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Redesigning the Dillon Mini Tunnel

The original Mini Tunnel was built in 2006, but is now owned and raced by Megan Halver. In an effort to enhance performance and safety, I launched this redesign project in the fall of 2010. The idea is to deepen the tunnel in order to raise the bow higher off the water, and hopefully increase aerodynamic lift.

Here is the original configuration: The tunnel is four inches deep from the transom forward to about 2/3 the length of the boat.

The first step was to strip the pad right off the sponson.

In the next step I created a pattern for the new tunnel side. The new tunnel will still be four inches deep at the transom, but will increase to nearly seven inches up forward.

This is as far as I got in the fall. Waiting for spring 2011 to continue.

April, 2011. Spring is here! But the mornings are cold, and as you can see there's still some snow in the woods.

Here I've added the new lower sides and some of the basic structure for the new sponsons. Not actually installed yet, just held in place with a few screws.

I figured this all out on one sponson first, then duplicated it for the other sponson.

These are upside-down views of the bulkhead where the tunnel will be deepest.

As you can see, I've stripped off the original sponson pad.

The new tunnel side overlaps the lower half of the original.

The new lower side is attached to the existing side just under the rub rail, then angles away from it so that the original pad width will be preserved.

The bulkheads will also be extended and new stringers installed (not shown is this drawing).

For most of its length, the new sponson side is let into a notch in the original side. The notch was cut with a rabbit plane. Toward the stern, the new side will lay right against the original.

Here's a view of the new and original sides.

In the aft-most sections, where the boat will feel the most punishment, stringers are added to reinforce the sponson pad.

Additional partial bulkheads will be added forward to shore up the pads in those less critical regions.

Just about ready to disassemble it all and then glue it on permanently.

Typically, I would bend stringers along the framework, then bend plywood over them. But in this case I installed the plywood first. So, instead of bending in battens at this point, I chose to glue in a series of small glue blocks.

Over most of the hull there is not necessarily any advantage to this -- it's a bit tedious. But the sponson side, as it approaches the transom, has considerable twist. Twisting a stringer into postition here would be a bit of a challenge. The glue blocks were especially helpful there.

The spray rails must be installed on these same twisting sponson sides, but they can't very well be built with glue blocks.

So, I built them up from four layers of 3/16" stock. The layers were glued together while clamped to the sponson side. The resulting assembly contains all the twist and other irregularities present in the sponson side.

After being cleaned up and shaped on the workbench, this spray rail is ready for installation.

One screw secures it at the transom. The rest is all clamps.

I was able to glue on a lot of pieces with Titebond III. But there were a bunch of pieces that really needed epoxy.

A couple of batches of epoxy glued in all these parts, plus the same set on the other sponson.

One new sponson pad cut and temorarily screwed on. The other soon to follow.

The sponson pads are installed, and the screw holes and seams filled with epoxy/microballoon sanding filler.

Sanding it all out.


Clear epoxy on the pads and extended tunnel sides. Paint on the sponson sides.

Just a little more sanding and it will be ready to set up again for racing. The season starts in just a few weeks.

Megan's dad, Dave Halver, came by to pick up the boat. The bunks on this trailer don't quite fit between the sponsons. But good enough to get it home.

Here Dave (left) confers with Gary Treanor, who was on hand to pick up the original Pro Vee, which he recently bought.

Megan going gunwale-to-gunwale with Tony Fortuna in his Pro Tunnel. Looks like the Mini Tunnel is getting lots of bow lift.

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Building the original Mini Tunnel
Redesigning the Mini Tunnel
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