Firstborne Revisited: a 7.5m Light Cruising Proa
10 or 12 mm is the right thickness. If you used the marine grade plywood (okoume moabi), the use of tissues is not necessary. Only enough protection 250g is for outside the immersed portion.
That is way more than needed. Madness is 6mm plywood glassed inside and out (9mm for the bottom), with 2 layers of glass at the joints. Jzerro is 2x4mm glassed outside (only?), and Cheers is 3x3mm strip planked glassed outside.
6mm for the vaka (except the hull bottom where it should be more) should be fine. Using 4mm plywood for the side panels of the 1.25m long bow sections in the vaka would shave off a total of ~4kg. Going from 6 to 4mm with the ama would save about 6kg, but with a large ballast tank and a potential ama foil I’m not sure if that’s such a good idea. Both measures together would make the fully loaded boat about 2% lighter, which isn’t much, but if everything still holds, then I might as well…
10 or 12 mm is the right thickness. If you used the marine grade plywood (okoume moabi), the use of tissues is not necessary. Only enough protection 250g is for outside the immersed portion.
That is way more than needed. Madness is 6mm plywood glassed inside and out (9mm for the bottom), with 2 layers of glass at the joints. Jzerro is 2x4mm glassed outside (only?), and Cheers is 3x3mm strip planked glassed outside.
Typical scantlings for offshore cruising plywood trimarans in the 1970’s - 80’s (Jim Brown Searunner—Russ’ dad), using marine grade 5-ply fir plywood were as follows:
25’ Searunner tri, appx 2 tons; all 1/4” ply with ~200gsm fiberglass on the outside
31’ Searunner tri, appx 3 tons, all 1/4” ply with 1/2” ply on the bottom only.
34’ SEarunner tri, 4T, all 3/8” ply with 1/2 on the bottom only
37’ Searunner tri, about 6 tons, topsides and decks 3/8” ply, bottoms 3/4” ply and “side-bottoms” 1/2” ply.
These were (still are) considered beefy, all oceans, all conditions boats—many crossed oceans and at least one survived a full-on hurricane. Several hundred were built and I never heard of a Searunner coming apart under sailing stresses. Point is, they were very heavy by today’s standards, and were NOT under-built.
Building a 7.5 meter proa in 6mm ply covered in glass is far, far overkill, in my view. Unless you plan to cross oceans in it. What will your boat weigh, perhaps 1 ton? Half that of a similar length Searunner? Proas develop far less loading in the akas and connectives than trimarans do, even if of the same mass.
It is a very common mistake to build one’s first boat—or boats—too heavy. I have done it more than once, and always to my detriment and shame. You can always beef up the framing later, add more fiberglass, etc. You cannot lighten an overweight boat, and you will pay the price for extra weight every time you go sailing. “If only I had taken Dave’s advice, I’d be making my own wind on lightest legs, rather than sitting here drifting right now.” 😉
LF Herreschoff once designed a “conventional” planked hull about 28’ long (ballasted keelboat). Very delicate and graceful, with finished solid wood planks just over 1/4” thick, over 1/2” x 1/2” oak frames. He was asked by a journalist, “But Mr. Herreschoff, how will such a boat fare should it go onto the rocks?” Herreschoff’s response, “Such a boat shouldn’t be taken onto rocks.”
Dave
Building a 7.5 meter proa in 6mm ply covered in glass is far, far overkill, in my view. Unless you plan to cross oceans in it. What will your boat weigh, perhaps 1 ton? Half that of a similar length Searunner? Proas develop far less loading in the akas and connectives than trimarans do, even if of the same mass.
It is a very common mistake to build one’s first boat—or boats—too heavy. I have done it more than once, and always to my detriment and shame. You can always beef up the framing later, add more fiberglass, etc. You cannot lighten an overweight boat, and you will pay the price for extra weight every time you go sailing. “If only I had taken Dave’s advice, I’d be making my own wind on lightest legs, rather than sitting here drifting right now.” 😉
LF Herreschoff once designed a “conventional” planked hull about 28’ long (ballasted keelboat). Very delicate and graceful, with finished solid wood planks just over 1/4” thick, over 1/2” x 1/2” oak frames. He was asked by a journalist, “But Mr. Herreschoff, how will such a boat fare should it go onto the rocks?” Herreschoff’s response, “Such a boat shouldn’t be taken onto rocks.”
Thanks for that great post! 😊
My goal is to bring the fully rigged boat itself in at 250-300kg max. Two guys adds about 170kg of weight to that, add some very spartan stores and gear, and a total of 500kg should still be within reach, if the boat itself ends up closer to the 250kg mark.
To be entirely honest, if after several years it turns out that the boat has proven itself to be seaworthy and that it handles well in a variety of conditions, then I could imagine attempting to cross an ocean with it. The Mini 6.50s are small and they do it too, and plenty of Polynesian craft that were no larger than this crossed hundreds of miles of open ocean as well. It’s definitely not something to be taken lightly, but if anything I think proas have the potential to be the most seaworthy vessels for a given size, and could make such a crossing faster, more comfortably, and much more safely (no violent slamming or sideways surfing down waves at 15 knots tearing your rudders off) than a Mini 6.50 with its horrendous amounts of sail area ever could. That said, I’d rank that whole thing as more of a pipe dream than anything else. The design brief is cruising in the North and Baltic Sea, and that’s what the boat should be designed for. If 5 years down the road I decide I want to head off to the Azores, I could still just build another boat then, or like you said, strengthen this one.
Maybe 4mm plywood, glassed both sides to make sure it has the required stiffness, is the better approach after all. For the actual intended use, 4mm may well be adequate, and saving 1kg/m^2 on all surfaces would bring the weight down almost 30kgs on the vaka alone, which would be an enormous difference. I think making some material samples is the only way to get a better impression of what’s realistic and what isn’t. If Dave Gerr says 6.5mm of plywood without glass is enough for this boat to really go to sea though, then I’d be inclined to think that 4mm with glass on both sides is as well. Maybe it’s best I make some material samples so I can see and feel and destruction test some real panels…
Marco
You have to remember that the formula’s from Gerrs book have safety factors already. So there’s no need to add safety factors on top of those. Have you tried looking at scantling rules like GL?
There is a really interesting discussion at the moment on the boatdesign.net forum called “Why can’t most catamarans get over the hump ?”
I will quote here: “Indeed…..length displacement ratio shows that shape plays no part, or very little, in such matters. Its all about length-displacement ratio (L/D). Shape is a major misnomer.”
Other posts go on to explain that by for slender hulls, L/D ratio has by FAR the major effect on resistance. 8/1 L/B ratio is already pretty much a “slender hull” and any higher is even better.
Most other hull shape parameters like Cp have a very minor effect compared to L/D. This post is a good summary. http://www.boatdesign.net/forums/multihulls/why-cant-most-catamarans-get-over-hump-50329-2.html#post687108
Conclusion: Saving weight is pretty important!
Here is a great article by Richard Woods about multihull resistance. http://sailingcatamarans.com/index.php/articles/12-to-be-published-mainly-technical/51-hull-shapes
“As an example a 30’ boat with a Cp of .63 will be 1/2 knot faster than one with a Cp of .55 when sailing in 25 knot winds for EXACTLY the same sail area”
Starting with an L/B ratio of 8 and going right on up to 20, reduction of wave making is continually more effective until you reach a point of diminishing returns, caused by increased form drag and skin friction,—unless displacement is reduced.
That is why if you have a very long skinny hull it has to be kept light.
From post #41 in the aforementioned thread.
An UL-airplane is much lighter then a fighter-jet, but still slower. I guess a more powerful rig can make a difference to. Not only D/L ratio.
Johannes
But airplanes are not boats (they don’t make waves). Also, the comparison is resistance, while speed depends on both power and resistance.
This is a heavy boat by multihull standards, but still sailing at 25 - 27 knots on autopilot.
I don´t believe you would let a Marström M-32 (light weight) sail at 20 knots on autopilot while making and eating breakfast. A slightly heavier boat will be much more manageable in waves then an very light weight boat.
I know there is a big difference in length, but the theoretical hullspeed is 7,5 knots for the M-32 and 11,7 knots for the 78 foot Beowulf. A difference of only 4,2 knots. With all that light weight, high tech carbon fiber structure, hugh sail-area and the works, it should be faster then a lead-laden comparable heavy aluminum monohull.
Sailed by a very skilled crew in reasonable flat water it is faster, but that is not the case when most people cruise, with all the food, water, anchors, cloths and gear needed.
Johannes
But airplanes are not boats (they don’t make waves)
Airplanes makes waves in the exact same way a boat does, but all around and not only from the lower part of the hull.
Cheers,
Johannes
You take a plane, big or small boat, light or heavy, duck, irons clothes,
to be always and invariably the same angle!
We called it the wake of Kelvin. The angle is 39 °
That said, I remain convinced that a light boat tired more structurelement
That said though, I doubt you could smash a hole through 6mm plywood with a layer of fiberglass over it in a single blow. Cracking of the epoxy, some local crushing of the plywood, and local delamination maybe, but a hole, no way. 😉
This is 12 mm plywood, and the extra 6 mm will more then compensate for the “one layer of fiber glass in epoxy”.
It is a good quality 9 layer fir plywood, with very few and very small voids. WBP glue.
12 mm plywood vs hammer - Youtube
There are some ways you can dismiss this very crude test, by saying it is not bent into a stiffer shape, lack of stringers etc etc, but 3 mm Weldox 960 is only slightly dented under the same circumstances. One blow with a hammer is nothing compared to running at 7 knots into a stone-jetty or rock underneath the water, or some old pipe, or a log or and old car that sank through the ice in the winter or any other of the million possible things out there hiding just below the surface.
Nobody needs to build a strong durable boat, but any decision should be made based on facts and experience, not a false belief that thin plywood is strong or durable.
Cheers,
Johannes
Tdem, thanks a lot for those really helpful resources! 😊 I wasn’t aware that the effect of the L/D-Ratio is that much larger than all other hull parameters, bow shape and everything. That’s really made me think a bit! For a boat this size, crew weight makes up a relatively large portion of the overall weight, so any weight savings are dampened quite a bit by that, but 30kgs would make for over 5% of the total displacement anyway. If you can carry the crew weight with an ama foil then that would massively reduce the L/D ratio of the vaka as well, and even with all the drag which the ama foil would produce, there should still be a noticeable improvement overall.
A ready to sail C-Class cat weighs about 160kg (360lbs), and those things are built about as light as you can build a boat with carbon fiber, honeycomb, and foam. The LOA of 25 feet, is about the same as my boat, so even without the heavy wingsail and an ama instead of a second hull, it’s reasonable to assume my boat will be quite a lot heavier than those 160kg, say 250, since I’ve got a cabin and am using wood + fiberglass. I’ll have to have a detailed look at Gerr’s scantling rules for framing etc (and maybe do a bit less than what he suggests), and calculate a proper estimate for that.
@Johannes: Thanks taking the time to demonstrate! I don’t believe that’s comparable though. I think a 6mm panel glassed both sides, maybe even a 4mm panel, wouldn’t break under the same test. The wood would probably be crushed/cracked, but my guess is that even though the area would be soft, the fiberglass would still hold it together. When I have the materials on hand (hopefully a few weeks from now), I’ll do a test to find out! 😉
Tdem, thanks a lot for those really helpful resources! 😊 I wasn’t aware that the effect of the L/D-Ratio is that much larger than all other hull parameters, bow shape and everything. That’s really made me think a bit! For a boat this size, crew weight makes up a relatively large portion of the overall weight, so any weight savings are dampened quite a bit by that, but 30kgs would make for over 5% of the total displacement anyway. If you can carry the crew weight with an ama foil then that would massively reduce the L/D ratio of the vaka as well, and even with all the drag which the ama foil would produce, there should still be a noticeable improvement overall.
It really does make you think, doesn’t it? I’ll speak to ama foils in another post, but a couple of comments on crew weight. Once upon a time I built and raced speedsailing boats. Crew weight is a very significant issue here; for instance, I eliminated the second person when similar boats had two. I also aggressively dieted before each campaign, aiming to reduce displacement 20-25 lbs. Both significantly increased my boat speed. The dieting concept also served to focus my thinking on taking weight out of the boat at every opportunity. Pounds and even ounces are worth considering.
I continue to advocate L/B ratios into the twenties and as high as 30—even though they eventually increase net weight of the boat. The boat design thread is interesting; again paraphrasing the Herreschoff family of designers/builders. They claimed, more than 100 years ago, that the drag of any boat was closely related to cross-sectional area of the midships section for any given length. Saying exactly the same thing as the forum thread.
Cheers,
Dave
Nobody needs to build a strong durable boat, but any decision should be made based on facts and experience, not a false belief that thin plywood is strong or durable.
Having a clear frame of reference from which to measure and judge things is important. Applying judgement without a common frame of reference is .... not fair. Your idea of strong and durable maybe based on your experience, your venue and common hazards you encounter. Your design goals may not place weight, speed, build effort and cost of build high on your priority lists.
There are lots of thirty year old (and older) plywood boats around that still deliver on their original design requirements. There are also many twenty year old steel cars here that are rotted out and unsafe to drive. These observations are anecdotal and should not be used to judge.
It is important to define what exactly is failure. Just using plastic versus elastic deformation as a criteria, steel deforms easily and permanently (plastic deformation) where plywood may deform elastically (bouncing back to original state) in the same type of impact.
—
Bill in Ottawa
Sorry for the late comment.
I think that for Cp increase, you have to focus as much, if not more, on rocker, than on entry angle. If you have a keel line which raises early on (from midpoint) to end up at the waterline at the bow (and the stern, on a proa…), you will by definition greatly limit your Cp. It is not only angle of entry in the plan view, but also, if I may say, angle of entry in the side view… that matters.
Cheers,
Laurent
@Skip: Could you have a look what the C_p of Nomad is? Also, do you have the values for the bow entry angles on hand?
The original hull I posted had an entry angle of 14.3°, which made for the relative low C_p value.
Edit: If I make the bows fuller to give them an entry angle of 19.2°, and decrease rocker from 175mm to 100mm, then the wetted surface goes up only very slightly (~0.5%), the bow sections maintain their nice deep v shape, the draft (boards up) decreases from 0.30 to 0.27m, and the Cp goes from the original 0.570 up to 0.647, which seems like a really good improvement to me. According to a figure in “High Speed Sailing: Design Factors”, for a boat of my size and DLR, running at (only) 10 knots, that should equate to a hull drag reduction of around 10%, which would be awesome. 😊
Does anyone have typical values the hull entry angles for proas or high performance cats or tris on hand?