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On my West Wight Potter 19 sailboat, I have a circa 1994 2-stroke long shaft Tohatsu M5BS motor. This is a 5HP motor that has served me well. Tohatsu makes this motor and it is also sold under Nissan Marine and Mercury Mariner brands. The 2-stroke is no longer made, but now there is a series of 4-strokes sold as 4, 5, and 6HP versions, differing in carburetion and propellers. This article applies to the 5HP 2-stroke as well as the 4-6HP 4-strokes.
Small outboards are usually used on lightweight planing boats. As such, the manufacturers usually ship these motors with a propeller appropriate for planing a light craft. In my case, the stock propeller was a 7.9 inch diameter with a 8 inch pitch, using skinny blades designed for planing.
On a relatively heavy boat like my sailboat, the motor will not get into its optimum RPM range with this propeller, and therefore won't make as much power as it could with the right prop.
Displacement craft tend to do much better with large propellers with wide blades and a low pitch. A lower-pitched propeller is like a lower gear in a car transmission, and a lower gear is more appropriate for heavy loads at lower speeds.
The manufacturer has always offered 7 and 9 inch props for this motor and I had always wanted to try out the 7 inch. I recently learned that two different 6-inch props were available. One is a standard-style with a 6 inch pitch. But the other is actually designed for sailboats and features the largest diameter practical on this motor, which is about 8 3/8 inch. Anything larger would come dangerously close to hitting the cavitation plate. This prop also features nice wide blades. Tohatsu calls this the 'Elephant Ear' propeller, and is the stock propeller on their 6hp "Sail Pro" motor. It is also called the 'High Thrust' or HT prop in some literature.
Depending on where you buy it, you may save money buying it under one brand vs. another.
Mercury/Quiksilver part #: 48-812951a (this is the one I bought).
Tohatsu/Nissan: 399C645121, 399B645121
I wanted a spare prop anyway, so I started looking for these. I found a seller on eBay offering a new-old-stock propeller at a decent price so in a few days, it was mine.
It is noticeably larger in diameter and blade size.
This picture shows the old (white) propeller atop the new (black) propeller. The picture understates the difference in size because the old propeller is physically closer to the camera lens. I would estimate the blade area is about 30% higher on the new prop.
I wanted to try this propeller and objectively test the following:
- RPM optimization (can I make more power by getting into the right range, without going over?)
- Noise level (would a higher RPM be louder?)
- Performance in forward and reverse
Baseline Test of 8" Propeller
I began with a baseline test with my old 8" pitch prop.
I put the daggerboard all the way down, but did not have the rudder attached, so I was steering strictly with the motor itself. Motoring dead upwind against an estimated 10-knot headwind in chop, at full throttle, I could achieve 5.3 knots over the ground, as measured on the GPS. This boat has a theoretical hull speed of 5.5 knots. The motor ran around 4750 RPM (measured on a multimeter I own that has a tachometer mode) varying slightly. I also measured sound level with a sound level meter from just below the boom, aimed straight at the motor. The measurement was consistently about 85.x dBA.
The owner's manual and service manual specify this motor should run 4500-5500 RPM, so I was at least at the lower end of that, and making decent speed.
Installation
I returned to the marina to attach the HT propeller. Installation is no different than any other propeller. Remove the split pin, remove the washer, and slide off the old prop, being careful not to lose the thrust washer (mine stayed on the shaft). I applied new grease to the shaft, and installed the new propeller.
Test of Elephant Ear 6" Propeller
Running the same route upwind against what felt to be the same amount of chop and wind, the first thing I noticed was I was revving around 5000 RPM. I had forgotten to check the owner's manual for maximum RPM so it concerned me when it occasionally popped up to 5100, but I later read that I was good up to around 5500, so I wasn't risking an over-rev.
GPS speed was definitely improved too, showing 5.4 and 5.5 knots, which is hull speed. So I was almost certainly making more usable power. Being able to hit hull speed that easily against chop is comforting.
The big surprise was noise level. I had full expected the additional RPM to create additional noise, but the meter was reading slightly lower, consistently in the 84.x dBA range. I can't really explain the drop. I suspect the different RPM affected resonance of the entire boat. But that was a pleasant surprise, especially with my expectation of more noise.
Handling
Handling in forward and reverse was slightly improved, but it is difficult to quantify. I especially noticed that when turning using the motor, the boat would turn right now, rather than a somewhat delayed effect. I didn't have a reliable means of measuring acceleration (it was much too windy anyway) but acceleration felt somewhat better.
Reverse felt solid too, and I seemed to get more response at smaller throttle settings.
Bottom Line
First, the numbers:
This prop DOES fit the older 2-stroke 5HP motors, even though that application isn't always listed by the manufacturers. This prop gets my motor higher into its proper RPM range. Handling is somewhat improved. On my boat, it was also surprisingly quieter.
If I had merely borrowed the propeller instead of buying it, I'm not 100% sure I would go buy it unless I found a good price (or felt compelled to carry a spare). Since I did find a good price and feel better with a spare, I'm happy that I purchased it.
I'd recommend this propeller if you're not currently able to make maximum RPM on a displacement craft and need a little more push, or need a spare or replacement propeller anyway.
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