holes are almost perfectly square front to back / angle etc., rake and general fit is almost right on. The top notch out area should have also been a clue, just plain dumb as can be.

All back to the way it should be with the many fine adjustment suggestions done, it sails perfect now. One finger is all you need in any condition as it should be. Thanks for the many tips all we know the boat much better than ever now.


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PostPosted: Sat Nov 10, 2018 4:45 pm 
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Rudder rake tucked to far forward under boat

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PostPosted: Mon Nov 12, 2018 7:27 am 
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Redline, your puzzle has bested all of us. It has also opened up a new wold of rudder pranking. Reverse your buddies rudders and watch them swerve erratically as they pick up speed!


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PostPosted: Mon Nov 12, 2018 10:12 am 
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redline wrote:
In the whole process of buying the boat and moving it we somewhere got the rudders on backwards (trailing edges forward) and never noticed or thought hard enough about the possibility until one day looking at the shape of the dagger pockets and boards, then noticing the gap from the rudder to the casting was huge and wondering about all the deflection under load that must be going on.

Well in that, I'm surprised the boat sailed as well as it did! :lol:

https://www.osti.gov/servlets/purl/6548367/

Looking at the coefficients of lift for foils at large angles of attack (pages 83-86), your foil on backwards would have reduced the amount of lift you were getting out of the foil at any given angle of attack, but surprising the stall angle remains about the same. However, to get X amount of turning force out of the rudders, you'd need to be more exaggerated with your tiller movements, and would run out of lift and end up stalling the rudders sooner than if they were on properly. You'd also get much, much more drag at any given rudder angle (page 92-95), so you would've been really putting on a lot of drag (much more than normal) whenever you had even a slight rudder angle. One thing that I'm not so sure of though is where your center of effort would be for a foil on backwards. I'm tempted to say it's still near the 1/4 chord length, but the rake may have been incorrect, adding tiller force.

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