Lakesailor
Well-Known Member
Perhaps the initial question is wrong. Maybe it's not that interesting.
Interesting Technical Question
- Thread starter Gludy
- Start date
Joe_Cole
Well-Known Member
Naughty! You'll be told off. /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/wink.gif
D
Deleted User YDKXO
Guest
You're welcome to the thread, gludy. Keep it, it's yours
hlb
RIP
Course he could always stick a set of mole grips on the shaft and let us all know what the result was, it's fairly simple to stop a shaft going round. For most!!
Gludy
Well-Known Member
I really do not understand why it upsets you so much when someone actually tries to get somwhere with a subject. Your personal attack in this case was without any foundation whatsoever ...... no sense of actually reading what has been written and then withdrawing your personal comments ...no, you simply take your ball away.
Gludy
Well-Known Member
yes- trouble with that is that you need fairly big mole grips and the means to measure fuel consumption ... a rare combination 
Major Catastrophe
Well-Known Member
[ QUOTE ]
My boat is GB42 Classic woody, with twin Lehman 120, approx. 5200 hours. My test ride has been carried out in very unstable wind & sea conditions, so that result may be accordingly inaccurate. I can provide complete table if someone interested, but real reason for this post is folowing simple experiment:
I choose speed 7 KN, and adjusted 1) only port engine, stbd freewheeling, 2) only starboard engine, port freewheeling, 3) both engines, to reach this same speed.
[/ QUOTE ]
To complete the experiment he should redone it with the second engine just in gear at idle speed. That gets over the drag/gearbox damage question.
My boat is GB42 Classic woody, with twin Lehman 120, approx. 5200 hours. My test ride has been carried out in very unstable wind & sea conditions, so that result may be accordingly inaccurate. I can provide complete table if someone interested, but real reason for this post is folowing simple experiment:
I choose speed 7 KN, and adjusted 1) only port engine, stbd freewheeling, 2) only starboard engine, port freewheeling, 3) both engines, to reach this same speed.
[/ QUOTE ]
To complete the experiment he should redone it with the second engine just in gear at idle speed. That gets over the drag/gearbox damage question.
hlb
RIP
It dont take the brain of britain to stop a shaft, a few lumps of wood would do, or an old vice, stiltsons???
It's quite easy to test. Go out on one engine till fuel runs out. Release prop and come back on other engine. you either make it or you dont!! /forums/images/graemlins/laugh.gif
It's quite easy to test. Go out on one engine till fuel runs out. Release prop and come back on other engine. you either make it or you dont!! /forums/images/graemlins/laugh.gif
Gludy
Well-Known Member
Yes- obvious but I did not think of that - that is an easy experiment to do as well.
In fact I would think that has to be the best of all solutions (for the record, I do not know) because the prop is at least being turned in the right direction - no gearbox resistance and there must be less resistance than even a locked prop - set aginst that is the tickover consumption of one engine.
You have introduced an intersting third way here ... thanks.
This would make an inteesting set of tests for a mag to do.
In fact I would think that has to be the best of all solutions (for the record, I do not know) because the prop is at least being turned in the right direction - no gearbox resistance and there must be less resistance than even a locked prop - set aginst that is the tickover consumption of one engine.
You have introduced an intersting third way here ... thanks.
This would make an inteesting set of tests for a mag to do.
jfm
Well-Known Member
Gludy we need to be careful, and be clear about parameters. If the constant against which you are measuring is a fixed 7 knots in a TSDY then 2 running engines might be better than one engine plus a load of drag from an undriven prop. In my case I was just referring to absolute highest mpg, regardless of speed, and that is achieved on our boat with one engine running at idle, 3.5knots,, and the other stopped.
I see you say it is accepted wisdom that a locked up undriven prop is less drag than one allowed to spin freely. I will happily be proven wrong but I gotta say i just dont get that. It seems intuitively correct to me that there must be less drag if the prop is allowed to spin. ??
I see you say it is accepted wisdom that a locked up undriven prop is less drag than one allowed to spin freely. I will happily be proven wrong but I gotta say i just dont get that. It seems intuitively correct to me that there must be less drag if the prop is allowed to spin. ??
Gludy
Well-Known Member
Now that sounds an intersting way of testing, you need a bit of faith in your theories though Mind you those with the wrong ideas simply do not make it back ... a sort of natural selection.
Mind you those with the wrong ideas simply do not make it back ... a sort of natural selection.
hlb
RIP
But most raggies seem to stop the prop for some reason. /forums/images/graemlins/confused.gif
Gludy
Well-Known Member
JFM
I too first thought that a free spinning prop offered less drag but ot was pointed out to me by boat designers that this is not the case and that such a prop slows the boat down more than a fixed one that is locked in place and simply has the water sliding over it. Since then anyone I have asked in the industry confirms it.
"In my case I was just referring to absolute highest mpg, regardless of speed, and that is achieved on our boat with one engine running at idle, 3.5knots,, and the other stopped."
And I accepted your point. I think that you are right - a boat with one engine at idle must consume less fuel, I would think, than a boat with boat engines at idle. But there again we are talking mpg so a boat with both engines at idle may go faster and make up for it. I think the point that the rules cannot apply throughout the speed range must be right and I tend towards agreeing with you but I am not at all sure.
How fast does your boat run with both engines running at idle?
I too first thought that a free spinning prop offered less drag but ot was pointed out to me by boat designers that this is not the case and that such a prop slows the boat down more than a fixed one that is locked in place and simply has the water sliding over it. Since then anyone I have asked in the industry confirms it.
"In my case I was just referring to absolute highest mpg, regardless of speed, and that is achieved on our boat with one engine running at idle, 3.5knots,, and the other stopped."
And I accepted your point. I think that you are right - a boat with one engine at idle must consume less fuel, I would think, than a boat with boat engines at idle. But there again we are talking mpg so a boat with both engines at idle may go faster and make up for it. I think the point that the rules cannot apply throughout the speed range must be right and I tend towards agreeing with you but I am not at all sure.
How fast does your boat run with both engines running at idle?
Gludy
Well-Known Member
I think its because it offers less drag stopped than free wheeling - although it may be to save the gearbox.
Its_Only_Money
Well-Known Member
[ QUOTE ]
That is wrong - locking decreases drag.
[/ QUOTE ]
The US Navy seem to disagree, look through this thread
(sorry but I don´t know how to link to a particular post...)
landaftaf quoted the US Navy towing manual 2001
"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."
So at best the jury seems out on that one, I suspect there are many differences depending for instance on the % blade area - which on mobos is generally much higher than on sailing yachts - and higher again I suspect on the vessels to which that manual is intended to relate. As usual it is dangerous to extrapolate evidence from one set of circumstances and assume it applies to all, in this area the results seem to differ widely.....
Just to spice things up I have found several references to pusher propellors being inherently more efficient than tractors....which might be a surprise in IPS-world... /forums/images/graemlins/shocked.gif
That is wrong - locking decreases drag.
[/ QUOTE ]
The US Navy seem to disagree, look through this thread
(sorry but I don´t know how to link to a particular post...)
landaftaf quoted the US Navy towing manual 2001
"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."
So at best the jury seems out on that one, I suspect there are many differences depending for instance on the % blade area - which on mobos is generally much higher than on sailing yachts - and higher again I suspect on the vessels to which that manual is intended to relate. As usual it is dangerous to extrapolate evidence from one set of circumstances and assume it applies to all, in this area the results seem to differ widely.....
Just to spice things up I have found several references to pusher propellors being inherently more efficient than tractors....which might be a surprise in IPS-world... /forums/images/graemlins/shocked.gif
Vara
Well-Known Member
[ QUOTE ]
It is an accepted fact that a fixed shaft is more efficient
[/ QUOTE ] Not on these forums it isn't, posts ad nauseam arguing the case, seems to depend on whether you drive a helicopter or not!
It is an accepted fact that a fixed shaft is more efficient
[/ QUOTE ] Not on these forums it isn't, posts ad nauseam arguing the case, seems to depend on whether you drive a helicopter or not!
Gludy
Well-Known Member
Interesting BUT what is meant by a free wheeling prop>
If its truly free, like in neutral than there is no clash but if its driving the gearbox then there is a clash and the designers telling me that the fixed prop offers less drag are challanged.
Can we find out if raggies reckon they have less drag with a locked prop - the racing guys would know.
If its truly free, like in neutral than there is no clash but if its driving the gearbox then there is a clash and the designers telling me that the fixed prop offers less drag are challanged.
Can we find out if raggies reckon they have less drag with a locked prop - the racing guys would know.
rwoofer
Well-Known Member
[ QUOTE ]
I dunno for sure but gut feeling says that locking the prop would increase drag, compared with just letting the prop spin. Surely the components are following the path of least resistance, so allowing the prop to spin as it wants to causes less drag than locking it?
[/ QUOTE ]
The reason the locked prop offers less drag than a spinning prop, is that a locked prop is effectively a stalled hydrofoil and the spinning prop is generating lift because it is acting hydrodynamically.
Exactly the same principles apply on raggie sails, just swap hydro for aero. A stalled sail if far less efficient at generating power than an aerodynamic sail.
I dunno for sure but gut feeling says that locking the prop would increase drag, compared with just letting the prop spin. Surely the components are following the path of least resistance, so allowing the prop to spin as it wants to causes less drag than locking it?
[/ QUOTE ]
The reason the locked prop offers less drag than a spinning prop, is that a locked prop is effectively a stalled hydrofoil and the spinning prop is generating lift because it is acting hydrodynamically.
Exactly the same principles apply on raggie sails, just swap hydro for aero. A stalled sail if far less efficient at generating power than an aerodynamic sail.
Its_Only_Money
Well-Known Member
Seems to be a mixture of avoiding the noise, reducing wear and tear on the gearbox, avoiding terminal failure of the gearbox, avoiding starting problems when stuck in gear....oh and some of them find the boat sails faster....but I think the props for which this is claimed seem quite different to a mobo prop so there may be a line crossed between the two.
hlb
RIP
Ah, well what you need, is fethering props, like the raggies have, course crap at stopping or going backwards, but you cant win them all.
