Single outboard stability

I have to say I was rather surprised at the mentioned rarity of counter-rotating twins too. I can't recall seeing anything but CR's on new rigs for many years.
We replaced the two 200hp mercs on our 23' Formula with a pair of 225 counter-rotating dfi mercs some years back (not to overcome any small ***** syndrome thanks IV !) and it transformed the boats handling. Just as important was the improvement in low speed maneuverability.

I disagree with the lack of enthusiasm for large outboards too.
They are lighter (a merc optimax 250 weighs 229kg, vs 450kg for a 260 hp sterndrive, for instance)
They free up enormous amounts of cockpit space &/or allow a lower cockpit sole.
And these days, they don't use any more fuel - in fact as often as not, they can use less.

For many of the reasons listed by others, I'd go for a single over twins...

 

Yes, outboards are lighter but the the center of gravity of the motorization is placed far behind and very high. With small (until 150 HP) outboards the total weight of the outboards plus the weight of gas remains reasonable compared to the total displacement.

It's not the same with big outboards. You'll have weights high and far aft from the total center of gravity of a small boat, thus augmenting the polar inertia and the necessity of amortizing this mass. That means that the stern will have to be designed wider and deeper. Not the best way to get good hydrodynamics even on planing boats as the power needed to start planing will be greater (bigger and "sharper" hump in the resistance curve).

I see 3 other drawbacks in big outboards;
- Raw water cooling with seawater inside an engine is not the best for the corrosion.
- Raw water cooling means than the engine will run "cold" as the seawater must never reach a temperature of 50 celsius degrees, or you'll have salt deposits inside the engine.
- The propellers are definitely to small and too fast spinning for a good efficiency.

I made for a client, owner of a dive shop, a study about motorizations, gas or diesel consumptions, reliability, TBO, duration of the engines and the financial consequences. His boats work about 1000 hours/year each. The gas is rather low quality.

I won't explain how I made the study. I'll give just a few results. The results are valuable for this particular case, and in the conditions prevalent in the Caribbean. Some brands were eliminated because locally known problems of reliability like internal corrosion, blowing head gaskets, injection problems and other annoyances.

In gas engines the Mercruiser 5.7L and 6.2L, with FWC (fresh water cooled) and injection with BRAVO II stern drives using the biggest propellers are the clear winners for the specific consumption; around 210 gr/HP/hour, and sometimes less. And even counting the price of the installation of a inboard engine financially it was the best solution over 3 years or 3000 hours.

A boat, a panga of 30 feet, which had 2 Yamaha Enduro of 150 HP before, had a 5.7L installed with a "big" 4 blades propeller. The study results were confirmed. The measured specific consumption was 205 gr/HP/hour and all the costs of the new engine plus installation were paid in less of one year of gas savings... The Yamaha 2S carburetted are terrible gas suckers, it was easy to do better, but even compared to 4S outboards of similar HP the over-cost is paid in 2 years max.

Other fact, the cost of maintenance was lower and the engine at 3000 hours was running well, so the Time Before Overhaul (TBO) is better than expected (the 2 Strokes have the greatest difficulties to go over 1200 hours before overhaul, that eliminates the Optimax which has also a very "sensible" injection system). I think that a 5.7L or a 6.2L has a good life expectancy if it has good accessories.

The stern drive BRAVO II asks for a careful and precise maintenance. At 3000 hours it needed to be taken apart and totally revised. Stern Drives are complex mechanisms with a lot of gaskets and articulations... That's the main drawback.

The 30 feet Panga has also a better sea-keeping, better top speed, gets out of the hole and stars planning sooner even overloaded with divers.

Lateral advantage of the inboard engine: the engine is enclosed, out of weather, and most important out of reach of mechanics killers, gas stealers, thefts and other saboteurs. Very common animals in this part of the planet

 

For a good anecdotal answer, look no farther than the US guard; they now operate what is by far the largest inventory of twin outboard powered vessels

(all CR of course..I've personally never seen a twin OB setup that was not and having owned many..I'm quite sure I would never care to see a setup that was not CR either..but I digress).

I would be interested to know to what extent they (USCG) overhaul their outboards..if at all. I could see a lot of reasons why wearing them out and simply replacing them would make a lot more sense, with the fact that craft are scattered far and wide and any facilities that can overhaul their outboard engines for them are scarce. But..three station crewman can swap out a pair of outboards in two watches. Hmmmm....how is that inboard looking now?

Me ...I like all three flavors (the IB-waterjet..the IO..the twin OB). Each has its place.

 

Ilan
Intersting study, thanks.
Just a couple of points the thermostat in an outboard and a stern drive are the same setting, 143F you still have the same problem if the sea water in your heat exchanger gets to a hot surface the salt will come out of solution as you say.

Except as you point out with a bravo II the prop is bigger all other stern drives have the same diameter as the V6 outboards have ratio's so they do end up spinning at the same rpm.
The whether a bravo 1 or 2 prop diameter will do a better job is just the load it must carry
A 30' dive boat for sure a Bravo 2
regards

 

USCG uses outboards with the goal to have a minimal number of boats not working for engine reparations or maintenance. The mechs exchange immediately the faulty engine. The Navies do the same for the fast intervention boats. It's similar to the system used on the planes like fighters or interceptors.
The budgets have nothing comparable with common man budget...the task is not the same also.

 

Yes Powerboat the thermostats have the same setting, 143F or 63 Celsius. But after the outboards runs "cold" while an gas inboard with exchanger runs at 92 Celsius (198F), the same temp as a car. For better efficiency an engine must run as hot as possible, adiabatic being the unattainable ideal.

Remains also the problem of a highly corrosive liquid (seawater) inside an engine and the electrolytic problems as you'll have always different metals bathing in seawater. I do remember 100HP Hondas badly damaged by the corrosion in 2 years. Here, in summer the seawater is at 29-32 Celsius (85-90F)...
Also FWC uses not water but an anticorrosive and antifreezing liquid (like the cars). If you use a cupro nickel exchanger and electricity no-conductive piping (the black rubber pipes can be conductive as they have a great amount of carbon) I can insure you that the internal corrosion of the engine becomes close to zero.
Add a good heat exchanger for the oil and overheating problems are just a souvenir even with air at 100F and seawater at 90F.

Here the 250 HP Suzukis explode their head gaskets in summer with the water passages in the heads clogged by the salt. And with the quality of the gas the Mercury Verado have a life expectancy of about one to two months...as unhappily some people experienced it at Cancun.

The lone brand that withstand is Yamaha, and they make 90% of the outboard sales.
And now we stop hijacking this thread...

 

IV - I'm not suggesting for a moment that ob's are superior to sterndrives in every application. But both have their applications... horses for courses as they say.
As for the CG argument, it's not as simple as that. Yes the mass is further aft, and higher, but of course there's also less of it, so depending on the installation, the moments can often be similar.

Now, as you say, enough thread diversion...

 

the thread is called 'single outboard stability' i would have thought any arguement re twins or sterndirive is relavant to the poster?
( assuming the poster realises some boats are made for twins and some are made for stern drives)

 

Here, 90% of the 200 -300 Hp commercially used outboards are leased.

-Tom

 

Just a quick thanks to everyone who replied to the post, your suggestions and opinions are greatly appreciated. Not sure what the etiquette is for these forums but it seemed rude not to give thanks.

 

Hi Ilan
Just wondered how the BRP Etecs do down there as you didnt mention them?
Cheers

 

Almost enough diversion. ;-) Your post reminded me of a conversion I did some dozen years ago. I took a pair of 140HP Volvo inboards out from under the largish center console on a 25' deep-V Rampone. Removed shafts, struts and the transom-hung rudders and filled all the holes up. I bolted a transom bracket on the stern and hung a pair of Yamaha 150HP outboards (2-stroke in those days) on the bracket. The trim changed so little, both running and still, that I was astounded how little.

Top speed went from 28 knots to 42 knots, although the most economical cruising speed remained unchanged at about 24 knots.

Just a data point...

 

BMcF,

How did the stability and seaworthyness fair after the conversions?

-Tom

 

Lateral stability was not noticeably affected but might have decreased a bit; that said, the deep-V Rampone hull (same as Formula 233, Sea Vee, etc) was never noted for its low-speed lateral stability anyway and were quite tender and tippy at low speeds. Once up on plane..like a rock. My boat had a full fishing tower and it was a delight to drive such a relatively small boat in the rough stuff whilst up in the tower.

Seaworthiness completely unaffected...and I can't think of any reason it would have been affected one way or the other by going from inboards to outboards. The Rampone is/was always a capable offshore boat. Although the top speed was increased quite dramatically as a result of the OB conversion, I seldom ran the boat much faster than the old Volvos would carry her; nothing like a set of fuel flow rate meters and a totalizer staring you in the face to temper your throttle hand a bit.

 

Interesting,

Thanks

-Tom