Slipper test sail

[rayaldridge]

I finally got to take Slipper, my 14 foot cartoppable cat, for a test sail. In general, I was relieved by the results, though she's too slow in stays. Today I'm cutting off about two thirds of my skeg area, in an attempt to make her a snappier tacker.



Details here:

http://slidercat.com/blog/wordpress/?p=222

[dskira]

Congratulation.
Hope you will have a good time sailing.
Daniel

[rayaldridge]

Thanks, Daniel.

As I mentioned on the blog, Slipper was somewhat reluctant to tack, and I've been spoiled by Slider's relative quickness in stays, so I feel like I have to address this problem before I can be satisfied with the design.

Slipper and Slider have very similar hull forms, except that Slipper has no daggerboard. However, even daggerboard up and with only the mainsail, Slider tacks without any effort. Slipper has to be nursed through the tack carefully, somewhat like a Hobie or other boardless beach cat. Since the design is aimed at novice sailors, it's just not acceptable for her to have any handling vices.

So, here is what the skegs looked like when we went for a test sail, but with the alterations marked:



This is what the redone skegs look like:



I hope there's still enough meat there to protect the rudders.

So what do folks think? Will this improve Slipper's tacking?

[dskira]

I think it will, and you can also add a little bit of rudder area if it not give satisfaction.
But anyway, be kind to the tiller when you tack, keep the speed, and don't move to much.
Daniel

[rayaldridge]

That's excellent advice, Daniel, but as I say, I've been spoiled by Slider. As long as you don't forget what you're doing halfway through the tack and let go of the steering line, she tacks every time. You can slam the tillers over, you can be sailing at less than a knot, you can be steering from anywhere on the boat, or any combination of these errors, and she still tacks.

I want that for Slipper, because even more than Slider, Slipper is a magic carpet boat, where being out on the water and immersed in the natural world is far more important than any of the admittedly fascinating technicalities involved in getting the most out of a cranky boat. With no jib sheets to tend, fixed rudders, no boards-- Slipper is aimed at the irreducible minimum of effort and skill needed to get where you want to go via wind power.

If cutting down the skegs doesn't do the job to my satisfaction, I have another area to explore. Slider's Ackerman geometry is a little stronger than Slipper's. With Slider, the rudders required stops to limit deflection-- otherwise the steering would overcenter and lock up. So I reduced the geometry slightly, and Slipper's rudders have full travel without
overcentering. That may have been a mistake. But it's one that's easily remedied.

[rayaldridge]

I finally got a few photos of Slipper sailing, though the sail is not set very well. We went out in light air yesterday afternoon. I landed my wife Nancy on a dock across the Sound, and she took a few snapshots.

http://slidercat.com/blog/wordpress/?p=223





The boat is sailing pretty well, though she still doesn't tack as quickly as Slider, and I may have to adjust my expectations in that regard. Still, I have a few things to try. Suggestions are welcome, though I'm trying to keep the sailing and the structure as simple as possible.

[rayaldridge]

I took Slipper out again yesterday afternoon. The breeze was decent, 10 knots in flat water, and we touched 6.5 knots often, close reaching. This was the first time I had a functioning GPS with me, and the track was a bit disappointing. She made okay progress to windward, but not nearly as much as I'd hoped, after my experience with Slider, which has similar hulls. For one thing, leeway was excessive. Tacking was still an adventure, and several times I chose to gybe, rather than risk getting up against someone's dock if she failed to tack. Unacceptable behavior in a boat meant for neophytes. The first thing I'll try is T-foils on the bottoms of my rudders. Then I'll strengthen the Ackerman geometry of the steering. Finally, I'll try adding horizontal foils, similar to those Bernd Kohler uses in his cruising cat designs. If all that fails to correct the flaws in Slipper's performance, I'll go to low aspect ratio keels, which will add a few inches to her draft. So I hope that won't be necessary.

Anyway, my experiences with Slipper have increased my conviction that amateur designers should never offer plans for a boat they haven't built and thoroughly trialed. It would be pretty embarassing to draw up plans for a boat like this, talk someone into building the prototype, and then have to hear about these problems.

Still, I think this is a worthwhile effort. It's amazingly pleasant to sail across the flats in less than a foot of water, and not have to worry about foils kicking up, getting them back down, tending jib sheets, and all that other stuff that detracts from the pure experience of being carried along by the wind over the water.

[rayaldridge]

My latest changes to Slipper have proven to be the opposite of helpful. Sad story can be found here:

http://slidercat.com/blog/wordpress/?p=224

Here's a pic of the rudders that didn't work:



I guess I could solve most of the problems by putting foils similar to Slider's on Slipper, but in a way, that would be giving up. I think I'm going to take the little boat in a different direction.

[daiquiri]

Ray,
That rudder is really too short and will produce much less lift for a given deflection angle when compared to deeper and more slender rudders.
Fenders are useless, will only add drag. Is there some particular reason which has made you choose that particular rudder shape?
What is the actual max. rudder angle?

[DarthCluin]

Ray,
Is there any chance that your end plate is too small? If you look at the end plate on Phil Bolger's Chebacco rudder ("Instant Boatbuilding With Dynamite Payson", admittedly a much larger boat) the end plate runs the length of the rudder, 21", and is 10" wide with a 45 degree point on the forward end.

[rayaldridge]

Originally Posted by daiquiri

Ray,
That rudder is really too short and will produce much less lift for a given deflection angle when compared to deeper and more slender rudders.
Fenders are useless, will only add drag. Is there some particular reason which has made you choose that particular rudder shape?
What is the max. actual rudder angle?

This rudder was patterned after the barn door rudders found on some traditional shallow draft boats. In its original form, there were no fences, and the rudder had a chord of 4 more inches. The boat went to windward better, but did not tack well enough to make me happy. Slider, Slipper's big sister, is a snappy tacker by any standard, let alone that of a small cat, so I'm spoiled. Someone used to tacking boardless beach cats probably wouldn't have seen much wrong with Slipper's tacking performance in her original form. Slider uses high aspect shaped foils, but the idea with Slipper was to avoid those complications, and use fixed rudders

If by "fenders" you mean fences, I wouldn't go so far as to say they are useless, despite my lack of success with them. When they're called T-foils, they've done well on the racing circuit. My planform for these appendages is probably badly flawed, but I wanted to avoid a form that would catch crab trap lines and weed.

[rayaldridge]

Originally Posted by DarthCluin

Ray,
Is there any chance that your end plate is too small? If you look at the end plate on Phil Bolger's Chebacco rudder ("Instant Boatbuilding With Dynamite Payson", admittedly a much larger boat) the end plate runs the length of the rudder, 21", and is 10" wide with a 45 degree point on the forward end.

I think you may be right, and the planform doesn't make as much sense as I'd hoped. In all likelihood, the horizontal component of the foil should probably have gotten wider as it approached the trailing edge of the ruder, since that's where the tip vortices are strongest.

Ah well. If you don't make mistakes, you're not doing much.

[DarthCluin]

Here are a couple of pictures of Chebbacco rudders. I don't know if this is the problem. Maybe you could rough something out to see if it helps.
Hang in there. Read a few of Bolger's books, and look at some of the wild ideas he tried (bow rudders, modular boats, etc.). Sometimes they work out brilliantly, and sometimes they flop, but most of the time they fall somewhere in the middle. If you don't try, you don't know.

[catsketcher]

Gday Ray

I think you are going about it wrong. Foils at low speed will stall unless they have low wing loading. By having small foils you increase loading on the rudder and board. Make the foils much much bigger and she will tack much better. Coming out of a tack your rudder is awfully prone to ventilation so reduce the load on it by increasing area and having nice big board.

cheers

Phil

[DarthCluin]

I think Catsketcher is onto something. If you look at shallow depth boats like sharpies and catboats, the rudders tend to be long if they are not deep.