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Old 19-06-2013, 20:24   #1
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Depth of Prop

Since a left hand prop pushes the aft end to port (I think) and vice versa with a right hand prop. the lower blade is deeper, thus it is in denser water causing the above action. OK, my question is: If I lower the prop deeper, will I have more forward force? My problem is simple. I have a 55# thrust electric motor, which will not push my 2,800# boat, unless conditions are perfectly calm. Before I lower the prop, I would like your opinions Puleez.
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Old 20-06-2013, 09:11   #2
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Re: depth of prop

G'Day Fred,

Actually, the water density does not change with depth, so your basic premise is incorrect. Prop walk is, I believe, caused by the proximity of the hull to the upper blades of the prop changing the flow characteristics of the water through the prop.

However, if your prop is so close to the surface that it ventilates (sucks air from the surface) the thrust will be compromised and lowering it could indeed help.

Cheers,

Jim
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Old 20-06-2013, 09:29   #3
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Re: depth of prop

Unlike air, water is not compressible. While the pressure changes at depth, the density does not. The only way that lowering the prop will improve performance is if there is cavitation at the current depth. Otherwise, you're better off experimenting with the prop's pitch and diameter.
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Old 20-06-2013, 10:54   #4
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Re: depth of prop

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Originally Posted by Bash View Post
cavitation
Ventilation is often confused with cavitation.
Cavitation = region of partial vacuum.
Ventilation = propeller sucking air from water surface, symptom: vibration and lost of thrust, can be cured by getting the prop deeper under the surface or reducing diameter. See Dave Gerr Propeller Handbook
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Old 20-06-2013, 14:13   #5
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Re: depth of prop

Quote:
Originally Posted by Capt.Fred View Post
Since a left hand prop pushes the aft end to port (I think) and vice versa with a right hand prop. the lower blade is deeper, thus it is in denser water causing the above action. OK, my question is: If I lower the prop deeper, will I have more forward force? My problem is simple. I have a 55# thrust electric motor, which will not push my 2,800# boat, unless conditions are perfectly calm. Before I lower the prop, I would like your opinions Puleez.
A right handed prop (Clockwise rotation) will push the bow of the boat to starboard when in forward and push the stern to port when in reverse. The opposite works with a left handed prop.

Looking from the rear of the boat the direction the prop turns is how to determine the left or right handed prop. Clockwise - Right Hand or Counter Clockwise - Left Hand.

Now back to your question....
Is the prop turning at full speed and not moving the boat at all... If so, like previously suggested, play with the pitch of the propeller.

The only reason to "lower" the propeller, deeper in the water, is if it is not completely submerged when running.

Also remember Trolling motors are not made to maneuver you boat in anything more than light winds and seas.

Here is a web page that talks about Trolling Motors

Interestingly enough this page talks about thrust, but then recommends Trolling Motor sizes by length of boat.... According to the above web page the T-55 should be used on no more than an 18 foot boat, but does not talk about weight/displacement.

The key sentence on that web page is: "If your boat, gear, and passengers are extremely heavy, going to a motor with even more thrust would be your best bet"
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Old 20-06-2013, 14:31   #6
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Re: Depth of Prop

In addition to the good information above, it should be noticed that prop walk becomes insignificant with momentum or with the prop wash directed over a rudder. Usually the prop walk only has a significant role when maneuvering in reverse with a single prop from a stationary position.
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Old 20-06-2013, 15:18   #7
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Re: Depth of Prop

Love pressure stuff. Not really helpful to your question, but interesting.

from ground to space is an atmosphere. around 14 psi ..if memory is correct, is like14.7psi

My math might be a little off, but I'm not giving a class, but more explaining the principal. My math is close enough.

so, that same amount of pressuare is at 33 ft of seawater too. so you would have 2 ata on top of you at 33 ft. the 14 psi the ground to space and the 14 psi from the 33 ft of water equaling a couple of ata.

The pressure changes, but the water will remain the same density. If the water changed density, it would go from water as we know it to an oil consistency to a jelly to a solid. That is not the case.

The pressure from 0-5 feet is not great. Think of this. You can go to the bottom (probably) of a 5ft pool and not feel your inner ear squeezing, but if you go 10 ft down, you would need to plug your nose and blow to "valsalva" and add pressure to "equalize".

In dive school a long time ago, we breathed a pure o2 rig. Was cool. You would wake up in the middle of the night and have to "reverse valsalva". Imagine inner ear was filled with pure o2 and your body over the course of hours while sleeping would absorb the 02 and create a vacuum in your inner ear. We called it dregger ear.

Sorry for tangent, I loved the physics behind diving! Still do.

I have an ob on my boat. You should not have propwalk with your trolling motor, so that is not a worry. If you wanted a gas ob, that would work great especially if you are used to a trolling motor. I have never used electric, but would love to test a torqueedo on my boat.

Let us know what you conclude for your motor if you change it. Get a video if you can.
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Old 20-06-2013, 15:35   #8
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Re: Depth of Prop

In fact, water is *slightly* compressible. See Properties of water - Wikipedia, the free encyclopedia

Keep in mind that the actual thrust provided by a propeller decreases when the advance speed increases with constant RPM: a propeller provides much more thrust at bollard pull than underway.

For this reason, I find it strange that trolling motors are sized using thrust in lbs instead of power in hp or kW.

Alain
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Old 20-06-2013, 17:30   #9
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Re: depth of prop

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Originally Posted by jeremiason View Post
A right handed prop (Clockwise rotation) will push the bow of the boat to starboard when in forward and push the stern to port when in reverse. The opposite works with a left handed prop. ...


My left-handed prop pushes the bow to starboard when in forward gear; in reverse gear the propwalk is to starboard also. When moving forward under power (not drifting), a left, three-degree rudder is necessary to maintain a straight course.

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Old 22-06-2013, 04:51   #10
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Re: Depth of Prop

Thank y'all for this information. I always have been a little hazey on prop depth. I've watch empty container ships in San Francisco Bay chugging along with their props almost half above the water. I suspected they would take on ballast before entering the Pacific.
I have an 8hp Tohatsu on my RR23 and I thought for just leaving and entering my bayou, I would hang the 55# thrust electric over the side and keep my precious Tohatsu high and dry. I fabricated a bracket and...well, it did not work. I was just contemplating whether I should put the prop deeper. Thanks again, you've saved me the bother.
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Old 22-06-2013, 05:49   #11
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Re: Depth of Prop

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In addition to the good information above, it should be noticed that prop walk becomes insignificant with momentum or with the prop wash directed over a rudder. Usually the prop walk only has a significant role when maneuvering in reverse with a single prop from a stationary position.
Especially when docking with lots of people watching.
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Old 22-06-2013, 14:03   #12
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Re: Depth of Prop

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Especially when docking with lots of people watching.
Yeah, but that's when you can use the prop walk to your advantage. Just stop off your slip and, with the steering kept hard to starboard, make short surges of forwand and reverse and your boat will turn in little more than it's length until your stern is just the next "prop walk" from backing easily in.

Prop walk belongs in the tool box!
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Old 23-06-2013, 07:12   #13
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Re: Depth of Prop

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Yeah, but that's when you can use the prop walk to your advantage. Just stop off your slip and, with the steering kept hard to starboard, make short surges of forwand and reverse and your boat will turn in little more than it's length until your stern is just the next "prop walk" from backing easily in.

Prop walk belongs in the tool box!
I know. I was trying to be funny.

The only way I can back into a slip in my single screw inboard sailboat is while backing and turning to port on my boat, due to prop wash. And, you're right, you can use the prop wash to your advantage. It does go in easy that way and won't go in at all backing to starboard.

I used to have twin inboard cruiser and the first time I had to back into the slip with one engine dead, it would have made a good funiest home video clip.
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Old 23-06-2013, 13:59   #14
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Re: Depth of Prop

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I know. I was trying to be funny. .......................
'sorry. Humor slips right by and over my head all the time!
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