Auxilliary Engine on RIB

[nigel52]

What do Forumers think about adding say a 5HP auxilliary outboard to a Zodiac Pro Open 550 with a 115HP .

Also would members normally use the main engine for steering and lock the auxilliary in the centre position.

How easy would it be to fit this type of set up.

Thanks for input

[mister p]

Originally Posted by nigel52

What do Forumers think about adding say a 5HP auxilliary outboard to a Zodiac Pro Open 550 with a 115HP .

Also would members normally use the main engine for steering and lock the auxilliary in the centre position.

How easy would it be to fit this type of set up.

Thanks for input

a Reckon it may struggle a bit but a 2 stroke will be better than a four, size and weight wise ( also cost wise ).
b Yes, but there are exceptions, your 115 will drag the 5HP like crazy.....don't really reckon you should use this set-up in an emergency. I have a 4HP wing for my 30HP on my 4m SR.
c Should be easy enough with the wee 5HP, maybe not so with an engine that would be more suitable.

Doesn't really help you much does it?

[Chris]

Where abouts do you do your boating...?

[nigel52]

Originally Posted by mister p

a Reckon it may struggle a bit but a 2 stroke will be better than a four, size and weight wise ( also cost wise ).
b Yes, but there are exceptions, your 115 will drag the 5HP like crazy.....don't really reckon you should use this set-up in an emergency. I have a 4HP wing for my 30HP on my 4m SR.
c Should be easy enough with the wee 5HP, maybe not so with an engine that would be more suitable.

Doesn't really help you much does it?

Sorry Mr P but I did not really understand your various answers.

[nigel52]

Originally Posted by Chris

Where abouts do you do your boating...?

Hi Chris, we will be using the RIB for Pleasure trips around the various islands off La Rochelle, West coast France. Also wake boarding and waterskiing. Some of the distances between the islands are about 15-20 miles hence the question about auxilliary/back up if the main engine fails. Sods law says it will when you are out at sea and we do not have a local RNLI or Seastart on easy call.

(BTW I would not call them as I like to be self sufficient but they are a luxury back-up in a real emergency)

[Locozodiac]

How about a Tohatsu 2 strokes 9.8 HP, 170 CC aux, best in it's class, light and with punch. Weights 26 Kg as their 6-8 HP younger brothers which are detuned same weight/sizeversions.

Happy Bopating

[nigel52]

Originally Posted by Locozodiac

How about a Tohatsu 2 strokes 9.8 HP, 170 CC aux, best in it's class, light and with punch. Weights 26 Kg as their 6-8 HP younger brothers which are detuned same weight/sizeversions.

Happy Bopating

Thanks - as I already have a 5HP Tohatsu I thought I might as well use this.

[Nos4r2]

If you're carrying more than 3 people you'll probably find 5hp isn't quite enough.

[nigel52]

Thanks something to think about

[DGR]

As a general rule of thumb, we always used to reckon 1hp for each metre - so 5hp for a 5.5m RIB doesn't sound too silly (after all, an aux is a get you home motor, or use it for trolling etc).

I don't think that you'll get that much steering effect using a fixed aux and the main as a rudder - esp at low speed, where you are directing the thrust when you steer. Probably better to lift the main out of the water, and use the aux for power and steering, if you can.

Is there any way that you can rig it temporarily to see whether it's up to it?

IMHO, obviously...!!

[BogMonster]

Originally Posted by mister p

a Reckon it may struggle a bit but a 2 stroke will be better than a four, size and weight wise ( also cost wise ).
b Yes, but there are exceptions, your 115 will drag the 5HP like crazy.....don't really reckon you should use this set-up in an emergency. I have a 4HP wing for my 30HP on my 4m SR.
c Should be easy enough with the wee 5HP, maybe not so with an engine that would be more suitable.

Doesn't really help you much does it?

IMHO there is not much to choose between them. As far as I know the 4/5/6hp models in most manufacturers ranges are often based on the same block, and the extra HP presumably derived from a different carb giving more fuel higher revs at the top end, so at 3000rpm (or whatever it sits at) the power/torque is probably similar.

An aux will never achieve full operating RPM and I see very little performance difference between the 4hp Merc that I use as an aux on the Vipermax, and the 6hp Johnson that I had on the old Humber. It is a whisker slower but I suspect that is partly to do with the VM being heavier - much bigger main engine, bigger fuel tank and generally heavier construction. It tootles along at about 4 knots with the main engine down and used for steering, and while you might lose half a knot from the drag, I'd much rather that in rough weather than be perched on the tube trying to steer the aux. Having said that I'd not like to be trying to get home on any aux in rough weather.

If I were buying again I'd seriously think about the 9.8 Twatsoo option, but I'd also seriously think about twins.

[nigel52]

Originally Posted by BogMonster

IMHO there is not much to choose between them. As far as I know the 4/5/6hp models in most manufacturers ranges are often based on the same block, and the extra HP presumably derived from a different carb giving more fuel higher revs at the top end, so at 3000rpm (or whatever it sits at) the power/torque is probably similar.

An aux will never achieve full operating RPM and I see very little performance difference between the 4hp Merc that I use as an aux on the Vipermax, and the 6hp Johnson that I had on the old Humber. It is a whisker slower but I suspect that is partly to do with the VM being heavier - much bigger main engine, bigger fuel tank and generally heavier construction. It tootles along at about 4 knots with the main engine down and used for steering, and while you might lose half a knot from the drag, I'd much rather that in rough weather than be perched on the tube trying to steer the aux. Having said that I'd not like to be trying to get home on any aux in rough weather.

If I were buying again I'd seriously think about the 9.8 Twatsoo option, but I'd also seriously think about twins.

----------------------------
Hi BogMonster, I guess in your part of the world an auxilliary is mandatory.

For me 2 x large engines is out for multiple reasons, cost, weight, space etc. As I have a 5HP Tohatsu two stroke knocking around I thought it would make sense to use it as an auxilliary when I get a RIB with a 115HP. There are the brackets that are sprung loaded to lift the auxilliary out of the water to avoid drag although a 5HP may be too heavy for these. Getting out of trouble at 4 knots is better than been stuck. I don't think we will be using it in very rough weather unless it blows up suddenly. Some replies have referred to have an auxilliary as a wing to their main engine. Not sure what they mean by that.

I must admit I am more in favour of doing what you do but also trying to get the auxilliary up out of the water when not in use.

[Chris]

A 5hp aux will struggle to push your rib past 4 knots and if you wanted more speed then you will need to look at 10-15HP for another 3 knots at this point you are close to the maximum speed the hull can achieve in displacement mode anyway and you will have about 35KGs dead weight hanging on the transom.

If you have a 5HP and can hang it on the back without any problems then guess its up to you, however having had a 5hp aux engine fitted to a similar size rib (Osprey Viper 5.25) with a 115 Etec I have to say it was a complete waste of time. I never used it and when I did it invariably had problems with throttle sticking etc as it was not getting enough use, also the extra weight on the transom will affect the way the rib performs on the water, and can cause the rib to become a bit tail heavy. If you have a 2 stroke without an inbuilt tank you have to carry premixed fuel (more weight) and then if you don't use the fuel regularly you run the risk of stale fuel issues.

I would rather spend the money on ensuring I had a really good fuel filtering system and that my electrical system was in top condition, also a twin battery set up.

[mister p]

Looked like you were set for a 5 hp.
I reckon you need 10 at least (9.9) .
Get the biggest you can that fits, IMHO for a wing get a two stroke, lots smaller and lighter plus forces you to use different fuel.
Get your wing running before you start your main from the off and if poss use it to navigate away from berth/mooring. That will keep fuel fresh and engine ready should you need it . Will also keep you familiar with wing. The wing is your emergency device to get you home and dry......look after it and be happy using it. It all helps when the seas are big and desperation seems the order of the day, albeit uninvited. A wing engine that starts at half a pull on fresh fuel is a good as a fleece and a hot chocolate..........even the sound of it running soothes.

[95gstnj]

I use a 3.5hp 2stroke on my 6 meter. it pushes it along fine at 4knots with no tide, and about 2 knots against a decent tide. I turn it slightly off-center to compensate for the fact that it's not centered on the transom. Then I use the main leg to steer. Works fine as long as you're making headway.

[mister p]

Oh, and use it when you don't need to. Just to prepare you for exactly how long it will take to get back the ten miles it took you twenty minutes to travel.
If it's a two stroke wing, keep oil on board.

[AK2AZ]

I have a 8hp Mariner 2-stroke on my Pro 530 with a 115 main. It is a tight fit but works. Max recorded GPS speed was 5.5mph with the main out of the water.

For your application I would recommend something between 8-9.9hp...any less will be a disappointment if you have to buck wind or waves.