kgf

[Dominic M]

kgf (kilogram-force) is used as a power rating for many bowthrusters - Vetus seem to rate theirs this way. I have scrabbled around on Google on occasion trying to find a conversion formula to horse power. Do any of you brain boxes out there know how to do it?! Wikipedia give an explanation of kgf (also pointing out that it is not an internationally recognised measurement but it only gives a conversion to newtons, which doesn't help me at all).

[Kraken]

Horsepower is a unit of power (i.e. a rate of doing work) where as kgf is a unit of force, like pounds or newtons, not power, so there can never be a conversion factor. They are giving you an indication of the amount of thrust developed, but not power.

[Chris Pink]

Gosh, you've got my brain rattling back to school days, 'cos i know what I mean but how to explain it.....

 

well.

 

Kgf is a unit of force, horsepower is a unit of power, which could be expressed as kg-ms^-1

 

so you need some metres and seconds to convert your kg to hp

 

Try Wiki;

 

http://en.wikipedia.org/wiki/Horsepower

http://en.wikipedia.org/wiki/Kilogram-force

Edited February 7, 2008 by Chris Pink

[dove]

Gosh, you've got my brain rattling back to school days, 'cos i know what I mean but how to explain it.....

 

well.

 

Kgf is a unit of force, horsepower is a unit of power, which could be expressed as kg-ms^-1

 

so you need some metres and seconds to convert your kg to hp

 

Try Wiki;

 

http://en.wikipedia.org/wiki/Horsepower

http://en.wikipedia.org/wiki/Kilogram-force

 

WHAT!!

 

I agree with Kraken, a propeller can't have an horse power rating because it slips, It can probably have an IHP.

 

Andrew

[Dominic M]

Thanks for the input. So there is no way of converting one to the other it seems. Yet some manufacturers do rate their bowthrusters in hp - look at . It remains a mystery!

Edited February 7, 2008 by Dominic M

[dove]

Thanks for the input. So there is no way of converting one to the other it seems. Yet some manufacturers do rate their bowthrusters in hp - look at . It remains a mystery!

 

That's the motor size to you a clue on cable and switching sizes

 

Andrew

[howardang]

If it helps to equate BHP with thrust, my boat has a BETA 1903 rated at 43 BHP and a Nobel Hydraulic Bowthrust rated at 6 BHP.

 

As others have said, thrust is presented in lbs, kgs or tons force as is bollard pull.

There is a crude rule of thumb formula for the relationship between bollard pull and horse power which is:

each 100 BHP equates to 1 ton in bollard pull. Therefore, for a bowthrust of 6 horsepower it would give a very rough figure of 135lbs or the equivalent of a 9 stone person pushing at maximum weight which seems about right!

 

HTH

 

Howard Anguish

[Kraken]

Some manufacturers tell you how much thrust or force the bow thruster can exert, while others tell you the power of the motor as Dove has said. There is no direct conversion factor. To give you some understanding of the relationship between power and force: Work is the measure of a force applied through a distance. For example one pound of force applied through one foot of distance is one foot pound of work. Power is a measure of the timed rate of doing work. For example 550 pounds of force applied through 1 foot in one second, or 55 pounds of force applied through 10 feet in one second is one horsepower.

[Dominic M]

Some manufacturers tell you how much thrust or force the bow thruster can exert, while others tell you the power of the motor as Dove has said. There is no direct conversion factor. To give you some understanding of the relationship between power and force: Work is the measure of a force applied through a distance. For example one pound of force applied through one foot of distance is one foot pound of work. Power is a measure of the timed rate of doing work. For example 550 pounds of force applied through 1 foot in one second, or 55 pounds of force applied through 10 feet in one second is one horsepower.

Blimey! I hope I don't come up against you in a pub quiz

Thanks for the education though - much appreciated.

[sideways]

kgf (kilogram-force) is used as a power rating for many bowthrusters - Vetus seem to rate theirs this way. I have scrabbled around on Google on occasion trying to find a conversion formula to horse power. Do any of you brain boxes out there know how to do it?! Wikipedia give an explanation of kgf (also pointing out that it is not an internationally recognised measurement but it only gives a conversion to newtons, which doesn't help me at all).

There is a standard for figuring horsepower to thrust. Most thruster manufacturers will begin with 21 lbs of thrust per horsepower. That is usually conservative and acurate with older style kaplan propellers. Some of Lewmars thrusters will run at 28 lbs of thrust and they have one of the best test tanks in the world.

Side Power has a pretty good tank and their reported thrust looks fairly accurate.

When Lewmar and Side Power tested Vetus they came up with results lower than Vetus claims.

Little things like propeller clearance between prop tip and tunnel make a big differance.

Basically use 21 lbs per horsepower and it is easy to convert to kgf

[howardang]

There is a standard for figuring horsepower to thrust. Most thruster manufacturers will begin with 21 lbs of thrust per horsepower. That is usually conservative and acurate with older style kaplan propellers. Some of Lewmars thrusters will run at 28 lbs of thrust and they have one of the best test tanks in the world.

Side Power has a pretty good tank and their reported thrust looks fairly accurate.

When Lewmar and Side Power tested Vetus they came up with results lower than Vetus claims.

Little things like propeller clearance between prop tip and tunnel make a big differance.

Basically use 21 lbs per horsepower and it is easy to convert to kgf

It's good to see that your rule of thumb agrees quite closely with mine (see above). As you say exact figures depend on a number of factors re propellertype, underwater hull shape, nozzle/no nozzle etc etc. It also depend too on whether you use BHP or IHP.

 

Howard Anguish

[sideways]

It's good to see that your rule of thumb agrees quite closely with mine (see above). As you say exact figures depend on a number of factors re propellertype, underwater hull shape, nozzle/no nozzle etc etc. It also depend too on whether you use BHP or IHP.

 

Howard Anguish

Years ago the outboard motor companies rated their units in several different ways. Horsepower off the crankshaft, horsepower off the propeller shaft, horsepower with different octane fuels, and on and on. Now it is off the prop shaft and ,I believe, Honda is the only company that will also use "Thrust".

Thrusters and windlasses do not have a shared "standard" as such. A couple of the companies are beginning to set the standard by using strain gauges in a test tank. The thrusters are mounted in a tunnel with propellers and run against one or more strain gauges. Both Side Power (in Norway) and Lewmar (Havant) allow visitors to see them.

Sometimes the tests incorporate a lever arm and lift weights.

It would be interesting to have a type of bollard pull but I do not see that happening. We have a tug boat builder very near and their finished boats must come in at a predetermined level on the bollard.

The lack of standards has allowed most of the thruster manufacturers to claim anything they want to. A few take it seriously.

It is much worse with windlass claims. The honest companies will actually pick up a given amount and rate how fast it happens before overheat. Some place the weight on a wood pallet and drag it across a warehouse floor and I heard of another that placed the weight in a shopping cart.

I am a bit prejudiced toward Lewmar as they have been the most open and honest of the of the four thruster companies we have delt with. I have been allowed to inspect the test equipment and get my nose right in the middle of actual tests.

I would love to be able to see BHP, IHP, amp draw, run time to overheat, decible level ect. on all thrusters but we are years from that.

In the mean time there really are some meaningful numbers if one knows where to look.

[Yoda]

It is interesting that someone has picked up on the point of blade tip to tube diameter. This is a critical diamension for a fan type thruster, Power loss is greatest with the gap between to the point of being like an open propeller. Thrusters should be measured as Kg force or from shaft horse power or the equivilant.

 

Propellers are different in that they are not ducted. Therefore the power should be measured as absorbtion. That is the power that the prop. can take until slippage occures. Absorbtion power should be plotted against output shaft rpm

 

Engines should be measured at the crankshaft for power if being sold as just an engine. If fitted with a gearbox and ancillories such as generators thus making it a power plant, the measured power delivered, should be from the gearbox output shaft.

[DHutch]

Although vetus now call the models by the rated force they produce, they still also quote the power in both kilowatts and horsepower.

 

For instance the equivlent model for our unit is now the Bow55 (previously the Bow50, previously there '4hp' model)

 

Vetus Bow 55

Technical Data:

Motor: Reversible DC motor,

3 kW output (4 HP)

Thrust: 121 lbf.

Length of tunnel (L): min. 12" (optional equipment).

Diameter of tunnel: 5 29/32" I.D.

Weight, excl. tunnel: Approx. 44 lbs.

Voltage: 12 Volt D.C.

Operating time: 4 min. continuously or max. 4 min. per hour at 375 A (12 Volt) resp. 205 A (24 Volt).

 

 

 

Daniel

[ChrisPy]

trying to rationalise the basic relationship between thrust and power -

 

if you had a very low-pitch fan or propellor it would be like using a car engine in low gear - you can produce a huge amount of thrust against a static or nearly static resistance, with limited horse power, but that is not very useful once significant movement is involved.

so the pitch of the fan or propellor is optimised to provide good thrust at a reasonable flow speed through the tube, with moderate horsepower, to give maximum effective thrusting effect taking into account the resistance represented by trying to turn the bow of the boat.

 

a comparison is a 50hp outboard swinging an 8" high pitch prop and driving a light boat at 40knots, with little thrust but lots of feet per second.

compared with a big workboat with a 50hp engine driving a low pitch 36" prop, with a maximum speed of 4 knots - lots of thrust but limited feet per second.

Edited February 11, 2008 by chris polley