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.

Prop Blade Profiles

Prop Blade Profiles

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).

Props side by side

Props side by side

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).

Installing new four blade prop

Installing new four blade prop

All that was left at this point was to put a new cotter pin in the prop nut and reinstall the lower unit.

Lower Unit Installed

Lower Unit Installed

You can see the effect the larger hub will have on venting the exhaust gases in reverse in this picture.

Larger Hub Diameter

Larger Hub Diameter

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.

Painted and ready for use

Painted and ready for use

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.

8 Responses to “High Thrust Prop”

  • art madresh says:

    i have a johnon 15 hp 2007 ooutboard model # j15rlsu on my 25 oday sailboat; it has little power in reverse. can you help me, your prop seems like the solution but johnson does not have one

    • It looks like Solas does make a 4 blade high thrust propeller that would fit your engine. Either the 4113-100-07 if you have a 10 spline prop shaft, or more likely the 2113-100-07 if you have a 13 spline prop shaft.

      Unfortunately they don’t make them any larger than a “7” pitch, which is probably going to be a little undersized for your boat / engine combination, and you would need to be careful as you could probably end up over-revving the engine at full throttle.

      If you’re going to give it a try (I would) You should try and see if Defender can special order it since boats.net A) doesn’t seem to carry it, and B) their website is currently hosting some malicious javascript that just pissed off my antivirus when I tried to do a search (not to mention C) they had untold numbers of customers CC info stolen earlier this year).

  • Don Whiteley says:

    I have a Tohatsu 9.8 on a 26 footer.
    I need to change the prop and was considering a 4 blader but Tohatsu’s data say this is only suitable for a long shaft.
    Is your engine long or short shaft?

    • My engine is a long shaft. I can see why a short shaft may struggle with the prop for 2 reasons. A) the prop is a larger diameter which means the blades reach closer to the surface than the normal prop does, which contributes to B) the engine has little problem moving so much water at low speed that it creates strong eddy currents and that turn into small vortexes, which allow air to get sucked down into the blades and result in cavitation. I sometimes have problems with this when backing out of the slip and going into gear forwards if I give too much throttle as the prop will aerate itself when the boat during the transition from reverse to forward motion.

      All in all, I would say given how much we all spend on our boats annually, a $65 prop is a very worthwhile experiment to see if it improves things, and I suspect that it will for you, just be aware that your problems with low speed cavitation may be worse than mine due to the prop sitting higher in the water, meaning those vortexes will be able to aerate the prop much easier. If I were you, I would definitely buy it and give it a try, I’d be shocked if you weren’t happy with it as a whole, even if it means you have to be careful not to give too much throttle anytime you are moving slow or not at all.

  • Andre Meyer says:

    My boat-engine configuration is similar to yours: First 260 (2800kg loaded) with a Tohatsu 8 hp 2-stroke long shaft.
    Tohatsu proposes two high-thrust propellers which I consider:
    8.7″ x 7 pitch four blade
    8.7″ x 5 pitch four blade

    Having calculated a bit I would go for the 5 pitch one.
    Why did you opt for the 7 pitch?


    • It was pretty easy to decide between them. I had both a 6.5 and a 7.5 pitch regular props that I had tried swapping between before, and found the 6.5 a little under propped, and the 7.5 a little over propped. I spoke with a couple guys from Tohatsu at one of the boat shows that had knowledge of the development of the new 4 blade prop design, and they told me that while the prop is not even close to being actually a 5 or a 7 pitch, it is rated that way so that you can swap it for a regular prop and match the pitch to the old prop. Since I knew that the ideal pitch for a regular prop was probably a 7 based upon previous testing, I decided on the 7 for the high thrust.

      Furthermore, if the prop isn’t quite right then it’s safer to be slightly over propped than under propped, as under propped can result in over revving of the engine.

      As for your situation, your boat is a little heavier than mine, but not by much. I have since upgraded the engine and swapped it for an otherwise identical 2002 Tohatsu 9.8 engine (write-up to eventually be done), which was always my intention for this boat. The little extra bit of power and the 7″ high thrust prop are a very good match.

      Given the little bit of extra weight you have, and that you don’t have the touch of extra power, the 5″ prop is a very safe option for you, but you may find it to be a little under pitched.

  • Ross Shaw says:

    A great write up which answered all my thrust questions perfectly, as I have just purchased a new 9.8 Tohatsu two stroke extra long shaft motor for my 25ft moored yacht. Thank you very much for your knowledge on this subject.
    Best regards
    Ross Shaw

  • Ross Shaw says:

    Thank you for a great write up on the thrust prop for Tohatsu 9.8 outboard.
    Best regards
    Ross Shaw

Leave a Reply for Brian (admin)

Subscribe to Blog via Email

Enter your email address to subscribe to this blog and receive notifications of new posts by email.