FatCat 21

14 March 2006     Editor    1 Comments.

GALLERY | Click images to enlarge

Chris Ostlind presents a catamaran design for plywood stitch and glue construction that continues the Manu Kai theme.

Just a few months back, our Proafile host, Michael Schacht, introduced a set of concept illustrations for a very nice, 23’ Hawaiian flavored catamaran called Manu Kai.

Michael’s vision was at once elegant in the manner in which it honored traditionally styled Hawaiian sailing vessels and it provided a set of unique solutions for how to blend the traditional with the modern in boat design.

Below the text of the article in the section for comments was a note from the author about the difficulties of designing this type of hull for a plywood build style. Since I design a lot of boats for stitch and glue plywood construction, I was immediately curious as to just how far down the path of the Manu Kai I could actually go with a possible design from that material/technique.

Coincidentally, I had been developing a pair of plywood cats at 21’ and 24’ that would be legal for trailering while providing energetic cruising potential for the owners. Taking on a challenge, like the Manu Kai form, seemed like a logical extension of the design process, so I pushed myself up to the computer to take a whack at Michael’s high standards.

The result is the FatCat21 pictured here.

Much of the territory explored by Michael has been addressed in this design with some exceptions for pragmatic materials use and simplicity of build. For instance, I did incorporate an upswept bow horn to establish a definite nod to the traditional Hawaiian, paired canoe forms from the past, but I dropped the canoe hull stems in the stern in favor of a transom build. This gives the stern more room for an emergency flotation chamber, slightly more room for interior applications like a single bunk for a kid and a very standard method for mounting beach catamaran rudders.

I changed the rig to a Gaff setup to set the design apart a bit from Manu Kai’s Gunter, while still drawing on the trad look of older sailing boats. The boat could just as easily be equipped with an affordable, modern and easily acquired Hobie20 rig if the owner so chose.

The displacement of the FatCat21 is set at 1850 lbs. This allows a significant load carrying capacity of 1000 lbs. over the estimated finished hull weight. I was targeting the load requirements on this boat to accommodate a family of four (mom, dad, a couple of small kids) on a long weekend, or two adults for a more extended coastal cruise. In case you’re wondering, I am not suggesting that this boat should be used for crossing oceans.

Build style is a very easily managed, stitch and glue style with external forms to hold the panels while they are being wired together. I am indicating a full, epoxy/glass laminate on the inside as well as outside of the hull. The three beams are owner built, glass re-enforced, box structures with one box beam able to slide inside the other to compact the boat for trailering. Sailing beam is 11’

LOA: 21’
BOA (sailing): 11’
Beam (trailered): 8’6"
BOA (hull): 32"
BWL (hull): 21.5"
Displacement: 1850 lbs.
Sail Area: 251 sq. ft.

The hulls each have substantial spray chines about midway from the water to the deck inside and out. These will provide considerable wave and spray deflection underway and enhance the ride comfort. They also enlarge the interior volume so that anyone dropping down into the hulls will not have to move sideways to dig through their gear.

When I looked at the Manu Kai’s integrated keel forms, I saw a perfect solution to something I’d been haggling with in the original FatCat design: What to do with the daggerboard/centerboard trunks so you could still get some useful space down below? I’m still going to offer the other board solutions to builders if they seek more performance from their boats. I just feel that the integrated keel form presents a very viable solution to boaters who are, shall we say, a little less attentive, when approaching a shore. Nobody likes the thought of clipping along at 10 knots and driving your boat onto the beach with the daggerboards still down. So, the integrated keels will do wonders for the sailors who don’t mind losing few degrees of headway in exchange for peace of mind in thin water.

I did change the rudder design and it has more to do with my personal preference than anything. Spade rudders are more efficient for a given size. They work off the hull as an endplate and have none of the cavitation issues one might experience with transom hung units. They are, however, much more susceptible to damage from underwater obstacles. Some of the concern has been addressed in the integrated keels of the Manu Kai design and for good sailors who pay attention, this will be a great solution. For others, (and that includes me) who don’t always pay close attention when sailing, transom hung rudders can really save your boating vacation, as well as your pocketbook, if you drive up a beach or tiptoe over reefs. I’m specifying transom-hung rudders of the flip-up variety for that reason.

There are footwells in the aft hull decks so that when the boat is driven, the crew does not have to lay their legs out straight on the bridgedeck. The sitting couches can be folded down for travel or removed completely from their mounts and stored in the hulls. The bridgedeck, itself, is a series of component platforms that can be removed for trailering.

There is a drawing for an additional, modular spray shield that installs across the main forward beam to give further comfort while under sail. Much like the Manu Kai, the motor can be carried in a pod amidships or simply hung from the aft beam according to owner wishes.

Below decks, there is ample room for a couple kids to sleep in each hull, storage for gear and room to setup a porta-potti if so desired. I also feel the bridgedeck is the place to hang a boom tent or setup a mountaineering tent. The boom tent option is especially powerful for anchored locations as it would full enclose the hull pods as well as the large deck, making for a really roomy camping cabin.

Plans for the FatCat21 are not completed yet. In the meantime, I’d love to hear comments and/or constructive criticisms on the design as I would like to incorporate anything powerful before I sit down to finish the working drawings.

Chris Ostlind
Lunada Design

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