Aja
Well-Known Member
Can confirm that the Gori on my boat will open and freewheel when moving. I have to put the gear in reverse.
Donald
Donald
best folding prop
- Thread starter mariog
- Start date
Aeolus_IV
Well-Known Member
No, surprisingly, its not a bad thing, its exactly what the prop is meant to do. If the hull on a boat is really clean then, even at tickover, the boat will slowly accelerate until the resitance of the boat through the water is equal to the HP output by the engine. Very efficient, and means that on the calm days when your tend to motor, then engine runs slower, using less fuel, than a setup with a fixed pitch prop.
I have an autoprop and this is exactly what it does. It is exceedingly good, but also rather expensive. I would recommend it for general saling folk. For a Mumm 30 though - its a heavy lump of metal, and a feathering prop too (and so still capable of snagging stuff when your sailing) and it does "spin" unless you lock the shaft (with brake or gearbox). I would suggest a good two blade floding prop is the right solution - prior to the Autoprop, I had the basic Gori 2-blade folding prop, which was also excellent. This is now in use on a UFO 31, and the owner is also very pleased with the results. I would say that the Gori had few of the "problems" which people attribute to folding props, and the way the UFO 31 "stops" when entering a lock, you'd think he had a 3-blade fixed prop!
Regards,
Jeff.
I have an autoprop and this is exactly what it does. It is exceedingly good, but also rather expensive. I would recommend it for general saling folk. For a Mumm 30 though - its a heavy lump of metal, and a feathering prop too (and so still capable of snagging stuff when your sailing) and it does "spin" unless you lock the shaft (with brake or gearbox). I would suggest a good two blade floding prop is the right solution - prior to the Autoprop, I had the basic Gori 2-blade folding prop, which was also excellent. This is now in use on a UFO 31, and the owner is also very pleased with the results. I would say that the Gori had few of the "problems" which people attribute to folding props, and the way the UFO 31 "stops" when entering a lock, you'd think he had a 3-blade fixed prop!
Regards,
Jeff.
bedouin
Well-Known Member
[ QUOTE ]
No, surprisingly, its not a bad thing, its exactly what the prop is meant to do. If the hull on a boat is really clean then, even at tickover, the boat will slowly accelerate until the resitance of the boat through the water is equal to the HP output by the engine. Very efficient, and means that on the calm days when your tend to motor, then engine runs slower, using less fuel, than a setup with a fixed pitch prop.
[/ QUOTE ]
That argument is a little flawed. Diesels tend to be more efficient at cruising revs so just because the engine is going more slowly does not mean that it is using less fuel. In fact for the same power output most diesels will be less efficient at 1200rpm than at 2400.
People often seem to quote low revs at hull speed as an indication of prop efficiency but that is a fallacy. When I replaced my old 2 blade fixed for the 3 blade Gori it actually increased the revs I needed to reach hull speed in flat waters, but that doesn't stop it being much more efficient and economical.
No, surprisingly, its not a bad thing, its exactly what the prop is meant to do. If the hull on a boat is really clean then, even at tickover, the boat will slowly accelerate until the resitance of the boat through the water is equal to the HP output by the engine. Very efficient, and means that on the calm days when your tend to motor, then engine runs slower, using less fuel, than a setup with a fixed pitch prop.
[/ QUOTE ]
That argument is a little flawed. Diesels tend to be more efficient at cruising revs so just because the engine is going more slowly does not mean that it is using less fuel. In fact for the same power output most diesels will be less efficient at 1200rpm than at 2400.
People often seem to quote low revs at hull speed as an indication of prop efficiency but that is a fallacy. When I replaced my old 2 blade fixed for the 3 blade Gori it actually increased the revs I needed to reach hull speed in flat waters, but that doesn't stop it being much more efficient and economical.
Aeolus_IV
Well-Known Member
Perhaps .. but if "at crusing revs" your doing 6 knots instead of 5, then thats got to be better efficiency in the litres per mile stakes. My recent experience of calm water motoring had Aeolus doing 6 knots through the water at about 2000 rpm. Looking back a fuel consumption I estimated I had used about 1.6 Ltrs/Hour. I thought this was pretty damn good. Especially as previously to get 6 Knots meant doing 2500+ RPM which definitely used more fuel (I have a Yanmar 3GM30F, rather than an MD2B).
But, putting the ecconomy aspect to one side, the way the Autoprop works does mean that ther boat always speeds up to the maximum speed possible for the HP being generated by the engine (as set by throttle/governor). A fixed pitch system will speed up until the effective "thrust" placed into the water matches the resistance of the boat which is basically proportial to the shaft speed, and hence engine RPM.
Regards,
Jeff.
But, putting the ecconomy aspect to one side, the way the Autoprop works does mean that ther boat always speeds up to the maximum speed possible for the HP being generated by the engine (as set by throttle/governor). A fixed pitch system will speed up until the effective "thrust" placed into the water matches the resistance of the boat which is basically proportial to the shaft speed, and hence engine RPM.
Regards,
Jeff.
Hoolie
Well-Known Member
[ QUOTE ]
Who specified the prop for the boat?
[/ QUOTE ]
Bruntons! The engine (Prima M50 just decoked) doesn't labour and 6 knots is with low throttle setting. If I wind it up to 2000 rpm the boat does 8 knots plus - close to hull speed. I have never tried finding maximum revs with this prop - perhaps I should?
Who specified the prop for the boat?
[/ QUOTE ]
Bruntons! The engine (Prima M50 just decoked) doesn't labour and 6 knots is with low throttle setting. If I wind it up to 2000 rpm the boat does 8 knots plus - close to hull speed. I have never tried finding maximum revs with this prop - perhaps I should?
bedouin
Well-Known Member
You can't equate higher revs with higher diesel consumption, it does not work like that. When I swapped props the revs went up but the consumption went down.
To reach hull speed at 2000 RPM in calm water sounds perfect (particularly with an engine that is perhaps a little large for the boat /forums/images/graemlins/smile.gif )- whatever prop you have - but to do so at 1200 RPM as the OP mentioned does not sound so good.
I had thought that the Autoprop worked rather like a CVT gearing - enabling you to maintain almost constant revs over a wide range of power outputs - but on reflection I wonder whether it doesn't actually do the opposite and take proportionally lower revs at low power than a fixed equivalent.
Note that even with a fixed prop the boat will also reach the maximum speed possible for the HP being generated - at least until you begin to get significant prop slip and in calm conditions there should be very little difference between the performance of any reasonable prop.
In theory the big benefit of the Autoprop ought to come in more difficult conditions where its ability to re-pitch itself should significantly reduce slippage and hence give much better drive
To reach hull speed at 2000 RPM in calm water sounds perfect (particularly with an engine that is perhaps a little large for the boat /forums/images/graemlins/smile.gif )- whatever prop you have - but to do so at 1200 RPM as the OP mentioned does not sound so good.
I had thought that the Autoprop worked rather like a CVT gearing - enabling you to maintain almost constant revs over a wide range of power outputs - but on reflection I wonder whether it doesn't actually do the opposite and take proportionally lower revs at low power than a fixed equivalent.
Note that even with a fixed prop the boat will also reach the maximum speed possible for the HP being generated - at least until you begin to get significant prop slip and in calm conditions there should be very little difference between the performance of any reasonable prop.
In theory the big benefit of the Autoprop ought to come in more difficult conditions where its ability to re-pitch itself should significantly reduce slippage and hence give much better drive
flaming
Well-Known Member
I assume you're not doing much cruising on a Mumm36!?
If so then a 2 blade folder is all you should need. Just give the prop manufacturers a call to suggest the correct pitch for your engine and gearbox. The Elan 37 I race has a 2 blade folder, no idea what type though, and will do 8 knots at full chat. Perfect when you're running a little late for those early winter series starts!
If so then a 2 blade folder is all you should need. Just give the prop manufacturers a call to suggest the correct pitch for your engine and gearbox. The Elan 37 I race has a 2 blade folder, no idea what type though, and will do 8 knots at full chat. Perfect when you're running a little late for those early winter series starts!
bedouin
Well-Known Member
[ QUOTE ]
[ QUOTE ]
Who specified the prop for the boat?
[/ QUOTE ]
Bruntons! The engine (Prima M50 just decoked) doesn't labour and 6 knots is with low throttle setting. If I wind it up to 2000 rpm the boat does 8 knots plus - close to hull speed. I have never tried finding maximum revs with this prop - perhaps I should?
[/ QUOTE ]
I had underestimated the hull speed (being used to smaller boats /forums/images/graemlins/smile.gif ) - if you reach hull speed at 2000 rpm that is more reasonable.
It would be interesting to know if you can get the engine to full revs with that prop on. However on reflection I suspect that your engine is simply bigger than you need for that boat with the Autoprop on (which will probably be much more efficient than whatever was provided as standard).
[ QUOTE ]
Who specified the prop for the boat?
[/ QUOTE ]
Bruntons! The engine (Prima M50 just decoked) doesn't labour and 6 knots is with low throttle setting. If I wind it up to 2000 rpm the boat does 8 knots plus - close to hull speed. I have never tried finding maximum revs with this prop - perhaps I should?
[/ QUOTE ]
I had underestimated the hull speed (being used to smaller boats /forums/images/graemlins/smile.gif ) - if you reach hull speed at 2000 rpm that is more reasonable.
It would be interesting to know if you can get the engine to full revs with that prop on. However on reflection I suspect that your engine is simply bigger than you need for that boat with the Autoprop on (which will probably be much more efficient than whatever was provided as standard).