F-Ribs And Sibs - Inflatable Boat Community

Morning chaps!

I've been Sibbing now for the past few months, even getting out without the kids in the colder weather. Absolutely love it!

I would like to see what your opinions are on propeller tuning and to see if anybody has done it?!

I have a Tohatsu 6HP MFS6D (26ish Kg) and a Tobin sports Mirovia Airdeck (circa 50kg)
With me and my brother (200kg between us) It planes out at around 11-12kts on the gps.
With me and the kids (120kg between us) It planes out at around 12-14kts on the gps.
these speeds taken off two separate GPS units, one my phone and the other a walking gps, measure the same bang on high tide on my local river with almost no current.

I also use a tiller extender, which allows me to sit at the front of the boat with my bro/kids, but to be honest have never needed to use it. It planes no problem wherever we sit. But it is handy for me to sit next to the kids!

The current prop is a 7.9x8". I do feel when I am out that the boat feels like it isn't getting to optimum RPM, particularly with me and my brother on, presumably due to the weight... And that with me and the kids its potentially the other way, revving almost too high. (has a limiter on it so it wouldn't go over max anyway but not the point...)

This suggests to me I could potentially get more speed with me and my brother with a smaller propeller, and more with me and the kids coupled with a larger propeller...

I have ordered a RPM tinytach online and expect to receive it over the next few days, will be doing a trial run to see what the RPM is in different circumstances.

Has anybody changed the prop size on their smaller outboard? Or will the difference be negligible?

Id be happy to change if it was going to give 3-4kts extra.

That said I am more than happy with the performance of my SIB, just like most people, want the performance to be the best it can be! A few extra kts wouldn't be a bad thing!

Photos!

Thanks chaps!

Changing the prop can have a dramatic effect. My Suzuki 20 came with a 12” pitch prop and I was struggling to get to 5000rpm and having problems getting in the plane with four adults. I changed it for a 10” prop and now it gets to full revs and the hole-shot one-up is phenomenal!

Take some RPM readings. Dropping pitch by an inch will typically give you an extra 500rpm

Cool mate. Ill deffo get some RPM readings at different loads and report back! Thanks for the reply

Grahamdavies1989 wrote: ↑

20 Dec 2020, 07:29

Has anybody changed the prop size on their smaller outboard?

Yes, I did.

I have the same engine on a Takacat 340LX (catamaran-style SIB).
With the standard 8" prop and my whole family (me, wife, 2 kids) and some equipement (payload about 220 kg) it did not plane.
It planed with less than 180 kg. Top speed with 100 kg payload was about 14 kt.
I changed to 7" prop. Now it planes with 230 kg (10 kt). Top speed at 100 kg is still about 14 kt.
For me the change was worth it.

But when I get my license I will buy a 9.8 hp Tohatsu

Thanks for that mate. Appreciated!

use a 7x8 x7 look on you tube ERICK GIVES YOU MORE TORQUE GETS YOU ON THE PLAIN WITH TWO UP BUT TOP SPEED IS REDUCED BY A MPH sorry for shouting oopsie

Nobody can advise you which prop to get until you’ve taken rpm readings at wide open throttle. (WOT)
Each engine manufacturer will advise you of the WOT rpm range, once you have obtained the RPM you then reduce or increase the pitch to obtain the correct reading.
As Chris says, expect a 500rpm difference per inch alteration.

Martin

well i have taken his word and bought one 7x8,7 prop ,

bun wrote: ↑

02 May 2022, 20:32

well i have taken his word and bought one 7x8,7 prop ,

I've got the same outboard, how was the result of a smaller prop