One or two motors (split from 'Is there a valid reason for a pony bottle')

[SomeGuy509]

I find the comparison to airplanes interesting since I come from an aerospace background. The older tri-jets existed largely because engine reliability had not reached point where you could safely go over the ocean with just two engines. Yeah there were some thrust considerations but a lot of it was redundancy.

From a maintenance and cost and fuel burn standpoint there is no question that 2 is more cost effective then 3 or four. If that translates to boats I don't know. I suppose there is a reason some large container ships seem to have a single massive engine rather than multiple smaller ones.

Even now a lot of having two engines is to provide redundancy in the even of the loss of one. You still have hydraulics, power, etc. You then light the apu in case you lose that one too. If all 3 fail you still have the RAT (ram air turbine) to provide bare minimum to control the plane as a glider.

So I guess I would liken a modern airplane to a set of manifolded doubles. And then an engine failure would be like losing a first stage. You shut down that post but still have most of your functionality and and all your gas.

For the plane you can still complete a takeoff and land and even fly for 3+ hours in some cases with one engine out. You are still getting your ass back on the ground as fast as you can though.

 

[Darnold9999]

Am firmly in the camp that wants two engines on a boat. Have been on an 18 foot boat in the middle of a shipping lane with a dead engine. Not an experience I want to repeat. Current boat has twin 350 Chev engines. Poor fuel economy running just one as the other prop provides significant drag and docking is almost impossible on one engine. However have lost a coil on one engine 30 miles from anything including radio contact (Desolation Sound) and made it back on one engine.

Not one engine with a redundant spare however. Boat does not work properly on one engine. If I had a single screw and then a kicker for trolling then the comparison to a pony might be more accurate.

Have also run out of fuel and had both engines die within seconds of each other. Two engines does not solve everything but for the things it does solve it is nice to have. (Running out of fuel was on the trip back to home port after buying the boat. Discovered the fuel gauge was not accurate and fuel consumption was WAY more than expected). Ran out 30 feet from the dock. Paddling a 10,000 pound boat must have looked pretty funny from shore but we made it.

I don’t even touch the wheel when docking, at low speed the rudders have almost no impact. Throttles only - with two engines throttles give way more control than rudders.

 

[clownfishsydney]

Only just saw this post. As you can see from my photo at left, I have a 6.2 metre catamaran which I use solely for scuba diving. It has twin 75hp 4 strokes. It cannot have one engine, virtually all catamarans are made to have twin engines. I can think of only one make that uses a single engine, and that is built differently to the others and cannot have twin engines.

Many advantages of twin engines, separate batteries, separate fuel tanks, separate controls. Even if steering fails on one, can normally use the other. Of course, as mentioned you do have redundancy, but as pointed out, even flat out on one engine, I can only do about 8 knots compared to normal 23 knots (with five divers and gear).

Disadvantages? Yes, there are some. Costs more to buy twin engines than one of same combined horsepower. Fuel use is probably more, servicing costs are doubled.

Summary, I would not have a single hulled boat, even with twin engines, as the catamaran has more room and is more stable in most seas (not as good in following swells).

 

[2airishuman]

Outboard motors are complicated things.

Having two of them means that there's twice as many things that can go wrong.

My boat weighs 7,000 pounds. It has an 8hp outboard as a backup. The primary propulsion is sails.

I also have a canoe paddle as a backup to the backup.

 

[MaxBottomtime]

And a tiller as a backup to the backup to the backup.