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Old 05 February 2011, 14:38   #1
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Advice sought on changing to single outboard

My 6m chinook currently has twin 40hp mariner outboards and id like to upgrade to a single engine. Would a single 80hp give equivalent performance or would I need to get a larger engine as there is only 1 prop now? Also Im hoping to change from 2 stroke to 4 stroke...

Thanks
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Old 05 February 2011, 14:50   #2
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A single 80hp would give more performance than twin 40's.
Just make sure you seal the holes left from the twin installation properly.
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Old 05 February 2011, 14:55   #3
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Originally Posted by diver 1 View Post
My 6m chinook currently has twin 40hp mariner outboards and id like to upgrade to a single engine.
Fair enough, there is no need to discuss the reasons, the single v twin debate has raged at much length in the past.
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Would a single 80hp give equivalent performance or would I need to get a larger engine as there is only 1 prop now?
9D820 is the one to answer that really but from what he has said in the past, at these sort of HP levels, yes, twin 40s will be the equivalent of around 75hp~ish. However the fact that there means there is only one prop does mean that there is a little less grip and blade area in the water however the drag is significantly reduced.
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Also Im hoping to change from 2 stroke to 4 stroke...
To have the same power as your twin 2/s you'd need at least a 90 4/ whereas a good 2/ like an etec or opti would do it for slightly less. You will no doubt be aware of the weight saving with the 2/ and the fact that modern ones have the same fuel economy/no smoke/greater efficiency than a four stroke. I imagine that your two 40s are quite light and they probably would weigh less combined than a single fourstroke of the same HP (speak to 9D280 for the exact science), wheras the modern two stroke would be less than your two engines combined and you would see much greater efficiency and retain/improve the balance of your boat which you would not if you went the 4/ route.

It can be tricky changing from twins to singles, especially when central elephants trunks and holes need to be sealed properly.
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Old 05 February 2011, 15:16   #4
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To have the same power as your twin 2/s you'd need at least a 90 4/ ...
Yes, it's well known that a single 4T-HP is worth a mere 0.88E of a 2T-HP. I'm not surprised that the manufacturers keep so quiet about it....

If you factor in the Wow Effect that you'll lose with a single and the smaller HP on 4 strokes (lets just call it PP, i.e. Pony Power) you may need to upgrade your RIB just to take a 4 stroke engine big enou...,

HEY, I'M CHANNELLING GOTCHIGIT!

Out of my scone, you pustulent wretch!
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Old 05 February 2011, 15:19   #5
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Yes, it's well known that a single 4T-HP is worth a mere 0.88E of a 2T-HP. I'm not surprised that the manufacturers keep so quiet about it....

If you factor in the Wow Effect that you'll lose with a single and the smaller HP on 4 strokes (lets just call it PP, i.e. Pony Power) you may need to upgrade your RIB just to take a 4 stroke engine big enou...,
Sorry, just the usual etec sales pitch. I'd have assumed you'd approve
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Old 05 February 2011, 15:20   #6
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at these sort of HP levels, yes, twin 40s will be the equivalent of around 75hp~ish
Very ish.
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Old 05 February 2011, 15:21   #7
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Very ish.
Please see my repeated referral to speak to 9D280 for the scientific reasoning.
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Old 05 February 2011, 15:22   #8
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Sorry, just the usual etec sales pitch. I'd have assumed you'd approve
Fair 'nuff. I'm busy selling a pair meself....
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Old 05 February 2011, 15:26   #9
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Fair 'nuff. I'm busy selling a pair meself....
Yes I saw. Your engines are mounted very far apart. I know that this is better than having them close for moment etc. I believe 1m is generally regarded as the cut off for optimum moment anyway isn't it? However most RIBs seem to have them mounted much closer together than you have yours?
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Old 05 February 2011, 15:37   #10
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However most RIBs seem to have them mounted much closer together than you have yours?
That's the result of a deep V and a 2.62m beam. Standard rigging on a Redbay, bear in mind that the Etec 90s are very slim

The crowd who commissioned mine, replaced her with this one, Dj vu all over again, eh?
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