By Nick Newland.
After the article on Juana Maria, the shallow draft double ender, in the last issue of Soundings, I began to look for more information on tunnel sterns as this seemed a possible solution to the tricky problem of powering a double ender.
I had a 1927 book on the design of Small Oil Engined vessels by Walter Pollock which had some profile drawings of a few tunnel sterned vessels and offered some rules of thumb for tunnels and I remembered that there were a few wooden boat articles on tunnel sterns and a very good article with a lines drawing in Watercraft (number 40) on Consuta the umpire launch.
It seems that tunnel sterns have come a long way since Consuta and have become an accepted feature in medium speed craft such as the modern RNLI lifeboats. Advantages claimed for tunnels include higher efficiency due to a combination of reduced shaft angle and increased propeller size and pitch (leading to greater propeller efficiency). These advantages come at a cost, in particular higher construction costs of the tunnel itself and the need for bigger reduction gears on the engine to take advantage of the prop
I have arranged this article roughly in chronological order starting with Consuta.
Consuta was designed in 1898 her principal dimensions are LOA 51.5 ft., draft 22 inches, prop 30 inch dia 40 inch pitch. To accommodate the large prop, the whole of the after body is sculpted to provide a tunnel for the prop the tunnel rising above the waterline and then sweeping down to the waterline at the stern.
The above sketch I hasten to add is not Consuta but is a representation of the type – Consuta's hull is rather more graceful see Watercraft article for lines.
Pollock's book provides only profile views so there is no evidence as to whether the whole afterbody was sculpted as Consuta's is or whether the props ran in a tunnel. For what it's worth here is a composite sketch of the Pollock stern in profile.
The problem with this wholesale 'carving out' of the underbody, is that it reduces the volume of the vessel just where it is most desirable to support the crew in most craft and clearly increases the surface area and hence skin friction.
William Atkin started to tinker with this problem in 1919 by putting a box keel into the hollowed out afterbody in the manner of the Seabright skiff. By 1942 he had perfected the process to produce the Rescue Minor excellently written up in WoodenBoatMagazine number 189 of March/April 2006 by Robb White who built a boat around the RescueMinor design.
Atkin designed these boats to be made from plywood so it is a very simple shape with flat sections. A crude representation of the bottom looks like this. A feature of the design is that the top of the tunnel at the transom is considerably above the waterline. This is commented on adversely by Naval Architects for going astern, but Robb White particularly draws attention to the astern performance as being excellent.
Dave Gerr designed a 40 ft launch on similar lines called Summer Kyle written up in WoodenBoat number 121 . Both these designs can be seen in Google Images - type in "tunnel stern boat design" or for Gerr's design "Summer Kyle"
The RNLI had experimented with powering lifeboats from about 1890 and had incorporated tunnels in three single screw steam driven lifeboats of 1898. The single screw Motor/sailing lifeboats of the 1931 Liverpool type had its propeller in a true tunnel concentric with the propeller.
Modern lifeboats still utilise tunnels but have evolved to suit the needs of the much higher speeds required whilst still being compatible with shallow draught. The stern of the Mersey Class lifeboat below seems typical of the current fleet although the new Shannon class has a waterjet see:
The proportions for these higher speed tunnels is suggested by Dave Gerr to be as below.
In 3 D
There are several sources with different rules:
Pollock suggests that prop dia < draft x 1.6
Angle twixt buttocks and the tunnel roof should be 7 to 8 degrees for high speed craft and 10 to 12 degrees for semi planning. In all cases the change of slope must be fair.
Note that Pollock drawing slope is 17 deg or so for displacement craft.
Tip clearance can be reduced if tunnel is radiused to the prop suggested 5 to 10% clearance.
All tunnel interior surfaces must be free from clutter and shaft struts should fair into the tunnel surface
Building tunnels in wood looks challenging and maybe the best way would be to build the boat without the tunnel then bolt in a GRP tunnel before cutting out the hull. Purists look the other way.