High torque engines

Discussion in 'Boat Design' started by Boston, Apr 27, 2009.

  1. Boston

    Boston Previous Member


    Im playing with the idea of a big steam launch and considering an engine that has the following statistics associated with it.


    the tricky part is that although it took a while, I figured out that this engines HP rating is based off of a watts/hp of 745.7 rather than the boiler HP of 9809.5 watts

    ( lovely that we have four ways to measure HP eh )

    so at 100hp 740rpm 410psi I get a whopping 710 lb/ft of torque, which is substantially higher than what the standard diesel engine would ever turn out

    so the question I've got for the group is a basic mechanical engineering one about propeller pitch and shaft speed

    the vessel is an elco 57 flat top and with a 3.5 foot draft and 57' at the water line and a 13' beam, it normally cruises at 10 knots with two 210 hp diesels pushing it. Max speed is 14 knots. There horse power rating total is barely more than half, balls to the wind, what the big steam engine can do in its sleep but there shaft speed is higher. Apples to apples the big 4 cylinder steam engine can rock at 1000 rpm 200hp and 1050 lb/ft were as the diesels ( Cummins 6BT5.9 ) can do 403 ft/lb at 1000 rpm although thats not its best range http://www.cmdmarine.com/Products/Commercial%20Propulsion/6B/fr90761.pdf

    they gear down
    Im going to be gearing up

    so the question is whats the optimal shaft speed and prop stats for pushing this hull and how does that effect the choice of engine
  2. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Consider a bigger prop.... Where is the prop....
  3. Boston

    Boston Previous Member

    prop size and speed is a complete mystery to me thats why I threw up all the data
    to collect a consensus view of opinions
    I would prefer to have a prop that operates most efficiently in the 700-800 rpm range
    on a steam engine I can play with the valves and get it to run forward or backwards
    although this particular engine is not set up that way
    if that works out I may not need a transmission
  4. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Boston, this is a single motor replacing the original two, correct?
  5. Boston

    Boston Previous Member

    so far thats what Im thinking Phil

    best I can do is use one of the online calculators and I come up with for a slip of 20% or an 80% efficient prop
    at 740 rpm on a 30 ton fully fueled boat at 57 feet a pitch of 21.41 for a speed of 12 knots
    slightly better than I was hoping for in terms of speed
    but what kind of torque does it take to drive that screw through the water

    for a slip of 15% I get a pitch of 20.15
    and for a slip of 25% I get 22.83 pitch
    so I think Im in the ball park at a slip of 20% and a pitch of say 21 inches with this engine

    concerning hp I get
    50-HP = 15 in. pitch
    70-HP = 17 in. pitch
    100-HP = 19 in. pitch
    200-HP = 23-inches or more. although the weight of the boat has got to have something to do with it
    but thats bound to be considering diesels which have half the torque of steam

    so whats an average slip I should be aiming for and whats up with this thing not mentioning diameter of prop

    course its the idiots guide to props
  6. Guest625101138

    Guest625101138 Previous Member

    The boat pictured with the dimensions provided will require 115kW to do 10kts and 354kW to do 14kts using realistic prop efficiencies.

    The ideal prop for 740rpm will not be achievable with the draft. You need to determine the largest diameter prop that can be fitted. Once you know this you can determine the power required with some accuracy.

    Rick W
  7. philSweet
    Joined: May 2008
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    philSweet Senior Member

    20 percent slip at speed would be a good target number for high efficiency. Don't worry too much if you end up with 30 percent at 12 knots due to various constraints. How does this motor perform at "idle" or whatever you call a steam motor ticking over. Does it run right down to 0 rpm like an electric motor? I'd like to know you can go slow enough before figuring on cruising efficiency. It may be worthwhile to use more rpm at cruise, sacrificing a bit of efficiency, in order to gain some additional control at low speed. If the replacement motor was a 4 cylinder diesel replacing two 6 cylinders, I would expect trouble getting it to go as slowly as smoothly as the old setup. Of course you could always put a Kitchen rudder on it! I'm not trying to be awkward, I just have no sense of feel for the steam motor.
  8. Boston

    Boston Previous Member

    actually the steam engine is famous for slow speed high torque
    basically it will not run quite as high a torque as an electric at low rpm but its close ( sorta )

    the two diesels were 6 cylinders each and the new steamer is 4 cylinders which maxes out at 1000 rpm and typically would run at 400-800 rpm between about 500 and 900 ft/lbs

    Rick thanks for dropping by
    Im working on finding out the prop diameter for the original boat but its going to be difficult
    Ill post it soonest
  9. Boston

    Boston Previous Member

    scaling it off the lines I have about 1.17' of room to cram a prop under this thing
    that is really not enough space
    I might have to modify the lines to fit a decently sized prop

    here is a picture of the prop under the same hull configuration but a different deck plan


    here's a shot of the lines plan


    you can sorta make out the water line ( its the line with all the tick marks on it and the keel, that distance represents 3.5' so scaling that from the picture I come up with about 1.17 feet prop diameter
  10. Guest625101138

    Guest625101138 Previous Member

    And this from a fellow who argued that sailing directly downwind faster than the wind was impossible. Going back a couple of months you seemed to know everything there was to know about propellers!! You were the expert on the theory involved.

    I would have thought the calculations involved for a simple boat propeller would be a cinch for one so at ease with theoretical physics.

    Rick W
  11. Guest625101138

    Guest625101138 Previous Member

    Attached is what you would get if you could fit a 1m prop. It would be 40 x 26. This is larger than I think possible with a draft of 3.5ft without going for large shaft angle.

    If this prop could be fitted then note the power is less than my original estimate. Torque for 83.5kW at 740rpm is 1077Nm.

    Rick W

    Attached Files:

  12. Boston

    Boston Previous Member

    nah I was arguing that the amount of energy in a mass being used to power a sled at whatever gearing will only result in a transfer of kinetic energy equal to or less than the original energy applied. Which is sound physics

    turns out I was wrong about that into the wind thing ( Im ok with admitting Im wrong from time to time, pretty sure we're all going to be completely wrong at some time or another ) you aught to hear folks going at it up a the university when there is a question about some of the finer points
    and yes I did get the patent on plasma insulation ( I got the number round here somewhere if you want to look it up )
    fat lot of good its doing me though
    but I am far from a plasma physicist, I studied cosmology mostly ( thirty years ago ) theoretical astronomy in layman's terms
    I knew Ild be hearing about that eventually
    what I did on that one was take the guts out of a fluorescent light bulb, put em in a window frame, add argon to the space between the pains, and tune it using a harmonic resonance frequency tracking system out of a stereo amplifier to radiate at the optimal temp difference between inside and outside in order to create the lowest strength convection loop possible
    but your a good sport to be trying to help out cause you got me
    I dont know **** about propellers
    so no worries on calling me on being so wrong about that one
    I was and Im ok with it
    ( I wasnt ok with thug spamming nor was I ok with those guys descending on our forum with the deliberate intent of tricking the folks here into there games, lets get real they could have come out and explained themselves like they were asked to from the start rather than refuse to give any explanations and launch into personal attacks because they themselves couldnt explain mathmatically how that thing worked after offering to do exactly that, I just called em on it, at least you and I went round and round over a physics issue. Those clowns were in it to pretend they invented something they didnt and patent it as such )
    writing a paper on someone else's work is just plane plagiarism

    props are a science in themselves and I have little experience in em so it never hurts to ask a few questions of people who have obviously studied them in the course of my self education in the subject

    well that is one crazy conversion
    11,669 N = 83.5 Klw
    1 HP = 746 watts

    83500/746 = 112 HP

    15.7" what ?
    pitch = 660 mm or 26"
    at 70% efficiency

    70% cannot be the best I can expect
    what if I put a cowling around it shouldnt that improve efficiency
  13. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect


    You're getting bogged down in too much propeller theory which you can't actually control or measure right now, and is bit of a red herring for your application.

    1) What is the maximum diameter you get safety get away with, or would like? (is it the 1.17ft prop mentioned above?...can you physically go measure or just use the old lines plan?)
    2) What is the most advantageous RPM you wish to use, you have quoted several RPMs for given power above?
    3) Do you have any actual performance data of this or sister vessels? Since this relates to the actual resistance of the vessel and hence what power gives what speed and at what displacement.

    Without any of these, you wont have a target to aim at to design a prop for your SOR.
  14. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    Fit largest propeller that fits under hull. Figure max rpm for shaft, number of blades and shape. Don't worry the pitch to get it perfect, it will be wrong. But then you have to something to do so real numbers from. You can't really guess the prop because you don't know how hull or engine will truly perform.

  15. Boston

    Boston Previous Member

    the original hull topped out at 14 knots with two 210 hp diesels and around 450+ ft/lbs of torque pushing one prop and eating fuel
    the prop looks like it had a diameter of about 14" maybe 16"
    I have no idea of the shaft speed
    I can fit a pretty dam big prop under the hull but Im not sure how close to the surface I can run a prop without getting into other issues

    I could gear up the shaft speed of the new engine Im considering with a transmission but given that steam engines run or can run in both directions I thought it would be kinda trick to leave off the tranny and just reverse the engine when needed
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