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Old 04 April 2016, 05:33   #1
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What size for a'get me home' engine?

After a level of b*ggeration I have finally managed to create a bracket which will allow me to fit a small 'get you home' ancillary motor to my RIB.

Which begs the question what size engine would or do other members use for their RIBs? I have a 5.4m avon sea rider and was thinking of about 6hp.

Any idea?
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Old 04 April 2016, 05:39   #2
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Or lower hp for rib. A 4hp in protected waters will push that rib up to 6knots! But if you're in the sea then it's a different matter. I'd go with 6 or even 8hp
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Old 04 April 2016, 05:48   #3
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Or lower hp for rib. A 4hp in protected waters will push that rib up to 6knots! But if you're in the sea then it's a different matter. I'd go with 6 or even 8hp
Hi

I am pretty sure 1hp per metre
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Old 04 April 2016, 07:30   #4
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6hp is fine or if you can find one a lighter 8hp 2 stroke would be brill.

i have an 8hp 2 stroke yam on mine, 25kg or so.

if your main engine is 2/4 stroke may also be a factor if you intend to use same fuel tank/s
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Old 04 April 2016, 07:37   #5
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Thanks for the feedback, I have a two smoke engine as my main unit with working fuel injection so I can use the same fuel source if necessary.
I am tending towards a 6hp simply because of the range of 4 strokes at a reasonable price, there seems to be a 'orrid hike when you go to 8hp, maybe a totally different set of mechanics/power unit etc, maybe not.
Thanks for your feedback!
RichW
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Old 04 April 2016, 08:25   #6
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Thanks for the feedback, I have a two smoke engine as my main unit with working fuel injection so I can use the same fuel source if necessary.
RichW
The general advice on RIBnet is 1HP per m of hull length. In many cases you can get away with less provided you aren't pushing against a big tide etc. Beware using the same fuel though - many a problem afloat is caused by fuel (running out, water in fuel, contamination in fuel blocking filters, failed primer bulbs etc) - if you plan to use the same tank you may not get the same resilience you are hoping for...
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Old 04 April 2016, 08:34   #7
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>>>there seems to be a 'orrid hike when you go to 8hp, maybe a totally different set of mechanics/power unit etc

Yes certainly true in the Suzuki range which I'm familiar with. 6hp is the same CC and powerhead/leg as the 4hp but the 8hp adds a cylinder, 50% more capacity and the larger powerhead/leg of the 9.9hp so almost doubles in price.

Also going from 6hp to 8hp in that range takes you from 25kg to 40kg.
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Old 04 April 2016, 11:53   #8
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Go 6 hp but get a high thrust engine more grunt for displacement speeds gear box is approx 3-1 as a posed to 2-1.

Cheers
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Old 04 April 2016, 13:56   #9
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there really is no need for a high thrust on a 5.4m boat, i wouldn't worry about that too much. be nice to have for sure if buying new though?

by all means order a smaller pitch/dia prop to save lugging the motor too much, if you are buying new your dealer can sort that no probs for same money or if a good dealer may get you a saildrive at good price if you wish.

you will find plenty of deals about on a 6hp merc/mariner and probably decent second hand options...some used options here- http://www.seamarknunn.co.uk/download/usedob/used.pdf

if you can find a yam 2 smoke i can recommend it as only 25kg for 8hp, this is on my boat and gets me upto about 7 knots quite easily. my boat is SUBSTANTIALLY heavier/bigger than a 5.4 so it would be very happy on there. i would like a bigger motor ideally but up in the 60kg range....sod that!

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Old 04 April 2016, 17:02   #10
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Even with a 2-stroke auxilliary with its own integral tank it's possible to decant fuel into the engine if you are worried about having to carry pre-mix. I run a Mariner 4hp on a 4.8 RIB. I always carry 2-stroke oil, so a quick glug of oil, and press the fuel connection and pump fuel with the fuel bulb straight into the reservoir.

Run the auxilliary engine as a matter of course, ideally before you leave harbour. I didn't use my auxilliary for a few months and found the impeller was siezed with salt. Salt water even spray is an issue. I also find the tell-tales on these small engines aren't particularly powerful, so flushing is absolutely essential, regardless of whether it's used. I now carry a piece of strimmer nylon in my tool kit for this very reason.

Ratchet the auxilliary engine down. If you're mounting on a bracket, then fit a stainless U bolt on the transom knee or similar. The tilt mechanism on the auxilliary so unless it's fixed in position when you're underway there's a good chance you'll damage it if you're out in anything remotely rough.

Couple of older but perfectly serviceable 8hp engines here: Engines (Outboard) items for sale at Andy Seedhouse Boats
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