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Old 08-08-2016, 09:34 AM   #1
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HP loss from engine to prop?

Did a search. Couldn't find what I was looking for. So...

Does anyone know the approximate percentage loss that you might usually see between the engine and the prop at displacement speeds for, let's say, a 30k lbs. trawler? That is, if I want, for example, 50 hp at the prop, how many hp is the engine going to have to be producing?

I realize this will vary with hull shape, LWL, etc. Just looking for a rough rule of thumb, if there is one.


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Old 08-08-2016, 02:06 PM   #2
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I'll take a shot at this.
My old Perkins T6.354 was advertised as 160 BHP (brake horsepower) and 130 SHP (shaft horsepower).
That's "around" 20% difference.

Jay Leonard
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Old 08-09-2016, 12:31 AM   #3
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Many "prop calculators" available on line ask you to enter engine HP and the number of bearings between engine and prop....in the Output section they then display the HP available at the prop (and recommend pitch & diameter). You could use one of those to give what might be a reasonable accurate estimate of the losses for your particular vessel.
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Old 08-09-2016, 04:00 AM   #4
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I would guess it would be a 10-15% loss in total on a 50hp engine. About 5% lost to parasitics (alternator, pumps). Another 5% at the transmission, and couple % at bearings & stern tube. More if you have engine driven refrigeration, or multiple big alternators.

The hull shape doesn't really come into the equation. It's all a matter of what is hanging off the engine.
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Old 08-09-2016, 06:15 AM   #5
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Refrigeration , water maker, multiple alts and hyd pumps are not part of propulsion losses.

"160 BHP (brake horsepower) and 130 SHP (shaft horsepower)."

This is a typical SAE number , where the engine water pump , a cooling fan are not part of the BHP measurement.

A tranny might loose 5% , but most will be down to 2% or 3% as this is something tranny makers work on.

If you can turn the shaft by hand in neutral the stuffing box is OK and the shaft bearings are too.

50 at the prop , perhaps 55 with a modern stuffing box packing like Duralast.

This is something you cant change , the temperature of the air in the engine room can be changed.
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Old 08-09-2016, 09:15 AM   #6
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Lower gear ratios consume more power.
Hydraulic operated gears have a pump oil pump that consumes power. 5hp max for a BW.
Takes power to sling oil around inside the gear box.
I do'nt think the damper plate consumes power .. but it may.


North Western Washington State USA
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