efficient 10m displacement powercat (build thread)

Discussion in 'Boatbuilding' started by groper, Apr 15, 2012.

  1. Ebeneezer
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    Ebeneezer Junior Member

    A great project that I have enjoyed following! If you are considering tramp netting check out net systems out of Washington state, they make fantastic knot less dyneema that lasts dam near forever.
     
  2. DennisRB
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    DennisRB Senior Member

    From what I have read 4 blade props would be better suited to pushing something with more resistance than this boat. Since this boat is light and easily driven it might be best to stick with 3 blades? Unless you plan on motoring with one motor then the 4 blade might be the way to go. But I have no real idea.

    Also in regard to fuel efficiency, I always thought that being slightly overpropped was good for efficiency on the condition that the operating speed was well below full speed. When overpropped the engine will be loaded right up in the higher end of the RPM range, but at slower speeds the increased loading is more efficient then revving the engine higher at low load. Kind of like how some cars actually have a slower top speed in top gear, but they still use less fuel in top gear at cruising speed. Remember the prop power curve bends in the opposite direction of an internal combustion engine power curve, so sub max speed there is a lot of surplus power available. But I am sure this is very dependant on individual scenarios. Less noise at the same speed too.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    If you only pulled 5000 rpm, it is too coarse as you say. You may get some more revs when the engines loosen up a bit, when "run in". I doubt you'd get more than 300-400 rpm extra from going down to 19" though.
     
  4. Brian@BNE
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    Brian@BNE Senior Member

    Why don't you ask to trial them before purchase? Maybe offer a few bucks for the hassle of the trial. Afterall, there may not be many buyers around.
     
  5. DennisRB
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    DennisRB Senior Member

    Props are so expensive. It would be awesome if the let you borrow them for a day before purchase.
     
  6. waikikin
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    waikikin Senior Member

    Hi Groper,
    Surely you can add some awning with a bit of self bracing geometry to simple pulpit/s(tripod style/whatever) over the tramp.
    I've got a boom awning on my Carter 33 & it's always worth the few minutes to rig especially in summer- cant sail with it though.
    Looks lovely running in that tropical water:).... just like you see on the glossy brochure.

    Jeff
     
  7. groper
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    groper Senior Member

    Yeah Jeff - i know i can rig up an awning when stationary, but at speed it would flog all over place... Cant really beat sails for that :)

    PS- the glossy brochure photos are coming... bikini clad girls and all, heres a teaser :D
    [​IMG]

    [​IMG]
     
  8. groper
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    groper Senior Member

    And back to the props - i gave solas a call and they were very helpful. Their recommendation is a custom propeller which is a modified V6 size prop which the hub and spline has been adapted down to suit the smaller gear case of the 4cyl outboards i have. This custom size they suggest for best efficiency for long distance island cruising would be something like a 4 bladed 14.25" x 17" pitch which is the largest thing you can fit under the cav plate. They also suggested that the modern engines have tuned exhausts and mapping which gives better fuel efficiency if the revs are kept higher - which is at odds with old school thinking. They said this applies to the modern small displacement engines like the suzukis and yamahas etc.

    Either way - i like the sound of swinging the largest possible diameter prop possible - thats always what ive understood to be most efficient provided were not considering outright top speed applications... Thats my next performance parameter to figure out - what the fuel consumption is compared to boat speed...
     
  9. Richard Woods
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    Richard Woods Woods Designs

    That's why I suggested you try running at higher revs on one engine instead of low revs on two at cruise speed

    do you have a fuel flow meter? if not you will be some time aimlessly motoring to and fro dispensing fuel in small quantities to get economy figures. Or it will take you all season

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  10. groper
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    groper Senior Member

    How will running on 1 engine help me figure out fuel consumption? And no, i dont have fuel flow meters... perhaps i will get some if they are not too expensive?

    otherwise, i will simply fill the tanks full, do a known long distance commute to one of our favourite diving spots (250nm round trip), and refill the tanks full again afterward - that will give me a pretty darn good idea and certainly wont be "aimless" as we get to enjoy our weekend in the process...
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    There is boat test data with those engines on the web, in several places. Planing hulls, but that is still useful. Best rpm, for an engine that will rev to 6000, seems to be no more than 4500 rpm. Fuel flow doubles from 4000 to 6000. There is a fairly wide band where fuel efficiency is not greatly different. I think you need to trial in "average" sea conditions to see what represents a comfortable cruising speed, but I can't see that a cruise of 20 knots would be killing the fuel figures, with suitable, probably 17" props, in good conditions.
     
  12. Richard Woods
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    Richard Woods Woods Designs

    Sorry to confuse you. I meant you need to see if running one engine at say 17knots at a high rpm is more fuel efficient than running two at the same speed but at a lower rpm.

    How fast can you go on one engine?

    You will need to do several trips to your reef at say 17, 20, 23 knots to find the optimum. Or you can do it in an hour with flow meters (which are expensive and are only accurate with big engines). But as I said, it's pointless unless you load the boat to your cruising displacement

    Having said that, on our much smaller (28ft) powercat with tiny engines (2 x 20hp) we have found that there is no difference in mpg whether doing 10 knots or 16. So we usually run at 12 when day cruising and 10 when live aboard cruising (as we are now).

    We have cruised 4000+ miles in the last 3 years and in a recent log entry I wrote "I see that since Port Aransas, Texas we have cruised 1491 miles including detours, to Lake Worth, Florida, and used 228 gals of fuel. So an average of 6.54mpg."

    About 3/4 throttle seems optimum, so 4500 revs on a 6000rev max engine.

    I doubt if you will go that slow, but you will want to avoid the hump speed, probably 9-11 knots in your case

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Groper's boat seems destined to get about 2-2.5 nmpg ( US gallons) . I think I'd have gone a little smaller with the engines, even as low at 2 x 70 hp. But his judgement seems to be stacking up pretty well so far.
     
  14. DennisRB
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    DennisRB Senior Member

    When slightly wrong engines come up at the right price its a good decision in my books when the prices of these engines is astronomical. Especially if the cheap engines happened to have a more power than optimal. Yeah its weight, which is not good, but it may mean less noise and if you are very lucky no more fuel burn or under certain conditions even less fuel burn.

    Slightly related. I hired a new Corolla on the weekend in Peth, which is a fairly heavy car with a small engine. It has a 7 speed auto (CVT but with ratios programmed in as users hate the way stepless gearing sounds). Its interesting to note the engineers chose to program the engine to run at absolute minimum RPM where possible. This thing is in 7th gear at 40km/h. Obviously the more you accelerate the lower down the gear ratios go. But I was amazed at how tall the gear programming was in normal cruising. The thing hardy went about 1500rpm even with max power at 6500 unless you were accelerating pretty hard. I was also amazed at the economy of the car. Obviously the lighter the load on the engine, the lower the RPM needs to be for best economy.

    So can any of this apply to a light boat with oversized engines? Seems the operating point for best efficiency will not be the same as a heavy boat with just enough power. For best economy, a heavy boat with just enough power will likely need to rev much higher than a lighter boat with ample power even when fitted with the same engine. So it would be hard to look at general performance/economy data for the engine and come up with much, unless that data is cross referenced to various boats and how much extra power is available at the measured speeds.

    I would also be interested in performance and economy on one engine. I know with many planing boats if one engine of two dies, speed might go from 30k to 3k thanks to one prop being overloaded and cavitating before any speed can be built.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Boat resistance is high compared to a vehicle rolling along a flat road at constant speed. As demonstrated by the fact boats don't need brakes !
     
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