Locking the prop

[BrendanS]

Re: Wayward Son

You seem to think people are argueing with you, without reading the content. The aircraft posts just illustrate that prop driven aircraft do have mechanisms in place to stop freewheeling. Your original question didn't say what type. The article also says, if you read it all the way through, that it depends on the prop type, and there is no one answer. In the past, on similar threads, I've posted info that says freewheeling props more or less drag can mean different things depending on size and pitch of props.

Don't judge too quickly, or assume people are argueing against you. sometimes we are just providing information.

 

[Wayward_Son]

Re: Wayward Son

I have nothing against what you said BrendanS. It was dexter who showed displeasure by implying it bothered him that I thought I was right with disregard to others. I have said twice now that I may have misinterpreted the information. What else can I say?

Anyway, I have put more thought into this. A prop moving a craft forward has force applied to the back of the blade. A prop moving a craft in reverse has the force applied to the front of the blade. We all know that a reversing prop is inefficient and cavitates resulting in prop-walk or paddlewheeling. A freewheeling prop also has the force applied to the front of it and when forced to exceed a specified RPM for the individual prop will cavitate and produce substantial drag. So, I believe both stands on the topic are correct depending on the speed the rig can move.

As far as the aircraft in the article are concerned, they are referring to an engine failure of one of the engines. The craft is still being pushed forward by the other powerfull motors. The force on the running engines is on the back of the blades, the force on the dead engine is on the front of the blades. The speed the craft can maintain with the remaining engines woluld overspeed and cavitate the dead one. When I mentioned our plane, I said if proper glide slope is maintained the prop spins happily along. I should add that you can overspeed the prop by descending to steep. So again, both stands are correct dependant on speed.

Be it noted that I enjoy the differing points, because they spur thought and open new avenues to learning.

 

[BrendanS]

Re: Wayward Son

Much of this covered in the articles on the site I posted.

As to propulsion being applied, then doesn't the same apply to vessel under sail?

 

[landaftaf]

Re: Wayward Son

I have more extracts from the us navy towing manual but am unabale to paste them here in the forum ... themanual specifically states the drag ratios and about drag forces re locked and unlocked props ....
maybe someone can let me know how I can get these extracts on here .... they are from adobe format pdf and keep on automatically going to my clipboard in picture type mode as opposed to notmal cut and paste in word mode ...

I can of course write it all out but knowing the sceptics on here ........ I have looked at putting it on my web space but its 700 mb

so ....... shall I precis it for those unbelievers ...... also goes into navy props and merchant props, speeds etc.

brilliant book to have on cd
/forums/images/graemlins/grin.gif

 

[fireball]

Lost in the Post

I'm repeating this post cos u're either ignoring me, missed it in the mass of posting at the time or I'm talking bo||ocks ...

An earlier post on this gave a link to some (very complex) research done into props (air) and the drag characteristics - it seemed to be inconclusive as the variables depended on the size, pitch and number of blades.
In the outboard case - (I'll assume) a 3 bladed outboard prop has a far greater & flatter surface area than most "normal" shaft/saildrive props and so a different drag characteristic may well be found. (can't you lift the engine up anyway?)

For those that what more to think about - it takes quite a bit of energy to stop the wheels on a car - but once your skidding the stopping power is not so great - hence the need to cadance brake or anti-lock brakes in wet/icy conditions.
Apply this to a shaft - Is this not because there is a lot of power transfered through the shaft when the prop is being turned, but once it is stopped it takes far less effort to keep it that way.

The power transfered through a freewheeling propshaft is changed into something (power doesn't just dissapear right?) - so in most cases it is turned into heat and noise, hence the need to service the gearbox more often. So where does the power go from a locked prop? Hmm ...

Me? I lock my shaft - reverse gear with a Yanny 2gm20 - no noise! Stuff the drag ...

 

[landaftaf]

Re: Lost in the Post

no, not ignoring you - I read it ...... I was wondering if you read my latest which I posted about an hour or so ago, and the one last night ...... but there we are ...... /forums/images/graemlins/smile.gif

what I do find interesting is some of the the forumites attitude of apparently sneering and gawforing towards this interesting thread - so I sat back and watched with interest how the 'discussion' grew /forums/images/graemlins/tongue.gif
parallelling of aerodynamics and hydrodynamics, indeed even the comparison with custard ..... /forums/images/graemlins/confused.gif
if one wanted to be pedantic .... helicopter theory and aerodynamics dont compute so helicopters shouldnt really fly - apparently /forums/images/graemlins/confused.gif

so, although I am not qualified in aerodynamics, IMHO air at high speed flowing accross variable or fixed aero propellors doesnt actually mirror the complexed forces involved in a ship/boats propellor .... (in fact if indeed that it is the case why did our r.n. spent many years and loadsa money researching ships propellor theory).

from personal experience I can concure that trailing a freewheeling prop when steaming on the other increases speed through the water in comparison with if it is locked. /forums/images/graemlins/grin.gif I can also state that locking the prop on a tow is sometimes done to increase the drag and enhance control over the tow.........
at the risk of repeating myself, having done hydodynamics 30+yrs ago, and also having obtained an unlimited tug masters certificate of competancy (in addition to a normal masters cert) and done more towing than you can shake a stick at, .... and trying unsucessfully to present written evidence from the us navys towing manual 2001 stating exactly the fact that a locked prop caused more drag than a feewheeling prop, I find that the debate is somewhat stifled by the theorists that seem to insist that ' as the previous result was that a locked prop caused less drag then so be it ........' /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

my earlier post was

I fail to see the connection, and respectfully beg to differ about drag/resistance on a rotating prop.

debate here hasnt changed my opinion, but there we are, thats life huh, I wonder if this debate has changed anyone elses ?


/forums/images/graemlins/grin.gif

 

[Robin]

Re: Lost in the Post

I'm a declared lock the prop man, for reduced wear on gearbox, seals and cutlass bearing as well as peace and quiet, plus with our prop (Brunton) if not locked the prop doesn't feather itself.

I can understand your tug and tow comparison but I think there are HUGE differences with big (sometimes over 100% area ie overlapping blade) ship props and the 'sailing' props that we are using where the blade area may only be 30% of the circle. Another factor is that one blade (even on a 3 bladed prop) can be hidden behind the shaft skeg or deadwood if there is one, even partially behind the strut of a 'P' bracket. The big ship comparison may therefore not be as valid as it would seem? Mind you it is easier to envisage than crashing helos or planes with dodgy motors!

Likewise with the outboard lifting analogy, I think outboard props in neutral spin easily (I saw one spin in the wind once!) whereas inboard props with gearbox, seal, stern gland shaft and cutlass bearing drag do not spin easily. I believe for example there is a 5% or so difference in HP measured at the engine and at the gearbox output of marine diesels.

 

[Ships_Cat]

Re: Lost in the Post

A good sailboat prop will have a blade width about or less than 25% of its length. Very different to a ship propellor or even a MoBo one. So quite high aspect ratio towards aircraft prop style.

Henley's, who design and build propellers for around the world, maintain that their sailboat propellers are designed such that they have least drag when locked and recommend that they be so.

John

 

[Robin]

Re: Lost in the Post

Hi John

I think I remember when fitting a different prop to my last boat and doing all the pitch v dia calculations there was also a factor which was the blade area to absorb the HP available at the revs chosen without risking cavitation and that this was expressed as a %age of the full 360 deg circle. I know with the prop I chose (got the dia/pitch spot on too, after lots of sleepless nights/research mathematical formulae etc!) I was aware that at max revs/HP cavitation was possible but the blade area was around 30% I think from memory, it didn't cavitate in practice that I could tell. The prop was changed to match the new engine (MD22L like yours I think) and had a bigger pitch than the old which to my layman's eyes meant that it was more 'feathered' compared to the old one therefore a bit less drag despite bigger engine bigger prop.

Robin

 

[Ships_Cat]

Re: Lost in the Post

Hi Robin

Yes is a MD22L. Do you remember what dia and pitch your prop came out to? Ours is 3 blade 18 x 14 in, and its Blade Area Ratio is 0.38 (or so I believe). For around 35 foot WL, 0.9m canoe draft, don't know what the max beam is on the WL, around 11 Tonne with half tanks etc boat.

The prop's performance drops off very noticably with even quite light fouling which as we normally only get lifted every second year for a polish up is a nuisance. Water is too cold for me to get in between times to do anything about it /forums/images/graemlins/smile.gif.

Regards

John

 

[ccscott49]

Yes, you are wrong, a fixed prop produces less drag than a turning prop! Cany remember the science, but it does, I have proved it on my own boat. On her there is a noticeable difference and I have two of them.

 

[ccscott49]

Re: Wayward Son

Cavitation does not cause paddlewheel effect or propwalk, that is a completely different thing, as is cavitation and ventilation, cavitation is caused by a substantial drop in pressure on the blades of the prop, causing the water to boil, which then attacks the surface ofn the prop and destroys thrust. Vebntilation iss drawing air in from the surface, destroting thrust from the prop, but normally doesnt cause any damage. They are commonly mistaken, if you wish me to explain prop walk send me a PM and I will, I do know a thing or two about the subject.

 

[landaftaf]

Re: Locking the prop - whoopee, here are some references

aha - dunnit /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif..... please find extracts from the yanky navy towing manual 2001 /forums/images/graemlins/blush.gif

also - please note that contrary to what other forumites have stated, this concept isnt only for large blade area props .... it is calculated for generic props as the value of Rp, and Rp is reduced by 50% for rotating props, and zero for removed props.

mind you - I would be suprised if anyone is still interested in this theoretical and dry subject.
/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif




5-7.1.8 Removing Propellers
For long-distance tows, fixed-pitch propellers
may be removed to decrease towing resistance.
For some hull forms, however, the added
drag of locked propellers may be desirable
for better directional stability. Tow planners
must also consider the economic feasibility of
removing the propellers.

6-3.3.7 Propellers
A locked propeller will create a larger drag
than a free-wheeling propeller, thereby result
ing in reduced towing speed. The additional
drag in the stern due to a locked propeller,
however, may decrease the tendency of the
vessel to sheer off from the intended track.
Refer to Section 5-7.1.7 through 5-7.1.11 for
information on preparing the propellers for
tow.

Item 7
List projected area of all propellers (AP) in
square feet. This value assumes that propellers
are locked. If propellers are trailing, reduce
this value by one-half. If propellers have
been removed, use a value of zero.

3-4.1.1 Calculating Steady Resistance of the
Towed Vessel
Steady resistance (RT) of the towed vessel
may be estimated using the following
approximation:
where:
RH = Hydrodynamic hull resistance of
the tow
RP = Hydrodynamic resistance of the
tow’s locked propellers
RW = Wind resistance of the tow
RS = Additional tow resistance due to
sea state

RT = RH +RP +RW + RS

 

[Wayward_Son]

Re: Wayward Son

[ QUOTE ]
Cavitation does not cause paddlewheel effect or propwalk, that is a completely different thing, as is cavitation and ventilation, cavitation is caused by a substantial drop in pressure on the blades of the prop, causing the water to boil, which then attacks the surface ofn the prop and destroys thrust. Vebntilation iss drawing air in from the surface, destroting thrust from the prop, but normally doesnt cause any damage. They are commonly mistaken, if you wish me to explain prop walk send me a PM and I will, I do know a thing or two about the subject.

[/ QUOTE ]

Read Chapman's Piloting - Seamanship & Small Boat Handling regarding efficiency of propellors in reverse, prop-walk and cavitation. Anybody with a single prop driven craft knows that when it is engaged in reverse the stern will drift to the direction the prop is paddlewheeling because it is inefficient at developing thrust due to the blades deing designed for forward travel.

 

[Lakesailor]

Re: Locking the prop - whoopee, here are some references

I too have read through the whole thread.
I have now lost the will to live, where's that hosepipe?

 

[PuffTheMagicDragon]

Re: Locking the prop - whoopee, here are some references

Well Done!!
This topic is like the monsoon or hurricanes - it comes round every season since I have been on the forum. Even when it was not yet in this form!
I have always believed that it was futile to argue with people who, at best, can only quote technobabble. They usually don't understand one half of what they're repeating and most times they are quoting someone else's opinion and not their own! /forums/images/graemlins/confused.gif

As far as I am concerned the answer is quite simple :- just look at the COG as shown on your GPS, and then choose whichever option you believe is best. Simple, no? /forums/images/graemlins/wink.gif
In my case the engine maker specifically recommends leaving the prop free and <u>mine</u> is a three-bladed affair with a cover factor of 85% !!
Fair Winds! /forums/images/graemlins/wink.gif

 

[landaftaf]

Re: Locking the prop - whoopee, here are some references

. /forums/images/graemlins/shocked.gif

it should be interesting to everyone .... cos the initial reaction was - <font color="red">well we have already done this subject and its dead .... when in fact its not really dead at all ....</font>

is it ? /forums/images/graemlins/confused.gif

/forums/images/graemlins/grin.gif

 

[landaftaf]

Re: Locking the prop - whoopee, here are some references

so why was my last post mysteriously removed ........ thought police huh

<font color="red">the issue was written off as done and dusted but it seems it wont lie down .......... new last minute evidence gives it a reprieve</font>

doesnt it ?

/forums/images/graemlins/grin.gif

 

[landaftaf]

Re: Locking the prop - whoopee, here are some references

s h i t - where did the first post come from ....... it wasnt there just now on my screen ?

 

[Wayward_Son]

Re: Locking the prop - whoopee, here are some references

It's been there for quite awhile.

What's a propellor?