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Mizzen mast foot failure


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  #1 rbappelbaum

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Posted 14 June 2012 - 11:06 AM

Last year I decided that the CS15 was the right boat for me and I bought a set of plans. Before I started building, I found Lapwing 16 #5 for sale on Sailing Texas and couldn't resist. After my fourth sail, which was the first in winds of 12-15 mph, I noticed cracks in mizzen mast step. I am considering cutting out the whole thing and installing a new stronger one. I am open to suggestions on how to proceed. Has anyone else had a similar failure? Attached File  mizzen step 1.jpg   244.2KB   18 downloads

  #2 PeterP

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Posted 14 June 2012 - 01:13 PM

Unless I am misreading the depth of the hole because of foreshortening in the photo I would guess the mast hole is too shallow. It should go right through to the top of the keel batten. If that is the case then you would just need new step with a deeper hole. That makes it simple -use the old one for a pattern. If you can cut along the CB case with out tearing it up you're halfway home. If you are not comfortable doing stuff like that cut back 1/4" from the case and clean up with a chisel. Go easy here - the case is probably 1/4" ply and a hole in that can be a pain to fix.The step bottom will come off easy - just find one of the ply veneers a bit off the bottom and drive a chisel into it. The whole step will pop right off - with a few licks. (Make sure the screws are out first) Sander will take care of the epoxy. Good luck PeterP
Spindrift 10, CS17 River Dancer, CS17 Red Beard, CS20, P28R under construction

  #3 Hirilonde

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Posted 14 June 2012 - 02:00 PM

It also looks like the inside of the hole was never painted. Add water to an unfinished hole and this may have been another part of the problem. Measure, scribe, sketch the existing step to preserve the info and carefully remove the old mess. I like Peter's chisel idea. It allows care to be taken in the process.

Dave Finnegan

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Spindrift 9N #521 -  many KudzuCraft SoF kayaks


  #4 PeterP

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Posted 14 June 2012 - 02:19 PM

Good point about the water. I like a little weep hole in the step so water has a way of draining out. Good epoxy soak is a must. PeterP
Spindrift 10, CS17 River Dancer, CS17 Red Beard, CS20, P28R under construction

  #5 Mike Vacanti

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Posted 14 June 2012 - 03:01 PM

The guys who built the "Dawn Patrol" will probably chime in but until they do, here is a pointer to a photo gallery of exactly this problem:

https://picasaweb.go...wMizzenMastBase

  #6 tom151

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Posted 14 June 2012 - 05:50 PM

Seeing the other failure (in the great link Mike provided) makes me think that at least some of the problem is related to the use of plywood for the mast base. This is not, in my opinion, a good place for using plywood.

I would use a strong (fir?) solid timber mast base (made up from three or four laminations of whatever thicknesses you have handy (grain running for/aft) All extended (structural filler) epoxy - do not rely on screws for strength here - use nice big fillets everywhere..

Make the mast base wall as thick as possible by making it at least as wide as the keel batten - the side away from the c/b case that is. Maybe even let it bow it outward a bit.

The earlier idea of adding a drain channel is good and proper - again, as done in the link by Mike.

  #7 Tom Lathrop

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Posted 14 June 2012 - 06:12 PM

This is actually a good place to use plywood but it must be properly engineered and built. The step looks strong enough if you follow Peter's advice and IF you have it well epoxied and IF you have an adequate drain at the base. No step should ever lack a drain hole.

  #8 Alan Stewart

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Posted 14 June 2012 - 07:45 PM

Second all of what Tom said. on dawn patrol our mizzen mast base crack not failure (mast never came close to coming down) was caused by builder error. we thinned the walls of the base when we added a glass tube to aid in stepping and removeng the mizzen mast but didnt glue the tube in for maintenence reasons. that turned out to be a bad idea. it took a very rough ride to crack the base after the modification that weakened it. We wanted to keep the glass tube so the new base was made a bit bigger. Just making sure it was clear that our mast base that cracked was not to designed specs and It stIll never failed.

Looks like your step may have suffered from water ingress. Make yourself a new one and go sailing !!!! :)

CS 17 'Southbound'

CS 20 'Dawn Patrol'

 


  #9 rbappelbaum

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Posted 14 June 2012 - 08:13 PM

Thanks for the input, especially the pictures of Dawn Patrol.

  #10 Joe Anderson

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Posted 16 June 2012 - 04:24 AM

This is a small sample but the cracks are occurring across the grain on the outside of the mast step. The plywood has a predominate grain pattern. I wonder if it would pay to align the grain direction of the laminates fore and aft. Or perhaps aligning the grain is part of the problem and the laminates should be alternated.

EC22 # 5

 

Spindrift 10 # 1005


  #11 PAR

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Posted 16 June 2012 - 09:22 PM

I used a three layer sandwich of HDPE to make the mizzen step on the CS-17 I built. The first layer was a square, full width of the keel batten with 5/16" flat head machine screws, bonded to the batten. The next two layers where shaped to the inside diameter of the aluminum tubing and through bolted to the base layer. The base layer was also beveled to met mast rake and bottom slope at the step location. My thinking was I didn't want an aluminum tube turning on the batten or a wooden step, grinding away with every tack. The lower piece of the HDPE sandwich serves as a bearing, the stub mounted above it keeps the mast in it's place with no wear on the tubing and when stepping the mizzen, there wouldn't be a tube trying to cut or gash the wooden step or batten, as you attempt to find the hole. The HDPE layers were 1" thick and after a few years of use, maybe a shallow 1/16" groove has been cut in the base, by the tube. This means it'll take 48 years before the base piece is worn complete through.

  #12 Gordy Hill

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Posted 17 June 2012 - 05:21 AM

Par,
If your mast plug is HDPE as well, there should be no wear at all.

  #13 PAR

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Posted 17 June 2012 - 11:24 AM

I haven't measured the groove, but a shallow groove is viable on the bottom piece. It's purely from abrasion as the mast rotates. It's very shallow and 1/16" is a generous dimension, but there is a slight groove after 3 years of sailing. I'd think a wooden bearing area, would have a considerably deeper groove by now.

  #14 tom151

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Posted 17 June 2012 - 03:13 PM

I haven't measured the groove, but a shallow groove is viable on the bottom piece. It's purely from abrasion as the mast rotates. It's very shallow and 1/16" is a generous dimension, but there is a slight groove after 3 years of sailing. I'd think a wooden bearing area, would have a considerably deeper groove by now.

PAR,
Thanks for the detailed info on the HDPE mast step and the wear data is very helpful.

I like the idea of having the mast fit down over the fixed plug you described.

Did you use HDPE for the bearing at the thwart as well? If so, what kind of wear are you seeing there? - what diam is that bearing?

Thanks a lot,




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