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Positives - the draft is very shallow, the boat tracks like a train, runs level, and is very smooth (no prop tip wash thumping off the hull).  The peace-sign propeller strut is very strong with low drag.  The shallow draft and fold-down transom allows an ATV to be driven off the stern ‘tailgate’ very close to shore.  The down-pointing aft end of the tunnel helps the boat run level and does not cause prop aeration when the propeller is in reverse.  There is no propeller aeration at planing speeds either - all the water feeding the tunnel comes from higher pressure water from the bottom of the boat and not from the waterline, where air and turbulent water meet.

Negatives - A bow-steering problem in a quartering wind has been mostly fixed by changing the tunnel to all-flat between the rudders.  Cutting away some of the fine keel forward (to narrow flat bottom) there should fix the rest.

(For the evolution of the Chunnel Keels, see the Sulis page.)

LeBlanc 24 “Welcome II”

Chunnel Keel Detail

Tunnel, Shaft and Strut

Remedies -  Lighter construction for future boats if planing speeds are required.  A grid-type keel cooler in a recessed area of the keel or hull, or raw water cooling, would be good for low-drag cooling (instead of the external workboat-style copper pipes).  Having the tunnel end before the rudders - the run is flat in these shallow boats - has gotten rid of rudder stalling (see Sulis Draketail version). The increase in power and fuel efficiency with the addition of one of my Water Reactors, the hoped-for 15 knots might be achieved.   A tip-cupped BPT Concept prop from Jacques Juan, of Sanary sur Mer, France would help.  More soon....

Conclusions - I had intended this keel/tunnel setup to be used for displacement-type cruisers and workboats only, and still feel it would be best for these hulls.  I think a boat like Mai Pehn Rai could have this arrangement and a draft of only 3’ (91 cm).  And with fuel or water in the keels, stability and cruising territories (range and shallow draft areas) could be increased.  For workboats - especially certain fishing boats - a low-drag angled grate could be placed before the propeller in the tunnel to keep flotsam from fouling it - replacing the drag-causing ‘cage’ or ‘basket’ now used by many boats. 

Copyright 2001-2021 inclusive   McGowan Marine Design, Inc.

7.39 m

6.67 m

2.96 m

0.49 m

2.72 t

82 kW

Welcome III is a modified LeBlanc Brothers workboat.  Neil LeBlanc was brave enough to try my ‘Chunnel Keels’ - a long tunnel between prominent twin keels,  with very shallow draft and rudders off the keels - on his own boat.

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LeBlanc 24 with “Chunnel Keels”