Outboard prop "UPGRADE"

Much obliged, FE!!

FE, you said you had blown up two outboards from over reving while power sailing. I know you had some sort of small two stroke before the Honda.

So did you blow up a stock Honda 2.3 outboard from over rev or were the ones that blew up some other 2 stroke you had previous?

It would great if anyone concerned about this did get one of those tachometers and measured the RPM for a few different TI boat speeds. First do this in no wind and no current for a range of speeds including peak speed.

Then try it again going as dang fast as you can go using the outboard, sails, pedals noting both peak speed and RPM. The RPM over the speed range is interesting but most interesting is RPM at peak speed with no pedaling (about 8 mph in my case) and then with again at peak power sailing speed (in my case, the fastest I saw doing this was just under 10 mph).

And. if at any time during the test you saw near 6000 RPM.. that is as far as you would want to go.

Hopefully we could get this info for at least the Honda 2.3 and the Suzuki 2.5. Im fairly sure we will find the Suzuki stock prop is just about perfectly matched.

I may go try another one of those tachs again on the Suzuki. I am fairly sure there is no problem that needs to be solved by changing props but you need that RPM vs speed info to be sure.

If someone buys one of those RPM meters and is happy with it, also useful info..

FE, I dont know if you missed this.. still curious if you have blown up a stock Honda 2 or 2.3 from over reving on a TI?

I thought it was worth it to bump this old thread because it is one of the few places (via google etc) where Suzuki 2.5hp props are being brought up anywhere online. There there was mention that the 4hp prop might be a possibility, unfortunately not true- really does seem to be only one option.

I took a gamble due to availability/shipping time (at the lowest price point) & result- the 6 pitch 4hp(5hp/6hp) Suzuki prop does NOT fit the newer 2.5hp suzuki 4-stroke (say ~2013+ with the rubber hub bushing) . Yes the spline is the right size but the length & setup of the hub is just too long on the 4hp to even consider trying to make it work (even with machine work or serious grinding). I suppose a really good prop shop could probably come up with something custom $$$, but I'm also considering looking into what could be out there or what would be required to go back to the early df2.5 (regular shear pin) setup..

The 7 1/2 prop does clear the lower unit etc. There is a 7 1/2" yamaha prop 4/5/6hp I think, that might be worth looking at, I suppose, but spline diameter is about 2mm too small so no-go for me there already

There was a misconception here that you can get more power out of the Suzuki 2.5 outboard on a TI if you change the prop.

Remember power is torque times RPM. You get the maximum power with the highest torque possible at the highest RPM possible. The best prop for a particular craft for speed is one where full throttle gets the RPM to just under the maximum RPM at the speed the craft is running at. This is how you get the rated HP out of the outboard.

So if a particular prop gets the craft to peak speed and is slightly under peak RPM, you are getting the maximum power from the outboard. Change to lower pitch prop and the outboard rev limits with less torque resulting in less power. Change to a higher pitch prop and the higher torque wont allow the higher RPM also resulting in at best case the same HP (if the power curve is fairly flat). If the reduced RPM takes you lower on power curve.. you end up with less power.

Take a look at how the experts match a prop to a power boat. They will always set the pitch (for a particular prop) so that the outboard gets to its design peak RPM at full throttle. If RPM is too low, go to a lower pitch prop. If RPM is too high (limiting), go to a higher pitch prop.

The stock prop on the 2.5 does in fact run at just under rev limit at full speed on a TI. The stock prop is matched fairly well to deliver the full 2.5 hp when used on the TI..

This video is only test I have seen with the TI where the prop pitch was looked at and shows that the stock prop is about right.

https://www.youtube.com/watch?v=kNbpoeaCe80&t=1s

If someone does want to mess with the prop, please make sure you do a good controlled experiment where you measure peak speed with the stock prop, then the ONLY thing you change is the prop and measure speed again. As far as I know, this controlled experiment was never done. I am going to guess that the higher pitch prop will just bog the outboard down and you will actually get a lower peak speed.