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Princess 26


Howard

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As I envision this concept, imagine a single strip of 1/2" plywood that is 2 inches wide and 6 feet long. Now imagine 36 such strips stacked on top of each other. Glued together, they would make a single plank that would be 18 inches x 2 inches by 6 feet long, essentially the same overall size as the blank I glued up to make the CB. The edge grain of each plywood strip would be facing out, not fore and aft as would be the case if you laminated up 4 pieces of 18" x 6' long 1/2" plywood.

 

Plywood CB's and rudders seems to lose their strength and fail when repeatedly flexed across the grain (grain running fore and aft with the board). My notion would to turn the grain 90 degrees where the strength of the grain would resist the flexing. Same principle as vertical grain vs. flat grain.

 

Taken one step farther, if you drilled holes in the centerline to register the piece in the lamination stack, then started shaving fractions evenly off each side, you could start to build in the shape of the board as you stacked it up.

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Plywood CB's and rudders seems to lose their strength and fail when repeatedly flexed across the grain (grain running fore and aft with the board). My notion would to turn the grain 90 degrees where the strength of the grain would resist the flexing. Same principle as vertical grain vs. flat grain..

I think you are making an assumption that just is't true.  Just because the first part is one way that plywood boards are weaker does not make the second part true.  And plywood is not related to vertical or flat grain wood.  The reason all solid wood center boards are stronger is that all grain runs opposite the loads.  No matter how you orient the plywood, this will not be the case.

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Curiosity got the better of me, so I decided to test my plywood flex theory. Attached are photos of a plywood lamination........the two pieces are the same and turned 90 degrees. Basically, they are 1 1/2 inches square.  Dave was right. Loaded, they flexed the same. These pieces were cantilevered out about 3 1/2 feet, with a 15 pound lead ingot on each piece. They also bent the same when supported on both ends and loaded in the middle.

 

Had I been playing "bet your a$$", I would have bet (and lost!) my a$$ the vertical grained piece would have been stiffer and flexed less under load. Not so.

 

What was stiffer was a solid piece of yellow pine. It didn't flex at all.

 

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Keep in mind that vertical grain lumber doesn't have grain in a different direction.  It has the rings appearing to be vertical.  There is no swirling appearance like most regular sawn boards.  It is more stable though not significantly stronger.  Some think it looks better too, but that is subjective.

 

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The notion that quarter/rift sawn (vertical grain/ringed) wood has the same resistance to sideways bending as flat sawn runs counter to everything I have ever been told. This is specifically mentioned and discussed in "Devlin's Boat Building" on pages 24 and 25:

 

"vertical grain will be harder to bend, but stiffer and stronger. A flat, or mixed grain gunwale would be easier to bend, but won't be nearly as strong. And when making a sheer clamp on a larger boat, where multiple layers of dimensional wood must be laminated to form the clamp, a flat grained wood will bend into place more readily, fastern easier and do so without as much splitting.

 

 

Then there are the OSHA specs for scaffolding lumber that require a scaffold plank to have vertical grain, with a maximum slope or lean of the grain allowed.

 

Strength and resistance to deflection issues aside, for most things on a boat, my preference is far and away to use straight, vertical grained wood, as so much the uses we make of it is glued on the flat side, as keels, carlins, clamps, etc. are done. Vertical grain is more stable and does not have the tendency to cup that flat sawn does, so a lamination is less likely to blow itself apart.

 

The problem for me has always been finding it. Since flat sawn makes for the most efficient use of the round tree, the vast majority of wood in any lumber yard bundle (construction lumber) is going to be flat sawn. A 3/4" x 1.5" carlin is no problem. Rip that from a larger, flat sawn board.....the closer to the edge of the tree and flatter the grain, the better. A large, wider plank? Those are almost impossible to find.

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Howard, that is why when you get a flat sawn board, you should rip it into strips, turn them 90 deg and glue it all back up. Control of stiffness and expansion properties. By the way I got the rudder almost done. I'll shoot you a pic one of these days. PeterP

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Howard, that is why when you get a flat sawn board, you should rip it into strips, turn them 90 deg and glue it all back up. Control of stiffness and expansion properties. By the way I got the rudder almost done. I'll shoot you a pic one of these days. PeterP

Question for you while you are on this thread,,, What stage are you to having your boat full size inside and even available for viewing? I ask you this out of ignorance of not knowing where you are at, but have a friend from a ways off that's going to be in your area in July and maybe interested in building one with strip plank..

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Bring him over Oyster. But give me a heads up so I'm home.

Thanks, he is a seasoned sailor and  has done a fine job on a smaller hull finishing it last summer. The boat building bug is a bad virus that you never really get rid of you know. ;<}.

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  • 1 month later...

Update on the sand casting. Settled on a recipe for the green sand and mixed up a batch to test the process. For this, I made a casting box out of some scrap lumber and made a mold of the same bread pans I've been using to cast my ingots.

 

Long story short, it went off without a hitch. With only two days for the casting to dry, I wasn't sure what to expect, but it was totally benign. Not a hint of steam or any pops like I've heard of when people use sand/cement mixes. Most of the concern I've had about casting the CB tip and the keel just went away. Not to say there isn't about 1,000 things that could go wrong with a stunt like this, but if a person is careful, this looks to be doable.

 

BTW, contrary to what you may see, the green sand really is green, or at least a green tint. The mix I used includes two forms of bentonite clay.......southern and western.........and they are both greenish in color. By weight, I used 90% silica sand, 4% western bentonite; 4% southern bentonite, and 2% water. Mix the sand and clay well, then add the water and let it rest overnight to stabilize the moisture content. I could squeeze a clump in my hand, but it would still crumble. I packed it in tight around the pan in the casting box. Compared to the ingots from the used pans, it came out the same shape, but with a sand pebble finish. Scuffed really well with a stiff wire brush and it should be fine.

 

Also, for some of the subsequent lead melts, I really cranked up the level of wax for the flux, and in the last batch, even let the wax catch fire once the pot had entirely melted. Once it burned off and the fire went out, I started skimming. That left me the smoothest, shiniest ingots of the bunch.

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I did 4800 lbs of wheel weights in the build of my Bristol Channel Cutter, I could have bought the keel fish for much less ( I think at the time $1200), but in the time it took me to pay off the hull and deck we were in the 1979 run up of metals, gold hit $900, lead of course followed. I poured mine directly into shallow molds that would fit in the keel, I used something like waterglass to cover the molds so little burning.

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  • 1 month later...

With work and excessive heat from the summer behind me, finally got a chance to resume work on this.

 

First, finished shaping the CB. Plans show an area towards the pivot end that is not shaped. So that leaves a transition area between the foil shaped leading and trailing edges and the unshaped board left in the trunk. . Since this will be glassed with 17 oz biaxial, I left a modest transition area below the area that will protrude beneath the slot, as I've never had much luck getting glass as thick as that to lie down and not pucker on sharp transitions. Hopefully, there won't be a great deal of drag from it.

 

Finished board looks to be very much like that on the pattern I cut.

 

Once I decided to quit on the shaping, next step was to snip the tip for the lead casting. Pondered a number of ways to do that, but in the end, used a skil saw riding against a fence to give me a square cut. Worked perfect.

 

 

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With a tip to work with, decided to have a go at casting the lead for it. Plans call for 50# of lead on the tip, and shows where the cut lines should be to get you there, but depending on the wood you use, that can and would vary. For example, my CB, made from relatively dense yellow pine should not take as much weight as lighter woods like pine, cypress or cedar. As I understand it, goal for weighting CB's like this is to add enough weight to get the CB to reliably sink, but not much more. (Curious to know how much lead Wayne added to what type of wood. Somewhere back in this thread he mentioned he didn't add enough and his board doesn't sink, so he has added a downhaul to it).

 

How much is enough? To start with, you have to get it to at least neutral buoyancy. In my case, the finished CB weighed 32 pounds, and after snipping the tip, it dropped to 30 pounds even. Interesting, but irrelevant unless you can relate that to the volume in water it will displace. I found some scraps left from shaping the blank and weighed those, then found their volume by displacement. It seems my yellow pine weighs close to .7 grams per milliliter. Doing the math, (30 pounds / .7) I figure my board will displace about 43 pounds of water. Since the board already weighs 30 pounds, I'd need to add 13 pounds of lead to get it to neutral buoyancy. How much more to sink it? Plans call for 50#,  but again, that does not specify what wood is to be used. I did the same test using a scrap of white pine and it was going to take 20# of lead to get it to neutral. So if I had used white pine, and my tip weighed 50#, I'd be at 2.5X what it would take me to get to neutral. On my yellow pine CB, I'll be at just under 4X what it takes to get to neutral. Bottom line is this CB should sink. Downside is I may be hanging excessive weight on the tip that will have to be overcome by the lifting tackle. That may come back to bite me. Is there a typical design percentage beyond neutral? Say the 2.5X number to neutral? I don't know.

 

Regardless, once the tip was ready, I could do the casting. In my previous test, I had decent luck with a greensand recipe of:

 

90% silica sand

4% southern bentonite

4% western bentonite

2% water (all of these by weight)

 

That recipe came from the supplier I used for the bentonite. The two clays are different and have different properties and uses.

 

For this casting, I put together a form and concluded it was going to take about 60 pounds of greensand to fill it. Close, it took about 50#. I was also curious if my tip was going to be close to the 50# and did some displacement tests, and decided it was going to be close. Time to give it a go.

 

Packed the form tight, then gave it 48 hours or so to dry out, then pulled the plug. Not as easy as it would seem. Despite the shaping that was tapered all the way to the tip, this did not want to release. Required putting in some lifting eyes and prying with a crowbar to get it to break free. If you do some research on casting, you find that a horizontal casting doesn't shrink much, but a vertical casting does. To allow some extra fill space, I also put in a shallow "tray" made from luan plywood to allow for some overfilling. (Casting still shrunk some, but not serious).

 

With the casting box level, I grabbed 70 pounds of ingots (didn't want to be short) and went at it. BTW, ingots melt way faster than the wheel weights did. Night and day different. Once melted, I simply ladled it into the mold, which was fast enough to stay ahead of it going solid. I did a messy job, but did it.

 

Bottom line is I am not thrilled with the results. A decent shape, but very rough texture. At first, I thought it might have been blowouts from residual water in the greensand mix, but upon closer examination, the rough texture comes from the sand. My guess is my mix was too dry and too crumbly. Did some more research and found a different greensand recipe that calls for more clay. The new one is:

 

84% sand

10% bentonite

6% water

 

As I have plenty of lead, I may give this another try with is this different mix. Cleaning this tip to a smooth shape could be done (if nothing else, fair it and glass it), but I think it can be made better. At this point, I'm more curious than anything. Part of on the job training. I also have the big keel to go, and would prefer to get a better job on it that I did here.

 

BTW, final weight of the cast tip, before any trimming of excess, was 48 pounds.

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More CB fun. Started to prep the board to receive the lifting tackle (drilling holes for lifting tackle eye screws and backfilling with epoxy), and immediately ran into a problem. Plans show an eye strap attached to the board in a vertical orientation, mating up with additional eyes on the king plank (one for the terminal end and another for the second turning block that makes up the lifting tackle). Problem is the connections on the blocks are not oriented that way. Blocks need to run on the horizontal, but with vertical straps, they would be vertical. Decided on a change in plans to a lifting eye to replace the straps for the blocks. Will probably do that for both ends.

 

Also, out of curiosity, I built a mock up of the whole CB plan. Never gave it much thought before, but this is essentially a crank mechanism.  As such, maximum torque on the board comes when the pull is at 90 degrees to the line from the anchor point of the forward block and the pivot pin. So as the board is raised, the lifting force is reduced in variable pattern until such time as the lifting tackle ever comes into a direct line of the pivot pin, at which point, no further lifting is possible. That never happens, but towards the end (about the time the board is fully retracted), the amount of force left is greatly reduced. Of concern, that is about the same time as the lead tip starts to emerge above the waterline. Ouch. Time will tell how that is going to work out. If that is not enough force, there are ways to increase the purchase, but that may also require beefing up some of the other parts. Again, time will tell.

 

Lastly, I also notice how most of the lifting strain on the CB itself is confined to the very most aft stave of the CB blank. The same one that after shaping is narrowed down to not much more than 1/4" or so along the entire length. Hopefully, I did a good job on the glue up and combined with glass, it will hold together. Not too concerned about anything, until the board is fully up. Looks to me like there is going to be some serious strain on the entire apparatus when the board is fully up. Again, a good reason to add enough weight to get the board to reliably sink, but not much more than that.

 

 

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Howard, I know it's just a mock up of the CB in the pictures showing the uphaul; however, in the last pic, it looks like the head (?) of the CB is fouling the lines where they exit the block that is attached to the CB. If your mock up is cut to the same angle/dimensions as the real CB, it looks to me like you'll need to either trim the head to a lower angle (to provide more clearance for the lines as they exit the block) or add a shackle or some other kind of extension to the head of the block so it can sit higher and clear the head of the CB. Otherwise, I think you're going to find that the lines will potentially bind or create unnecessary friction, and certainly wear faster than desired.

 

Forgive me if I'm misinterpreting the pictures somehow.

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Wes:

 

The mockup was not to scale. Otherwise, you would be exactly right. As designed the blocks should always run in a straight line, with no bends or kinks in the run to foul them. I made about three iterations of that mockup to play with the variations in leverage, etc. That one was the last one, and yes, that bend in the run was a problem.

 

One additional aspect of this mechanism, as I see it, there isn't anything in the design to limit the forward range of CB. My current boat has the old "pie slice" or wedge shaped centerboard that drops until it hits a stopper pin. The newer boats to the same design eliminated the stopper pin and simply tie a stopper knot in the lifting pennant. You drop the board until it hits the stopper knot, which carries what little weight there is on it.  For the Princess, other than a situation where the boat was being backed with the board down, that should not be a problem. I expect it will take some trial and error to find the sweet spot where the board needs to be to balance with the sail plan. That is where you would want your stopper knot. But not leaving a board like this down while backing up is important. It can kick up when going forward. It can snag and break something when in reverse.

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Howard I've just caught up on these postings regarding weighting the centreboard. My experience thus far is the board works just fine with a small amount of lead,say 10 pounds. The downhaul which at present is a bit makeshift (needs larger dia rope) pulls the board down quite easily, cleats into a auto release cleat and releases well on any impact. It is comforting to know that the board will float into the up position on entering shallow water- less to think about in times of crisis. The boat seems to have adequate ballast without the additional lead and I did worry about that high loading on the uphaul in order to fully retract a weighted board.I have run the downhaul thru the same tube as the uphaul so there is some risk of snagging and I intend to run a separate tube next haulout.Cheers and good luck

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I think it would be a good idea to limit the amount of forward travel on the centerboard. I've seen a boat (I won't say whos :) ) that was pushed backwards by a good breeze until the centerboard hit a shallow spot and rotated past vertical. Once the board was foward, it wouldn't retract because the weight of the boat was holding it. It was a bit of an ordeal but eventually all was well, and the builders have since added a definite stop so it can't happen again.

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Wayne's solution to use a downhaul to hold to hold the board down, vs. a lead weight, is an interesting thought. Most of the time the board would be up and I suspect the tension on his downhaul system will be less than what it will be to hold the weighted board up. You really only need the board down when going to weather. The rest of the time it would be up.  A means to hold it up would be necessary when it is being loaded and unloaded on the trailer. Essentially, what Wayne has is the same thing as the rudder, which is not weighted at all.

 

Wonder how much lead is used on the Princess 22 board? When I hitched a ride with Tony, his board went up and down with ease. I'm hoping this will work about the same.

 

Back when I qualified for diving, I seem to recall it took about 20 pounds of lead weights to get me, the wet suit and all the diving gear to sink. In the water, it felt like nothing. Ouf of the water, it felt like a ton.  Even when fully up, this board will only have about 1/3 of the weight out of the water. Hopefully, in practice, it work fine. If not, it wouldn't be a huge ordeal to build another board with less weight on it. I've always intended to build a spare anyway, in case anything happened to this one. It can be roughed out, then snip the tip once we get a feel for how well the first one works. Over the long haul, getting this dialed in will be important.

 

The CB on my existing boat is 180 pounds of cast iron. Even when submerged, it is heavy. The lifting mechanism is a 3/8" rope pennant on a straight pull. A strong guy can do it in an emergency, but the normal way is to take three wraps around a sheet winch (installed for that purpose) and crank away. It is not easy and is a strain. And it is always hanging there.  The new board design replaces the cast iron with a glass board laid up in a mold. The new board, including the lead to sink it, only weighs about 50 pounds. Seems to work just as well and is a whole lot less strain on everything. That would be the goal for this boat.

 

For a trailered boat expected to operate in shallow water, a CB is the only way to go. But they do have their issues. Even more so if you have a high profile design such as this one.

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