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Old 06 October 2011, 11:18   #11
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It is an interesting topic. I have been perusing several sites to get an overview. The advice seems pretty universal, nice idea but very problematic. I have also been looking at Outboard jets as a possible way forward. Does anyone have any feedback of the current models, they would care to share? If they work, that could be a good solution.
They work with a substantial loss of total HP power and loss of some general performance in the mid-rpm ranges. Idling they are highly manuverable and fine. At WOT or nearly so they are enjoyable to use IMHO. Mid-range expect some sluggish responses, difficulty staying on plane, and general finickiness compared to a prop outboard.

They will suck up stones in shallow water and seaweeds something fierce. Most of this debris will lodge in the inlet grate and stay there until you shut the motor off, tilt it up, and clear it by hand or with a rake. Which pretty much requires hanging over the transom.

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but would mean in the unfortunate case of comatosed sailor meeting it they are infinitely less likely to get "chopped"
As the RYA link points out, arms and heads etc can still get struck anyway. AND may actually be more likely to get struck by a ringed prop since the operator <thinks> the prop is immune to such things.

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the ring round the blades means it may survive a bottom strke a little better, unless of course it just collapses.
The rings I have seen don't collapse nor is the prop bent typically. The aluminum casting of the lower unit just cracks instead.
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Old 06 October 2011, 12:23   #12
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We are not looking for blistering performance, the RIBs spend long periods just chugging about, but they do need to be able to tow and be able to get a dash on when needed. They usually have a 2 man crew. The worse case scenario would probably be to tow a keelboat such as a SB3, or Flying Fifteen back over the Chi Bar in high winds and big waves against a 5-6 knot tide with 2 or 3 more people on board. It is not unusual to have a string of smaller dinghies with their crews on board being towed, but that is usually in a no wind situation but may be against a strong tide. The current boats have their hands full in these conditions, we would need to have the equivalent of around at least 60 HP at the jet IMO. Based on the sort of losses being suggested, I imagine it will need around 90-100 Hp from the motor?
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Old 06 October 2011, 12:59   #13
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Rokraider,

As an afterthought - I'm not sure how a jet outboard handles, if it is like a "real jet" then its quite different to an outboard in which case you might have some training issues anyway.
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Old 06 October 2011, 13:02   #14
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we would need to have the equivalent of around at least 60 HP at the jet IMO. Based on the sort of losses being suggested, I imagine it will need around 90-100 Hp from the motor?
A 90hp jet is roughly = a 70hp prop in actual thrust.

While jets are amazingly manuverable at low speeds, I am not sure how the performance will be towing loads at displacement speeds. I would guess rather anemic.

Sounds like you have a pretty big RIB, most of the sailing club safety boats around here are much much smaller. 3.5 to 4.5m, 9.9 to 25hp type things.
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Old 06 October 2011, 13:04   #15
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I was wondering about that. Do they have a bucket for reverse? The spec sheets make it sound as though they have a forward facing port that opens for reverse and close the rear outlet?
We are taking all our drivers through ongoing training programmes, so I don't see a problem with some additional training. In fact it would be a compulsory refresher course before they would be allowed to use the new setup.
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Old 06 October 2011, 13:08   #16
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Rokraider,

As an afterthought - I'm not sure how a jet outboard handles, if it is like a "real jet" then its quite different to an outboard in which case you might have some training issues anyway.
They are similar in forward although a bit more manuverable I think (depends on deadrise). They are pretty weak compared to a prop outboard in reverse. I have a few hundred hours on jet outboards in the ~75hp range although only on 2 different hulls.
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Old 06 October 2011, 13:09   #17
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I was wondering about that. Do they have a bucket for reverse? The spec sheets make it sound as though they have a forward facing port that opens for reverse and close the rear outlet?.
Yes, commonly called a "clamshell" which swings over the outlet. The engine and impeller always turn in the same direction.
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Old 06 October 2011, 13:16   #18
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A 90hp jet is roughly = a 70hp prop in actual thrust.

While jets are amazingly manuverable at low speeds, I am not sure how the performance will be towing loads at displacement speeds. I would guess rather anemic.

Sounds like you have a pretty big RIB, most of the sailing club safety boats around here are much much smaller. 3.5 to 4.5m, 9.9 to 25hp type things.
All of our RIBS are around 5.5 - 5.8 metres and the proposed new ones will be around 6.0.
They have to operate inside the harbour in strong tides and occasionally lumpy water as well as out in the bay where it can get very rough and windy. To get out into the bay, you have to cross the notorious Chichester bar which can get extremely rough with waves that are quite capable of rolling a RIB if caught off guard. The 5.5's are pretty uncomfortable out in the Bay in rough weather, the 5.8's are better, but still too small really if you are on duty for 8 hours or so.
The club is quite large by UK standards and hosts a lot of World cup events. Last year we hosted the Laser Worlds which involved (I believe) around 700 Lasers (Olympic class) over a 3 week period, all out in the Bay. As you can imagine, the club RIBS got quite a hammering.
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Old 07 October 2011, 04:45   #19
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As the RYA link points out, arms and heads etc can still get struck anyway. AND may actually be more likely to get struck by a ringed prop since the operator <thinks> the prop is immune to such things.

The rings I have seen don't collapse nor is the prop bent typically. The aluminum casting of the lower unit just cracks instead.
Not sure my description was so great. I'm not talking about a toob attatched to the leg / skeg, more like what those in the pumping world would describe as a "shrouded impeller".

To describe explicitly: Take your prop off the boat, and into a workshop. Then take a length of sheet material slightly longer than the prop hub length by <pi x blade tip dia>. Wrap it round the blade tips, and weld to the tips, thus enclosing your prop in it's own toob. (of course if you have cast it from scratch the toob will be cleverly shaped to improve the thrust - that's the one I can't for the life of me find the link to). Replace the prop.

This then gives you what from the outside is a rotating cylinder, no blades to catch stuff that drifts past. As I said, it will still ingest, but there's a lot less there to potentially go wrong, as any unconcious limbs would need to get past the gearbox and be pretty much parallel to the keel at the keel to be fully entrained. If you look dead astern at the midpoint of your gearbox, that's only about a 3"-ish gap on a 50Hp.

Mt comment about collapse was more related to if you hit something like a rock, the outer ring could transmit the force to all three blades, which will either help as you have more metal to move, or collapse the whole thing as the force is transmitted round the ring.

It also behaves more like a prop, so no re- training on operation required. Point out to the drivers that it's still a prop & will still mince, so treat it like a normal prop & risks should be reduced.
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Old 07 October 2011, 04:59   #20
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9d280,

I think you are possibly thinking of: RingProp? They seem to have dissapeared. There is some history here: RingProp | Propeller Guard Information Center [a rather weird american site which seems to spend more time telling you what everyone else offers than they do]. Whilst 'waybackthen' says: Ringprop - News
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