thrust and bhp

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1charan
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thrust and bhp

Post by 1charan »

My forty featherweight is quite loud. So I was going to make a SEEgull. The project died because I did something wrong with the electrical stuff, didn't take the time to do it right. But, my girlfriend (and I too) wanted a more quiet experience on the water. So i bought myself a secondhand electrical outboard. A Rhino VX34. That's right, it's not rated in horse power, but in pounds of thrust!

The featherweight is supposed to be about 40 lbs of thrust, max boat speed about 5.5 km/h. With the Rhino the max boat speed was not that much different, max boat speed about 5 km/h. Being an electrician I couldn't resist measuring the power used by the Rhino. At full speed it uses a little over 20 Amps at 12 volts, making about 250 watts. One bhp equals 735 watts, so the Rhino is 1/3 bhp (roughly, depending on too many factors). This means the featherweight is max 1/2 bhp!

I think BS was right in rating their Seagulls in thrust instead of bhp.
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Gannet
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Re: thrust and bhp

Post by Gannet »

Interesting topic. My take on it is this:-

20A input at 12v (is it actually 12v at the motor?), is as you say 240W. But one ought to take into account the loses within the electric motor (would it be about 10-20%?), because output power is what one wants to know. Additionally, of course there are gear loses (perhaps 5%? - assuming a bevel gear is used, although the motor might be mounted on the prop shaft) , so that the power put into the prop might be about 190W ie 0.26hp!
Then assuming the Rhino prop efficiency is the same as the Seagull prop (probably unlikely) at the same conditions, then one can make the comparison.
But, in the end of the day, it is thrust at a given speed that counts.

I do not know exactly how engine outboards are rated, but it is a function of the engine's maximum output power rather than thrust. Prop efficiency is often very poor and always highly variable on speed, boat type etc.

It appears to me as if BS got into a bit of a tangle with their hp figures. I can well understand why, because its thrust that is really important and not the hp of the engine.

Jeremy
1charan
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Re: thrust and bhp

Post by 1charan »

But how to measure thrust? I could put an electronic scale between the boat and the thrust block to measure forward thrust, but that does not take into account that the outboard pulls on the transom backwards. I'm willing to build something that will work, but don't know what to build.

For the Rhino I used a Lithium battery, 12 volt 50 Ah. The voltage as measured at the battery was about 12.5 volts. Between the lowest speed and the highest speed there was only 0.1 volt difference. Since the copper leads are less than a metre long and of sufficient size, the voltage won't have sagged too much near the electric motor. There is no gearbox on the Rhino, it's a direct drive with the electric motor where the gearbox would be in a Seagull. We used it continuously for three hours without the voltage going down, but at a slightly lower setting, using 15 amps and going 4.5 km/h.
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Charles uk
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Re: thrust and bhp

Post by Charles uk »

Seagull measured their bhp using a Ricardo Cussons dyno that measured the power output on the drive shaft without any gearbox losses, that I belive they purchased in the 1970's.

Before that point in time they must have made educated guesses & comparisons perhaps on the generous side to come up with their power output numbers.

When Seagull measured their 45lb thrust figure for the featherweight in the early 80's, they used a spring balance device that was built into a transom bracket so the thrust could be measured at peak revs, & just like a teenager with his first means of transport the max performance would not be an uphill against the wind figure.
Plus the motor measured probably had a 416 carb, CDI ignition & a sealed gearbox running EP 80 & had been picked from that weeks production as the best example for testing.
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Adrian Dale
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Re: thrust and bhp

Post by Adrian Dale »

Boat speed is directly related to the water line length (WL) of the vessel being propelled through the water (1.33 X Square root of the WL measured in feet) = speed in Knots. This equates to a 3 meter dingy having a maximum speed of approximately 4.5 Knots. This speed is reached when the bow wave, running down the side of the hull, reaches or approaches the stern of the vessel. Although different boat shapes and materials will make slight differences to the formula due to fluid friction the quoted formula is sound for most purposes.
To exceed this speed the boat needs to climb its own bow wave and lift onto plane. This means a very much larger engine with a completely different gearing.

So in the question of the featherweight and the electric motor, it is probably that the vessel in question reaches its hull speed at say half throttle for the FW and, opening the throttle fully will not necessarily increase the hull speed but simply change the trim of the boat as it attempts to climb the bow wave. The little electric motor however may require its full rated capacity to reach the same speed. Put simply, one engine is running at full power the other, is at best, half power to achieve the same result. Running both engines at full power does not change boat speed. In adverse weather the featherweight will have sufficient reserve power to continue driving the vessel at it hull speed whereas the electric motor will see the speed fall off.. make sense?

If a 102 were to be placed on the back of the same boat the resultant speed would remain the same, this is because the extra power available from the 102 is still insufficient to over come the hull speed.

From this we can see that a direct comparison of two engines on the same boat running at it's rated hull speed will not provide a comparison of engine power.

On the subject of Torque; changes in gear ratio and propeller pitch and diameter provide a wide range of different torques for any given engine. The simple analogy is that of a 4X4. when in high range it may have a speed of 150Kl/hr; However go into Low range and for the same revs the speed is now perhaps only 30 Kl/hr. Attach a heavy trailer and in high range try to go up a steep incline, the vehicle may stall or labour heavily, but in low range it will pull the trailer with little effort.

This is why, as Charles indicates, BS and indeed any engine manufacture needs special equipment to first measure and from the results then calculate the power and Torque.

AJ
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Re: thrust and bhp

Post by Gannet »

As Adrian and Charles have said, special equipment is required to measure power. However I am sure that we have all observed how a piece of wet and dry against the flywheel slows down the engine - even the high power FVs!
So what about a strap or rope around the flywheel, with one end fixed to a solid object, and the other connected to a spring balance. There are various practical considerations of course. Run the engine at full throttle, pull the spring balance so as to get a constant load (ie constant torque), measure the speed and hey presto, you have the data to calculate the power. In other words, a classical brake dynometer. If you remove the prop and crownwheel, then you will have the engine power. Do it at various torques to produce a power curve.
A good project for somebody - Adrian?

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Charles uk
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Re: thrust and bhp

Post by Charles uk »

Would that it were that simple

I already have 2 dynos, Seagulls original Cussons one, that I can't seem to calibrate & might no have been fit for purpose & one I built myself, getting an accurate reading of something as simple as the revs is no simple matter, there is a lot of engineering & computing in a rev counter that is better than 1%.
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Mike Killay
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Re: thrust and bhp

Post by Mike Killay »

What I have difficulty with is that the 40 power head coupled to a 40 minus leg and propeller will produce (allegedly) 45 lbs. thrust.
Swap the same power head onto a 40 plus leg and the power apparently climbs to 55 lbs.
Gannet
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Re: thrust and bhp

Post by Gannet »

Crikey Charles, what accuracy are you trying to achieve in reading revs? What accuracy does a good optical rev counter give? Probably not 1%.
I would have thought that achieving a power value to within 10% would be quite acceptable for everybody - except of course for those deeply involved in the racing scene.
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Charles uk
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Re: thrust and bhp

Post by Charles uk »

If your rev counter can't show you a difference in revs when you try a different carb jet, your just playing.

When you give some thought on how optical or spark plug impulse revcounters work, ignoring the quality of their components, you will see where the problems lie.

Let's put one on your favourite FV, on the back of your boat at full throttle, 4000 revs exactly.

This figure is arrived at by counting electrical pulses over a 1 second period & multiplying the total by 60 to give RPM, pausing for 5 or 10 seconds so you can read it & then do it again.

So 66 pulses times 60 = 3960 RPM, so not counting the .66 of a rev gives a 1% error, if your realy doing 4019 RPM it will still read 3960 & that 1.5%, more than 2 degrees of timing change or 1 jet size will show, this error gets smaller as the revs rise 8000revs = 133 revs per second. 133 x 60 = 7980 RPM, not the best example but you get the point.

The problem gets much worse at lower revs 2033 revs, 33 pulses per second x 60 = a reading of 1980 RPM, thats an error of getting on for 2.5%.

Depending on the chip design/cost several other glitches creep in alongside this making matters worse.

Seagull used a vibrating spring rev counter for their dyno work, Charles P has posted pictures of one he owns, I'd guess accurate to + or - 100 RPM, where as mine, counts in 12 degree increments of prop shaft rotation but requires the gear ratio to be input, so the engine revs can be calculated & takes all readings, torque & RPM from the prop shaft after the lower unit & water pump losses.

If your going to spend a lot of time doing a job you might as well do it right!
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Gannet
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Re: thrust and bhp

Post by Gannet »

I thought that the initial post by 1charan was interesting as it was a slightly sideways/original view of comparing power and thrust, but certainly thought provoking. I don't think that there was any expectation of great accuracy.
My 'James Watt' dyno was a just little bit tongue in cheek and again accuracy would obviously be 'approx'; rather in the manner of the BS brochure statements of '1.5 - 2.5 hp category'.

Yes, Charles, I am sure from a racer's viewpoint of assessing changes, great accuracy is absolutely critical. Your comments on rev counter accuracy is very knowledgeable and detailed as well as interesting.
However, for the rest of us, in discussing the determination of power or thrust, to say 10 or 20% or so accuracy; the measurement of engine speed to within 1% is not necessary.

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Charles uk
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Re: thrust and bhp

Post by Charles uk »

Then take the electronic bathroom scales to bits, the load cell goes into the thrust block, the screen & battery, where ever you can see them.

The numbers will be hard to read as they will dance about a lot, don't worry about leverage & the powerhead wanting to move backwards, this is effectively a static installation.
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Re: thrust and bhp

Post by Collector Inspector »

You are being a Toad Charles.

B
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1charan
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Re: thrust and bhp

Post by 1charan »

[quote="Adrian Dale"]Boat speed is directly related to the water line length (WL) of the vessel being propelled through the water (1.33 X Square root of the WL measured in feet) = speed in Knots. This equates to a 3 meter dingy having a maximum speed of approximately 4.5 Knots. This speed is reached when the bow wave, running down the side of the hull, reaches or approaches the stern of the vessel. Although different boat shapes and materials will make slight differences to the formula due to fluid friction the quoted formula is sound for most purposes.
To exceed this speed the boat needs to climb its own bow wave and lift onto plane. This means a very much larger engine with a completely different gearing.


Adrian is of course absolutely right in his calculation, except for one small thing. I measured the boat speed in kilometers per hour, Adrian is talking about knots. One knot is 1.852 kilometers per hour. Thus my dinghy has not reached its max boat speed. Max boat speed would be just over 8 kilometers per hour. I can see this in the wake of the boat. With the Rhino the created waves are nearly non-existant, with the FW the waves are noticeable, but not disturbing for other boats, not even for ducks. Though most ducks fly away because of the noise!
SJP11446LL parts only, TC52234L unrestored, THC67581L unrestored, FP1986JJ4 running, GF2355EE7 running,TC63272 work in progress, AD52014 unrestored, sEEgull work in progress.
1charan
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Re: thrust and bhp

Post by 1charan »

Charles uk wrote:Then take the electronic bathroom scales to bits, the load cell goes into the thrust block, the screen & battery, where ever you can see them.

The numbers will be hard to read as they will dance about a lot, don't worry about leverage & the powerhead wanting to move backwards, this is effectively a static installation.


The location of the load cell is important. If the load cell is high on the transom, the measured force will be higher than when the load cell is placed lower on the transom. So what am I measuring? Or; how to translate this into something meaningful? It probably has something to do with the distance between the mounting bracket and the propeller and the relative place between these two where the load cell is placed?

Anybody dare to guess in which range this would be? Are we talking kitchen scale or bathroom scale numbers?
SJP11446LL parts only, TC52234L unrestored, THC67581L unrestored, FP1986JJ4 running, GF2355EE7 running,TC63272 work in progress, AD52014 unrestored, sEEgull work in progress.
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