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Laker SC12 Tunnel Race Boat Plans
Page Four
Sponsons

The spray rails are installed three inches above (or below on your upside-down hull) the edge of the tunnel side and extend from the Transom to Bulkhead #3. Besides getting spray off your boat (water running along your hull equals drag), the spray rails strengthen the tunnel sides.

The suggested shape of the rails is shown on the Construction drawing.

The sponson keels are attached to the edge of the tunnel sides.

If you used butt blocks to join the panels for your tunnel sides, you will need to offset the keels over them.

Like the Tunnel Battens, the Sponson Battens are 3/4" thick from the Transom to Bulkhead #4 and then are reduced to 1/2" thick.

The aft-most part of the sponson pads should be as flat as possible, and the installation of the sponson battens is a good time to start thinking about this.

With the battens nestled in their notches, lay a level or other straight edge on top of them and look for high and low spots. Adjust the notches in your bulkheads as needed -- file down those not quite deep enough; shim those too deep.

You're not looking for perfection at this point. There will be a second chance to make adjustments after the battens are glued in place.

To avoid cutting up the sponson tip bulkhead too much, only carry one of the battens all the way forward. The block underneath the middle batten (as seen in this upside down boat) is glued to all three battens.

The forward ends of the one Batten that does extend all the way to the Tip is attached with a small cleat.

NOTE: All the sponson longitudinals CAN be notched into the sponson bulkheads, much as they are notched into the plywood bulkheads and the transom. However, for APBA racers, rules require that the ends of the sponson stringers be covered by a bulkhead at least 3/4" thick. See drawing Sponson Tips.

A sheer clamp being installed.

A chine being installed.

Shave or sand the stringers flush to the aft face of the transom.

An outer transom is added to strengthen this critical part of the boat, and to cover the ends of the stringers. Hold a scrap of plywood snug against the extension of the tunnel side and trace the shape.

Install the sides.

I used butt blocks, temporarily secured with screws, to get the required length. Scarf them together if you prefer.

The side spray rails, stretching from the Transom to Bulkhead #4, are also installed three inches below (actually, above) the chine. Install them with screws driven through the plywood and into the rails.

Limbers cut into the sponson battens allow water to run to the low corner of the sponson, where a drain will be located later.

Note the filler blocks added between the Battens to provide attachment of the sponson pads.

The sponson pads going on.

NOTE: I installed a single layer of 5mm plywood for the pads on my Dillon Sport C. However, early in my first racing season, I noticed a slight "softening" of the pad between the transom and Bulkhead #5. A Batten had broken.

I added a lifting stake to shore up the pad (and added one to the other sponson as well) then continued racing, and encountered no further problems.

In spring 2013 I repaired the Batten and added a second layer of 5mm plywood to both pads (see it HERE).

Depending on how hard you intend to use your boat, you may want to consider a second layer on your boat as well, especially if you are building with 5mm plywood.


You may want to finish the bottom of your boat at this point. See the Page Thirteen.

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