I’ve VERY much enjoyed seeing this thread.  Keep it comming PLEASE

Thanks! My next step is to build a 16 foot Advanced Sharpie Proa to my kids. My oldest son wants a sailboat, and its a good testbed for my ideas. Its going to be a 3:1 scale of the large proa. 4,7 X 0,4 X 0,6 (LxWxD) meter Vaka, and a Wharram Tiki-like 3,6 meter ama. (in the picture).

Regards.
Johannes.

This has been a terrific thread, Johannes, thanks for posting. At first, I hated the idea of the blunt bows, but now I see the method to your madness with the rudders - excellent!

I’m curious to know how the boat balances now that the mast is centered fore and aft?

I’m curious to know how the boat balances now that the mast is centered fore and aft?

When i pull it from a point halfway up the mast, the boat balances very good. It seems to be very directionaly stable, whatever i do with it.

I have not sailed it in any wind with the mast centered fore and aft yet. I will when i get some time of. This last week has been crazy.

Today at work i wrote down a couple of points about why i want to build something like this.
The blunt bows might seem very strange at first, but i have my reasons…. I just want to say “David Raison and Teamwork Evolution”... There is a lot more thought behind it.

Hard Chines:

# Tracking - Directionaly stable.
# Enables the hull to function as an extremly low aspect lifting foil. It gives windward ability.
# Hard Chines and sharp corners creates large power-sucking eddies, that transfer a lot of energy from the hull to the water. They damp a lot of unwanted motion. The chines are realy important, and that is the reason i belive a dory-hull with a lot of flare is not as good as a square section advanced sharpie type hull.
# The hard sharp chines does not resist forward motion (sailing) but will create very powerfull eddies of any type of rocking motion or pitching motion, and greatly damp those.
# Imagine two 46 feet long hard chines heaving in a turbulent sea. Thats a lot of energy transfered to the water in the form of eddies.

Flat Bottom:

# Shallow draft. (who would have thought of that?!? 😉  )
# All that heavy cruising gear. - Will carry a lot of weight.
# Simple build. Rational use of plywood.
# Dynamic lift of bows, Will let go of its wake-wave and semi-plane. Wide stern will have a lot displacement very far aft and will flaten the wake a lot.

Blunt Bows:

# High PC. A lot of volume very far aft and forward. It will lift the bow up and allow the stern to semi-plane (kind of).
# Will resist hobbyhorsing by pushing aside a lot of water when pushed down into the surface. Similarly will create a lot of eddies when pushed into the water or lifted out of the water. Imagine how the underwater-body changes shape when hobbyhorsing. All that changing of shape and form, will damp out any motion fast.
# Protection of rudders in heavy seas.
# Maximizes load carrying ability and usefull space.

Slab Sided:

# Less resistance in waves.
# Simple build. (Plywood)
# Sharpe chines and windward ability.
# I believe it will make the boat go better to windward in heavy seas. It will behave the same way regardless of waves and wind. The hull is the lifting foil, and it wont stall easily.

These are some of the points i consider important.

Regards Johannes.

At first, I hated the idea of the blunt bows,

They look strange on a Proa. My tests has shown me that they work as expected. They will probably send a lot of spray up in the air when beating hard up against the wind in a steep chop. I hope it wont be a serious problem. There is ways to deflect the spray, if needed.
I will build a 16 feet Advanced Sharpie Proa soon. I hope to be able to sail it within a month or two.

Regards
Johannes.

Here are two images of the Mini Trans-At design Johannes referred to

Thank you James Shanahan! Thats the boat i mean.
I know a monohull is a very different beast compared to a proa. I just want to point out that a blunt bow can work realy good in waves. It has a lot of advantages compared to a sharp bow. Its all about balance and tradeoffs.
Today i bought 7,75 kgs of Nils Malmgren Epoxy.

Regards.

Johannes.

Everybody needs some epoxy on the kitchen table! I think epoxy, bread and té mix very well!!! 😉

I was reading through the “Box Keel” thread and fund this from Luckystrike118 in post number 12:

If you have the right rocker shape the bow will rise the forefoot out of the water and the forward bottom will create some dynamic lift. At the stern the rockerline will suck the back part of the boat into the water so that the aft bottom is parallel with the flow of the passing water. It leaves the hull with nearly no resistance. The boat will get a stable trim with a slight bow up tendency.

Very interesting, as this is what i experienced when i sailed my model.

This forum is a goldmine!

Regards
Johannes.

I thought the preceding comment was also very interesting, Johannes

If your rocker is curved too much ...  the bottom will “stick” to the passing water like a laminar airfoil does and your bottom is sucked into the water which causes much waves = drag. Did you ever wonder why this funny PDRacers or Optimist dinghys are unable to get into real planing mode while other scow type boats just a little longer are planing like mad? There is the answer ... too much rocker.

So how does one go about determining the ‘right amount’ of rocker, I wonder?

I think Matt Layden have got it right in his monohull-designs (Paradox and Enigma). I think my proa has a little too much rocker, and i plan on making the hull longer on my next design. 50 cm (20 inch) rocker on a 40 foot long hull seems to be ok. 20 inch rocker on a 46 foot hull looks better on paper. I have to try.

Regards

Johannes.

Johannes,

lately ive been playing with a 12:1 beam/length on a 24ft hull, meaning a 2’ beam on the box.  if i follow bolger curvature, this puts me at a 12” or 24:1 length/rocker ratio.

seems to be right where you’re ending up 😉

Tom

My next proa (scale 3:1) will be 4,7 meter long and 0,6 meters wide with a 0,16 meters rocker. 470/16 = 29,375:1 length/rocker ratio. 24:1 seems to work very well at slightly lower speeds. I hope 29,4:1 will work better at slightly higher speeds without adding to much wet surface. Its a trade of, and im having a hard time trying not to go all crazy about speed. Speed is way too addictive.

I tried to sail my proa totaly filled with water, and its still very directioanly stable and sails at about one knot of speed.

Johannes.

johannes,

you’ve inspired me to go buy balsa 😉  This is 1/12 scale of a boat 24’ long, 4’ at it’s deepest, the “box” keel is 24” wide with the waterline drawn at 12”.  the “cabin box” is 8’ long x 6’ wide.  it’s bottomis 12” above the waterline.

she looks a bit bulky but very roomy.  not as bad as it looked on paper.

and one more…..

Here’s a couple showing the Heel angle where the pod starts to engage.  While I have concerns as to the looks and possible pounding of the flat underside, it sure meets the water nicely, almost a “tunnel hull"effect that seems would actuall tend to head the boat up instead of possibly dragging it down wind. 

Tom