Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 08-03-2015, 15:00   #31
Registered User
 
Nicholson58's Avatar

Join Date: Jan 2011
Location: Live aboard
Boat: Camper & Nicholson58 Ketch - ROXY Traverse City, Michigan No.668283
Posts: 3,466
Images: 83
Re: Prop Brake

Quote:
Originally Posted by iancoombe View Post
There's been talk of reduced drag if the prop is allowed to spin, I'm not sure about water but if the engine in a helicopter stops I want the rotors to keep turning to slow down the decent rate, known as auto rotation.
This is counter-intuitive but totally correct. There is more turbulence and more work being done if its rotating - more drag. Locking it is less bad.
__________________

__________________
Nicholson58 is offline   Reply With Quote
Old 08-03-2015, 15:34   #32
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,330
Re: Prop Brake

Quote:
Originally Posted by vinayfm View Post
Actually a revolving prop applies more brake than one that is locked and capitate. In my humble opinion



Sent from my iPad using Cruisers Sailing Forum
Quote:
Originally Posted by GILow View Post
And here we go again...
Quote:
Originally Posted by Nicholson58 View Post
This is counter-intuitive but totally correct. There is more turbulence and more work being done if its rotating - more drag. Locking it is less bad.
And you need even more effort (work) stop it rotating which means stopping it increases drag.

The OP wouldn't need to apply any effort to stop the shaft rotating if it produced less drag stopped rather than spinning.

Let's not worry about what is intutive or not, let's just just use some science and think the issue through from first principles and oh, let's leave aside airplanes and helicopters; their engines and transmissions systems are quire different and the fluid dynamics are not the same.

So cutting to the chase, fixed blade props create more drag when braked than when allowed to free wheel.
__________________

__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 08-03-2015, 15:57   #33
Marine Service Provider

Join Date: May 2012
Location: New Orleans
Boat: We have a problem... A serious addiction issue.
Posts: 3,940
Re: Prop Brake

Quote:
Originally Posted by vinayfm View Post
Actually a revolving prop applies more brake than one that is locked and capitate. In my humble opinion

Sent from my iPad using Cruisers Sailing Forum
Then you opinion is wrong. This question has been debated ad nausium on sailing forums, but the scientific debate is settled. The only real open question is how much additional drag is produced. This is a function of the specific prop in question, and boat speed, but the results are always the same. Allowing a prop to spin creates less drag.

NOTE: less drag is nice, but not destroying your transmission is likely more important. So only allow your prop to spin if your transmission allows for it.

The best study i have seen recently below. But it is in line with the rest of them.
http://www.plaisance-pratique.com/IM..._sdarticle.pdf

"The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% more drag was observed, this being at higher speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic resistance is significantly affected by the amount of frictional torque on the shaft, low torque being accompanied by low drag."
__________________
Greg

- If animals weren't meant to be eaten then they wouldn't be made of food.
Stumble is offline   Reply With Quote
Old 31-03-2015, 12:22   #34
Registered User

Join Date: Sep 2014
Location: Tromsø, Norway
Boat: Långedrag steel, 1988, 40ft
Posts: 86
Re: Prop Brake

Hi there,

The setup with the belt breaking and stopping the prop is very interesting since possible to remote control and simple and I'm considering copying that system. I wonder if that doesn't introduce a lot of pull upward and can misalign or otherwise do something to the gearbox or shaft seal? The same goes for belt driven alternators?

We used to have a bike disc brake from the previous owner. However, the disk was so thin and didn't hold up, so we used the string around a bolt solution as described by others and it works perfectly and is cheap. It is awkward to climb down to the engine room each time, and a remote controlled solution is most preferable.

Phil
__________________
phipseml is offline   Reply With Quote
Old 31-03-2015, 23:27   #35
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 1,373
Images: 7
Re: Prop Brake

It appears that having a split disk with a split bushing to fasten it onto the shaft would not be much of an engineering problem to implement and even do yourself.

On the caliper part it appears that if you could find the pads to mount on a couple of pieces of flat bar which were fixed to an engine bed with a pin with the brake cable fixed onto the other end, you could implement the calipers very cheaply. You are not trying to brake a heavy vehicle by dissipating a lot of kinetic energy just one little old prop and shaft. Might even be simpler than making a mount for a motor vehicle caliper?
__________________
RaymondR is offline   Reply With Quote
Old 31-03-2015, 23:30   #36
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 1,373
Images: 7
Re: Prop Brake

PS

Doesn't a Roberts 53 have room for another bunk away from the prop?
__________________

__________________
RaymondR is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Barlow Winch w/ Brake? What's This For? kevman1421 Deck hardware: Rigging, Sails & Hoisting 15 19-08-2015 13:58
How to Create a Shaft Brake ? Geoff H. Monohull Sailboats 14 12-05-2010 22:23
Lewmar Steering Brake Seizure seanie20 Deck hardware: Rigging, Sails & Hoisting 0 18-03-2010 17:59



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 22:08.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.