Inboard /outboard

[beargrylls]

looking for a offshore rib and was wondering about peoples views regards the benefits of twin outboards 4 strokes to a single diesel inboard? ie : fuel costs v reliability/performance
Thanks

Bear

[geoff]

I had a boat with twin 50hp outboards and now have one 100hp and a small backup. In my opinion one main engine with small backup is the best unless you have to be out and able to operate in all weather, e.g. the lifeboats.
Two engines are twice the service charge, one always runs better than the other so you wonder what is wrong with the other and you lose a lot of power when you drag a second leg through the water. My new boat is bigger and heavier than the previous, but goes faster with the single 100hp than the 2x50hp. My advice would be to go for a single outboard
Dont have much experience of diesel inboard, there are plenty in the forum who do who will no doubt post.

[John Kennett]

I can't think of many offshore RIBs that use twin outboards for long distance work. Serious RIB cruising is overwhelmingly dominated by single inboard diesels.

Modern diesel sterndrives are proving to be very reliable, and are more economical to run. It's not just the price per litre for diesel (which is a huge bonus in the UK though), but the actual consumption which has a big impact on the range for a given fuel load.

Another factor for consideration (possible the single most important!) is the availablility of fuel. Where are you planning to go? Diesel in available on the quayside just about anywhere, whereas petrol will inevitably mean dragging jerry cans for miles!

John

[Brian]

I can thoroughly recommend a Scorpion with a single inboard diesel for long range offshore cruising.
Reasons:
Scorpion very well built, designed for the task
very "dry" boat due to deep V hull and design and positioning of hull and tubes
Inboard diesel v. reliable, goes on for ever
Uses inexpensive fuel with good availability
Also usually built with a small outboard stowed inside the engine compartment either for (really!!!!) occasional use or to power an inflatable tender (I use a Zodiac Fastroller 3.1m)
E-mail me direct if you need more info.

[Alan]

I can also thoroughly recommend a Diesel Scorpion, its really the only way to travel! Particularly the executive lounge rear seat on Cyanide (Brian's boat).

One serious point to be made in the eternal "twins vs singles" debate ... which can apply to diesel inboards as well as outboards on larger RIBS. You may carry an aux engine on a large RIB but don't expect it to provide any serious ability to get you home from far offshore unless a) its a very hefty outboard and b) you are carrying a lot of fuel!

I'm not sure whether you've ever tried the aux on Cyanide Brian? IIRC its a 6hp? Suspect it will not give you more than 3-4kts flat out and that the 10L tank will not last more than an hour or two. That gives a range of 4-8 miles, not taking into affect any tidal effect. Means that a small aux is really only useful if you are very close to a refuge or to manoeuvre you away from a hazard or towards assistance. I'm using Cyanide as an example but the same applies to the 5hp we have on YellowTang (21ft BWM). The furthest we've gone under aux power was 2-3 miles back into Portsmouth harbour against the ebb. Which was fairly exciting and another story!

Alan.

[Brian]

Totally agree with all Alan says there. Aux. more a phychological prop. than anything else (scuse the pun). OK for tender though in an abandon ship situation or I'll go get "x" you guys stay here scenario.
Two engines, it would appear to me to give only one benefit-some measure of redundancy. Apart from that limited advantage, all the other factors seem negative, fuel consumption and therefore weight and performance (range), more to go wrong, cost of purchase, servicing etc. Even the one advantage is limited in that if you have a fuel or electrical problem, these are often shared resources between the two engines, so both egines are effected. Unless of course you have total redundancy and cross-switching, in which case, costs, weight, more to go wrong.......

[sirzap]

Alan,
Would it not be best that the AUX engine burn the same fuel as the main engine? Then be able to tap into the main tanks?

[Alan]

Yes I agree with you and with an out board driven RIB that isn't a problem. You may have to muck around with premix (for a 2 stroke outboard) which isn't the easiest thing to do at sea but you should be able to burn of the main tank which I agree would considerably extend your range.

My comments related more to inboard/outdrives which in this country are usual diesel (petrol not being free like it is in the US!). Clearly if you run a petrol outboard as an aux in a diesel rib you have to carry a completely seperate fuel supply to run the aux. Diesel outboards are expensive and usually fairly large so are not really an option!

Alan

[beargrylls]

thanks for all the very sound advice
it seems to be the single diesel option is winning!
Are they alot slower? and do they compromise interior space as well? and finally my experience of inboards is getting plastic bags caught and melting impellers! a problem unlikely with an outboard with visible water outlet jets?
thanks guys
Bear
it's great to use all your expert knowledge

[John Kennett]

it seems to be the single diesel option is winning!

That's because it's the right option

Are they alot slower?

Not noticeably. An 8 metre RIB with a 240hp (Yamaha) or 300hp (Yanmar) diesel will easily do 40 knots and more. From your original question, you're not looking for an out-and-out race boat in which case outboards might be the answer!

and do they compromise interior space as well?

You do lose some space for the engine box, but in a RIB over 7 metres it's not likely to be a problem (can make a very comfy seat too!)

and finally my experience of inboards is getting plastic bags caught and melting impellers! a problem unlikely with an outboard with visible water outlet jets?

An outdrive leg is basically the same as an outboard, so there will be not difference. It's not something that I've found to be a great problem.

So, are you going to tell us what you are planning?

John

[Alan Priddy]

We use an air cooled diesel outboard as a back up. This pushes us along @ 6-7 Knots when fully loaded ( 6 ton)
I do not see the sense of a petrol outboard with a diesel main engine Example.... 35 kgs for ta petrol outboard + 10 gallons of highly inflamable petrol = 45kgs, which will not get you far = 80kgs
Our spare engine all up is only 82kgs and will run forever., and only cost £3000
Contact me if you want details..
Alan P

[beargrylls]

thanks John- very helpful
You'll be the first place I'll come for advice when a plan is definitely in the offing!
Stay well
Bear