Getting out on the water this year

What does your design brief look like? What are your requirements for the boat? What do you want to use it for? What does your budget look like?
old school
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Re: Getting out on the water this year

Post by old school » 06 Feb 2015, 22:00

Here is a quick description of the sketch I have made as basis of my input on this thread –

Plywood vaka, with hull sides made up full length before bending over parallel sided bulkheads. V bottom is built onto this structure, so no strongback needed, and hull is glass skinned before turning over to do * optional * glassing inside.
Squared off shape allows cabin structure as a separate modular construction , possibly to be mated to vaka during final assembly stage. But cabin top is a curved ½ tube shape imparting structural stiffness to support both mast step and safety ama frame………this frame could be an outrigger structure if attached by lashing, as can the straight configured beams (kiato) to windward.
But of course it is possible to bond or bolt box beams, or akas, onto the vaka to attach both amas ( calling it a shunting “double outrigger” if you really want to confuse terminology, like the trimaran crowd has done).
Ama is a flat bottom, running straight in the mid sections and having rocker in the ends (surfboard like, but double ended) with inverted V deck to shed water easily.
Attachment of ama to connecting beams is by stanchion connectives, keeping the ends high and helping to decrease shroud angle.
Single (Sloop) mast is supported on lee side by stays to each end, and duel axis SO rudders are hung to w/ward of vaka.
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Manik
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Re: Getting out on the water this year

Post by Manik » 07 Feb 2015, 00:19

Thanks for all the posts guys!

Originally I'd considered having an underwater section like this:
section.jpg
section.jpg (8.29 KiB) Viewed 1981 times
which results in a hull with a 2-3% penalty in wetted surface over a semi-circle. I don't know how my current flat bottom with flared sides fairs by comparison, but I think two chines less will result in big savings in construction time, and the flat bottom leaves the door open to possibly adding rudder- or daggerboard trunks to the vaka later, for experimentation. A v-bottom makes adding trunks later during a refit considerably more difficult. That is not the major argument here though, the primary concern is just getting construction time down to the point where I can finish the boat on time.

As for the ama, I don't know who is right with regard to spray and planing lift. I'll have a look at the equations in 'Elements of Yacht Design' at some point and calculate roughly how much lift is to be had there, but I recall having done a rough estimate before and getting such an abysmal lift coefficient I just altogether discarded the notion of having any sort of planing lift for a double ended hull, especially since getting a positive angle of attack on the aft part of a hull with even a slight bit of rocker is probably nearly impossible. I admit I may be mistaken though, I'm no naval architect.

As for the pod: I really like the arguments for a windward pod, the weight is on the right side, and the mast base is a good point indeed, especially since the alternative is to build a mast base that rests ontop of the beams anyway. Having a sea kayak as a safety ama would be a cool extra, but I'd have to build that first. Slamming would probably be a serious issue for a windward pod though, since there would only be 30cm of clearance to the water. I know Pacific Bee's cockpit was made narrower from the original, because the slamming on the underside was so unpleasant, and even with the narrower cockpit it persisted.

I'll experiment a bit in CAD with some different designs for lee pods and see if I can find one that produces a reasonable compromise between heel angle and weight to leeward. Just because I have the pod there, doesn't mean I have to use it the entire time, so if the pod itself is built lightly, then there's not a lot of reason for concern there. I'll have to see how the aesthetics pan out though, my initial attempts with a 1m wide pod just looked plain wrong, on a boat this small.

Cheers,
Marco
"Man's mind and spirit grow with the space in which they are allowed to operate." - Krafft A. Ehricke, rocket pioneer
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Manik
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Re: Getting out on the water this year

Post by Manik » 07 Feb 2015, 00:19

@old school: Any chance you have a cellphone camera, scanner, webcam, or anything you could use to take pictures of the boat or of hand-drawn sketches? With your rudder explanation you lost me after about 3 lines unfortunately. I think when you try to explain any mechanism as complex as a door, you really need a sketch, I think with words there's just too many ways to misunderstand what a device is supposed to look like. Maybe that's just me though. ;)
"Man's mind and spirit grow with the space in which they are allowed to operate." - Krafft A. Ehricke, rocket pioneer
petermirow
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Re: Getting out on the water this year

Post by petermirow » 07 Feb 2015, 11:19

Sorry, for that confusion, Old School.
It was a lapse, that now that I read the posts back with more time, I can not reconstruct, how I came to it.
...
Any chance of you sharing a picture or scan of your sketch with us?
old school
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Re: Getting out on the water this year

Post by old school » 09 Feb 2015, 18:22

OK guys, I guess I need to do some halfway decent drawings and get them copied for posting here. This is not going to happen quickly though, since I am not able to justify sitting down at the drawing board right now………. too much laborious stuff to get done.
I was running (in my mind) with the concept of quick build from flat panels of vaka, ama and control foils along with a modified borrowed sloop rig, and the suggested design was appealing enough to steal some time. But I shouldn’t be putting effort into something that is maybe never going to be built.
My own mission is to get away from using outboard engine power, while the need is to have auxiliary power and live aboard amenities.
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Manik
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Re: Getting out on the water this year

Post by Manik » 13 Feb 2015, 09:47

Hey guys,

I've been extremely busy the last week, so just a quick update from me. Last night I finally had the chance to sit down and spend a bit of time with the CAD program to do some measurements regarding the waterline, the pod, and heeling. I recall Sven once made the recommendation to design a pod so it carries 100% displacement at 45°of heel. I played around a bit, dropped the clearance of my pod way down to 150mm, widened it to 900mm and gave it a high volume shape (fuller ends), to the point it looks absolutely ridiculous on such a small boat. Under the assumption of 700kgs of total loaded displacement I still ended up with a heeling angle of 50 degrees before the pod carries 100% of the displacement. The pod I had in the initial renderings, probably couldn't carry that displacement even at 90° of heel.

Out of curiosity I also had a look at how much skin surface area different options would have in total:
  1. 19.4 m^2 for the bare hull without the pod
  2. 23.0 m^2 with the initial pod
  3. 24.8 m^2 for a 650mm wide pod which is extended all the way to the bows, like Sidecar
  4. 26.3 m^2 with the 900mm wide monster pod

6mm Okoume plywood weighs about 3kg/m^2, add exterior fiberglass, paint, and interior epoxy coating to that and we're probably around 4kg/m^2, not including stringers and the increased weight of the hull frames, so from no pod to the initial pod, adds at least 14kgs to the displacement (just the extra hull skin on the leeward side), and from there to the monster pod adds a further 12kgs. Going down to 4mm plywood for the pod would shave ~3 kgs off each of these figures, but again, stringers aren't factored in yet.

Overall it seems the cost in displacement for the pod is not that large, but the gains in usable cabin space definitely are. The heeling angle at which the pod carries 100% displacement will necessarily be quite large on such a small boat though, or you pay a moderately high price in terms of extra displacement. I think that leaves me with two lines of investigation to pursue:
  1. Is the cabin too cramped without a pod?
  2. How much would beams to leeward and a safety ama weigh? Can I get / make an inflatable safety ama for a reasonable price, or do I have to make one out of plywood/glass or foam/glass for instance?
Cheers,
Marco
Attachments
top.jpg
Top view of the monster pod
pod_underside.jpg
Underside of the monster pod
"Man's mind and spirit grow with the space in which they are allowed to operate." - Krafft A. Ehricke, rocket pioneer
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Rob Zabukovec
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Re: Getting out on the water this year

Post by Rob Zabukovec » 13 Feb 2015, 11:30

Marco,

Full length pods might per se might add extra area and weight when compared to Jzerro type pods, but the extra weight can be retrieved by reducing the height (and therefore the windage waveage and CG) of the bows because you have already have extra buoyancy to leeward. You also have spray deflection and potential dynamic lift (when you want it most) plus less resistance when immersed for the same reasons as a short hull vs a long hull for the same displacement. The space provided inside by having a full length pod is also very significant...Sidecar might fall apart, sail like a dog, but even in the raw unfinished state that it is now, the amount of usable extra space throughout the vaka is already very apparent. Just as I thought it would be.

Cheers

Rob
Attachments
vaka 15 02 12 C.jpg
vaka 15 02 12 B.jpg
vaka 15 02 12 A.jpg
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Manik
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Re: Getting out on the water this year

Post by Manik » 13 Feb 2015, 13:37

It looks very roomy indeed... I've said it before, but I think your cabin layout is genius, and I think it's going to make for a very comfortable interior indeed! :) I'm very eager to hear about how your boat sails. ;)
"Man's mind and spirit grow with the space in which they are allowed to operate." - Krafft A. Ehricke, rocket pioneer
old school
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Re: Getting out on the water this year

Post by old school » 13 Feb 2015, 21:32

Not wishing to get into a debate or make an issue of the feature, but rather just telling it from an alternative view…………Before internet and the idea of a “Russel Brown pod’ I was convinced that a proa pod to leeward was the greatest thing, and something to base design safety on. This was from observing the pod on Cheers and imagining that it would be so much better to use the feature as a way to increase cabin space. Since I was also sold on the idea of free-standing masts, or mast singular, the structural requirements for both, created complications that kept me pondering for years. Unwilling and unable to spend money on something which may not help to get me out sailing quickly and more economically I never committed to either, and have always been looking for quicker, cheaper, lighter options.
Getting usable bunk space means keeping the pod lower, having negative bearing on clearance from water. At the same time, safety buoyancy when heeled is better the higher it is (free-standing buoyant masts like Cheers has, are good in this respect) so I find a compromise is hard to reach as scale diminishes………on a proa of 30ft or more a pod is practical (as proven with Russel Brown’s craft) but almost impossible on a 20 footer.
Talking of 20 footers……………….this discussion has fuelled my thoughts in a way that makes sense of a minimalist approach, including size just big enough to sleep inside (for max 2 pers) seaworthy and quicker/cheaper to build due to smaller scale.
I see that 24 ft suits the 3 length plywood sheet criteria, and which might be the better option because of the extra displacement capacity and load carrying ability, although the 20 footer will reduce rig costs…………so as a design exercise the 20 footer could tick more boxes in getting a proa built speedily.

In the smallest size a kayak filled with some inflatable bladder is just fine as a safety ama instead of a pod, and can be positioned as high as is required....no problems there with having to design the correct volume for any particular heeling angle , because it can be varied and even increased with more inflatable volume. This variability factor is a good way to determine if a greater heel angle (where sails have spilled more wind) is actually better than lee buoyancy acting sooner but also losing effectiveness sooner (at lower heel angle) than buoyancy most effective close to knockdown.
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Manik
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Re: Getting out on the water this year

Post by Manik » 14 Feb 2015, 22:27

I agree with Jeremy that pods seem to become more and more problematic the smaller the boat gets. They either become excessively large for such a small boat, or then they fail to really deliver the benefits of a pod for lack of volume and beam. I've come to the conclusion that the pod won't deliver what I want for this boat, so I'm going to have a safety ama. I do need some extra internal space though, so now it's just a question of whether I'll feature a pod to leeward regardless, just for the space, or if I for example step the hull on both sides to get a bit of extra elbow room (on both sides!) that way. If I do step the hull on both sides, then I probably wouldn't do it full length though, just near the center of the hull, and I'll probably drop the hull splays then just to cut down on the number of panels needed for the hull...

Which configuration would you guys recommend? Still have a single-bunk pod even with the safety ama, or give the hull a flared hull step on both sides for extra elbow room? Or do both?
very_rough_midsection_sketches.png
Possible midsections I'm considering...
very_rough_midsection_sketches.png (2.36 KiB) Viewed 1859 times
For the safety ama I'm thinking either an inflatable, like a RIB tube (self made? or repurposed?) or a cheap, simple and very light plywood/glass hull. Since the safety ama is out of the water most of the time, and thus not under constant cyclic loads, using a cheap foam / glass sandwich seems like an option here too. Out of the water it should last quite a while before it starts delaminating / failing. I think weight, and the possibility to flood it to allow rerighting of the boat by rolling it over the safety ama (using a winch, pole, and water ballast bag for instance) are factors worth considering...

Cheers,
Marco
"Man's mind and spirit grow with the space in which they are allowed to operate." - Krafft A. Ehricke, rocket pioneer
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