Kobus Thai Longboat Build

Thai Longboat / Bangkok Longtail Build

I worked for a while in that part of the world and we often used these pilots with their boats to get around and later rented the boats only to explore the area. In the process I became very interested in these boats.

Here are some pictures of these beautiful boats. They are mainly used for transporting people but we did some fishing as well.

The Search for Plans

Once I decided to build one of these boats, I started looking for plans or at least some layout and waterlines. I soon found out that these boats are built by hand and the skill is carried from generation to generation, mainly through an apprenticeship running in families. The Maritime Museum in Bangkok put me onto Mr. Scott Carter of Cape Pakarang Boatyard in the south of Thailand. It turns out that many boats were lost or badly damaged in the Tsunami a few years ago and this Mr. Scott is running an organization, called Tsunami Boat Project which helps local people build replacement boats. Mr. Scott referred me to Umberto Petricich, an Italian naval architect, who spent a while measuring as many of these boats as he could find to establish basic lines and dimensions. As wood was now also scarce due to the losses in the Tsunami, he also developed a mould so that they could build glassfibre boats where wood was unobtainable. When I explained what I wanted to do, they kindly sent me a full CAD design package of the typical Thai longboat. Below is a photo of one of the boats built from these plans being launched. Below are also some downloads of the CAD package.

Download and Print the Plans

From the CAD package I could decide on the scale of the model. 1/16 gave a hull length of 622 mm, just a nice size boat for a 400 size motor. The plans were downloaded at 1/16 and printed.

Making the Frames

The paper prints were stuck to 1/8 ply with a gluestick. Then each frame was cut on the fretsaw both inside and outside contour. The paper patterns were removed & the frames were sanded and stained the desired wood colour that matches the original wood used on the full size boat that I was using as prototype.

Building the Hull Frame

The keel was laid out on the plan, covered by a thin sheet of plastic to avoid it sticking to the paper. (7950). Then each frame was attached at its station and held square and at the correct height with relation to the waterline. (7954) Note the crosspiece (unstained section) to give some strength to the very thin and flimsy scale frame. One by one the frames were added. (7956). Once all the frames were attached to the keel, a strong-back was built out of 1" square balsa to hold the fragile frames in place. (7979) & (7981). Now the complete hull frame could be removed from the plan and turned over. (7976).

7950

7954

7956

7979

7981

7976

Spiling and Planking

As these boats are Carvel planked, I had to use spiling to mark the width and profile of each of the planks with a batten. (7985) & (7987) The planks were then cut to profile and shaped by soaking in water with a bit of ammonia (7991). First plank is attached with plenty of clamps (7989). Second and subsequent planks are added with no gap between the planks for a watertight hull (7993).

7985

7987

7991

7989

7993

Hull Painting and Lettering, Fitting Electronics & Decking

While the hull was still on the strong-back it was sanded and stained with a mixture of wood stain and black ink to simulate a weathered boat. Lettering was copied from a 1;1 boat to get Thai writing. It was drawn on airbrush frisket and cut with an Exacto knife & applied to the hull as a mask. White airbrush spray was applied & frisket removed. (8574 & 20140804).

The frames were cut with a fine-tooth saw and the hull removed from the strong-back. Fitting the floorboards, receiver & steering mechanism and speed controller. (8361, 8632 & 8633).

Fore and aft decks made from basswood & stained with diluted black ink to simulate bleached wood. Pilot's deck added with motor wires (AKA fuel lines😊) routed through the deck to the speed controller. (8700 &8702).

8574

20140804

8631

8632

8633

8700

8702

Longtail Engine Build

These boats are mostly powered by modified automobile engines. The guys add a water-to-water heat exchanger which cools the coolant that is in the normal auto engine coolant loop. Some have a gearbox & shifter, others drive directly to the long shaft. I calculated that I would need a 400 brushed motor, geared about 3;1 to get scale speed plus some get-out-of-trouble extra when needed. I had to hide the 400 motor plus 3:1 gearbox inside a scale looking auto engine shell. This whole contraption had to balance the motor with the shaft and prop and had to swivel via a servo connection to facilitate steering. I drew up a plan that gave the outline of the motor/gearbox/steering and then drew a fake automobile engine shell that would fit over the electric motor/gearbox.

Decks/Canopy/Rub-rails and Finishing Details

8700, front deck,8701 front lower deck, 8702 aft lower deck and platform on which the pilot stands. 8703 shows the aft upper deck where the engine is attached and I added a few oil-spills/leaks with a dry brush dipped in diluted matt black paint. 8703 and 8704 also shows the rubstrips being nailed in place. The brass nails were cut with a flat sidecutter & filed flat to the surface of the rail so as to simulate the scale nail heads. 8707 shows the complete deck and passenger seating. The basswood was given uneven brush coats of diluted black airbrush ink to simulate the bleached wooden surfaces. 8709 is the brass frame for the canvas canopy being flame soldered. 7813 is the frame installed and painted to simulate rusty once galvanized tubing. 8712 are the typical traditional streamers in Thailand's colours that are added to the stem of these boats.

8700

8701

8702

8703

8704

8707

8709

8713

8712

Motor/Gearbox/Prop-shaft Build

I used an old Graupner 400 motor found in my 'parts' box and added a 3:1 Graupner gearbox obtained from E-bay. Made up a brass frame (7438) and scale looking propshaft with skeg (7436) that protects the prop from digging into the mud etc. in the shallow canals where these boats are used. 7433 shows complete drivetrain with bronze bearings.

Next step was to make a yoke (which allows the prop to be lowered and raised) and a steering mechanism which swivels the prop/motor assembly via a servo link.(7440 , 7441 & 7444).

7446 and 7449 shows the parts disassembled painted & re-assembled. A 'scale' false auto engine was built over the electric motor/gearbox assembly using plastic pieces from a toy car engine, styrene sheet and and plastic model parts from my 'bits' box. Simulated straight exhaust and cooling loop was made and fitted. All painted by airbrush & that dry brushed with black ink & 'rusty' brown paint mix the get the old, used, rusted look.(7871 & 7868).