boffin thrust question

[deep denial]

Does anyone have any rough idea of how much static thrust in kgs or lbs I should expect from a 12 hp engine (dolphin 2-stroke) with a fixed 3-blade prop? we have been a bit disappointed with performance under engine and I though I might try to test the thrust available by attaching a strain guage to the boat whilst moored and revving up in gear.
precision not required - any rough ideas?

 

[Evadne]

I've not tried to look this up on Google as I assume you're able to do it yourself. My 24lb thrust electric motor is rated at rather less than 1hp, so I would expect 12hp to come out at about come out at 250lb thrust. It will all depend, inter alia, on prop efficiency at 0kts. Don't forget that you only develop max. hp at optimum revs and boat speed.

 

[Pye_End]

Will this work?

If the boat is stationary you may end up with so much cavitation that you do not end up with anything that will be accurate enough?

If you are trying to problem solve engine performance then (assuming the prop is clean!!!!!) maybe better to start with max achieved engine r.p.m., then check prop size against theoretical best and start from there.

 

[deep denial]

thanks - at least that gives me some guide, and it will be an interesting exercise if nothing else.

 

[EdWingfield]

Kevin, out of interest, what max speed are you getting in smooth water + no wind, and what type of boat?

 

[milfordman]

I would not pretend to know very much about props and their thrust, and I am quite sure that there are some forumites who work with them every day. However I think that assuming your prop is 3 ft under water then the pressure absolute will be approx 16 psi, so if the prop is cavitating you will have pressure differential of 16 psi, so if you multiply 16 by the area of your prop that will give you a max static thrust.

I know that as others have said this is different to operating under normal conditions. However I recall that when some outboards were compared in pbo they used static thrust running to compare them, so presumably this should give a reasonable result.

p.s. Pick a good post to pull against, your motor may be better than you think.

 

[billcowan]

I have often pondered this:

How DO they measure 'bollard pull' on ships?

Obviously not by bulling a static bollard, as noted this would result in prop cavitation and a very falsely low reading.
anyway be dangerous to have a ship thrashing about near a jetty.

 

[Stoaty]

You get very little cavitation on ships. Big props turning slowly and deeper in the water give less cavitation and are more efficient. My boat's static bollard pull is rated at 1.5 tons (imperial) on a 30" prop, max prop revs 700 rpm, 88hp engine.

 

[William_H]

A propeller is surely like a gearbox in a car. In so much as it is the interface between the rotation of the shaft and the water moving aft.
If you were interested in max bollard pull then a very fine pitch prop would give full engine power at the rated RPM. (very low gear ratio)
However we normally choose a prop pitch that will give forward thrust when the boat is moving at normal (cruise or max) hull speed.
Now most boats don't have the luxury or variable pitch propeller like a tug boat.
So to try to tow a way a fixed jetty means that the boat is in high gear all the time. The engine labours unable to reach max RPM for max power and all the rest of the enginnepower is taken up in moving water past the prop and around through the prop again or in cavitation.

So real thrust is a measure of how much power in the forward direction the prop can achieve when the boat is moving at normal speed. I t might perhaps be measured or at least indicated by a spring balance in a tow rope to a boat being towed where the towed load can be increased to slow down the towing boat by say 10%.

Or it could be measured by measuring the thrust bearing push on the hull as the boat accelerates to cruise speed. ie thrust will be less at or above cruise speed but max a bit before cruise speed but again less at lower speeds.

I seem to be waffling here but you can see thrust is not an easy one to measure. A suggested to the original poster engine RPM and hull speed if plotted and then used as a reference over a period of time will establish a normal for this boat propeller combination.
A finer pitch prop will give more thrust at lower speeds (towing ability) but the engine will over rev while the torque is not being extracted from the engine at max hull speed. A course pitch prop will slow the engine down with excess load and at max hull speed the engine will not be going fast enough to produce full power.

olewill

 

[johnalison]

Just to give you a rough idea, we used to get somewhere up to 5 1/2 to 6 knots max with a Dolphin on a <26-footer (Mystere) and usually cruised around 4 3/4 to 5. Great little engine. We once motored half the way to Ijmuiden, about 15 hours continuous.

 

[vyv_cox]

<<< You get very little cavitation on ships >>>

Where did you get that from? Ships' propellers and rudders are very susceptible to cavitation damage. Materials improvements (to nickel-aluminium bronzes) and hydrodynamic computer predictions have made the situation better than it was, but cavitation erosion of these components remains a considerable problem. I can't show you any examples, but if you Google 'cavitation ship' you will get some idea of the scale of the problem.

 

[Talbot]

The thrust at the bollard is not just a function of the engine, The gear ratio will have an impact as will shape and size of the propellors. I would be very surprised if accurate figures are available from anyone other than the prop manufacturer, so might be worth contacting Steel Developments ltd.