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26-08-2018, 11:52
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#31
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Senior Cruiser

Join Date: Mar 2010
Location: gettin naughty on the beach in cornwall
Boat: 63 custom alloy sloop,macwester26,prout snowgoose 37 elite catamaran!
Posts: 10,598
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Re: propshaft driving an alternator , when sailing
Quote:
Originally Posted by bailsout
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you would need to regulate voltage through a range of different rpm's.
generators generally work at a fixed rpm and rely on the torque of the engine to compensate for loads.
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26-08-2018, 12:10
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#32
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Registered User
Join Date: Dec 2014
Location: Summer Europe Winter Florida
Boat: Dufour Nautitech435/FP Venezia42/Baltec Visiona 40/Catalac 10m used as a river cruiser
Posts: 183
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Re: propshaft driving an alternator , when sailing
My sailing club asked me to let install a prop shaft alternator on a Contest 42 by Conyplex in the Netherlands. That thing produced not much power and failed very often, as the engine bilge was very small and not deep. If water (salt water!) intruded through the stuffing box the huge pulley on the prop shaft sprayed the salty water around. What a mess! Better not to find out the price of the W produced. For me it is clear: never again.
Today there are several trailing systems available. Some of them good value for money some very pricey but very powerful (Wattandsea) and used during round the globe races.
My two cents but good luck
onavegador
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26-08-2018, 12:28
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#33
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Registered User
Join Date: Oct 2015
Posts: 2,007
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Re: propshaft driving an alternator , when sailing
Quote:
Originally Posted by tml
Max Prop used to mention in their literature that it would work with a shaft driven alternator. If you stop the engine in reverse the Max Prop will spin in reverse driving the alternator with the engine off.
Stop the engine in forward the Max Prop feathers as normal and the shaft and prop don't turn.
Never saw one hooked up but always seemed an elegant solution.
Tom
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This was actually quite a common arrangement on Amel boats before they shifted from MaxProp to Autoprop. The Autoprop worked fine with a propshaft alternator without fussing. All you did was NOT lock it from rotation when sailing.
The propshaft alternator was a very popular option on Amels delivered to European customers. More so than Americans for some reason.
Of course the Amel has a lot of room in the engine space for mounting, and there is no shaft packing gland to spray salt water around either. Kind of the ideal setup for this.
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26-08-2018, 12:37
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#34
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Marine Service Provider
Join Date: Dec 2011
Posts: 1,541
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Re: propshaft driving an alternator , when sailing
As mentioned several time on this forum, a free-wheeling prop on a boat, or an airplane, creates parasite drag equal to a solid disk the size of the prop's diameter. Add a Mickey Mouse alternator lash-up to that and you are really sucking the power out of your sails. Wind generators do the same thing unless you are anchored or in the marina.
Solar? No drag, of course.
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26-08-2018, 12:53
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#35
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Registered User
Join Date: Dec 2008
Location: SC
Boat: None,build the one shown of glass, had many from 6' to 48'.
Posts: 10,206
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Re: propshaft driving an alternator , when sailing
Long story short it is folly!
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26-08-2018, 12:57
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#36
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Registered User
Join Date: Jun 2007
Location: Western Caribbean
Boat: 48' Alu Cat
Posts: 218
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Re: propshaft driving an alternator , when sailing
I have had a small 25A car alternator connected via some idler pulleys ( to increase the RPM ) to the prop shaft. it worked reasonably well, except the noise was driving me nuts.
Chucked it all out, too noisy!
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26-08-2018, 16:15
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#37
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Registered User
Join Date: Nov 2009
Location: Bundaberg, Qld, Australia
Boat: Moody 425
Posts: 21
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Re: propshaft driving an alternator , when sailing
Hi,
When I purchased my Moody, it came with a propshaft alternator, although not fitted. The couple I bought it from were Norwegian and he, a Rolls Royce marine engineer, had fitted it. Through some correspondence and Google translate, I learned that he had the alternator rewired in New Zealand to excite at a much lower speed due to the lower RPM produced. After a while he modified the field circuit to include a thermistor to drop excitation as the alternator was getting hot due to reduced air flow. I am yet to set it up so don't know how much power it will produce. I have just been given the generator off a trolling generator system and am thinking that may be an easier alternative.
Regards
Russ
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26-08-2018, 17:29
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#38
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Registered User
Join Date: Jan 2009
Location: San Diego, California - Read about our circumnavigation at www.rutea.com
Boat: Contest 48
Posts: 1,064
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Re: propshaft driving an alternator , when sailing
We crossed several oceans with some friends on a Najad 39 that had a very simple system of a freewheeling prop shaft driving a small alternator. My friend, René, said that it developed enough power to run his autopilot.
Fair winds and calm seas.
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26-08-2018, 17:40
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#39
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Senior Cruiser

Join Date: Mar 2010
Location: gettin naughty on the beach in cornwall
Boat: 63 custom alloy sloop,macwester26,prout snowgoose 37 elite catamaran!
Posts: 10,598
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Re: propshaft driving an alternator , when sailing
Quote:
Originally Posted by nhschneider
We crossed several oceans with some friends on a Najad 39 that had a very simple system of a freewheeling prop shaft driving a small alternator. My friend, René, said that it developed enough power to run his autopilot.
Fair winds and calm seas.
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the system i built provided the constant 16 amp 24v draw 24/7 on the boat required for the auto pilot,radar , plotter and 3 fridges on their atlantic crossing
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26-08-2018, 18:19
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#40
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 21,515
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Re: propshaft driving an alternator , when sailing
Quote:
Originally Posted by jmschmidt
As mentioned several time on this forum, a free-wheeling prop on a boat, or an airplane, creates parasite drag equal to a solid disk the size of the prop's diameter. Add a Mickey Mouse alternator lash-up to that and you are really sucking the power out of your sails. Wind generators do the same thing unless you are anchored or in the marina.
Solar? No drag, of course.
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And discredited every time by posters who understand physics and by empirical data. Let's keep this thread on topic.
The power is sucked from the wind as you state but as others have posted, it would hardly be noticed. In most trade wind and passage sailing conditions, some sails are partly reefed so if you are slowed, add a little more sail area.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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26-08-2018, 18:21
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#41
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 21,515
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Re: propshaft driving an alternator , when sailing
Quote:
Originally Posted by a64pilot
One question though, alternators have brushes correct?
It’s not usually good to turn a brushed component backwards as the brushes wear faster. It’s obviously done, look at a reversible drill, but I think I’d still look at a way to disengage when motor was running, cause the the alternator is turning backwards
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This more of an issue with brushes on commutators rather than on slip rings; alternators use slip rings.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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26-08-2018, 18:35
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#42
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Registered User
Join Date: Feb 2016
Posts: 90
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Re: propshaft driving an alternator , when sailing
Just an observation which may not be relevant. Harmonic vibrations at certain conditions will destroy 'Stators" in Alternators" in certain conditions. It is probably not worth the effort to remote mount the alternator away from the engine ie. on the engine bearers as the suspension of the alternator to prevent the transfer of vibration will require some allowance for belt flex. Which will only work with lots of experimentation?
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26-08-2018, 18:40
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#43
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 21,515
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Re: propshaft driving an alternator , when sailing
Question for the engineers on the thread.
Is there any meaningful side load on the shaft with coupled to the alternator as described up thread and if so, is this an issue?
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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26-08-2018, 18:50
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#44
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Senior Cruiser

Join Date: Mar 2010
Location: gettin naughty on the beach in cornwall
Boat: 63 custom alloy sloop,macwester26,prout snowgoose 37 elite catamaran!
Posts: 10,598
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Re: propshaft driving an alternator , when sailing
Quote:
Originally Posted by Wotname
Question for the engineers on the thread.
Is there any meaningful side load on the shaft with coupled to the alternator as described up thread and if so, is this an issue?
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generally the pulley is on the output flange of the gearbox between the prop shaft coupling and gearbox coupling with the alternator supported by mounts on the engine and gearbox so no appreciable side preassure apart from on the output bearing of the gearbox.
the one i designed was directly above the thrust bearing and commected to the bearing mount of the flexible coupling mounts, so no side load on the shaft
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26-08-2018, 19:58
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#45
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Moderator Emeritus

Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,348
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Re: propshaft driving an alternator , when sailing
Quote:
Originally Posted by Wotname
This more of an issue with brushes on commutators rather than on slip rings; alternators use slip rings.
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Yes, I was actually thinking of the admonishment of don’t turn a turbine engines starter / generator backwards as it will break off the sharp edge of the brushes, but as you said those are commutators, not slip rings.
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