Affordable seaworthy cruiser

still looks and sounds mental to me. but it's your money and your life. i just hope your not persuading others to 'sail' this thing with you.

love n peas

 

I just couldn't help myself any longer, so I decided to throw some numbers at the latest round of sketches. The live loads are from the ABS publication "Guide for building and classing motor pleasure yachts"

I'm looking at just the cross section shown on the previous page, which shows no deck beams, and the cabin entirely unsupported. Using the self-weight of the materials (the dead load) and the ABS suggested live loads for the deck, I calculated the bending stress and shear stress in the decking member where it meets the hull.

The good news, which surprised me, is that the dead load alone was not sufficient to exceed the bending stress or the shear stress of the decking.
So that's nice, it won't collapse directly after being built.

The bad news, is that when you combine the dead load with the required live load, the bending stress at the deck exceeds the allowable stress by almost three times.

So.. the design shown on the previous page has a factor of safety of roughly 0.35, and that's not ideal.

Additional thoughts.

For my allowable bending stress, I assumed Douglas Fir #1, that's the good stuff. In all likelihood, your wood will be weaker.

I did not add any additional dead load allowance for epoxy, paint, fittings, insulation, etc. It's bare wood only, so in reality, the dead load will be higher.

Also, the calculations do not account for the fact that the T&G boards are arranged along their weak axis, the plywood skin on top of them may hold them together, but I would guess that the T&G boards would fail along the grain at a much smaller load than I calculated.

The calculations I worked out only look at the strength of the decking in section. I would guess that the decking will be much weaker down the long axis of the boat, as it is unsupported over this length.

Cheers,
-Matthew

 

Greetings Matthew,

Thankyou, for that info, very good observation.

Knew t&g was in wrong direction for arc strength, easier to build going longitudinally. Had reservations of only one layer of 1/4", will go with two 1/4" (as hull sides) and change wood direction, not sure of how then to easily build deck.

You mention stresses at deck to hull joint, is this where your calculations suggest failure? Have to say deck is not entirely unsupported with the angled layers of ply. Does your calculation say deck has greater loads than hull sides which have water pressing against them?

Hull sides are plenty strong and only point of concern are chine logs transmitting loads and adequately attaching four hull planes.

Reviewed Ruell's book and saw cross section drawing of Teresa (about 44'x11') and picture of the hull turned without frames. Bottom slightly arced using 5/8 t&g and two ripped 3/8 ply, hull sides are straight and two 3/8 ply. They are connected with a chine log and deck connect stringer that is weaker than ones proposed here. Ruell's book is excellent, the building concepts are taken from it but, with a different slant because of hull geometry.

Coldmolding book is maybe 25 years old and wonder if lloyds or abs has updated specifications for that type of boat building? If not, they are way behind the times.

 

I notice that it still has sails. How do you intend to distribute the rigging loads?
Also , how about a foam core deck to reduce weight and provide insulation?
Why not integrate the interior into the structure as Bolger did in the AS39? Your design is very similar to the AS39 , and it might be worth at taking a look , at the very least.

 

Yes, I would think that loads would concentrate there .

 

I checked stress at the deck to hull joint because it looked like an obvious point of failure, I did not check any other areas.

Regarding the unsupported deck, the angled layers of plywood at joints simply reinforce the joint, they do nothing to support the deck. Loads always take the weakest path, in your case, the deck would fail in bending just in front of the angled plywood reinforcements.

I have attached a few sketches to help you understand the load conditions.

Also, if you are using Parker's "The New Cold-Molded Boatbuilding" as your guide, did you happen to notice chapter 13, which explains how to install your deck beams? Or his description in the last paragraph of page 10 regarding the beam spacing used in the designs?

Again, as has been mentioned a million times before on this thread, I would caution against simply copying details from existing designs without understanding how they work. The Theresa is far more egg-shaped than your sketches, and as a result, is stronger.

 

This is how I see it.

rectangle is very weak shape on its own. Think of cardboard box (or plywoood for that matter) without two sides (bottom and lid) - takes minimal load to skew this into a "diamond shape".

If this shape is used for boat hull with no framing and minimal curvature it is likely to fold even if the surfaces themselves were stiff. I have illustrated a situation where a wave comes from screen left and strikes the side of the hull. Even if the water didn't have much momentum you can see that the pressure would be very asymmetrical and would want to fold the structure. Add rigging forces and mast itself for example would only add more forces in same direction.

I would like to know (goodwill this is to you) what kind of advances in cold molding has happened in last 25 years that allow very different structures than early 80s? Glues better? wood stronger?

Why would one cold mold a box in the 1st place is beyond me. Or worry about bending deck beams? I would NEVER think that I could build 50ft boat as a 1st building project but I can think several ways of building those deck beams.

Don't you think its worrying lack of perspective that laminating curved beams (or sawing them) is a "challenge" that drives the design? But otherwise designing hull is not a big deal...

 

Greetings,

Thanks, Matthew & Kerosene.

Made changes to cross section drawing and will post when on pdf. That could have almost been a catastrophy if deck construction proceeded. It would have been noticed right away but needed heavier modifying.

The new drawing shows an extra 2" of camber along with the strips aligned properly and (2) 1/4" ply layers on top rather than only one, this causes some loss of interior space and ports are lowered as well. There is an extra 45 degree structural at bottom that is 26" across and enlarged one at the deck level (these perform as trianglur shaped stringer frames), and should completely correct concern of racking.

Frank, if you took off 3' @ bow, 7' @ aft, and raised hull 15"; you get an almost perfect proportioned loose moose. Acquiring diesel engine puts emphasis on diesel propulsion instead of sail, the as39 is pretty good design but would not trust it for extensive travel because it gets away from the proven ark proportions.

Do own t-27 rig (376SF)and can use it alone until decent extra rig is found and rerig with galvanized or; if safe, construct square unstayed masts, partners built up with ply layers above and below deck for necessary strength.

Later on would add extra samson post at stern along with cleats at midboat using same type construction only smaller. Thought about the mesh screening for fabric and would still need to use more pliable xynole for chines.

 

Figured out construction wont be more complicated for building deck since stringers are needed anyways to align temporary frames, t&g strips will go perpendicular to them.

 

Matthew,

As far as deck cantilevering, when you look at bathroom (head), notice that lockers built in, align with deckhouse, sheet of ply is installed upright and will use adjoing ply layer to connect area over galley. The deckhouse sides and triangle connectors will act as stiffners to carry most load to ends rather than concentrating it all on unreinforced cant.

This might not make sense, but am ready to add additional support at galley/seat area if needed when built. Might lose little headroom there.

Peace.

 

Greetings,

Looking at S&S site for T-27 info and came upon these two interesting designs.

 

One more. The Norwegian burns one gallon at seven knots.

 

and what does a round bilge heavy displacement motorsailer have to do with flat sided light displacement plywood boat?

let me guess! 1/6 beam to length ratio!

 

Goodwilly- Nice designs you picked there though I don't know why you are calling them to our attention. They are nothing like what you are struggling to design.
Now about your plywood box-boat, one factor you consistently ignore in your ideas is the low life span of cheap plywood in a marine environment. IF you totally saturate both sides of every piece with expensive epoxy it will slow things down a great deal, if not your dream boat could be a mildewed soggy mess in a couple of years.
This adds a lot of time and expense to the use of supposedly cheap plywood, that is prone to voids, internal overlaps and other hidden faults that can greatly decrease its strength anyway.

 

Didnt feel like looking around to post in more proper spot, so did it here.

Cut scrap 2x4s on sunday to see how they would bend and also tried to make t&g's on table saw. At same time did think about possibly going with 1/4" diagonal strips instead of plywood and not sure if thats a good idea.

All plywood would be well encapsulated and for reasons you mention, going with two layers of 1/4" rather than one 1/2". At lowes there is a type of 1/4" luaun underlayment at $20 a sheet vs. $11 a sheet for the regular luaun type, it has X's all over it and is supposed to be more water resistant. Fir 1/4" ply is also about $20 a sheet.

165 Gallons of epoxy to work with will give enough protection.

Peace