One or two outboard engines?
That’s an essential dilemma that concerns many sea-friends who have decided to buy a larger rigid inflatable boat. And the answer is not as easy as it may seem.
This dilemma has become even stronger in recent years due to the huge increase in horsepower of outboard engines, which now allow even a 9m or 10m Rib to run effectively with the installation of a single engine.
So, among the most important decisions we have to take when choosing a rigid inflatable boat of 8-10 meters is whether we will equip it with single or twin outboard engines.
Of course, it should be clear that when we plan to equip our boat with two engines we must bear in mind that the sum of horsepower must be at least 30% higher than the horsepower we would have chosen if we were to install a single engine.
Otherwise, if the total engine power of our twin-engine Rib is the same as the horsepower of the single-engine it is certain that the performance will be poorer and the only thing we will achieve is to add more weight and expenses in our setup, while at the same time the chapter «safety», which is usually the main reason for buying a second engine, is automatically defeated because our boat will be unable to get on plane with only the one engine.
A twin-engine Rib with the same horsepower of a single-engine one, considering all other factors are equal and the Ribs are of course right and properly set-up, will give us a lower top speed, lower cruising speeds and higher fuel consumptions, with the exception of better performance at low rpm in terms of throttle response, acceleration and planning times.
The above results are expected because at the same horsepower, in the case of twin-engine setup we have two gearboxes which create more drag in the water than one, additional weight and of course worse horsepower-to-weight ratio.
So let's take a look at the benefits we get in case we chose two engines for our Rib, which will have a much higher overall horsepower than we would have if we had only selected one engine.
Advantages of twin-engine Ribs
For many people, Safety is the main reason for choosing two engines. When one engine malfunctions or suddenly «refuses» to start, we still have the other one that can safely return us into the harbour.
But does this really work?
The answer is unfortunately negative, and the argument of additional safety in twin-engine boats is weakened if we consider that the reliability of today’s engines is at very high levels (which means that very rarely a single-engine craft will present a mechanical problem leaving us adrift), most of the twin-engine boats are unable to get on plane with only the one engine, while 90% of the outboard engines’ problems are fuel related and as both engines are fed by the same tank the bad fuel will take both engines down.
We must also not forget that, according to the probability theory, in case of having two engines the probability of damage to one of them is always greater than a single-engine Rib.
Nevertheless, twin-engine Ribs have a safety edge.
We can highlight this lead to a huge comparative advantage if we take care to set up our boat in strict compliance with the following preconditions:
- Each engine - as we have already mentioned - must have the appropriate horsepower so as to be able to get the boat on plane on its own
- Avoid raising the engines to excessive transom heights
- We must always have two larger diameter and shorter pitch spare propellers stored on the boat, one clockwise and one left-handed, so that first the boat can jump on plane with the one engine and secondly to be held on plane at the lowest possible rpm for being able, in case of emergency, to travel many miles without overstressing the engine
- Have two fuel tanks with completely separate lines and filters, so as the one fuel supply line to be 100% independent of the other. Thus, in the case of any impediment to the fuel supply, we will not risk losing both engines at once. In addition, make sure that one of the two tanks is permanently filled with gasoline from a gas station we trust. So in the case of bad fuel or the presence of water in the second tank, which we use on a permanent basis, all we have to do is turn the switches and feed the engines from the tank with the pure fuel
- If our vessel has only one large tank, it is prudent to equip it with a second one, even portable, 100 or even 50 litters depending on the distances we usually cover. In this case, a permanent «T» with a selector valve must be fitted just before the fuel-water separators, to which we can directly connect the spare tank in case of emergency.
If the above conditions are not met, then we have safety only in theory and basically we have the same degree of safety that we would have with a single-engine setup accompanied by a small auxiliary engine.
An equally important reason that leads many people to the choice of two engines is their desire for better performance. Assuming the twin engines we will choose offer overall horsepower that exceeds by far a comparable single-engine setup, our boat performance will be clearly much better.
And of course, the term of performance includes top speed, cruise speed, throttle response, acceleration, planning time, the ability to hold the boat on plane at lower rpm having the engines to work effortlessly, while in some cases even the fuel consumption is lower.
In a test we've done in the past, comparing the performance of the same Rib, a Technohull SeaDNA 999, with two different set-ups: one with a single Verado 350hp and the other one with 2x300hp Verado, the twin engines:
- achieved 56 knots of top speed versus 46 knots for the single engine
- were running with 31 knots at 3500 rpm burning 2.1 litters per nautical mile, while the 350hp was running with 28 knots at 4500 rpm burning 2 litters per mile. We can see, therefore, that twin engines achieved higher cruising speed at lower rpm –meaning much less stress for them- while the fuel consumption was almost the same.
- clearly showed quicker hole shot and greater acceleration, attained 30 knots in 10 seconds compared to 16 seconds of the single engine
- got the Rib on plane at 2600 rpm, while the single 350hp engine at 3000 rpm.
A twin-engine Rib has a clear advantage in handling and low-speed maneuverability in limited places or crowded marinas, since it is capable of moving around its center without the need of many «backing» and «filling» of a single-engine one. This is a great advantage which will be appreciated especially in cases of strong wind or currents, as well as the greater thrust in reverse twins offer.
On a twin-engine Rib also, we have the option of adding a joystick control system that maximizes our handling capability by allowing us to move our boat in any direction, even to pivot it 360 degrees in place, and docking quite easy.
Twin-engine setups provide more choices when navigating, offering much better control of the Rib. They offer greater stability at high speeds, inspiring great confidence to the driver, while the steering torque is absent due to the inversely rotating propellers and thus the need for continuous corrections is minimized.
When we are travelling in windy conditions, we can improve our ride quality and, depending on the direction of the waves or the wind, we can control the boat-listing by trimming inward the one engine while at the same time we have trimmed outward the other one.
The great throttle response at low speeds, which is mainly due to the huge blade area offered by the two propellers, as well as the excellent «bite» of them allow us to climb effortlessly onto the big waves and travel holding the Rib on plane at the lowest possible speeds, which means lower fuel consumption and more comfortable cruising.
When we are travelling in following seas, our ride usually is improved because of the much heavier weight on the transom which leads the bow to stand higher in the water, minimizing thus the possibility of stuffing on the backside of the next wave.
On the other hand, a single-engine Rib is more versatile, and of course it can perform more tight turns with a much smaller radius of rotation, elements that are also very crucial when travelling in bad weather conditions.
After all, which is best?
Should I choose single or twin engines for my Rib?
The answer, as always, is: it depends!
Let your Needs lead your final choice.
Weigh out carefully your desires, the purpose for which you buy the Rib and the area in which you will most likely navigate.
If your goal is to ride in lakes, nearshore or to very close destinations then a single-engine setup with a small auxiliary engine is the best choice.
Otherwise, if you are one of those who enjoy high performance or if you plan to travel in the open sea alone or with your family by marking distant destinations on the map, then the twin-engine setup is one-way. In this case all you have to «weigh» is your budget
Sure, twins seem to be the best choice in terms of Safety, Performance and Boat Handling, making «Ribbing» easier, but all of them have their price and they cost for sure much more.
In addition to the purchase price of the second engine that can reach up to 70% more, you also need to take into account the extra equipment and working hours that a twin-engine Rib requires: extra batteries, filters, spare propellers, electrical installations and fuel supply system and of course, more time and expenses to set it up properly, i.e. props, engines’ mounting height, sea-trials.
And of course, whenever the time for the annual maintenance comes, the extra engine will start to hurt «double