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Old 17 August 2014, 02:26   #1
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Twin engines

Hi, not a problem I'm just curious why some ribs are fitted with say twin 100 hp outboards instead of a 200 hp outboard, surely the single motor weighs less
It's a slow process but I'm learning : )
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Old 17 August 2014, 02:39   #2
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People generally think that having 2 engines is the safer option, I.e. if 1 breaks down, the other will get you home.

However, the reality is that unless you have 2 completely seperate systems (electrics, fuel, controls etc) its possible that if 1 stops so would the other.

Because of weight and drag factors you wouldnt need a 200 to replace 2 x 100, a 175 would do it or even a 150.

All my boats are single engine, I believe that as long as its looked after (usually better than my kids if im honest ) it wont give you a problem. I do however carry a tender or take a wing engine if im doing some serious offshore trips.
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Old 17 August 2014, 02:42   #3
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Thanks for the swift reply, and another nugget of information
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Old 17 August 2014, 03:32   #4
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Of course, when you're looking at 2x250hp or 2x300hp , it's because there's nothing out there big enough to generate that much HP in a single motor.
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Old 17 August 2014, 04:44   #5
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Of course, when you're looking at 2x250hp or 2x300hp , it's because there's nothing out there big enough to generate that much HP in a single motor.
well there almost is

though over that you need two
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Old 17 August 2014, 04:45   #6
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Ive got twins partly for the backup, both haveing seperate fuel, electrics etc.

But mostly because my RIB wont work as well with one. Its a cat, so a single mounted centrally would cause a lot more drag than the two running behind the two seperate hulls.

Downside is twice the service cost
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Old 17 August 2014, 05:15   #7
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My Current boat has twins & was built & specced for offshore diving, when you are putting people in the water you want to be sure that you can get them back out again. The boat has completely separate fuel systems, batteries, controls & will plane on a single engine. Now I've more or less given up diving, we are looking to change boats & I will have a single + aux on the new boat.


PS Any body interested in buying an extremely well specced dive RIB
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Old 17 August 2014, 06:00   #8
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I did hear of a fast rib that on its maiden passenger trip hit a rock in shallower water that took out both props (Twin 250's) so twins are no guarantee... I have seen some power boats with triple 300's!


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Old 17 August 2014, 11:28   #9
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well there almost is

though over that you need two
No dealers in this country-and it's cheaper to buy 2 300 hp motors!
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Old 21 August 2014, 03:18   #10
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People generally think that having 2 engines is the safer option, I.e. if 1 breaks down, the other will get you home.

However, the reality is that unless you have 2 completely seperate systems (electrics, fuel, controls etc) its possible that if 1 stops so would the other.

Because of weight and drag factors you wouldnt need a 200 to replace 2 x 100, a 175 would do it or even a 150.
Just remember that what applies up at the umpteen hundered horsepower end of the scale is not necessarily relevant down at the 5m / twin 30s end of the market:

http://www.rib.net/forum/f8/engine-drag-21160.html (tho' we never did get any dimensional data!)

http://www.rib.net/forum/newreply.ph...ote=1&p=640205 - Post #9

Search for "twins drag weight cube" and you'll find plenty posts about it.
Bottom line is - size is relevant in these discussions.

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PS Any body interested in buying an extremely well specced dive RIB
Stop it....... my garage isn't big enough!
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Old 21 August 2014, 03:46   #11
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Quote:
Originally Posted by Pikey Dave View Post
My Current boat has twins & was built & specced for offshore diving, when you are putting people in the water you want to be sure that you can get them back out again. The boat has completely separate fuel systems, batteries, controls & will plane on a single engine. Now I've more or less given up diving, we are looking to change boats & I will have a single + aux on the new boat.


PS Any body interested in buying an extremely well specced dive RIB
It certainly is well equipped Mate!!...Best spec'd,and maintained Dive RIB I've come across by some way!...spose OCD does have it's upside
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Old 22 August 2014, 16:07   #12
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I have twins because it looks cooler and sounds better.
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Old 22 August 2014, 16:16   #13
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I have twins because it looks cooler and sounds better.
Ooh, is she finished? You out and about next week?
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Old 22 August 2014, 16:18   #14
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Ooh, is she finished? You out and about next week?
Yes at long last!! Although had starter motor failure on first run out i kinda though it was going to play up! new one should be in next week!
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Old 22 August 2014, 16:22   #15
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Old 22 August 2014, 16:29   #16
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[QUOTE=Landlockedpirate;640035]People generally think that having 2 engines is the safer option, I.e. if 1 breaks down, the other will get you home.

However, the reality is that unless you have 2 completely seperate systems (electrics, fuel, controls etc) its possible that if 1 stops so would the other /QUOTE]

Not always true! most boats rigged with twins would also have 2 filters & often separate tanks
We recently blew a head gasket 20 miles north of oban & drove 100 nm back to largs on the other engine
Our plan was to head out towards St Kilda had we not done the head gasket but it's reassuring to know we could have got back had it happened in a more remote area

If we were just staying fairly local & didn't need the 100% redundancy then single is far cheaper in every respect & often makes more sense

Depends where you do your boating I suppose !
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Old 22 August 2014, 16:50   #17
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We have twins on ours that run off same tank,which i dont see as abig problem as i would fill 2 tanks at same pump anyway If i go on long trips we have a couple of on deck tanks as well.
It suits me I am not sure having a small aux engine on a big rib is much use anyway I would want a 25 hp for mine
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Old 22 August 2014, 16:55   #18
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I have two also because I am psychologically damaged! I only usually see broken engines! Mine engines are rigged separately, with individual fuel tanks and filters etc. the only thing linking them is the steering tie bar and the diode on the killswitch
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