single or twin engine dive RIB

[IanMcDean]

Hi,
My dive club are looking into a new RIB at the moment. We currently own 2 6m boats and one is getting a little long in the the tooth and is creating alot of work. Both run twin 45Hp hondas. So we want to replace with something similar size wise and have been looking very closly at the RIBcraft boats.

One question I would like to guage opinion/experiance of is the use of twins and single engines. There is alot of feeling in the club that we must have twin engines for safety reasons. However when we look around virtualy no one seems to be using this config. Which leads to the question why?

Well i can see the performance, drag and weight issues around this but does this outweight the (possibly percived) safety increase?

Best regards

Ian

[captnjack]

Most engine breakdowns are fuel related, water, sediment, filters clogging... 90% is the factoid I hear bandied about, although I have no real data on this issue. But twin engines, in addition to being 2x the maintenance to keep up on to really be truly more reliable, are frequently hooked to the same tank. So you just end up with 2 dead engines and not much genuine redundancy.

In the life of the existing RIBs+engines, how many times have you be forced to return to port on one engine? That's the best metric I know of to gauge whether your use and maintenance habits dictate redundant power.

Richard

[Andy]

Maybe you need a large engine with small aux to let you pick up divers if they were in the water when you lost the main. This is the only reason I would want a second engine.

In my experience getting back to port with a single 45HP on the back of a loaded 6m boat of divers is a very very slow process. But you my be able to get back solo.

We tend to use 2 RIBS, each gives the other cover, and each has a single.

[PeterR]

Why do the RNLI use twins is something to look at.May be someone can through some light on this

[landlord]

I chose twins over single for the knowledge that if one goes down I have a back up. Both engines are completely independant to each other, running off thier own batteries and fuel tanks.

I have seen a number of vessels with a small second engine as a back up to the main. If your going to have a back up might as well use it on a regular basis so you know it works when you need it !

I have driven many vessels with twin engines, maintained by onboard engineers and they still go wrong. Nice to have the other engine to limp back to port !

[JIY]

I've driven well over one thousand hours commercially now and would have been towed in at least 5 times if I'd been driving a single engined RIB! All the failures were mechanical, none were fuel related.

An auxilliary is a great comprimise, but they do take a beating if mounted on the transom in rough conditions. Storing securely for mounting on the transom in the event of failure of the main outboard is another option, but if you have a big RIB you need a fairly heavy outboard (10-15hp?) to make meaningful progress in any sort of sea. Lifting this auxilliary onto the transom might be a problem, especially if the RIB is rolling around in lumpy seas.

Twins do have the advantage of torque free handling (provided they are contra-rotating), so there is no tendency for the RIB to come down on one side in rougher seas. They are also excellent for alongside handling, and once you get the hand of "tank-turning" you can almost spin the boat in its own length.

The downsides are of course all expense related. More expensive to buy than one big single, more expensive to service, more expensive to run. For instance, the XS850 I have on order for private use will have twin 200 Verados. The perfomance will be pretty similar to the single 300hp, but the purchase price is £6K more and I expect it will burn about 20 litres/hour more than the single at 30 knots.

I expect that most of us would choose twins if the significant extra costs were not a factor!

[Cookee]

The positive side to twins are:

Extra grunt from two props when fully loaded with dive gear

Extra manoeuvrability from a twin rig

Safety of two engines to get you home

The downsides are:

Extra purchase cost of engines and separate systems (fuel tanks, batteries, filters etc)

Extra servicing costs

Extra fuel consumption

For a single 150 hp you will need 2 X 100 hp to give the same performance due to extra weight and drag. Get counter rotating gearboxes and props - much better handling!

We would be happy to give you a price on a 6m if you would like a comparison!

[9D280]

Originally Posted by Cookee

...due to extra weight and drag. ........

More weight is a myth - it depends what engine(s) you choose. Tohatsoo if you work your way all the way up the range form twin 15(!), the "heavier" option alternates from single to twin - simply 'coz a lot of engine "lumps" are tweaked to run at two or three different Horesepower levels, as it halves the number of different blocks etc the OEM has to manufacture. Then don't forget to add the weight of the Aux when you are doing the maths......

I'm still researching drag, but It seems the drag through water is related to something nearer the cube of the speed for certain shapes, rather than air which is nearer the square.
So, bigger rib, bigger engine - higher speed, so it won't take much to double the drag when you are travelling faster anyway...... Also as engines get bigger, the frontal area of the leg in the water gets bigger... Then mix in the point in the previous paragraph and you find yourself with HP:leg area ratios bouncing back & forth as you climb the HP ranges. I may ask you lot for some dimensional data over the winter to do a comparion, but right now time ain't on my side.


Assuming like Landlord your engines are TOTALLY independant, you will at least have enough power to get you & all your cylinders, weight belts etc home, even if it don't plane. Just aim for twins at the top end of a "HP range" for a given CC of engine.

Might be woith trying a 6 or 8 on the back of your current rib into a headwind one afternoon?

[IanMcDean]

Hi,
thanks, some good thoughts there (especialy trying a aux before we go anf get one). One thing that occurs to me is that our current engines are not counter rotating. Just wondering would there be any performance gain from doing this as well as the handling?

thanks

Ian

[PeteSmith]

Ian

What people are tell you is all good stuff, Ive had a lot of dealing with lots of dive club ribs and unfortunately they (generally) dont get the proper servicing on either the engine or the steering.
When Ive been teaching on RIBS ive even had the steering wheel come off when we were going along!

Most dive clubs with a 6m boat go for a single engine around the 120 - 150hp with a small auxillary (4- 6hp) this is enough to pootle round a dive site and pick up divers, or maintain position in bad weather - thats what your VHF is for.

The transom mounting for the aux engine needs to be thought out as they do get alot of bashing. Id recommend extra bungie fastened between the engine and the A frame, the transom plinths arnt that strong.

Also check that you have full steerage (due to the tiller) and check that the aux engine uses the same fuel fitting as the main tank, that way you can use fuel from you main tank or an external spare fuel tank.

Remember that fuel "goes Off" so your better using an external (well cycled) fuel tank than one thats been sitting there for 6 months, hence why not to rely on the small internal tanks alot of aux engines have.

The main thing is make sure people know how to lower and start the aux engine. IMHO when people come along for Diver Cox'n Assessments most have no idea how to use the auxillary.

Pete
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[PeteSmith]

By the way it must have taken you ages to get to dive sites with 2 x 40hp

Did you have to wait for the tide?

Pete

[landlord]

Not sure whether this helps.

Data recorded in not ideal conditions but gives an idea of the fuel consumption for one engine. Both engines were in use at the time so the consumption figure has to multiplied by 2 to give a total consumption.

RIB a Tornado 7.5m