How Strong an Autopilot do I Need?

[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

[Bash]

That's a good drive. I had the smaller one on my previous boat and the larger one on my present boat. Never a problem running downwind with a spinnaker, but I tend to put the spinnaker away before seeing 20 knots apparent wind. My previous boat was just a scootch heavier than your current boat, fully loaded.

If the Valiant Esprit is as well mannered downwind as its older sister, I don't see why you'd need to oversize the linear drive unless you're the type who runs a chute in strong winds and/or high seas.

[Cheechako]

How much weather helm does that boat have... that's what you need to know. If I remember right the V32 had a lot.

[sailorboy1]

I have the core pack you are talking about with a rotary drive and my boat weights more than yours and I don't have a problem. It steers just fine at a low response level of 2 and doesn't use much power normally.

But it comes down to how balanced your boat sails, if you have a lot of weather helm and are a storm chaser type of sailor you should go up in size.

Otherwise you have a 15% margin and are going to be fine.

[S/V Alchemy]

You might also consider the concept of duty cycles. The larger drive will not be, under normal operation, be working as hard as the smaller one to perform the same corrections.

On the other hand, helm and sail balance can make a huge difference in this regard. "Trimming for the pilot" can give you a pilot that budges maybe a quarter of the time as a pilot that's "fighting" the boat to stay to a course.

[jeremiason]

Becareful estimating you vessel displacement... My vessel's displacement is listed by the factory is 28,500. Twice it has been wieghted at 37,500. My sister ships have all had the same experience.

I have also have a RM linear drive and frankly it drives the boat downwind better than I do...

The Auto Pilot worked great for about 11,000 NM, before I had a failure. The connecting bolt between the linear drive arm and the rudder post tiller sheared. We ended up driving the boat for 250 NM in about 30 hours... It gave me a new respect for that device!

I checked with my sister ships and six of them also reported the same problem. If you are going crusing I would suggest you purchase a spare bolt (Part #K065), which costs $53 US.

[Journeyman]

Not much weather helm on a Valiant Esprit 37 when trimmed and reefed properly.

Sailing downwind and cross wave on a short period wave, seems to be the worst case autopilot load. My existing Raymarine wheel pilot I have had for 10 years can struggle n those conditions. I have assumed when that happens, it is for lack of power. However, I am exceeding the design limits of that unit as it is rated at about 16,500lbs. It does fine in other conditions, but then I have never been in a gale where I might have a lot of hard steering for a long time, despite being well reefed and trimmed. I intend to keep this unit as a parallel, completely independent backup, with the new linear drive control head mounted next to it.

The issue of power consumption and duty cycle is of particular interest. 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?

[zeehag]

my simrad was praised by a well travelled sailor---we had to bash into seas and winds on our way to barra, as barra de navidad is uphill from zijuatenejo---we didnt have any problem with autopilot--only with wind direction.....we bashed wnw into 20 kts of headwind for most of our first day. our prevailing current ws 1 1/2 kts to south/south east......
we didnt do electricity numbers but the unit uses so little electricity that i dont measure it .

[donradcliffe]

If you want to cross oceans, you will need to carry a spare linear drive. The electro-hydraulic drives are much more reliable.

[goboatingnow]

Go for the larger drive

Dave

[Bash]

Originally Posted by jeremiason

I checked with my sister ships and six of them also reported the same problem. If you are going crusing I would suggest you purchase a spare bolt (Part #K065), which costs $53 US.

Excellent advice.

[donradcliffe]

Originally Posted by Bash

Excellent advice.

Even better advice would be to upsize the bolt so it won't break again. My bolt was about 10 mm and there was a nylon sleeve over the end. I bought a 1/2 inch bolt and had a machine shop turn down the end so it was the same diameter as the outside of the sleeve--probably increased the meat by 50%.

[jeremiason]

Originally Posted by donradcliffe

Even better advice would be to upsize the bolt so it won't break again.

We were sailing in a broad reach with quartering seas... The waves were really square and close togther. One wave rolled the boat and the second hit the rudder. I would imagine if I had been hand steering, I probably would not have been able to hold the wheel.

So in other words, if the bolt had not broken, something else might have, like the arm on the linear drive.

Although our bolt is beefier than yours, sometimes improving things isn't the answer. This bolt lasted 6.5 years with over 11k NM on the boat, with a couple of storms under our belt.

I now carry a spare so the next time I can fix it underway!

[jeremiason]

Originally Posted by donradcliffe

If you want to cross oceans, you will need to carry a spare linear drive. The electro-hydraulic drives are much more reliable.

I have never carried a spare Linear Drive and have not heard anyone report a linear drive failure, except during a lightning strike.

I have heard of the auto pilot brains, control heads or hydraulic units going out, but usually after 5-7 years of use.

If you are a belt and suspenders kind of person or you drive starts getting older, then think about carrying a spare. Otherwise you are investing a lot of money for something that will sit rusting in the bildge.

[Bash]

Originally Posted by donradcliffe

Even better advice would be to upsize the bolt so it won't break again.

I'd be worried about a situation where you ran the rudder into a semi-floating obstruction while under auto-pilot and something had to give. Sometimes it's good to have a sheer point.

Of course, I also advocate carrying an emergency rudder.