Beachable 1-2prsn. design using minimal materials?

[Seafarer24]

I'm not seeing how this thing will turn? There isn't enough rocker in the hulls to allow it, and since they're a "deep V" design (sort of) they'll really dig in.

Of course, I've been wrong in the past.

[VBAjedi]

Yep, that's a legitimate question, all right. I of course WANT it to dig in since I don't want to have to use daggerboards. But between the hull design and the likely sail-plan choices, I do anticipate difficulty tacking. We'll see when I get it out on the water, but I'm hoping that with a boat this small I'll be able to shift my weight to the rear to help it come around. In case I have serious difficulties, I am also playing around with a half-baked idea for a bow-mounted "tacking rudder" (for lack of a better term) that I can drop into the water via a line while tacking. Sounds odd and needlessly complex, perhaps, but this is an experimental little boat, so I don't mind. . .

[gggGuest]

Originally Posted by Seafarer24

I'm not seeing how this thing will turn?

Do you think a 12 foot Catamaran with minimal rocker is likely to turn much more slowly than say a 20 foot Tornado Cat that has a good 12 feet of low rocker in the middle of the hulls plus more rockery bits at the ends? If so why?

[VBAjedi]

gggGuest,

I'm certainly no expert, but two things I would think the larger boat has going for it are: 1) larger rudders proportionate to that 12' of low-rocker hull and 2) more weight/momentum to carry it through the turn (unless that's offset by more wetted surface/friction). There's also the question of hull cross-section. My boat is hard-chined, and I can't remember what hull shape Tornado cats have. Rounded hulls should pivot in the water better.

Still, you have a good point. . . I do hope you're right, even while I make plans for what I'll try if Seafarer is right!

[Seafarer24]

Tornados will tack on a dime because:

1) They are round-chine hulls which do not have much grip on the water at all
2) They have center-boards which create a central pivot for the rudders to work with
3) They are wide enough that the momentum of the turn does not force one of the hulls down into the water creating a long brake
4) They have headsails to help bring the bow around, and high-aspect ratio mainsails to keep the power up through most of the turn

VBAjedi. Try to find pictures of a G-Cat catamaran, the 5 meter would be most relevant for you. It has symetrical Deep-V hulls with a lot of rocker to them. They are surprisingly fast boats, for all that they look very "old-school". They also go to windward amazingly well for a boardless boat. I often got my 5.7M G-Cat to point as high as my friend's Prindle 18-2 (which is essentially a 18x8.5' Tornado). It is more sensitive to crew placement, and I can't make it tack very quickly. However, the designer of these boats (Hans Geissler) still races them and and roll-tack them faster than you'd believe.

Notably- the shorter these are the faster they tack, and they've built them from 16' to 36'!

[sharpii2]

Dear VBA:

If your little beastie refuses to come about into the wind, you can always jibe her (wear ship). Using a crab claw, boomed lateen, or even a ballanced lug sail, jibing is far les dangerous because there is sail in front of the mast as well as behind it. This will help slow it down once it starts swinging due to the wind getting on the other side of the trailing edge of the sail.

Narrow monos are very vulnerable to jibes. I did one in my Super Snark(r) and was looking at the bottom of the lake before I knew it. Wider monos and multis are not. In wider monos and multis, jibing is often the only reliable way to get it to change tacks in strong winds and lumpy seas. My light beamy centerboarder was jibed many a time to save her from going on the rocks.

It may not look pretty and may take more sea room, but it does work. I was usually able to get the bow up into the other side of the wind before the boom even reached the end of its sheeting bight.

As for sail type, my vote would be for a boomed lateen sail of about 60 sf.
In strong winds, she would sail fast without much worry of stuffing the lee bow.

The boom and yard would be exactly the same length (about as long as the boat) and the Center of Area (CA) would be considerably lower than it would be with a crab claw and would, therefore, give you more drive for given amount of righting moment.

It's interesting to note that the polynesians usually cocked their crab claws up only when sailing down wind. When sailing up wind, they usuall brought it down so that the boom was nearly parrallel to the water line. This accomplished two things:
1.) It shifted the CA aft to make it want to sail up wind, and
2.) It lowered the height of the CA considerably.

Both were very desireable for boats that, even in the small sizes, were expected to do a lot of hands off sailing.

When Westerners started studying the thing in a wind tunnel, they discovered that it produced a lot of aerodynamic lift in its cocked up state, so Western designers started designing their versions set permenantely cocked up.

Polynesians had permanently cocked up crab claws too, but only on large two masted double canoes which would be, by our standards, considered very much under rigged.

To design your sail, you have to get its peremeter right. You need to get it set up so that, when it is sheeted in, its CA is about 6 in aft the Center of Lateral Area (CLA) of your boat. The CLA of your boat will be the underwater profile area of your leeward hull. To find it, you need to make a scaled cutout of its underwater profile out of stiff cardboard, then ballance it on an edge like a teter toter. Where it ballances at is its CLA.

Next, you have to figure out where the mast is going to go. On your boat, I imagine it is going to go on top of the 1st crossbeam. You need to find out how far the mast is from the CLA.

Once that is done, you can scale sketch triangles slung from the mast until you get one with the correct CA, and the desired sail area. The reward for getting this right is that you will end up with a boat that not only has good 'ballance' (easy to steer), but may be able to be controlled by sheeting the sail and shifting your weight fore and aft alone and may be not need rudders at all.

If you want, you can post how far aft the CLA is from the mast, how much sail area you want, and what kind of sail you want, and I can run it through my spread sheet and give you a workable peremeter.

Bob

[gggGuest]

Originally Posted by sharpii2

Dear VBA:
Narrow monos are very vulnerable to jibes. I did one in my Super Snark(r) and was looking at the bottom of the lake before I knew it.

Its much more complex than that I think you'll find. Critical factors for ease of gybing seem to include boom length and boat speed. If the SuperSnark you mention is the Lateen rigged type boat with minimal overlap in front of the mast that I just found with a quick google then I'd regard that as being pretty much a worst case setup for gybing, being low aspect ratio with a long boom, dubiously controlled, on a slow boat. The high performance boats with high aspect ratio rigs, well controlled short booms and the rest of it are actually relatively much better disciplined on a gybe, to the extent that, as you rightly point put for other boats, a gybe can be the preferred choice, especially reach to reach.

Now we are talking about a relatively slow boat here, length being what it is, so gybing needs thought, but given a more conventional rig I'd always go for a short boom, a battened sail, and a very firmlly controlled boom with a very efective kicking strap/vang. Things like Crab Claws I have no practical experience on, so would avoid simply because I don't like implementing things I don't understand.

A lot depends on your priorities - the traditional rigs were often relatively good with the wind on the beam, because traditional craft sailors would tend to sail where and when the wind suited. If on the other hand I have often to get home upwind against the tide then a modern full battened large roach short boom sail is the only game in town.

[VBAjedi]

Wow. . . I haven’t been in here in a while! But my little self-designed catamaran project is still moving forward (just a lot more slowly than I’d like – you know how that goes!). I now have both hulls built and painted, and I’m just finishing a nice set of mahogany oars. I also decided to go with a hard “tramp” on the back half of the boat (easier to mount sailing hardware, and adds some much-needed stiffness), so I’m starting construction of the 3 beams I need. I’ll try to post a couple pics in a week or so.

My current issue has to do with the rudder. I abandoned twin rudders as needlessly complex once I realized I now had something to mount a single center rudder to. But I’m not quite sure how to calculate the size of rudder I need. I read that small boats like a rudder with an aspect ratio between 1 and 2 and with a NACA 15-25 profile. I do want it big because I anticipate difficulty tacking (hard chines and crab claw rig). My gut says to use a rudder with a submerged area that is 10” wide (chord) and 20” deep (giving me an aspect ratio of 2), with about a NACA 21 shape (making it slightly more than 2” thick). That would be 200 sq. in. of submerged area.

Anybody know how to calculate (or have enough experience to guess) whether that will be a good, slightly oversized, rudder for a 11’ 6” cat? I plotted out a NACA 21 profile on cardboard and it sure looks fat, but I guess it creates less drag than it would appear.

[lewisboats]

Descriptions go a lot better with pictures to relate to

Steve

[VBAjedi]

True, true, Steve! I'm going to take pics in a couple days when I finish laminating up the crossbeams (a ply/basswood/ply sandwich) and test-fit them onto the hulls.

Anybody got any input on that rudder size? By the way, before you give me TOO much flak about abandoning twin rudders, let me just restate my comment from an earlier post that I'm not intending to sail this thing hard enough to fly a hull. Besides, I don't think I have the materials to build the boat strong/light enough to do that even if I wanted to. So I'm hoping I can make one rudder adequate (I can always fit twin rudders later if one doesn't cut it).

[messabout]

Jedi;
There are far too many varables in boat design to permit a formula or recipe to provide "sure thing" information. I have plenty of experience with experimental boats and what sort of rudder was needed. In my quest for minimum wetted surface, drag, etc. I often made them way too small. I also placed too much emphasis on foil shape. I have disabused myself of all that and now adopt realistic shapes and proportions.

I'll make some wild assed guesses and say that the rudder you describe is too small. I'd start with something on the order of 12" x 30" or 15 X 24. Dont worry about aspect ratio. and for sure dont waste time with NASA sections. If you had an all out race boat the section is worth agonizing over. In truth an old beer sign will steer your boat. So dont sweat it. Make the rudder big and cut it down after a few trial runs if desired. A flat section of 3/4 ply will do just fine. Taper the leading and trailing edges for about an inch and a half into the flat part. One of my favorite boats of the past was a Windmill. Windmills are plenty fast and respond to steering inputs very nicely. Both the daggerboard and the rudder is made from 3/4 inch stock and tapered as described above. The design of both of those parts are proscribed by class rules. There are lots of others small boats that use simple plates with perfectly good results.

When your boat does not tack well it will be largely because you have too little rocker and/or the sail is in the wrong place. A cat like yours should have enough rocker to get BOTH ends out of the water when the boat is fully loaded. Banana shaped like Hobies. If you have a layout that produces much lee helm then the tacking thing becomes tedious. You may indeed be able to trim the boat by moving your weight fore or aft to balance the CE/CLR. And yes you may be able to steer it that way too. There is a catalonian cat , very popular in Spain, that has no rudder at all. Those cats have some tricky mast locater gadgets that can be adjusted on the fly. In your case just fool around with it until you get it right.

Keep us informed.

Gene

[VBAjedi]

The project that never ends is slowly (thanks to surfing season) ending! Here's a picture of my latest on this little "build-a-boat-with-minimal-materials-in-Africa" project.



The hulls are done. The three crossbeams (laminated beams from three layers of 3/4" ply) and the hard platform on top of them are done (the two rear beams and the platform between them are one solid unit, and the front beam and front platform are a second unit). In this picture I'm mocking up the crabclaw sail with PVC pipes representing the top and bottom edges so I can take measurements and get a sense of proportions. Rather than try to make the A-frame mast rigid enough to be freestanding, I decided to go with fore/aft stays on both sides.

The sail as I have it drawn now will be about 61 sq feet, with the CA about 6" aft of the boats CLA (got that right on my first guess!). Trying to find the bamboo now for the spars. For my test runs I plan to use one of the oars (lashed into a fork at the stern) instead of a rudder. . . we'll see how that goes and add a rudder if necessary.

I'm to the point now on the boat that I'm thinking about rigging and all the associated hardware. This being Liberia, sailing hardware is not to be had, and even the raw materials for it (like brass, stainless steel, etc) are not available. But since I'm going to be in salt water, I need rustproof hardware.

Last night it finally clicked in my head how to make the two blocks (pulleys) I need for my mainsheet. So today's project was actually making them. The wheels are made from a kitchen cutting board. I cut them out with one of those doorknob drillbits (can't remember what the name of those is - the hollow cup with teeth on the lip and a normal drillbit in the center), then figured out how to chuck the resulting discs into the drill so I could turn the groove in them for the line to sit in. The housing was what took me so long to figure out. Not having metal, I had to figure out a way to make a housing that would hold up to the load of a mainsheet. I finally settled on Plexiglas (it was that or wood, and this is thinner and better looking). The key to the strength of this design is the hollow spindle the wheel rotates around (courtesy of a dollar-store screwdriver I never used that happened to have a hollow aluminum shaft - ha ha!). So the line that attaches the block to the boat passes through the center of the wheel and takes all the load - all the housing does is keep the line from jumping off the wheel. The attachment line looks way too thin, but it's from a little piece of Spectra-cored line I had that is over 1000lb test, so it's as strong as the line running through the block!




I was really happy with how they turned out - they should be very strong, rustproof, and thanks to the clear housing they look cool! It's a simple but effective design, and it's fun to know I can make stuff like that here in Africa. Heck, I'd use these on a boat in America, too. They only cost a couple of bucks, but I probably spent more than six hours on them, so it's only something to do for the art of the thing, or if it's your only option (both were true in my case!). (Should I post these as a seperate thread somewhere so people can use the idea?)

[LP]

I'm impressed. Nice job on the blocks.

Here's a couple of links to small cat designs. One has a centrally mounted rudder.

http://www.svensons.com/boat/?p=SailBoats/CatsPaw

http://www.svensons.com/boat/?p=SailBoats/Hobby_Kat