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Old 09 February 2021, 16:22   #1
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Which / what aux engine

Decided Iím going to get an aux engine for the back of my Cobra 7.5 for this season.

Thinking something about 10/15 hp. With a separate tank but will make it so I can draw from the main tank as well.

Decent secondhand ones seem expensive so maybe just buy a new one

Is there much between them?
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Old 09 February 2021, 16:57   #2
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Get a high thrust one since you'll be at displacement speed
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Old 09 February 2021, 17:35   #3
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Is there much difference between an 8, 9.9 or 15 in this situation?

None of them are going to be able to push it above displacement speed.
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Old 09 February 2021, 17:55   #4
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Consensus is 1 hp /m of boat so 9.9 would give a bit of redundancy size of cowl would be the limit if mounting on the transom
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Old 10 February 2021, 04:31   #5
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Iím making this up a little as Iíve never had an aux but have bought one now. Different manufacturers measure boats in different ways

Some itís now tube to stern tube others now to transom. It can make a difference.

So I subscribe to 1hp a metre but you may well find that 6hp wonít be far off at displacement

And I do agree with Jeff the size of the cowl may well be the limiting factor
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Old 10 February 2021, 04:32   #6
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Bow not now predictive text doh
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Old 10 February 2021, 05:00   #7
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just for info on my ribquest 585 [6m bow to tube end] 90hp susi and 4hp aux mounting on the transom 15mm between engines on full lock but the aux hard up against the A frame so steered with the main engine 5 knots max speed good conditions not a get you home just out of trouble
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Old 10 February 2021, 09:40   #8
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I've never seen anyone do a "nice" job of attaching an Aux to a Cobra - the swim platforms don't lend themselves to an obvious solution. If you go ahead please show me your setup.
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Old 10 February 2021, 13:27   #9
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im just thinking about a new aux outboard but im more restricted by weight of it than physical size .my humber 5.5 has a max transom load weight of 190kg my mariner 115ct weighs 165kg dry so at the most i have 20ish kg for an aux i have a lovely mariner 5hp four stroke but thats 25kg dry so over the recommended weight. a new 3.5hp is 18kg so just about on the money but would that get me along at the 4 to 5 knotts the 5hp would achieve. i have seen on an american site that recons a 5hp carb will bolt straight onto the 3.5 and with an upgrade to an alloy prop should give it near 5hp performance ?
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Old 10 February 2021, 14:24   #10
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Quote:
Originally Posted by charliee View Post
I've never seen anyone do a "nice" job of attaching an Aux to a Cobra - the swim platforms don't lend themselves to an obvious solution. If you go ahead please show me your setup.
Iím going to weld up a removable stainless steel bracket as a kind of Ďstalkí that will come out of the transom and behind the swim platform. I donít want to attach it to the swim platform itself.

The only thing Iím not sure about is how I will reach it to steer / control it. Thatís still a work in progress.
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Old 10 February 2021, 14:30   #11
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Originally Posted by beerbelly View Post
im just thinking about a new aux outboard but im more restricted by weight of it than physical size .my humber 5.5 has a max transom load weight of 190kg my mariner 115ct weighs 165kg dry so at the most i have 20ish kg for an aux i have a lovely mariner 5hp four stroke but thats 25kg dry so over the recommended weight. a new 3.5hp is 18kg so just about on the money but would that get me along at the 4 to 5 knotts the 5hp would achieve. i have seen on an american site that recons a 5hp carb will bolt straight onto the 3.5 and with an upgrade to an alloy prop should give it near 5hp performance ?
I’m no expert but I’d suggest if 20 vs 25kg is the difference between your transom failing or not, it’s probably not a very good idea to hang anything extra on the back.

The position / angle of the engine during transport and how you drive the boat will have much more impact on the transom than 5kg.

I doubt many calculations were done to establish 190kg as the max weight, more I expect they looked at the average weight of most engines at the time and thought ‘that’ll cover it’.
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Old 10 February 2021, 15:28   #12
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Quote:
Originally Posted by jeffstevens763@g View Post
just for info on my ribquest 585 [6m bow to tube end] 90hp susi and 4hp aux mounting on the transom 15mm between engines on full lock but the aux hard up against the A frame so steered with the main engine 5 knots max speed good conditions not a get you home just out of trouble
Never had an auxilary but i have towed a rib for ten miles and the most notable thing was how much easier it was when the towed boat raised his outboard ,without appearing too dim ,can we assume a small aux outboard would be under similar strain if the main outboard was still lowered ??
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Old 10 February 2021, 17:11   #13
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It would but with most rigs itís much easier to use the main engine for steering.
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Old 10 February 2021, 17:12   #14
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Sorry, someone already said that?!?
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Old 10 February 2021, 18:25   #15
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6.5m rib using 6hp Auxilliary

We have a 6.5m rib with a 200hp main engine. We use it a lot up in Scotland offshore between Gairloch and Ullapool whale watching. There aren’t many people around up there for a friendly tow so we always fit an Auxilliary portable engine just as a ‘Get you to a safe place’ engine. It’s a 6hp Tohatsu with an integral tank. We have had to use it twice and both times it has got us to a harbour while we sorted out a fuel or prop problem on the main engine. It definitely won’t get you on the plane and we get about 5 knots max out of it. It is painfully slow when you are far from a shore but better than nothing. We use the main engine as a rudder and to be honest we’ve never raised it to see if it increases the speed from the auxiliary. It weighs in at 25kg so we just roughly offset it with a 25l petrol container on the opposite side in the well.
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Old 10 February 2021, 19:04   #16
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Originally Posted by jakew009 View Post
Iím going to weld up a removable stainless steel bracket as a kind of Ďstalkí that will come out of the transom and behind the swim platform.
I'm going to fabricate a stainless aux bracket as well. Going to bend some 20mm thick wall 316 pipe because that's what I have left over from when I replaced my wobble roller spindles.

I can't decide what material to use as the mini-transom. Choice are 25mm HDPE £17, or 25mm Nylon or Acetal £35/£34 for a 10" square sheet. Hard to decide which material has the best properties for this application.
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Old 11 February 2021, 01:46   #17
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Quote:
Originally Posted by jakew009 View Post
Iím no expert but Iíd suggest if 20 vs 25kg is the difference between your transom failing or not, itís probably not a very good idea to hang anything extra on the back.

The position / angle of the engine during transport and how you drive the boat will have much more impact on the transom than 5kg.

I doubt many calculations were done to establish 190kg as the max weight, more I expect they looked at the average weight of most engines at the time and thought Ďthatíll cover ití.
thanks i am not too worried about the transom failing its a fairly new rib (2019) more the back end sitting too low in the water. but thinking more about it 5kg is no more than having a spare container of fuel back there i will try it with my 5hp before i buy anything.
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Old 11 February 2021, 15:04   #18
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I'm going to fabricate a stainless aux bracket as well. Going to bend some 20mm thick wall 316 pipe because that's what I have left over from when I replaced my wobble roller spindles.

I can't decide what material to use as the mini-transom. Choice are 25mm HDPE £17, or 25mm Nylon or Acetal £35/£34 for a 10" square sheet. Hard to decide which material has the best properties for this application.
20mm pipe sounds a bit puny? I think that will twang around like crazy. I was thinking more like 100mm.

As for material, I’d just use a chopping board cut to the right size - £1.99 from home bargains.
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Old 11 February 2021, 16:27   #19
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Quote:
Originally Posted by Orwell boy View Post
Never had an auxilary but i have towed a rib for ten miles and the most notable thing was how much easier it was when the towed boat raised his outboard ,without appearing too dim ,can we assume a small aux outboard would be under similar strain if the main outboard was still lowered ??
Not sure what you mean but my aux was a short shaft way up on the deadrise still in water when down and well clear of the road when towing
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Old 11 February 2021, 17:38   #20
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20mm pipe sounds a bit puny? I think that will twang around like crazy. I was thinking more like 100mm.

As for material, I’d just use a chopping board cut to the right size - £1.99 from home bargains.
The design will be super strong having four fixing feet. I prefer thicker nylon vs 6mm nylon of a chopping board. Also it's thick wall tube, not sure my own pipe bender will handle it.

Copying the bracket shown below:
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