Hello Swaylockians

I have been a huge swaylocks fan/addict for years now but have always just lurked around since I am not a surfboard builder or designer. Also I am kind of a computer idiot.

But a recent post concerning outrigger design has finally inspired me to make the leap. I have a paddleboard which I designed and had built which I would like to share with you and get some comments on. It is a double hull configuration (see attachments) which, I believe, has not been tried before. If it has been done, I figured that this would be the place to find out.

I was motivated to build this when I became interested in paddleboarding. I had a velzy surftech, then two Barks which I really enjoyed but I have always found too unstable to knee paddle consistently. I also did not like the way they handled in the windy ,choppy conditions that we often get in florida during the afternoons, which is usually when I am able to work out. So I thought why not take advantage of the superior stability and rough water capability that catamarans are known for?

The 1st photo is of the prototype I built myself about 4 yrs ago. I simply took a 12 ft clark blank, cut it in half, stood them on end and tied them together with a platform between. It was not very stable since I made it much too narrow and it had way too much buoyancy. But it did the things I wanted well enough to want build a better one. It was really swaylocks that motivated to follow through and make it happen. I knew that I didn’t have the shaping and laminating skill to do it myself so I had the crew at Oceansportsworld do it for me. It is all epoxy construction and weighs 25 lbs. The dimensions are 12 ft long x 22 in. wide x 9 in. deep.

I have only had it a few weeks but so far I have been pretty pleased with its performance. It is extremely stable when paddling. I can get up and knee paddle easily under any conditions and it cuts though chop like butter. I recently took it out one afternoon when the surf was around waist high with onshore winds around 10 to 12 mph. I was able to paddle through the surf with no difficulty and once I was outside the surf line it easily cut through the wind chop regardless of the direction.

It does have some problems. It has too much buoyancy and gets pushed around by the wind too much. It also can be difficult to maneuver since it alternately doesn’t track enough or it tracks too much. I attribute almost all of that to my complete ignorance of hull design. Since I really had no reference to work with, I consider this a baseline to improve on. I would really appreciate any comments/suggestions you could offer either good or bad (I can handle the negative stuff). I will be working on this through the summer as I find the time. Thanks in advance!

What if you had it set up as a double hull in the nose with more of a traditional paddleboard tail?

Actually that issue came after the board was built.I had wanted the square stern in case I wanted to make a double rudder setup.That would give me a flat surface to work on. When we went to test it in the water the guy who did the laminating brought along his surfski and you could really see the difference. The water flow off of the square stern of my board created a lot of turbulance while the flowed off his pointed stern cleanly. In retrospect it should have been obvious since all fast paddleboards, surfskis, kayaks etc. have that feature. Hopefully I will be able to redo the shape in the future and correct that.

I’m the surfing world’s weakest paddler but…

I thought narrow and low draft was two keys to paddling speed, at the expense of rough water stability, of course. Length is important, as once parted, it takes little additional energy to slide your craft thru.

your two craft are really intrigueing and interesting

I’d go with a 16’ long, 17" wide paddle board with really deep knee wells to lower center of gravity.

But your two are much more interesting and worthy of analysis

Interesting… while it’s not applicable to monohull paddleboards, the twin hull version has some possibilities - have you seen these? http://www.swath.com/

Might be useful, might not. In relatively small chop/swells it might be a power-saver, though in larger stuff you might have problems staying on it. Platform design could be kinda interesting too - laminated fin-like structures perhaps, or shaped foam pylons ( might be a natural for hot-wire cut foam, as they’d be fine as straight sided or straight-tapered sides shapes)

Rudder cable(s) run in inset plastic tubing channels glassed into the upper hull surfaces? A few other ideas bouncing around…

hope that’s mebbe of some use


the variables explode my immagination,cant and toe in fifst then flat planing surfaces on bottoms of the catamaran hulls 18’’ wide for reach ease…what is the float line?how deep is the draft on these interstellar units…these begin to look like those hawaiian downhill sleds for that big Island track!..you are onto an intrigueing cariation here… Kudos…ambrose…wow

Hey Doc… maybe you could set us up with one of those big cats for S.A. 2005 on the east coast?! Maybe cruise Puerto Rico, Nova Scotia etc???

Hey, John -

Actually, a buddy of mine is about to launch a ~40’ sailing catamaran, after a 10 year/2 wives/3 houses boatbuilding ordeal. Another is doing a ( again, around 40’ ) lapstrake gaff ketch on the model of a Norwegian Baltic boat of some kind, similar sort of process…

Me, I’m a monohull guy. Built a few catamaran work platforms ( twin-hulled barges) for others, but for myself I like to have just one hull to keep track of. And these days I don’t sail off too far: My current project is redoing this old girl for myself…

She’s just right for what I’m gonna be doing with a sailboat for the next 10-15 years or so. Stash a board, some fishing stuff, a book or two and a supply of grub aboard and sail out through the estuary, anchoring inside the barrier beach and plain chilling out for a few days.

Now, if you wanted a boat to put a bunch of people aboard, I can mebbe arrange that…


Sorry I am taking so long to reply but I get pretty busy sometimes. Especially during the first of the week. I am interested in your comment about length. If most of the energy used to part the water occurs at the entry point is it the length that allows more speed or is it how quickly the bow area fairs back to a straight line at the waterline. Isn’t the lenght to width ratio a bigger factor? The reason I ask is that using the catamaran design allows me to achieve a better L/W ratio since I don’t have to worry about making it too unstable to paddle- like what would happen on a monohull. And I had hoped that with this design I would to make as fast as a longer monohull. Thanks

Doc- thanks for the link to the SWATH hull design. I love stuff like that. I will file that in my Future Cool Projects file since I have this affliction that causes my to take on more projects than I can possibly finish.

As for rudder cables, I think I could simplify that by simply running the cables under the deck, between the hulls. There is plenty of room under there.

Ambrose- When I built my 1st prototype I made it 18" wide which, along with the high CG, and extreme rocker, made it very unstable and just wobbly bobbly. On the 2nd one I made it 22" wide based on a lifeguard paddleboard width. The entire bottom is very flat- basicly a square profile. we kind of guesstimated the buoyancy going on the assumption that too much is better than not enough and ended up with about 6-7" of freeboard which is too much. We basicly WAG’ed the whole design. On my next one I probably need to take the middle ground between the two. I think I can go to a 20" width without affecting the knee paddling( which I am kind of obsessing over with this) and I know I can go with a much lower freeboard.

By the way all the relentless raving about surf mats by you and the others on swaylocks has forced me to have to order one for myself from Dale. I have been testing the fun factor on a surfster for the last 6 months or so and that has convinced me that it is to get the Real Thing. Thanks.

you have made a wise choice gettiing a mat…make a paddle for your cat paddle board …glue a fillet of boogie board to sit kneel on …ambrose

‘More projects than’…yeah, that sounds familiar. Real familiar.

Answering a few questions for ya;

Length kinda governs maximum speed for displacement hulls. Roughly 1 1/3 times the square root of the waterline length in feet gives you a number- for instance an 81 foot hull, square root is 9, multiplied by 1 1/3 gives you 12, and that in statute miles per hour gives you the maximum speed you can get before you start climbing your bow wave and wasting power: 12 miles per hour. A really skinny hull can do better than that, say up to 1.7 or so, say a really well developed light displacement sailboat, but length is good, basicly. Absolutely straight, blade like hulls give you a way around that but the tradeoff is a lot of wetted area and thus drag.

If you have two rudders, which you will need on two hulls, then it’d be best to have 'em on or better yet in the hulls, say run inside plastic tubing. Less chances of 'em snagging, dragging or whatever. http://www.infinitysurfboard.com/inf_waveskidesign.htm gives some rudder illustrations that’d be a good starting point, Think in terms of tension-only cables in many ways run like a bicycle shift cable. With hulls completely immersed you seal around 'em and if the plastic tubing gets water in it, so what? Use vinyl-coated stainless wire and a little grease. It can be a real simple system if it’s set up right.

Hope that’s giving you a few ideas to play with.


I too have considered starting a paddleboard project. I also have been toying around with the “paddle-cat” idea, but am still not sure it would be worth all the trouble to build two hulls if the performance gain aren’t worth it. In the end, though, at least the “hey, look how cool this is” value would be worth it!

Here’s some things to consider:

In general, Cats are designed to offer maximum stability while generating the least wave making drag. But, they usually have a higher wetted surface area then a monohull of the same displacement. For a craft as narrow and light as a paddleboard, it may not have a great affect.

As far as entry goes, the finer the better. Most kayaks have the wide point aft a few inches, which sort of “tricks” the water into thinking the boats is longer than it really is. Be gentle though, 'cause a board with a long fine bow and a fat stern won’t do you any good, either. Also, leave the stern fine, too. You don’t want to square off the stern and “drag transom” so to speak. You want to leave the water as smoothly as you entered it.

Doc is right on about waterline length. Longer is generally faster.

Sounds like your buoyancy was throwing off your stability like crazy. I would think a paddle board should probably only be about 7"-8" thick from keel to deck at the thickest part (depending on length, of course). Every 1/4" you get closer to the water surface, the stability will increase drastically. Perhaps that’s why your kneepaddling efforts were met with failure.

Oh, and round that hull bottom out (cross-section wise). Rounded bottoms lower wetted surface, and make for a faster boat. The more round, though, the less stable. Compromise.

Where in Florida are you?

Doc- thanks again for the info. That cleared up some confusion I had about hull speed and length. Do you know of any books about hull design that might useful? Maybe something like Hull Design for Dummies. Also thanks for the link. That gave me some other good ideas.

Anton- the stability problems you refer to were with my 1st prototype. My 2nd one is very stable and handles rough seas very well. It has some other problems, such as too much buoyancy, but I think I will be able to dealwith them as I work on it more. The rough sea capabilities actually make it fun to paddle in chop instead of being a chore. I am really in an early stage of developing this and since I am doing this as a spare time kind of project it will take some time to refine the design.

I had been told about round hulls being faster but I didn’t understand why. Thanks for explaining that for me. It looks as though I need to look more closely at kayak designs to help me develop this. By the way, photos of my paddleboards are attached to my 1st post.

I live in Merritt Island, across the river from Cocoa Beach.

Thanks again for all the helpful info.


You might want to pick-up a copy of “Kayakcraft” by Ted Moores. It is available from WoodenBoat magazine. The magazine has a web site store. The book’s early chapters contain an excellent summary of design of the everyman.

Hope you have success with your idea.


‘Hull Design for Dummies’…hee, now there would be a difficult book to write. Kinda like Practical Neurosurgery for Beginners.

Still, if you pick up a copy of Skenes Elements of Yacht Design by Francis Kinney ( A used copy of the out-of-print 8th edition rather than the reprint of the 6th edition that’s currently available) it has the basics of hull design maths for planing hulls and displacement hulls. It’s a fair introductory text with the rudiments of marine engineering in there. Don’t be put off by the math; while there’s a lot of it it’s not like any of it is quantum mechanics, rather it’s a lot of number crunching and the concepts are presented pretty well.

After that there’s a few introductory textbooks, Zubaly’s Applied Naval Architecture and Tupper’s Introduction to Naval Architecture which are set up more for the professional , either as a refresher or an introductory text. The discipline is evolving rapidly these days, what with new numeric methods made available on account of our little silicon pals like the one I’m using to write this, so I’d have a look around the SNAME ( Society of Naval Architects and Marine Engineers - www.sname.org ) for what’s up to date.

While there is hull design software available, the free version of HullForm for instance, it’s best to have a working familiarity with the numerical processes involved, so that you can do rough, back of the envelope calculations and check the results that pop out of the software. That way, a dropped decimal point won’t sink ya, if you follow my drift.

hope that’s of use

To paraphrase Jim Phillips, I’m ‘just a water rat who followed his dreams’ and wound up knowing a very little of the subject.


Patrick thanks for the recommendation. I will look that up.

Doc- thanks for the book recommendations. Sounds like I may be biting into more than I can chew since I am kind of allergic to math. But I will check them out just the same.