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Old 26-11-2006, 06:30 PM   #1
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Underpowered Zodiac Medline 2 with DF 140 HP

I have a new (Dec 05 – approx 40 hours) Zodiac Medline 2 (6 Metre) with a Suzuki DF 140, 4 stroke engine.

My problem is that she will only rev to 5,000 RPM at a max speed of 30knots at WOT.

We have checked the throttle cables - both at the hand control and at the engine and they appear to be o.k. – no restrictions, fully extended!

We have disconnected the throttle cables and operated the throttle lever on the engine with the RIB in the water - using a screw driver and hanging on tight! - Yet we can still only get the above figures at WOT - which eliminate the cables!

We have disconnected the throttle cables and revved the engine in neutral and we can get the engine to rev to the recommended 6,200 RPM - so now we know that the engine is capable of revving to that figure – but it will not rev to more than 5,000 RPM in motion.

I have emptied the bilge – it contained about 10/15 litres of water.

I am running a 21” x 14” Alloy prop and have tried a 19” prop which gave me an increase of about 200 RPM but no real speed increase.

On trying a 20” stainless steel prop the revs remained at 5,000 but the speed increased by one knot to 31 knots

When I tried a 17” aluminium prop, the revs increased to 6,000 and the speed went to a maximum of 34 knots (one way – 30 knots the other) giving an average of about 32 knots – two people and a quarter tank of fuel

The Engine has been trimmed fully out (until cavitating!) and has been tried in all different trim positions

We have dropped the engine by one notch Ύ” without any variation in speed or revs
The bottom cavitation plate is level with the base of the transom.

Speed is measured with a Garmin 198 C Chart plotter. This has been checked with a hand held Garmin and at worst was 1 knot out in speed (it varied up and down – sometimes my Garmin was quicker other time it was slower)

The RIB is trailered and always washed down and the engine flushed after each use. The hull is spotless.

My local Suzuki Distributor has put the engine on the computer (or visa versa!) and he is convinced that the engine is operating correctly.

I have removed all extra weight from the RIB (i.e. porta pottie, extra life jackets, oars, snorkeling gear, etc

Does anyone have any suggestions? Can you help? Surely this setup (Zodiac Medline II with a 140 HP four stroke DF Suzuki outboard) should be achieving at least 40 knots at a minimum of 6,000 RPM?
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Old 26-11-2006, 06:48 PM   #2
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Country: UK
Boat name: Johnny-Rude
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Engines: Seagull with hi five prop

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it is a pretty big rib for a 140hp four stroke swinging a 14" diameter prop. 17" pitch seems to suit from your extensive tests. Maybe it should do 40 knots but maybe you are expecting to much. Mr Cookee is the guy that races ribs so he sould have the best advice
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