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Old Dec 17, 2012, 01:17 PM
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As I was writing the previous entry, I remembered working on the stern. It was actually more than two panels. It was a jigsaw puzzle of four panels. The amazing thing is, they fitted perfectly. And the foam inserts that took several hours to make were not needed at all.

At the same time I was working on the topside, I noticed that some of my underwater panels started curling a little bit along the keel. The problem was that they were not glued to the keel frame. There was very little surface area where the panel meets the keel. I tried in the beginning to pore glue on that joint, but it would just flow out, so I generally left that area alone. But as soon as I noticed the curl (it was not much of a curl anyway) I took two long strips of old thin cotton fabric and laminated them along the seam between the hull and the keel. This way I reinforced the panels, prevented any further curling, and added some degree of waterproofing in a critical area.
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Old Dec 17, 2012, 01:26 PM
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Old Dec 18, 2012, 07:14 PM
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I think you should reinforce the garboard strake area (where the panels started to curl). I suggest slathering triangular pieces of wood with glue, and then placing them inside the hull along the seam that was curling. The triangles are to match the hull plates and the inner keel backbone. Each will have to be custom fit since you may have a bead of glue down there already.

There will be a Lot of prying force on this area once you start sailing. The fin keel with ballast is your friend and foe: friend as it keeps the boat from falling over in a knock-down, foe as it imposes large forces on the hull. The ballast will be prying downwards, counterclockwise, to oppose the force on the sails prying downwards clockwise (for a port tack, switch the clocks for starboard tack). In other words, it's trying to open the garboard strake seam.

The torque is taken up by the garboard strake area, so this seam must be very strong. Think of holding your boat by the masts, at deck level. Now rotate the hull until the fin keel is horizontal. If the keel breaks off, you'll know the hull was too weak. But this is exactly what happens when the ship is hit with a big gust. The forces are ameliorated to some extent by the buoyancy of the elements in the water. And a free-body-diagram would show that the rotational point is actually the center of buoyancy (not the garboard strake only). But even with these refinements, the forces involved are pretty big.
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I'm assuming that you are going to use a fin keel with ballast at the bottom. If you are eschewing a finkeel, and going with all internal ballast, then you won't have to worry. But no model with internal ballast will be as stiff as one with external ballast (on the end of the fin keel). Lack of stiffness will limit your sailing to zephrs. The Pamir's finkeel gives the same righting force as 32 lbs of internal ballast would give. My hull would sink under that amount of ballast :-).
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Old Dec 19, 2012, 02:42 PM
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Brooks:
But of course I am going to use a fin keel with ballast bulb. If you noticed, on my model I have extended the keel down by 3/4 of an inch. In that space I have made slots that will become fin wells. I covered them from both sides with flat thin fiberglass panels. Then, I filled them with molten paraffin to make sure that no glue will stick to them during construction. I plan to melt the paraffin with a heat gun and pore it out when the hull is finished. I laminated a layer of fiberglass over the hull, including the whole keel. I am planning at least one (more likely two) additional layers of fiberglass. That will give me 5-6 layers of lamination on each side of the fin wells. I hope it will take the stress of the forces acting on the fin.

One of the upcoming operations already planned for the hull is to laminate an additional layer of my fashionable cotton strip on the inside of the hull to reinforce the garboard strake area. Originally, I planed it only for the sake of waterproofing, but it will work as the triangular wood pieces that you suggested. I can throw one more layer of lamination, if you think it is necessary.

By the way, I am in owe of your theoretical knowledge. I can follow and understand most of you say about the physics of sailing, but I could never derive it, or, many times, even to know that I need to pay particular attention to this or that particular area of nautical theory.
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Old Dec 19, 2012, 03:07 PM
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As long as we started talking about keels and fins. I need help in deciding on a shape of my fin. I did figure out where the CLR is for the original hull shape (easy to do in SolidWorks). I know (I think) that the CLR of the fin should stay on the same vertical. My problem is deciding on a shape. Narrow and deep? Long with very acute leading edge? Somewhere in between? Which would be the shape for a novice square sailor? I am sure in the future I can experiment with variety of shapes, but what do I start with? My intuition suggests that since the hull itself is very long and narrow, maybe the fin should also be long. But my intuition is not based on knowledge, so I do not trust it one bit.
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Old Dec 19, 2012, 06:16 PM
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First, thank you for the kind words :-). The theoretical and practical concepts are really simple and elegant, but my writing is not, alas. I confess even I find my descriptions confusing when I revisit my posts several years later, darn it.
--------------
Re finkeel size and placement: I talk about my reasoning in Ropanoch's Wawona build, starting at Report #351:
http://www.rcgroups.com/forums/showt...168568&page=24

If you'd like to play with the worksheet, let me know if it needs changes or explanations to meet your needs.
-------
Re finkeel shape: All mine are rectangular. Tapered wings (like your finkeels #2 & #3) have some theoretical advantages in airplanes. I don't pay attention to those advantages for my boats, I just use the simplest form to construct. Unless you are planning on racing, you probably won't need the advantages, but if you want to experiment, go right ahead. Finding the center of lift of a tapered wing (CLR for a boat) is a little trickier than finding the c.o.l. for a rectangle, but there are programs on the web that will do it for you. Some programs call it the "aerodynamic center" which I think means C.o.l., or CLR for us.

Narrow deep keels: These are more efficient, L/D-wise, than wide shallow keels. That is, you get more lift per unit of drag with a narrow deep keel. Deep keels also make the ballast more effective for combating heel. Their only disadvantage is that you have to wade out deeper to launch and recover your ship; I've topped my hip waders launching Aldebaran with her deep keel... wet socks are not that bad, I guess :-).
Narrow keels also make it easier to change course, though I don't find that necessary with my sailing. I don't whip around to tack (rudder hard over, racing sloop method), but sail my squareriggers around, trimming yards and jibs as I go...more fun, more realistic, usually works better.

Note that a narrow keel will likely have it's CLR in a different spot than a wide keel installed in the same location. You can see this in your keels #2(wide) & #3(narrow) the CLR : the 1/4 chord pt of the wide is a bit forward of the 1/4 chord pt in the narrow. You can use this if you want to move the CLR a little bit w/o moving the keel attachment point.

The main problem, besides wet socks, with narrow keels is that it's tempting to reduce wing area, thinking that the extra efficiency of the narrow wing will compensate for less wing area. That is, you are tempted to make a 1/2 width narrow keel shorter than 2xlength of the wide keel. Less wing area=less surface drag (nice), but also=higher wing-loading (not so nice). The higher wing loading really hurts squarerigger performance. This is because squareriggers move slower than Bermuda sloops. The total lift off a slow narrow keel at high wing loading at speed X will be less than the total lift off a wide keel of low wingloading running at the same speed. So, an ideal keel for a speedy sloop will be a failure in a squarerigger in any winds less than a scale gale (when the squarerigger will really move out, yippee!). There will simply be insufficient lift off the narrow highwingloading keel at slower hull speeds (lighter winds), and boat leeway will be excessive. So, if you go with the narrow, efficient keel, make sure you don't increase wingloading. A speedy sloop can handle it, but the slow squaredragger can't.

Drag in a keel: Drag = a keel with the cutaway shape at the forward end of the keel (like your 1st keel diagram). Drag allows the designer to move the CLR aft, compared to a rectangular keel w/o the cutaway. I don't use drag because I just move the rectangular keel aft (drill another hole in my aluminum L shape) if I find I need to move the CLR aft. I think math will only take you so far, though, so precise calculation of keel size, shape, and location will not solve all problems. I think it's easier to make several simple rectangular keels for testing under sail before you go to the trouble of making a dragged keel. My gut feeling is that you won't need that much wing area (your dragged keel). I'd start with a rectangular version of #2 if the ballast depth (=righting force)seems sufficient. Then, make a #3 to see what a stiffer hull sails like, finally make a #1 if you feel the need :-).

There is lots of room for variation in keel size and shape. Your own sailing style will tell you which shape makes her respond like you want. There is less freedom in choosing the CLR/CE alignment. But your boat will tell you what is correct. For me, "correct" alignment yields a boat that will tack and wear with equal facility.
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Old Dec 19, 2012, 06:48 PM
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Quote:
Originally Posted by Brooks View Post
...Finding the center of lift of a tapered wing (CLR for a boat) is a little trickier than finding the c.o.l. for a rectangle, but there are programs on the web that will do it for you. Some programs call it the "aerodynamic center" which I think means C.o.l., or CLR for us...
I use SolidWorks to find the location of a CLR of a complex shape. I really simplified the calculation be assuming that I am only interested in a CLR of a shape (such as a hull or a fin or a rudder) that moves in one direction only, ignoring the effects of drift, current etc. This way I only include the profile of a vessel in my calculations, and ignore the latitudinal sections. Than CLR would correspond to a center of mass of a flat uniform shape. I know, this is an oversimplification, but for a model it might do. I can tweak the position or shape of the fin later, if I discover that she does not sail in a balanced way. I could also play with size and shape of the sails, especially jibs and mizzen sails.
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Old Dec 19, 2012, 09:15 PM
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The center of Drag of a stationary object is the center of area (that area perpendicular to the force). But the center of Lift (Center of Effort of sails, Center of Lateral resistance of underwater bodies) is located at a different place. Generally, it's assumed to be at the 1/4 chord point, ie. aft of the leading edge 1/4th of the distance from leading edge to trailing edge. The center of static drag is, using this terminology, at the 1/2 chord point.

For a tapered wing, it's easy enough to find the 1/4 chord line, but where on the line is the CLR spot? With a rectangular wing, the spot is halfway between the ends of the wing. But a tapered wing may not have the CLR spot at the halfway point, but at a spot closer to the widest end of the wing. This is where a computer program helps (provided the programer knew what he was doing).

If SolidWorks finds the center of area (c.o.a.), you can probably visually place the CLR spot at the 1/4 chord point estimated off the c.o.a. 1/2 chord point with sufficient accuracy. This works really well for mostly rectangular objects (like square sails, rectangular fin, "rectangular-like" hull). For triangular sails, finding the center of area, then mentally estimating the point half way from the c.o.a. to the mast is good enough. For quadralateral sails, eyeball is good enough. You can formally find it via splitting the sail into triangles, and proportioning the area and c.o.a.'s up, if you want.

Sails change shape with angle of attack, so the CE changes slightly too. That's why eyeballing is ok.
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Old Dec 19, 2012, 10:15 PM
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That's why I do not trust my intuition. I was sure that center of area would more or less coincide with CLR, and that COA of a sail would do the same. I even thought myself very clever when I used to find a center of area off a "silhouette" all the sails together, thinking it was a CE of all the sails. Live and learn.

I have placed my fin where I did because I made the COA of a fin to coincide with COA of an underwater part of a hull. So now I need to move it a bit forward, right?
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Old Dec 19, 2012, 11:08 PM
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Meanwhile in the shipyard...

I finished sheeting the hull. I secured the garboard strake area. I covered the hull with one layer of fiberglass. At first I thought of doing each side separately, but in the end I just draped the fiberglass over the whole hull. My plan is to use bondo to smooth all the areas where the hull panels overlap, and where the shape is not up to scratch. When the hull shape is ship shape I plan to add another one or, maybe, two layers of fiberglass. I know that bondo is based on a resin different from epoxy, so I am wiping the hull with denatured alcohol and acetone before every application. So far I have only started "bondoing" one side of the hull, but I have had no problems. It has been a long time since I worked with bond, so I is going slowly, one little batch at a time. But I am getting there.
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Old Dec 20, 2012, 02:12 AM
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After the epoxy set on the first layer of fiberglass I removed all the foam frames. The inside of the hull immediately looked nice and spacious. Next operation was to install the stuffing tubes. But before I did that, I installed partial plywood frames with stuffing tube supports into the slots where foam frames used to sit. The idea is to provide additional support for the bottom and dampen the vibration in the long tubes. I was surprised how easily they slipped into place and how well they alined with the tubes. I reinforced the place where the stuffing tubes exited the hull from the inside. First I put a layer of cotton fabric, and than a layer of fiberglass. I drilled the holes and enlarged them with small files. After I inserted the tubes I secured them with epoxy mixed with fine saw dust.
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Old Dec 20, 2012, 11:53 AM
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Location of finkeel on my Pamir model

Our intuition works good for statics - find the cg of a long 2x6, find the cg of a puppy. It works because we have a lot of practice picking things up and carrying them around. The cg, we know from experience, is about in the middle of the object (the 1/2 chord point). From experience on the teetertodder in the playground, we get a feel for how to balance gravity forces about a fulcrum. When it comes to statics, we have gained experience from the time we could walk.

When it comes to flight, or anything involving lift forces, however, we don't have as much experience from childhood. We don't have as much to base our dynamics intuition on. But, many boat modelers have experience with model airplanes. We know or learned that the plane flys best when the cg is set at about the 1/4 chord point of the wing, not at the 1/2 chord point. Setting the fulcrum at the 1/2 way point worked for static situations, but not for dynamic situations involving lift. Sailboats are all about lift, from the sails and from the underwater surfaces. If you just readjust your intuition to think about airplanes when you design a sailboat, then your intuition will work much better :-).

My Pamir photo. The yards are braced for a beat, the finkeel is placed where it'd be when attached to the hull. Hull length underwater 33", finkeel 10"x12":

I've marked:
The hull center (of the underwater part, not counting the overhang at the stern) labeled "Half underwater Hull". This would be the static balance point of the underwater part of the hull alone.

The "Hull CLR". Notice it is fore of the hull center, in fact it's at the 1/4 chord point (halfway between the bow and the hull center). This would be the dynamic balance point of the underwater part of the hull, that is where the hull would balance while moving and generating lift. The hull generates lift on a beat because, due to leeway, the hull has an angle of attack.

The "Finkeel CLR". Ditto above, at the 1/4chord point of the finkeel. You can see the several sets of holes in the bright aluminum L that I use to attach the finkeel to the hull. Since I tried several keels, I needed different sets of attachment holes. Plus, I was experimenting with the location, trying to find the spot where the ship would both turn upwind and downwind via rudder. The final "best position" for this finkeel puts the center of the finkeel slightly fore of the center of the hull.

The Summation CE, eyeballed, labeled in red. I did not include the jibs or staysails in my eyeball estimate. How to eyeball: first, make it easy on yourself. Ignore the jiggermast, and jibs. Then, since the 3 square sail masts all have the same sail area, you can treat them as one big single sail. Brace the yards for a beat. Measure the distance from the leading edge of the forecourse to the trailing edge of the mizzencourse. 1/4 of that dist aft of the leading edge of the forecourse would be your summation CE for just those 3 masts. For Pamir, that measured just about at the Hull CLR. The addition of sail area aft, ie the jigger, would move the SumCE aft, too. Likewise, addition of jibs forward would move the SumCE forward. You can calculate the final SumCE using sail areas and lever-arms, or just guesstimate it, like I did.

How good is my guess? Well, I know Pamir tacks and wears easily. This shows that the red labeled SumCE must lie very close to the blue SumCLR (which is not shown). The SumCLR will be between the two blue lines. If the hull and finkeel are equally effective lifting bodies, then you can just use the ratio of their areas to figure out where the summation CLR must be. When I do that, I find that, miraculously?, the guesstimate SumCE and the SumCLR do in fact lie very close to one another.

Square riggers are always moving the SumCE around when they change their suit of sails. Removing a sail aft (from the jiggermast) will require removing a sail forward (say a jib) to keep the ship balanced. You'll know you made the right sail choices if you can still tack and wear after the changes.

Likewise, you'll know how to correct the situation if you can't tack and wear afterwards: too many sails aft mean you can't turn downwind to wear because the ship weathervanes more than the rudder can overcome. This also means when you get to the pond for the maiden voyage, if your guess for the finkeel location is wrong, just remove sails to "make it right". If she won't turn downwind, remove sails aft. If she won't turn into the wind, remove sails from the bowsprit (or from the foremast).

Tuning a squarerigger is like tuning a tettertodder. We move weights around (the sails) or we move the fulcrum around (the finkeel). The only real difference is that the fulcrum is never in the center of the ship when the ship is moving.
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Old Dec 20, 2012, 01:54 PM
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Brooks:

I understand about CLR being at 1/4 of a chord. I tried to keep the fin CLR (which I estimated wrongly) on the same vertical line as the original CLR of the hull (which I also estimated wrongly). My thinking was that this was the sum CLR will move vertically, but not horizontaly, and will not affect the original relation to the original sailplan. I know full size capitains could use full size sailors to alter the sail configuaration to suit the current situation, but still, I assume the bark was designed reasonably ballanced. I understand that your situation is a little different: your hull or mast positions or sails sizes are just an approximation so you had to do the hard work of experimenting to find the proper ballances. Maybe my job will be easier because I am working with more precise model of the original (I really, really hope).
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Old Dec 20, 2012, 03:57 PM
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For propulsion I am using two 400 size 12V motors with gear boxes. They will run counterrotating propswi. I made a motor mount that integrates the motors and gearboxes to the hull. Trick shapes, but easy to do with laser cutter. I also built a mechanical speed control holder that fits into a slot left from one of the foam frames. I am only planing to use motors on rare occasions, so I decided to go with cheaper option. Two speeds forward and one backward should be enough to assist in maneuvers and get Kruz to the shore in an emergency. I need to decide between sealed lead acid batteries and NIMH packs. In both cases I will make battery holders that will fit into foam frame slots.
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Old Dec 20, 2012, 04:40 PM
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Pamir scale: The mast locations, spar locations, and sail size are all scale. The hull is the biggest deviatant - it has the general rectangular cross-section of bulk cargo ships (like Pamir). But the aft run is not as fine as on a real ship, nor is the entry (at the bow).

What you are saying, I think, is that a scale model should sail the same way as the real ship. That's only true to a point. There are scale effects that affect the similarity between the model and the real. I don't fully understand them. The Froude number (waves adjustment), and Reynolds number (viscosity/inertia adjustment), are used by professional modelers to mathematically adjust model results so as to predict real results.

To give a feel for the magnitude of scale effects, consider the Navy and it's subs. They have found that any model sub smaller than 1/3 scale does not give results that the naval architect can use to predict performance of the full size sub. Ouch. My Pamir travels, on a reach, about 3.5x as fast as the real Pamir, calculating on a hull lengths/unit time basis.

In general, I think that a modeler who faithfully follows the design of his prototype should be in the balance ballpark. But fine-tuning the balance will be necessary unless the model is big enough to overcome scale limitations. I don't know what the minimum scale-invariant-size will be for a squarerigger. All I can say is that for my boats, putting the finkeel in the middle of the hull results in a boat with lee helm; the bow is always trying to fall off the wind, and you have to use rudder to keep it on course for a beat. Why the real ships did not, apparently, need to hold lee helm is a mystery to me.

At any rate, you will be able to move the finkeel CLR and still retain your attachment system. If you find you need to move the finkeel, just re-engineer your attachment point on the finkeel so that it can be offset from centered on the hull. If I was building your boat, I'd make a short aluminum L with lots of holes. This would attach to your boat via the existing method, and the finkeel would hang from the aluminum. Once you've found just where the finkeel needs to be to give your ship the manueverablilty you want, then replace the swiss-cheesed aluminum L with something more elegant.
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