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Scoop
United Kingdom
1 Posts |
 Posted - 20 Jun 2017 : 00:17:30
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Well the 'long and the short' of the matter is the size of my 'shaft', no pun intended. Currently my Norman Conquest is fitted with a 9.9 Yamaha outboard with a long shaft. However I have been advised that on upgrading, I fit an outboard with a 'shorty shaft'.
Could anyone please advise of the proper configuration - is it to be long - or short.
Scoop
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cliveshep
Thailand
1324 Posts |
Posted - 20 Jun 2017 : 03:04:49
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Normans are long shaft. The horizontal plate on the lower leg above the propeller is called the anti-cavitation plate and should be around one inch or 25mm below the bottom of the hull.
If the propeller is not in a clear "swim" of water it will thrash uselessly and you'll only creep along.
Finally living the dream!
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Edited by - cliveshep on 20 Jun 2017 04:23:01 |
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df
United Kingdom
5994 Posts |
Posted - 20 Jun 2017 : 09:11:15
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Clive is spot on unless of course your outboard well has been cut down which is not unheard of, always worth a measure up on boats of this age unless it all looks very original around the transom.
NBAS--The communal colostomy bag of the boating community.
 Visit leomagill.co.uk |
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Mad Harold
United Kingdom
228 Posts |
Posted - 20 Jun 2017 : 20:46:56
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Not trying to be clever,I'm not an experienced boater at all,but I have grounded my prop several times and hit solid objects twice resulting in a prop overhaul.
My boat is a Norman 20 with a longshaft Yamaha 9.9.Cliveshep is correct with his diagram,it's exactly what my Yamaha handbook says,and mine is fitted like this.I have measured the depth of my prop and it is 27 inches from the skeg to the surface.I also have a 6hp auxilliary on a trapezium mount,depth of prop 22 inches from skeg to surface.I have also grounded this too.My local waterways are the Huddersfield broad and narrow canals.The narrow is rarely full and the broad has many submerged pieces of rubbish.
I don't dare use the 9.9 anymore,but have raised the 6hp trapezium mount and secured it about a third of its travel upwards with chains and a screwjack to get the tension the same both sides.
I tried the 6hp at a prop depth of 15 inches and 12 inches and yes there is a decrease in thrust,but not much.The decrease is the same at 15 inches and 12 inches,so I have left it at 12 inches.So far no prop strikes.
If you are lucky enough to operate in deep water,go by the book and use a longshaft,if like me your local waterway is a muddy ditch or a flooded rubbish tip,then leave your outboard in the up position and buy a Gondolier's pole. |
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cliveshep
Thailand
1324 Posts |
Posted - 21 Jun 2017 : 05:42:42
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quote:
Originally posted by df
Clive is spot on unless of course your outboard well has been cut down which is not unheard of, always worth a measure up on boats of this age unless it all looks very original around the transom.
NBAS--The communal colostomy bag of the boating community.
Visit leomagill.co.uk
The crucial info is that the anti-cav plate is 0" - 1" (0mm - 25mm) below the hull. so if some idiot has cut down your boat's transom years ago then you would choose a shorter shaft outboard and possibly pack carefully under the clamp to get it to the right position underwater in relation to your hull.
Harold - you could change your outboard to an inboard/Hamilton jet that works in whatever depth floats your boat or sadly change your home waters to some with sufficient depth. Failing that get a short shaft but accept that much power is wasted, that it will cavitate like crazy as the prop will suck down air instead of push water because the transom blocks it. You'll move but not very well.
An alternative on a deep-vee like the Norman is to move the engine to one side and mount it higher as the hull is higher nearer the beam, or put a small engine on either side which would be better for control. That way the boat hits the bottom before the engines.
You need at least 3 feet of water to operate a normal outboard boat, and even then only slowly as the more power and speed the more you lower the level at the prop and the stern sinks until the prop hits. Sounds like your home waters are silted, clogged and full of rubbish and not maintained.
Finally living the dream!
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df
United Kingdom
5994 Posts |
Posted - 21 Jun 2017 : 08:08:57
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Problem solved!
NBAS--The communal colostomy bag of the boating community.
Visit leomagill.co.uk |
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Mad Harold
United Kingdom
228 Posts |
Posted - 21 Jun 2017 : 09:55:33
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Cliveshep,thanks for your advice on this subject,as usual you are correct with your analysis.
I have searched the internet for jet drives and found several types,all of them unsuitable for various reasons.
1.The manufacturers of these state that they are about 30% less efficient at producing thrust than a prop.
2.There are jet drive outboards,but in larger capacities.Mercury's jet drives start at 25hp.
3.There are jet drive conversions for outboards,but again,I havn't seen one for an engine of less than 25hp.
4.The cost of an inboard conversion is difficult to justify on a 1979 Norman 20,also,a jet drive outboard is expensive.
The solution to my situation is to find a mooring elswhere,and I am pursuing this at the moment.
In the meantime I will putter along the cut with my 12inch draught outboard,bubbling and gurgling and dream of the orgasmic moment when I can lower my main outboard and unleash 9.9 horsepower.
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Long and Short
