Jada, Fuel-Efficient 24-Foot Displacement Fishing Boat
Four years after launching and starting to fish the original 56-foot Tropic Bird, Tim realized the world was not going to beat a path to his door for sailing fishing boat designs and construction.
So, Tim thought he’d meet the motorboat people halfway: and he built a motorboat that was designed to use as little fuel as necessary to do the job. In contrast to the local 24-foot planing hull fishing boats that could easily use 4 to 7 gallons of fuel per hour (even when trolling at only 9 to 10 knots), Jada’s 24 horsepower Yanmar diesel engine only used 3 quarts per hour at 3/4 throttle, and hit her top speed of 10 knots.
Jada was 24 feet long, a displacement hull design of simple marine plywood construction with a thin skin of fiberglass and epoxy resin on the outside to waterproof her. She had a 1,500-pound capacity insulated fish box in the center of the main hull that would easily fit five or six 200-pound yellowfin tuna, or a seven-foot-long, 450-pound marlin with the bill cut off.
She had a hydraulic line puller that could haul pot lines or longline, or even anchor lines (that’s the “bump” on the boat’s rail just behind the guy who’s steering the boat in the photo below). This made it really easy on the fisherman; he could use his energy for finding fish rather than exhausting himself just humping line back into the boat.
She had a live bait well that could easily keep 100 pieces of bait alive for a couple of days; this is because (as every Hawaiian fisherman knows) live bait has about a 10:1 catching advantage over dead bait.
You readers who are knowledgeable about displacement hulls, and know that the theoretical top speed of any displacement hull is roughly equivalent to the square root of the waterline length times 1.34, may be wondering “how the heck did he get a 24-foot displacement hull to do 10 knots? Her theoretical top speed is only 6.7 knots! (1.34 X 5)”.
Get ready for some technical details: Tim put a 5:1 reduction gear on the engine so Jada was able to turn a huge 20-inch prop, with ends squared off in the machine shop to match the 16-inch inside radius of the Kort knozzle that he put on the prop, then located the prop so far aft that the aft end of the Kort knozzle was even with the vertical line of the transom.
What this did was to shove a HUGE amount of water very efficiently, and where the shoving happened was behind the back end of the boat, instead of forward and under the boat as with most displacement hulls. The rudder was also hung directly on the transom instead of back under the boat.
As a result, Jada didn’t “squat” the way most displacement boats do because their props are back under the boat, and are kicking water out from under the boat. Conventional displacement hull design doesn’t use all the propellor’s energy to push the boat forward, but diverts some of it to “sucking it down”, and creating more drag on the hull as a result. You wanna argue with this theory? OK, whatever; but it worked; she did 10 knots both ways over a measured course of 13 n.m.
This was thirty-five years ago, in 1982, and all the photos we had of Jada have disappeared into the mists of history, except for the one below.
(Below) Jada underway in Kawaihae harbor, early 1983.