What size Aux. for a 5.8Mtr RIB with 130 Yam?

Shaggy · 16 March 2005, 18:18

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

Louise · 16 March 2005, 18:19

Hi Shaggy. I believe a general rule of thumb is 1hp for every metre of hull length so 8hp should be ample, IMHO.

Aquapower · 16 March 2005, 21:04

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

Nick Hearne · 17 March 2005, 07:29

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

Louise · 17 March 2005, 07:34

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:
http://www.rib.net/forum/showthread.php?t=7543
http://www.rib.net/forum/showthread.php?t=3452
http://www.rib.net/forum/showthread.php?t=2189
http://www.rib.net/forum/showthread.php?t=736

Nick Hearne · 17 March 2005, 07:42

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

Nick

Aquapower · 17 March 2005, 11:35

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
Aquapower

codprawn · 17 March 2005, 12:47

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.

danny · 17 March 2005, 13:02

I think this one should do the job. With or without any tide.

danny · 17 March 2005, 13:21

Or build one inside ? Don't have to worry about the available space on the transom

jwalker · 17 March 2005, 15:54

Quote:

Originally Posted by Louise

......of the order of 9kg......It doesn't sound much but it could be the straw that broke the camel's back!

If an extra 9kg upsets the balance of your boat, it must be pretty marginal. After all, wasn't it Jono who said he takes that many sarnies on his boat for a snack?

Louise · 17 March 2005, 15:59

I was thinking about the strength of the transom rather than the balance of the boat.

Or are you just teasing me Jeff?

danny · 17 March 2005, 17:13

Too expensive ? Maybe this one then.

Jono · 17 March 2005, 17:32

I saw that, JW....

.

jwalker · 17 March 2005, 19:27

Shaggy · 23 March 2005, 14:08

Hi All
Thanks for all the great advice; I will have to have think about this one

Cheers Shaggy