Seeking Stability, Maneuverability and Speed Feedback on Design

My colleague has been looking at something along those lines
http://www.cyberiad.net/library/pdf/hawaii_seadov_presentation.pdf

 

I too was talking about hull shape. Since this particular form has zero inherent stability, sans it's appendage, you'll need some sort of stabilization program. This can be approached in a few different ways, but practicality wasn't my concern with the tongue and cheek giro remark.

If you'd like to discuss practicality, then lose the appendage, as it's not adding what you think it might to the design. Instead, make a stable hull form with good efficiency through the water, at the target speed you envision. This will maximize the capacity, decrease draft and greatly simplify the engineering and strains on the hull.

Simply put and has been pointed out, you're going to roll around quite a bit, even with the great appendage mass. You see, the mass (in regard to resisting movement) doesn't really come to play until you've got a bit of heel on and since your hull form doesn't resist initial rolling moments at all, she roll from side to side like a barrel. On the other hand if you had a substantially higher initial stability hull form, you'll gain huge benefits from the hull form dampening the roll initially, when the appendage mass is at it's least advantaged state. If the sea state (pool state) causes more then a little movement, then the appendage can use it's leverage, but in a calm pool, entirely unnecessary. Simply put, a well shaped entry and exit, with a square sectioned hull form, will do all of this without the need for an appendage and the weight savings can be used to carry cargo.

 

Thanks for your reply PAR.

Sans appendage? Why discount what is there. The "appendage" (I'll assume you mean pods) are all that saves this hull design from immediate roll-over. They are integral to my design.

Perhaps you don't see what I think the pods are adding to the design: Lowered centre of gravity, roll damping (inertial resistance), righting moment, 12% buoyancy (reducing surface wave), deep prop eliminating ventilation and reducing cavitation, directable thrust, pitch and yaw damping, impressive self-righting characteristics and anti-roll in dynamic stability while turning due to lateral, vectored thrust.

There is nothing to be gained by maximizing capacity as capacity is fixed.
The draft you reference is of little concern as the only constraint is 20 cm maximum.
For the model, the engineering and hull strains are of little consequence.

There would be no weight savings as the 3 kg below the waterline ballast is mandatory as is the rest of the gear (including the tiny rudder).

This is not an ideal design, it is a hugely constrained design aimed to accommodate the requirements in the original post.

Let me ask you this PAR, where do you think the boat will roll, the "centre of role" if you will, and what do you think the hull speed to be?

Thanks again for your comments.

-Tom

 

How do I explain this more clearly. Take a string, tie a bolt or nut to the end, then let it dangle. Move the weight, say 5 degrees to one side of the centerline and let it go. It will swing back and forth with progressively smaller angles, until it eventually comes to rest, aligned under the string. It does this because nothing is resisting the pendulum movement initially.

Now your appendages do offer some resistance to lateral movement and water is considerably more dense then the air example, but the hull will do a similar dance, because it has little "initial" resistance to this movement. It will swing from one side to the other until the weight aligns again, with a slight assist from the appendages.

On the other hand, if the hull had a high "initial" resistance to rolling, then the weight would be kept in column more easily and the cycling dramatically reduced. This is the whole crux of stability; form and CG. You can use one or the other or both. Most everyone uses some level of both and in this case you'd be wise to consider a higher percentage of form stability, as to counteract the pendulum motion that will exist in you design as currently envisioned.

I also think there is no advantage to the pods. If the ballast is contained in both, then you're pitching moment is increased, not decreased. A flat floored hull with a shaft out the butt, small rudder and ballast spread between station 3 and 7 will make the boat much more stable. It'll have less form drag and it will not roll like a drunken sailor either.

If I was approaching this problem I'd consider a burdened cat hull form or maybe an inverted V, if forced to use a mono hull configuration. Initial stability would be high, straight line tracking would be high, the drive could project through the bottom into clean flow, improving it's efficiency and it would be more clever then the container ship models this problem is intended to develop.

My point is you have two forms of stability to use. Why not employ both to best advantage. I would still ballast the mono, cat or inverted V hull in a similar way, spread out along the bottoms, basically centered to keep movement in this axis minimized. The hull form and ballast both contribute roughly equal, to the stability aspect of the project. And of course waterlines that will permit the speed you desire, with a minimum of fuss. Those bulbs will make a lot more turbulence than you might think. You really want as little down there as you can get away with, for a clean, efficient approach.

 

Interesting.

My reply within:

The drag of the bulbs is miniscule.

What do you think the hull speed to be?

-Tom

 

It will serve no purpose to guess at things you should already have targets and shapes for.

The oscillations you seem to think will not be there, will come from every wave you encounter, motion through the water, uneven pressure distribution along the sides and bottom of the hull, engine torque, etc., even in a "calm pool" you're going to get motion on each axis. You have about as much surface area on those pods as the hull, which to me is nearly twice as much as necessary; decreasing efficiency. The frontal area alone on those pods is more the the hull itself. Without a significant advantage brought to the table, they are superfluous and should be reconsidered. Again, anything hanging below the hull, should be eliminated as much as possible for best efficiency, which means a single pod or better yet, no pods, just a prop, shaft and rudder. Maybe a streamlined skeg housing the shaft, but that's about all you want down there.

 

PAR,

Thanks for your response.

I have targets and shapes as clearly presented in my original post.

I am asking your opinion of what the hull speed would be and where the roll centre might be. I have asked three times and you refuse to answer.
Why do you judge the pertinence of my questions? It is my thread, why shouldn't I ask questions. Perhaps it's because you don't know the answers. It's okay not have an answer but just say so, don't simply dismiss them as irrelevant.

Let me try and summarize your response to my original post thus far:

Stability: Dismal
Maneuverability: Poor
Speed: Unknown and irrelevant.

Would this be a fair representation of your opinion or have I miss-interpreted your insular response?

Thanks again for your interest.

-Tom

 

Without running numbers it's silly to suggests precise parameters for your design and why I don't bother. It's also irrelevant in this discussion, which has been mostly based on hull form and other design choices.

Your stability is all based on weight, which is only half the equation, in regard to how we get stability and the whole point of my contributions on this thread, is to find out why you've elected to go with just the weight aspect of the equation, while completely ignoring the other half of the stability equation (form stability). Another issue is the placement of your "ballast" at the ends of the vessel, more specifically in two pods, so they can increase pitching moments, rather than the the preferred idea of minimizing the pitching moment.

Your insistence at following this approach, is perplexing and likely why my replies have become more abrasive, as the thread has progressed. My apologies, I should know better. I have no issue with new ideas, if they can be qualified, but do with blind adherence without reasonable justification. I don't think you've quantified the lack of hull form stability, but again I haven't run any VPP's or other substantiation on this design. I do think the idea is unique, but I also think you could get better "numbers" if the hull form was more stable (both longitudinally and athwartship) and the appendage drag removed.

 

PAR,

I accept and appreciate your apology, and reply, thank you.

I put the aft pod as far aft as possible to achieve the highest yaw (steering) arm. (Note: the motor, shaft, prop are fixed variables and may not be altered, nor can they extend past the transom - 90 cm LOA) I ballasted it accordingly to balance it. The forward pod was placed according to the remaining mandatory ballast and it's effect on longitudinal (pitch) balance of the boat. The lay out looked reasonable.

I can assure you form stability was carefully considered and for reasons previously stated, led to my design.

I persist because I am endeavoring to communicate with you (and others) my thought process and that it wasn't willy-nilly ignorance that whimsically spewed randomly onto paper as you seem to have perceived.

Why do you ignore my responses to qualify your questions? Is it because you don't like the answers?

It is unfortunate that this dialogue has alienated others from contributing to my thread especially when it seems to be going nowhere other than around in circles "like a drunken sailor".

Your reply's really haven't become any more abrasive. They already where when you started. I'm not sure why, I always thought we got along okay.

Something has set you off and for my part in it, I am sorry.

-Tom

 

Does the motor/shaft prop combo need to be so far below the hull? If so, I owe you another apology, because I missed that portion of this thread. If not, then you'll get better efficiency numbers, if the drive assembly was incorporated into the hull, of course with only what needs to dangle, being out in the flow.

My first post questioned the "zero stability" hull form usage. Your reasoning was to suggest it would stiffen up with the "huge righting arm" and that you were attempting to improve flow, while having sufficient capacity. My point since has been, eventually the mass will come on line, but not until she's rolling a bit. Stumble made similar notes and Leo offered a jab as well.

I think where we got off was my tongue and cheek remark to Stumble about giro stabilization, in reply to his also noting the lack of form stability and the likely roll motion she'd experience.

Lets start again, post the drawing on this page so I don't have to flip back and forth, if you would. Does the drive package have to be in a pod below the hull? How about the ballast? I too always thought we got along fine, so lets try again.

 

PAR,

No, the drive-train does not have to be in the pod.

The 3 kg ballast must be "on the bottom of the hull", the other 3 kg "above the waterline".

I squared up the mid section and gave the bow a Nordic look. (It's a Norwegian competition.)

-Tom

 

I have to agree with PAR, I don't really understand your design. To get the best stability and speed from the constraints, maximize LOA at the waterline, make the cross section a rectangle with a beam somewhere around twice the depth. Stability should come from hull form as the drag coming off your bulbs will be tremendous, and adding weight to make it more stable will only slow you down. I would centre mount the motor and just give it a long shaft with a skeg as PAR said, that way you still have the maximum turning moment from the rudder. I'm fairly certain as well that your large bulbs and fins greatly reduce maneuverability compared to a conventional ship. Also I would go with a plumb and very sharp bow because unless that bulb at the stem has been very carefully design it will create a lot of turbulence.

 

I just can't figure out what this is supposed to acomplish. The boat will roll like an empty oil can.

Putting the drive in a pod like that is almost begging for a leak to destroy your propulsion. And I can't figure out what it could possible solve. The pod would create so much turbulence that the prop is going to be highly inefficient, and the lever arm on the force would cause the bow to lift and the stern to squat constantly.

The front bulb combined with the weight in the rear bulb will cause huge amounts of twist in the hull, forcing you to heavily reinforce the hull to keep it from twisting.

As far as I can see this is a perfect example of a hull that takes one desirable element and takes it to a ridiculous conclusion. There is a reason that all boats are a compromise, and this is a perfect example of why.

 

I am loathe to wade in, but what the heck?
1) - - compare the surface area of all the appendages - reapply that to give a squarer section in the hull = better NATURAL shape that does not need a lot of "fixes" to make it "work as a semi-stable boat" to start with... - the long drive shaft on your engine to propeller (in real-life-size), is, to put it mildly NuckinFuts...
2) - - consider the strengthening necessary to secure the legs that hold the various pods - would never work in real-life so why make a model supposedly representing real-sized needs and applications?
3) - - If your model represents a bulk carrier, a 'more rectangular' section makes it easier to dig out (as most bulk solid cargoes are emptied)...
4) - - Even greater stresses in rotating the aft pod... An engineering nightmare in real-life-application and even then NO SIGNIFICANT advantage and many major disadvantages - Just sit back and quietly look over the ideas as presented with minimal thought for the overall objective...

OK there is lots of interesting thought gone into your drawing and basic parameters, but the BIG picture has not been seen for the forest of distractions... KISS is important... adding complexity to fix an unnecessary problem does not make sense...

It is still not clear what you are endeavouring to achieve - just a model for fun's sake? - then do not try to make it look like something else... The less complex something is, (PARTICULARLY a BOAT), the better... To demonstrate a concept build a minimum model to demonstrate that concept... There are rules of thumb to calculate demonstrated model behaviour (speed/power/windage/roll etc), to approach what may be expected in "life-size" scale...

 

The Nortic look is "interesting", though maybe a bit over the top, at least for my tastes. Possibly a broken sheer, with a dramatic sweep forward to keep boarding waves at bay and low freeboard aft to help loading/offloading.

I'd also lose the bulbous bow thing and go with a destroyer style bow. In fact, dig up some lines for the Fletcher class and see if you can adapt them. The B/L ratio will need to change (assuming the beam and length are fixed parameters), then punch it through the sub routines on FreeShip and see what you get at the speed you anticipate.

The whole thing pretty much revolves around how efficient can you make it. With the micro apps in FeeShip, you should be able to fine tune the waterlines, to give you very close to the best you can get. The square section and reduced wetted surface of a self contained drive, will offer the most stability, least resistance, least draggy drive combination you can ask for.

Back to the Fletcher class, you'll be amazed at their lines, considering the midship shape. One look and you'll realize why they were so highly regarded. Better yet would be the destroyer escorts and the frigates that replaced them, which typically could do 1.7+ S/L with a war load. Your long lean shape with super high ballast/displacement ratio should be quite capable (even without the pods), so focus on a really slippery shape, for your target speed/weight combination and let the frivolous stuff, such as sheer sweep and extraneous accoutrements, fall where they may. In this vain, consider wind resistance if being tested outdoors. A tall bow (or stern) profile can cause steering issues. If it was me, I'd develop an effective underwater shape and just enough freeboard the carry the load, not much else. Boring as hell, I know, but efficient.