Single outboard stability

I wonder if anyone can settle an argument or at least challenge the views of me and one of my colleagues. We are debating the pitfalls or putting a large single outboard (probably a 350hp) in place of two smaller outboards (200hp) on an 8.5m rib. We are debating whether the larger engine would give the boat problems at a high speed, where as the twins would perform better because of counter rotation of the props and because propelling the boat from two points increases stability. Will the boat be more likely to porpoise and handle badly even make the vessel unsafe or are we just worrying about nothing.

 

Counter rotating twins would be unusual, typically they both have the same rotation and one is "toed" a tad to help offset torque steer. Also twins will be heavier then a single of the same HP so you pay a weight penalty. The extra lower unit generates more drag so you pay a speed penalty. Twins also have to be synchronized to run optimally. Porpoising issues wouldn't likely be related to the engine installation, unless the twins drag the CG too far aft, which isn't likely if the boat is designed to handle this much power. Simply put, a single will probably be a less troublesome installation in every regard. This isn't to say you can't have trouble free twins, but it is to say there's a lot more that can go wrong with twins then a single.

 

I think I've seen maybe four boats with counter-rotating twin outboards. Virtually all multi-outboard boats I've come across just use identical engines at all positions. Properly aligned and with the torque tabs correctly set, this seems to pose no problem.

As PAR says, porpoising would only be affected by the engine choice if the twins option moves the centre of gravity too far aft.

To be honest, I'm generally not a big fan of outboards in the larger sizes (north of about 180 hp or so). But if that's what the boat is meant for, I'd choose a single over twins in many cases- simply because there's half as much stuff to maintain. In any case, when outboards die it's usually because of crappy gas (which kills both at once). The low-speed handling advantage of twins is only valid if they're widely spaced, which is not the case on most RIBs.

 

A single engine will have more top speed. Two lower units will have more drag. The overall weight will also be higer.

 

I've seen dozens of counter rotating set ups from 150 Hp through 350 Hp.

It is much more the norm around here rather than the exception.

Chine hopping is more of an issue than porpoising and easily resolved by trimming down slightly.

It probably would make a difference one way or the other but I have no idea which way and I think it would be miniscule.

In thousands of hours of driving these boats I've only had to limp home once when the bottom end gears went on one motor.

If it was my RIB, I would only have one engine for weight and cost reasons.

It is nice to have twin, directable thrust at the dock in a storm but nothing that can't be replaced with good boat handling skills.

-Tom

 

I would say that on most of the twins I see, one is counter rotating. Which side you mount them on can make difference, but it comes down to the exact hull, the exact motors and the exact props, change any of them and you can change how it performs, for better or for worse.

Most people go with a bigger single than smaller twins for the reasons already listed.

 

Don't know why, but this made me think of the set up used by Navy Seals now days, I think they have twins with a 26/28 foot RIB, I know there is a larger one as well, but not familiar with the set up now days. I would think they would have the best set up possible for the purpose at hand. Anyone know?

 

They may need to think about being shot at, if one motor is disabled you still another one. Plus a 28' RIB may be able to use more HP than any current single motor can supply.

 

Military spec is usually quite convoluted and redundant. I wouldn't base anything off of military offerings, as you can rest assured it will cost twice what it needs to and be overly complex, using ridiculously redundant parts. For example for a VCR to pass military spec, it needed to survive and still operate after a 100' fall. Now I imagine this is a good thing for clumsy fingered helicopter loaders, but ridiculously over engineered for the vast majority of it use. Have you ever priced a water proof SPST 12 VDC switch that meets military spec?

 

Like Marshmat I'm not fan of big outboards. I do not see the utility and advantage, except of relieving the owner's small ***** syndrome.
They are so heavy that they are as hard to move than an inboard engine, they are not efficient with the raw water cooling (plus corrosion problems) and the too small propellers. And they are more now expensive to buy and to maintain than a equivalent inboard engine.

Stability problems of planning boats come generally from poor design; center of gravity, center of planing and shape of the hull. A possible stability problem with a single engine is the lateral torque induced by the propeller. The boat will slide laterally from the stern, the skipper over-corrects and a "snaking" may start. It's cured with a slight lateral angle at the expense of a new loss of efficiency. On big outboards as most of the power is used to make foam in the water is not a big problem, and the effects of prop torque will be seen in the marina while docking (very funny to observe...).

On small boats with twin outboards and small very fast propellers there is not notable difference between not-counter rotating, supra divergent or convergent rotation of the props. On fast patrol boats it's another matter...

When you need 350 ponies, a single inboard with fresh water cooling (FWC) well protected under the hood, with stern drive and dual prop, is an interesting option. The global efficiency of the motorization is better, it's less expensive, and it's easier to maintain at least for the engine.

 

PAR

"The extra lower unit generates more drag"

Why would two small lower units have more drag than one large one?

Easy Rider

 

Counter rotating props are the norm here in australia with all the motors available L or R hand rotation. In South Africa you must have 2 motors each capable of planing the hull if you go over 5 miles offshore.....it will come down to $$$$$

 

There's physically more surface area on two 100 HP legs then a single 200 HP leg. Typically a 200 HP leg will be about 20% - 30% larger then a 100 HP leg, which is considerably smaller in total area then two 100 HP legs.

Ilan is absolutely correct in larger HP ranges, you desire globalization control of propulsion elements, not division in small craft.

 

Yes the drag issue is large
When you look at any ofshore race boats that would have 2 v6's and you add a third, ( maybe to move to another class) thats a 50% inrease in power, it will only add a few mph.
What twins can always do is get the boat on the plane when it's overloaded when the single wouldn't

 

PAR surprised you would say that, Merc OMC and Yamaha have had counter rotation gearcases available for V6's since the 80's
Smaller engnes came a lot later and only the next size down as making the counter rotation gearbox is not a very neat solution hence you dont see them on small engines, counter rotation powerhead is the better solution but none do that anymore.
I would recommend every twin rig to be counter, much lesss steering torque and no prop torgue trying to lay the boat over to port when planning.