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We raced in Sport C OPC CBF when the class was introduced in Canada in 1992. It was a good class for newbies wanting to try racing. Commercial stock 30-40 hp motors. The class still races today in the USA but not in Canada. In the Twin City area they race small tunnels and overseas small tunnels are very popular.
We don’t have any OPC races in Canada so it’s better to have the small tunnel class fit under stock outboard and follow their basic rules.
The boat
12 to 14 ft constructed of wood or molded fiberglass or a combination of both
Homebuilt or commercial built boats allowed
Must be tunnel design...pure tunnel or Mod VP
Driver option of sitting in a seat or knee-down.
Foot throttle or crash throttle allowed
Minimum weight boat, motor and driver 500lbs
No power trim, motor must be in a fixed position

The motor
Commercialy sold 50 hp maximum....maximum displacement 43.9 cubic inch.
No motor mods allowed...motor must remain bone stock.
Gearcase can have a nose cone ( no lower water pickups)
Gearcase skeg can be enlarged

There are lots of fiberglass tunnels around, you could build your own .
Lots of motors...the 43.9 cubic inch rule eliminates the twin cylinder T750 omc.
Keep the class simple rules...no grey areas....stock means stock.

Here’s a picture of one
https://www.riverdavesplace.com/forums/ ... le.270909/

This boat is very easy to build.....family project.....get a group and build a few.
https://www.dillon-racing.com/laker-sc12/plans-m.htm

Now this is an interesting video......it dates back to the development stage of T class 2007 I believe . We were not allowed to have a race, but we were allowed to run time trials. The 60 cubic inch rule was not in place at that time. The UIM rule was 53 cubic inches maximum. We wanted to see how various engines and boat types compared including a 60 hp 59 cubic inch mercury. The yardstick was the 911 boat....70CES Yamaha critchfield. It’s performance numbers matched the top T850 UIM boats racing in Europe.
https://www.youtube.com/watch?v=Zd3o72N ... Nlcw%3D%3D

In the video
SpeedCat tunnel....700 series 70 hp mercury..Paul K
VooDoo ....650XS...Bryan G
Delta...70 hp Yamaha with a 55 hp Yamaha gearcase..Rick C
Critchfield...49 OMC...Ryley D
Critchfield...60 hp 59 cubic inch mercury ..Jimmy A
SLT tunnel...49 OMC...Steve H
Critchfield..70CES Yamaha...Jim W

I recorded they’re lap times and I put a GPS in the two boats with the best lap times.
Jim W had the fastest lap time. Ryley D had the fastest speed in the straightaway...

One thing is very noticeable about some of tunnels is the motor appears to be too high for it to pickup water for cooling.
On some of the boats the motor picks up water from the spray off the sponsons inside the tunnel. That water hits the sides on the gearcase.
https://www.youtube.com/watch?v=dS25HfgGZmI

On others they use externally sponson mounted water pickup or pickups . The side water pickups on the gearcase are sealed up and tubes run from the sponson pickup(s) to the flush inlet on the gearcase.
Here is a picture of a pickup
https://bobsmachine.com/product-tag/water-pickup/
Here is one on an F4 mercury 60 hp 4 stroke.
http://i00.i.aliimg.com/photo/v0/600897 ... t_with.jpg