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Sport C Tunnel Race Boat Plans
Page Four
Sponsons

The spray rails are installed two 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.

Install the sponson keels slightly below the edge of the tunnel side so room is left to bevel both parts.

You may want to bevel the keels before installation. The angle shown is constant the entire length of the boat.

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

There are two sponson battens that extend from the transom to the sponson bulkhead. A third sponson batten extends from the transom to a point a few inches forward of Bulkhead #4.

Like the tunnel battens installed earlier (see previous page), the two long sponson battens are 3/4" thick at the transom, but are reduced to 1/2" thick between Bulkheads #3 and #4.

The forward ends of the long battens are attached to the sponson bulkheads with a small cleat, much like the cleat on the aft face of the bow beam.

NOTE: All the sponson longitudinals COULD 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.

A sheer clamp being installed.

A chine being installed. These can be pre-beveled as well. The angle is constant along the entire length.

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 lower sides.

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

The side spray rails are also installed two inches below (actually, above) the chine. TEMPORARILY install them with screws, then remove and put them aside for later.

Wait until the hull is turned over and the upper sides installed before permanently installing these spray 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.

The sponson pads going on.

NOTE: I installed a single layer of 5mm plywood for my pads. However, early in my first racing season, I noticed a slight "softening" of the pad between the transom and Bulkhead #5. I believe a stringer broke.

I added a lifting stake to shore up the pad (and added one to the other sponson as well) then continued racing, and encountered not problems.

In spring 2013 I repaired the stringer 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.


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

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