thrust and bhp

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Re: thrust and bhp

Post by 1charan »

Charles uk wrote: Providing your using the same gearbox & prop.
Not at all. That is just what we are comparing!

Static Bollard Pull for various Seagulls. Just tie a rope to a tree, put some sort of measuring thing between the rope and the boat, and start your Seagull. See how hard it pulls at full throttle, mention the model and the gearbox and the propeller. This should become a nice list. I suppose the Barge Pusher will top the list, but at least we will know by how much. For the people with enough time and propellers, try the same model with various propellers, rpm's would be nice too. I'm willing to sort the data and put it in a spreadsheet, with graphs and maybe even conclusions.
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Adrian Dale
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Re: thrust and bhp

Post by Adrian Dale »

A good comparative exercise but it won't tell us much about thrust or BHP.

A suggestion:

Take a heavy displacement vessel which will not pop up on semi displacement or plane and calculate its hull speed

Tow the vessel with a heavy line having minimal stretch (Small nylon rope is of no use here) Fit a load cell into the line and tow at the calculated hull speed and again at 75% of hull speed

Tow the vessel at constant speed for each test in two directions over a set distance and record the load cell and speed.

The result will provide an accurate figure of the actual force required to tow the vessel at hull speed and 75% of hull speed.

With these results, connect your test engine and again carry out the same test over the same distance and course. Carefully monitor the speed to match the tow test.

Record engine revs and boat speed (accurate engine reves are critical). Conditions must be the same as when towing therefore smooth water is essential

The force required to propel the vessel at the corresponding speeds will therefore be equal to force required to tow the vessel.

These tests are also carried out in places like Marin, Wageningen, Holland see in tow tanks. in this case the tow vessel is connected to an overhead trolley that pulls the vessel down the tank, different wave and wind conditions can be impute to achieve a wide spectrum of conditions.

When the force is obtained it can be used to calculate the engine power. By having the two speeds, coupled with the engine revs we can then compute engine performance curves.

Points to note: the test vessel must have a clean hull and not be dragging an outboard engine. The tow line must be short and kept taut, it must not be so long that the catenary drags in the water. The towed vessel must not be so large that the test engine cannot reach its calculated hull speed. The towed vessel must be loaded and trimmed identically for all tests

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Charles uk
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Re: thrust and bhp

Post by Charles uk »

The condition of the gearbox & the engine revs makes quite a difference to the amount of energy it takes just to turn it, as does the prop blade area.

If you note the engine revs when performing this test it should be possible to find a rev count that most models can achieve so that all static bollard pulls can be found at these revs, so that like is compared with like.

It should then be possible to extrapolate the free running thrust with just a revcounter.

Any Seagull racer will tell you that 3 identical Seagulls will all run at different speeds under identical conditions on the same boat, often with more than a 10% difference between the best & the worst.
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