PROA SCANTLINGS
As I’ve been looking at all the proas that have already been built out there, i see 6mm (1/4”) plywood used up to some pretty large sizes. Is this just because everyone is trying so hard for speed, or is it really plenty strong enough in a 30’ Proa with a 1500kg loaded displacement? what particularly worries me is the bottom when encountering non h20 objects…..
Sven, What’s Pacific Bee built out of? There’s a boat that’s been around long enough to have smacked PLENTY of stuff 😉
Tom
Hi Tom
Of course Sven is the best man to answer this question, but by looking at the consturction pictures, you can more or less guess ( Sven will come back on this I am sure) that the hull was cold laminated probably with 2/3 layers of 1/8”-1/16” or something along the lines of 4-6 mm per layer. The mean thing though is the spacing in betwen the ribs and stringers that seem to be fairly close and that would impart a tremedous amout of resilience and strength WITH VERY LIGHT WEIGHT.
Also, consider that the craft is very light so the inertia build into it is moderate at LOW SPEEDS! Another factor is the very streamed lines, ...if you hit from the front (+/- 15 degrees ...et al) the projected area is very narrow and any object would tend to slide along the hull…..IF IS NOT FIXED AND ANCHORED TO A SHORE FOR EXAMPLE!
Again, another factor, both bows don’t want to be loaded so therefore is very easy to either make them water tight or fill them say, up to the water line with foam and flotation material.
cheers
Hi Toni,
I figured Bee for at least a 3/8 skin being 37ft and a cruising boat, but I of course don’t know for sure. It looks like John harris used 1/4” for madness, which is @ 30’ and was entered in wooden boats “Expedition Cruiser” competition. I know John wants a very fast boat though.
I’m hoping to do a more “tack and tape” or “stitch and glue” setup keeping stringers to a minimum. I was gong to do something @ 24’ to begin, and once I had all of the kinks worked out scale the basic design up to 30’-36’, or Bee sized. I guess at that point more of a frame and skin approach might be more appropriate.
I’ll be sailing on the Columbia River in the NW corner of the US, which has many shallow areas as well as wing damns that are barely submerged some parts of the year. There’s also quite a bit of debri for several days after a good storm, including entire trees. Smack factor is worth more to me that a couple of extra knots, once I had enough speed to embarass the local Hunter fleet.
Tom
Hi Tom
I understand what you are saying about the Columbia…you might want to consider Kevlar. I think that it would be very efficient in your case to lay a couple of layers of Kevlar, lengthwise, say from 10cm above to 20/30/40 cm below the WL under the cloth 6/10 ounce that I presume you will lay on top of you ply?
It sounds somewhat complicated but is really not. Many French and some american monos do just that around the bow simply for impact protection, and because of the small quantity needed is cost effective.
cheers
Hi, Tom ,Toni , I took the liberty to answer in the topic, PACIFIC BEE construction
cheers Sven
FWIW, P52, a 22’-6” proa was built out of 5.2mm underlay ply with a bottom of 3/4” plywood.
cheers,
Skip
It would make sense to me in a Dory/Sharpie/multichine hull if the bottom were heavier than the topsides. In a Vee’d hull it would be more homogenous, but as Sven says the bottom jiun can be heavily filleted and glassed for a very strong “blade edge”.
Tom
I’ll be sailing on the Columbia River in the NW corner of the US, which has many shallow areas as well as wing damns that are barely submerged some parts of the year. There’s also quite a bit of debri for several days after a good storm, including entire trees. Smack factor is worth more to me that a couple of extra knots, once I had enough speed to embarass the local Hunter fleet.
If your bow profile is vertical at the waterline, the impact is determined by how long it takes for boat and obstacle to match speed. That gives you the deceleration, which gives you the impact force (assuming constant deceleration as a first approximation). So if you want maximum waterline, you need a crashbox: a foam-filled sacrificial section in front of a collision bulkhead. You’ll have to repair after each collision hard enough to crack your crashbox.
Alternatively, you may slope the hull profile at the waterline. Then your hull will ride up and over a floating obstacle or a spill dam. It will take a lot longer for your boat to come to a stop, reducing deceleration. Some of that deceleration will come from friction, rather than impact. Kevlar would still be useful, but more for abrasion than impact resistance.
A hull profile like Wharram’s Spirit of Gaia (front page at http://wharram.com/site/) or others in the Pahi line of designs shows such a sloped waterline. You will give away waterline length and some prismatic coefficient. But it would save you lots of repairs, and a boat sitting in the workshop waiting for the crashbox to be repaired is not fast. Average speed over a season may be better with the sloped profile.
I once sailed a Hobie 14 onto a barely submerged stone wall at about 8 knots. The boat rode up and came to a stop balanced on the wall. I pushed off again, sailed back and took the boat out, expecting major damage. The boat being a bit elderly, I couldn’t find the new scratches that had to be there among the old ones.
An acquaintance with a deep interest in tanks once explained to me what an advance sloped armour was. This is the same principle.
Regards
Robert Biegler
Robert,
all absolutely true. I’m actually trained as a mechanical engineer, so I can even do the math on the size and direction of the forces 😉
your points are one reason I’m veering back towards a Bolger type sharpie layout, but with more beam. the 16:1 box had the vertical ends immersed, presenting some dead vertical bow to absorb 100% of the kinetic energy of any (mass of obstruction)xV2. at 12:1 the ends come out as Bolger suggests, meaning those objects hit a bottom somewhere @ 15degrees instead of 90 degrees. the sin (portion of force at 90 degrees, or ” smashing”) is only .259, or 26% of the total energy of the collision. at 10 degrees it comes down to 17%.
this is all assuming my desired displacement on a 24’ hull.
Tom
your points are one reason I’m veering back towards a Bolger type sharpie layout, but with more beam.
I have noticed that the proa wants to lift its bow when moving through the water. The amount of lift depends on the speed, so when sailing fast the bow will lift a lot making a grounding or hitting something semi-submerged less of a danger.
Johannes.
Don’t forget at your considerations, that a raised bow may overcome an obstacle easily, but this will hit therafter the unprotected rudder. Hence a crashbox may be the better solution. The boat will stay swimming AND navigable.
I prefer a broken rudder over a holed vaka. Rudders should be able to lift clear if it hits the bottom. One should always carry a spare rudder, and there are always two rudders on a proa to start with. The foam crush block seems like a very good idea. I am probably going to carry a large steering oar, as a backup or to steer in very shallow water. I always seem to need a long pole for a lot of different tasks, and its very easy to shape it into a oar on one its ends.
I will try the daggerboard rudder on my model. I just need some energy to get going again. Im very tired after this weekend of sailing.
Johannes.