old heavy"lugger" vs high reving & more hp

Discussion in 'Diesel Engines' started by globaldude, Apr 7, 2006.

  1. FAST FRED
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    FAST FRED Senior Member

    "it just seems logical to me ."
    When what seems "logical" is wrong its called counterintuitive.

    Any prop freewheeling is presenting the blade surfaces in an unstalled condition for the full diameter of the prop. EG a speedbrake of propellor diameter.

    This will absorb far more HP than the stalled prop .

    Even with a large 4 blade prop with BIG BAR 71 (Blade Area Ratio of 71%) the stopped & stalled prop willl have less drag as the stalled blades will not absorb the HP .

    A fine experiment would be to lock the prop with a pipe wrench and sail at speed in moderate wind conditions (under hull speed), then luff up & knock off the wrench and get back on course. The results may suprise you.

    There will be little difference in higher winds where the boat is at hull speed as usually you are producing more power than the hull can stand.

    Freewheeling MUST be aproved by the transmission MFG , as many will overheat from lack of pumped oil.

    FAST FRED
     
  2. globaldude
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    globaldude court jester

    Ok fred, so I'm " counterintuative "as well as thick :p [ I like it - LOL ] you quote a 4 blade prop with a blade area of 71% having less drag stalled than freewheeling because ---- what ?, the remaining 29% is "Gaps" the water can freely [ relatively ] flow through !?, as opposed to a turning prop presenting a 360 degree [ 100% ] barrier - reads resistance to the water.
    to quote you below, the stalled pail, doesn't it present a 360 degree resistance to the water also ?

    "To contemplate the speed reduction, tow a heavy duty pail astern the same diameter as your propellor , and remember the filled pail is "stalled" the turning prop is far more efficent at pulling out power , slowing you down , even in half a gale."

    There must be some other dynamic going on here I can't see.:confused:
    "absorbing HP" ?,
    Prehaps it would help if you explained what a " speed brake" is & how it works .

    Give me an argument that explains why the " stalled " 360 degree, pail has less resistance than the yeilding turning prop .
    Isn't this all about resistance, and isn't the turning prop yeilding to that same resistance thereby offering less or obsorbing less resistance !?

    I also ask myself why the chap, who's log, he claims, is acurate to 1/100th of a knot, would have his prop freewheeling if it induced MORE drag !?.
    I 'll try to email him and ask him to test/ confirm our -- opps, my speculations
     
  3. marshmat
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    marshmat Senior Member

    The same effect is noticed on twin-engine aircraft that have the engines mounted at opposite ends of the fuselage, rather than on opposite wings.
    When one prop is not in use, it must be feathered and braked. If it is left to freewheel, it is operating in an unstalled condition and so is extracting power from the flow over it, creating drag and causing wear to its transmission and mechanicals. If it is braked, it stalls- but at a high stall, it will create less drag than if it's spinning and extracting power. The same principle should apply to boats.
     
  4. Guillermo
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    Guillermo Ingeniero Naval

    Not so easy, I'm afraid. Let's remember a Larry Zeitling post, I think in these forums:

    "To freewheel or not to freewheel has been debated in the sailing community since Ericsson invented the screw propeller. As you say, a two bladed prop that can be locked behind the deadwood offers less drag than one allowed to freewheel but not as low drag as a folding or feathering blade prop. The British journal Yachting World published a series of tests about 20 years ago that compared the drag of freewheeling and fixed three blade props. The experiment was quite ingenious. A boat with a three bladed prop was anchored in a tideway. A spring scale was inserted in the rode. As the tidal flow reached a speed of six knots, the prop was allowed to freewheel and the scale force measured. Then the prop was locked and the force measured again. The process was repeated with several props and several degrees of resistance to rotation as might be provided by a shaft in a stuffing box and turning a transmission. The conclusion was that a freewheeling prop with NO RESISTANCE offered the least drag, followed by a FIXED PROP. The most drag was offered by a SEMI-FREEWHEELING prop rotating about 1/2 to 2/3 the speed of a free spinning prop."
     
  5. FAST FRED
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    FAST FRED Senior Member

    The conclusion was that a freewheeling prop with NO RESISTANCE offered the least drag, followed by a FIXED PROP. The most drag was offered by a SEMI-FREEWHEELING prop rotating about 1/2 to 2/3 the speed of a free spinning prop."

    The above is quite correct.
    Any boat with a stuffing box will be far from "freewheeling", and the added drag of a shaft alternator places the most drag on the boat.

    So ,
    "love the regeneration aspect - "free" / parasidic energy."


    Is ONLY true at gale conditions where the huge drag of a working prop wont slow the boat at all.

    Otherwise DRAG costs SPEED.

    But a loss of speed for a box of cold beer might be a worthy trade?

    FAST FRED
     
  6. Guillermo
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    Guillermo Ingeniero Naval

    Well, we have the gearbox, shaft bearing and stern gland to induce resistance. If those elements are well conceived, a relatively big propeller may be not so far away from freewheeling. And if the coupled alternator is not a big one, (The idea isn't run a lot of high consume electrical appliances at the same time), just to keep your batteries up, maybe the thing is not so dramatical, as the owner of Calypso V states, and you call allow yourself a couple of 'luxuries' like cold beers when in a long passage. :)
     
  7. globaldude
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    globaldude court jester

    I'm so glad those Pomme buggers did that test, It was bugging me so much I thought I was going to have to go and do the exact same test. :mad:

    The results are conclusive and I'm sure the test boat had a stuffing box [ or a big water pump :eek: ] & most likely a G/B attached to the prop shaft !

    We all know for every resistance [ in this case] there is an equal and opposite reststance , the yeilding prop must have less resistance .
    The stalled prop allows some "clear" flow through the blades --- very much so on the thin bladed aircraft prop , and the " parasidic " power sapping ,interfering , arse dragging, semi freewheeling prop will give me cold beer, nice music, lots of fresh water and an extended - slower - cruising range .
    Of course this is all just dreaming - if I listen to the scoffers who abound - but I take heart in the likes of you lot, who are , or who has " been there & done that" .
    I have been a member of an "Inventors trust " for about 6 years here in NZ and like this Forum, find like minded people don't scoff [much] at " different " thinking.
    That's not to say we agree with all we hear, and there are many a lively debate as to the pro's & con's of a thingy bob .
    I do think Brian Eiland is having a hard time convincing the sailing bods the merits of his " aft mast " , but I like his thinking , albeit unproven .
    Pete.
     
  8. globaldude
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    globaldude court jester

    Update, I got the old lister going today, the original starter motor she came with was stuffed and a new one was quoted at $700, virtually what I paid for the engine Nah !!. I had a similar sized starter so adapted and mounted it on the other side of the engine.
    Problem is without the decompresion buttons pulled [ engaged ] the starter "bounces" on the compresion and can't turn her over.
    I suspect the same would happen to th original starter also.
    By engaging the decom buttons, spining her up & dropping one - two - three etc buttons she fires.
    All good !?. Not really, I don't want / expect to be button pushing in the engine room when I'd rather be at the bridge.
    So maybe this engine is not suitable -- what say you all ? .
     
  9. FAST FRED
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    FAST FRED Senior Member

    Probably the starter is weak.

    Plan B a solenoid to push the button as needed.

    FAST FRED
     
  10. ron17571
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    ron17571 Junior Member

    I really like the lister,the ones i read about with the big flywheels were cranked over by hand,i imagine with the compression release thrown on to make it catch and start.Seems like a fuel sipping diesel,as an auxilary it sounds great,im not sure how to use it as a main engine,unless hooked up to a gen.to make electric for some sorta hybrid setup.try a search engine for info.
     
  11. globaldude
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    globaldude court jester

    I'm going green, throwing away the monster

    Having started and run the lister - well - I know it would " see me out" the bloody thing is built like a tank !!. Annd welllll , that's part of my problem with it actually.
    She does run well, but it's not what you call a balanced engine - to be fair, there's a lot of weight spinning around in her guts and well like I said, having heard her running ---- I've decided to sell her !

    I have two Kubota 3 cyl 667cc diesel engines that I couldn't resist buying while on a trip to Japan years ago.I'd never started them.
    I dragged them out from under a bench, put oil in them, hooked up an IV [diesel] and threw 24v at the 12v starter to give it a good spin ------ splutter varoom , instant start !!. But oh so smooth !!! , and quiet .and all at probably 1 - 1.5 liters per hour.
    So, back to the electric motor / alternator main engine, with the twin Kubota's either side running 15kva alternators.
    I will have , in effect, 4 propultion means. Sail , electric, & two small diesel emergency engines that can be coupled to the drive shaft if needs be.
    I have a controlable pitch prop so would be able to dial up a fine pitch to enable the small diesel/s to get me "home".
    I'll try to post a pic of the electric engine. The vert axis tower is one my friend builds and the alternator [ reads soon to become my engine ] is driven from this beast. Oh and it just glides silently - the turbine - unlike the noisey wind mill types.
     

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  12. globaldude
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    globaldude court jester

    I forgot to say, we're planning to have two 48v banks [ coupled to 96v for engine] with around 3000ah of reserve. They will be AGM batterys set low as possible in the bilge - around 2.5 ton . All house power will be 240v, std here in NZ = cheap fittings and tools appliances same as [ real ] house.

    Hey BILGE BOY, thinking about the "excess fuel" in the combustion chamber [ old post] I think it's entirely possible with the Lister because it's possible to crank it over at full throttle [ puting lots of fuel in the chambers] with the decompresion buttons engaged [ without realising ] and therefore no compression to fire.
    when disengaged there's a lot of fuel to go bang !!. --- No, I didn't, but I could imagine it being done !!
     
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