Power / energy demand autopilot


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johnenvanessa
johnenvanessa
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Hello, I am just calculating my energy demand for a Dufour Grand Large 380 for blue water cruising. I would like to ask for your experience on what your autopilot draws (energy demand) in cruising mode. We have a "Raymarine SPX-30 Autopilot with ST70 display at helm" and I am assuming 5 A with well-balanced sails in "cruising" mode. What is your experience? It's obviously dependent on sea state and rudder balance. Thanks! Vanessa
Dick
Dick
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johnenvanessa - 26 Jun 2021
Hello, I am just calculating my energy demand for a Dufour Grand Large 380 for blue water cruising. I would like to ask for your experience on what your autopilot draws (energy demand) in cruising mode. We have a "Raymarine SPX-30 Autopilot with ST70 display at helm" and I am assuming 5 A with well-balanced sails in "cruising" mode. What is your experience? It's obviously dependent on sea state and rudder balance. Thanks! Vanessa

Hi Vanessa,
You bring up an interesting question with lots of variables. Short answer, 5a may be a tad conservative offshore, but is probably a good guess. That means you must generate at least ~~ 125 amps per day just to steer the boat.
You mentioned an important variable, well-balanced sails.
Another important variable is how your boat responds to changes in wind strength. Increase in wind strength causes boats to heel and this immediately changes the balance that you correctly point out is important. Even offshore, the winds are rarely steady. On a close reach, the difference between 12 and 15 knots TW is significant (more so close-hauled, and less so the more downwind you go). These slight variations in winds speed are always occurring.
A boat whose fore and aft profile (in the water) remains largely the same (think double ender) will just smoothly lean over and increase speed in increased wind. It will demand little attention. A boat which carries her max beam aft (to accommodate double berths aft) (more wedge type boats) will lean over, but her fore and aft profile changes significantly. The stern gets lifted, the rudder can lose its grip (hence dual rudders on some of these designs) and the balance is thrown off and sails will need adjusting (if the change in wind speed persists). Sometimes the boat, in the extreme, will try to bury her bow or tack with the autopilot working hard to control and using amps.
A boat like that will make its autopilot work hard. It is also the reason so many of the modern designs have tired crews at the end of passages. They must be tweaking sails often to achieve balance. Well managed, these boats can achieve fast passage times at the expense of increased work load.
Casual observation is that your vessel falls in the middle (beam is carried aft, but not to the extreme of many) and will be fairly well behaved. The more attention you give to the making sail controls easy to adjust (traveler, jib lead, halyard tension, outhaul, reefing etc.), the happier your autopilot will be and the fewer amps will be used.
Random thoughts, Dick Stevenson, s/v Alchemy

johnenvanessa
johnenvanessa
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Dick - 27 Jun 2021
johnenvanessa - 26 Jun 2021
Hello, I am just calculating my energy demand for a Dufour Grand Large 380 for blue water cruising. I would like to ask for your experience on what your autopilot draws (energy demand) in cruising mode. We have a "Raymarine SPX-30 Autopilot with ST70 display at helm" and I am assuming 5 A with well-balanced sails in "cruising" mode. What is your experience? It's obviously dependent on sea state and rudder balance. Thanks! Vanessa

Hi Vanessa,
You bring up an interesting question with lots of variables. Short answer, 5a may be a tad conservative offshore, but is probably a good guess. That means you must generate at least ~~ 125 amps per day just to steer the boat.
You mentioned an important variable, well-balanced sails.
Another important variable is how your boat responds to changes in wind strength. Increase in wind strength causes boats to heel and this immediately changes the balance that you correctly point out is important. Even offshore, the winds are rarely steady. On a close reach, the difference between 12 and 15 knots TW is significant (more so close-hauled, and less so the more downwind you go). These slight variations in winds speed are always occurring.
A boat whose fore and aft profile (in the water) remains largely the same (think double ender) will just smoothly lean over and increase speed in increased wind. It will demand little attention. A boat which carries her max beam aft (to accommodate double berths aft) (more wedge type boats) will lean over, but her fore and aft profile changes significantly. The stern gets lifted, the rudder can lose its grip (hence dual rudders on some of these designs) and the balance is thrown off and sails will need adjusting (if the change in wind speed persists). Sometimes the boat, in the extreme, will try to bury her bow or tack with the autopilot working hard to control and using amps.
A boat like that will make its autopilot work hard. It is also the reason so many of the modern designs have tired crews at the end of passages. They must be tweaking sails often to achieve balance. Well managed, these boats can achieve fast passage times at the expense of increased work load.
Casual observation is that your vessel falls in the middle (beam is carried aft, but not to the extreme of many) and will be fairly well behaved. The more attention you give to the making sail controls easy to adjust (traveler, jib lead, halyard tension, outhaul, reefing etc.), the happier your autopilot will be and the fewer amps will be used.
Random thoughts, Dick Stevenson, s/v Alchemy

Hello Dick, Thank you for your reply with such a great level of detail! This is very helpful to know and - as a seasoned racer - I will try to trim as well as possible to ease the work for the autopilot. It will keep us busy on long trips then ;)
GO

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