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Old 28-12-2009, 12:31   #1
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SHP and Transmission Mechanical Advantage

I apologize in advance if this ends up being a stupid question, but there seems to be a big gap between the horsepower requirements (on paper) to move a displacement hull through the water, and what everyone estimates for diesel engine horsepower.

The main thing that "appears" to be missing, is the mechanical advantage realised through the transmission.

If I have a 3 - 1 transmission (prop runs at 600 RPM, while engine runs at 1800 RPM), wouldn't I also be getting a 3 - 1 mechanical advantage?

If the calculations say you need 30hp at the prop, wouldn't you only need 10hp at the shaft of the engine?

I am looking at trawlers, and having a hard time understanding why there are such high-horsepower engines with these significant mechanical advantages via the transmission.

And yes, I understand it's not a 1-1 relationship - friction reduces the efficiency of what can be expected.

Thanks in advance for the education I'm probably getting ready to receive!


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Old 28-12-2009, 13:06   #2
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Large engines on trawlers...

I suspect that one of the reasons for large engines on trawlers is the need for those on board to get to a nearby destination quickly and then back in time to go to work.

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Old 28-12-2009, 13:40   #3
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A Gear Ratio can increase the output torque, or output speed, of a mechanism, but not both.
The law of energy conservation dictates that one can never get more energy in the output, than provided by the energy source. Indeed, one always has some energy loss in a Power Transmission.
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Old 11-02-2010, 22:13   #4
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Often it depends on the size and pitch of the prop-in the end its the prop not trany or motor that pushes boat- there are some good articles and threads on along this line The hp needed to reach hull speed or planning speed in a boat can be closely estamated along with the usual safety margin for extra wt. and adverse conditions and motor size and torque needed- then the prop size has to be determined (usually by how much clearence room) and based on prop size an appropriate trans. ratio is selected. Small props need to turn faster to push boat large props slower for same total thrust. Yes many of the newer trawlers are overpowered - reasons #1 many are not true trawlers they are semidisplacement hulls which with enough hp can be driven beyound hull speed.#2 Many true trawlers also have bigger motors some for good reason(hydrolics-extra power take offs etc) and the ability to push a little over hull speed with a big quarter wave and a sucking sound as fuel goes down the drain #3 Same reason we have cars(family sedans with over 300 hp) I actually own one- people like power even wastful power.
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Old 16-02-2010, 14:33   #5
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Well, a 3-1 ratio (or any ration) is just gearing At any given RPM 3 engine turns to 1 prop turn would give more torque than 1-1 but not more horspower, likewise 3 prop turns to 1 engine turn gives less torque but not less horsepower. the power produced is your constant the torque is the variable. Yes, there is some lost horsepower in the transfer but not significant I believe for our applications.
There are two main reasons for the bigger engines, 1. Heavy boats regardless of displacement require gobs of torque which is more easily produced from lots of cubic inches at low rpm's than from a smaller engine at low rpm's (think of cubic inches in terms of displacement) so a big engine running de-tuned can economically produce lots of torque
2. It is more economical to produce engines of any kind in quantity, so by detuning the same engine that would plane out a 45' Sports Fish to power your 36' displaement trawler makes sense to the builder. the same engine can be used with/without turbos, intercoolers etc. and serve both purposes. also the great majority of diesel engines come from a modification or slight redesign of an existing power plant and in the Western world that more often than not, means bigger from the start.
Just my $.02
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