wbta - tunnel sterns

8/11/2019 WBTA - Tunnel Sterns

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http://www.wbta.co.uk/tunnel-sterns

Tunnel Sterns

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 andI 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 theumpire 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 ofthe 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 toprovide 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.

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The problem with this wholesale 'carving out' of the underbody, is that i t reduces the volume of

the vessel just where it is most desirable to support the crew in most craft and clearly increasesthe 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 li feboats 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


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see:

http://rnli.org/aboutus/lifeboatsandstations/lifeboats/Pages/The-fleet.aspx

The proportions for these higher speed tunnels is suggested by Dave Gerr to be as below.

In 3 D

Rules for tunnel sterns

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.
http://rnli.org/aboutus/lifeboatsandstations/lifeboats/Pages/The-fleet.aspx


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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.
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wbta - tunnel sterns

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