Gearbox Problems :-(

Discussion in 'Diesel Engines' started by Leela Belle, Oct 25, 2014.

  1. pdwiley
    Joined: Jun 2008
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    pdwiley Senior Member

    Agree. Not sure what reduction ratio the g/box has, and I'm very far from being any sort of expert, but 11 or 12 inch pitch seems way too little.

    I would have thought top speed for that hull should have been in the 6 to 7 knot range.

    PDW
     
  2. whitepointer23

    whitepointer23 Previous Member

    I may be wrong but think the po may have been using the boat with a clean hull and achieved hull speed. It may have not been used for a while and collected a bit of growth before gerome got it. I just can't imagine the po being happy with 2 knots. 2 to 6 kts with a small pitch adjustment doesn't sound right. Whats it like for bottom growth down there pd.
     
  3. Leela Belle
    Joined: Oct 2014
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    Leela Belle Junior Member

    Hi Brendan,
    Before slipping the boat would only achieve 3 knots at 2800 rpm (full throttle) on calm water. The bottom of the boat was very dirty with a lot of growth which wouldn't help. The engineering firm I went to, coincidentally had made the prop for the previous owner. I gave the engineering all the details about the boat ie boat length,engine size and boat weight. I have used this business before on a another boat with great success. As I said in the previous post i am yet to get the prop on and hope to in the next few days, where I will do sea trials. Fingers crossed that it makes the difference.

    Cheers Jerome
     
  4. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Well that's good news it wasn't an expensive gearbox problem,along with a lot of work and hassle in pulling it out and refitting.
     
  5. whitepointer23

    whitepointer23 Previous Member

    Hopefully its all good now jerome. Waiting for a report of a successful sea trial next.
     
  6. AndySGray
    Joined: Jun 2014
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    AndySGray Senior Member

    Sorry to be the voice of doom but I doubt the pitch change will help on its own.

    Pitch is simply the distance that a prop would travel in a perfect medium for a complete revolution.

    i.e. your old prop would go 10 inches

    the new one would go 11.8 inches or 18% further per rev.

    So if a 10" pitch prop gives 3 knots an 11.8 will give 3.5 approx. best case scenario.

    worst case is the new prop cavitates and 'slips' - it is turning your engine power into foam and bubbles and not thrust - you could actually drop speed!

    But cleaning the hull well might yield a further 1.5 knots - just don't attribute that to the magic new prop.

    Use a GPS if you can and measure the speed for a given rpm, repeat in opposite direction to minimise wind and current, increase rpm repeat,

    you'll get a graph with speed vs. rpm which will be straighter at low revs and as drag and cavitation kick in it will flatten out (may even dip!).

    good luck - a few pictures of the prop and undersides might give us some other ideas if poss.?
     
  7. Leela Belle
    Joined: Oct 2014
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    Leela Belle Junior Member

    Hi Andy,
    I read your post and yes it left me feeling very disheartened. Not being a mechanic or having an extensive knowledge about prop size, pitch and how they are married up I felt very depressed. I have owned the boat since September and have been doing my best to get her up and running.
    Given that you seem to a be wealth of knowledge on this issue, could you give me some idea what is the problem. I understand that from a big distance from me makes it hard, what am I missing? Leela Bell is a 32ft steel ketch. Weighs around 8 ton, Has a 36hp Lomatec diesel engine coupled to a TMC260 2-1 gearbox.
    As you can imagine I am becoming frustrated at not being able to resolve this problem.

    Cheers Jerome
     
  8. whitepointer23

    whitepointer23 Previous Member

    no need to panic. there is a 99 % chance it was the dirty bottom causing your trouble, its usually the cause of speed loss. pretty silly getting frustrated when you have not tried it with a clean bottom. you should get 5.5 to 6.5 knots now.
     
  9. whitepointer23

    whitepointer23 Previous Member

    you confuse me with your first post, you said the prop was clean yet the bottom is dirty when you slip it, did you dive or get a diver to check the prop when you said it was clean because that statement was misleading which had us all looking at mechanical problems when it looks like it just needed a clean.
     
  10. AndySGray
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    AndySGray Senior Member

    :!: What he said :D

    A lot of issues are dealt with by the normal maintenance procedures and folks don't analyze what they do - that was all I was trying to say.
    Cleaning the hull, changing the anodes, applying new antifouling will get you much more 'bang for your' buck than a tweak of the prop.

    Remember that for motor boats, getting the prop into clean water is no hardship, the prop will always be running so there is no drag issues. They use the biggest prop possible.

    On a sailing boat the prop is not used all the time so a balance is required.

    A bigger diameter prop in clean water will give much better performance when running but will act like a drogue anchor with the motor off (creating drag), so the compromise is something a little smaller - less performance on motor but much better on sail.

    Understand the benefit that comes from using a prop a couple of inches smaller in diameter, how much less drag there is from (at the design stage) simply removing a one inch strip on the outer edge of each propellor blade. :?:
    OK, Now compare that to the effects of a 1/4 inch of fuzz over the whole area of the bottom of the boat! :eek:

    If that's not enough of a problem, it also creates eddies and vortices so the flow of water over the prop is not the clean (i.e no turbulance) water you'd like it to be (or was anticipated by the designer when he specified a 2" smaller prop) ...


    The round the world racing yachts actually use a special hard antifouling which they hand polish and flat with fine 'wet & dry' paper to get a glass smooth finish and this can be the difference between a win or a loss.

    As you have it hauled, change (or at least check) the anodes. Especially on a steel hull. It's very easy to fool yourself that they are OK - used ones look almost the same size (but they go porous) - a good check is to take one off and weigh it, they can loose 70% of their weight and still look physically as big as a new one.


    Don't get stressed - even 3 knots on a boat is better than the top speed of the most expensive office desk!:rolleyes:
     
  11. pdwiley
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    pdwiley Senior Member

    Yeah, true, but I cannot see *any* way that a 10" to 12" prop spinning at a maximum of 1800 rpm - more realistically 1250 to 1500 - can ever get the boat anywhere near 6 knots, even with a pristine clean hull.

    My guesstimate of pitch is in the 16" to 18" range. FWIW.

    Which, funnily enough, is what is on my friend's 36' boat with 40 HP Nanni engine and 2:1 gearbox.

    PDW
     
  12. AndySGray
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    AndySGray Senior Member

    Yes, that's an excellent point. Maybe Jerome is working to the wrong baseline :?:

    Reminds me of a friend who did a fine rebuild on a boat, was cruising by an old gentlemans dock and the guy starts waving and calls him over - turned out to be the original owner (my friend being # 5 or 6). They talked at length about the rebuild and took him and his wife out for a cruise, but on returning the original owner was adamant the props were wrong. He told him to wait there, and disappeared for about 10 minutes and came back covered in dust but holding a RH prop with the factory stamp.
    The LH one had lost an argument with a submerged object and the boatyard fitted the only props on hand that would go on the shaft albeit the wrong pitch and diameter, a replacement was ordered and there was supposed to be a call when it arrived... A temporary fix which lasted 20 years ! :(

    So my friend spent almost 2k on a pair of props to match some random boatyard spares! :mad:



    Jerome, is this a production boat or built from a plan? I think you may need to go back and do a little research as to what was originally spec'd - a fair few sail boats use either self feathering or controllable pitch props and if the builder strayed from plan on that, you could be going in circles.
    Also understand that it is a different thing to have an engine mainly for motorsailing versus a sails down setup.
    Many boats have an emergency kicker outboard which can be as small as a 9Hp - doesn't create drag as the prop is out of the water when not in use.

    If the prop is in an aperture that can give a good idea what prop size is envisaged - about 115-125% of the anticpated prop diameter (though for smaller Hp ranges that may not apply), similarly if just a shaft and cutlass bearing, what is the distance to the nearest part of the hull. That will give a clue if the diameter is way off.

    Now if we take the pitch, 10"
    10" per revolution
    and say 1500 rpm [revolutions per minute] (I don't know your gear ratio so this is AN EXAMPLE) multiply;-

    gives 15000 inches a minute, (x 60)

    = 900,000 inches per hour (/12)

    = 75,000 feet per hour


    NOW 1 knot = 1 nautical mile per hour = 6076 feet per hour

    gives you a THEORETICAL speed of circa 12.34 knots

    So an actual speed of 3 knots is 75% slip or 25% efficiency, not great but also not that far out of the ballpark - 30% - 40% is considered 'normal' for a sail boat.
    PDW is spot on - 6 knots would NEED 50% efficiency and that is not normal for a sail boat!

    Everything you do to improve efficiency will add drag - all will add blade area so a 4 blade will slip much less than a 2 but your penalty will be a slower speed with the engine off and sails up.

    Ditto larger diameter IF the boat will accomodate...

    Your new 11.8 pitch will yield a Theoretical 14.56 knots but unless the design is radically different you are likely to get a similar efficiency and the aforementioned 3.6 knots.

    PDW's suggested 18" pitch could get you to 5.4 knots but could also become a whisk generating cavitation rather than thrust - without details we are ALL just guessing.

    ;)

    Best bet is to find a boat yard that will let you borrow a few props to find something closer to what you expect.
     
  13. AndySGray
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    AndySGray Senior Member

    Hi,

    just saw your other post where you state the box is a 2:1

    so if you got 3Kt at 2800, thats a shaft speed of 1400

    Gives 14000 inches a min and 11.5 knots theoretical, 26% efficient

    So the previous guess was pretty close.

    You can work it the other way -

    desired is 6 Kts

    slip is 74%

    required theoretical is therefore 23Kts.

    140,215 feet per hour
    1,682,584 inches per hour
    28,043 inches per min
    20 inches per rev (provided you don't get more slip)

    i.e. 20 pitch

    but thats just theory - best way is to find what props are used on sister vessels - they've already done the trial and error bit (and believe me the error part is not to be taken lightly).
     
  14. pdwiley
    Joined: Jun 2008
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    pdwiley Senior Member

    I kind of know that boat - at least from pictures. It's probably a John Pugh design built from plans by an amateur (Build A Boat magazine had lots of plans for amateurs). So who knows how it was done or how well it was finished. It was sold quite cheaply - a friend of mine was looking at it but couldn't get hold of the owner at the time.

    Anyway while I'm quite ignorant of props et al in general, I'm prepared to stick my neck out & say that a prop less than 16" pitch with a max shaft speed of 1800 rpm is not going to deliver the speed desired.

    This is why I have an Autostream adjustable pitch prop with a ton of blade area! However, given the price of that boat, a new Autostream might well be regarded as over-capitalisation.....

    I have a spare prop or 2 here with much bigger diameter and pitch. I think one of them is an 18 x 19 - might work. I'd sell it for what I paid for it, less than $200.

    PDW
     

  15. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    ....so from the very start it had a fouled bottom?

    I'm used to lots of horsepower where a dirty bottom makes little difference,so with small hp on a slow boat I'd expect it to make a fair difference.
     
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