Repowering the Aquarius 5.5 metre RIB

[Maranoa]

I have inherited my son's 2000 era Aquarius 5.5 metre twin hulled RHIB minus engines (it was powered by two 26 year old 60 horsepower Mariner 2 strokes) and am debating what to re-engine it with.
My wife and I want the boat for light game trolling off the Gold Coast here for mackeral/tuna/mahi mahi with speed not high on our agenda.
With the hightened price of outboards here in Australia due to the pandemic my choices are limited:
- pay about $19-$22,000AUD for two new Mercury/Honda/Yamaha 60hp 4 stroke engines (and wait until March for arrival in Australia)
- pay about $5000 to $7000 for a single used 90hp Evinrude, Yamaha/Honda/Mercury mounted centrally between catamaran hulls on transom with jackplate.
- Pay about $14,000 for a single new 90hp Yamaha/Honda/Mercury/Suzuki/Tohatsu mounted centrally between catamaran hulls on transom with jackplate.
Thank you for reading and any advice is appreciated.

[Jon Tallis]

Difficult to comment without seeing the boat (can you post pics - sounds interesting) but I'd be concerned about mounting a central single unless you know the boat was designed with that as an option.
Assume if the boat originally ran twins it has twin fuel systems, electrics etc
I'd be tempted to stick with twins for the extra reliability and manoeuvrability. That all assumes you can absorb the extra weight (esp going from 2-str to 4-str), cost and fuel consumption, and wait for them to arrive!

[Jon Tallis]

Just found some photos online. Ignore my comment about singles - clearly an option.

[Maranoa]

Thanks Jon, zero to no information in Australia on this issue so I thought I would pick the RIBnet knowledge bank

[Jon Tallis]

Maranoa,
In the absence of other replies....
Just wondering which boat you've got - you described it as a catamaran rib designed for twin motors but looking at Aquarius website https://www.aquariusinflatables.co.z...ll-semi-rigid/
they seem to do two models - a 5.5 conventional hull designed for twin motors and a catamaran (like a zapcat) designed for a single.
I'm guessing you've got the 5.5 monohull linked above.
That being the case, the boat was clearly designed for twins and the website doesn't show a single as an option - which is odd.
It's rated for up to 115hp so you should be good to that, even moving to 4str - just check the weights of your intended motor Vs a pair of 1990s vintage 50hp and I'm sure you'll be well in.
Given the motors were designed to be very close-mounted (not much space to do differently on a 5.5) the transom structure is probably fine for a centrally mounted single but if in any doubt, get a local boatbuilder or surveyor to have a look.
In the UK at least it would be pretty unusual for a boat of this size to have twin motors - the extra cost (purchase and running) isn't really justified unless doing serious safety / rescue work. Your do of course get a benefit in terms of redundancy / reliability but this is somewhat eroded if your don't have separate fuel & electrical systems - again, unusual on a small rib (dont know if you have these). Also it is very hard (IME) to get smaller outboards (below about 75hp) with a contra-rotating gearbox. Having a pair of "right handed" engines will make the boat a pig to drive Vs a single. If you're getting quotes for twins, make sure the supplier bis quoting you for a pair -i.e. one conventional "right hand" rotation and one contra / "left hand" rotation.
If it was me I think I'd find a nice secondhand 100-115hp, pop it on and go have fun

[Limecc]

The end product's not worth a new engine/s because of it's age. In my view unless there is some sentimental reason, simpler to live with, replace or repair an engine like for like or simply start over with a new (or used) boat and let someone else take it on. Pure economics. Doubtful fuel savings will cover the cost for the remaining life of a 20 year old boat. Unfortunate that decent used engines are rare and overpriced, as this could work.

[Maranoa]

I should have been more specific, the boat is the 5.5 metre catamaran here https://www.aquariusinflatables.co.z...an-semi-rigid/
We have received mixed advice on how viable it is to put a single engine on a catamaran. Some say no way due to hull wake issues, others say it will be fine but we will get wet with spray at high speeds (which will be rare).

Thanks for your inputs.

[Jon Tallis]

Yup, you need someone who's done it and that's not me so will drop out at this point