Proa hull lines, suggestions/guidance

[Skip Johnson]

Light weight and speed are admirable traits but there's a couple of other things to consider. EG windward ability and whether or not you need to sail in really shallow waters.

First @gonzo is right, skin on frame works well on closely spaced stringers, truly sucks as a replacement for a flat panel.

Second, I look at your current design and see two things.
One, there is a lot of extra wetted surface in the ends of the boat.
Two, the boat will be extremely difficult to turn.

There are some very good reasons Gary Dierking's T2 has such pronounced rocker in the hull. Helps minimize wetted surface and enhances turning ability . In addition the pronounced "vee" of the rocker adds some vortex flow to improve leeway resistance.

That being said, I really, really don't want to discourage you; there aren't that many proanauts as it is. My first proa, ACDC, had a simple vee cross section and was a wonderful learning experience. So much so that proa number 8 or 9 (depending on how you count modifications ;-)) sits in the boaat shed waiting for me to finish other chores before I make one more round of adjustments.

 

[Thule]

I would thing that particularly on skin of frame. Otherwise the water pressure will make the panel concave.

Thanks Gonzo. I guess makes sense, considering it does not have inherent rigidity. I will modify the plans.

 

[Thule]

Light weight and speed are admirable traits but there's a couple of other things to consider. EG windward ability and whether or not you need to sail in really shallow waters.

First @gonzo is right, skin on frame works well on closely spaced stringers, truly sucks as a replacement for a flat panel.

Second, I look at your current design and see two things.
One, there is a lot of extra wetted surface in the ends of the boat.
Two, the boat will be extremely difficult to turn.

There are some very good reasons Gary Dierking's T2 has such pronounced rocker in the hull. Helps minimize wetted surface and enhances turning ability . In addition the pronounced "vee" of the rocker adds some vortex flow to improve leeway resistance.

That being said, I really, really don't want to discourage you; there aren't that many proanauts as it is. My first proa, ACDC, had a simple vee cross section and was a wonderful learning experience. So much so that proa number 8 or 9 (depending on how you count modifications ;-)) sits in the boaat shed waiting for me to finish other chores before I make one more round of adjustments.

Hi Skip

I read in one of the posts from the designer of Harry Proa that a rocker is not needed since a proa doesn't tack. I may have misunderstood what he was trying to say, and decided to remove the rocker!

Gonzo's point well taken, I will modify the hull lines and post back.

It is not a discouragement in the least. I am not an expert in boat design and the reason to post here is to take advise from people who are. I would love to bring this project to fruition with help from here. No point in posting here for advise and then getting offended!

So, please keep with the critiques and advise flowing. It is the lack of it that can be discouraging
Thanks

 

[TANSL]

A well-designed flat panel doesn't necessarily have to be heavier than a curved one designed by someone who doesn't consider certain factors. What I mean is that to achieve a lighter overall structure, it's not necessary to use only curved panels. Simply changing the shapes isn't enough to create a lighter structure. You can save a lot of weight through good design.

 

[Thule]

Taking in to account about rocker and curvature of the panels, here are the lines for modified shape. I wonder whether the lee side is too curved - looking at Ailuk Atoll and Marshallese outlines, mine does not look too curvy.

Thanks

 

[Thule]

A well-designed flat panel doesn't necessarily have to be heavier than a curved one designed by someone who doesn't consider certain factors. What I mean is that to achieve a lighter overall structure, it's not necessary to use only curved panels. Simply changing the shapes isn't enough to create a lighter structure. You can save a lot of weight through good design.

Hi TANSL
I was thinking of strong gunwhales/keel and some strong supports to stay in compression between them.

Then plan on running a lattice of thin line perhaps dyneema or some other strong rope to create a wide support. On the outside of the lattice support, I am thinking of putting some 0.5in blue foam strips - these strips will be held in between the lattice of ropes behind and the nylon skin on the outside, providing a wide surface area for load/impact transfer and some rigidity.

Thoughts?

 

[TANSL]

What I've understood seems quite incorrect (please forgive me if I'm misinterpreting), but I know enough about structural design not to offer any advice without knowing the complete structural design. A structure is a set of elements that never function in isolation. It's the entire assembly, well-designed and with properly calculated scantlings (which is a slightly different topic), that determines whether the structure is correct or not.

 

[Thule]

What I've understood seems quite incorrect (please forgive me if I'm misinterpreting), but I know enough about structural design not to offer any advice without knowing the complete structural design. A structure is a set of elements that never function in isolation. It's the entire assembly, well-designed and with properly calculated scantlings (which is a slightly different topic), that determines whether the structure is correct or not.

I am not sure how the images came across but essentially what I was thinking is to use rope woven between fixed frames/stringers to act as ribs, and this rope net supports a board of blue foam and the skin is on the outside of the blue foam. Hoping this will be lighter than the ribs. Did I explain better or did I make it more confusing? If the latter, I can see if I can draw better than the model and try.

 

[Robert Biegler]

I had the odd idea of running a lace between the chines like a lattice and place a 1/2 inch blue foam between the spans, foam being locked between the lace netting and the skin. That way, I will have a surface for pressure on the skin to dissipate, something to stop skin from rubbing on the ribs/lace and lighter weight.
Sounds reasonable?

Depends. If the lace runs between stringers and you use constant thickness foam, it does the same job as the skin. You might as well use a stronger or more tightly tensioned skin.

In theory, you might save some skin weight by having a double skin. The inner skin would sag, like the cables of an underspanned suspension bridge (see Suspension bridge - Wikipedia https://en.wikipedia.org/wiki/Suspension_bridge#Underspanned). The foam would have to transfer compression loads from the inner to the outer skin. The outer skin would bear no load, it would just be a waterproof barrier (which may make it too fragile to be practical). The foam's cross section would have to form a catenary. Maybe that can be approximated by multiple layers of constant thickness foam. That would take a lot of work, the foam may weigh more than you save by being able to use a lighter skin, and I don't know how you would shape the inner skin to have just the right amount of sag.

If the lace runs between frames, as you say in response to TANSL, I expect this to create the starved cow look you sometimes see on metal boats. The lace can only provide support in tension, so when it is being pushed in, it has some way it can move between frames, but not where it is attached to frames. So the frames would stick out and disrupt water flow along the hull.

re: asymmetry, yes, I am trying to avoid a dagger board or other appendage and use the lee side for resisting slipping sideways. I was trying to minimize the draft but I can see why all the fast proas like the T2 have a fair bit of draft - needing volume in a narrow hull.

Shallow draft and leeway resistance are in conflict. For a given amount of lift, induced drag is inversely proportional to span, which translates into draft.

You can, to some extent, do things with endplates and maybe non-planar wings (I barely understand how, so you would have to research that thoroughly), but I don't see a way of doing anything like that with a skin on frame hull.

 

[Thule]

Depends. If the lace runs between stringers and you use constant thickness foam, it does the same job as the skin. You might as well use a stronger or more tightly tensioned skin.

In theory, you might save some skin weight by having a double skin. The inner skin would sag, like the cables of an underspanned suspension bridge (see Suspension bridge - Wikipedia https://en.wikipedia.org/wiki/Suspension_bridge#Underspanned). The foam would have to transfer compression loads from the inner to the outer skin. The outer skin would bear no load, it would just be a waterproof barrier (which may make it too fragile to be practical). The foam's cross section would have to form a catenary. Maybe that can be approximated by multiple layers of constant thickness foam. That would take a lot of work, the foam may weigh more than you save by being able to use a lighter skin, and I don't know how you would shape the inner skin to have just the right amount of sag.

If the lace runs between frames, as you say in response to TANSL, I expect this to create the starved cow look you sometimes see on metal boats. The lace can only provide support in tension, so when it is being pushed in, it has some way it can move between frames, but not where it is attached to frames. So the frames would stick out and disrupt water flow along the hull.


Shallow draft and leeway resistance are in conflict. For a given amount of lift, induced drag is inversely proportional to span, which translates into draft.

You can, to some extent, do things with endplates and maybe non-planar wings (I barely understand how, so you would have to research that thoroughly), but I don't see a way of doing anything like that with a skin on frame hull.

I never knew about the underspanned bridges - thanks for the link.

Apologies in advance but comically bad drawing but hope it shows my thoughts. Black lines are the keel and the gunnels. blue lines are athwarts and framing members. Thought is that the keel/gunwhales and framing members are in and of themselves rigid. The purpose of the red netting is to hold the blue foam on the outside of the netting, with skin being draped over the foam. I thought that this can help with a water pressure acted on the skin being transferred to foam from outside in, and the pressure of blue foam against the netting transferring the load at each point where the netting is tied to the framing/gunnels etc.

So, in effect, an odd "I frame". Am I way off?

I gave up on shallow draft, realized that boats with shallow draft have other means of lateral resistance are in conflict as you say. So, most recent lines are in #20

Thanks Robert

 

[TANSL]

The panel calculations assume that all four edges are perfectly fixed and cannot rotate due to the reinforcements. If these perimeter reinforcements are not rigid, they are considered nonexistent from the panel's perspective. However, if you create a single panel spanning from bow to stern and from the bulwark to the keel, you can calculate it as if it were a nutshell, requiring no reinforcement, wires, or frames (your image suggested this example)..

 

[gonzo]

The foam is a flat panel, so you may get an approximate shape by cutting strips. However, it is a complicated system with little advantages. At that point you might a well forget the skin and build a stitch and glue or a cored fiberglass hull. Lacing in diagonals is sufficient. The canning or starved cow look is inherent to the construction method.

 

[Thule]

The panel calculations assume that all four edges are perfectly fixed and cannot rotate due to the reinforcements. If these perimeter reinforcements are not rigid, they are considered nonexistent from the panel's perspective. However, if you create a single panel spanning from bow to stern and from the bulwark to the keel, you can calculate it as if it were a nutshell, requiring no reinforcement, wires, or frames (your image suggested this example)..

Thanks TANSL. I was planning to create long foam strips as the blue foam is unlikely to follow a bilge curve smoothly, and approximate the shape. So, already, it is not a single panel from bulwark to keel and is likely to move with reference to each other. So, perhaps a bad idea afterall :-(

 

[Thule]

The foam is a flat panel, so you may get an approximate shape by cutting strips. However, it is a complicated system with little advantages. At that point you might a well forget the skin and build a stitch and glue or a cored fiberglass hull. Lacing in diagonals is sufficient. The canning or starved cow look is inherent to the construction method.

Looking like not enough return on the weight savings - by the time I add the weight of the foam, likely needing more robust keel/gunwhales and perhaps some support in the mid for the curve, it will probably come about the same as a foam core or ply stitch and glue.

On a similar note, if well done, are there limits of a skin on frame sailing vessel in terms of speed? I assume that at some speed the starved cow will come out and affect the water flow no matter how tightly built she is.

 

[TANSL]

So, perhaps a bad idea afterall :-(

I think so.