Optimax 225 legs not long enough??

qcamel · 18 July 2005, 14:47

I have an Optimax 225 engine which is the long shaft model. However I have cavitation problems when not running at high speed. I have done a lot of investigation into this and the only solution would appear to be a longer leg/shaft. I need to install an extra long shaft onto the engine but my local engine dealer says that there is not a conversion kit available, his only solution is a new engine

I am a mechanical neanderthol and have no argument against this.

If any of you can offer specific mechanical advice I would appreciate it.

Kind regards

Ollie

CJL · 18 July 2005, 15:17

Can the engine not be mounted lower on the transom to stop this?

Just make sure the exhaust is above waterline.

Chris.

philip · 18 July 2005, 15:28

Ollie,

I’m not an expert here but I can guess the first few questions for clarification.

How high is the engine - ie how high is the cavitation plate from the bottom of the hull at the transom.

What prop are you running – If it is a Laser 2 it could be worth experementing with the grommety type things on the prop – described better here as the perfomance vent system :
http://www.mercurymarine.com/chapter...opeller_parts2

Cheers

Phil

Edit - Sorry Chris I'm too slow - just saw youre engine height comment

Richard B · 18 July 2005, 15:37

If it's just better grip that you want, perhaps consider a different prop, for example maybe a 4-blade? Bear in mind that any "tuning" that you do is likely to lower your top speed as your boat appears to have had considerable effort put in to maximise this.

qcamel · 18 July 2005, 15:45

Without hacking into the boat structure there is no way to lower the engine, it is already at it lowest setting. Already running a 4 blade prop, problem is simply that the engine sits too high which reduces drag at top speed and presents no problems when doing that. With the stepped hull and low speed manouvers causing slightly confused water there is a tendancy to cavitate. I am happy to loose some top end to obtain better ow end performance but a new leg or engine does appear to be the only solution.

Ollie

Richard B · 18 July 2005, 16:07

Maybe the only thing that would avoid major modifications would be a jack plate to allow you to raise and lower the engine.

A word or two of caution though... lifting too high can loose water pickup pressure, and lowering too much could immerse the exhaust port. Race boats manage to overcome these problems though.

Brambles · 18 July 2005, 16:34

Hi Ollie,

Have you spoken to Graham regarding this? He has recently sorted the same problem on an 8.5 stepped hull with a new prop. If it can't be resolved like this, he may be able to alter the transom for you.

Ian

qcamel · 18 July 2005, 22:02

Brambles: Thank you for that thought, I will contact Scorpion tomorrow and check it out.

Richard: Excuse my ignorance but what is a jack plate??

Ollie

Richard B · 18 July 2005, 22:11

Quote:

Originally Posted by qcamel

what is a jack plate??

OK, I'm not an expert on this, so here's my attempt at a description...

It's a bracket arrangement which slides up and down (hydraulic ram I think) so that you can adjust your engine height as you're going along. They are widely used on race boats so the driver can lower the engine to grip on a turn, then accelerate out of the turn and raise the engine again for maximum top speed.

Have a look here: http://www.k4plus.com/shop.asp?catid=2

qcamel · 19 July 2005, 06:16

The jack plate looks rather clever!!!!

I have a busy daya ahead, avoiding all work responsibilities and trying to resolve my boat issue, the perfect day...

Ollie

Searider · 19 July 2005, 21:24

Quote:

Originally Posted by qcamel

The jack plate looks rather clever!!!!

I have a busy daya ahead, avoiding all work responsibilities and trying to resolve my boat issue, the perfect day...

Ollie

Hi Ollie,

I would second the comments about propeller vents. If you propeller has vents in the hub near the leading edges of the blades then block them up with some plastic plugs - about 50p each from your friendly Mercury dealer.

I found that my Laser 2 performed much better once plugged.
The Vengence seemed to cavitate a lot - but no holes to plug.

Wayne Rogers · 19 July 2005, 21:40

I'm running a Optimax 225,had the same problem no grip,changed to a four blade revolution prop and it cured the problem.

qcamel · 19 July 2005, 21:58

No luck today with Scorpion, will try again tomorrow.

Already running a stainless steel 4 blade so not sure thats the solution.

More news to follow

Ollie

jwalker · 19 July 2005, 22:42

Ollie, it wouldn't be difficult to have a bracket fabricated to bolt onto the original holes which set the engine back a little and allowed the repositioning of the engine mounting holes.

codprawn · 20 July 2005, 01:55

http://www.cook-mfg.com/

Have a look at these - will give you some ideas - I have gone for a 10" setback jack for my 225.

qcamel · 20 July 2005, 10:20

Had a good chat with Graham at Scorpion today. Stepped Hulls will always suffer a little because by nature they are putting air bubbles into the prop area, however my Stepped Boat was an early one and they have learnt and continued to develop since then. He put me in touch with Steel Developments in London who now produce props with 'cupping' on the tips of the blades this has pretty much solved the problem on boats produced now. I have ordered one and will let you know how it performs.

Interesting point from Graham was that when he first put my boat in the water the only way to get her to plane was by putting the trim tab setting to full down. Amazing how much inpact the current prop has made.

codprawn · 20 July 2005, 12:46

Quote:

Originally Posted by qcamel

Had a good chat with Graham at Scorpion today. Stepped Hulls will always suffer a little because by nature they are putting air bubbles into the prop area, however my Stepped Boat was an early one and they have learnt and continued to develop since then. He put me in touch with Steel Developments in London who now produce props with 'cupping' on the tips of the blades this has pretty much solved the problem on boats produced now. I have ordered one and will let you know how it performs.

Interesting point from Graham was that when he first put my boat in the water the only way to get her to plane was by putting the trim tab setting to full down. Amazing how much inpact the current prop has made.

If this is the case with a stepped hull then I would have thought a bracket which moves the engine further back would be ideal - the theory is the prop is operating in much cleaner water.

Richard B · 20 July 2005, 13:40

Quote:

Originally Posted by codprawn

...a bracket which moves the engine further back would be ideal - the theory is the prop is operating in much cleaner water.

A perfect theory for a hull travelling at precisely zero knots!

codprawn · 20 July 2005, 14:35

Quote:

Originally Posted by Richard B

A perfect theory for a hull travelling at precisely zero knots!

So it is speed dependent then????

qcamel · 20 July 2005, 15:07

I think the point being made is that if you move the engine backward to find cleaner water then you would need to move it back more than a couple of inches to have any effect...