Height of CG
Where is usually located the center of gravity on a cruising proa? I know, there are not many of them out there to get an answer from.
Longitudinally, it is obviously located at 50% of LWL (duh…)
Transversally, it depends on your design assumption of how much displacement is carried out by the ama; but you can determine where it should be from your design assumptions.
But what about the height?...
I am currently sketching a cruising proa in FreeSHIP! freeware, and I am trying to put together the stability curve. I need to locate the CG vertically to do this.
My sketch is only that; a sketch, not a design. In other words, I put together a skin of developable surfaces, but no structural stringers, bulkheads, etc… Giving a uniform weight to the skin, I end up with a CG located fairly high.
All the pay load will make the CG go down, but by how much?
Is there a rule of thumb that I could apply as a first round approach as, for instance a % of LWL above waterline?
Right now, the leepod start kissing the water at 22° heel, but because the CG is so high, at that angle of heel, it is towering above the main hull center of buoyancy, and the leepod has not had the opportunity to make its magic yet, so the stability curve goes negative for a few small degrees of heel, until enough buoyance transfers from the main hull to the leepod to stop it going belly up…
In other words, there is a tiping point at about 23° of heel, and then the boat is stopped doing a complete overturn by the leepod.
I looked at the stability curve of the Bieker “Jester Class” proa, and I can tell that the leepod is starting its magic way before the CG is towering above the main hull…
So either I have my assumptions on the CG height completely wrong, or the leepod is supposed to provide buoyancy, starting way before 22° of heel.
Any actual real life experience sharing is welcome!!!
At what degree of heel do Russel Brown proas start to have the leepod touch the water?
Cheers,
Laurent
15 degrees, says Sven.
http://proafile.com/forums/viewthread/25/
see post #9
This is the most interesting thing I have ever read about pods.
Thanks Chris,
That’s exactly what I was looking for.
Laurent
Where is usually located the center of gravity on a cruising proa? I know, there are not many of them out there to get an answer from.
Longitudinally, it is obviously located at 50% of LWL (duh…)
Transversally, it depends on your design assumption of how much displacement is carried out by the ama; but you can determine where it should be from your design assumptions.
But what about the height?...
I am currently sketching a cruising proa in FreeSHIP! freeware, and I am trying to put together the stability curve. I need to locate the CG vertically to do this.
Laurent
A quick and dirty way to get a location of CG to windward is to take moments using your ama displacement on one side of CG with vaka displacement on the other side…...So for example if you had: ama 200kg and vaka 1000kg with the distance between C/Ls at 6 metres, then CG is located 1 metre to windward with an ama moment of 200x5=1000kgm and a vaka moment of 1000x1=1000kgm. You can do it graphically this way as well, much as you would for finding the CE of a sail plan.
Vertical is not easy at all, but maybe if you draw a like from the C/L of ama deck amidships to the C/L of vaka deck amidships and where it crosses your plan CG line (in above case 1metre out to windward) that gives you a very dirty vertical CG position. Most of the boat and loads will be below this level, so it should be on the safe side. After that you really have to guestimate how (un)safe you want to be unless you put in a lot more work itemising as many weights and loads as possible and their positions and take moments in the usual way.
See how you go….
Rob
Where is usually located the center of gravity on a cruising proa? I know, there are not many of them out there to get an answer from.
Longitudinally, it is obviously located at 50% of LWL (duh…)
Transversally, it depends on your design assumption of how much displacement is carried out by the ama; but you can determine where it should be from your design assumptions.
But what about the height?...
I am currently sketching a cruising proa in FreeSHIP! freeware, and I am trying to put together the stability curve. I need to locate the CG vertically to do this.Laurent
A quick and dirty way to get a location of CG to windward is to take moments using your ama displacement on one side of CG with vaka displacement on the other side…...So for example if you had: ama 200kg and vaka 1000kg with the distance between C/Ls at 6 metres, then CG is located 1 metre to windward with an ama moment of 200x5=1000kgm and a vaka moment of 1000x1=1000kgm. You can do it graphically this way as well, much as you would for finding the CE of a sail plan.
Vertical is not easy at all, but maybe if you draw a like from the C/L of ama deck amidships to the C/L of vaka deck amidships and where it crosses your plan CG line (in above case 1metre out to windward) that gives you a very dirty vertical CG position. Most of the boat and loads will be below this level, so it should be on the safe side. After that you really have to guestimate how (un)safe you want to be unless you put in a lot more work itemising as many weights and loads as possible and their positions and take moments in the usual way.
See how you go….
Rob
I got the first part, no problem…
My hurdle is about the height. I agree with you that if you want to do it in an accurate manner, you have to itemize the whole boat and payload, with weight and position, and calculate the overal CG from there.
But I am not there yet, far from it.
I was hoping for something like a rule of thumb, i.e.:
- CG height is % of LWL above waterline
or
- CG height is % of vaka height, from keel line
or something like that…
I did a sketch is FreeSHIP freeware; it is just a “skin” at this stage. I know what total displacement I want to hit. Giving a VERY conservative weight to that skin, not taking into account bulkheads, stringers, etc… just the boat shell, I end up with more than 50% of total displacement for “everything else”. I am happy with that for now; but I am trying to draw the stability curve and see if I get to something similar to the Bieker 32’ proa curve, which is the only one I know of…
I did a first set of “semi-automated” calculation for that curve in FreeSHIP, (you rotate the boat manually in FreeShip, then adjust the draft to get back to the intended displacement and get the CG and Center of Buoyancy Y axis location from the hydrostatics calculations; repeat at every 5° of heel).
But because my assumption for the CG height is… high, when the boat heels, the CG gets on top of the vaka before the leepod hits the water.
So either my assumption for the CG height is wrong; or my leepod does not do its job early enough when the boat heels (which is most likely true, compared to Pacific Bee).
Or both…
If I could get some information on CG height for true proas… that would confrim where I am wrong…
Cheers,
Laurent
.
My hurdle is about the height. I agree with you that if you want to do it in an accurate manner, you have to itemize the whole boat and payload, with weight and position, and calculate the overal CG from there.But I am not there yet, far from it.
If I could get some information on CG height for true proas… that would confirm where I am wrong…
Laurent,
To my knowledge, there is no average for proas. And what is a “true” proa anyhow….There are so few proas about, and they vary so much in size and approach that even if there was an average, it would be pretty meaningless.
1) I assume that my suggestion gives you a higher CG than you already have? If not and it is sufficiently lower, try it. You know that in reality it should be lower still.
2) Do the test with different CG heights till you get a stability curve that makes sense or you are comfortable with and then look at the CG height. If it looks plausible or achievable, then maybe your pod is OK, if not, then obviously it isn’t or something else is wrong. You will get a good feel pretty quickly.
3) If it means that much to you, there is no substitute for doing the donkey work as best as you can thoroughly at least once. Then you have a good base CG height relevant to your specific design approach and program on any other designs variations or modifications that you do in the future. As an example, using a blanket shell or plating weight will give you a higher CG height because as an indicative crude rule of thumb, slamming and underwater areas ought to be around 20% stronger / heavier, keel area even heavier still, say around 40%, and decks and cabin tops around 20% lighter than your blanket shell weight assumption. If you are confident that your assumed shell weight is so conservative, then reassign the weights accordingly and you will get a lower CG overall.
Good Luck.
Rob
I was hoping for something like a rule of thumb, i.e.:
- CG height is % of LWL above waterline
or
- CG height is % of vaka height, from keel line
or something like that…
Laurent
Rule of thumb for Proa Design !?!?!
You know better 😉
Cheers,
Skip (we really missed you today)
IMHO concerns about vertical CG somewhat overstated. The bottom line is the vertical CG is going to be high, actually very high. Such is the case for all multihulls by the virtue of not having a hunk of lead strapped to their bottom. Except for a rare few, the hulls of multihulls tend to be very narrow, shallow, and tall. Put all the heavy stuff you want in the bottom of the average multihull hull and it’s still going to flop on its side without another hull attached to steady it.
Vertical CG is important design consideration for a monohulls because their static lateral stability and its largely dependant on the counterweight affects of ballast which is negated by moving vertical cg upward away from the counterweighing ballast negating it’s stabilizing affect.
The stabilizing force of multihulls has little to do with a counter weight system. It’s all about beam, and lots of it- i.e. a horizontal. lever arm. One way to think of it is that you’re trading a vertical lever arm for a horizontal lever-arm.
Where the whole vertical cg thing becomes a significant concern for a multihull is when that horizontal lever arm starts moving towards the vertical, as in flyin an ama. Cause that’s when you start loosing the lever arm which provides stability.
In addition, an ama along with the mast(s) is a significant hunk-O-mass which if now is way up in the air, (not an uncommon situation) any concerns/calculations regarding static vertical CG look pretty insignificant compared to the mass and angular accelerations of a flying ama and mast.
You touched on one point that I think IS important, and that is the safety/lee pod. The heel angle, the volume and how that volume is distributed I think is something worth giving a lot of thought to, more so than static vertical cg. You mentioned that the pod started “kissing the surface” at 22 deg. which all well and fine but the real issue is how quickly or at what rate of angular rotation does the lee pod submerge a given volume so as to, 1.) Significantly shift your lateral CB to leeward, or 2.) How quickly does the pod create a buoyant upward counterforce to counteract all that flying and rotating mass of an ama and rig.
So the way I see it the whole static vertical CG thing is a red herring. Good seamanship (storing heavy stuff as low down), good craftsmanship ( building light and reasonable choices regarding spares), will get your vertical CG about as low as it’s going to get in any multihull unless you decide to hang lead from the bottom of your boat. Regards JRT
Rob,
What I meant by “true” proa would most likely be better said by a “real” proa; something that is not only a drawing, but a boat floating somewhere, and eating miles away…
1) yes it is higher than mine, and mine is already too high on a static stability point of view.
2) and 3); sure I can do that, and most likely will do that, once I worked out a bug in my design that gives me some funky “cross section area” curve. I was just trying to find an easy way out… as a first approximation.
Skip,
One would think I know better but wishful thinking sometimes takes over…
How was the sailing trial?
JRT,
I agree with you that dynamic stability and the LARGE amount of rolling inertia we can have with a multihull is an important factor to take into consideration; but static stability should be a pre-requisite… If you do not even have that, dynamic stability becomes a mute point.
Once again, I am not there yet…
Cheers,
Laurent
Right now, the leepod start kissing the water at 22° heel, but because the CG is so high, at that angle of heel, it is towering above the main hull center of buoyancy, and the leepod has not had the opportunity to make its magic yet, so the stability curve goes negative for a few small degrees of heel, until enough buoyance transfers from the main hull to the leepod to stop it going belly up
IMHO this is a fairy tale. During my examinations for P12 I tried to check out the righting moment, and I find out, that there is no hump in the curve (http://proafile.com/forums/viewreply/3179/). The cause is, that the turning point moves also to leeward during heeling, also if the leepod dunks. See attached sketch for the ama. If you are do the same for all relevant weight like beams, cockpit, and vaka, and then calculate the moments, you will get a good prediction of the whole (static) righting moment of your boat. And you will see too, that the COG of the vaka brings only some percent of all rigthing moments because of the short leverarm.