Zodiac Medline 2 underpowered with Suzuki Df 140?

Beitsta · 04 October 2006, 22:07

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.

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 - yet we can still only get the above figures - 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 am running a 21 º x 14” Alloy prop and have tried a 19º prop which gave me an increase of about 200 RPM but no real speed increase.

Speed is measured with a Garmin 198 C Chart plotter.

Does anyone have any suggestions?

Martin

codprawn · 04 October 2006, 22:23

What prop did you try? Maybe you need a 22 or 23? More likely though your engine is mounted too deep - try raising it a few holes.

Hugh Jardon · 04 October 2006, 22:41

Quote:

Originally Posted by codprawn

What prop did you try? Maybe you need a 22 or 23? More likely though your engine is mounted too deep - try raising it a few holes.

i dont think going up the pitch to 22 or 23 will increase the revs! unless i have missed something in all this!

measure the cav plate or take a pic of it and post on here so the experts can cast their eye over it

sparkey · 04 October 2006, 22:48

prop too big.
A 2" reduction should increase rpm by 200 rev's aprox.
You might also try a diam reduction as well.
When your WOT is correct your speed will increase.

Richard B · 04 October 2006, 22:56

Without seeing your RIB, it's almost impossible to guess what the problem is!

However, making the bold assumption that it's been sensibly rigged, 30kts from a 6m RIB powered by a 140 sounds a bit low. Meme who posts on this forum has the same Zodiac and the same engine so it may be useful to find out how his performs.

If the engine revs to the max with no load, but won't rev under load, it sounds like there's a problem with the engine. Hugh is right about his prop comments. Codprawn could be right about the engine height. But there may be a fuel starvation problem - check your fuel lines for kinks, make sure there's no obstruction (maybe try running the engine from a portable tank?) and if you have a filter, maybe replace it.

codprawn · 05 October 2006, 02:04

Quote:

Originally Posted by Hugh Jardon

i dont think going up the pitch to 22 or 23 will increase the revs! unless i have missed something in all this!

measure the cav plate or take a pic of it and post on here so the experts can cast their eye over it

WHOOPS - swot I meant - teach me to pay proper attention!!!

pathalla · 05 October 2006, 02:28

try a 13 1/2 inch wheel yet? How is your fuel tank vented? Spiders occasionally like to build egg sacs on the fuel vent screens... have seen this create a problem which is easily overlooked.

alt · 05 October 2006, 08:13

Check you've got a spark on all plugs. I had a 10HP that would rev really high in neutral, but when under load would hatdly move. Turned out one of the plugs wasn't getting a spark

Beitsta · 06 October 2006, 23:26

Thanks for all the replies!

Codprawn:
Thanks Codprawn, but I only want to move the engine as a last resort!

Hugh Jardon:
I’ll post a picture of the engine - showing the cav plate - in a few minutes

Sparkey:
I’m sorry now, but as a fellow Cork man I have to ask what does WOT mean. What Diam prop do you suggest?

Richard B:
I’ve emailed Meme with a query on his prop size but am still waiting for a reply.
The fuel lines are under the floor and appear to be o.k. where I can see them

Pathalla:
How do you mean to try a 13 1/2” wheel yet?

Alt:
Hiya Alt! The computer says that she is firing on all cylinders – even under load!

I am awaiting a 17º x 14” prop which I should have next week. It will be interesting to see the result of this!

Keep the suggestion coming – I have to solve this problem soon

yoyo · 06 October 2006, 23:33

WOT: Wide Open Throtle.

Beitsta · 06 October 2006, 23:38

I attach a picture of the engine as she is mounted on the transom

Limey Linda · 07 October 2006, 00:05

Photo should have been taken with engine down. Just a guess, but based on photo the engine is mounted too high or wrong shaft length.

Beitsta · 07 October 2006, 17:04

Hold tight - photos on the way!

Limey Linda · 07 October 2006, 18:56

Yup, it is on the high side. I would guess 2-3 inches. Cavitation plate should be level with bottom of transom. You can always trim out for more performance once you have got the engine postioned properly.

alt · 07 October 2006, 18:57

Whoever mounted that engine should be shot

Beitsta · 07 October 2006, 19:21

The engine is mounted on the centre of five holes on the engine mounting plate/transom (see photograph).

There is approximately 1.5" of movement on the engine bracket if the engine was lowered - would this solve the problem?

DM · 07 October 2006, 20:22

Quote:

Originally Posted by Beitsta

My problem is that she will only rev to 5,000 RPM

If the engine was mounted too high, I'd expect it to over rev and cavitate.

There are three fins on the Suzuki and the antivent plate (bottom one of the three) looks more or less level to me. Level with the keel is only a rule of thumb anyway as lots of ribs (Ospreys included) run with the plate 1"-2" above the keel line.

To find the extra 1000rpm I'd try a 17" pitch prop, more out-trim when on the plane or better balancing of the boat before messing with the engine height.

Also those transom add-ons look like they are set too low.

DM

Downhilldai · 07 October 2006, 21:17

The lower unit on that Soozook looks as if it's done a lot of work. Is the engine older than Dec 2005? If it's done a lot of hours, then it may be worth doing a conpression test, to see whether you're losing something there.
I wouldn't get concerned about fuel starvation - if there's enough getting through to run it at 5000 rpm, there should be enough for 6000+. Fuel starvation will cause the engine to virtually cut out, rather than lose a few rpm at the top end of the range.
As DM says, mounting the engine high will reduce drag, resulting in higher, rather than lower peak rpm, so dropping it is unlikely to help you.
A drop in prop pitch will probably be the best way forward, if the powerhead is making the full 140 ponies.

Beitsta · 07 October 2006, 22:03

Davidmanning:

'Also those transom add-ons look like they are set too low.' What do you mean??

Beitsta · 07 October 2006, 22:06

Downhilldai:

I reversed the unit on the trailer against a kerb in my own driveway and just caught the Skeg ... the engine was brand new Dec 2005 when I purchased the RIB