~~PACIFIC BEE~~ STABILITY
Height of CG
[answering to a question about height of CG]
case ; 53ft PP HEIGHT OF CG
At 0-4 degrees [ama is scimming the water][99.95% of the time ] height of CG in less relevant as there is almost no shift in CG versus CB.
read also post #09
Onwards from 4 degrees [ama leaves the water] [00.05% of the time] it is increasingly important to have all weights down low. in vertical sense ;the nearer the CG approaches the CB the better.
although the angle you will finally go over is nearly impossible to get to [unless you have no pod/leeside floatation device] I consider the following important;
- rig to be as light as possible, don’t overdo your sail cloth weight, as proa rig is loaded lightly anyway [ take note of flat cut sails] you don’t needs catamaran or trimaran heavy cloth.]
- having sturdy underwater panels [Iam totally into chined plywood these days]
- having lighter vaka top panels and deck
- carry heavy equipment low stores,cooling area ,batteries under the floor, etc
More important is a well though-out ama ballast system and a platform loading plan with the ability to fine tune the moment of heaving the ama out of the water.
when getting to know your own pacific proa , you get a pretty good “feel” of power versus remaining reserve weight of the platform and ama to play with.
When in doubt reef early.
regards Sven
Sven,
Thanks a lot.
I am toying with the idea of a 45 footer cruiser right now, and I am playing with FreeShip 3D software to look at different options and volumes etc…
When moving the CG from 4.5 to 5.9 ft above keel line, you are right, the righting moment up to 15 degrees of heel does not change much, 3.58 T.m in one case and 3.28 T.m on the other case. It is less than 10% difference.
I found more difference on the ultimate righting moment, when the leepod is taking most of the buoyancy; in my case, around 50° heel. I know, in that case, it is catastrophic recovery time, at that angle of heel!
In that case, my righting moment is 4.16 T.m with CG at 5.9 ft above keel line and 5.06 T.m if CG is 1.5 ft lower, so about 25% increase…
Cheers,
Laurent
PS: I am playing with developable surfaces as well, buy my hulls look definitely more “agricultural engineering” than yours!!!
Aloha Laurent,
I was just checking on my 45ft design ; at 32 degrees heel the pod takes over the total displacement of the proa.
I have the impression your pod is attached very high ,or ??
cheers Sven
Not Funny !!!!
Sven,
I do not think it is because it is too high, but rather because it does not “protrude out to the leeside” as much as yours.
If I look at the bottom chine of the leepod, the edge between the bottom of the pod and the leeside of the pod, the mid point is 7 ft from the centerline of the main hull, and 2.3 ft above waterline.
On your 45 ft version, for comparison, where does this chine mid-point lie? My gut feeling is that it is much further to the leeside.
I most likely could put some pictures here on your thread, but I do not want to “pollute” it with my pictures…
Regards,
Laurent
height of CG in general is almost irrelevant for a pacific proa.
Based on my experience with my different scale-models I would say that low CG is important to resist pitching and hobbyhorsing. A high CG will make a hobbyhorse out of any hullshape.
Cheers,
Johannes
Sven,
I do not think it is because it is too high, but rather because it does not “protrude out to the leeside” as much as yours.
If I look at the bottom chine of the leepod, the edge between the bottom of the pod and the leeside of the pod, the mid point is 7 ft from the centerline of the main hull, and 2.3 ft above waterline.On your 45 ft version, for comparison, where does this chine mid-point lie? My gut feeling is that it is much further to the leeside.
I most likely could put some pictures here on your thread, but I do not want to “pollute” it with my pictures…
Regards,
Laurent
Ok , yes I see what you mean. basically [for ease of building] the cockpit wing = same pod wing.
this gives me 2 double bunks in the pod, and a great cockpit area w/ table chairs etc.
on top I get great RM figures from that pod volume
the pod lower chine= 2300mm of the CL , 900mm above CWL , so Iam a bit wider , but also higher.
your pod must be shorter ,or ?? 50 degrees is too dangerous ,I think. I would make a pod which takes over the total displacement at about 30 degrees. in my case 5000kg displacement.
cheers Sven
height of CG in general is almost irrelevant for a pacific proa.
Based on my experience with my different scale-models I would say that low CG is important to resist pitching and hobbyhorsing. A high CG will make a hobbyhorse out of any hullshape.Cheers,
Johannes
hello Johannes, may I suggest that full size versus scale models versus wave period and height , mast inertia, drive of the sails etc makes comparing pretty difficult.
I can tell you that Pacific Bee has absolutely no tendency to hobby horsing.
I think one needs to have sufficient forebody fatness designed in .
Also it are very specific wave directions,height,period. which could trigger some hobby horsing, because its so specific you can easily change a few parameter , and away is the problem .for example change the course.
further , because you sit in the center of movement on a proa , you won’t encounter much movement anyway.
my 2 cents
cheers Sven
my two cents
They are two very valuable cents then.
I lack the experience you have got, so i read everything you write with great interest.
Pitching might not be a big problem., but i tend to use much more rocker then Russell Brown or you, and that might make height of CG more important for my hullshapes.
Cheers,
Johannes
Laurent wrote; to use much more rocker then Russell Brown or you
Hi Johannes,
A boat design is an integrated whole and full of interaction and compromise, I do not suggest there is a specific good or bad way, as pacific proas are really unexplored territory .
about rocker in proa, ; I purposely design less rocker for exceptional tracking, ideal for covering long distances in a straight line.
lake or river use could ask for a quicker turning hull shape. so there you might be right in having more rocker.
my the recent vaka design is more a shallow canoe shape…....
cheers Sven
Hi Sven,
Very cool renderings! Does Pacific Bee have two dagger boards in the ama, and two more in the main hull? The mains I presume are cheers-style rudderboards, like the ones in Jzerro? Do you control the ones in the ama in any way, or just up and down? Do you find it helps helm balance with one of these raised and the other down, or…
Thanks!
Dave Culp
Hi Sven,
Very cool renderings! Does Pacific Bee have two dagger boards in the ama, and two more in the main hull? The mains I presume are cheers-style rudderboards, like the ones in Jzerro? Do you control the ones in the ama in any way, or just up and down? Do you find it helps helm balance with one of these raised and the other down, or…
Thanks!
Dave Culp
Hi Dave
Pacific Bee is setup like Jzerro on which you have sailed on the San Francisco Bay. one ama centerboard , 2 dagger rudders.
The renderings seen are of a few new designs a 36ft , 45ft , 53ft ,58ft in which we plan to put multiple foils in the future. the ones in the ama will go up/down and can change the angle of attack slightly.
Cheers Sven
Pacific Bee is setup like Jzerro on which you have sailed on the San Francisco Bay. one ama centerboard , 2 dagger rudders.
The renderings seen are of a few new designs a 36ft , 45ft , 53ft ,58ft in which we plan to put multiple foils in the future. the ones in the ama will go up/down and can change the angle of attack slightly.
Cheers Sven
Very cool. Lots of adjustability. I like that!
Dave
Sven,
I do not think it is because it is too high, but rather because it does not “protrude out to the leeside” as much as yours.
If I look at the bottom chine of the leepod, the edge between the bottom of the pod and the leeside of the pod, the mid point is 7 ft from the centerline of the main hull, and 2.3 ft above waterline.On your 45 ft version, for comparison, where does this chine mid-point lie? My gut feeling is that it is much further to the leeside.
I most likely could put some pictures here on your thread, but I do not want to “pollute” it with my pictures…
Regards,
Laurent
Ok , yes I see what you mean. basically [for ease of building] the cockpit wing = same pod wing.
this gives me 2 double bunks in the pod, and a great cockpit area w/ table chairs etc.
on top I get great RM figures from that pod volume
the pod lower chine= 2300mm of the CL , 900mm above CWL , so Iam a bit wider , but also higher.your pod must be shorter ,or ?? 50 degrees is too dangerous ,I think. I would make a pod which takes over the total displacement at about 30 degrees. in my case 5000kg displacement.
cheers Sven
Sven,
Your are right, it is fairly short, in the longitudinal direction. I played with it in many forms and maybe I went too far in the shortening phase…
Do you have a stability curve for your designs? I am getting inspired by the Bieker proa information I got here.
http://biekerboats.blogspot.sg/2013/12/study-plans-ready-for-32-proa-it-has.html
If you look at the different drawings and print them, you can see that the Centerline to Centerline distance between the hulls is 50% of waterline length.
Also, the stability curve, with ballast, is very interesting, but I believe somewhat incomplete… They obviously plotted no point between 0° of heel and 10° of heel. So it seems that 10° of heel is the highest righting moment… In fact, the highest righting moment will be as soon as the ama is just above water, which for this boat, should be around 3 to 5° of heel. My design show at least a 10% increase in righting moment between 10° of heel and max RM (for my boat, at 4° of heel)... So taking into account the displacement of the boat, loaded, and the amount of ballast, also in the table on the drawing, and the righting arm, one can see that ballasted, 30% of the total displacement is in the ama… Isn’t it quite high?
It is much more than what Philippe Guillard told be about Des Jours Meilleurs…
What about yours?
There are not that many REAL blue water proas to compare to; this is why such information is so important and difficult to get.
Cheers,
Laurent
Aloha Laurent, iam in a big hurry , and give you a precise answer later.
But think about this; have an as light ama setup as possible ( good for light weather ghosting along) and a relatively large ballast tank(s) for all other moments….. Thats what I design to. Benefit a very big
RM range to keep the ama scimming or at least in light mode, an the other benefit is you don’t have anything dragging out there to windward, except a foil(s) keeping you to weather….................
Note; I accepted to have a lightweight military high tech diesel genset in the ama ( for propulsion) , for a 53 footer that weight is peanuts. And the noise far away, I hate that noisy outboards.
Cheers for now, Sven
Aloha Laurent, iam in a big hurry , and give you a precise answer later.
But think about this; have an as light ama setup as possible ( good for light weather ghosting along) and a relatively large ballast tank(s) for all other moments….. Thats what I design to. Benefit a very big
RM range to keep the ama scimming or at least in light mode, an the other benefit is you don’t have anything dragging out there to windward, except a foil pulling you to weather….................Note; I accepted to have a lightweight military high tech diesel genset in the ama ( for propulsion) , for a 53 footer that weight is peanuts. And the noise far away, I hate that noisy outboards.
Cheers for now, Sven
Sven,
I agree with all your statements, of course.
The question I am asking myself though is; how far do I have to go? how much light is “light enough”?
That’s where the comparison by % of displacement becomes handy, even if it has to be taken carefully, especially when you scale up and down… Let’s not forget that if you do a (somewhat stupid) direct scale up of a 32 ft proa (like the Bieker design) to 45 ft proa, the result is 2.8 times bigger than the original design (45/32 to the cube).
I am looking forward to hearing more from you!
Laurent