Building an Autopilot Base

[AJ_n_Audrey]

With some help from a friend, I finally got the auto-pilot base project going, and two pics are attached - one with supports attached to the hull and the second with platform in place. Thanks to those (Halien) who pointed out the need for balsa between plywood and and the hull - I would not have known about that otherwise.

In the pics, you can see the propane tank well, which is directly over the attachment point for the linear drive. Fortunately, there is about a 1/2 inch of clearance for the top of the drive (and yes, I've gone from hard port to hard starboard, and it does not touch), and that clearance gets it within 5 degrees or less of level. Everything is doubled 3/4 inch marine plywood, through-bolted to the stringer. The design has evolved, and the platform was lengthened in the forward-aft dimension so that it can attach to the aft bulkhead. Monday we'll start doing fillets.

This thing is going to be massively strong, but I'm still thinking about attaching to the underside of the deck to definitely stabilize it in the vertical axis. Anybody know of a reason not to do that?

[AJ_n_Audrey]

With pics:

[deblen]

Looks like a good design once it's fully glassed. A "post/brace" from the overhead down to the area of the red drill may make you feel better,but I don't think it's required. If you do add a vertical,try to leave room enough to access the ram on the new shelf & the propane locker,etc behind.

Re the propane locker,is that copper line the the propane supply line??
The supply line is supposed to come out near the top of the locker & the drain to overboard should come out the bottom,in most cases.
Safe Boat Propane System Installation

Len

[AJ_n_Audrey]

Thanks Len.

No, the copper fitting at the bottom of the well is the drain line that vents through the transom. The feed line, as you would expect, is at the top of the well (and tank).

I know I'm probably over-engineering the thing, and I may well leave out the brace from the platform to the underneath of the deck, but in any event I need to restore the support that was originally supporting the bottom of the well and stabilizing it in three axes (plural of axis).

[DeepFrz]

Originally Posted by AJ_n_Audrey

Thanks Len.

No, the copper fitting at the bottom of the well is the drain line that vents through the transom. The feed line, as you would expect, is at the top of the well (and tank).

I know I'm probably over-engineering the thing, and I may well leave out the brace from the platform to the underneath of the deck, but in any event I need to restore the support that was originally supporting the bottom of the well and stabilizing it in three axes (plural of axis).

Over engineering in this case is a good thing IMO. The last thing you want when the going gets rough is the auto pilot mounting platform to break loose. I would consider inserting stainless steel annuli in the bolt holes to prevent the plywood collapsing over time and the mounting bolts loosening.

Edit: A good backing plate will also do the job.

[noelex 77]

Originally Posted by AJ_n_Audrey

I know I'm probably over-engineering the thing

Failure of the autopilot attachment point is a common problem on cruising boats so I think you are doing the right thing.

This was the solution that our boatbuilder came up with for the autopilot drives on our new aluminium boat. The mounting point is a 20mm thick aluminium plate welded to ribs, stringers and the rudder post itself.

Aluminium construction does not have a lot of relevance to fibreglass boats, but it does show that some boatbuilders obviously feel a strong mount point is important.

[AJ_n_Audrey]

Originally Posted by DeepFrz

Over engineering in this case is a good thing IMO. The last thing you want when the going gets rough is the auto pilot mounting platform to break loose. I would consider inserting stainless steel annuli in the bolt holes to prevent the plywood collapsing over time and the mounting bolts loosening.

Edit: A good backing plate will also do the job.

So maybe an 1/8 inch SS backing plate, 6 inches square? I agree that sounds like a good idea. Also, I do think I will add the brace between the platform and underside of deck. It's really an interference fit (i.e., jammed in) with the glass just holding it in the place I will jam it.

[deblen]

Agree with DeepFrz & Noelex. Long term use of a ram bolted to wood or GRP will eventually result in the ram mounting bolt holes elongating.
Have seen it often on my customer's fish boats.
One solution that worked very well was to mount the ram assembly on a metal plate that dropped right over the rudder stock (with a bearing or bushing). The whole assy. is then securely bolted to the hull stringers & preferably,to a vertical surface also,to decrease shear force to the hull mounting bolts.
Some typical pics from Google,though there are various ways to accomplish the same result & needs to be customized to fit different boats.

[AJ_n_Audrey]

Originally Posted by deblen

Agree with DeepFrz & Noelex. Long term use of a ram bolted to wood or GRP will eventually result in the ram mounting bolt holes elongating.
Have seen it often on my customer's fish boats.
One solution that worked very well was to mount the ram assembly on a metal plate that dropped right over the rudder stock (with a bearing or bushing). The whole assy. is then securely bolted to the hull stringers & preferably,to a vertical surface also,to decrease shear force to the hull mounting bolts.
Some typical pics from Google,though there are various ways to accomplish the same result & needs to be customized to fit different boats.

Attachment 158827

Attachment 158828

Thanks - I'm pretty far along with the project, but I will definitely incorporate a stainless backing plate. I might even be able to get a welder to produce stainless plate with flanges I can bolt to the ribs.

[antoha]

I used an aluminum backing plate - figured that since the drive base itself is aluminum, that should suffice.

[AJ_n_Audrey]

Originally Posted by antoha

I used an aluminum backing plate - figured that since the drive base itself is aluminum, that should suffice.

I have come to the same conclusion. Also, I can lay hands on 1/4 inch 7075 T6 aluminum plate fairly easily. After a little reading, I found that 7075 T6 is actually stronger than mild steel, so that should be good enough.

Now I have a strength question regarding fiberglass. All the plywood I'm using is doubled 3/4 inch marine plywood, so 1.5 inches thick. To attach the ribs to the hull and the base mounting platform to the hull and ribs, I'm doing 3/4 ounce mat and then 10 ounce Hexcel cloth, and repeat. I've got two layers of mat-cloth combination layers on. I've searched and searched for some kind of strength table for fiberglass, and I've read the chapter of Don Casey's book on this, but there does not seem to be any rule of thumb on how many layers to apply. In the opinions of those of you with experience, would three layers be enough, or do I need more, and if more, how many more? Or, can you recommend a reference text?

Thanks in advance; remember, this is my first real fiberglass project, so I really am as ignorant as I seem.....;-) But I'm learning a ton from this work.

[antoha]

FWIW, I put on about 7-10 layers of triaxial cloth. Mat has little strength, you can use it to build up volume but I wouldn’t do it here. Just layers of cloth, every next layer extending further onto whatever it’s gripping (hull, stringer, bulkhead, etc) than the one before. I also laid cloth in continuous pieces (which covered the entire base), not just tabbing (where surfaces meet).

[AJ_n_Audrey]

Thanks Antoha. I'll drop the mat and go with cloth only. I'll get some triaxial.

[DeepFrz]

The West System site has some very good advice on using epoxy. Here is a link to the surface preparation page.

https://www.westsystem.com/instructi...e-preparation/

[noelex 77]

Originally Posted by AJ_n_Audrey

I have come to the same conclusion. Also, I can lay hands on 1/4 inch 7075 T6 aluminum plate fairly easily. After a little reading, I found that 7075 T6 is actually stronger than mild steel, so that should be good enough.

Be aware that 7075 is not very resistant to corrosion. One of the marine grade aluminiums such as the 6 series for example 6082 or even better one of the 5 series such as 5083 would be a much better choice.

Different alloys of aluminium have very different properties. 7075 is strong but even in aircraft (where it is commonly used) corrosion can be an issue.