How Strong an Autopilot do I Need?

[S/V Alchemy]

Originally Posted by Journeyman

While we have lots of power available (twin 150A alternators/860Ah batteries) I note that the Linear Drive motor units for the Type 1 and Type 2 are really different.

Type 1 linear drive "Power Consumption" = 18-36W
Type 2 linear drive "Power Consumption" = 48-72W

But I assume for a given amount of work output, and the displacement of my boat, the power consumption should actually be lower for the bigger unit?

Picking the midrange of 24W, that looks to be about 2A at 12V, or about 50Ah per day on passage?

That's what I was getting at: The bigger unit, working at a lower output, might eat fewer Ah over a given day.

It's like the larger battery bank argument giving you a broader band of useful (above 50%) power to play with, at the cost of weight and initial cost, but the benefit of fewer discharge cycles.

[sailorboy1]

Originally Posted by S/V Alchemy

That's what I was getting at: The bigger unit, working at a lower output, might eat fewer Ah over a given day.
.

I don't think it matters what the size the controller is as far as power consumed for a given boat (assuming it can handle the boat to start with). The unit will use the amount of power it takes to steer the boat regardless. The difference is the power needed to steer when conditions are bad, and that is what you need to consider as that is when you what the AP to really work.

Most of the time your AP shouldn't be using all that much power. If it is sucking down power during "normal" sailing conditions you need to spend more time on sail trim etc. My AP can do 20 amps I think, but in normal use only uses about 4.

I bet most installation could get a 5% improvement in safety margin on their AP just by increasing the wiring size to reduce voltage drop! That is a lot less expensive than going nuts in selecting a larger unit (that will need the larger wires anyway).

[brianb00]

I have a 10000 lb fin keel boat equipped with RM linear drive and RM hydraulic type 1, this is for redundancy. In a recent 5000 mile trip the linear drive failed as an internal keeper spring came off. The hydraulic drive motor began to fail due to brush wear. I estimate the total miles on the hydraulic motor at about 8000NM under aggressive racing conditions. I also carry a tiller pilot as back/back up. It was useful on the aforementioned trip.

Brian

[donradcliffe]

Originally Posted by brianb00

I have a 10000 lb fin keel boat equipped with RM linear drive and RM hydraulic type 1, this is for redundancy. In a recent 5000 mile trip the linear drive failed as an internal keeper spring came off. The hydraulic drive motor began to fail due to brush wear. I estimate the total miles on the hydraulic motor at about 8000NM under aggressive racing conditions. I also carry a tiller pilot as back/back up. It was useful on the aforementioned trip.

Brian

I think your experience is more typical than Jeremiason's. How long did the linear drive last?

I had a Simrad linear drive where the internal plastic gears failed in a blow off Columbia--they wouldn't give me new gears so I fiberglassed them back together and that got me to Australia where I fitted a Simrad hydraulic drive. The first motor on the hydraulic drive lasted 50,000 miles and several gales, but the brushes on the replacement motor died at about 10,000. Brushes are cheap, so I just replace them every year.

I don't let RM on my boats, but I had a friend whose brand new linear drive, installed by RM technicians, failed within a month. He had the autopilot steering by waypoint, and put in the wrong data so that the
pilot had to make a near 180 degree turn. It tried to put the drive hard over (the manual says its not supposed to) but the drive broke internally and RM said it was not repairable--there was a fight over getting it replaced under warranty.

Before you depend on a electro-mechanical linear drive as your primary drive, take one apart and see what's inside. If there's any plastic, forget it.

Only a masochist would cross oceans without autopilot backup(s).

[Journeyman]

Good input. Thanks.

I went with the Type 2 linear drive after reading these posts, and talking with RM tech sales that told me the Type 1 has been recently de-rated from 24,000lbs to 22,000lbs displacement.

We are installing the Type 2 drive now in anticipation of buying the new RM Evolution autopilot package to go with it (EV Sensor Core, ACU-400 Control Unit, and p70 Control Head), coming out next month.

I am thinking to leave my 10 year old Raymarine ST4000MKII Wheel Pilot in place as a backup. That is less expensive than carrying a second Type 2 drive backup that may never get used, and should be a completely parallel redundant system.

It will be interesting to switch back and forth between the two units to see how much better the below decks Evolution steers in various conditions vs the old wheel pilot.

I don't really like being on the bleeding edge with a new generation autopilot, but the marketing blurb is pretty compelling.

Putting a powerful new autopilot into a 35 year old boat with its original rudder stock, rudder bearings, quadrant, etc. makes me wonder. The quadrant wire was replaced 8 years ago after 27 years of seasonal coastal cruising, and looked fine. But the steering system is likely to get the workout of its life over the next 2 years.

Anybody got any installation anecdotes that resulted in trouble?

[brianb00]

Originally Posted by donradcliffe

I think your experience is more typical than Jeremiason's. How long did the linear drive last?

I had a Simrad linear drive where the internal plastic gears failed in a blow off Columbia--they wouldn't give me new gears so I fiberglassed them back together and that got me to Australia where I fitted a Simrad hydraulic drive. The first motor on the hydraulic drive lasted 50,000 miles and several gales, but the brushes on the replacement motor died at about 10,000. Brushes are cheap, so I just replace them every year.

I don't let RM on my boats, but I had a friend whose brand new linear drive, installed by RM technicians, failed within a month. He had the autopilot steering by waypoint, and put in the wrong data so that the
pilot had to make a near 180 degree turn. It tried to put the drive hard over (the manual says its not supposed to) but the drive broke internally and RM said it was not repairable--there was a fight over getting it replaced under warranty.

Before you depend on a electro-mechanical linear drive as your primary drive, take one apart and see what's inside. If there's any plastic, forget it.

Only a masochist would cross oceans without autopilot backup(s).

>> My linear drive lasted about 2000 miles estimated. It was used prior to the long trip and intermittently on the long trip.

>> I would not go far offshore without several redundant systems.

[brianb00]

Journeyman;
I went with the Type 2 linear drive after reading these posts, and talking with RM tech sales that told me the Type 1 has been recently de-rated from 24,000lbs to 22,000lbs displacement.

>> good choice. The new generation of RM control heads seem much more seaworthy than the older generations. My policy used to be no RM on the boat after losing a full instrument set to a single wave on a Trans Pacific event. (RM was nice about it, except I was standing in a Honolulu WM with a bag of waterlogged instruments after having no instruments for 2000 miles).

We are installing the Type 2 drive now in anticipation of buying the new RM Evolution autopilot package to go with it (EV Sensor Core, ACU-400 Control Unit, and p70 Control Head), coming out next month.

I am thinking to leave my 10 year old Raymarine ST4000MKII Wheel Pilot in place as a backup. That is less expensive than carrying a second Type 2 drive backup that may never get used, and should be a completely parallel redundant system.

>> good idea. You may want to make real sure the ST4000 control head will not be exposed to salt spray or salt behind the panel. I have had two of those generation control heads fail (st4000) in very moist conditions. They absorb moisture and seem to distort the compass signal (a very sensitive circuit). I have repeatedly had to bake my st4000 at sea in a plastic bag in the sun to drive the moisture out.

It will be interesting to switch back and forth between the two units to see how much better the below decks Evolution steers in various conditions vs the old wheel pilot.

>> THe difference, with a rate gyro, will make you smile in a big way.

I don't really like being on the bleeding edge with a new generation autopilot, but the marketing blurb is pretty compelling.

Putting a powerful new autopilot into a 35 year old boat with its original rudder stock, rudder bearings, quadrant, etc. makes me wonder. The quadrant wire was replaced 8 years ago after 27 years of seasonal coastal cruising, and looked fine. But the steering system is likely to get the workout of its life over the next 2 years.

>> My experience: Had a rudder failure on a 15 year old boat, that was only doing coastal cruising in early life, 500 miles from Oahu. Internal rudder welds failed and rudder sheared off. Upon replacement I found bearings were pretty sad. Went with oversized everything and roller bearings. Super strong post. No issues in the last 15000 miles.

Anybody got any installation anecdotes that resulted in trouble?

>> AP installs are notorious for tearing themselves apart at sea. Just pretend the AP is putting loads on all the fastners/welds/epoxy joints that you can generate at the helm by cranking as hard as possible. A good friend recently hit a bridge stanchion with a 50' vessel when the AP took a sudden turn to port. He grabbed the helm and in the ensuing attempt to over steer the AP he snapped the quadrant cables and bent the hydraulic arm on the very large RAM.

BB

[Crimea Cruiser]

Originally Posted by Journeyman

I am buying a Raymarine linear drive autopilot this week.
There are two options, a unit that is good up to 24,000lbs of displacement and one that is good to 33,000 lbs.
My Valiant Esprit 37's dry weight displacement is 17,000 lbs., at least according to the designer's documentation.
I have totaled up the additional weight of everything I can think of that I have added to the boat (see list below) and get a laden cruising displacement of 21,000lbs.
If I go with the autopilot rated for 24,000 lbs of displacement, will I be happy with it crossing oceans?
Thanks!

Esprit 37 displacement 17000lbs, plus:
860 AH Batteries 520
65 gals water 500
40 gals fuel 320
250' chain 250
Two anchors 50
Sails 100
Head stay furler 50
Inner stay stuff 50
HVAC 80
Books 40
Lines and winches 100
Hot water heater 50
Tools 100
Inverter 30
Cushions and bedding 50
Watermaker 30
Dinghy 100
Outboard 40
SUP 30
Holding tank 100
Electronics and radome 100
Two autopilots 50
Refer compressor 30
Dodger 40
Fishing poles and tackle 30
Clothes, wet suits 60
dive tank & gear 50
4 people & sea bags 800
Total .... 4,000 lbs
Engine & tranny?
Total dry ship + cruise config =
17,000 + 4000 = 21,000
Max Type 1 autopilot rating 24,000 lbs

I know I am going to set of a raging debate, but weight and displacement are not necessarily the same thing. I will borrow Charles Burgess' analogy to explain.

"Let's take two metal cubes that both sink completely.

Take a cubic centimeter of lead and a cubic centimeter of aluminum. The lead cube will weigh considerably more than the aluminum cube, but each will displace exactly the same amount of water...a cubic centimeter of water = 1 gram. How much the cube weighs has nothing to do with its displacement.

Now take the same lead cube, and an aluminum cube that weighs exactly the same as the lead cube. The aluminum cube will be much larger in volume than the lead cube (this can be calculated for the exact dimensions from periodic/atomic density...if anyone so desires). Each cube, although weighing exactly the same, but the aluminum cube will displace more water than the lead cube...because the aluminum cube has a larger volume.

Weight vs displacement are not equivalent factors. Weight is concerned with matter under the acceleration of gravity, where the principle factor of displacement is volume...whether it is displacing air or water the principle is the same.

Does weight play a factor in displacement? As a practical matter of boats that float - Yes, but not a primary factor. The role of weight is only to push the hull deeper into the water, but does not directly cause the displacement of the water. It is the volume of the hull that is the direct cause of displacement of the water. "

And this does not even account that the same same hull weight in fresh or salt water displaces different volumes and weights of water by attribute of the buoyancy factor of the water being different.

Do not shoot the messenger.

[Vivid]

A vessel that is afloat on the earth's surface will displace the mass of water that is equal to the vessel's mass, as Archimedes is said to have discovered. At the same place on the earth's surface those masses will each have the same weight, by definition of those terms. Your analogy of the solid cubes of different density metals is inappropriate, since neither is afloat. Hollow them out until they float and you will find the principles just stated will hold true.

[Crimea Cruiser]

mass and weight are too different things. one square centimeter of balsa wood has the same mass as one square centimeter of lead. Mass is volume, weight is gravitational pull. One kilo of feathers weighs the same as one kilo of bricks. Same weight, different mass.

[Vivid]

You are right about definition of weight, but wrong about mass. Mass is the measure of amount of matter and can be thought of as the number of molecules in a physical object. As you say, the object's weight depends on gravity and other accelerational factors (it will weigh less on the moon, on Mt. Everest, in a falling elevator) but, absent fission or fusion or attrition, the mass won't ever change. You are wrong in thinking that mass is a measure of volume. The relation between mass and volume you are thinking about is called density, how many grams of mass (number of molecules) for a given volume, usually cc (cubic cm). Pure water has a density of one gram per cc. (Volume changes with temperature and sometimes with airpressure, so density of substances is defined at STP, standard temperature and pressure, which is 20 degrees C. and 14.7 psi.)Things that float have a lower total density, like floating boats. Things that sink, like blocks of metal, have a higher density than water or whatever they are sinking in. Different waters can have different densities (salt water is denser than fresh, cold water is denser than warm) so a given boat with the same contents will float lower on its lines in the Caribbean than in Maine and lower in Lake Michigan than in Maine at the same water temperature.

[Crimea Cruiser]

Originally Posted by Vivid

Different waters can have different densities (salt water is denser than fresh, cold water is denser than warm) so a given boat with the same contents will float lower on its lines in the Caribbean than in Maine and lower in Lake Michigan than in Maine at the same water temperature.

And thus you prove my point that simply adding weight to a boat does not equate to equal weight or volume in displacement. Buoyancy and density of water have effect on displacement formula. And as you have reiterated my point that it is not simply weight=displacement, I will now withdraw from debate.

[brianb00]

Originally Posted by Vivid

You are right about definition of weight, but wrong about mass. Mass is the measure of amount of matter and can be thought of as the number of molecules in a physical object. As you say, the object's weight depends on gravity and other accelerational factors (it will weigh less on the moon, on Mt. Everest, in a falling elevator) but, absent fission or fusion or attrition, the mass won't ever change. You are wrong in thinking that mass is a measure of volume. The relation between mass and volume you are thinking about is called density, how many grams of mass (number of molecules) for a given volume, usually cc (cubic cm). Pure water has a density of one gram per cc. (Volume changes with temperature and sometimes with airpressure, so density of substances is defined at STP, standard temperature and pressure, which is 20 degrees C. and 14.7 psi.)Things that float have a lower total density, like floating boats. Things that sink, like blocks of metal, have a higher density than water or whatever they are sinking in. Different waters can have different densities (salt water is denser than fresh, cold water is denser than warm) so a given boat with the same contents will float lower on its lines in the Caribbean than in Maine and lower in Lake Michigan than in Maine at the same water temperature.

Very succinct explanation.
Thanks.

[Greggegner]

Originally Posted by Crimea Cruiser

I know I am going to set of a raging debate, but weight and displacement are not necessarily the same thing. I will borrow Charles Burgess' analogy to explain.

"Let's take two metal cubes that both sink completely.

Take a cubic centimeter of lead and a cubic centimeter of aluminum. The lead cube will weigh considerably more than the aluminum cube, but each will displace exactly the same amount of water...a cubic centimeter of water = 1 gram. How much the cube weighs has nothing to do with its displacement.

Now take the same lead cube, and an aluminum cube that weighs exactly the same as the lead cube. The aluminum cube will be much larger in volume than the lead cube (this can be calculated for the exact dimensions from periodic/atomic density...if anyone so desires). Each cube, although weighing exactly the same, but the aluminum cube will displace more water than the lead cube...because the aluminum cube has a larger volume.

Weight vs displacement are not equivalent factors. Weight is concerned with matter under the acceleration of gravity, where the principle factor of displacement is volume...whether it is displacing air or water the principle is the same.

Does weight play a factor in displacement? As a practical matter of boats that float - Yes, but not a primary factor. The role of weight is only to push the hull deeper into the water, but does not directly cause the displacement of the water. It is the volume of the hull that is the direct cause of displacement of the water. "

And this does not even account that the same same hull weight in fresh or salt water displaces different volumes and weights of water by attribute of the buoyancy factor of the water being different.

Do not shoot the messenger.

Not shooting the messenger, take those cubes and flatten them out and make a hull. The lead will displace more water than the aluminum. Displacement is the weight of water moved by the hull.

[brianb00]

Lead or aluminum cubes and attempting to steer same with an autopilot are going to require a significantly more robust design than what was considered in the opening of this thread.