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Old 09-12-2008
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Question for all our Mech. Engineer types ???

Hi All,

I have a couple of questions.

#1 I am looking for a prop thrust calculator that would show me how much thrust my boat can develop. Can anyone point me to a link that might help me determine this how much thrust my boat has?

#2 Does thrust equal pull? By this I mean, if I tied my boat to a dock and put a device in the line, then put her in gear and let her rip does 1000 pounds of thrust turn into 1000 pounds of "pull" on the line???


If anyone can answer these two questions I'd really appreciate it..!
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Last edited by Maine Sail; 09-12-2008 at 11:54 AM.
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Old 09-12-2008
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Hal,

When I was re proping a few years ago I looked, and could not find any.

But being an Mech. Engineer I set up a spread sheet to estimate the approximate differences between various propellers I was looking at. I did not calculate the out put in LBF as that would be extreamely difficult and was more interested in speed differences at several given RPM settings. The problem is friction, the flow of the water around and near the propeller, propeller slip, counter forces etc. . .

This is also why I do not believe that static thrust in water is equal to dynamic thrust, it may be close but the water flow around the prop will be very different between the two. I have the feeling that the static may read higher that what can actually be achieved when the boat is in motion. Other Mech's specializing in fluid dynamics should able to correct me on this.

What I had done was calculate the therortical speed based upon the measured slip rate of the old propeller (current propeller at the time) was giving me. This was an approximation at best, since all propellers have different slip rates, and friction properties. This estimation helped me deterime which new propeller was what I was after. Interestingly enough the projected speed at various settings was within .2 knot of observed. I got very lucky with my estimations.

If you do find a calcuator or even some equations I would be very interested in them. Everything that I found was not very useful, I never did purhase Gerr's book.

Best of luck.
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Old 09-12-2008
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I'm..

I'm not really trying to determine through the water thrust but rather from a dead stop how much pull would be exerted on the dockline and if thrust and pull are the same from a dead stop?

I guess what I'm asking is stationary thrust less than through the water thrust and if so by how much and how do you determine it somewhat accurately?

I know, this is a tough one!!
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Last edited by Maine Sail; 09-12-2008 at 12:11 PM.
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Old 09-12-2008
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halekai-
Mercury Propellers | Props Fundamentals

And they apparently sell a $5 cardboard slide rule to do the work, too.

"boat propellor thrust calculator" brings up all sorts of hits. Apparently static performance (tied to the dock) won't quite be the same as free performance. Still, "push" is the converse of "pull" which is close to thrust. Your scale in the line should at least tell you which prop is giving you the best static performance. It woulod be the same reading as a "compression scale" ahead of the boat.
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Old 09-12-2008
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Hal,

Agreed It is a tough one,

First, the pull on the dock line would equal the thrust of the propeller plus or minus any current / wind / external forces being applied, also being careful of the vector of the propeller thrust / relative to the vector of the pull measurement (minor but still an influence).

As far as the second part Experimentation. I would think a quick experiment is in order. Tie boat to dock measure pull at say 2000 rpm. recording steady state tension in LBF. Using the same force gauge and a friends boat pull yours at measured lbf as recorded in at the dock and record the speed. Next crank up your boat to 2000 rpm and record your speed. Now you got three data points and two unknowns. The calculations should be pretty straight forward. But this will apply to your boat, and your propeller at that RPM. Other settings will be different.

Now that you got me curious, I will help with the with calculations as time permits. I believe that a model can be gernatered to map the whole performace of the engine if multiple settings are taken.
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Old 09-12-2008
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Patrick

My reasoning for this is two fold.

#1 I am trying to determine how much pull I can get at about 80% reverse thrust while setting my anchor. I want to be able to compare the pull exerted to say 30 knots of windage load in ft lbs. I know my engine can't develop the same load as 30 knots of wind from a stop but I'd like to know just what I can replicate with the engine? Hope that makes sense?

#2 To be able to compare the thrust of my fixed three blade prop to a new adjustable pitch feathering prop and to be able to dial it in for the best possible performance. Simply dialing it in so I can turn max rated RPM is not the whole story so I want differing variables to compare to at different pitch settings.

For instance I may be able to achieve max RPM with four to five different settings but not all will be the most efficient and develop the most thrust for getting up rivers with large tidal currents. Hope that made sense?
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Real results

For real results with your boat I think you need to rent a digital dynometer (where???) like they use in anchor tests. While your at it you can test your anchors.
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I have a lead...

I have a buddy who works for a company that has one that has not been used for years. Says he can grab it any time...
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Old 09-12-2008
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Hal,

The thought process makes sense.

For item one, the static measurement of reverse thrust from the engine should be straight forward. Both are static forces, i.e. the thrust from the propeller in reverse at 80% is a static anchor setting force, the wind load in assumed ideal conditions of a 30 knot wind for comparision purposes would be treated as a static load as well. And a direct comparison can be made. But, one thing that does not to me is the windage load in foot lbs. That is not what the anchor line will see in a loading situation. I would not think that the resultant force from the windage would not be a torque on the rope but a force.

On item two, I sort of disagree. While thrust appears to be the output to compare, I would almost say that the output to compare would be speed. This is the thought process that I went through as well when I reproped. The relationship of the hull to the water and the force it takes to move the boat varies with the speed of the boat. As well as the efficiency of the propeller varies at different speeds. Some of the things that I read term this slip, efficiency, what ever, you get the picture I lumped both variables together and assumed drag on the boat / propeller slip was one variable.

What I found when I was mapping my existing propeller I had 32% slip at 1000 rpm, up to 45% slip at 3600 rpm. By using this, then could I compare the various propeller options assuming that the slip model remained the same for the new propeller. I am limited to a 13/15HP, Yanmar 2gmf.

When I think of achieving max RPM at wide open throttle and your last example couple thoughs come to mind at least with limited HP, lets I have two pitch setting that achieve and slightly exceed Max RPM at WOT. Then the higher pitch would be the best for conditions with no wind or current (could run the motor at a lower rpm to achieve the speed required for maxium speed to fuel usage), but against a strong wind and heavy seas the lower pitch would be more be most effective, since at the higher pitch maximum rpm would not be able to be achieved. Overall I tend to agree with the maximum pitch setting that achieves slightly greater than WOT. A rough sea will increase the slip or the thrust required.

I hope this made sense, I have five interuptions while I was typing.

Feel free to email me.

Pat
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Hal,
I'd recommend acquiring a copy of Introduction to Steel Shipbuilding, by Eiljah Baker. You can find a copy cheap or expensive through Amazon and it will provide you with good entry level formula to deduce what you desire.
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