Re Hjälmarsnipa, Skerry cruiser etc great pictures thanks for that Johannes, always appreciate a pretty boat.

With respect for proas and northern latitudes, I would prefer an lightweight and easily unsinkable, shallow drafted (assume retractable / sacrificial foils) proa than a heavy, deep drafted, sinkable monohull. Hopefully pushing on an open door there.

The other aspect of safe seamanship ultimately has the be the, crew who naturally should be comfortable, well rested and have reserves of energy to make prudent decisions. Warm spacious accommodation goes a long way to achieving that.

Traditional proas were obviously based on materials available, round trees, (I oversimplyfy for effect) we have the benefit of strong and light sheet materials and exploring alternatives to half circle forms deserve serious consideration.

Thinking in a craft might build, a would assume a decked bow and ultimately the ‘front’ of the hull becomes increasingly un-usable, I think I would reduce the beam at the bow to maybe 1/3rd the hull beam.

Anyway thinking outloud keep up the great work Johannes

Tink

Hi Johannes,

Did you get to any conclusions about how effective are the chine runners on this kind of hull?

cheers

Did you get to any conclusions about how effective are the chine runners on this kind of hull?

I have not tried chinerunners on a barge hull. With two hard 90 degree chines the hull tracks very straight. I am more worried that the proa might be hard to turn without large and powerful rudders. It seems to sail reliably at 50 degrees to the wind without chinerunners. With the unbalanced rocker there is much more crossflow over the chines, and I believe chinerunners will create much more turbulence and drag. The chinerunners is a hydrodynamic fence on a lifting body, and can (should) not be used on other hull-shapes.

I dont know how these properties scale, but I am very curious to find out.

Cheers,
Johannes

(Posted from my Raspberry Pi http://www.raspberrypi.org)

Here’s the results of some preliminary runs in Mitchlet and Godzilla for a barge hull and a totally flat bottomed sharpie and a rockered sharpie.

Displacement was 400 kg optimised for 5 meters/sec.

Results, dimensions in meters

hull.      length   draft   beam
Barge     13.00   0.183   0.25
SharpieF 11.24   0.137   0.364
SharpieR 13.43   0.166   0.33

I haven’t figured out how to post a graph of drag vrs speed ( 2.5 to 7.5 m/s) but trust me 😉 there’s not much difference, Barge is draggiest, 0.42 newtons @ 7.5 m/s SharpieF 0.41 newtons and SharpieR 0.39 newtons.

As always take it all with a large grain of salt. Absolute numbers are meaningless, relative relationships valuable, along with the multitude of other factors not addressed here.

Skip

Skip - 06 June 2013 12:23 PM

Here’s the results of some preliminary runs in Mitchlet and Godzilla for a barge hull and a totally flat bottomed sharpie and a rockered sharpie.

Displacement was 400 kg optimised for 5 meters/sec.

Results, dimensions in meters

hull.      length   draft   beam
Barge     13.00   0.183   0.25
SharpieF 11.24   0.137   0.364
SharpieR 13.43   0.166   0.33

I haven’t figured out how to post a graph of drag vrs speed ( 2.5 to 7.5 m/s) but trust me 😉 there’s not much difference, Barge is draggiest, 0.42 newtons @ 7.5 m/s SharpieF 0.41 newtons and SharpieR 0.39 newtons.

As always take it all with a large grain of salt. Absolute numbers are meaningless, relative relationships valuable, along with the multitude of other factors not addressed here.

Skip

Very interesting Skip, I naively designed TP02/03 vaka with a flat mid section and a rocker for the front 1/3. The idea being the rocker would have a good angle of attack to give some bow lift. Nice to know it isn’t a total disaster. Was the analysis you have done from some freeware like Delftship? I use Delfthip and have seen some drag analysis tabs but my models didn’t return any meaning full results.

Tink

Analysis is done in Michlet (freeware for the truly nautically obsessed) which can describe fairly simple forms mathematically. Godzilla is an optimization module within Mitchlet. All my design work is in Autocad 14, an old friend. Recently switched to Sketchup for 3D presentation since my old plugins just won’t cut it anymore.

Like I said before take it all with a grain of salt. Mitchlet is pure math from the waterline down, but it gives good consistent relative numbers to cogitate on.

Skip

Thanks Skip I will try and look into those, I would like the minimal proa to be based on something a little better than intuition

Tink

Thanks Skip for some interesting numbers. It does seem like the barge-hull is not so slow compared to more elaborate shapes as some might think. This verfies my experience from my models.

Today I have begun a new longer and more slender Barge proa.
I use the same amount of material and try to get the wet area and total weight as close as possible to the red barge proa.

It is going to be 244 cm long, 7 cm wide and 8,7 cm high. There is 3,2 cm rocker and 3,5 cm overhanging stems at each side.

I am very curious about what happens when it gets longer and more slender. How does the barge-shape behave when trying to build a racing multihull out of it?

Cheers,
Johannes

The new hull came out 600 grams heavier than the old one. I guess that is because of different quality of the plywood.

I hope I can test-sail it today…

Cheers,
Johannes

A disastrous first sail. I guess the spars in my CC was exhausted from being pulled under the water many times.
Sometimes when i pull the proa back, the CC cuts down into the water. It has so much power that it will pull the hull down under the surface, and I imagine the spars bend a lot. Today one of them snapped.

The new vaka tracked like it was on rails. It was fast, but nothing impressive. I only got two runs before the spar snapped, so I don’t have a lot of data to interpret. The proa could sail a reduced speed with what is left of the CC.
The steep bow and the lack of rocker created a bow-spray that was at times several times higher than the hull. This is not good. I consider this an instructive but disastrous first sail.

Now I need two carbon-spars for the next CC.

Cheers,
Johannes.

John pizzeys canted rig seems like a good fit, no pod, maybe a board in the ama, controllable sailing heeled for traction, ideal for a box. 😉

Skip - 06 June 2013 12:23 PM

I haven’t figured out how to post a graph of drag vrs speed ( 2.5 to 7.5 m/s) but trust me 😉 there’s not much difference, Barge is draggiest, 0.42 newtons @ 7.5 m/s SharpieF 0.41 newtons and SharpieR 0.39 newtons.

I think you made a typo with the units. My guess is that should be kilo-newtons? 0.4 newtons would be a 40g mass, which is like the weight of quarter of a glass of water… I think the total drag force is bound to be a little higher than that. xD

0.4kN (=400N) on the other hand sounds about right. We can do a rough calculation example to see what ballpark we should be in: if you were on a reach with the sails at 45° to the hull, with 30m^2 of sail area up, a lift coefficient of 1, a relative windspeed of 20kts (=10m/s), and the boat was no longer accelerating (meaning propulsive forces for the sails are in balance with the drag), then the sail would be generating around 1050N lift in the forward direction (lift equation * cos(45)). It would have somewhere between 300-600N of drag in the axial direction (depending on aspect ratio). That would leave something on the order of 400-700N of forward lift which is cancelled out by the drag of the hull. While this is an artificial example, I think the numbers are realistic enough to say that the hull drag for a boat of that size, is on the order of magnitude of several hundred newtons, so it’s definitely supposed to be 0.4kN. 😉

All that said, the results are interesting…

Skip - 06 June 2013 12:23 PM

Barge is draggiest, 0.42 newtons @ 7.5 m/s SharpieF 0.41 newtons and SharpieR 0.39 newtons.

I’m a bit surprised the barge came out so well in all this compared to the SharpieF.

Where the SharpieF and SharpieR are concerned: When playing around with fine flat-bottomed trapezoid cross-section hull shapes for Firstborne in Inventor, I made the observation that if you add enough rocker to the just get the bottom panel of the boat to the waterline, then you decrease flat-water wetted-surface area of the hull by around 5%. At that speed, friction is bound to be about 1/3 of the total hull drag, so a 5% decrease in friction would get you about 1.7% less hull drag. That said, 390N and 410N are a good 5% apart, so a reduction in wave-drag is probably playing into the whole thing here too, which is interesting.

One does have to wonder though if the model used by that program is any good for proas…

I’m a bit surprised the barge came out so well in all this compared to the SharpieF.

As probably quite obvious by now, I got very surprised when I “dared” to try something as “wrong” as a barge-proa.
No other hull-shape has impressed me as much as the barge. Even if it is not the fastest shape, it has a lot of very nice properties. A great damping of any unwanted non-forward motion, a great load-carrying capacity. Extreme simplicity in almost every aspect of construction. A very efficient use of sheet-materials like steel, aluminum, plywood or foam. A very nice calm behaviour when sailing through confused breaking waves near the shore, and a very usable interior.
It is quite fast, but I don`t think it will sail regularly at 2 X hullspeed like Jzerro seems to do. I have been sailing my model at about 1,3 X hullspeed, but I have no idea how the speed scales from a scale-model. I have been using crude and simple sails, without any real effort to sheet the sails properly. I think there is a lot more potential speed available if I worked some more with sheeting angles. I have seen short bursts of much higher speeds, but nothing I could sustain without flipping my proa upside down. I would love to sail a large barge-proa with one hand on the tiller and one on the main sheet. I think it could be very fun.

Cheers,
Johannes

Someone might wonder why I did not start building a barge considering my very positive experience of my my models. It was only due to available materials. 7 mm plywood lacks stiffness when not bend. I could have used it with a lot of stringers, but I prefer to use thicker panels instead. A barge has to have enough weight to get some lateral area into the water. If it is to light it will pitch much more violent. All the displacement near the ends of the hull can create very large vertical acceleration when sailing through waves. The barge needs to be slender and slightly more heavy then a sharpie or other more elaborate shapes.

Cheers,
Johannes

Very interesting!  Based on your observations sailing the barge, how do you think paddling the barge shape would be compared to a sharpie shape? 

I definitely have to consider a barge style hull for a future proa.  That would make a nice nesting proa.  I am currently working on a nesting proa (15’length, 19” beam, 19” height) with a sharpie shape, and the extra displacement in the ends would be quite nice due to my load (I am a giant).