Prop pitch

Steve509926 · 14 September 2020, 18:57

Hi all, firstly thanks to everyone who has given me so much advice so far, but I'm now after a bit more!
I was out today just off Conwy and Anglesey for 4hrs, calm sea, little wind, sun out had a brilliant day, 2 up, dog, fuel, kit etc. and I achieved 16kts with a WOT at 5720rpm. My current prop is 9.25x9
The operating range for my OB, 15hp EFI Merc, is 5700 - 6200.
Would I be right in thinking about changing the prop to a 9.25x8 to bring it closer to a mid range of 5950, or should I leave if as it is?

Roflhat · 15 September 2020, 07:15

Dropping down a pitch will help, especially if you're carrying a fair amount of weight in the boat, will get on the plane faster and possibly give more top end

Fenlander · 15 September 2020, 09:57

Roflhat knows way more about pitch tuning than me but I can say with all the 15hp motors I've had on similar size sibs with similar loads a 9" has always done the job and a drop to an 8" only ever been of benefit on a 10hp on a SIB with average load.

The nuisance with an efi motor is if your pitch is a little under you will bounce off the rev limiter.

If you do decide to change beware changing pitch in another, particularly cheaper, prop brand. The pitch will only be fully comparable using the same make/style. I have a 9" cheapo for my Yamaha which behaves more like an OE Yamaha 10.5".

Steve509926 · 16 September 2020, 09:52

We could do with having a try before you buy prop library! Could work out as an expensive trail and error excercise.

Fenlander · 16 September 2020, 10:56

Indeed... see posts #30-33 here for my prop trials...

https://www.rib.net/forum/f36/yamaha...ice-81611.html

I think I spent about £180 on the three extra props and sold one as used for trial only and the original used one that came with the OB for about £75 total on Ebay.

For £105 I now have the perfect pitch (for our setup) OE new Yamaha and a spare aftermarket so not too bad overall.

Well I say £105 but just remembered later on I accidentally bought a new OE Yamaha 9" for the rare times we are 4-up with eldest daughter... so it might be £195... but hey it's boating... who's counting.

Steve509926 · 16 September 2020, 11:15

Interesting Mr F, so would I be safe to assume a new Mercury OB would be fitted with an OE Mercury prop? and I'd be better to try an OE Mercury 9.25x8? Would Tohatsu and Quicksilver be the same?

Fenlander · 16 September 2020, 11:30

All my new outboards and many I've bought used on the original prop have maker's own brand props. Can't say for Tohatsu but the Mercury/Quicksilver are nice quality props. And yes the best chance of seeing an accurate change for a meaningful result when pitching down will be to keep to the brand and style.

Steve509926 · 16 September 2020, 11:41

Thanks Mr F, spending a bit more will probably save in the long run.

Fenlander · 16 September 2020, 12:12

I think so. And in my experience if the result wasn't right you will get back about 60% of the prop cost selling once-used and boxed on Ebay so the loss becomes acceptable.

Of course ideally you need a spare prop anyway and it's not the end of the world if it is a second choice pitch if it saves looking for a prop mid holiday in Scotland.

Steve509926 · 23 September 2020, 14:10

I'm struggling to find a mercury OE prop at 9.25x8.
I can find a couple of non OE aternatives ranging between £40 - £100.
If I left things as they are, with the prop that's on now 9.25x9 and 5720rpm at WOT, am I likely to cause any premature wear or engine failure?
All out speed isn't that important to me and the SIB planes easily.

Fenlander · 23 September 2020, 14:31

No you won't do any damage... the engine's not labouring at 5720rpm.

I thought that 8" would be an easy prop to find... try these guys and they can advise on the OE availability...

sales@palynch.co.uk

Steve509926 · 23 September 2020, 14:41

Thanks David.

Steve509926 · 15 April 2021, 20:28

So today I tried a new prop 9.25x8, that I bought on ebay over Christmas (yep from China)
It only cost £35 so I wasn't expecting much, but after being quoted £175 for a new Merc/Quicksilver 9.25x8 from 2 different suppliers I thought it was worth taking the small risk and it could always be used as an emergency spare.
What a pleasant surprise, tested under the same load and the same conditions
Current prop, 9.25x9, WOT, RPM 5725, 15.6 knots.
New prop, 9.25x8, WOT, RPM 6160, 16.2 knots and popped up on the plane noticeably quicker.
Thanks Roflhat and Fenlander for you advice.