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Old 04 April 2016, 04:33   #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, 04:39   #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, 04:48   #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, 06:30   #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, 06:37   #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, 07:25   #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, 07:34   #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, 10:53   #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, 12:56   #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, 16:02   #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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Old 05 April 2016, 02:41   #11
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tohatsu do a 6 hp with a high thrust propped engine 36 kg designed for displacement craft you just e-mail them with your application for them to advise, you can buy with remote or integral tanks.

cheers
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Old 05 April 2016, 13:10   #12
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this may be of some importance to the OP.

I have read many times on here the 5.4 is sensitive to weight on the transom, this may well make the difference so buy carefully!!! especially since you have the max outboard/weight on there already (90hp your sig says?)

freeboard may be a major problem with another 40kg ish on there. please note, typically the weights quoted for 4 strokes are minus oils.
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Old 05 April 2016, 17:22   #13
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I acquired an old Johnson Seahorse 4hp long shaft over the winter as a refurbish project. I tried it out last weekend on my RC 585. Was surprised that it gave over 4 knots in flat conditions. Have yet to try it in waves and headwind.

This is a very light engine at only around 16 kg.

Realistically this will probably only enable holding position while help comes or running downwind, but a lot better than nothing.
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Old 06 April 2016, 05:04   #14
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tohatsu do a 6 hp with a high thrust propped engine 36 kg designed for displacement craft you just e-mail them with your application for them to advise, you can buy with remote or integral tanks.

cheers
http://www.tohatsu.com/outboards/6_4st_spec.html

The standard 6HP Tohatsu/Mercury/Mariner is 25KG (6hp boat motor) maybe an extra Kg for the longshaft and stick a high thrust prop on it.
I'm currently using one of these on my 6.5m Coastline. It's only giving 4.5Kn but that's with a tight new engine (only two hours on it), standard 8" pitch prop and a heavier boat.
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Old 06 April 2016, 05:19   #15
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Silly question time !!!!!!

How are these Aux engines attached to the transom, and what do you do if towing on trailer.

Does anyone have pictures, am looking to do this on my own boat.
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Old 06 April 2016, 06:51   #16
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mines is right on the transom as my boat is wide enough, it is a rare thing though that you have room. i was able to squeeze a new 5hp 4 stroke JUST on my last avon adventure though.

i would only use a aux outboard bracket as a very last resort and i wouldn't be leaving the engine on it when trailering if avoidable.

i take mine off to trailer it and it is ratchet strapped up when tilted up when out with boat.
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Old 06 April 2016, 14:35   #17
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Silly question time !!!!!!

How are these Aux engines attached to the transom, and what do you do if towing on trailer.

Does anyone have pictures, am looking to do this on my own boat.
Not a daft question at all. Ideally you want it on the transom itself. 2-stroke auxilliary engines are perfect considering they're slimmer, lighter, etc. An adjustable bracket can rattle like a bag of spanners, so if you're pushed for space, opt for a fixed bracket.

If you look at the picture it's secured (while underway) using a stainless steel U bolt on the transom knee and a ratchet strap. I put a ratchet strap around the outboard block and feed it back through the pull-handle hole, nice and tidy. It's not going anywhere even in the lumpy stuff.

It's no different to the main engine. When trailered its in the up position, orange prop bag, etc.





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Old 09 April 2016, 03:12   #18
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OK, this is getting a bit silly now. Where's all the Searider owners run off to to talk sense?

Realistically, there's bugger all room on an SR5.4 to fit an aux, even with a bracket. Remember the main engine still needs to turn to full lock.
Post pics of the bracket and I'll give you some sensible suggestions. To get 6hp on there without it hanging so far astern as to get a dunking constantly will be a miracle.

Without seeing the bracket, a 4hp will be adequate, but realistically you're probably looking at a the largest single cylinder 2 stroke you can find, which from memory is the 3.3 Tohatsu/Merc thingy. It's not ideal but needs must.
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Old 09 April 2016, 07:45   #19
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OK, this is getting a bit silly now. Where's all the Searider owners run off to to talk sense?
Going for the subtle reentry approach for your come back then ;-)
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Old 09 April 2016, 08:24   #20
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Yep.
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