Reply
 
Thread Tools Search this Thread
 
Old 24 March 2008, 10:17   #11
Member
 
Country: UK - England
Town: Fleet
Boat name: Hudson
Make: Ribeye Sport
Length: 6m +
Engine: Yamaha 150
Join Date: Sep 2007
Posts: 128
Quote:
Originally Posted by SeaSkills View Post
Prop guards limit your performance,
I'd never really thought about this before but why is this?

I ask because ducted fans on air propellers are quite a bit more efficient than an open propeller. I seem to remember this is due to a combination of factors such as directed airstream / reduced radial vortex shedding at the prop tips etc.

So why not a ducted prop on a boat??? The obvious guess would be due to the much higher viscosity of water creating substantially higher drag that negates any advantage. Or is it simply that prop guards are just built as guards and are not engineered for efficiency (e.g. a ducted air fan requires clearance between prop tip and guard to be minimised and the inlet shape is v important)?

Just intrigued...
__________________

__________________
RichardB is offline   Reply With Quote
Old 24 March 2008, 16:37   #12
Member
 
Country: UK - England
Town: Colchester
Boat name: Pamela
Make: Valiant
Length: 5m +
Engine: Mariner 80 EFi
MMSI: 235059536
Join Date: Jun 2007
Posts: 38
The following link is to an MAIB safety digest for 2007. Case 25 involves a RIB and discusses prop guards:

http://www.maib.dft.gov.uk/cms_resou...007_Part_3.pdf

One of the conclusions was:

"This accident highlights the dangers
posed by unprotected rotating propellers.
Had a propeller guard been fitted to the
safety boat, the terrible injuries would
probably have been prevented. Such
guards can lead to reduced acceleration,
speed and manoeuvrability of the boat;
however, the benefits of a safely guarded
propeller have to be given consideration
compared to the boat’s potential loss in
performance. It is suggested that the
requirement for a propeller guard will
depend on the exact role and particular
operational conditions that a safety boat
is likely to encounter."

It is an interesting read, although the safety digest version is much abbreviated.

Personally, I use one occasionally at work if I am worried about fouling/obstructions in very shallow water, I also use one when I have kids in the water playing on boards/tubes etc, and I also recommend them on the sailing clubs dories where performance isn't such an issue. Otherwise, if I am driving the boat myself I am happy not to have one on.
__________________

__________________
J.C. is offline   Reply With Quote
Old 25 March 2008, 09:27   #13
Member
 
pressman's Avatar
 
Country: UK - England
Town: helston
Boat name: pressman
Make: Carson 900
Length: 9m +
Engine: twin 370 yanmar
Join Date: Jul 2005
Posts: 373
Best go for a water jet instead
__________________
pressman is offline   Reply With Quote
Old 25 March 2008, 12:53   #14
Member
 
Country: UK - Scotland
Town: West Scotland
Boat name: Orca
Make: Humber Ocean Pro 5.5
Length: 5m +
Engine: Tohatsu 115 TLDI
Join Date: Oct 2007
Posts: 168
Quote:
Originally Posted by BruceB View Post
A Rib stopping 20ft from the shot and using a diver to tow the Rib to the bouy (against a tideway) because they didn't think they could come alongside without "hitting" our Rib. This was about 5mins of utter amazed laughter watching a diver swimming as hard as he could and getting further away until I took pity and threw a rope.
On the Beagle perchance?
__________________
Mr Flibble is offline   Reply With Quote
Old 25 March 2008, 12:55   #15
Member
 
Country: USA
Town: Seattle
Boat name: Water Dog
Make: Polaris
Length: 4m +
Engine: Yamaha 60hp
Join Date: Apr 2007
Posts: 1,152
Quote:
Originally Posted by RichardB View Post
I'd never really thought about this before but why is this?

I ask because ducted fans on air propellers are quite a bit more efficient than an open propeller. I seem to remember this is due to a combination of factors such as directed airstream / reduced radial vortex shedding at the prop tips etc.

So why not a ducted prop on a boat??? The obvious guess would be due to the much higher viscosity of water creating substantially higher drag that negates any advantage. Or is it simply that prop guards are just built as guards and are not engineered for efficiency (e.g. a ducted air fan requires clearance between prop tip and guard to be minimised and the inlet shape is v important)?

Just intrigued...
They are used all the time on tugs. On this side of the pond they're called "Kort Nozzles". But a tug is a slow moving, high power application. The nozzles increase efficiency in this use by controlling the vortices of water coming off the blade tips and directing it strategically. A planing boat/prop is actually designed to have a certain amount of ventilation (air entrainment) to reach proper RPMs.
__________________
captnjack is offline   Reply With Quote
Old 25 March 2008, 13:02   #16
Member
 
Country: UK - Scotland
Town: Inverness
Boat name: none
Make: none
Length: 5m +
Engine: none
MMSI: none
Join Date: Mar 2007
Posts: 1,908
Quote:
Originally Posted by Mr Flibble View Post
On the Beagle perchance?
Don't tell me it was you
You never know sometime who is listening sometimes.......... In saying that after looking at your profile I reckon I know who you are now though, at least 95% certain anyway
What were you up to that day anyway, gave us all a laugh wondering what you were all up to..........
We were up to no good that day as you may have noticed, the viz was quite good till we went in..............
__________________

__________________
BruceB is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

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





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 21:31.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.