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Styrons 15' Tunnel Hull
Submitted By: 2manyboats
Date Created: 12-06-2005
Description: This hull was designed and built in Creole, LA (now devastated by Hurricane Rita) as a commercial fishing, shrimping and trapping craft for use in the flats, swamps, marshes and shallows along the Louisiana Gulf Coast. My purpose will be fishing and duck hunting Louisiana swamps and marshes, as well as North Texas lakes and the Red River. The beauty of this project will be in the eyes of the beholder. As much as I love the sweeping sheer of a classic sailboat and the bold lines of an artfully designed powerboat, this ain't one of them...

This project will attempt to solve three issues: 1. No original floatation. Compartments will be created to contain 2 part, 2 pound polyurethane foam. 2. Old, worn and nasty carpet. This will be replaced with a rubberized polyurethane coating. 3. Small planing surface due to the 15' length and long bow sweep. This means the boat porpoises when lightly loaded, and when getting on step the stern drops too much. Bad for real shallow water. After much deliberation, including thoughts of cutting the hull and adding a few feet of length, I decided to add planing pods to the stern for a variety of reasons.

The hull is built of 1/8" welded alum. plate with welded longitudinal stringers and sheet alum. floor riveted to tops of stringers. OB is 2000 Merc 40 w/custom SS surface piercing prop. Tunnel allows running through about 12" water depth depending on the bottom. Rock bad, mud and sand good.

Floatation Discussion: My goal for floatation is to keep the craft salvageable and hopefully bailable after a swamping. After a fairly detailed dimensional survey of the hull, I conservatively estimated its weight to be approximately 480 pounds, based on aluminum weighing 165 pounds per cubic foot. In a swamping, let's say half the hull is under water as a conservative measure. The hull when swamped thus displaces 86 pounds of water, with water weighing 59 pounds per cubic foot. For the hull then, I need 394 pounds of floatation/ displacement. The OB weighs 204 pounds, so I need 598 pounds of floatation/ displacement minimum for the boat and motor. Obviously, more floatation will be required at the stern. Additional floatation will be required for gear, etc. As shown in the following photos, 710 pounds of floatation/ displacement was added. Hope my math is correct, although I do plan a controlled test swamp...
Tools / Materials: Go to the Boating Store


Styrons Port Side before
Note the sweep of the bow, decreasing planing area, although great for getting through vegetation in shallow water when planed.

Sweep of bow. We're talkin' flat bottom here. Well, I didn't buy her for offshore, you know...
Starboard bow


View from bow before
Old, worn and nasty carpet. Where it wasn't worn through, the glue seems to be 3/16" thick. The carpet glue defies all chemical removal systems. Elbow grease, here we go...

Carpet and glue has been removed, aluminum sanded and ready for primer. The aluminum floor is riveted to the stringers. After several years of hunting, fishing and shrimping, some of the rivets are loose. SS screws with threadlock were used to tighten the floor up and reduce rattling.
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Bow Storage Compartment before
Bow storage is open inside all the way forward with no floatation. A bulkhead will be added inside so foam can be added to very front.


Bulkhead is 1/2" treated plywood fastened to aluminum angles with SS screws and washers (don't forget the Threadlock). The edges were sealed with aerosol foam. Polyuretane foam (2 part, 2 pound) was poured into the void, creating approximately 250 pounds of floatation/displacement.
Forward bulkhead for floatation


Aft Seat before
Note the space under the seat as evidenced by the gas tank below it. A bulkhead and bottom will be added to contain floatation foam, while allowing water to pass below for drainage.

Space under seat was utilized for floatation by adding a 1/2" treated plywood bottom attached to the tops of the stringers. This allows water to pass through below. Also, as shown in this photo, is the vertical bulkhead to complete the enclosure. Foam was poured into the holes for the seat mounts. These small holes created quite a challenge to keep up with the rapidly expanding foam. Approximately 320 pounds floatation/ displacement was created.
Aft Seat with floatation added


Stern before
Transom configuration. Tunnel is 5"ht. x 9"w, tapering to zero about 5' forward of the stern.

This portion of the project was designed to solve several issues: 1. Add floatation: The pods are constructed as tanks and pressure tested, then welded to the transom. Approximately 140 pounds floatation/displacement was added. 2. Add planing surface to reduce or eliminate porpoising and aid in planing in shallow water. 3. Provide dog platform for waterfowling. 4. Provide steps to get back into boat for dogs (or people). 5. Every good yacht has a party platform!
New Stern Pods


New Stern Pods
Note the tunnel at the transom. The tunnel as designed was ventilated, which I don't understand, as air is introduced where a channel of water is being created, thus causing cavitation. The ventilation hole was welded shut when the pods were added. The skeg protects the OB and helps keep out weeds, sticks and small logs.

Note the prop position relative to the boat bottom. With new pods, the boat now floats dead level, with the water line just below the top of the bottom step. Sea trials in Galveston Bay proved porpoising was eliminated. The boat was tested with just me, a half tank of gas and a bottle of water in it. One side issue arose in that the Merc 40 is too much power for that light a load-especially going upwind. The bow came up on me...
New Stern Pods


Durabak liner paint job
Carpet removed and Durabak Roll on liner applied. Beige (sand) and grey were used. The product was very easy to use. Be sure to mask areas not to be painted and wear gloves, as it sticks to everything! Aluminum deck was sanded to bare metal, washed with water, then washed with a 50/50 water/vinegar mix. Then a zinc chromate primer was applied. The first coat of Durabak was applied within 12 hours, starting with the grey trees, then beige background. Two coats of each were applied. The smaller branches were painted after 2 coats of both colors were built up.

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Pod Detail


View from Bow
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Bow detail


Doel Fin Before
This Doel Fin was put on the OB to reduce porpoising before adding the pods, and it worked great. The boat still needed floatation and quicker planing, so after much thought, sketching and considering other ideas, the pods were added.

After the pods were added and sea-trialed, I thought a modified Doel Fin would work with the pods. These cuts were made to allow full turning of the OB. May be belts and suspenders, but worth a try.
Doel Fin after

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