UPDATE: I replaced the 8hp with a 9.8hp, kept the same prop. will get a write-up of the engine build sometime, but the prop is great on the new engine too.
I have no issues with the power output of my Tohatsu 8hp 2 stroke engine in calm water, but when I’m motoring into chop and a strong headwind I’ve often wished I had a couple extra HP of power to move the boat along. I’ve always subscribed to the formula that the ideal minimum power for any given sailboat is right about 2hp for every 1000#’s of displacement with a fin keel boat. You can get by with less, but that should be enough to push you through the rougher weather. Since Four Points displaces 5000# before crew and gear, I usually go off the assumption that I am displacing at least 6000#. Using that math I should ideally have a 10.5hp engine.
In addition to the slightly less than desired power output in rough weather, I have always suffered a distinct lack of thrust in reverse, and when I first bought the boat resulted in a couple higher speed than desired arrivals at the dock. Since purchasing a new higher power engine is not something I’m willing to do at this time, I’m fortunate that this is not so much a problem with the engine power as it is a combination of two design choices with the propeller design common to most outboard engines.
First reason for poor reverse performance is the blade shape, Outboard propellers are normally designed with speed in mind, and are designed to provide a high velocity thrust so that you can get your boat up on a plane and travel at high speed. They achieve this by moving a relatively small amount of water a rather large distance aft with each rotation with a cupped blade to further help direct the thrust aft. This design is necessary if the propeller will continue to produce forward thrust while the boat is traveling at a high speed since the prop output speed must always be higher than the forward speed of the boat. This design makes for a very good high speed forward blade, but when turned the opposite direction makes for a very inefficient blade as the cupped blades tend to direct nearly as much water out as they do forward, resulting in little reverse thrust.
The second is the exhaust gas discharge. To reduce engine sound most of the exhaust gas is vented through the prop hub, and to make everything as low drag and efficient as possible the prop hub actually has a flange that extends forward and inside an overhanging lip on the lower unit to make the water flow as smoothly as possible from the lower unit onto the propeller. The downside of this design is that when you put the engine into reverse, the exhaust gas needs to come out the center of the hub aft and then gets drawn forward over the prop blades, leaving them to spin in the exhaust bubbles, partially out of contact with the water, and further reducing the amount of reverse thrust generated.
Since I have a sailboat that should never exceed a speed through the water in excess of 7.2kts (semi-displacement hull, and I don’t think I want to be onboard in any conditions she would go faster than that!), a prop designed for high speed travel is simply not the right design for a sailboat. What I need is lots of low speed thrust, and that is best achieved by moving large volumes of water at relatively low speed vs. the traditional outboard prop designed to move small volumes of water at very high speed. The idea being that the larger the volume of water you are moving the more low speed thrust you generate. Think of the design of a tug boat propeller, low speed, massive thrust. I had considered adding one of the various prop rings / guards, but the lack of any conclusive proof that it would actually provide me more thrust than it cost me in additional drag has made me very hesitant on that idea, and while I have not 100% ruled it out, I don’t think I’ll be exploring that route anytime soon.
Previously in an effort to get a little additional thrust out of the engine I had swapped out the 6.5″ pitch prop installed when I bought the boat for a brand new 7.5″ pitch prop that was included with the spare parts that came with the purchase. This proved to be slightly too much prop for the engine when running at higher RPMs, but had noticeably improved performance at cruising RPMs, leaving me to believe that a 7″ pitch would be the ideal pitch, but I left the higher pitch prop on to gain the performance at the speeds I most use it at.
This year I purchased the recently designed Tohatsu 8.7″ x 7 four blade high thrust propeller. I confirmed with the Tohatsu rep that the design is not really a 7″ pitch (I estimate it more like a 4.75″ pitch), but is in fact only labeled a 7″ to make it easier to determine which prop is better suited to your boat based on which 3 blade high speed prop you are switching from, as a propeller with an extra blade but the same actual pitch would require much more power to drive. As you can see they blades are also not cupped so they should provide the same thrust in forward and reverse.
Another benefit with the new prop design is that the hub is larger than the gear case housing allowing exhaust gas to get sucked/pushed forward without first going over the propeller blades and reducing. With the addition of the fourth propeller blade, and the significantly different blade design, I calculated that the new propeller will move approximately 20% more water per blade (plus one additional blade), or an additional 113% larger volume of water per revolution than the 3 blade propeller, but move it 36% less distance (if my 4.75″ estimation is correct), for a total increase in thrust of 41%, with (according to the Tohatsu rep) no additional torque required from the engine, and possibly even less since I’m going down by 0.5″ in pitch (theoretically).
How the engine actually performs will be the real question, but my research leads me to believe that I should be quite happy with the results. I was told that when they were testing the new prop design they tied off a 26′ sailboat with a 9.9hp engine to the dock and put the engine in reverse and revved up to full throttle, and pulled the dock cleat out.
While also doing the repair work on the lower unit I installed the new propeller. I first greased the prop shaft quite liberally with a marine axel grease so the prop comes off the next time I try to remove it (I did this with the old prop when I installed it and after being on there for 3 years it came of with nothing more than a single smack from the palm of my hand).
All that was left at this point was to put a new cotter pin in the prop nut and reinstall the lower unit.
You can see the effect the larger hub will have on venting the exhaust gases in reverse in this picture.
Once the lower unit was reinstalled I gave everything a nice coat of Petit Alumaspray (grey) aluminum lower unit antifouling paint. Since I don’t leave the engine down when not in use I could probably get by without any antifouling paint, but it is just easier to spray some of that on and not worry about it.
This prop is nothing short of amazing! I have always felt the boat was under powered with the 8hp engine, and really wished I had the 9.8hp, but with this prop there is no sense that I am under powered anymore (I’m sure I could still use more power in rough weather, but I am no longer under powered). Reverse is absolutely sunning, and backing out of the slip the boat just leaps back and quickly accelerates reverse giving more than enough speed through the water for the rudder to function in only about half a boat length, where the old prop would take 2 whole boat lengths of slow reverse acceleration before the rudder was really doing anything. In forward the benefits are not as profound, but certainly still significant. Most importantly is that the engine hits proper RPM without bogging down now, and I can easily run at 5kts at a comfortable RPM, and 6.5kts is achievable at full throttle. The boat also powers through wakes much better than it did before, and won’t feel like comes nearly to a stop if I hit a large wake with the new prop.
Another bonus is that it doesn’t seem as prone to cavitation as the old prop was when motor sailing on a port tack, despite the fact that it can actually cavitate slightly at a stand still if I hit the throttle too hard, where the old prop never once did that. I believe that is a result of it just moving so much more water than the old prop.