Hard Spots

Power Boats only. Please include the boat type in your question.
Dougster
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Post by Dougster » Wed Jul 05, 2006 4:14 pm

JGB's question is on my mind too. For that matter epoxy is hard as rock too, so I'm missing something. I get the wood on wood wet thing but don't we edge seal all the panels and frames with epoxy?

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Post by tech_support » Wed Jul 05, 2006 4:36 pm

The "hard spot" made by a Popsicle is not significant enough to matter at all :D

As far as sealing the edges of the plywood; yes you should. Lets say that its the bottom of a frame that is being filleted and glassed to the hull - its going to be encapsulated anyway, even if the edge itself was never coated. Thats about the place I would worry about moister damage.

The designs have a very wide safety margin, so even if you left a lot of hardspots its going to be OK on the smaller boat. The bigger, the boat, the more attention you pay to things like hard spots.

With foam core construction, avoiding hardspots is VERY important

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Post by jacquesmm » Wed Jul 05, 2006 4:36 pm

Ideally, the loads should be transmitted by the fiberglass seams not by the panel edge.
In our designs for foam sandwich we show a foam strip. That foam strip is shaped like a trapeze and eliminates the need for a putty fillet.
In plywood composite small boats, there is no need to go that far, all what we want is to avoid a hard spot.
A hard spot is where part of a panel (stringer, frame etc.) sticks out and pushes on the hull. That concentrates the loads.
The panel edge should push evenly all along it's length.
To guarantee that, Joel proposes to slightly lift that panel with whatever you want then to fill the gap with putty. Yes, the putty is hard but the panel edge pushes evenly on the hull, we eliminated the hard spot.
We have a picture of that somewhere in our HowTo files.
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Post by jacquesmm » Wed Jul 05, 2006 4:38 pm

Here is a picture of the foam strip:

Image

It's used in the PH15, foam version:
http://bateau2.com/content/view/104/28/
Last edited by jacquesmm on Wed Jul 05, 2006 4:42 pm, edited 1 time in total.
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Post by jacquesmm » Wed Jul 05, 2006 4:39 pm

And here is a picture of a hard spot:
Image
They can happen on a stringer too.

This is extracted from one of our HowTo files:
http://bateau2.com/content/view/72/28/
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Post by CET » Wed Jul 05, 2006 8:46 pm

Dougster wrote:JGB's question is on my mind too. For that matter epoxy is hard as rock too, so I'm missing something. I get the wood on wood wet thing but don't we edge seal all the panels and frames with epoxy?
jacquesmm wrote:A hard spot is where part of a panel (stringer, frame etc.) sticks out and pushes on the hull. That concentrates the loads.
The panel edge should push evenly all along it's length.
To guarantee that, Joel proposes to slightly lift that panel with whatever you want then to fill the gap with putty. Yes, the putty is hard but the panel edge pushes evenly on the hull, we eliminated the hard spot.
Please forgive me for beating a dead horse here, but I am still a bit confused about the concept of hard spots and how a putty fillet between a stringer or a bulkhead and the hull surface is better than the stringer or bulkhead itself touching the hull surface. I would think the epoxy fillet would be harder than the wood (a foam strip is a different matter), so in the case of the most recently posted picture above with the red arrow pointing at the hard spot, it would seam to me to be more of a “soft spotâ€

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Post by ArizonaBuilder » Wed Jul 05, 2006 9:51 pm

CET wrote:
Dougster wrote:JGB's question is on my mind too. For that matter epoxy is hard as rock too, so I'm missing something. I get the wood on wood wet thing but don't we edge seal all the panels and frames with epoxy?
jacquesmm wrote:A hard spot is where part of a panel (stringer, frame etc.) sticks out and pushes on the hull. That concentrates the loads.
The panel edge should push evenly all along it's length.
To guarantee that, Joel proposes to slightly lift that panel with whatever you want then to fill the gap with putty. Yes, the putty is hard but the panel edge pushes evenly on the hull, we eliminated the hard spot.
Please forgive me for beating a dead horse here, but I am still a bit confused about the concept of hard spots and how a putty fillet between a stringer or a bulkhead and the hull surface is better than the stringer or bulkhead itself touching the hull surface. I would think the epoxy fillet would be harder than the wood (a foam strip is a different matter), so in the case of the most recently posted picture above with the red arrow pointing at the hard spot, it would seam to me to be more of a “soft spotâ€
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Post by ks8 » Wed Jul 05, 2006 10:21 pm

jacquesmm wrote:And here is a picture of a hard spot:
Image
They can happen on a stringer too.

This is extracted from one of our HowTo files:
http://bateau2.com/content/view/72/28/
Let me see if I can reveal the mystery by wording it a different way...

In the illustration above, which Jacques posted, there appears to be only one spot, chiefly, where the frame is touching the hull panel. When hulls are assembled, stitch and glue, depending on the design, the frames are also dry installed and stitched to form the hull shape, in preparation for filleting and taping. Now here is, I think, the critical point....

When the filet is formed on either side of the frame which is already stitched in place and immobile (so that the hull does not go out of square), it is an iffy thing to successfully get filler material *under* or *into* the entire frame to hull panel junction. If one is very good at progressive angle joinery, you may get a perfect frame to panel wood fit before the fillet is formed on either side, but how many people will take the time to do that? Barring the joinery labor, there are two alternatives...

1- Devise a construction sequence and method so that you can thoroughly embed the frame into a full length bead of filler putty, and then build the fillets on either side as well...

or...

2- Somehow prop up a distance between the frame and the panel, so that filler putty can completely and successfully be worked under the entire frame joint, so that loads will be equally distributed along a wood/putty continuous support between the frame and the panel(s).

Method *1* requires that the build method be significantly adapted, and quite possibly more complicated from the simple stitch and glue method. I built my hull right side up in a cradle, and used battens to hold the sides in shape while I pushed the frames down into a thick bead of filler. The one or two that I did not or could not do this with, I did some more accurate joinery, or I seriously worked and jammed and oozed the filler under and in the entire component joint.

Method *2* simply props the components away a bit and makes it much easier to work the filler putty into the actually joint junction, and not only on either side.

I think the question of *Isn't wood putty or filler even harder than a hard spot of wood on wood?* is a moot question with this understanding. I think the entire issue arises out of the difficulty of actually working a consistent load bearing material evenly under or *in* the joint of a stitched in frame when building fillets on either side for taping. A quick fillet job may not involve taking the care to work fillet putty *between* the joint of the parts as well as on either side of the joint, and I think that is where the problem arises with power boats and larger boats. In this sense, the statement *gaps are good* is because it makes it much easier to work the putty in *to* the joint itself for even load bearing so that a wood on wood hard spot is no longer a singular load bearing point in that area. Yes, the tape bears the loads, but if the fillet putty isn't universally consistent between or *in* the joint, and a large load does bear on a hard spot due to a tape joint slightly flexing under the load, now you've got a possibility of damage. A softer interface material in the joint (foam or relatively soft popsicle stick) can absorb (in momentary compression) some sudden loads while the tape bears most of the load. In foam construction, it is not a hard wood frame hard spot pounding into a hard hull panel, as all the foam can give slightly while the tape *skin* rightly bears the load. But with plywood core, if there are large gaps (a structural loads would see them as large) that do not get rightly filled, then there are also hard spots, and while the loads on smaller boats may make this relatively unimportant, when you get into larger power boats or larger sailboats, where the potential and magnitude of sudden loads becomes geometricly greater, having a consistent load bearing joint interface is more important, so to utilize stitch and glue, we need some way to make that joint (not just the fillets on either side) an equal load bearing structure. Then, if there is some flexing of the tape and fillets in a load, we know that if the frame to panel joint bears some of the load, it is a consistent and even bearing of the load and not focused in one small area because filler putty could not be rightly worked in evenly in the build.

Yes cured putty is just as hard as wood in a *hard spot*, but if you can easily and assuredly get the putty in the joint fully, then the hard spot of the wood is neutralized, unless there is severe moisture intrusion to cause severe swelling of only the wood, or the wood is extremely more temperature sensitive in expansion than the filler. We are aiming to completely seal all structures though, right? It seems the main point is to *ensure* that the build method and strategy easily and effectively fills that space, that joint, completely with filler putty, hence *gaps are good*, or some material is used to prop open the joint when stitched so the putty easily gets in there and fills it completely (not only on the sides).

I know that is wordy, but I'm hoping I got it right, or close enough to demystify the subject.


edit-- I see Terry clicked on submit while I was still typing, but then, everyone knows I am *short post challenged*... :lol:

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Post by CET » Wed Jul 05, 2006 10:58 pm

Ah, I see the light. :D

ArizonaBuilder and ks8 - thank you for the excellent explanations! I see now that a "high spot" in a frame or stringer that has a void next to or near it that is NOT filled with putty or foam would indeed create a 'hard spot'. Sorry for the confusion, and thanks again for the very thorough explanations. I'm sure others will benefit from them as well. 8)

Cheers,

CET

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Post by ArizonaBuilder » Thu Jul 06, 2006 12:24 am

edit-- I see Terry clicked on submit while I was still typing, but then, everyone knows I am *short post challenged*... Laughing
But your posts are so informative. :D :D
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