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Old 06 June 2003, 17:37   #1
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Back-up Engine

Hello again,

On the basis that I am thinking of putting a 115HP on a Ribcraft 5.45, with a 2.5 mtr beam, would an 8hp outboard be a sensible size of back up?

I currently have a 2 stroke Yamaha 8hp for my "tender" but - if I am to have a 4-stroke main engine - it would seem logical to replace this with a 4-stroke and to use a single fuel supply. (or have I opened a can of worms here?)

Does anyone have a view about the ideal size of back up?

Would a "high thrust" 8 hp 4-stroke be a better option than a standard version?

What is the best way to fix it to the transom? I am concerned that it could twist into the main engine. Is there a cunning way to keep it in place? (the friction adjustment does not seem very reliable on my present Yam).

Thanks for your ideas!

Chris.
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Old 06 June 2003, 18:10   #2
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I too have a two engine setup on my rib, but went for a smaller engine.
Somewhere along the line, I think it was perhaps one of the guys at work said that a general idea for aux is to get something like 1hp per metre, so in my case ideally I would be looking at 6hp.
In the end I went for the 4hp Suzuki 4-Stroke, which leaves the look on the back of the boat pretty good, since both engines are from the same era, and have black styling - more asthetics than anything here I'm afraid.
Where technicalities are concerned, I've had 4.5 knots out of the aux so far, and that's probably just around 3/4 throttle. I am not running full throttle yet since I have only put 2hrs on it
Mounting it was easy - Humber cut a flat section of the transom away on the port hand side which the aux engine mounts on perfectly. Rather than use an aux engine bracket, the fixed option was preferred given the fact that these ribs fly quite a bit sometimes, which could result in the aux bracket laden with aux engine sheering off
It's held on by the normal 2 hand screws, plus 2 bolts that go through the transom. As for turning, the main engine is not affected by this extra unit on the transom, and the aux engine will just about do 90 degrees in both directions which makes the rib very very monouverable in tight spaces - I have used the aux engine to berth before now since it has a far better lock than the main engine
Just reading the Suzuki mag, they now do a 5 and 6hp 4-stroke too which is exactly the same size as the 4hp unit - it's easy to carry, and pretty light btw.
I don't have the aux engine plumbed into the main fuel tank - If I ever put a batch of bad fuel in the main tank which the main engine consumes and stops with, I want to make sure that the aux will still run......

Hope that's of help or sense

-Alex
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Old 06 June 2003, 18:55   #3
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I have a 6HP Mariner 4 stroke on a 4.8m Ribcraft and a 6HP 4 stroke suzuki on a jointly owned Ribcraft 5.85. If you are going for a four stroke then any more than 6HP and the weight jumps up considerably. The mariner and suzuki both weigh in at 25KG.

The Suzuki in my opinion is a better outboard. It starts easier than the mariner and has a nice carrying handle. The design is much better thought out.

Incidently the 5.85 goes faster on the auxillary motor probably due to the greater hull length as it is in displacement mode.
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Old 07 June 2003, 03:54   #4
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We use a Mariner 4sail - 4hp 2 stroke and designed specifically as an auxillary for small sailing craft (hence the name). Main engine is four stroke like yours but went for 2 stroke because its lighter. We bolt the engine onto the deck, just ahead of the transom so its good and secure and does not get thrashed around. When needed, reach over and hang it off the back so you want a light engine. Also very easy to remove from boat when its left outside to stop it being nicked.
True, top speed is only 3kts, but we take the view that it will either keep us out of trouble or get us to a safe shore until help arrives. We have a Yamaha main engine and have only used the auxillary once in three years when we wrapped our fishing line around the main engine and use the spare to keep us away from rocks while we sorted out the line.
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Old 07 June 2003, 06:43   #5
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Hi Chris, their have been many posts on here about aux engines, do a search.

The idea of 1HP per meter is quite solid but I am carrying only a 6HP 4 stroke on my 7.8. The reason being as boats get bigger the HP needed makes them to heavy in my opion to fit to the transon when needed. This is even harder with a diesel as you cant just lowerit onto the transom but need to cross then engine box and lower it onto a bracket.
I mave made a pulley system up to the A frame to make the job easier.

As far as speed goes, as long as you have fuel you will get their. I used to cruise yachts and 3.5Kts was somtimes our max speed in light winds, you just are more aware of currents tides etc and use them to your advantage. When last did a RIB owner plan a passage based on tidal flow atlas. If you have 5Kts of tide against just anchor up and wait, you will soon have 5 Kts with you.

As far a 4 stroke V 2 stroke

The main differences is weight, although out 6HP 4 stoke is not much heavier than a 2 stroke at 27KG. The main down side of 4 stroke is the oil, not 2 stroke oil but engine oil. Often they can only be stored one way up without the engine sump oil draining out.
As far a common fuel, just carry neat petrol and a bottle of 2 stroke oil. Make a guess on how much you need and add a bit more. If you put too much in and get a smoky exahust who cares if it keeps you of the rocks.
As far as ecomony goes again who cares how often do you plan to use it and how much fuel do you carry for the main engine.
We have motored from Conway to Fleetwood on a old 2 stroke 8HP in a yacht on 8 gallon.

As far as storeage goes I am not a fan of keeping it on the transom. Many months of salt water spray and damp but never getting run much does not go for easy starting when its needed.
If you have space I think a waterproof bag on deck is better.

Regards Gary
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Old 07 June 2003, 07:08   #6
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Gary

That was a very good answer.

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Old 07 June 2003, 07:38   #7
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Here's a couple of thoughts:

1. If you want to mount an aux on the transon, why not mount it on the starboard side to balannce against the prop. torque twist?
2. If you've got a diesel engine, then some manufacturers like Scorpion and Revenger put mounting points INSIDE the engine box. Now, if you have an integral tank and it leaks, what would the effect upon the diesel engine be? I reckon it would be a bit like using easy start...
3. I've seen Geoff's floor mount points, and it's an excellent system.
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Old 07 June 2003, 15:04   #8
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Geoff,

Any chance of a snap or two of your floor mounting arrangement
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Old 08 June 2003, 12:53   #9
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Thanks guys!

Your replies have made me think that the location and use of a back-up engine is less straight-forward than I had anticipated. Perhaps I should look into twin engines, before coming to any hasty conclusions?

Problem is that the Yam 115 is, in my opinion, a far superior engine to the "new" Yam 60's, which would be paired, so I need convincing that a single + back-up really is the way to go - or that the conventional 60's would offer a more practical solution.

I have therefore opened another thread "One lump or two?", in the hope that the wide experience of Ribnet users can help with my dilemma.

Thanks again for your input - I shall get there in the end!

Regards,

Chris.
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Old 09 June 2003, 03:17   #10
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Picture of auxillary engine mounting.
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