centre-board cheek clearances

[b.o.a.t.]

I have never had cause to think about pivoting centre-board cheek clearances until now.

What is reasonable for a board with cheek thickness around 39mm,
and a cheek radius from pivot to outer edge of around 240mm.

Both the board cheeks & the bearing face of the case are LDPE.
The boat is expected to occasionally take the ground on mud or sand.
The cheeks will be set back about 20mm into the case from the lowest edge once I add hardwood strips to the edges.
There will be some form of fairing flaps across the case opening - probably light rubber or neoprene secured under ally or s/s grounding strips.

070720131818sm.jpg 070720131819sm.jpg 070720131820sm.jpg

My concern is sand/mud jamming the boards.

[PAR]

This is a butt kicker in design. I've found you want less than an 1/8" (3 mm), but not much, so shoot for 3/32" (a slightly fat 2 mm) on each side of the board. This will keep out debris, for the most part, with a slot seal helping in this regard. Much more than this clearance and you'll offer enough room for stuff to wedge in and the board will have enough slop to bind on the case sides.

My boards usually have a well fitted "bearing" area (cheeks) and I use Teflon sheet, HDPE or UHMPE on the case sides and or board. The bearing area I make a very close fit, on this style of board/case assembly, because the lack of friction can permit a snug fit. I do reserve snug fits for weighted boards, while leaving a bit of room for unweighted. This is because weighted boards usually have tackle or a winch, but unweighted are man handled.

Slippery case sides can mitigate much of these types of problems, if the board is a very close fit. It also assumes the case and the board will not change dimensions with use. It's for this reason, I've been using more and more inert materials for boards. I just made a board with a HDPE bearing area (solid plastic), which resembled a big disk with a pivot pinhole in it. This was mechanically fastened to a 'glass board that had blind nuts buried in the laminate. The HDPE disk was literally screwed (threaded rod actually) to the 'glass blade portion. with a bedding compound between the two.

I guess this is hard to picture and I don't have images handy. Envision a 17" (43 cm) diameter disk, with flattened bottom, which matched the boats rocker and a pivot hole about an inch above the bottom, but inline with the center of the disk. This is the bearing area (cheek) and a snug fit inside the all 'glass case. Three holes drilled perpendicular to the pivot pin axis, through the HDPE receive the threaded rod. The board portion is a solid casting, with blind nuts in the locations of the eventual fasteners. These where molded with open cell foam and removed with solvent once the goo cured, exposing the blind nuts. The rods travel about half the length of the blade portion, clamping the HDPE disk on it's forward edge.

Since the HDPE is self lubricating and pretty tough, but not as tough as the case sides, it'll slowly wear over time, but a couple of decades from now, someone else can worry about the slop in the board. Lastly the pivot pin hangs from above, inside the case on two custom brackets. The board, pivot pin and brackets can be removed through the top of the case and the pivot doesn't penetrate the hull anywhere, so no leaks.

[b.o.a.t.]

Thanks Paul
I have a shade under 2mm clearance on one board, and shade over 2mm on the other.
LDPE place-mat glued to each bearing surface, so it's LDPE to LDPE contact.
Pivot bolt is 3/8".
From the sound of it, I'm on about the right track at this point.
Will start assembling in the next day or three
IMG_20130709_001502.jpg
this is the lifting tackle hardware.
Boards only weigh about 5kg in their socks & undies, so the 150kg rating on the bottom-of-the-range blocks should be plenty.

[PAR]

Rather than the polyester double braid line, I'd recommend a double braid sheath over Spectra line. Stretch is minimal. If going this line route, then large diameter sheaves would be desirable too.

Lastly on a light board like this, a straight up pull, rather then tackle is all you need. If you must, just a whip tackle (two part) so you don't have that much line to pull.

I mention this because I've done a new, 220 pound (100 kg) ballasted board for my ketch, which has made a world of difference, but I needed to haul this darn thing up. Not wanting a winch, I used a gun tackle (3 part), which still has about a 50 pound (23 kg) pull on the line. The worst part of this is I now have to pull about 4' (1.2 m) of line to bring the board from full down to full up. A single part tackle would have been on about a 15" (38 cm) pull, but I wouldn't have been able to haul it up, without the tackle advantage.

The system I use, allows a line to come through a "key hole", cut in the bridge deck. The line is knotted at specific points, full up, dead run, broad and beam reach, close reach. close hauled and light air (all the way down). This means, with no moving parts, except the line, I can pull the line and let the knot rest in the key hole slot, for the point of sail I'm on. Simple, easy, but it does have the draw backs mentioned. I have a lawn mower pull start handle on the end of the hoisting lanyard, making for a comfortable grip.

[b.o.a.t.]

Hi Paul
Planning to try a lift without mechanical advantage first.
Just two turning blocks to aim tension in the right directions.
That would be about a 500mm pull on the cord, probably about 4-5kg.

Ummed & ahhhed about spectra over the static polyester.
Couldn't tell the difference between them for feel & stiffness.
The extra strength of the spectra isn't needed.
When trailing, I'll put a pin through the CB & case to carry the weight.
Maybe even a bed under each board & let them rest on that.
The two cords ran through the little blocks about the same, so price won.

cheers

[PAR]

Stretch is the issue, more than anything else, with line lanyards. On big boards, I use 7/19 stainless, but on smaller stuff I use Spectra, as the stretch is near the same as the stainless and way better then polyester. On a casual cruiser, not much of an issue, except you'll have to adjust it's full up location, you insure it's actually full and you wouldn't be able to mark the line for various points of sail.

[b.o.a.t.]

Cut a bit off it last night.
The outside is polyester, but the core is straight fibres,
not braid, and of something which doesn't melt.
I suspect it is an aramid core of some type.
And thinking further, I paid rather more for it
than the $1.20/M sticker price on 4mm D/B Polyester.

Anyway, the cases are dry assembled ATM.
All seems reasonably well aligned. Will glue in the next day or so.
A test lift came in at 10Kg without purchase, so I will probably
put a 2:1 block in to make things a bit lighter for SWMBO.
Then I really must get the paint lifted off the boat where the cases will go.

thanks & cheers