an outboard question

Discussion in 'Outboards' started by boatenthusiast, Aug 3, 2015.

  1. boatenthusiast
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    boatenthusiast Junior Member

    i know with inboards some hp gets lost through the shaft so a 300 hp engine isnt rlly giving 300 hp, does the same thing happen to outboards?
     
  2. FMS
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    FMS Senior Member

    After 1985 outboards are all rated at the propshaft.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Frictional losses are simply a matter of fact with any and all mechanical devices.
     
  4. boatenthusiast
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    boatenthusiast Junior Member

    thanks
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Apt reply, and it should also be noted that there is leeway allowed in the difference between the stated output and the actual, from memory could be + or - 10%. So a 150 could really be 140 or 160 and still be within the limits. So it is impossible to accurately discriminate between engines hp without testing them side-by-side. The label on it is only a rough guide.
     
  6. powerabout
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    powerabout Senior Member

    I would have to add that in the last few years large outboards have at least as much as 10% which is meant to be the allowance
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    In the past there have been outboards of 90 hp that didn't perform much different to the 120-140 in the same brand line-up, except the higher hp one would rev more freely into higher rpm, with different porting and carbs etc, but at cruise revs there wasn't much difference, except they sucked more gas !
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    The main problem of efficiency on outboards is the size of the propellers, so the losses are in their propellers. The limitations of the cooling system (direct salt water so the engine must run"cool" to limit the salt deposits) lower also the global efficiency.
    Outboards have comparatively small diameter propellers for their power and thus can't get good efficiency ciphers compared with other transmissions as for example the Bravo2 or Bravo3.
     
  9. powerabout
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    powerabout Senior Member

    Outboards are not designed to shift houseboats
     
  10. CDK
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    CDK retired engineer

    Since the OP had his question answered, hijacking this thread doesn't seem impolite to me.

    My Yamaha F100 came with a 13x19 straight edged prop that needed 4000 engine rpm for stable planing at 20 knots and could reach 42 knots at 5500 rpm.
    To reduce noise I installed a 13.2x21 Solas prop. The mileage is somewhat better and the engine rpm is now 3300 for stable planing, but top speed is reduced to 32 kn. @4200 rpm.

    I know the engine now cannot deliver all 100 horses, but does this harm the engine or its gearbox in any way?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    4200 is very low, and a sin in the eyes of the engine makers. Reason being it loads up the innards in much the same way as using too high a gear in climbing a steep hill in a manual transmission car.
     
  12. CDK
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    CDK retired engineer

    Nice comparison!
    I've always believed if you can accelerate in a climb you have selected the right gear. We use the RIB with 3300-3500 rpm because of fuel efficiency, or around 1500 rpm if the waves are too high to plane.
     
  13. Ilan Voyager
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    Ilan Voyager Senior Member

    Are you sure of your ciphers? 42 knots is 1296 metres per minute, at 5500 RPM that gives 2423 RPM (gear 2.27) at the propeller, 1296/2423=0.54m per revolution WITH NO SLIPPAGE, it's a 13x21. If you count 10-12% of slippage that gives a 13x23.3 for the propeller on the outboard, but you have a 13x19. Except for a miraculous over-gripping propeller it's impossible.
    If you calculate the theoretical speed with a 13x19 with 10% of slippage that gives (2423*(19*2.54/100)*60)/1852)*0.9=34.1 knots not 42...34 knots seems to me a more reasonable speed with 100HP on a inflatable.
    A 13x21 gives at 4200 rpm (1850 rpm at the propeller) = (0.5334*1850*60)/1852= 31.96 knots WITH NO SLIPPAGE and 31.96*0.9=28.77 knots allowing a 10% slippage. I'm sure that this propeller is cavitating when you open the throttle at low speed. A curious noise at the propeller and nothing happens are the most common symptom of and over loaded propeller.
    In all cases 4200 RPM at WOT is not enough, you should have 5500-6000 RPM and you're forcing on the engine, like trying to climb a hill in 5th speed...A small front wind and some face current and the poor engine is making a very sad noise in the air filter...Yes you're harming the engine and the gear box which is not particularly strong on the Yamaha F100.
     
  14. Ilan Voyager
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    Ilan Voyager Senior Member

    You have outboards designed to move heavy slow boats; the primitive Yamaha Enduro line for third world fishermen, or the professional line by Tohatsu for Japanese fishermen. Nothing to do with sporty flashy outboards for Sunday yachtmen, they have very flat torque and power curves. Nothing fancy, the equivalent of small truck engines, the Tohatsu is far better than the Yamaha. I can say that the 70 HP 3 cylinders 2S carbureted is indestructible, even pushing a more than 10 metric tons oysters barge 6 hours/day/6 days/week during years with 2 positions of the throttle; stopped and WOT. I have been said that the Tohatsu 2S injection are excellent with outstanding consumption even in professional conditions.
    If you use the good propellers, you can move barges with the right outboard...
     

  15. CDK
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    CDK retired engineer

    As a matter of fact, I'm not so sure about the figures.
    The speed dial is an LCD display. I expected knots, but now that I look at it with my reading glasses I see 'mph' behind the large numbers. So the 42 should be 36 and the 32 is 28. Thank you for pointing that out.

    The engine rpm is probably the only reliable figure because it comes from the engine's ECU, the speed is measured with a pressure transducer and there is a display option to select a correction factor. But that isn't relevant here.

    By playing with the hydraulic trim I can reach 4400 rpm at WOT but getting over the hump becomes more difficult with the engine trimmed out.

    The 13.2x21 Solas prop has a conventional shape. Would a cleaver style prop be a better choice? I do not need the top speed, just less engine noise at planing speed.
     
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