Navtec tie rod block material

[mbowser]

I'm in the process of rebuilding a 1986 Niagara 35 and I've started thinking about the rigging. I'm replacing all the above deck rod rigging (probably original) with wire and Sta-Lok fittings, but plan on keeping the underdeck tie rod assembly intact. They are in good condition except for the aluminum tie rod blocks that tie the underdeck cold headed rod to the deck mounted chainplate. Several of them have corrosion and need to be replaced. RigRite has some photos of the aluminum blocks, but they don't appear to be available anymore, so I'm designing my own.
Using one of the old ones I took off all the measurments and did a CAD model. To test the model tolerances, I 3d printed the block in PLA to make sure the tie rod and bolts fit properly and at this point I'm satisfied that they will work. So the next step is to determine what type of aluminum they are and I'm hoping someone here can help. Is 6061 enough or should I go with something with more strength like 7075? Ideally, I'd like to do stainless but I believe it will be out of scope budget wise.

Any ideas on aluminum type or is there a way to test what type it is?

[Cap Bill Des]

I see that no one has responded yet. I own a 1982 Ericson 33 with the same standing rigging. I surely have no concrete answers for you, but having minor in Materials Science decades ago two thoughts come to mind:
- find a nearby University with a Materials Science or Mechanical Engineering Dept and ask someone there to tell you what grade of aluminum you have ( If the school is near water there will be numerous staff members who are sailors!)
- have you checked the density (specific gravity) of the possible grades of aluminum? If the vary enough, you might get away with the Archimedes trick- weigh the part in grams, then measure the volume of the part by by the volume or weight of water it displaces from a full container. Water is 1 gm/cc. Weight in grams divided by cc of volume gives you the specific gravity listed for grades of metals. Having written all this, I suspect the S.G. of aluminum alloys are two close to distinguish.

Best bet may be the University faculty. If you're on the East Coast of the U.S. there are numerous good schools...

[mbowser]

Thank you, I hadn't considered that, but I'm certainly close to a number of good engineering schools (about 50 minutes from Boston). I'll make a few calls tomorrow. I have no experience with NavTec rigging (I've always had standard wire and chainplates), and most of the knowledge I've found online is through the Ericson forum.

[Cap Bill Des]

Ah, given your location, then it is simple. Dial 617-253-1000 (MIT), and ask for the Department of Materials Science, which is also referred to as "Course 3".

[Steel sails]

5083 is the standard non heat treated marine Aluminum grade.
How thick are your blocks

[mbowser]

They are .75" thick. I just got in touch with the rigging foreman for the Hinterhoeller yard back when they were installing NavTec on the Niagaras and he thought they used 7075 for the blocks with a hard anodized coating.
As I've been pondering this work, I started thinking that because several of these blocks were heavily corroded on the port side, clearly water was getting in. I stopped over at the boat yesterday and took a look at the deck where the chainplates were mounted.
The following is clear:
1. There is depression on the deck where the chainplates mount indicating at least that the core is compressed at that location. I'm guessing that a previous owner overly tightened the underdeck tie rods when tensioning the rig or after they noticed water getting in, but not sure since I've never had a NavTec system.
2. The core is likely wet in those areas and will have to be addressed.

My current thought is to pull out the bad core and replace it with solid core from the bottom skin of the deck up. On my last boat (Alberg 35) I did this and also built 'islands' for the chainplates to mount so they aren't subject to standing water. I think I want to do the same thing, but take a different approach on implementation.
This time I would cut out the top skin and core using the dimensions of the tie rod block (2"x3.32" rectangle) and then cnc g10 plates so that they fit in the rectangle with an overlapping flange around the edges. This would raise the area .25". Then epoxy the plate in place to the lower skin, the core on each side, and the flanges to the top skin.

Does this sound like a viable approach?

[Cap Bill Des]

Our Ericson had varying wetness in the chain plate and side deck areas. The decks were cored with Balsa, the chain plate coring was plywood. The boat had vinyl headliner. We took down the vinyl and cut from below, recorded with ply and Balsa like original. The nice thing here was that the deck did not have to be cut at all to so the project.

[mbowser]

With the Alberg I owned, I re-cored 100% of the deck because it was a spongy mess. Every place where there was a through hull fitting (chainplates, cleats, tracks, etc...) I removed the core and replaced with 8 layers of 1708 biax glass so the core would be effectively isolated from any fittings with sealant that would inevitably become compromised over time.
By doing it from above I can at least have gravity working with me, and the overlapping flange on the G10 would cover up any sins from the deck cut. Another benefit of using solid core in this area is that you don't have to worry about compressing the core if you crank down too hard on the bolts holding whatever fitting is installed there.

[Bowdrie]

Originally Posted by mbowser

1. There is depression on the deck where the chainplates mount indicating at least that the core is compressed at that location. I'm guessing that a previous owner overly tightened the underdeck tie rods when tensioning the rig or after they noticed water getting in,

and also built 'islands' for the chainplates to mount so they aren't subject to standing water.

With that type of set-up, the deck can be easily stressed in either direction, be careful/judicious when setting up the rigging.
Islands? Yes, chainplates/stanchion bases/cleats/chocks/windlass/winches, all are best mounted on raised pads.

[mbowser]

Originally Posted by Bowdrie

With that type of set-up, the deck can be easily stressed in either direction, be careful/judicious when setting up the rigging.

It seems to me that this type of rig inherently creates this condition given that tension can be adjusted from above and below. Do you think there is any difference between what I'm suggesting and what was in place originally (balsa core)? It's pretty clear to me that the port side tie rods were over tensioned and consequently crushed the core causing the low spot on the deck and water to get in. I just don't want to create additional problems and want to make sure I think it through before making it happen.

Islands? Yes, chainplates/stanchion bases/cleats/chocks/windlass/winches, all are best mounted on raised pads.

I really liked them on the Alberg, they never leaked a drop once I had those in place. The problem was that I laid up each one by hand and it took quite a long time. I think sending a CAD drawing off to a CNC machine shop would make for a much better use of my time and give me more accurate results.

[mbowser]

I spent a bit of time putting my thoughts in CAD and trying to find problems with this implementation. At this point I don't see any problems with it and really the only difference between the original configuration and the new one is that I'm isolating the core from any water intrusion by the addition of the G10 insert epoxied in place.
Thoughts? I'm going to be sending this off to a rigger in the morning who specialized in Navtec rigging.

[s/v Jedi]

Originally Posted by mbowser

I spent a bit of time putting my thoughts in CAD and trying to find problems with this implementation. At this point I don't see any problems with it and really the only difference between the original configuration and the new one is that I'm isolating the core from any water intrusion by the addition of the G10 insert epoxied in place.
Thoughts? I'm going to be sending this off to a rigger in the morning who specialized in Navtec rigging.

Do you know the company SendCutSend? Check them out and their materials choices.

There should be no core in the deck where it is compressed like that… that’s a design flaw and indeed a good application of G10 indert

[mbowser]

Originally Posted by s/v Jedi

Do you know the company SendCutSend? Check them out and their materials choices.

I've used Xometry in the past, but will check out SendCutSend. Thanks.

There should be no core in the deck where it is compressed like that… that’s a design flaw and indeed a good application of G10 indert

Agreed on the flawed design, but I've seen more boats than not with chainplates or other structural deck fittings going right through balsa or plywood cores and hoping that a glop of sealant will protect the core forever.

If this was a standard chainplate that passes through the deck from the bulkhead and attaches to a shroud I wouldn't think twice about doing this, but I have no experience with Navtec assemblies and while the concept is the same, I just want to make sure I'm not missing something. Thanks for the feedback.

[Bowdrie]

Don't see any real downside in your plan as expressed in the first drawing.
I would offer a couple/three points for your consideration.
1, give yourself LOTS of flange area covering the cut-out in the deck.
2, make the deck cut-out an oval, (two hole saw holes joined by straight cuts between them,) this eliminates stress risers that form at acute>90 degree angles, and it's faster and easier to do.
3, the G10 CNC part? make it oval also, (both insert section and raised flange, no hard corners,) and bigger than the pic shows.

[mbowser]

Originally Posted by Bowdrie

Don't see any real downside in your plan as expressed in the first drawing.
I would offer a couple/three points for your consideration.
1, give yourself LOTS of flange area covering the cut-out in the deck.
2, make the deck cut-out an oval, (two hole saw holes joined by straight cuts between them,) this eliminates stress risers that form at acute>90 degree angles, and it's faster and easier to do.
3, the G10 CNC part? make it oval also, (both insert section and raised flange, no hard corners,) and bigger than the pic shows.

Good points, I hadn't thought about stress risers, but that makes sense. I'm a bit limited on available area because of adjacent stanchion fittings so I can't go much larger that the footprint of the tie rod block underneath (2"x3.32").