Outboard prop "UPGRADE"

I have a Suzuki 2.5df comes with prop 7.5 x 5.5
The Tohatsu 2.5hp comes with 7.5 x 7
My understanding it will improve my top speed since the AI is light.

Anyone know if the Tohatsu prop would fit Suzuki above?

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where does your understanding come from? References? Accurately measured data?

My gut instinct tells me that increasing the diameter and not the pitch could ONLY increase top speed if the smaller prop was being overwhelmed by too big a load. After all, propellers work like a screw, so on a light load like an Island, the prop's path through the water will be the same with either diameter, offering zero gain in speed.

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker

I am going to do the test myself. I just need to figure out which 7.5 x 7" would fit the Suzuki.

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Tech, why not do that test.. and I dont know if the props are interchangeable or how to find out. Hopefully you can get an accurate measure of peak speed in ideal conditions with the only variable being the different prop.. I would be interested in knowing this also.

The two props you listed differ by pitch only and are the same diameter. We dont know if the blade area's are the same which is another variable but assume they are.

I will assume the TI and the AI are about the same. My guess is the higher pitch will be slower than what came on the Suzuki. Here is the video I did within the last year trying to make sure that the Suzuki was not exceeding 6000 RPM with the stock prop - I dont think it does. If you put on a higher pitch prop, most likely you will get lower RPM and lower HP out of the outboard. https://www.youtube.com/watch?v=kNbpoeaCe80 The only reason to go to higher pitch with the Suzuki 2.5 is if it was rev limiting - but that is NOT the case.

That Tohatsu 2.5 displacement is 85 CC compared to the Suzuki 2.5 at 68 CC so is a larger motor (same as the 3.5 hp Tohatsu). Both outboards have the same gear ratio of 2.15 to 1.

The Suzuki gives its wide open rpm as 5250 to 5750 and the Tohatsu 2.5 WOT is 4500 to 5500. Ie, the 2.5 Tohatsu has lower peak rpm.

The larger Tohatsu can deliver the same HP as the Suzuki at lower RPM because of the larger displacement and higher torque. Notice also that the 3.5 hp Tohatsu has nearly identical specs but the WOT rpm is higher at 5000 to 6000.

Remember that HP =torque times rpm.

Anyhow.. I think the higher pitch prop will result in lower rpm on the Susuki than the stock prop so less HP and slower top end speed. If the blades are different area or shape, then we have a new variable and who knows then. More area would have a similar effect as higher pitch

But.. I could be wrong.. give it a try and see what happens..

Boy o boy, my comment was far off the mark, since I confused diameter for pitch. Consider my head fully pulled in!

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker

I have a Honda 2.3 hp and it does not have a rev limiter; therefore, you can easily unloaded the the motor quickly under sail which could allow the motor to over rev and distroy itself. The Honda comes with a 7.5 x 4" prop and that seems to be the only prop that Honda makes for that motor. I bought a Suzuki 7.5 x 5.5" prop to help minimize the risk of an over speed. The Suzuki prop has a larger diameter drive shaft than the Honda so I made a brass shim that has the OSD of the Suzuki prop shaft but the ISD of the Honda shaft. The Suzuki prop also required the base to be be sanded down about 3/16" and a new Cotter pin hole to be drilled. Bottom line is that at wide open throttle the Honda sounds like it is turning less RPM but my gps shows almost the same top end speed as the original Honda prop. It is hard to push a displacement hull faster than hull speed. By the way the prop does run fine on the Honda with no cavitation or vibration and I think I have decreased my chances of blowing up a great little engine. PS. I never run more than 1/2 throttle while under sail.

CobraIP:
Been there done that, I've exploded two motors now over revving them under sail.
Gets really expensive really fast.
FE

Curious, did you record any peak wide open throttle speeds for the two props? If so.. what are they? Im assuming this is also with no wind and no current..

FYI, when I find a peak speed, I actually watch the GPS speed number. The number will bounce around a little and you have to average. GPS automatic peak speed readings are always suspect..

THis is a very interesting subject to me. Thanks, folks.

I did some non-scientific experiments over longer (for me) distances recently; sailing with a nice 7-10 mph wind, I wanted to see what the incremental speed was between the motor and peddles.

Base case: motor at about 1/3 throttle, and peddling. Boat did about 6.5 mph per my gps.
Alt #1: motor unchanged, but peddles out, boat did about 5.8 mph.
Alt #2 motor up, peddles at the same cadence as the base case, boat at about 5.1

I was surprised that the motor was adding only about 1.4 mph when the other two forms of propulsion were working well. My (naive?) assumption was that I need a bigger pitch in order for the motor to add more speed when used in conjunction with the other forms of propulsion.

What do you all think?

Bobagain:
Yep you are correct, when powersailing you will need a higher pitch prop.
You have ridden in my boat, you know what it's about and how it all works.
It's pretty easy to fit a new prop on, if you look on Iboats.com you will find replacement props for a toatsu (not spelled right) outboards, they come in 5", 6", and 7" pitch. I recommend ordering the 6" pitch, (I think they are about $25 bucks).
You will need to make a sleeve to convert from the Honda 10mm hub shaft up to 12mm, (I'm going from memory back 7 yrs here, it's probably best to actually measure to get the correct diameters).
You will need to hack saw off about 3/16" off the face of the prop hub. The cotter key hole lines up fine so no need to modify that. Once you make that sleeve you can run most any common pin props out there for Toatsu's, Mercs and several other brand aftermarket props. I don't know much about the Suzuki, don't know if they run the standard 'pin prop', (that's what the prop style is called)
When used for powersailing with multiple power sources you can ignore all the rest of the mumbo jumbo about max speeds, max hp, etc, none of that means anything at all.
With the right prop you only need 1hp of engine power, (around 1/4 throttle) and you should get 2-3hrs of run time with one tank of gas, (1 liter).

I despise the engine noise at high rpm, at very low throttle our motors are very quiet (you have ridden in my boat and heard the motors) we had no difficulty talking over them. I refuse to run my motors at anything over 1/4 throttle unless it's a dire emergency.
If you had twin engines I would recommend the 7" pitch prop, but I think you will do better with only one engine with the 6" prop.
Just tryin to help here
FE

My sailing buddy and I are both large ( old and fat ). My gps speed no wind or current is about 6.4 mph with either prop.

Thanks Cobra IP! The loading conditions also are important for the data.. two people, one person in the front or the back.. etc.

If you just motor, probably nothing to worry about with either the Honda or the Suzuki but you have a good point about the Honda not having a rev limiter when power sailing.. I would worry about that also.. I dont motor sail often but did do it once when we were in a hurry to get somewhere. On the Suzuki 2.5, the stock prop is about perfect for just motoring as you also have to worry about the higher torque that comes with higher pitch being not good long term for the outboard. The Suzuki 2.5 does have the rev limiter so its near optimum for just motoring but you never need to worry about damaging anything from over rev when power sailing.

You may have seen this video in the past https://www.youtube.com/watch?v=kNbpoeaCe80 I tried to get a tachometer to work with the Suzuki and had problems with it but had used an oscilloscope on the Suzuki at home (prop in a bucket of water - very important since it has a water pump that will burn up quick out of water) to confirm the working rev limit and it looked like the electronic ignition did something to the spark timing at 6000 RPM. Clever in that this was a cheap way to accomplish this since the electronics in the electronic ignition can also easily measure RPM.. Since I could not get the tachometer to work, I had make sure the outboard was not rev limiting by going by sound. With just one person in the back seat and at 450 ft elevation, I think the stock setup works well - see the GPS bouncing around 8 mph. But I wouldn't be surprised if you could get the rev limit to kick in if you added in sail power in a fair breeze. But with the rev limit.. no need to worry at least from over reving.

I dont know much about the Honda since I dont own one but if you really wanted to understand what was going on, you would need to measure RPM. The instrument that I bought a few years ago is in the link below

https://www.amazon.com/KEDSUM-Tachomete ... tachometer

You can also try amazon and search for outboard tachometer, maybe better options.. https://www.amazon.com/s/ref=nb_sb_noss ... tachometer

The Kedsum that I owned had a wire that wrapped around the spark plug wire. It worked on my Nissan 9.8 four stroke but I had problems with the Suzuki. Im not sure I would buy it again except that is was cheap and maybe it would work with the Honda. Its programmable for sparks per revolution as two strokes spark once per rev and a four stroke can fire either once or once every two revs..

Thanks for the info. I will check it out.

I pulled out the suzuki prop, took it to a few boat dealers.
The only one that will fit is the suzuki 4HP prop. The new prop cost $75, I did not buy it to try out.

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