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Old 02 June 2014, 07:11   #11
Country: UK - England
Town: Dorset & Hants
Boat name: Streaker/Orange
Make: Avon/Ribcraft
Length: 4m +
Engine: 50Yam/25 Mariner
Join Date: Apr 2007
Posts: 5,551
I'm running a 25 hp 4 stroke on a 3.6m rib.

I just think your asking/expecting too much of 20hp.

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Old 02 June 2014, 11:01   #12
Locozodiac's Avatar
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
Join Date: Sep 2007
Posts: 1,706
My 430 rib with just a 18 HP tuned to perfection with 3 adults planes easily on most sea conditions. Unless you have a built in tach inside yoiur ear get a physical one, are cheap, will make your boating day.

If you're lost not knowing where to start rigging a small sib/rib with a 20 HP check this : + add an induction tach

ABC Sib-Rib Installation Guidelines.-

Happy Boating

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Old 02 June 2014, 14:53   #13
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Poly's Avatar
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: 2 stroke YAM 20 HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 9,419
Originally Posted by Northsearibber View Post
Engine is a Yamaha f20 2013. Prop size is 9.25 x12.Speed - would have to let you know exactly but about 12/13 mph.
OK - Yamaha says your engine max rpm is 5000-6000 rpm. With a 2.08 gear ratio. So at 5000 rpm your prop should be spinning 2400 times per minute. Assuming no slip that would mean your prop moves forward 12x2400" a minute = 23.7 knots. Bearing in mind that 10% slip is about the best you will get you should be getting 20ish knots (23 mph). On a small boat it feels like more. You think you are only getting 1/2 that - so I think there is a problem.

Is it sitting bow up?
Is it generating a huge wake behind it?

Its going to take actual tacho and speed data to work out more - and ideally some video footage so people can see how she is sitting.

Originally Posted by PeterM View Post
I'm running a 25 hp 4 stroke on a 3.6m rib.

I just think your asking/expecting too much of 20hp.
I'm running a 20 HP 2-str on a 3.9m and think he's not!
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Old 02 June 2014, 19:01   #14
Country: Canada
Town: Trois-Rivieres
Length: 3m +
Join Date: Jun 2014
Posts: 2
I have a Brig 360 with the same engine (2013 Yamaha 20 HP 4 strokes). Two adults and three kids (total weight around 200 Kg) I get 25 mph with a 9 1/4 x 10 propeller. When I tow a large 3 persons towable tube with the three kids on it I get 20 mph with a 9 1/4 x 9 propeller. I never had problem to get on plane. I think your engine is appropriate for the size of your rib, you just have to find a good setup.

Quebec, Canada
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Old 03 June 2014, 11:06   #15
Country: UK - England
Town: wetherby
Boat name: s
Make: ribeye
Length: 3m +
Engine: outboard
Join Date: Oct 2012
Posts: 7
Thanks guys - all those comments are really useful and have been a lot of help.

The general opinion seems to be that boat/engine combo should be capable of 20 mph with a couple of adults on plus 1 maybe two kids. That is what I am after really so it seems the issue is in the set up . I have ordered a induction tach and with advice given will do a few tests when I next get the rib out - hopefully next weekend. Hopefully I will be able to sort the issue now but I may need more views and advice! Either way I will repost with some results

Again - many thanks to all who have offered the benefit of their experience

Cheers for now!

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