blocking up mast base

Moderator: GreenLake

Re: blocking up mast base

Postby GreenLake » Sat Oct 03, 2015 1:32 am

Your images are kind of small and fuzzy, so I'm just responding to what I'm imagining I might be seeing :)

Looks like a clever solution!

Now, remember, each shroud will be tensioned to, say 15% of its breaking strength, or around 300# So, your mast-jack has to be able to handle 900# of compression, or some higher value (to include a safety factor). On my DS, the mast jack is a 1" threaded rod, with a bronze nut that itself is about 1" thick.

I have to keep the rod and nut well lubricated, or else the friction is difficult to overcome. The nut for the mast base is circular, with about 3" diameter, so that it pretty much supports the entire mast foot. The mast foot has a casting with a circular opening in it, that centers the mast on the jack, and it provides a good transition for the load from the thin aluminum walls of the mast profile. However, by extending the mast jack out as far as it goes, the mast foot does not have to distribute the load from its center to its rim, it mainly supplies a flat surface. There's a washer or two to make sure that the jack doesn't scrape at the mast foot while being turned.

The mast jack does not use a wrench, but has holes drilled into it for a short handle that is inserted. Mine looks a bit "aftermarket" as it resembles nothing so much a nail punch (used blunt end first). Looks like one of the POs misplaced the original :)

Other than some worries about how sturdy your setup is, the basic configuration you have in your image looks about right. Needs some heavy washers. I also don't know how well the galvanized would work when loaded. I'm thinking friction here.
~ green ~ lake ~ ~
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Re: blocking up mast base

Postby MookaCB » Sat Oct 03, 2015 11:01 am

Thanks GL!

I was a little concerned about the 3/4" vs 1" threaded rod, but if the theory is sound I will try it out and then upsize the rod. The 3/4" was just an off the shelf solution.

Galvanized will cause some extra friction but I hope to almost set it and forget it, or there is always a bigger wrench...

I will look for a more massive bolt and larger washers to add some support.

For me, the project, back yard engineering, and problem solving is 75% of the fun. If it works, great. If not, I can go get a mast section.

Here is a bigger picture.
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Re: blocking up mast base

Postby GreenLake » Sat Oct 03, 2015 7:15 pm

I get you on the backyard engineering, just wanted to throw some sense of the stresses.

The standard purpose of the mast jack is not simply adjustment of the mast position, but it is the source of the tension in the rig. So, I raise and lower my mast jack each time I launch the boat. If your design fails gracefully or doesn't quite perform as smoothly as you'd like then you can go back and redo it. A fun process in its own right. So, let's hope you're staying away from catastrophic failure modes. :D
~ green ~ lake ~ ~
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Re: blocking up mast base

Postby MookaCB » Mon Oct 05, 2015 12:30 am

Today was a light air day but everything worked. I had pretty bad lee helm so I need to get a better time and more mast rake, but I think it just might work...

So the garage engineering ended up costing more in the end but a good project. After the new turnbuckles to extend the shrouds, it was probably about $100.

I'll report back again when I get it tuned and out in a heavy air day. Thanks for all the help and input.
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Re: blocking up mast base

Postby MookaCB » Sun Sep 18, 2016 11:31 am

Just a follow up to this repair...

It worked great. The bolt didn't loosen, everything held great, and the moveable base helped to get the proper mast band and rake.

Thanks again for the help.
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Re: blocking up mast base

Postby GreenLake » Sun Sep 18, 2016 2:17 pm

Glad to hear that.
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