Tohatsu 8hp 2-stroke, prop is stamped "8.5", pitch?

[dgiles]

All that is stamped on the prop is "8.5". No other markings. I know it is a 8.5" diameter, but I assume the stamp is for the pitch? I want to drop about 1" in pitch and I think Tohatsu makes 6.5, 7.5, and 8.5 pitch sizes, while Solas has all pitches at 7, 8, or 9. I just want to know if I should drop to a 7.5 or an 8, based on what I have. I have a 3 person dinghy that planes very easily with 1 person, but with 2 kids and an adult, it struggles a little, but still planes. Thinking of trying a smaller pitch and see if the out of the hole is a little better.

 

[steelespike]

Probably makes sense 8.5 pitch.
Whats the wot speed like, GPS. While you can prop for a stronger hole shot you may lose some top end and the motor may over rev
Especially with a light load.

 

[Sea Rider]

Some issues with inflatables :

To start with : which is your dingly size, is it a air deck, alum, wooden deck or RIB ?

- If not a RIB, tubes must be fully pumped to min 3.0 PSI on all air chambers including keel.
- One story is fast displacement speeds other plane speeds, worst if loaded.
- Size matters, less size more hull drag, larger much less specially with under powered engine.
- You need to tach that engine with you alone on flat calm no wind waters while being lightly loaded, check achieved wot rpm.
- Can go from there maximizing prop to be at min middle 5000 to max 5500 wot rpm.
- Going one less pitch achieves 400-600 + RPM on very light boats.
- Buy a new 9.8 carb and bump that 8 into a 9.8 HP.
- Middle rpm is 5500, max wot 6000 RPM which is a better improvement over an 8.
- Before investing blindly on a new prop, better order firstly and induction Tiny Tach or Hardline tach with programmable firing sequence.
- Bear in mind that if you dial a prop that runs middle to max wot numbers with 3 up, when alone will over rev that engine at wot.
- It's a good idea to have an induction tach installed next to throttle grip.
- Will need to move to a prop pitch around 7.5 to 7.0 with 8 HP and 7.5 with 9.8 HP.
- Dialing best prop pitch is not exact rocket science, must test props under trail & error with given load.
- Plain Theory doesn't apply here...

Happy Boating

 

[dgiles]

Appreciate the help. This is a Titan 8.5' RIB that I have on the back of my other boat. Really, it is mostly only used for my kids to drive around while I am relaxing on the other boat. The only real reason I am considering dropping the pitch, is that it struggles only with a decent load and it goes a lot faster than it needs to with only my kids on it. I haven't GPS'ed it yet, but i would estimate close to 20mph. As far as over-reving, yes a risk, but i try to ensure the kids are only using part throttle anyway, after it gets up on plane. I'll tach it to make sure we are not far off.

In knew the 8hp are very close to the 9.8's. Is it really only a carb change? Bolt on, bolt off? For a 2-stroke?

 

[Sea Rider]

If you drop one pitch revs will rise 300-400 theoretical RPM, will achieve better hole shot and slight less top speed than with an 8.5 prop. If your current 2 stroke Tohatsu is a M8 model can swap for a M9.8 carb. Yes, bolt off, bolt on + 4? more time advance.

Both engines are same 170 CC, carb + timing makes the HP difference. If maintaining 8 HP better go for a full one pitch down to 7.5 as opposed to a 8 prop pitch. 0.5 pitch should bump revs half of what will be achieved with 7.5. Will depend entirely on HP and load to achiev +.- revs.

You could even keep same 8.5 prop and bump that engine to a 9.8 HP, should improve hole shot, a matter to test.

Happy Boating