Best Outboards for Displacement Craft?

Discussion in 'Outboards' started by CatBuilder, Apr 27, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    Wow, great posts. Thanks, guys.

    Keys: Yup. Prop diameter and blade area are things that need to be maximized too. I agree. For the install, I think I'm going with a well. I really want to protect the motors from spray and/or dunking under. I also want to get those props as deep as I can, so I don't pop up and draw air in when motoring off the nasty lee shore under power.

    Marshmat: I choose "C" - want to be able to motor off a nasty lee shore under power. I spend all my time at anchor, so if some storm pops up in the middle of the night and my anchor slips (for the first time in 10 years of full time living aboard at anchor), I do like to have the option of getting the engines going to deal with the problem. I think if I had to rely on sails for that, I'd be breaking up on the rocks before I was able to get the mainsail cover off. So yes, I need to be able to power off a nasty lee shore.

    Random side question:

    If I get a pair of 50's, will they consume a small amount of fuel when in "cruise mode" at half throttle or so?
     
  2. marshmat
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    marshmat Senior Member

    Fuel consumption at 1/2 to 2/3 throttle is probably going to be on the order of 300 to 350 g/kWh for all modern direct-injected or four-stroke outboards; this figure won't deviate much between brands and power ratings. The modern designs all throttle down very nicely and tend to be optimized for running at 1/2 to 2/3 throttle. Near wide-open throttle when pushing a heavy load, they'll all get much thirstier.
    Twin 40s or even 50s does give you the option to run single-engine on long trips at 6-8 knots, which is often more efficient than running both engines at idle or pushing the hull to higher speeds. And a 40 running at half-throttle is a fair bit more fuel efficient, and quieter, than a 20 at wide-open. I think more than 100 total hp would be wasted in a boat that size.
     
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  3. DCockey
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    DCockey Senior Member

    I've been looking at published data for fuel consumption of 4-stroke outboards and wide open throttle on virtually all from a few horsepower to 150 HP runs right around 0.1 gal/HPh which is about 370 g/kWh. So wide open throttle doesn't look too bad compared to part throttle.
     
  4. hoytedow
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    hoytedow Carbon Based Life Form

    You don't want to have to run at wide open throttle all the time, so the engine doesn't work itself to death prematurely.
     
  5. CatBuilder

    CatBuilder Previous Member

    Again, wow. Thanks for the input on the fuel consumption.

    Marshmat: I did just as you are talking about on my last catamaran, with Yanmar inboard diesels. I would get much better fuel economy running one engine at 2800-3000RPM than both engines at a lower RPM setting that gave me the same hull speed. Works very well on cats to save fuel.

    For clarity and my own calculations, is "g" per kWh grams? As in... the weight of the fuel?
     
  6. marshmat
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    marshmat Senior Member

    Yes. Grams of fuel burned per kilowatt hour of power delivered at the output shaft. (1 horsepower = 0.75 kW, 1 horsepower for 1 hour = 0.75 kWh)
    DCockey- if they're down to 370 g/kWh @ WOT, that's not bad! It's certainly better than the carb and early EFI 2-strokes did at cruise speeds. (Still, you don't want to run up there all the time- noisy, and speeds up wear on the motor.)
     
  7. CatBuilder

    CatBuilder Previous Member

    The consumption, based on these figures, is far worse than I would have imagined:

    50HP *.75kw/HP = 37.5kW

    37.5kW/h * 350g/h = 13,125 g/h

    A gallon of gasoline (sorry, I have to think in terms of gallons for comparison to other boats I've owned and tank sizes) is 2,766 grams.

    So, a SINGLE 50HP outboard running will use 13,125g/2,788g or 4.75 gallons of gasoline per hour?!?

    4.75 gallons of gasoline per hour gives a very small range. This is a bit of an issue.
     
  8. marshmat
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    marshmat Senior Member

    MY BAD: The figures I mentioned earlier (300 to 350 g/kWh) are typical of older EFI two-strokes at cruise, not DI and 4-stroke types. (It was late, I pulled out the wrong table...)

    Try 250 g/kWh at cruise (putting out 1/2 to 2/3 of rated power) and 300-350 g/kWh at high load and WOT, if you have your eye on a modern Evinrude E-tec, or a Honda / Merc / Yamaha 4-stroke. The manufacturers are notoriously secretive about releasing real numbers but these seem to be typical average values. It really depends on what they tune the engine for; some have a sweet spot at moderate load WOT, most at moderate load near 2/3 throttle, and all will drop off at low load idle or high load WOT.

    Two-stroke carb outboards and some older designs with bad throttling losses can be a bit different- they might get as good as 250 to 300 g/kWh at WOT, but be much worse (ballpark of 400 g/kWh) at low throttle....

    (For comparison, the best modern small diesels get to about 200 g/kWh and a few older 2-stroke diesels go as high as 275 to 300 g/kWh. And 215 g/kWh is the often-stated "20 hp per gallon per hour" rule of thumb.)

    Also worth noting: a "50 hp" outboard could be anywhere from 45 to 55 hp, on average; they're allowed a fair bit of flexibility in how close they are to their rating. There was a time when the Merc 90 and Yamaha 100 were identical 96 hp engines, just with different labels.
     
  9. Steve W
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    Steve W Senior Member

    I dont think you need or want the larger engines as they are going to suck too much fuel,a pair of 9.9 yamaha high thrust or similar should be enough. I have some experience with a single 9.9 on a macgregor 36 cat and that thing had so much thrust we could motor out of our ship canal in a 25 knot northeaster funneling lake superior directly between two seawalls at 1/4 throttle,(we had to,it was mounted on the stbd transome and would be underwater when the bows reared up) At cruising speed in the 7-8k range it only used 1/2 gph. I saw a very nice instalation in New Zealand in a Shionning cat of aboult 44ft (Mcmoggie) which had 2 x 9.9 yamahas in wells ahead of the rudders, This is a very good performing cat with an emphasis on sailing,it was built with a lot of effort to keep weight at bay, so their motoring expectations may be less than i think many of the large horsepower proponents have. You may want to check out the bolt on Kort nozzle systems such as "the Handler" which claim to allow you the same thrust at a lower throttle setting,hence, lower fuel burn. I have no experience with these but they are intiguing.
    Steve.
     
  10. marshmat
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    marshmat Senior Member

    Those 9.9 Yamahas are nice motors, for sure.
    CatBuilder's boat is 8.3 tonnes loaded, though; a Mac36 is 1.5 t empty / about 2 to 2.5 t loaded. 20 hp (1.8 kW/t) will get it in and out of a marina, but will not push an 8 tonne boat into a head sea with a decent wind.

    Twin 50s is the absolute maximum I'd recommend for a boat like CatBuilder's, and will be overkill for most situations. Twin 30s (high-thrust style) would be my choice, if it were my boat; perhaps twin 40s if a lot of motoring and a lot of nasty weather are anticipated. They'll rarely be run above 2/3 power- the main reason for going with a 30 or 40 is the larger diameter prop it can swing, which does wonders for efficiency on a large, heavy boat, and the quieter running / longer life you get by not having to use WOT all the time.
     
  11. Submarine Tom

    Submarine Tom Previous Member

    I have a Merc Bigfoot 25 on my 25', 12 000 pound houseboat. It'll do 6.3 knots through the water on a calm day. With an 8Hp Honda (new) it'll do 5.0 knots. Being a fairweather sailer, I'm considering downsizing to a 9.9...

    I wouldn't go over 25's on the cat.

    -Tom
     
  12. Easy Rider
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    Easy Rider Senior Member

    On the power issue I think it needs to be known if this sailboat is 75% displacement hull or more like a 50% planing hull. Would need to see a picture of the stern quarter underneath to begin to properly analyze the power requirement. My 8 ton 30' Willard is properly powered w 40hp but it's not particularly slick or clean from a design standpoint so on a cat w slick full disp hulls 20 hp would be plenty. Heavy into the semi-planing would dictate 40 to 100hp total. Ther'e was talk of 10 knots.
     
  13. philSweet
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    philSweet Senior Member

    From other threads, this boat is for charter service with captain. I think the fifties would be a reasonable choice. I wouldn't be at all surprised if this boat doesn't end up at 18 tons fully loaded at some time in the distant future. See what options the mfgs offer for programmed replacement contracts. Those are the models you should look at closely. They may differ from the stock offerings.
     
  14. CatBuilder

    CatBuilder Previous Member

    The boat will never be at 18 tons. It's term charters, so limited to those who can comfortably sleep aboard. I will never exceed max designed displacement.
     

  15. philSweet
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    philSweet Senior Member

    Didn't mean to imply irresponsible behavior. Just expressing my penchant for large amounts of built in versatility.
     
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