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Old 16 March 2005, 14:18   #1
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Boat name: Spare RIB
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What size Aux. for a 5.8Mtr RIB with 130 Yam?

Hi All
I am looking for a Aux engine for my 5.8Mtr Narwhal which has a 130 Yam, I though something like a 8HP should do the trick, is this the right HP for my size of boat?

Cheers Shaggy
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Old 16 March 2005, 14:19   #2
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Hi Shaggy. I believe a general rule of thumb is 1hp for every metre of hull length so 8hp should be ample, IMHO.
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Old 16 March 2005, 17:04   #3
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Quote:
Originally Posted by Shaggy
Hi All
I am looking for a Aux engine for my 5.8Mtr Narwhal which has a 130 Yam, I though something like a 8HP should do the trick, is this the right HP for my size of boat?

Cheers Shaggy
Hi Shaggy

To give you a bit of performance against the elements (ie the solent tides) a good rule is to multiply the length by the beam..... the wider the boat the more drag. Too many people go that little bit too small with an aux, its there to help you when all else fails, so an extra couple of hp goes a long way.

Aquapower Dave
www.aquapower.co.uk
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Old 17 March 2005, 03:29   #4
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Quote:
Originally Posted by Aquapower
Hi Shaggy

To give you a bit of performance against the elements (ie the solent tides) a good rule is to multiply the length by the beam..... the wider the boat the more drag. Too many people go that little bit too small with an aux, its there to help you when all else fails, so an extra couple of hp goes a long way.

Aquapower Dave
www.aquapower.co.uk
Dave are you saying a 5m X 2m = 10M so a 10hp Aux?
Nick
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Old 17 March 2005, 03:34   #5
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Thanks for the advice Dave.

Shaggy, if I were you I'd also check how much available space you have on your transom and also check the weight of the aux. Not sure if you have a 4st or 2st but in the Yammie 2st range there is a jump in weight from the 8hp to the next one up (9.9hp) of the order of 9kg. The 15hp is the same weight as the 9.9hp. It doesn't sound much but it could be the straw that broke the camel's back!

If you do a search on 'auxiliary engine' you'll find several threads where members give actual figures, ie. what speed they make with their aux on their rig. You'll probably find at least one with a similar set up to you! HTH

Edit: a few old threads to have a look at:
back-up engine
Auxillary Outboard
Back-up Engine
Alternative propulsion
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Old 17 March 2005, 03:42   #6
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Well when I get round to it I intend to fit my Yamaha Malta (5hp) to the transom as it pulls along the rib just fine the I have towed it using my sib!
Just have to cut away the wave deflector off the back of the transom
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Old 17 March 2005, 07:35   #7
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Quote:
Originally Posted by Nick Hearne
Dave are you saying a 5m X 2m = 10M so a 10hp Aux?
Nick
Yep, 5 x 2 = 10hp, at least you'll make a bit of progress if a strong tide is running. But as Loise says, do check for available space and weights. Its a fine balancing act.

Dave
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Old 17 March 2005, 08:47   #8
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Quote:
Originally Posted by Louise
Thanks for the advice Dave.

Shaggy, if I were you I'd also check how much available space you have on your transom and also check the weight of the aux. Not sure if you have a 4st or 2st but in the Yammie 2st range there is a jump in weight from the 8hp to the next one up (9.9hp) of the order of 9kg. The 15hp is the same weight as the 9.9hp. It doesn't sound much but it could be the straw that broke the camel's back!

If you do a search on 'auxiliary engine' you'll find several threads where members give actual figures, ie. what speed they make with their aux on their rig. You'll probably find at least one with a similar set up to you! HTH
Good point Louise - weight is the most important point - why have a 9.9hp when a 15hp is the same size engine?

HP isn't everything though. Thrust is far more important and as a RIB with a small aux isn't going to be very fast thrust at low speed is what counts.

A mate had a 17' fishing dory with a 4hp aux on it - wouldn't move against any sort of tidal race. He tried out his father's old Seagull and it pushed him along quite nicely even though it is only about 3hp.
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Old 17 March 2005, 09:02   #9
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I think this one should do the job. With or without any tide.
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Old 17 March 2005, 09:21   #10
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Or build one inside ? Don't have to worry about the available space on the transom
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