Ericson 25, Centerboard, Basic Observations

The profile of the centerboard’s trunk (as opposed to the centerboard itself) is usually depicted as:

In fact, it looks like this:

The trunk is, of course, open to the water except for the 7″ x 3″ area depicted by the red stripe faintly seen in the picture above (the red stripe at the bottom of the diagram, not the red triangular area at the top). Here, a substantial piece of fiberglass once covered the opening .  Many boats have lost this cover piece over the years.  Whether this creates a minor turbulence problem is open to debate. One assumes that the boat’s manufacturer had justifiable reasons for incurring the extra cost involved in closing this small part of the trunk. Here is a picture of where the missing piece once lived, looking up and forward. The green dots indicate the outline of the missing piece.

The dotted green outline shows the shape of the missing piece which was probably integral with the deteriorated horseshoe-shaped strap shown. John Cyr points out that the strap is there to reinforce structural rigidity of the high stress area of the hull trunk joint (i.e., the front), and that these stresses include (but are not limited to) the side loads and twisting of the board as well as the pivot point. The red dotted and dashed line shows where the forward edge of the centerboard comes to rest when fully retracted.

Below is a photograph by Mark Kelsey of the centerboard trunk of his boat, Ruby. Here we see the piece still intact, although loosened from the hull:

Getting back to the sketches: the stop block (below indicated in red) prevents the centerboard from swinging too far forward:

Also notice that the rectangular trunk tube (that extends up into the mast compression box) slopes to accommodate the cable’s movement when the centerboard is lowered.

This shows the centerboard fully retracted into the trunk:

The cable tang on the centerboard point moves approximately 13.25″ from the fully lowered to the fully retracted position.

When lowered, the angled bottom of the centerboard is perfectly horizontal: this leads one to speculate that the NACA foil profiles (more about this later) of this foil might originally have been computed parallel to the bottom edge as shown below, and not perpendicular to the straight back edge. This is an unverified assumption and may be a moot point: the two foil profiles are quite possibly indistinguishable.

The tang on the centerboard attaches to a keel cable with a twist pin shackle. The top end of the cable then attaches to a block with a shackle:

This block in turn is rigged as follows:

The centerboard weighs 150lbs. We can do a rough estimate of its volume by comparing it to the volume of a rectangular slab of wood 61″ x 11″ x 2.5″ = 1677 cubic inches. As there are 231 cubic inches in a gallon, this equals about 7.25 gallons. Because this rectangular slab of wood doesn’t a have a foil profile, the centerboard’s volume is probably on the order of 60% (?) of this. We therefore get 4.3 gallons as the approximate volume of the centerboard. When underwater, the centerboard will displace 4.3 gallons of fresh water weighing approximately 36lbs. This reduces its effective weight underwater to 114lbs. In seawater, the displaced water would weigh about 2.5% more, but seeing our estimate is very approximate, we can safely ignore this.

On a side note, a centerboard of this shape and size would float if it weighed less than 36lbs.

The effective weight at the tang will be 114lbs provided the center of gravity of the centerboard is directly under the tang position, as Roy Sugden points out. If the center of gravity of the board were to be further out from the pin, the effective weight of the board will be greater than 114lbs. We have not had an opportunity to determine the position of the center of gravity and these calculation assume, for the moment, that it is directly under the pin.

The block in the mast post confers a mechanical advantage of 2:1, hence the effective weight of the centerboard is 114 / 2 = 57lbs. However, the amount of line needed to haul the centerboard from the fully lowered position to the fully retracted position increases from 13.25″ to 26.5″.

The Barlow 16 winch with a 10″ handle confers a mechanical advantage of 8:1, hence the effective weight of the centerboard now reduces to 57 / 8 = 7lbs. So, if you’re grinding away on the winch with the standard setup, and it feels as if you have much more or much less than a Yorkshire Terrier on the other end, there’s probably something amiss.

This estimate ignores the water resistance on the centerboard foil as it is hauled up: so the effective weight of the centerboard will probably be greater — this still has to be determined/calculated.

Here is the original page provided by Ericson on the centerboard.  The highlighting of certain items of interest is not original:


CENTERBOARD ASSEMBLY

FIXED-KEEL MODEL: 
The Ericson 25 is available as a fixed keel mode) with 2,500 lbs./1,134 kg of lead ballast and a draft of 3'10"/1.17 m. While the fixed-keel model may be legally trailered, it cannot be easily ramp launched. Accordingly, most owners prefer to keep it in a slip or on a mooring.

KEEL CENTERBOARD MODEL: 
The keel-centerboard model incorporates many of the qualities of the fixed-keel Ericson 25, combined with the ease of towing and launching offered by a centerboard boat. Stability is provided by two sections of lead with a combined weight of 2,500 lbs./1,134 kg which are fiberglassed low in the hull on opposite sides of the centerboard trunk, lending additional strength to the trunk itself.

The centerboard is constructed of a steel web core with lead plate weighing 150 lbs./68.04 kg attached to prevent positive buoyancy and is covered with fiberglass over foam.

The centerboard itself does not provide ballast, but acts as an underwater foil section to increase draft and improve windward sailing ability. Accordingly, even if the centerboard is damaged or lost, the stability of the Ericson 25 is not compromised.

The centerboard is secured on the underside of the hull near the forward end of the centerboard trunk by a 1/2" (12.7 mm) stainless steel pin which passes through a U-shaped reinforcing plate which is held in place by two stainless steel screws. Before each launching, the pin should be checked for condition
and security.

The centerboard is raised and lowered by a stainless steel cable connecting the trailing edge of the board to a 2-to-1 block and tackle which is carried up an integral fiberglass tube inside the mast post support and out the after side of the mast to a winch. Access to the entitle system is possible by removing the inspection access plate which forms the back of the mast support
post.

Care should be exercised when lowering the board to prevent possible damage to the centerboard stop. Always lower it slowly. 
When sailing in shoal waters, extra caution should be used to prevent the board from grounding, particularly laterally, as this may damage the board and pivot pin.

Under certain conditions, particularly when beating into a heavy chop, it is possible for small amounts of water to be forced up through the centerboard control tube. This is to be expected as a normal condition since the opening is necessary to allow access to the hoist mechanism for repair and maintenance.

The centerboard is likely to have some side-to-side play within the centerboard trunk which is necessary to allow ease of operation when raising and lowering the board. Under sail this movement will be less noticeable, and at anchor or while powering, it can be eliminated by raising the centerboard.