One lump or two?

[Chris Murray]

Following the helpful comments of other Ribnet users, including checking out earlier threads on the subject of auxiliary engines, it does seem that - like boats themselves - auxiliaries are very much a compromise.

I appreciate that one engine is more efficient than two, but why does there seem to be a substantial school of thought that a single large engine plus a small back-up is the way to go, when there are obviously major issues over location, storage and/or manhandling of these engines?

I was at Port Hamble yesterday (on Aries II), when a 6mtr RIB cruised in and out of the marina, with hardly a sound. It was running a single Yamaha 60 four stroke.

The boat that I hope to buy (eventually) can take either a single 115hp or twin 60 hp (all 4-stroke). If you add say an 8hp aux to the 115 (even 2 stroke), the combined weight is very similar for both configurations.

The concept of twin 60's is beginning to look quite appealing - less hassle, always in use, twin 20 gal under-floor tanks, real "get you home" power. However, I would not wish the boat to suffer severely impaired performance, so I would welcome any views on the performance (and fuel consumption) of the two options.

BTW, the Yam 115 is fuel injected multivalve, whereas the Yam 60's are more conventional, albeit they are "new" (inc. 4 carburettors) for 2003.

On the same subject - what is the perception of "noise" at cruising speed (say 30kts) with these two set-ups?

Your thoughts would be greatly appreciated, thank you!

Chris.

[Charles]

My personal opinion is : IF you can afford two outboards and the rib is capable of supporting twins, this surely must be the way to go. It is a fallacy that some would suggest your consumption is double - think about it - with a twin arrangement the engine's do not have to work as hard to get you through the water. The inherent safety of a rib, first and foremost (I have proved that!) is that they are virtually unsinkable and with a twin set-up you are merely increasing your safety factor.

[DJL]

I would try and stay away from the 60HP four strokes with carburettors. The new EFI models are a lot more fuel efficient and much better on the power/acceleration side too. You will have to go for Mercury or Suzuki though if you want a EFI 60HP, the Suzuki begin a lot heavier than the Mercury.

I have to agree with Charles on the twin side of things. Unless you really want to push every last knot out of the boat twins are the way to go. You have to remember though twins cost more to buy, service etc.. and fuel consumption will be greater than with a single engine (not twice as much but still more).

[Garygee]

Hi Chris

As far as I am comcerned ther is no question, if you value saftey over everything else, go for twins.

So if thats the case why does not evryone go for them.
Well their are a number of cons,

While they dont use twice the fuel the extra drag of the drive makes you loose about 25% of the extra power, i.e. a twin 50's will give you the same power as a 75hp single.

The cost of buying and servicing is higher

To make full use you need twin redundent systems, fuel, battery etc.

Given this you need to ask yourself the following questions.

How far do I go solo, would a 6HP aux get me home?.
Do i usually travel as a group
If I cant get home how close is help
If you do have to get home on a aux, are you set up to spend say 10-20 hours in a open boat?. Dont forget 1 hour at 40Kts is 10 hours at 4Kts, assuming their is no tide etc

But in real terms most people would be able to get a tow.

These days outboards are very relieable and if well looked after and serviced should ramain so, but one is better then two IF set up right.

Answer yourself the above question and then decide. If in dought and you can afford it go for twins, it the safe descision.

Regards Gary

[geoff]

I had twin 50hp on my first boat and now have a single 100hp. I much prefer the single engine set-up. I take the point about safety and with a single engine you need to plan your cruising accordingly. If you are going to remote places, go with another boat is the best thing to do.
Twins are expensive, more servicing, more drag, more weight and one engine always runs a bit better than the other so you worry about that engine.

[gavin]

I would have to say twins are the way to go manouverability with regard to turning and manouvring is excellent as they never rev quite in scyro there is a slight ressonance as the revs build, but be warned twice the maintaince and certainly twice the cost, two batteries two gear oil fills two sets of plugs.Another good thing about twins is you can swop between them if you have a fault i.e compononts etc.With twins you can stagger the trim tilt depending on weight distribution in the boat also. gavin

[Mike C]

One other point..... (I had twin 60's on my last boat - now a single Diesel)
The idea that you'll get home quick with just one remaining engine may not be true either. Since there are two engines, you will have much higher pitch props on them than if there were only one. I had 15" on my old 5.8 Ribcraft. Great when they were working together but with the 15" and only one engine on the go I could only get about 3,000 revs and a top speed of about 8 knots. Better than your average aux perhaps.... but not much! Probably a single 60 would have been sufficient to get the boat on the plane and get 20 or so knots.... IF PROPPED CORRECTLY! but you're not going to want to change props after one engine conks out!
Two engines are psychologically safer though!.. and I would probably still prefer them myself if money were no object but……!

Mike C

[Chris Murray]

Thanks again for some excellent input, both technical and practical.

I have just come across Yamaha's own tests for a 6.5mtr RIB, comparing the 115 four stroke (fuel injected) with twin 60s (carburettor). Have a look at this sample:

Single 115 * * * * * * * * Twin 60

RPM * MPH GPH MPG * MPH GPH * MPG

3000 * 14.7 2.4 6.13 * 14.9 5.4 * 2.76

4000 * 24.6 4.3 5.72 * 25.8 8.2 * 3.15

4500 * 28.7 5.4 5.31 * 30.3 10.0 * 3.03

5000 * 33.1 6.5 5.09 * 34.7 12.8 * 2.71

6000 * 41.7 10.8 3.86 * 42.7 16.4 * 2.60

I imagine that the figures will be slightly better for a 5.5 mtr boat.

Incidently, the single took 2.76 seconds to plane and the twin took 3.49 seconds.

Looks like I shall have to go back to the "one big lump" (+ small back-up) idea, based on this set of figures, although I wouldn't mind seeing comparative figures for 60 EFI's, before making a final decision. Any takers ???

Chris.

[Manos]

Twin engines, twin trouble, twin headake, twin petrol bills, twing service epenses, twin b@!#@!@y everything that may go wrong with them.

My advise whatever it worths single large engine with a small aux
on the side.

Had 3 bottle of wine this evening and hope this makes sence

[Powerboat]

Because of the extra drag of two engines etc a rough rule of thumb for power output from twins is 1.5 by engine HP.

So twin 60s should approximate to a single 90 etc.

Twin 100s = 150

Lots of other factors of course but it is a good rule of thumb!

Best wishes,

Stuart