Reply
 
Thread Tools Search this Thread
 
Old 23 March 2008, 17:21   #1
Member
 
Country: UK - England
Town: Portsmouth
Length: no boat
Join Date: Mar 2008
Posts: 6
Prop gaurds good or bad? (Discuss)

Hello all, I was talking to a friend the other day and we got on to disscussing prop gaurds I was heavily against the idea of fitting one and he seemed pro for it. My view is that a propeller will throw a limb away that is in the water e.g if there is a MOB. If however the prop gaurd is fitted it would be like a food blender on any limb trapped inside. What are peoples feelings on this?
Love Rasher
__________________
Rasher is offline   Reply With Quote
Old 23 March 2008, 17:27   #2
Member
 
Country: UK - England
Town: Bristol
Length: 5m +
Join Date: Mar 2006
Posts: 338
http://www.yorkshire-divers.com/foru...ay-jinxed.html

posted yesterday.

Propellors will NOT "Throw a limb away"....................

Regards,

Orve.
__________________
David G is offline   Reply With Quote
Old 23 March 2008, 17:36   #3
Member
 
SeaSkills's Avatar
 
Country: UK - Scotland
Town: Oban (mostly)
Make: Ribcraft, Humber,BWM
Length: 5m +
Engine: Outboards
Join Date: Jan 2008
Posts: 632
Send a message via Skype™ to SeaSkills
Prop guards limit your performance, and that can also be dangerous. Rope around the prop inside a guard is harder to remove. A prop will certainly NOT throw a limb away (although it might distribute pieces of it around the area) - I've picked up several people with prop injuries over the years and they are horrible injuries.

So the question isn't just about prop guards, it's about how to avoid damage to a MOB

I do think that prop guards have their place, and for me that place is for surf rescue. In virtually any other situation I'd vote for a properly trained driver as the safe way to protect people in the water.
__________________
SEASKILLS TRAINING
Web; www.seaskills.co.uk
Email; info@seaskills.co.uk
Tel; 07525 012 013
SeaSkills is offline   Reply With Quote
Old 23 March 2008, 18:18   #4
Member
 
Country: UK - England
Town: Portsmouth
Length: no boat
Join Date: Mar 2008
Posts: 6
Thanks guys that is pretty much how i feel, My bad on the trowing away issue i more ment in apposed to holding a limb between the prop and the guard. I think the only place I would consider using one is with divers, I hadnt even thought of surf rescue. As mentioned before it shouldnt be an issue if you know what you are doing when you approach someone in the water.
Rasher
__________________
Rasher is offline   Reply With Quote
Old 23 March 2008, 22:10   #5
Member
 
Country: UK - Scotland
Town: Inverness
Boat name: none
Make: none
Engine: none
MMSI: none
Join Date: Mar 2007
Posts: 1,908
Divers are a funny lot, some of them are their own worst enemies when driving boats. In saying that non-divers can be just as bad.
I can remember several incidents offhand, only one of which I wish a prop guard was fitted to the boat concerned.
A Rib backing in stern first onto a shot to pick up their divers, one being asked "not very politely!" by myself what the F** they were doing (I had divers including the wife coming up the same shot) and got the answer of "its easier to get away from the shot bow first" This generated a fair number of expletives from myself questioning his and his BSAC instructors parentage!
Boats coming over to ask what the blue and white flag I was pointing to as they came nearer meant..........
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.
I did think of a prop guard but eventually came to the conclusion that they weren't worth the fuel and reduction in speed and they only made some folk think of the prop as now somehow "safe" with a reduction in the care taken with it in manoeuvering around divers.
Then again it is really just a matter of opinion, some folk swear by them.
__________________
BruceB is offline   Reply With Quote
Old 24 March 2008, 06:26   #6
Member
 
Country: UK - England
Town: Scillies
Boat name: Freedom
Make: Searider
Length: 5m +
Engine: Yam 2st 90
Join Date: Jan 2007
Posts: 335
Sounds like some of the BS I regularly observed diving on the SS Breda.


Quote:
Originally Posted by BruceB View Post
Divers are a funny lot, some of them are their own worst enemies when driving boats. In saying that non-divers can be just as bad.
I can remember several incidents offhand, only one of which I wish a prop guard was fitted to the boat concerned.
A Rib backing in stern first onto a shot to pick up their divers, one being asked "not very politely!" by myself what the F** they were doing (I had divers including the wife coming up the same shot) and got the answer of "its easier to get away from the shot bow first" This generated a fair number of expletives from myself questioning his and his BSAC instructors parentage!
Boats coming over to ask what the blue and white flag I was pointing to as they came nearer meant..........
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.
I did think of a prop guard but eventually came to the conclusion that they weren't worth the fuel and reduction in speed and they only made some folk think of the prop as now somehow "safe" with a reduction in the care taken with it in manoeuvering around divers.
Then again it is really just a matter of opinion, some folk swear by them.
__________________
walruz is offline   Reply With Quote
Old 24 March 2008, 09:45   #7
Member
 
Country: UK - England
Town: Portsmouth
Length: no boat
Join Date: Mar 2008
Posts: 6
Do powerboat divers involved with diving not have a scheme similar to safety boat? it sounds to me like the people you were with didnt have a clue about boat possistioning etc?
__________________
Andy (AKA pipe down Rasher)
Powerboat instructor, Advanced, Safetyboat
Advanced Beverage mixer
...and consumer
Rasher is offline   Reply With Quote
Old 24 March 2008, 10:40   #8
Member
 
Country: Other
Town: Stanley, Falkland Is
Boat name: Seawolf
Make: Osprey Vipermax 5.8
Length: 5m +
Engine: Etec 150
Join Date: Mar 2006
Posts: 3,726
As I observed to the other half the other day while watching the occupants of a visiting Royal Navy inflatable rowing frantically 100yds offshore in the middle of a large kelp bed and an onshore wind, they don't appear to work well in kelp! If you are tangled up with a normal outboard, a quick blast on the throttle ahead then astern will clear all of it and spit it out in pieces, but apparently not with a prop guard on...
__________________
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...

Sent from my Computer, using a keyboard and mouse
BogMonster is offline   Reply With Quote
Old 24 March 2008, 13:10   #9
Member
 
Country: UK - Wales
Town: swansea
Boat name: Too Blue
Make: BLANK
Length: 8m +
Engine: Suzuki DT225
Join Date: Mar 2004
Posts: 12,791
Nah it wasn't that - the government just can't afford petrol any more!!!
__________________
codprawn is offline   Reply With Quote
Old 24 March 2008, 13:41   #10
Member
 
Cypman's Avatar
 
Country: UK - Isle of Man
Town: Douglas
Length: no boat
Join Date: Aug 2006
Posts: 339
We were forced to fit prop-guards to our 2 club powerboats in 2005.
The boats were a 5m Ocan-Pro with a Suzuki DF90, and a 5m Destroyer with a Suzuki DF70. We lost between 15 and 20kts top speed and had to reduce prop diam from 21ins to 17ins to obtain WOT !! We then put a case forward and removed them after 4 weeks. During this episode I found an RYA Information Bullitin on the use of prop-guards by Training Establishments/ Rescue Boats which indicated that guards were not recommended and that correct training for operations close to persons in the water was the preferred solution. I had a quick look on the RYA Site this morning but was unable to find this guidance letter, maybe someone elsemore competent can find it !!
__________________
Cypman is offline   Reply With Quote
Old 24 March 2008, 14:17   #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, 20:37   #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, 13:27   #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, 16:53   #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: 172
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, 16:55   #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, 17:02   #16
Member
 
Country: UK - Scotland
Town: Inverness
Boat name: none
Make: none
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




All times are GMT. The time now is 05:50.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.