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Old 07 January 2011, 12:01   #11
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Country: UK - Channel Islands
Town: jersey
Boat name: Martini II
Make: Arctic 28/FC470
Length: 8m +
Engine: twin 225Opti/50hp 2t
MMSI: 235067688
Join Date: Jun 2005
Posts: 3,021
I'm not an expert on performance boats and certainly won't claim to know half what the guys at BMC do but I'll air my thoughts anyway!

Steering: Assuming the helm pump is correctly matched to the cylinder, which at 2.1cu sounds like it is, then it's fairly safe to assume this is not where your problems lie re heavy steering. The Seastar pro system is suitable for your application and certainly up to the job.

As far as weight distribution is concerned, we don't know how much extra is in the reinforced transom but I doubt it's a significant amount, not enough to ruin the boat anyway.

So what's left to look at?

Engine height: I know performance boats run with motors set higher than the usual 2.5cm above keel but 8cm at the lowest? Assuming your CMC gives 5" (12.5cm) of travel that means your range is from 8cm right up to 20cm (8") Sounds really high to me, has the jack been fitted to allow you to move the engine down on the jack, or lower the jack on the transom? Worth a try I think.

Engine offset: I can't be bothered to work out if BMC moved it off centre in the right direction, I'll assume they have. But 30mm doesn't sound much for a 300 hp motor. Saying that I don't think the effect of going another 10-20mm would make a "massive" difference.

Prop selection: If I had to put money on it, I would say this is where most of your problems lie. I've experienced how heavy steering can get with the wrong prop, you would be suprised. It can also effect how the boat steers, eg start a turn and the prop can try to tighten the corner forcing you to have to forcibly pull the wheel back to centre.
I'm not that familiar with all the props you mention, but some I think are stern lifting props designed for bigger, heavier boats. This could be causing the back of the boat to get skittish, something like a Laser II might be worth a try, it's a fairly predictable, standard issue powerboat prop designed more for giving bow lift. I don't know much about your hull design, if it is very performance oriented then you can expect it to get flighty under certain conditions, specially when you back off the throttle at high speed.

With a set up like yours you need to accept that there's a trade off for all that top end speed and it's going to demand more user input with the controls to get everything right and allow the boat to perform properly.
With the addition of variable engine height as well as trim in/out, it's going to take a long time to get to know the boat and learn to interprete what she's telling you.

Getting your hands on as many props as possible to try out could be one of the keys to success, unfortunately you've shot yourself in the foot buying a counter rotating leg, although if you can find the right ones second hand they tend to be much cheaper. Pairs of props often get split up to use the RH, leaving the LH sat on the shelf gathering dust. The other problem is CR legs tend to wear out much quicker than RH legs, something about direction of the thrust acting on bearings etc.

I'd also suggest getting on to the Scream & Fly forum and repeating yourself there, there's a lot of guys there running the type of set up you have (just without the tubes) and might be able to offer advice.

Hopefully that gives you something to think about
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Old 07 January 2011, 12:03   #12
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Country: France
Town: Southampton
Boat name: Alea Jacta Est
Make: Osprey
Length: 7m +
Engine: mercury 300xs
Join Date: Jan 2011
Posts: 27
Thanks for your precious advices guys.

The problem is that if I lower the engine too much the steering system will block on the transom.
Even with the current height if I trim too much without lifting the engine up first the steering block and there is a big risk of damage the hydraulic cables.

Then I am reaching my top speed at less than half way up on the jack plate. Indeed if I lift more I am loosing speed. The pitch is clearly too small.

The boat is currently in South Bretagne in France.
Sorry for my poor english. But do you mean by "putting it in clean water" Andy ?

Thanks.

Regards.
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Old 07 January 2011, 12:13   #13
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Country: UK - Channel Islands
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Boat name: Martini II
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Quote:
Originally Posted by Vipermax View Post
The problem is that if I lower the engine too much the steering system will block on the transom.
The big problem with using jacks. Can you rearrange the hoses or even fit 90 degree elbows etc? Or will the cylinder still hit?

Quote:
Originally Posted by Vipermax View Post
Then I am reaching my top speed at less than half way up on the jack plate. Indeed if I lift more I am loosing speed.
This is telling you your engine is too high. You should only start to lose speed near the top of the jack's travel. The prop obviously needs more "clean water".

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Originally Posted by Vipermax View Post
.
Another username I thought would have been taken already
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Old 07 January 2011, 12:13   #14
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Country: France
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Boat name: Alea Jacta Est
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Yes you are probably right Martini. I should check several props first. It will be certainly the cheapest way to begin. Unfortunately prop such as Fury or Bravo 1 xs do not exist in counter rotation. That is a shame !

Other rib riders in France told me that the pb could comes from the height of the shaft (25). They recommend me to take a 20. But that means that in the lowest position the anti cavitation plate would be at around 12 to 15cm above the hull....

I have also heard about "torque tab" you can put at the end of the gear box in order to reduce torque effect which could be the cause of the "heavy steering" problem.

Thanks.

Regards.

Jean-Charles.
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Old 07 January 2011, 12:19   #15
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Country: France
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Boat name: Alea Jacta Est
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I think I can lowered the engine from 1 hole in the jack plate. (I am at the middle position for the moment). But if I do more I think I will damage the steering or/and the overall hydraulic system.

Yes you are certainly right about the height of the engine. But I was thinking, because I was using a 10cm thick jack plate; that I will able to "take" the water further behind the boat and the propeller will evolve in the rear wave of the boat which is higher than the botom of the hull. (I am not sure I have explain correctly just let me know).

Am I wring ?

jean-charles
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Old 07 January 2011, 12:21   #16
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Here there are couple of pictures of the set up.
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Old 07 January 2011, 12:32   #17
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Country: UK - Channel Islands
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Boat name: Martini II
Make: Arctic 28/FC470
Length: 8m +
Engine: twin 225Opti/50hp 2t
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Posts: 3,021
Quote:
Originally Posted by Vipermax View Post
But I was thinking, because I was using a 10cm thick jack plate; that I will able to "take" the water further behind the boat and the propeller will evolve in the rear wave of the boat which is higher than the botom of the hull. (I am not sure I have explain correctly just let me know).
I get you mean, you're right but the water level will only rise an inch or two (2.5-5cm)
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Old 07 January 2011, 12:33   #18
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Originally Posted by Hightower View Post
Offset is to the correct side. I have a CCW engine on my 6.2 Vipermax and it sits 1.25" (3cm) over to the port side. My engine is a 150 .
This guy has a 800mm longer hull and TWICE the gee gees!!!! YEEEHAAAA!
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Old 07 January 2011, 12:41   #19
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Offset is to the correct side. I have a CCW engine on my 6.2 Vipermax and it sits 1.25" (3cm) over to the port side. My engine is a 150 though so a bigger outboard might need to sit further over.
This is what I said you wind up Nobber
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Old 07 January 2011, 12:44   #20
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The boat is currently in South Bretagne in France.
Sorry for my poor english. But do you mean by "putting it in clean water" Andy ?

Thanks.

Regards.
So that the prop is biting into undisturbed water.

Where in Bretagne do you live? Vannes?
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