Tongdee Boats

long tail boats

The Longtailcat design


I am Sean Walker Tongdee the designer and builder of many small unusual boats. This craft is a long tail powered sailboat built for exploring the lakes rivers and coastal areas of Thailand. It is a cruising catamaran designed to make excursions into the jungles areas of Thailand possible. I had a dream and this is what takes me there.

                            The Longtailcat

This catamaran is eighteen feet long and nearly eight feet wide. The unique thing about it is that it has a 13 horse power long tail engine set up.

The boat was designed for easy access to rivers lakes and oceans. It is a sailboat and a powerboat. On rivers sailing is limited so the engine comes in handy. On lakes and oceans there is nothing that compares with sailing but the engine is of good use in times of no wind. It combines the best of both worlds without compromising either.  

An important feature of this boat is that the engine, hulls, mast, rudders, and deck, are detachable for transport. I load the hulls on a rack atop my pickup truck. Then I drive into the jungle alongside a river or lake, remove it from the truck and launch it alone. The hulls are long and awkward to handle but it is doable.

The reason I don’t use a trailer is that when it is assembled it is nearly eight foot wide. The additional width for the trailers wheels makes it even wider. Also as you might guess there are few launch ramps in the jungles of Thailand.

the purpose

It was specifically designed for expeditions lasting a week or two and traveling several hundred kilometers distance into remote river and lake areas. Its six large stowage compartments provide ample space for the necessary camping gear. It has a two piece deck that allows the shaft to pivot up and down. This broad deck provides a rigid surface for numerous activities like cooking, fishing and sleeping. This boat will be used extensively for continuing my exploration of the rivers lakes and ocean areas of Thailand.


The design

The hulls were designed using fluid dynamics and I often describe them using the fluid air. They are somewhat the shape of a high speed bullet. The hulls are narrow at the bow allowing low parting resistance of the water. They are widest at the pivot point where the centerboard and engine is located and cargo is stowed. Heading aft the hull narrows somewhat allowing the parted water to rejoin, easing water resistance. This shape provides a low resistance trajectory requiring only a small amount of power to cruise reasonably fast.

                                      the tail

The long tail engine and shaft setup does not swivel horizontally to steer but it does pivot up and down. When anchored the prop is raised up out of the water and locked in the up position. When you are ready to get underway the procedure is to start the engine and lower the shaft and prop into the water. While underway the engine and shaft is balanced so it will seek its own depth. This balance is desirable for operation at different speeds and conditions and it is self governing. When you throttle up the shaft and prop pivots up progressively in relation to speed. At high throttle the prop is only halfway submerged and it is surface propping for maximum speed. In other words the boat chooses the prop depth as per speed and water conditions. If the prop skeg, centerboard, or rudders run aground they will harmlessly pivot up and spring back down. They can be locked in an up position if desired. Powered the boat cruises effortlessly at 15 knots. When sailing its speed is comparable with most cruising catamarans.

                            MAIN SAIL DESIGN

The mast boom and sails have the characteristics of the standard Bermuda tacking set up. The sail is a modern performance airfoil design. I designed it myself and had it made to my specifications. This was the only part of the boat that I did not make myself. The Luff is attached to the mast using the track and grommet method and the foot is attached to the boom using a boltrope. The luff is rounded to provide the sails camber that forms its airfoil shape and the foot is rounded as well. The leech is rounded to expand the sail area and the battens help retain the sails shape. The sail is also set up to be reefed. There are cloth and strap reinforcements on the head, tack, clew and all reefing grommets. There is also a reinforced Cunningham grommet.  The sail material is high grade 6oz. polyester sailcloth. The main sails area is 135 sq. ft. less the roach area and totals 180 sq.ft.


Comfortable efficient transportation is its design performance criteria. The boat was deliberately built not to sail like a Hobie Cat or a high performance sailboat. Hobie Cats frequently capsize due to the tall sail and other balance considerations not to mention pushing the envelope for sport. On a cruising catamaran if you capsize, the engine, gas and stowed objects are subject to damage and righting the boat is nearly impossible. So you don’t want to capsize this boat but if you do both hulls have sealed bulkhead chambers that will prevent sinking. The way I have effectively lessened this catamarans tendency to capsize is by making it as wide as possible without compromising the width to length ratio. A catamaran that is too wide will be stable width wise but it will pitch fore and aft. Another way I minimized the tendency to capsize was to make the mast a bit short and the boom long keeping the larger sail area lower to the center of gravity. This reduces the wind leverage up the mast that causes the boat to be leveraged over into the capsize zone. Also all gear is stowed in the hulls as ballast at close to the water line.

This boat is a testimony to following a dream to its fruition. Whatever your dream is may you be as pleased with it as I am with mine.


The Longtailcat was dreamed designed built and played with by Sean Walker Tongdee in Chiangmai Thailand.