Advice sought on changing to single outboard

diver 1 · 05 February 2011, 19:38

My 6m chinook currently has twin 40hp mariner outboards and i´d 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 I´m hoping to change from 2 stroke to 4 stroke...

Thanks

chewy · 05 February 2011, 19:50

A single 80hp would give more performance than twin 40's.
Just make sure you seal the holes left from the twin installation properly.

gotchiguy · 05 February 2011, 19:55

Quote:

Originally Posted by diver 1

My 6m chinook currently has twin 40hp mariner outboards and i´d 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.

Quote:

Originally Posted by diver 1

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.

Quote:

Originally Posted by diver 1

Also I´m 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.

willk · 05 February 2011, 20:16

Quote:

Originally Posted by gotchiguy

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!

gotchiguy · 05 February 2011, 20:19

Quote:

Originally Posted by willk

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

Channel Ribs · 05 February 2011, 20:20

Quote:

Originally Posted by gotchiguy

at these sort of HP levels, yes, twin 40s will be the equivalent of around 75hp~ish

Very ish.

gotchiguy · 05 February 2011, 20:21

Quote:

Originally Posted by Channel Ribs

Very ish.

Please see my repeated referral to speak to 9D280 for the scientific reasoning.

willk · 05 February 2011, 20:22

Quote:

Originally Posted by gotchiguy

Sorry, just the usual etec sales pitch. I'd have assumed you'd approve

Fair 'nuff. I'm busy selling a pair meself....

gotchiguy · 05 February 2011, 20:26

Quote:

Originally Posted by willk

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?

willk · 05 February 2011, 20:37

Quote:

Originally Posted by gotchiguy

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, Déjà vu all over again, eh?

gotchiguy · 05 February 2011, 21:12

Quote:

Originally Posted by willk

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, Déjà vu all over again, eh?

Are you going for another RIB then or as Polwart has suggested converting to the SIB on a full time basis?

willk · 05 February 2011, 22:03

Quote:

Originally Posted by gotchiguy

...Polwart has suggested converting to the SIB on a full time basis?

gotchiguy · 05 February 2011, 22:22

Quote:

Originally Posted by willk

I'm glad to hear it!!

BogMonster · 06 February 2011, 09:58

Quote:

Originally Posted by willk

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, Déjà vu all over again, eh?

If they carry on heading towards that rock at full moo they're going to need another one in about 5 seconds

Anyway mr willk I have a bone to pick with you - why didn't you sell your boat last year

Downhilldai · 06 February 2011, 11:02

Quote:

Originally Posted by gotchiguy

Fair enough, there is no need to discuss the reasons, the single v twin debate has raged at much length in the past.

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.

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.

The OP ought to pay little regard to this load of bollox.

willk · 06 February 2011, 11:07

Quote:

Originally Posted by Downhilldai

The OP ought to pay little regard to this load of bollox.

I figured that unless he'd been underwater since May last, he probably got the general picture there...

Pikey Dave · 06 February 2011, 11:20

A 6m Chinhook with only 80-90hp on the back of any combination/flavour, sounds a bit weedy to me, especially for carrying divers. Stick a single 150 on the back

blueboy758 · 06 February 2011, 11:36

i have a 5.3 with extra large tubes high bow and deep 'v' with an evinrude 115!!! goes like stink !!! and very efficient when eased back!

willk · 06 February 2011, 11:41

Our old dive club 6.1 had/has the same 115 ficht. She fairly clipped along when lightly loaded, but a full compliment of divers (6-8) and kit meant that she was at WOT most of the time in the mid 20s

For 4 divers, I'd say 115 was enough, but 150 would have a big smile factor

blueboy758 · 06 February 2011, 14:40

would that have had the right prop on? i've seen the same engine on a ocean pro 6.3 with 4 divers doing 43 ish kt's! even though i have been in boats all my life i'm just getting to grips with different props abnd how much better they can make things