Scamp LUNA was dismasted yesterday!

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dsimonson
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Re: Scamp LUNA was dismasted yesterday!

Post by dsimonson »

Bcbimmer wrote:
Mon Jun 14, 2021 5:08 pm
I assume from your discussion about the addition of internal blocking that the fish tail design at the upper end somehow dissipates the stress otherwise it seems to me that you are just recreating the issue higher up the mast. Is there a link to a discussion on how to add that type of stiffener somewhere?
lhav wrote:
Tue Jun 15, 2021 1:19 pm
This is particularly interesting to me, as we are anticipating our own mast construction. What exactly does "fishtailed" blocking entail? Is the blocking itself tapered inside the mast so there is no edge directly against the mast interior?
I've grabbed a couple of photos of that "fish tail stress riser", added below:

ImageHollow birdsmouth mast -internal blocking by Dale Simonson, on Flickr
(photo by Robin Lefferts Dale Kidd)

ImageDave Ender's internal blocking by Dale Simonson, on Flickr
(photo by Dave Ender)

Just looking at these samples, and thinking about the flexing I've seen when sailing Luna, I prefer the length of the tails on the first photo, but I imagine the ones on Dave Enders' would be better than none at all!
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dsimonson
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Re: Scamp LUNA was dismasted yesterday!

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Bcbimmer wrote:
Mon Jun 14, 2021 5:08 pm
... my wife always says not worth the bother of trying to train a new husband.
Thanks, Dan. I'd better get my wife talking to your wife...

(Hoped to get to the Island with Luna once the restrictions lifted, but I have a bit of a delay. I'll get in touch if and when we can make it.)
All the best to the both of you,
Dale
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Re: Scamp LUNA was dismasted yesterday!

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Hi Dale, I'm glad to hear that everyone made it through your adventure ok. And thank you for posting the great photos of the break. It seems pretty clear that it snapped at the partner, so I think adding some internal blocking is a great idea. I just want to share what Michael Storer calls for in his plans (designer of the Goat Island Skiff, OzGoose, and other light dinghies).

He calls for an internal block running from the step to about 500mm above the partner. instead of the complicated "fishtails" you've shown, he says to cut the top off at an angle of about 12 degrees, so there is a long-ish tapering end at the top - which helps to spread out the change in stiffness from plugged to hollow. Finally, the mast gets a layer of fiberglass around the outside at the partner height (3-inch tape is enough for this). It's a design that's stood the test of time. The Goat Island Skiff mast is a hollow square design, with sides of 12mm thickness, and it carries a bigger sail than a SCAMP. So I expect that adding those features (plug from step to above the partner, and a fiberglass layer at the partner) would give the mast plenty of strength.
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Re: Scamp LUNA was dismasted yesterday!

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Dale, thanks much for sharing your "adventure" with us. We appreciate the learning experience!!

I thought the observation that you were "coming about" through the wind might be key. This is usually thought of as a direction where the sail is projecting much less wind area, and thus less threatening. But there are several elements that make that untrue. For one, the full length of the boat is resisting the force of the wind, but the boat doesn't rotate easily that way. Another is the effect of wind waves coming at you. There are some interesting mechanics when the boat is rotating in the fore-aft directions So, the question, did you sense that you were falling off a wave when the mast broke? What sort of level were the wind waves? It did sound like there was close land, and did that cause a confused wave situation, like a big hole?

I realize you had other things to do than measure the seas, but any thoughts about this would be appreciated. Thanks! Bob "Wave Guide" #343.
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Re: Scamp LUNA was dismasted yesterday!

Post by dsimonson »

Hi Wesley, and thanks for that info from Mik Storer.
A single 12 degree cut on top of the internal blocking is interesting. I wonder if that makes most sense for a square section mast. I calculate that to be a 1 in 4.7 slope, which looks quite similar to the cuts in the sample photo from Dave Ender, posted above (except there are 4 cuts in his).

Cheers,
Dale
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dsimonson
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Re: Scamp LUNA was dismasted yesterday!

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Hi Bob, great to hear from you!
I do find it interesting that the sail ended over top of us, with boom to port and both yard and mast to starboard. I concluded that we were bow to wind, or nearly so, as I was in the process of coming about, to get onto port tack. The sea state was not that big, perhaps max 2 feet crest to trough, The wind had not been very strong for the previous hour or two, so, even with about 4.5nm of fetch between the islands to the south east, the squall that hit us hand't had time to build much wave. The gap between those islands is only about 1.3nm wide,so there would have been some refraction, and we were only .2nm off a lee shore, to the northwest of us, so there could've been some reflective interference in the wave patterns. But I really didn't notice any big "holes" or large waves.

I did find some data from the nearest wind instruments, at Pam Rocks. It reported 26kts with gusts to 32kts at the time of our mishap, clearly too much wind for an un-reefed dinghy. The rain came driving at us on force, the gusts were very strong, quite variable in direction, and rolled through very quickly, and the tops of the waves were frothing, it's no wonder something undesirable happened!

As you say, a monohull resists a fore-and-aft rotation (pitch) a lot more than a side to side (roll). I surmise we hit one of those big gusts just before the sail unloaded (still on starboard), and the boat pivoted under the sail and perhaps the gust veered then backed and the mast gave way. But it all happened so fast, I'm just guessing.

It's a weird one. Nice to ponder, though, in the comfort of my armchair.
Cheers,
Dale
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Re: Scamp LUNA was dismasted yesterday!

Post by dsimonson »

Looking at the chart, again, it's likely the wind was coming through the gap, and then over the island, then through the gap again. It felt more like it was coming from due south, at times, before the squall hit. We were gradually bearing off as we approached the bay, and, as the wind built.

ImageNavionics screenshot showing track of LUNA, with red dot at position of dismasting by Dale Simonson, on Flickr
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Re: Scamp LUNA was dismasted yesterday!

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Thanks for the thoughts, Dale. When I think of big gusts, I think of the boat going over on its beam. As exciting as it is, it is very nicely releasing the breaking forces on the mast. Your experience being different, and the mast haven broken, makes this fascinating to speculate about!!

Speculating that the boat was allowed/required by the water to rotate bow down quickly, this can add substantially to the velocity of the air against the sail. The resulting friction force and the wind friction force are both trying to break the mast The friction force involved goes up as the square of the total velocity. This becomes a bit like a hammer driving a nail with the hammer being the weight of the stern and contents. As we know from getting our fingers under the hammer, the impulse force can be much more than the weight of the hammer. People that do these kind of problems get into the conservation of momentum. Maybe one of them could guess at the numbers. Or maybe this is all wrong,

Dale, I thought that your observation of the break being started by tension in the leading edge is what we see in the pictures. This is consistent with there being too much tension force there and that force being straight back. That appears to be good quality quality Doug fir. A rough calculation of the force involved is possible.

Again, thanks for sharing. Bob "Wave Guide" #343
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Re: Scamp LUNA was dismasted yesterday!

Post by dsimonson »

Hello all,
In the process of developing my plans for a replacement mast, I've been conversing with Howard Rice about an alternative to the solid internal blocking for a Scamp mast. I'll paste the conversation here:

——— Howard ———
In all the Scamp masts I have built or instructed the build of I lay in thin strips of hard wood coated in thickened epoxy from the mid height point of the mast trunk and up above the top of the mast partners by about six inches. These are not in a solid glue core state, they purposely fill most of the space and I believe offer a X factor of strength and a touch of flexibility. I’d worry about solid core as another hard spot is then built into a very flexible mast. My main mast was built this way and has been through a few blows.

——— Dale ———
…that sounds great, I'd be interested in more info. How thin, and how are the held in place during glue-up?

——— Howard ———
Since the mast is glued on the horizontal it is easy to have a sheet of plastic nearby with the glue covered pieces on it. I rip (or keep from other work) thin strips of wood approx 3 feet long, very thin, 1/4" and less. When I get to the point where half the staves are in place I lay about half the glued bundle of sticks in. I place then next two staves and then add the rest of the pieces before placing the last two staves and clamping. I know this sounds like a lot less work than making a specific load spreading insert but simple is good and it has worked to date as all masts I have built are still standing without issue including my severely stressed main mast on SCAMP #2. Hope this helps.

Oh I also add crumpled up aluminum foil in the mast to act as a built in radar reflector.

——— Dale ———
I'm starting to understand, re-reading your comments several times, it sounds like just a bundle of a somewhat random number of strips, somewhat square in dimension (1/4" x 1/4"? or less), nestled together in thickened epoxy. Sounds like the whole bundle would set up as a solid, so do you vary the length to reduce the stress riser at the ends? placing somewhat longer ones on the "outsides" of the bundle?

And, I have a bit of white oak, would that work? (no other hardwood at hand) Or, how about nice close-grained Doug Fir? Which would be preferable, in your mind?

——— Howard ———
Either would work but this said it is always best to keep species and their characteristics as close as possible, I would use the fir.

I would also extend up a little higher above the top of the mast partners, say 10 or 12 inches.

Another note.
The internal blocking fishtail riser element gives me pause because there will still be a hard spot although. If it was low enough to be of value the points in a SCAMP mast trunk due to the height of the trunk would be quite short. They should still work.

The method I use ends up being one flexible unit within and partially attached to the inside walls of the mast. SCAMP mast scantlings means it is going to be a bendy spar no matter what. So I went for a simple solution and so far so good. I think there might be 12 or 13 masts in field now built this way including mine. The sticks I bundle are about the dimension of a chop stick each or a bit bigger in diameter, just buzzed up scraps using the table saw.

——— Dale ———
Is it OK with you if I put this info on the SCA Scamp forum?

And, do you vary the lengths, with longer ones outside, against the staves? Seems that would work!

Love the concept, sounds perfect for our bendy masts!!!

——— Howard ———
Feel free to post it where you wish. I hope it might help someone else.

I haven't varied the lengths but your take on it is a good idea.

Best,
howard


Thanks, Howard!
Cheers,
Dale
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