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Old 31 January 2012, 16:28   #1
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One or two motors...?

I have purchased a 6.7m rib and will be using it inland mostly but will go to the coast every now and again. Can't make up my mind on one motor with a small emergency motor, or two motors. Any advice on this subject.
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Old 31 January 2012, 16:36   #2
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Mine has 2 but not everyone likes 2 there are different thoughts if you have 2 you should have 2 fuel tanks etc
Then if you just av 1 and aux get you home engine that might not start when you need it.
I didn't choose to av 2 that's the way it came second hand but I am happy with it
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Old 31 January 2012, 16:37   #3
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Two 90hp emergency engines will work.
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Old 31 January 2012, 16:42   #4
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I didn't choose to av 2 that's the way it came second hand but I am happy with it
DM - I meant to tell you. Depending on how you're loaded and propped, that rig should do about 22-24kts on one wing, if you raise the dead stick out of the water.

OP - For inland use and occasional coastal use in not-too-remote areas, I'd go with an auxiliary - just be sure to test it before you go to sea.
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Old 31 January 2012, 17:24   #5
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What size aux would you suggest if I went with this option?
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Old 31 January 2012, 18:11   #6
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What size aux would you suggest if I went with this option?
Well, if the RIB is an open boat and not too heavily loaded, then somewhere from 6hp up to 10hp, depending on your budget and mounting options. 10hp is running into 35kg+ for a four stroke, as low as 27kg for a 2 stroke. Personally I'd look at a nice secondhand 6HP four stroke. It will probably give you most bang for your buck and be in the lighter range of about 25-28kg. The tank of gas will go further and you can run it off your main with the right kit (assuming your main is 4 stroke). Your stored gas can be used up easily that way. It should move you along at around 5kts?

Guys, 5-6 knots? Less?
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Old 31 January 2012, 18:26   #7
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Guys, 5-6 knots?
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Old 01 February 2012, 02:56   #8
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Considering most engines die through fuel or electrics I'd go for one
Unless you're going to have two of everything there's no point of two and all that kit would seem a waste on something that will be mostly governed by inland speed restrictions. With a few exceptions

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Old 01 February 2012, 04:37   #9
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here we go again.....

Rem, Firstly, welcome to ribnet. You have opened with one of the more controversial discussions on this forum, so expect some strong opinions!



There is no generic answer...... Do a search for "twins", there is a good afternoon's reading out there. Also be aware that comments relevant to umpteen hundred HP rigs are not necessarily valid at the "100-ish total" ballpark.


A high level summary:-
beware the generic "nn% useable HP" figure..... with smaller set-ups the top speed is less and the bits being dragged through the water are usually smaller. (therefore the absolute drag for the rig is substantially less proportionally to the Hp available to overcome it)

The powerhead may be shared with another HP, and the leg / gearbox may be shared with 5 or 6 other engine sizes! e.g. my old Yam 55 I got spares for the gearbox end off a 90! - So how overkill was the gearbox on the 55? - to put it in perspective, the 'box & leg on my 60 is about the same cross section as the bottom end on my Suz DT25 So when I swapped engines I got more gee gees & less drag.....

If you are looking at 60 or 70 HP, most are still emergency pull start-able, and will likely run without the aid of a battery once going. (beware some of the new machines tho' -some (Optis?) need a healthy battery or it's no go.

Weight wise you have to look at individual combinations (I have posted examples in the past) - and look at different manufacturers too. Also as said above, don't forgret to add the deadweight of the aux.



What may be the deciding factor if your hull is secondhand tho' is the inevitable cut - out for a single. I nearly went for twin 30s when the 55 died, , but the woodwork involved to fill the transom cut was going to be too much hassle so I am still running single + aux.

Search the forums, and have a read with an open mind. One proverbial man' s meat & the other's poison is a phrase that springs to mind here!
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Old 01 February 2012, 14:23   #10
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I'd go for 1 & an Aux, & that's coming from a bloke with 2.
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