Shaft vs Outdrive vs Jet

bumpy_the_dog

bumpy_the_dog

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I'm a rag and stick sailor looking to move to a MB, but the thought of a rope around the prop leaves me fouling my slacks.

On one of the boats I'm looking at there's a choice of shaft drive (with the prop nicely protected behind a keel complete with a metal strap that joins the base of the rudder to the keel, so completely enclosing the prop), outdrive, or jetdrive.

Does any of these have an advantage in terms of avoiding a snarled up prop, and is there a trade off with low speed manoevring or fuel consumption etc?

And while I'm droning on, what's the advice regarding shaft strippers?

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Jets are an excellent propulsion system but expensive and to optimise efficiency the hull needs to be designed for jets it is not practical to use the same hull for jets , outdrives or shafts so rule out jets.

If you want a prop protected behind a keel with a plate from keel to rudder this is either a semidisplacement monohull with a single engine or semi displacement catamaran with twin engines. Assume monohull some boats such as mitchell 31 have an access trap over the prop that allows you to access the prop from above the waterline with a knife to cut away rope, net, plastic etc.

On a monohull boat twin shaft drive are inexpensive to maintain and robust but as they are in clear water liable to foul with rope, netting or plastic sheeting. You can foul both engines in one incident , i have done it. You need rope cutters.

Rope cutters are depending on shaft size expensive say £300 to £500 each, they cut down your speed by about a knot or two on a planing boat but they are generaly the difference in cutting through a rope or net or plastic shet with at least one engine adn limping to your destination.

Out drives are more efficient than shaft on planing boats but exepensive to maintain allow aprox 70% more than engine and drive for shaft drive . But if you foul something you can lift the drive up and reach it to cut away debris from the props.

It really depends on the type of boat, speed of boat and size of boat you go for but if shaft i would definitely go for rope cutters, stripper are good I have no confidence in the round sharp disc type.

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Jets are an excellent propulsion system but expensive and to optimise efficiency the hull needs to be designed for jets it is not practical to use the same hull for jets , outdrives or shafts so rule out jets.

If you want a prop protected behind a keel with a plate from keel to rudder this is either a semidisplacement monohull with a single engine or semi displacement catamaran with twin engines. Assume monohull some boats such as mitchell 31 have an access trap over the prop that allows you to access the prop from above the waterline with a knife to cut away rope, net, plastic etc.

On a monohull boat twin shaft drive are inexpensive to maintain and robust but as they are in clear water liable to foul with rope, netting or plastic sheeting. You can foul both engines in one incident , i have done it. You need rope cutters.

Rope cutters are depending on shaft size expensive say £300 to £500 each, they cut down your speed by about a knot or two on a planing boat but they are generaly the difference in cutting through a rope or net or plastic shet with at least one engine adn limping to your destination.

Out drives are more efficient than shaft on planing boats but exepensive to maintain allow aprox 70% more than engine and drive for shaft drive . But if you foul something you can lift the drive up and reach it to cut away debris from the props.

It really depends on the type of boat, speed of boat and size of boat you go for but if shaft i would definitely go for rope cutters, stripper are good I have no confidence in the round sharp disc type.

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Jets can get fouled. The massive KaMeWas tend to munch anything they imbibe but smaller jobs can suck rope thru the intake grill then it winds round the impeller. A massive job to clear. And of course they're always in gear, no neutral

What size/make boat you looking at?

Gludy on this forum has researched this subject to death and loves jets, but not the boats they are attached too

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Most commercial water jet applications have gearboxs giving drive and neutral (no reverse as the bucket achieves that) and most dont have grills , blockages of the jets with debris is rare. If you do get a man made fibre rope stuck in it can be a b@*^%r to clear but in general very reliable and little cavitation compared with high speed props.

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What about a surface drive? seem to be about same cost as outdrives, prop relatively easy to get at, and a lovely rooster tail !! very efficient at speed too. check out Lancing Marine and Pulse drive sites.

<hr width=100% size=1>Mike....... V8s are sweet music
 
Ahh, didn't know all that Bandit, thanks for info. Do the gearboxes tend to step down the rpm on commercial applicatons? Or are they just there to provide a neutral?

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Generaly a small step down to give optimum rpm for jet but mainly to give neutral and to warm up engine without jet in gear.

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Some Scandinawian boating mags made recently a joint test of three similar boats. One with shaft drive, one with outdrive and one jet. - Ranking 1. Outdrive, 2. Shaft, 3. Jet. Outdrive is the fastest, by far most economical to buy and run and you will be able to remove possible snarls from your prop at sea.
If you are about to buy a boat below 35 feet I would not consider anything else. The talk about the higher service intensity of outdrives is highly exaggerated - If you are mostly interested in speed ranges above 25 knots, jets start to become an alternative economically, but getting something blocking the impeller at sea...

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"There will of course be another option shortly, courtesy of Volvo Penta."

Do tell...

<hr width=100% size=1><A target="_blank" HREF=http://static.photobox.co.uk/public/images/45/99/10714599.s.jpg?ch=97&rr=16:00:39>Nirvana</A>
 
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