Slipper test sail

Ok, but are those traditional boats monohulls or cats?
I'm asking this because catamarans are naturally lazier tackers when compared to monohulls, so rudders with more authority (more lateral force for a given angle of attack) are necessary. That means rudders with higher aspect-ratio (more slender). The reason is explained in the pic I've attached. It shows the relationship between the angle of attack and the resultant lateral force for two rudders with different aspect ratios.
The steeper curve is for a deeper and more slender rudder (which means high aspect ratio), the flatter curve is for a shallow rudder (low aspect ratio), like the one you have here.

Now look at the case pictured with red lines. You can read that for the same angle of attack (called Alpha_fixed - which is, approximately, the rudder deflection) a deep rudder will produce much higher force (F_high) than a shallow one (F_low).
Vice versa, the blue lines tell you what happens if you need to obtain a certain fixed lateral force (F_fixed). A high aspect ratio rudder will need a much lower deflection (Alpha_low) to produce the desired force, than a low aspect ratio rudder.

The back side of the medal is that a high aspect ratio rudder will stall more easily, so you need to avoid hard tiller maneuvers.

Generally speaking, a side force generated by rudders is reduced by the vicinity of the water surface, due to the fact that:
1) surface-piercing rudders have half the aspect ratio of an under-hull rudder of the same shape and size
2) ventilaton reduces the pressure difference between ventral and suction sides of the rudder.
Knowing that, you can understand that the effect is more severe for very-shallow-draft rudders like yours.

If you still want or need a shallow-draft rudder, then I would suggest you a slender rudder with a kick-up mechanism, which is not so difficult to manufacture. For example, see how this guy did it:
http://marina.fortunecity.com/breakwater/274/2001/0715/index.htm
It would also allow you to completely eliminate the skeg (if it was intended for protection of the rudder), which is also slowing down your boat's turning rate.

Yeah, you're right about the name. I also like to play electric guitars, so...

Flat plate fences, particularily in this case, are nearly useless. The ones you have (and the way you have made them) are not foils, just flat plates which create friction and vorticity drag due to hard-angle joint with the rudder blade. But that's another story and needs a full page with fundamentals of aero/hydrodynamics to explain it better. Maybe the next time, right now if I were you I would head in the direction of kick-up rudder.

P.S.
You have said that it tacks slowly when compared to Slider. What can you say about jibing? How does that compare to the other cat?

 

Slipper

Look at successful versions of this type:

Hobie Wave and Getaway. M & M have a pretty good track record as designers of big cats and despite the complex hull shape, the "skeg" is what gets some of the lateral bite. However, the fact that all H14's, H16's, and Wave owners rake the mast back and load up their kick up rudders for more windward ability ought to point us in the right direction.

M & M's other design with the same hull form stretched out to 16', the Escape playcat, has underslung rudders that do not kick up. they didn't achieve good market penetration with this cat--especially compared to the Wave, but it also seemed to be a decent performer. Look at it's hull and rudder profile.





Dan

 

Phil, I think you're right. I cut 4 inches off the ends of the rudders, in a misguided attempt to see if the fences actually contributed to the rudder efficiency. Before I cut them off, the boat did not tack as well, but did go to windward better.

This is what they looked like originally:



Folks told me they were too big, but maybe they weren't big enough. Certainly they were not heavy to steer, as many folks warned me they would be. The cutoff versions are actually heavier to steer.

Slider, which is the best-behaved boat I've ever sailed, has pretty big high-aspect rudders, with a NACA 0012 profile

 

Both boats jibe smartly, and Slipper seems to jibe pretty much in her own length.

I see your point, though I think aspect ratio may be only part of the issue; the profile is important when considering rudder stall. For example, Slider has modestly high aspect ratio rudders, at least compared to Slipper, but her rudders are shaped to NACA 0012 (her daggerboard is 0009, for the obvious reason.)



Of course, cats are notorious for tacking poorly, but again, Slider is an exception. She tacks effortlessly with any sail combination and in any conditions, whether the daggerboard is up or down, and no care is needed to steer through the tack, as is the case with many small cats. You can slam the tiller to the stops and she'll tack just fine.. Slipper has almost identical hulls, slightly scaled down (though with less freeboard, proportionally) which is why I had hopes that a somewhat off-the-wall approach to the rudders might work out. It would appear that Slider's good rudders have a lot to do with her great handling.

The idea with Slipper is simplicity while sailing. I'm beginning to think that I should put a little ingenuity into designing self-tending rudders, which would be almost as convenient as fixed rudders.

 

Daggerboards

Gday Ray

Does Slipper have a daggerboard too? If not then this would be a problem for tacking.

I agree with Daquiri (I have a Fender strat) No about aspect ratio as well but I am convinced about loading coming out of tacks as well. I probably have mentioned a story about my little 6 m cat but I will retell it here.

My little 6 metre cat can do something few cats can do. It can go into the wind with just a main up and you can sit it there 30 degrees off the wind all stalled. Then two pumps of the tiller and gently pull the main in and she will bear away - just like a good mono. She has never missed stays. One day I tacked her with one of her rudders up - all of a sudden she is a normal cat and needs gingering round.

When tacking the rudders need to provide lift. They are working at high angles of attack (which increases drag) and they operate at lower speeds than normal (which reduces lift). Coming out of the tack a normal cat with weather helm will load up its rudders and then they are at their most highly loaded condition ever - slow but loaded. This is when a Hobie with the main on will round back up into the wind. What you need now is a set of rudders down the back that are more lightly loaded than the centreboard. This is what Jim Brown does on the Windrider which is a dream to tack. So to reduce the loading on the rudders make them bigger so that they are loaded at least somewhat less than the board. But you can't make the board too small or the boat won't pivot around it going into the tack so the only way around it is to have efficient (like Sliders) rudders that are large.

I would like to take credit for the way Cats-paw tacks but it is just because I like the feel and look of large efficient rudders. Worry over extra drag is a nonsense because you get so much better control. Look at mono sailors and their rudder sizes on dinghies and yachts and their boats are nowhere near as directionally stable. We need proportionally bigger rudders than they do to make our boats turn properly.

Make em big and deep Ray then let em kick up. She will spin your hat off.

cheers

Phil

 

Hi Ray,
One of the reasons I've asked you about her jibe behaviour is because I've had a suspect that she might had a lee helm which is acting against the tack maneuver and in favour of jibe. In fact, you have replied that jibing is pretty responsive, if I understood it well. Then I saw these 2 pics in your site and it does indeed look like you're sailing with a lee helm:



Which could be the second partial reason for her reluctance to tack (the first and the main one, as we seem to agree, is the too shallow rudder). It could mean that the center of effort is placed a bit too forward (or, vice versa, the daggerboard is a bit too aft). So, just to clarify that, what happens when you release the rudder - does she wants to go winwards or leewards?

Cheers

 

Phil, I'm really coming around to the idea that I need Slider-like rudders on Slipper. I don't know that the daggerboard is an issue, since Slider tacks just as smartly with her daggerboard all the way up. She goes to windward fairly well with the daggerboard up, too. As I mentioned in the blog piece, the other night we came home in Slider after an afternoon out fishing, and I took down the jib to enter the little bayou, as I usually do. The entrance is a narrow seawalled cut just a few feet wider than Slider, and prone to fluky winds from unexpected directions. I'd pulled the daggerboard, since there's a bar with just a couple of feet at the entrance. Anyway, once inside the wind was very light-- hardly more than a whisper, but we tacked up the bayou under main alone, tacking effortlessly, and looking at the GPS track, we were tacking through 90 degrees-- with no daggerboard. I guess this sort of performance is why I was unrealistically optimistic about Slipper, but if I'd wanted a mini-Slider, I should have built one.

Ah well. If you don't try, you don't know. I cut off the remaining skegs this afternoon, and I'm going to use rudders almost identical to Slider's for the next go-round. They might be a little oversized, but I'm still interested in the horizontal fins that Bernd Kohler has used to good effect, and I suspect that Slider's decent windward performance without boards is due to her big deep rudders. My hope originally was that Slipper would do about as well as Slider does without boards, and I planned to add the anti-vortex panels to see if they improved windward ability noticeably.

Anyway, Daiquiri recommended rudders like Jim Michalak's, which are weighted to keep them down. That's an appealing solution, but I don't know if they'll stay down just from weight if the boat is going 8 or 9 knots. What do you folks think?

 

That's an optical illusion, caused by the fact that the tillers rise substantially to clear the rear beam. Like Slider, Slipper has a modest amount of weather helm. If the tillers are released, she rounds up and luffs.

I'd have been far more distressed had she had lee helm. That's a fatal flaw, in my view, and would have raised the dismal specter of having to move the rig and cross beam.

You may be overestimating my ignorance when it comes to boat design, just a little. Slider has been a fairly massive success, for a unique boat from a novice designer, with many plans sold. That was probably because I made use of conventional wisdom in almost every respect. In only two ways does she diverge from contemporary multihull thinking-- her adjustable seating within the hulls, which as far as I know is not found in any other open 16 foot cat, and her modest sprit rig-- though the latter is not unique, as it was featured on Wharram's Hinemoa design almost 40 years ago-- I bought plans back then, but ended up finding a Tane with a good fractional rig that I could buy for less than the cost of the Hinemoa's materials. My reasons for picking the sprit/sloop rig are detailed here:

http://slidercat.com/blog/wordpress/?p=52

Slipper, of course, is another kettle of fish entirely. One of the reasons I drew her was to provide experimenters who might like to dabble in cats a simple cheap platform for such experiments-- as Michalak has done with small unballasted monohulls. (I built Slipper for about $500.00 US.) Suiting my own approach to this theoretical purpose, I decided to try barndoor rudders. I'm beginning to think they might have worked okay if I'd dispensed with the skegs, and made them a little longer, and finally, used an appropriate NACA profile. I might have had to add a little balance to the rudders, had they proven heavy, but the original ones were quite light on the helm and only got heavier when I cut down the chord. And windward ability deteriorated.

Ah well. I have a direction to pursue, and with any luck, putting Slider's rudders on Slipper will result in Slider-like performance, since the hulls are almost identical in form.

By the way, do you think Michalak's weighted rudders would really stay down at 8 or 9 knots? That's a simple and appealing approach, but I wonder if they'd ride up at higher speeds.

 

Oh, absolutely no!!! You have created a nice and simple boat, and deserve all the respect for the result. I even like the coloring scheme.
I was just trying to make the complete picture of what's below and above waterline, in order to root out any residual doubt and to allow you to concentrate on the most important issues.

As about your doubts about rudder staying down at 9 kts, you could easily calculate the weight required by calculating the total moment due to rudder's weight and drag around the kick-up hinge. The sum of the two must be zero, because hinge doesn't transmit moments, which allows you to calculate the necessary ballast weight. If you need a help with that, send me a PM.

But the blade tilting upwards at high speeds would not be a problem at all - it would give you an automatic reduction of wetted area (and the relative drag) when it is not needed. When the boat slows down, the blade will then automatically fall down under it's own weight (if the hinge is made to allow an easy and smooth rotation), increasing the area again for the low-speed maneuverability.

Cheers!

 

Daiquiri, that's a very interesting thought. I'll have to mull that over. One problem I can see is that kick-up rudders with the pivot point above the waterline become very heavy to steer when kicked up, because only the tips are in the water, well behind the rotation center of the rudder-- bad leverage. I suspect at high speed, you don't want the helm to get really heavy.

In fact, this tendency to be heavy when kicked up irritated me enough with Slider that I added a couple blocks to the tiller bar so that the steering line would have a 2 to 1 mechanical advantage. Didn't need it when the rudders were down-- you can hold the steering line lightly between thumb and forefinger, but it's pleasant to have the extra power when you're sailing across a shallow flat-- which we have a lot of here in the FL Panhandle.

 

I'd like to congratulate you on going out and building rather than just writing about what you might do....

Also you are very honest about the problems you are finding, which should benefit everyone. I am also impressed at the speed you are making your mods.

Having said all that, I agree with the others. You cannot "cheat" theory, a fence on a LAR "barn door" rudder will never get you the rudder force you get from even a moderate AR. Same applies to wings on a LAR keel which will never be as good as a daggerboard.

So I think you'll need kick up rudders. I don't think you need worry about them kicking up at speed (I assume you fit a downhaul), after all, conventional beach cats use them without problems. There are a number of ways to lock the rudders down automatically, ranging from the Hobie system to the Topper (much simpler and maybe suitable for you) or even shockcord.

And don't forget that so far you have only sailed in light winds and flat water. In strong winds and waves it is usually much harder to tack

I will continue to read of your progress with interest

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard, I appreciate the kind words, especially from a designer whose work I very much admire.

Daiquiri suggested a kind of rudder that's held down only by weight-- I don't know, but I think that at speed such a rudder would at least partially kick back. Slider has a somewhat complicated system of lanyards to allow her rudders to kick up. It uses a sort of euphroe on a continuous lanyard to vary the hold-down tension, and the rudder can be raised or lowered with a single yank on another lanyard. It's failry simple in operation, but does require you to get out of your seat and cross the deck to reset the other rudder.

I'm sure you and the others are right that high aspect kick-up rudders like Slider's are the smart way to go. If it comes to that, I'll do it cheerfully, and if they work as well as Slider's do, I'll be very happy.

But as a professional contrarian, and also because one of the purposes of the little boat is wild-eyed experimentation, I've decided to try something else first.

One of the interesting factoids that was revealed by my initial fiddling about was that the boat went a lot better to windward before I cut 4 inches off the ends of my barndoor rudders. This was a modest change in overall rudder area, but the result was disastrous, in terms of windward performance. Further, the helm was lighter when the rudders were longer. Neither of these changes were expected, though perhaps if I were a smarter guy, I'd understand them better.

In any case, what I took from the observations is that perhaps I need more area (or more effective area.)

I've always been fascinated by the working boat tradition, which I suppose is one reason why Slider has a sprit/sloop rig. I've particularly admired the sharpies that plied the American coast over a hundred years ago. I have Reuel Parker's book, and looking at the designs for sharpie rudders, I was struck by how similar the design brief for those shallow draft boats and mine appeared to be.

In short, I spent the afternoon cutting out a couple of New Haven sharpie rudders for Slipper. They seem enormous, but it will be easier to cut them down than to make them bigger. They have a small balance area ahead of the pivot point, to help ease the heavy helm that their greater length ought to induce. I'm just going to rough them out and try them, and if they work at all, I'll give them a better profile.

It's probably time down a rathole, but although Slider works great, I would be a little disappointed if Slipper turned into a mini-Slider.

 

You know what, Daiquiri has also thought about it and has changed his mind about the usefulness of having an automatic rudder area reduction as speed goes up.
Actually, quite the contrary - when the speed grows and sails start giving all the power they can, I don't want the rudder area to diminish - I want to have a maximum control of the forces involved. So, an amount of rudder kick back is ok under speed, but the rudder area is better not be compromised.

I have don a preliminary calc of how much weigh you might need to keep the rudder in the water. Say that you have a rudder blade 80 cm deep (31.5 in) with a constant 30 cm chord (11.8 in) and 12% thickness. It should weigh about 4 kg (circa 9 lb), based on NACA0012 foil's area, and would have a positive buoyancy of around 6 kg (circa 13 lb).
In order to keep it kicked back by an angle no bigger than 45° when running at 9 kts (which would assure that all the blade area would remain submerged), you would need an additional 4 kilos (9 lb) weight fixed near the tip of the blade. That doesn't sound too bad when the blade is in the water (it's apparent mass, thanks to Archimedes, would be just 2 kg - 4.5 lb), but once you pull it out you will have a beast weighing (blade only) around 8 kilos (17.5 lbs). You decide if that is acceptable. If it is not, as I suspect, then you might actually need some other locking mechanism to keep it down at 9 kts.

Of course, all the numbers are approximate and strongly depend of the actual rudder geometry. If you want, I can give you the Excel sheet I've created to perform these calcs, so you can play with numbers yourself.

 

Daiquiri, I'd like to see the spreadsheet.and I think other folks would find it useful as well. Could be helpful in other design problems.. If you have a place to post it, you could add a link to it on your next post, or if not, you could send it to me, and I could post it on my site..

So give me your best guess: will the New Haven sharpie rudders prove disastrous, or just silly? I figure they're worth 15 bucks of plywood and a few hours, just to satisfy my curiosity about them.

 

Hello Ray,
I see that you have requested it 2 days ago, so I hope that you have already made a new rudder without waiting for a reply.
Here is the excel sheet, I have added an explanatory pic inside, and have split the spreadsheet in SI and Imperial units, so you can choose which one to use. Hope it will be somewhat useful to you.
It does have some simplifying assumptions in it, like (for example) different forces acting in different Y stations but through the same X coordinate, buoyancy acting through CoG, zero hinge friction (which is conservative).

As about New Haven sharpie rudders
(http://upload.wikimedia.org/wikiped...g.svg/800px-New_Haven_sharpie_drawing.svg.png)
it must be terribly inefficient. That's all i can say on the basis of my general knowledge of hydrodynamics, since I have never seen anyone use it for a practical boat. Sorry.

I'm really looking forward to read further news about the progress you've done so far in improving the handling characteristics of your cat.

Cheers!