An Old Worn Out Boat, Prime Candidate for Cold Molding?

Hi @WhiteRabbet,

In the absence of specifics on your boat I have guessed some numbers:

• LOA....15.2 m
• LWL....13.1 m
• BOA......4.1 m
• BWL.....3.9 m
• DWL......1.0 m
• Cm.........0.70
• Cp..........0.63
• Cb..........0.44
• Disp.......22.5 t
• V............13.5 kn (25 km/hr)
• Pe...........42.61 kPa (Edmunds)
• Frame Spacing..................0.35 m
• Sheer to Keel arc length...3.0 m
• Intermediate stringers.....3
• Stringer Spacing...............0.75 m

Estimated plywood (F14, MC 15%) thickness with all good frames:

t = s*sqrt(P/S/2)

= 350*sqrt(42.61/14000/2)
= 13.6 mm​

Estimated plywood (F14, MC 15%) thickness with all bad frames (worst case):

t = 750*sqrt(42.61/14000/2)

= 29.3 mm​

Estimated plywood (F14, MC >=25%) thickness with all bad frames (even worse!):

t = 750*sqrt(42.61/8600/2)

= 37.3 mm
​

Estimated plywood (F14, MC 15%) thickness with all bad frames but old hull take 75% of the load (upside case):

t = 750*sqrt(25%*42.61/14000/2)

= 14.6 mm​

This would seem to be the most reasonable position if only a few ribs/frames are damaged.

Estimated plywood (F14, MC 15%) thickness with all bad frames using shell buckling:

R = (4*DWL^2+BWL^2)/8/DWL

= (4*1.0^2+3.9^2)/8/1.0
= 2.4 m​

t = R*sqrt(P/0.16/40%/E)

= 2.4*sqrt(42.61/0.16/0.40/12500000)
= 17.5 mm​

Check skin hoop stress:

S = P*R/t

= 42.61*2.4/17.5
= 5.8 MPa (okay) ​

Regards AlanX

 

Rumars,

Indeed, I am confused a tad by the addition of the option to strip plank the boat as an alternative. I don't understand, why would you want to take a plank off, cut it into strips and then glue it back together and reinstall it? This option seems to say re-plank and re-fasten the entire boat, and meanwhile fixing all the frames - It is interesting, that you can do it from the outside while not disturbing the interior. But it sounds like an incredible amount of work. It would seem like less work just to replace the compromised planks and sister the frames while doing this - basically keeping it traditional...

What is the benefit of the strip planking method?

Also, how many planks would need to come off at a time, to reveal enough of a plank to scarf or sister in a new section of frame? I imagine at least as much as the length of scarf, is that right?

And would doing this lessen the urgency of replacing the deck, as it seems going the sheathing route means glassing the topsides at the same time...

 

Fallguy,

So far I haven't seen any cracks in the floors so far... I hadn't thought of the possibility that for frames only beginning to crack that it might somehow be a able to just inject epoxy in that crack and it would be a sufficient fix?

I also wonder, with a method that implies a much longer period of time to accomplish, ie, doing piecemeal myself, vs hiring an outfit to come in and glass her, the boat would have to be sitting on the hard for much longer... Is there any issue to this? Next to my boat is a much bigger boat, all teak, that sat out for 6 years, and her sternpost checked so bad that it would have to be replaced... also the boat, the weight pressing on the keel, deformed the keel at each point that it rests on blocks, and the keel itself has checked. This seems to be from it sitting out so long? How to you ensure this does not happen?

Thanks

 

AlanX,

Thanks for your in-depth analysis - This seems hopeful - that not necessarily too thick a sheathing would be necessary, is that correct? In the upside case...
I will be getting back to the boat in a couple weeks and will assemble the measurements and locations of broken and cracked frames.

Cheers

 

@WhiteRabbet,

Your not going to be compliant with any boat design/building standard, but if you feel comfortable with an assumption that the old hull, as is, is basically okay (i.e. the 75% factor I used) that it is safe is so long as you don't go at top speed!, then the 14.6 mm case is for you.
But remember my numbers are guesses for your boat, you need to measure the numbers for your boat and calculate the nominal skin thickness for your boat.
If your hull is round then the skin/shell buckling calculations suggest it will work also. But skin/shell buckling analysis is not used in any boat design/building standard as far as I know. It just a backup calculation in my mind.

If it was my boat, I would sheath the boat with the minimum thickness fiberglass to seal/support the planking and then run full length external chines on a spacing similar to the existing transverse frames/ribs. The chines would have a section modulus similar to the ribs. Here is an example:
https://2.bp.blogspot.com/-j_SzJU-BxSE/TkG7qX0ETLI/AAAAAAAAAYw/2x9_8tBAhmw/s1600/Otter II 005.jpg

I would also look at timber preservation options before sheathing as well.

Regards AlanX

 

depending upon the severity and the cause; epoxy may be a helpful solution

For some face checking or minor rot in a frame; the rot can be removed [completely], a rot stop agent like cpes saturate the area, epoxy fill and epoxy sistering of the damaged area if needed.

All depends on the depths of the problem. It should be rather easy to determine the needs for a surveyor if he has access.

I can't ensure anything. The way the boat is supported and the protections provided are up to you, but bear in mind; the sun is the great enemy to raw wood.

As for the alternative Rumars provides; if some of the planking and some of the floors are rotted or cracked and you don't wish to gut the interior; the method he proposes offers you access to the bones of the hull and a very permanent solution that should extend the boat's life beyond your needs and at a high labor, somewhat lower material cost option than new or added planking.

 

Interesting, I'm curious, how do the external chines replace the effect of frames when instead they run parallel with the planking?

 

Why would I want to cut up my good planking and then glue it back together and re-install it? I just dont understand how that is any better than the planking already being as it is. Or is the idea that for planking that has been shortened, it can be re-lengthened by slicing up the two portions and laminating them back together in a way that makes one longer plank?

 

If its not compliant with any building standard, how difficult do you think it would be to be able to get the minimum liability insurance required to haul out at a shipyard?

By timber preservation options, do you mean something like cuprinol? And are you talking about on the external surface?

 

Are you saying completely remove the entire deck down to the deck-beams replace with plywood?-- Would it be possible to just take up the top layer, get the old 5200 off, and then lay a sheet of plywood over the deck and glass that? Ive seen this done on smaller craft, such as @Jespersenboats

 

The option @Rumars provided was a way for you to access floors and frames from the outside AND to repair planking in a way that is very permanent. The method allows you to not gut the inside of the boat and make a more rot resistant boat. This is my favorite method because all the inside finicky business is avoided.

 

"By timber preservation options, do you mean something like cuprinol? And are you talking about on the external surface?"

Yes - worth considering with plywood stitch and glue new builds as well.

Not ideal of course but:

• The external chines do not act alone.
• The planking is "generally" held in place by the internal frames/ribs, internal chines and stringers, and the external chines.
• Reducing the average chine spacing with external chines will reduce the section modulus required for the frame/rib.
• So it will improve the situation and uses the existing planking ($$$).

On a more esoteric/philosophical basis:

• Water pressure induces compressive hoop stresses between the planks, thus they can take some bending stress across the plank (think of a stone arch).
• Replacing the chalking with epoxy and sheathing a thin fiberglass sheath will do wonders to prevent the planks from buckling across the planking.
• The allowable bending stresses across the grain are of course much lower but then planking is much thicker than plywood.

Back in my university days long long ago, a question was asked how strong (i.e. section modulus) does a support strut needs to be to prevent bucking of a compression member. The answer was not much, assume 5% of the compression member and it will be fine.

AlanX

 

So much depends on what is there. A lot of spongy spots or rot requires removal. The concept is simple. You cannot bury rot, generally, because it will creep thru a system. And you cannot entrap moisture or rot will occur more rapidly. So removing the entire deck ensures it will not rot from the old stuff. Also, if the decks are left and then rot and the plywood system is fully encapsulated; the fastenings will fail easier because they are supporting the loads and are not designed for it..etc.

By the time you get going on removing the top layer, and searching out and repairong bad spots; it may have been more fastidious and thorough to just remove it all and lay the new decks down, etc.

Templates can be made as soon as you understand the sizes well. And so, a section of deck can be removed and a new piece of ply laid very quickly versus piecemal rot repair and hoping you find it all..or the entire deck can be templated and taken up and relaid in a couple of days by a fast crew.

If you think through the job with the constraints of no moisture and no rot; it may help you determine best action. Also, plywood edges cannot be exposed, so that must be considered. Does the system allow another layer physically, etc.

I have only done small plywood replacements; never anything 50' long. But the concepts are probably similar. However, I would think you would template before full removal in heavy paper or cardboard. Perhaps, with certainty, even some of the plywood could be cut before pulling up existing. Another thing full removal affords is a review of the structures to make sure no beams need sistering or the use of cpes or rot removal, etc.

In the work I did, we chiseled away all rot and those areas were saturated with cpes in hopes to stop further rot when we were not able to replace. This was, for example a tiny bit of rot on a beam on the edge. After cpes, we repaired as needed with epoxy fillers or as needed. Consider a deck leaking onto a beam below and causing a wee bit of rot for say 3/8". Replacing the entire beam seemed extravagant, for example, when we knew stopping the rot and fixing the area to as good or better strength sufficient. But leaving the subdeck, you would not see this issue..etc

 

The deck will have to go, complete replacement is in order. Trying to repair what's there is more trouble than a new glassed ply one.

The planks you cut up to stabilize them, the narrower they are, the less they move. This would enable a thinner overlayer, be it wood or glass. Of course simply splining or otherwise glueing the seams also works.

@AlanX Why should he put external chines on the boat? A carvel boat can be considered to be longitudinally framed with zero spacing between stringers. Once the planks are glued together there is also some limited transverse strength that we increase with additional wood or glass layers.
I estimate planking thickness in this case to be around 25mm, maybe more. That's over half of your calculated ply skin. Plywood has only half the wood going longitudinally, so he already has more longitudinal strength then needed. He just needs to add transverse strength, replacing the interior frames. The boat is already metal braced, he could in theory just run all additional layers at 90°, decreasing frame spacing to zero and thereby decreasing frame thickness to the minimum acceptable.

 

Hi @Rumars,

@WhiteRabbet has not provided any information on the current internal structures, I was presenting a process using some guesses for his boat.
He indicated that he did not want to fix or replace the internal structures (ribs at the moment).
He is asking for "out of the box options" to extend the life of his boat:

1. One option was cold-molding over the existing hull and (weakened) internal structures. And I think this is an option.
2. He was also wondered if cold-molding could act as a shell and not depend on the weakened internal structure. And again, if the hull is rounded, I think it is an option, especially as the existing internal structures and hull will support the shell.
3. Finally I considered the option of supporting the internal structures with external chines (the low cost option).

I like it! Yes, I know some boats are all transverse framed. But usually traditional caravel boats need internal chines.
Especially around the bilge (ignoring the keel and sheer for obvious reasons here,they have other purposes).
The ribs hold the planks in place and the chines hold the ribs in place (unless the boat is all transverse framed).

In the last option the chines do the same thing whether they are located internal or external to the hull. The support the ribs.
Usually for caravel planked boats the transverse frame spacing is much less than the longitudinal member (chine) spacing.
If you reduce the longitudinal member spacing (with properly spaced external chines, then the loads on the transverse frame (ribs) are reduced.

Lets image that the ribs are spaced 350 mm and the internal chines every 700 mm.
If you placed an external chine between every internal chine then the average chine spacing is 350 mm.
What have we achieved? The loaded span of the rib is one half of before and the moment carried by the rib is now one quarter.
I think it is safe to assume that the loads of a broken rib will be distributed to near by good ribs.

Regards AlanX