Ganging Outboards - Does it work?

Discussion in 'Propulsion' started by CatBuilder, Jan 14, 2012.

  1. CatBuilder

    CatBuilder Previous Member

    I have a theoretical question.

    Does ganging outboard, like this, work?


    If I were to gang four 10HP outboards on the back of a boat, would I get the same forward propulsion as a single 40HP outboard or a pair of 20HP outboards?

    There would be 4 props in the water. Would that increase efficiency over a single prop, with a decrease due to 4 outboard legs dragging?

    Does the above picture have the same power as a 900HP engine would?

    What about the spacing between props? Are there minimum clearances from one outboard to the next?

    Would this work as well on a displacement boat with lower gear ratio outboards?

    What are the dynamics of an install like this?
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Conventional wisdom, which seems to be well supported by experience, is that for a given total power fewer outboards will result in higher speed due to less drag from the lower units. I assume there are exceptions.

    Would a single 900 HP outboard be better than three 300 HP outboards? Probably if there was a 900 HP outboard available with space for a suitable prop. Highest power "production" outboard to date appears to be 557 HP from Seven Marine.

    Triple outboard installations are fairly common in some areas and quad installations are not that hard to find. Outboard manufactuers provide gauges and controls for such installation.
  3. michael pierzga
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    michael pierzga Senior Member

    You get a weight penalty.

    2 times 10 is heavier than one 20.

    2 times ten uses more fuel than one 20.
  4. cor
    Joined: May 2008
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    cor Senior Member

    I think it depends on what you're looking for. If speed is what you are after then one motor is better, less stuff in the water causing drag.

    If you were trying to pull water skiers then you would probably be better with more propellers and fins in the water. Ski boats often use multiple motor setups.

    I think that outboards are so limited on prop size that maybe spreading the load out onto two props may get better effiecency?

  5. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    Some of the small outboards don't have very good silencing compared to larger models. There may, or may not, be a noise penalty. I 'spose a dB meter could be useful. It seems engines with more cylinders tend to be smoother. Harder to predict are structure borne vibration and noise issues.
  6. CatBuilder

    CatBuilder Previous Member

    Ok, time to open up the can of worms after reading the responses. Thank you for those.

    The application is not speed, but 8-10 knots motoring, pushing an easily driven, 45' catamaran with a max load displacement of 9 tons.

    A sistership to this one has been using a pair of 18HP Yanmar diesel outboards for the past 10 years or so and they are happy with them.

    It's for the catamaran I'm building. It has dawned on me that if I am already going with outboards and already have a large, diesel generator aboard, why not just run a gang of Torqueedos off my generator, thus eliminating fire hazards (at least gasoline ones) and adding a small level of redundancy?

    Torqueedos come in 9.9 max HP, but I note they have nice, wide props.

    A gang of 4 (2 on each side of the catamaran) might be a good way to power the boat electrically without all that weight associated with inboard diesel/electric, straight diesel and even weighs less than the normal outboards I would have been fitting.

    The logic and advantages seem just overwhelming. No gasoline tank, yet I still have outboards.
  7. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    An internet avoiding older friend was interested in these..I priced out the 6 hp ones @ $3k each and IIRC the 9 hp ones were above $4k.

    So you're looking at ~$16k or so.
  8. CatBuilder

    CatBuilder Previous Member

    Aw. I thought it was 3k for the 9.9. Sigh.

    The other regular outboards were about 12k.
  9. WestVanHan
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    WestVanHan Not a Senior Member

    One on ebay for $3800..says it's new.

    There are Yanmars out there,if you know where to look.
  10. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Keep in mind that electric outboard power numbers are based on what the manufactuer claims a generally not identified "comparable" gasoline outboard's rating would be in some situation, not on a directly measured power. Gasoline outboard power ratings are generally based on the maximum power at the propeller shaft.

    Looked at the Torqeedo website today and their most powerful single unit is the Cruise 4.0R While they show 8 - 9.9 HP for this motor I don't think they are claiming that as the actual power at the propeller, which is what a power rating for an IC outboard generally means. The technical data page for that motor shows:
    Input power in watts 4000
    Propulsive power in watts 2240
    Comparable gasoline outboards (propulsive power) 8 HP
    Comporable gasoling outboards (thrust) 9.9 HP
    Maximum overall efficiency in % 56
    Static thrust in lbs 189

    4000 watts of input power is 5.4 HP of input power. The shaft power has to be lower than the input power. If the controller and motor are 90% efficient than the shaft power would be 4.8 HP.

    2240 watts of propulsive power is 3.0 HP of propulsive power. Power at the shaft would be propulsive power divided by propulsive efficiency, and less than 5.4 HP.

    Bollard (zero speed) thrust may well be equal to that of some 9.9 HP outboards since the Torqeedo has a relatively large propeller which appears to have an efficient space. But I doubt it would have the same thrust as the Yamaha High Thrust 9.9 with a 11-3/4" diameter propeller.

    Likewise it may have the same power/thrust at some, probably relatively slow, boat speed as some 8 HP outboard. But I would be surprised if it's equivalent to half an 18 HP Yanmar Diesel outboard.
    Last edited: Jan 14, 2012
  11. CatBuilder

    CatBuilder Previous Member

    Ah, so these aren't rated as reliably as the regular electric motors I was looking at, which far exceeded combustion engines at the prop based on input energy ratings?

    What about this thing, powered by a pair of the smaller Torqueedo 2.0's? 32ft power cat.

    You may, however, have to disregard the commentary... ha ha ha.

    From what I recall from inboard electric motors I did the math for, a 5.4HP input rating is about equivalent to a 9.9HP rated ICE. Remember, these are very different ratings, measured at different places. I believe my example from the old electric inboards was that a 12-15KW (16-20HP) input electric motor, used to directly drive a prop, is equivalent to a 30HP Yanmar diesel in thrust/shaft power at the prop.

    So, the ratios are pretty much the same for these Torqueedos.

    I think, DC, you might be confusing shaft rating at the crankshaft with shaft rating at the prop shaft. Internal combustion based inboards and outboards are rated for crank shaft brake testing. They deliver much less than rated at the prop shaft. Electric, being directly coupled to the prop shaft (prop shaft is "crank shaft") have lower HP/KW power requirements to achieve the same power at the prop.

    Also, I think I read the Torqueedos have 16" props. Tasty! :)

    Not really arguing for them, and would like to see why not to do it, but I am not seeing a lot of reasons they wouldn't work, except from WestVanHan! ha ha ha ;)
  12. CatBuilder

    CatBuilder Previous Member

    I don't know where to look, but would of course be very *very* interested.

    I thought they stopped making and supporting these Yanmars, so there were no part available. Is that not true?

    That would be my first choice - maybe - except they're bound to weigh a lot more than gas outboards and certainly about twice as much as these Torqueedos.
  13. masalai
    Joined: Oct 2007
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    masalai masalai Sanctuary Magazine.pdf
    At the Australian website on Torquedo are several applications "Schools Out" a Bob Oram 44ft sailing cat had 4R Torquedos, - - as a pair and worked briefly but seemed to have problems with the long control links which were adversely affected by the 12M loom and long leads for the power supply from the battery bank forward of the mast with voltage drop and digital signal issues - he now has a 60hp outboard in a nacelle midships, under the aft duck-board...

    I was going to install torquedo's, had the batteries (48V agm,s) and generator to charge the agm bank at 6000 watts - reconfigured the batteries to 24 volt for house, and the generator is still in the garage... (cost me Au$12000 for the genset using a pair of 24pole delco alternators and a special programmable controller)... Electric is not quite mature enough for a Live-aboard-cruising-yacht, in my observations and experience...

    I came across the 27 hp diesel outboards, but they were heavier than the 50hp saildrive Nanni setup, and had to be specially imported as nil stock were carried in Australia... - I ended up selecting 20hp Nanni saildrive and now with larger fixed 3 blade propellers, get fairly reasonable performance from my 40ft of sail-less 'sailing' cat of 4 to 5.5 tonnes (depending on fuel and supplies)...
  14. CatBuilder

    CatBuilder Previous Member

    Masalai: Now that's what I was looking to hear. What year did Schools Out put those Torqueedos in?

    So they are not up to quality standards as compared to petrol/gasoline outboards?

    I was hoping to get some input from actual Torqueedo people (or second hand info like you provided) who have had problems with them. Thanks for the great post.

    I was most of all, worried about leaks into the motors. Secondly, I don't even know what type of motors they are. Brushless? I hope!!

    The battery power issues were a concern for me as well. I was wondering how pure power they need and was worried I could not get those remote controllers up to my helm station (near the mast) without signal degradation. The batteries would be located within 6 meters of the drives and the generator would charge the batteries and supply instantaneous power through the DC system directly to the Torqueedos for diesel/electric running.

    As I've mentioned before, I need a whopping diesel generator to power air conditioning and heating. The Torqueedos seemed right, in theory, but I don't know about the reliability.

    If they are anything like my German Mercedes Diesel Sprinter, they will have lots of little faults to chase down and fix, but will actually run very well once you work the bugs out. :)

    Thank you for the post. That's some of the direct input I was hoping for once I "opened the can of worms" here.

  15. masalai
    Joined: Oct 2007
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    masalai masalai

    About 2 to 3 years ago... The Quality appeared to be very good, but the technology had issues for long loom distances and battery supply lines (batteries forward of the mast - see the link - -) and controls similarly way forward and complex cabling allowing interference across engines and via capacitive/induction issues in the long cable runs... resolved partially with the placement of NiFePO4 or "whatever" batteries close to the engines, but then voltage loss and charge management became an issue - is all that I know - rather garbled & second-hand was all the explanation I got - so I deduced long control circuits and distant placement of chargers were not viable YET... The engines seemed quite OK, but I figured 4 would have been desirable for me and the cost then became prohibitive when batteries, charging and charge controllers as well as placement of throttle controls... - Not a plug and play set-up for LARGER boats yet either...

    The actual motors are 'tiny' and brush-less (something like this ) and have a planetary gearbox between motor and propeller to reduce the motor revolutions down to 1000 at the propeller (at a wild guess about 10:1 reduction?) sail-drive leg seems well built and sealed for a long and trouble free life - see comparison and maintenance schedule on the Sanctuary Cove security patrol boats Sanctuary Magazine_1.pdf (a pdf file)... Similar electric motors may be found in suppliers for electric motors to model boats/aircraft that draw 4000 watts peak...

    I am still having issues with my DIESEL installation... - Mostly attributed to the long 10M extension to the engine control loom, and the voltage drop for the 22m long circuit controls... Another thing is the stray electrical leaks between INDEPENDENT ENGINE systems because of the common earth via the throttle and gearbox cables which are METAL !!! :D :eek: - So separate the throttle/gear leavers, (do not use "twin controls"), if your engines are to be isolated from each other....

    The batteries (LiFePO4 are not heavy for the Ah capacity and can take HUGE amperage for charging... Contact Bert from SA, or a few of the others into that technology, as the batteries DEMAND a special charging regime) should be mounted VERY close to the motors and the control cables likewise kept very short... - - Throttle controls midships between the engines...

    An option could be to run 'mains AC power' close to the engine batteries (one for each engine) with an automatic individual charger for each battery to run off your mains electrical power... That could work nicely (Pity USA uses 110VAC as European & Australian mains AC is 220/240V and carries greater wattage on the same cable pair... I guess a capacity to pump 6000 watts or more to each battery would be nice... - 2 x 4000R motors may suffice but more would be desirable for redundancy and ??? expect no more than 6 knots...
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