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Old 23-04-2019, 16:25   #1
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Designs for a Strong Autopilot Drive Mount / Shelf?

Does anyone have pictures or designs you can share of mounts you've used for a Type 1 autopilot drive (Raymarine linear mechanical, but examples with other drives would be helpful too). I'm talking about where the base of the linear drive (the platform with 4 bolts) is mounted to.

I've done extensive Internet searches but only found a few examples. I know there must be thousands or even 10s of thousands of these mounts in existence though. There was a prior thread on this but I could use some more examples.

I've already built a shelf but I'm not sure it's strong enough. People say make it bulletproof, but of course that's a relative term - how do you know when it is? The particular positioning requirements of the linear drive make it difficult to find an existing structural member to bolt it through.

If you don't have an existing thing to bolt it through, I thought fiberglassing a shelf into the hull was fairly common with these installations. So I'm trying to figure out why mine doesn't seem to be working.

I'll post some pictures, and if others have pictures of your own installation, that would be super helpful.
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Old 23-04-2019, 16:28   #2
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

Design #1)
Plywood shelf I fiberglassed into the hull. It seems to be failing, as it is moving slightly (about 1mm) when the drive is in use. I found the wood itself was buckling, so I added 2 layers of fiberglass cloth around it and added backing plates. But I have this slight movement now which I can only guess means either it's pulling away from the stern wall or the wood itself is flexing.

It's 3/4" birch plywood, epoxy coated, then the joints received a GFlex fillet and 5-6 layers of fiberglass cloth at each edge to hull joint, using overlapping pieces of cloth.

I could grind out the shelf rebuild it with G10 fiberglass board, which would be stronger, but I'm not sure if I could still have an issue with that pulling away from the hull?
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Old 23-04-2019, 16:43   #3
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

If you go to most one design boat websites and use a Google custom search, you'll find lots. Like this one from our C34 website:


Search Results – Catalina 34 International Association


Good luck.
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Old 23-04-2019, 17:21   #4
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

Idea #2) Custom stainless steel 90-degree angle bracket (1/2" thickness) thru-bolted to the stern transom wall with exterior backing plate. This would obviously be the strongest, but requires drilling through the stern and sealing that properly (wood core). A bit more risky than just building an internal shelf.

Idea #3) I have a vertical bulkhead located about 12" to port of where the drive base needs to be. The bulkhead is fore-aft pointing and is the structural bulkhead the backstay chainplate ties in to.

It would be much easier to drill through this and keep my backing plate interior, but it would mean the SS angle bracket would be at a 90-degree angle to the drive load direction. Would this put too much compressive force on the bulkhead? To the point where it might crush the wood or divet the fiberglass? It's a typical 3/4" wood core with about 1/4" fiberglass on each side.
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Old 23-04-2019, 17:22   #5
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

I think the problem with your existing mount is that the force of the ram runs at an angle to the face of the wood which is both flexing and overloading the area of the plywood that has failed. I'd probably realign the plywood to be in the same plane as the ram along with the addition of gussets underneath to increase overall strength and to resist flexing. If you have enough room, you can also glue two pieces of ply together to increase it's strength.


I've got a rotary style drive that - tbh - hasn't hit the water yet, but I went to great pains to ensure that mount stayed in plane with the loading. It uses 18mm ply bolted to angle iron on the bulkhead side and is coach screwed to a piece of hardwood fibreglassed to the hull at the stern. By my calculations, the vast majority of the loading is shear at the bolted connections which I reckon is the key to providing a rigid mounting.
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Old 23-04-2019, 17:25   #6
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

Quote:
Originally Posted by Stu Jackson View Post
If you go to most one design boat websites and use a Google custom search, you'll find lots. Like this one from our C34 website:

Search Results Catalina 34 International Association

Good luck.
Thanks, I have seen a few threads on the Beneteau site but hadn't discovered the c34 threads. Still, many of the autopilot install threads discuss other aspects (ex, tiller arm) but don't go into details on the mounting shelf. Noah's pictures on the c34 thread "below deck autopilot" are excellent, but just boggles my mind even more - his mount looks way more vulnerable to transverse loads than mine is. So he must have done something different than what I have, because his is holding up and mine isn't. I'd love to know what that difference is.
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Old 23-04-2019, 17:49   #7
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

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Originally Posted by Tessellate View Post
............................
...................- his mount looks way more vulnerable to transverse loads than mine is. So he must have done something different than what I have, because his is holding up and mine isn't. I'd love to know what that difference is.

Tesselate, why not ask him directly? You can join our C34 forum, for free, just like here, and ask away. He's very active on our C34 forum and would be happy to answer. He's our C34IA Commodore, too. Nice guy, he never pulls rank!


https://c34.org/bbs/index.php/board,11.0.html


Just register, make up your own un & pw, then I'll "approve" you (we do manually, not automatic, but no long waiting period) and you can go ahead.


The https://www.catalina36.org/ site has similar stuff, and I'd bet all the Catalina one design Associations with websites (i.e., most of 'em), do, too.
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Old 23-04-2019, 18:17   #8
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

Needs a lot more fiberglass top and bottom of the shelf not just the joint.

I've only installed 4 of these in fiberglass boats, but the shelves I built in the past were with two legs on either side shaped to the hull and the opening out the front of the shelf (opposite of your shelf). This allows you to glass the top and bottom of the shelf (large fillet for the bottom joint) and wrap the cloth up and down the hull. You can then glass both legs to the hull from side to side passing over the self top. The tabbing extended to the hull around 4-5" in all directions.

The bottom of shelf with the wrap to the hull was three or four layers of 17oz, the top gets four going from shelf top up the hull, and four from the legs over the shelf top (8 total on the top of the shelf...a little over 1/8" total thickness).

It's not scientific, but was a gut install on the first boat and has worked on the other three without incident. So, good luck

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Old 24-04-2019, 09:10   #9
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

You mount seems fine in PRINCIPLE, judging from you pic.s. The plywood shelf has to be a BOX with the three (or four) components not only screwed together as yours are, but also GLUED together. It doesn't look as if your shelf components are glued as well as screwed. I would put glue cleats in the interior corners where the components meet.

The edges of the box that meet the interior of the hull then have to be shaped so they match the curvature of the hull, both in curvature and in bevel. When the faying surfaces are correct, glue the box to the inside of the hull and let cure. Then, when the glue has set, lay in a fillet of thickened lay-up resinin every corner twixt box and hull. When the fillets have cured, lay in a narrow strip of mat into the corner and wet it out. Repeat with 3 successively wider ones plus a strip of woven tape, the widest of all, on top. They have to be laid in quickly enuff that the resin in the nether layer has not yet "kicked off" as you lay in the topmost tape.

Think about the fact that if you double the length of the distance from the centre of the rudder stock to the pin that connects the ram to the tiller or quadrant, you HALVE the forces acting on the installation. Check your geometry and see if you can reduce the forces.

Remember when you do that that at rudder deflections greater than about 20s the rudder will stall and act more like a brake than like a rudder. When underway at normal speeds the rudder deflection required to steer the boat is usually less than 5. At dead slow speeds greater deflection, up to the magic deflection when a stall (disruption of water flow over the blade) occurs, is required. Best if you can disengage the AP for slow speed maneuvering.


Have fun :-)

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Old 24-04-2019, 09:31   #10
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

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Originally Posted by funjohnson View Post
Needs a lot more fiberglass top and bottom of the shelf not just the joint.

I've only installed 4 of these in fiberglass boats, but the shelves I built in the past were with two legs on either side shaped to the hull and the opening out the front of the shelf (opposite of your shelf). This allows you to glass the top and bottom of the shelf (large fillet for the bottom joint) and wrap the cloth up and down the hull. You can then glass both legs to the hull from side to side passing over the self top. The tabbing extended to the hull around 4-5" in all directions.

The bottom of shelf with the wrap to the hull was three or four layers of 17oz, the top gets four going from shelf top up the hull, and four from the legs over the shelf top (8 total on the top of the shelf...a little over 1/8" total thickness).

It's not scientific, but was a gut install on the first boat and has worked on the other three without incident. So, good luck

Matt
Wow, that's a lot of fiberglass. I used 5 layers of 6oz cloth (Evercoat's "SeaGlass"). Fisheries Supply, a very good marine store where I bought it, actually doesn't sell biaxial 17oz. I could've ordered that online though.

Your design of two legs below the shelf is good, because those legs are aligned with the direction of force (rather than the support leg I have which is perpendicular to direction of force). I'm dealing with a very tight installation space however, and the shelf needs to be short in height because there's no room to move the drive up. I also have two thruhulls adjacent to it to port (pic attached. note, they are slightly above waterline).

If you have two legs and only an opening in the front, I imagine it's hard to fit a hand in underneath to attach the bolt nuts? I'm assuming your shelf is significantly wider than the 4" drive base, which would give you a bit more room to work.

@TrentePieds, I did glue the two pieces of wood together, unless I'm misunderstanding what you mean by glue. I filleted the inner corner joint with GFlex and then 3 layers of overlapping fiberglass. The outside of the corner joint also has 2 layers of fiberglass.

The wood was contoured and beveled to match the hull surface. This was quite difficult (and it's not perfect) because of complex cuts. I did a cardboard template first and did a dozen iterations of adjustments on the wood to fit. This is one concern with trying to switch to G10 or Coosa board - it would be tougher to cut and sand to match random hull contours.
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Old 24-04-2019, 09:47   #11
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

Quote:
Originally Posted by Tessellate View Post
Design #1)
Plywood shelf I fiberglassed into the hull. It seems to be failing, as it is moving slightly (about 1mm) when the drive is in use. I found the wood itself was buckling, so I added 2 layers of fiberglass cloth around it and added backing plates. But I have this slight movement now which I can only guess means either it's pulling away from the stern wall or the wood itself is flexing.

It's 3/4" birch plywood, epoxy coated, then the joints received a GFlex fillet and 5-6 layers of fiberglass cloth at each edge to hull joint, using overlapping pieces of cloth.

I could grind out the shelf rebuild it with G10 fiberglass board, which would be stronger, but I'm not sure if I could still have an issue with that pulling away from the hull?

This link may help you. Attaching ram to quadrant


The plywood "shelf" in your pic will never stand the strain of a M81131 ram on a 38ft boat,for 2 main reasons:
1. The ram mount bracket bolts are mounted thru the plywood @ 90 deg to the ram thrust,causing a pry & flexing of plywood. The ram should be mounted to a strong vertical, or
2.The ply shelf needs to be boxed in on 3 sides & completely glassed over. It needs to be rigid.


Hope the link & suggestions help. / Len
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Old 24-04-2019, 09:48   #12
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

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Old 24-04-2019, 09:49   #13
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

Agreed with Matt and Trente, you need more glass. But more importantly, and I'm not sure they mentioned it, you need to grind or sand all of that paint off and get to bare glass regardless of where you put the shelf. If not, it's only as strong as the paint's adhesion to the hull.

I would make a new shelf identical to what you have but make make a middle divider underneath. This would be split right between the drives bolts so you can access nuts. Glass the new shelf with multiple layers of glass top and bottom. I would go as far as glassing the top board on its own first before epoxying it to the other two pieces so this piece will be stronger on its own. When it cures, sand the bottom parts that will be against the hull. And make thick, thick epoxy "mud" and press it against the hull that been ground bare. Smooth the edges and put your fillets and let it cure. Then do the glass lay-up like Trente prescribed. When it's cured, sand the sharp edges off, drill your holes, paint and mount your drive. This should not move, ever.
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Old 24-04-2019, 10:05   #14
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

I wouldn't wait for the fillet to cure before applying the glass. If it's cured, then any imperfection in the fillet, unless filled, will create a void in the glass. If you glass before the fillet kicks, you can lightly push the fiberglass along the fillet and get a perfect bond on that critical joint. The bigger the fillet the better. Stick with your cloth, there is no need for mat with your epoxy.

Treat the plywood like a core... make it like the bulkhead next to it.

I hate to be that guy, but If you haven't already, you're going to want to address those thru-hulls while you are in there.

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Old 24-04-2019, 10:30   #15
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Re: Designs for a Strong Autopilot Drive Mount / Shelf?

Here's mine:
https://drive.google.com/file/d/1Zef...ew?usp=sharing

Boat is a Tartan 33. That's .75 inch Baltic Birch plywood. All sides of the ply are tabbed to the hull with two layers of 1708 biaxial cloth/mat tape and sealed in epoxy. I used a bit of balsa between the ply edges and hull to avoid hard spots. The sound deadening mat works well; the drive is inaudible under sail. I'm happy with this mount. The piece on the left side of the pic is a thinner ply mount for the rudder position sensor.
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