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Old 01-19-2013, 03:49 PM   #1
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Prop position from hull

Guys

In a trwaler with slow speeds like 7 or 8 knots, what is the minimum distance from the prop to the hull? Is there a rule for that as there are distance rules for planning hulls?

Thanks
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Old 01-19-2013, 04:16 PM   #2
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Some pictures of the power train
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Old 01-19-2013, 05:22 PM   #3
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I don't have the exact measurements but that's an 15" prop and it's 24" from the bottom of the boat to the bottom of the skeg at the rudder.
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Old 01-19-2013, 11:59 PM   #4
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I thought somebody'd jump in here w some hard numbers but no. There are rule of thumb clearances recommended but since it varies w prop size most won't be able to say exactly what they are.

If you get the prop too close to the hull each time a blade passes by it slams water onto the hull bottom causing noise and vibration. It can be a lot. Metal and other thin skinned hulls are more sensitive to this than wood.

Slam a heavy rope up under the hull and see how much noise it makes. If it's loud I'd lay up something heavy (like thick FG matt) over about 4 sq ft inside the bottom of the boat right above the propeller. Just an idea as I've never done that but I think I'd do that. Uncover some stones and do a bit of research. I see you've already got a 4 blade prop. If I were you I'd wish your propeller shaft was lower. Big job to change that though.

Your local prop man can probably tell you the rule of thumb clearances and If you're good to go in that respect you'll probably just go w what you've got and see how it works out. On this the fix (if there is one) is a lot of rework so I'd want to see how much there is to "fix" in the first place before I'd consider a rework.

Lets hear what your prop man says and I'll look on BoatDesign.com.

Already found it. One guy said 15% of wheel diameter and a NA suggested 20% for a minimum. Do you have more than 20%?
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Old 01-20-2013, 12:45 AM   #5
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[QUOTE=manyboats;


"Already found it. One guy said 15% of wheel diameter and a NA suggested 20% for a minimum. Do you have more than 20%?"

Maybe I'm a bit slow today, but I don't understand that.
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Old 01-20-2013, 06:06 AM   #6
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If the prop is 15" diameter, 20% would be 3" clearance from hull.
This is in response to Andy, not the OP.
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Old 01-20-2013, 07:08 AM   #7
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Hi Eric and gang

Thanks for the input. The Prop is 34" diameter and the clearance above is 5 3/4" which puts the prop within the so famous 15%. I'd like more but I thnik I am ok with the clearance.

Thks guys
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Old 01-20-2013, 08:21 AM   #8
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With a slow turning wheel and trawler speed you can be at 10% and be fine. If you find you are "burning" the bottom paint off above the prop, or if you get vibration above the prop at higher rpm then you have too little clearance.
Even if you do get some vibration, it is fairly easy to remedy. I did that when I repowered my old Mainship 34. A few well placed gussets glassed and the vibration was gone. I was at about 10% clearance.
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Old 01-20-2013, 05:47 PM   #9
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Trawler type boats do have slower moving props but they also have much bigger blades pushing more water than our go fast cousins. This fella installed a shoe from keel to rudder to protect his prop. Within 100 hours the blade tips were half as thick as what they should be and well on their way to disintegrating. Your spacing is much more generous that that shown in the photo and will probably be OK but given the cost of a prop, I'd suggest you monitor.
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Old 01-20-2013, 08:15 PM   #10
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bluebyu;If the prop is 15" diameter, 20% would be 3" clearance from hull.
This is in response to Andy, not the OP.

Thanks for that,it was the reference to a 'wheel' that through me. Hadn't heard it referred to in that way before.
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Old 01-21-2013, 10:12 AM   #11
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Thanks for that,it was the reference to a 'wheel' that through me. Hadn't heard it referred to in that way before.
Sorry, the term wheel is used around my area interchangably with propeller.
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Old 01-21-2013, 10:39 AM   #12
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I've thought that was kinda silly. What's wrong w propeller? Where did did we start calling props wheels? We talk a lot about efficiency and having multiple words for the same thing seems inefficient to me. A lot of people call cars rides now and it's stupid as the word car is WELL established. I think most or even all of this is just one little fad after another. Stupid stupid stupid as John Grisham said in his book The Client.
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Old 01-21-2013, 10:47 AM   #13
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Greetings,
Mr. Eric. A proponent of "newspeak" are we? That's double plus good.
Newspeak - Wikipedia, the free encyclopedia
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Old 01-21-2013, 11:26 AM   #14
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I looked for the word wheel and didn't see it. I almost didn't read more than 5% of it but I kept on looking and reading.

So it is all fad ... in w the new out w the old. I hate fads and vouge-ness it's so childlike. As time goes on (or should I say "goes forward") lots of newspeak has good reason and justification but the majority of new words (like "ride" for car) is just people trying to be cute, to be vogue, to fit into some group and basically to be cool.

To me unless there is a good reason for a new word oldspeak is just fine and should be left well enough alone. Newspeak is mostly sillyness (Marin's word) and the greatest amount of newspeak is found among teenagers. If you want to identify w that group adjust your wheel and go home in your ride.

Thank you though RT I did learn something. I read "lodging" on the road and it comes out "logging" in my head and "newspeak" I think was "newsspeak" until your post.
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Old 01-21-2013, 12:56 PM   #15
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I don't know what the correct percentage is but you surley would want enough room to at least apply a shaft zinc.

And isn't there something about cavitation causing implosions from small bubbles breaking Or exploding and actually rupturing off small particals of metal. So you would want it at least far enough away to prevent the cavitation.

Be advised disturbances in the water flow forward of the propeller can result in blade damage which appears to be blade cavitation, but is actually due to non-favorable water flow into the propeller.
As per boat poker's post

A little help here on the cavitation thing. Not sure.

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Old 01-21-2013, 02:13 PM   #16
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This is from Dave Gerr's Propeller Handbook. I recommend buying and reading the book. It is one of the better texts I have read, with lots of helpful information. It does read a bit like an engineering text though. Sit down with a pad of paper, engineering calculator, a large cup of strong coffee and enjoy good reading.

RPM SL Ratio Min. Tip Clr.
200-300 under 1.2 8%
300-1800 1.2-2.5 10%
1000 & above over 2.5 15%
high speed planing craft over 3.0 20%

These clearances represent absolute minimum. For propellers in an aperture, or with a protective skeg below, tip clearance should be at least 12%, 15% or greater if possible. High speed planing craft must have over 20%. Tip clearance should never bee less than 2 inches on any vessel.
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Old 01-21-2013, 03:40 PM   #17
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I looked for the word wheel and didn't see it. I almost didn't read more than 5% of it but I kept on looking and reading.

So it is all fad ... in w the new out w the old.
If it's a fad, calling a propeller a wheel is an old fad, dating back about 200 years anyway.

The propeller tip erosion shown in the picture looks like it may be more the result of that "ramp" just forward of the wheel than the clearance. A Kort or Rice nozzle equipped vessel might have a tip clearance of less than 1 percent of propeller diameter and benefit tremendously from that condition.
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Old 01-21-2013, 06:17 PM   #18
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Quote:
Originally Posted by RickB View Post
If it's a fad, calling a propeller a wheel is an old fad, dating back about 200 years anyway.

The propeller tip erosion shown in the picture looks like it may be more the result of that "ramp" just forward of the wheel than the clearance. A Kort or Rice nozzle equipped vessel might have a tip clearance of less than 1 percent of propeller diameter and benefit tremendously from that condition.
Agree with both the "wheel" comments (been around yards calling them wheels for over 50 years) and the ramp more so than clearance...but hard to imagine that kind of cav erosion on a trawler...wonder if the pic is from a 10 knot and under or much faster boat.

This is right from one of the major propeller manufacturer's websites...

About Michigan Wheel Marine

Producing the finest boat propellers possible, while maintaining the highest standards of quality.

Company Overview.....



http://www.miwheel.com/home/
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Old 01-21-2013, 07:07 PM   #19
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Quote:
Originally Posted by manyboats View Post
I looked for the word wheel and didn't see it. I almost didn't read more than 5% of it but I kept on looking and reading.

So it is all fad ... in w the new out w the old. I hate fads and vouge-ness it's so childlike. As time goes on (or should I say "goes forward") lots of newspeak has good reason and justification but the majority of new words (like "ride" for car) is just people trying to be cute, to be vogue, to fit into some group and basically to be cool.

To me unless there is a good reason for a new word oldspeak is just fine and should be left well enough alone. Newspeak is mostly sillyness (Marin's word) and the greatest amount of newspeak is found among teenagers. If you want to identify w that group adjust your wheel and go home in your ride.

Thank you though RT I did learn something. I read "lodging" on the road and it comes out "logging" in my head and "newspeak" I think was "newsspeak" until your post.

Eric,

Try this one on for size............

I just got new skins for my sled... all my peeps gave me props for the rad bling...

regarding the term "wheels".. it's a single vs. twin thing... a single has a prop.. twins have wheels


I couldn't resist!
HOLLYWOOD
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Old 01-21-2013, 07:22 PM   #20
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Greetings,

Got it!

Single...


Twins...

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