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Old 26 May 2015, 17:44   #1
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Offset or not?

Hello,

I'm slowly coming to the end of a transom replacement saga on my 6.4m Concorde rib. I'm going to put the merc 150 v6 back on it and I'm wondering whether I should offset it or not.

The service manual and my OCD suggest it should go bang in the middle, but previously on this boat it was around an inch and a half to starboard and it seemed to drive pretty well at speed.

However when I got the boat the prop was way too high, the engine would hit the well when it was raised, the control cables were too short to turn the engine one way and the hull was a bodged up mess so I don't have much faith in the previous owner!

I have extended the engine and changed the transom height so the prop is now 4in lower, which I understand will make prop torque worse.

Thanks

Mike
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Old 29 May 2015, 07:30   #2
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anybody??

Are large engines routinely offset? i'm pretty sure someobdy on here must own a rib...... what did they do on yours?

help, please?
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Old 29 May 2015, 08:09   #3
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Quote:
Originally Posted by mdearman View Post
anybody??

Are large engines routinely offset? i'm pretty sure someobdy on here must own a rib...... what did they do on yours?

help, please?
My 150 is offset.
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Old 29 May 2015, 08:10   #4
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Yes. I would think that for an engine of that size the offset that was originally used is about right, although I have no experience of your hull/engine combo. My 6.5 Osprey/200 etec is offset the same amount (1.5 inches).
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Old 29 May 2015, 15:08   #5
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[QUOTE=mdearman;679588]Hello,

I'm slowly coming to the end of a transom replacement saga on my 6.4m Concorde rib. I'm going to put the merc 150 v6 back on it and I'm wondering whether I should offset it or not.

The service manual and my OCD suggest it should go bang in the middle, but previously on this boat it was around an inch and a half to starboard and it seemed to drive pretty well at speed.

However when I got the boat the prop was way too high, the engine would hit the well when it was raised, the control cables were too short to turn the engine one way and the hull was a bodged up mess so I don't have much faith in the previous owner!

I have extended the engine and changed the transom height so the prop is now 4in lower, which I understand will make prop torque worse.

Mike, you post has raised some points I think it may be worth looking at.

- You said the prop was too high, yet the boat handled well ? this suggest to me that the prop was in the right place its just that the engine sits very high on the transom ?

- If you are then dropping the engine 4" this is a significant change and could lead to a significant loss in performance as well as affecting the handling of the rib (not in a positive way) for example it could cause the rib to lean to Port and come down on the Port chine when it comes out of the water.

Ideally you want the Anti Ventilation Plate to be level with or higher than the bottom of the transom (most Ribs like then plate to be 1-2" above the bottom), is this the case now you have dropped the engine 4" ? Where was it before ?

Most engine fitters no longer bother with offset and it can vary depending on hull type. However if the previous set up had offset and handled well I would be inclined to keep it that way.

However first I think you really need to check what the engine Vent Plate height is going to be if you drop 4"...

It sounds to me like the engine may be an XL shaft and that your transom is built for a long shaft. This results in the engine having to be mounted very high to get the vent plate high enough.
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Old 29 May 2015, 16:06   #6
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Thanks for all the responses

With regards to prop height, the boat handled well in that on flat water up to speed at 40kts it felt very stable. however it would ventilate alot, when accelerating, turning, or hitting any sort of chop.

At the time i started all this i had no intention of replacing the transom and really didn't want to make it any lower than it was, so I bought a bay extension kit, adding 5". Since then I've replaced the transom and had the option to pick a new height. The service manual recommended a height 3" lower than it was originally mounted, and my best judgement at the time put it 4" lower. I think it probably has gone from way too high to a bit too low, but I suspect that is better for towing and rough seas which i would prioritise over top end speed?

Mike
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Old 29 May 2015, 16:12   #7
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and to answer the question about the AV plate, I believe it will now be in line with the bottom of the transom, but no higher
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