Feathering Prop for Fuel Savings?

Discussion in 'Props' started by myk, Nov 8, 2011.

  1. myk
    Joined: Oct 2005
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    myk Junior Member

    I was looking at an old 57' Chris Craft Roamer and was thinking of doing some long range cruising with it. She holds 500 gallons of diesel and I'm told that at low speed (8 kts) I could expect a mile a gallon. If I wanted to increase her range the usual suggestions are that I could either: increase fuel tankage or install smaller more efficient engines.

    My question: what if I installed controllable pitch props on one or both engines. Then I could feather one prop for low drag, while running on one engine more efficiently. Does that make sense and what might be the cost (thousand, tens of thousands)?

    PS - still reading through the thread on CPP, but searches in these forums didn't turn up any answers.
     
  2. rubenova
    Joined: Aug 2011
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    rubenova Junior Member

    Try automatic feathering props for sail, expensive but available. Some are diver adjustable for fine tuning pitch. Retrofitting CPPs might make sense if existing prop shaft tubes could be used, that would probably require a repower with smaller engines.
     
  3. rubenova
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    rubenova Junior Member

  4. bertho
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    bertho bertho

  5. sabahcat
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    sabahcat Senior Member

    How many NM will you need to do to recoup the cost of that prop?
     
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Sell one engine and make it a single.
     
  7. rubenova
    Joined: Aug 2011
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    rubenova Junior Member

    When it comes to machines there is always a trade off, the trick is to balance some factors. It comes down to dollars vs whatever, speed, mph, gph, range, room for another head etc. If you are looking for slightly better range for the fuel you carry, slow down, leave your boat alone. If you want to go farther at the same speed, add tankage, temporary or permanent. If you have money to burn AND want to, have a naval architect do a study to see where your boat speed would need to be for your needs, repower, replace the props, to match. I like your question for the simple reason I like to ask "what if", if it gets even one person to think out of the box and come up with a point/counterpoint then that is progress. Keep the creative juices flowing and the questions coming
     
  8. eyschulman
    Joined: Jul 2011
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    eyschulman Senior Member

    Check with the people who built your transmissions some can free wheel some can not. Not all is lost if not you can put a stopper on shaft even some tire tube type rubber under a good pipe vice grip. If you do that be sure to tie key to that motor on vice grip so nobody can start same without removeing it. Then run on one motor.
     
  9. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    Yep... agree wholeheartedly. Anything else would just be $ down the tube...
     
  10. tunnels

    tunnels Previous Member

    Id be looking at a whole system overhaul and a variable pitch prop! single get rid of the two oldies and all there mechanics ( gear boxes shafts struts etc etc and using a more modern up todate fuel efficent model !!
    One you have lightened the boat a little !!
    Two you have halved the drag with just one prop shaft and strut and
    third a variable pitch will be able to go to a optimum pitch so even more efficency .
    Twin rudders ?? that might also have to get the old heave ho and go to a single system maybe
    All of a sudden your 500 gallons could go lots further where ever you are going !!. worth looking at the fesability of going the whole ten yards !!:D
     
  11. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    I assume we are talking about a boat a bit like this in which case, what's the point? It's worth 100K... maybe 150? You could very easily spend 1/2 that again on changing over the drive system and it would still be worth 100K.
    Even if you keep the boat for 10 years and do 5 times as many hrs a year as the average boater, you'd never recoup the money you've spent in fuel savings.
    Run one engine (with the proviso's noted above), slow down a knot or two and you'll be better off by a mile. If that's still not enough for you, then buy a boat with a more economical powertrain... they're for sale by the thousand and it's a buyers market....
     
  12. tunnels

    tunnels Previous Member

    A boat is a hole in the ocean ,the bigger the boat the bigger the hole to pour you money into .
    If you are a true boating freak or enthusiast spending money on your pride and joy is never a problem .
    Its your pride and joy and a way to escape the realities of everyday goings on !!!
    A boat is a Freedom machine !!
     
  13. MechaNik
    Joined: Jan 2011
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    MechaNik Senior Member

    As there has been no reply from the OP I guess he realised the error of his thinking.
    Out of curiosity I knew a guy who put feathering props on a sport fishing boat ( Not CPP). He wanted to trawl on single engine and maintain load at slow speed. He maintained load but efficiency was so bad he put the old props back on :(
     
  14. myk
    Joined: Oct 2005
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    myk Junior Member

    OP is Back

    Not gone, just deployed and now back.

    Two clarifications: it's not about fuel cost but it is about not being wasteful where I don't have to be.

    With regard to the latter, I like the look of a lot of the old hulls. I've just wondered, why not just reuse the hulls if they're still good? Gut and modernise, like we (or at least I) do with houses. You don't necessarily save money, but you do create less waste and extend the life of previous good engineering work. I'm not talking about bringing a boat back to its original spec for antique shows, more like salvaging what is still useful. My perfect scenario: decent Elco woodie hull (1920-30) and update it with this superstructure in modern materials, modern engine/electrical/electronics. Failing that: modernizing a pretty (to my eye) Roamer will do.

    I'm not worried about saving fuel costs per se, it's really a question of how much mileage I can get. If I can't make it to the next fueling station, how much I paid to fill the tank won't be my chief concern.

    About engines: I mentioned changing engines and one suggestion is to upgrade and remove one engine, but I figured that would affect trim and handling. From the reading I'd done here, I thought the engines on the Roamers were considered pretty decent even all these years later, so I didn't really consider an engine overhaul.

    About slow speed: My reading on this was that diesels don't like slow (a lot of debate, but this belief seems to prevail) and that the hull shape of a Roamer isn't efficient at low speed either.

    About props: Ah ... hadn't thought about free wheeling. Does it reduce drag appreciably? From the post it sounds like it is not a standard feature on a transmission. Is there a penalty to have a transmission that does this eg. more wear?

    Anyways, thank you for the replies. A lot of ideas for me to pursue.
     

  15. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Myk,

    Diesels are fine at low speed, just not low loads. Ideally a diesel should run above 60% of its load, which may mean putting in a smaller diesel to achieve a lower speed. For instance, you could take out a 450 designed to run the boat at 22kn, and install a 120 to run it at 8.

    Swapping props can be pretty complicated, and complete drag numbers are hard to find, but generally a prop allowed to rotate (if the transmission can handle it) will generate 1/2 the drag a stationary prop will. A feathering prop will generate 2% of the drag a fixed prop will, and a folding prop will generate about .5% a fixed prop will. Typically though folding/feathering props are intended for use on sailboats, and so there may be size limitations when looking to install them on power boats.

    As for free spinning transmissions. Some are fine with it, other will destroy themselves, it is model and manufacturer dependent, so you just have to look on your model.

    Personally if you are ok with lower speeds, i would consider removing one engine, center lining the other, and in the scavenged space adding two new fuel tanks. This will keep your trim pretty close to the same, decrease fuel consumption, and give you more tankage to further increase range.
     
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