Join Date: Nov 2007
Location: Hamburg and Wilson NY
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HT (high-thrust) props, especially for the small motors, are a recent thing. Many are available from only from Solas. In fact, the HT props for the Tohatsu, and the other flavors of the Tohatsu (Nissan, Merc) 8/9.8 are in fact remarketed Solas Amita-4 4-blade HT props. We sell them as part of our "sail" configuration. My personal 7,000+ pound sailboat is equipped with a 9.8 (the 15 would not fit in the lazarette, or that would be the aux of choice) and gets adequate speed performance with the 5-pitch HT prop... That and absolutely stellar slow speed control.
Compare that with my old 2-smoke, points-and-condenser '74 'rude 15, and the 'rude lost by every measure. And there wasn't any Solas HT 4-blade prop for the 'rude until very recently. In cruising dress, at 8,000 pounds, the Tohatsu 9.8 beats the 'rude 15 in all categories -- noise, economy, smoke, weight, reliability -- but top speed is a wee bit (roughly half a knot) lower.
One thing I can personally verify is that subtle changes in prop pitch have a HUGE effect when pushing a heavy sailboat with a small motor. The 7-pitch HT prop will limit my 9.8 to 4,800 RPM at WOT, while the 5-pitch HT prop allows the motor to go all the way to 6,000. This effect will be exacerbated with the smaller 1-cylinder models. If you want a full 6 hp from the 6, it really does need the correct propping to allow it to reach correct WOT RPM.
The 3-blade HT "merc" prop for the 4/5/6 Tohatsu works well. But again, the 4/5/6 is primarily a 1-cylinder dinghy motor, and was not designed to push a 4,000-pound (with crew and gear) displacement hull. There is a 6 hp configuration with 25" UL shaft, which I have rigged on 2,000-to-3,000-pound sailboats (with the EE "elephant ear" 3-blade "merc" prop), which works OK. I would still opt for the (much smoother) 2-cylinder 8/9.8 if the budget and boat can handle the weight (82 pounds).
To clarify, the difference in the Tohatsu 4/5/6 is the carburetor, not just jets. And yes, the 6 hp carb does give you 50% more, government-measured, prop hp, compared with the 4 hp carb. FWIW, the most-overlooked power loss item on the 4/5/6 is not adjusting the valves after break-in. Makes a big difference in top end. The "hot setup" in the 4/5/6 family is to buy the 4, with the onboard fuel tank (which the 5/6 does not have), and then upgrade the carb to a 6 version. Then you have the option of reserving fuel to get you home when you forget to fill up. Happens more often than you'd think, because the 6 uses only about 1/2 gph at WOT.
Just don't expect the little 6 to push a barge to planing speeds.
Paul Van Voorhees
Certified Tohatsu TLDI Technician
Mgr, Obersheimer Sails
Buffalo, NY USA