Theoretical question on boat speed with 1 or 2 motors

[KC8QVO]

If there was a choice between having a single vs twin motor set up, for matched hp, what set up would run the boat faster (same boat, just 1 or 2 motors)? Example: one 125hp or x2 60's on a 20ft aluminum boat. I know there is a lot of left out info and variables. 2 motors will weigh more than 1, 2 motors probably will burn more fuel than 1, but if one motor quits with 2 you still have one and with 1 you have no way to get home. Just curious

 

[Frank Acampora]

Common wisdom says that a single engine will be slightly faster than two engines totaling the same horsepower. Fuel usage should be close---it takes a certain amount of fuel to generate a certain horsepower and go a certain speed. However, as you said in cases of emergency the redundant engine will get you home. Then again, given the reliability of today's engines and the availability of pre-paid towing services, twin engines are not necessarily an absolute necessity for long trips. From a strictly aesthetic viewpoint, twin engines sound way nicer than single.

 

[KC8QVO]

Towing services are great on the great lakes or coastal areas, but there is no such thing on remote waters. When the other boats you see are in the same situation - running on limited resources - it wouldn't be real easy to get a courtesy tow. Spare fuel and, a spare motor, is a more secure option. I was just throwing the idea around instead of a kicker just split the mains.

I would think 2 lower units in the water would make more drag, but with 2 props pushing I guess that is where I'm at a bit of a loss. When I get some time I will see if I can find out what the drag is of the lower units and thrust is. When the thrust balances the sum of the drags the speed is constant, just where that speed lies is the question. I would imagine the fuel burn rate would be more because an engine is not 100% efficient. If you have a 50% efficiency (energy in [fuel] vs usable energy out [boat going forward] - all factors taken in to account - heat loss, prop slip, etc) in each "motor" you have 25% total efficiency. I am not sure where the available thrust is, either. With 2 motors you can crank the pitch up because each one has half the load so that may get the speed up a bit too. Lots of questions.

 

[Chris1956]

Two engines weigh more, burn more fuel, have twice the maintenance and will run slower than a single engine of the same HP. If you have twins set up with independent wiring, batteries and fuel systems and tanks, they shouldn't break down at the same time, allowing you to get home.

If you boat in an area with a good towing service, stick with one engine. If you go to the ocean canyons or other places far offshore, get twins as described. Remember a VHF radio is limited by antenna height and transmit power. It will also not work over the horizon. Cell phones have much less range.

 

[mrdancer]

The single engine would be faster, unless the hull is specifically designed for two engines. A prop is most efficient when run in clean water, which is typically where the hull is deepest in the water. A catamaran would probably be better off with twin engines.

Additionally, as pointed out above, the drag on the motor can be significant, especially at higher speeds (racing lower units are very thin).

I would opt for a single large main engine, plus a small kicker. The kicker can be used for trolling, low-speed cruising, and as an emergency limp-home backup if the big motor did go down. Ideally, the kicker would be able to run with no battery.

 

[midcarolina]

I would opt for a single large main engine, plus a small kicker. The kicker can be used for trolling, low-speed cruising, and as an emergency limp-home backup if the big motor did go down. Ideally, the kicker would be able to run with no battery.

+ 1

 

[dingbat]

A "small" kicker is pretty useless offshore. Even if you somehow managed to get back, good luck navigating the inlet

 

[midcarolina]

A "small" kicker is pretty useless offshore. Even if you somehow managed to get back, good luck navigating the inlet

Tell that to the hundreds if not thousands offshore fisherman that have made it back due to a kicker................And the word " small " is a relative term, most kicker hp is based on boat size/weight

 

[KC8QVO]

A "small" kicker is pretty useless offshore. Even if you somehow managed to get back, good luck navigating the inlet

Can you explain? On what grounds to you base a "small kicker" as being "useless"? When I make my rounds on the lake I have a 9.9 as a spare, 16ft open tiller boat with 25hp main. My travels are all freshwater lakes also.

 

[smokeonthewater]

Yes twins are always slower ALSO keep in mint that if you want to come back on one of a set of twins at much more than no wake speed you will need to swap props at sea to a much lower pitch..... example my twin I/O tops out at 48 mph but on one engine I can't plane and top out IIRC around 12 mph or so SERIOUSLY lugging the engine and would likely overheat it in a matter of minutes if I persisted tho it will do 8 or 9mph at 1/4 throttle or less

 

[ssobol]

After hitting something, I would think that the next most common cause of engine failure these days would be a fuel supply problem. Most twin engine installations are supplied from the same fuel tanks with fuel purchased from the same place. So a problem with one engine will likely manifest itself in both.

Modern outboards have a number of "get home" modes that can allow a malfunctioning engine to run a low speed to provide some form of self rescue. Of course, this won't help if there is no good fuel supply.

A single engine and appropriate sized kicker motor that has a separate fuel supply and can be manually started is probably the most efficient redundancy solution. Even if the kicker can't get you all the way home, it'll probably get you somewhere suitable or where you can call for a tow.

 

[Frederic Lynes]

When I was a kid, we had a Sundancer 270 with twin 5.7 260 Mercs...it was pretty damn fast. But, in keeping with the OPs question, I've never seen a single 520HP engine, so I couldn't say whether it would be faster.

 

[KC8QVO]

ALSO keep in mint that if you want to come back on one of a set of twins at much more than no wake speed you will need to swap props at sea to a much lower pitch.

I thought about that too, good point.

I'd still be curious what dingbat's thoughts are for what would be considered a "small" kicker. From what I have seen on remote kickers they start around 8hp and I even saw a boat once with a 40hp kicker (38ft, twin 350 mains). The boats in the range I'd consider, though, all seem to have 9.9's, some 15's but very few.

That also having been said, I have a 9.9 I carry as a spare in my 16ft boat. Last summer I ran it just to see how the set up would run and it would surely get me home. If I didn't have the 25 on the transom (instead removed and put up further in the boat) it would have run even better. I don't think it would really get up on plane, but close, and that is with a non-ideal prop (too high pitch).

 

[frantically relaxing]

Back in my youth I bought this '73 Reinell, 24' hardtop, which had a pair of 240 hp 318 Chrysler V8's w/Chrysler (Volvo) 270 drives and 21/15 props. I could only get the speedo to 48 mph. Even tho that's not fast by today's standards, not many boats outran me. But that was thee best ski boat I've ever owned, those V8's would yank your arms out if you weren't ready! Bit of a gas hog too, 1.4 mpg on a 30 mph trip from Wahweap to Bullfrog. And no, it would not get on plane on one engine. But it would get you home. Eventually.

 

[bobdec]

No clean answer , depends on the boat..putting other safety,, etc concerns aside . In answer to OP efficiency basic question the numbers vary but I have read there is a 15%-30% efficiency loss with two engines verses using a single engine of equal HP. The percentage will vary by type of propulsion, inboard, I/O or OB, gas , diesel, hull design and size of the boat.. Factors to consider are prop pitch efficiency, cavitation differences (prop placement), lower unit drag or rudder drag, weight of 1 Vs.2 power plants including batteries, props, shafts, transmissions, coolers, etc that will vary w/type of propulsion. My guess is the 20 aluminum he mentioned would be more efficient with a theoretical single 120 than two 60's by a measurable percentage..

 

[roscoe]

When I was a kid, we had a Sundancer 270 with twin 5.7 260 Mercs...it was pretty damn fast. But, in keeping with the OPs question, I've never seen a single 520HP engine, so I couldn't say whether it would be faster.

Here you go, 557 hp each.



https://www.youtube.com/watch?feature=player_embedded&v=xoYJ_4VzUNw

 

[bekosh]

Merc Racing sterndrives start at 520hp and go up to 1650hp.
http://www.mercuryracing.com/sterndrives/

 

[dingbat]

Can you explain? On what grounds to you base a "small kicker" as being "useless"? When I make my rounds on the lake I have a 9.9 as a spare, 16ft open tiller boat with 25hp main. My travels are all freshwater lakes also.

Different experiences, different perspective. The deer hunter sees the .30-30 as a powerful weapon. The elephant hunter sees it as a joke.

I sometimes use a 25 hp kicker while trolling in the Bay. The winds have to be 10 kts. and below or it doesn't have enough thrust to keep the boat on course at my trolling speed of 2.5 to 3.0 kts. This past Spring, I fished 12 of the 15 days of Trophy season. I was able to use the kicker twice. Something that might work 20% of the time doesn't give me the warm and fuzzes.

Tell that to the hundreds if not thousands offshore fisherman that have made it back due to a kicker................And the word " small " is a relative term, most kicker hp is based on boat size/weight

As someone who spends a good bit of time offshore, I think hundreds, if not thousands of my colleagues would agree that offshore is no place for a "small kicker".
Unless you can plane the boat, your at the mercy of the sea. Not a good place to be in a place so unforgiving.

Case in point.....we blew a fuel pump on one of the motors coming in from the Washington Canyon (90+ miles out) 2 weeks ago. It's took us almost 6.5 hours to come the final 20 miles using a single 150 HP outboard on a 24 ft. boat. "Plowing" our way back burned a pretty big hole in our fuel supply on what most would consider a "calm" day. Had to throw anchor outside the inlet and wait for the tide to subside before attempting to run the inlet.

Don't know about Mid-Carolina, but around here, a 150 HP outboard isn't considered a kicker and we struggled as it was.

 

[midcarolina]

The general point was that basic common sense dictates that one would rather have some propulsion opposed to NONE...............I never said it would be a fun timely ride.............

On another note if you could only manage 3 miles an hour with a 150 on a 24' boat, then you have other issues with the boat!

 

[smokeonthewater]

On another note if you could only manage 3 miles an hour with a 150 on a 24' boat, then you have other issues with the boat!

The ONLY issue they had was that they were propped for twin engines and had to come in at just over idle speed (because there was no way they could plane out) to keep from seriously lugging their one remaining engine.... lugging the engine can destroy it in short order....

I would BET they were also dealing with wind waves current etc... all at displacement speed ... That can be a REAL PITA


Depending on the exact boat and load.. 150 hp MIGHT not plane a 24' boat even if propped correctly