Prop gaurds good or bad? (Discuss)

[Rasher]

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

[David G]

http://www.yorkshire-divers.com/foru...ay-jinxed.html

posted yesterday.

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

Regards,

Orve.

[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.

[Rasher]

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

[BruceB]

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]

Sounds like some of the BS I regularly observed diving on the SS Breda.

Originally Posted by BruceB

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.

[Rasher]

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?

[BogMonster]

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...

[codprawn]

Nah it wasn't that - the government just can't afford petrol any more!!!

[Cypman]

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 !!

[RichardB]

Originally Posted by SeaSkills

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...

[J.C.]

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.

[pressman]

Best go for a water jet instead

[Mr Flibble]

Originally Posted by BruceB

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?

[captnjack]

Originally Posted by RichardB

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.

[BruceB]

Originally Posted by Mr Flibble

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..............