Transom Extensions

Adding buoyancy and planing surface

Written by Mats Vuorenjuuri

From Issue Small Boats Annual 2019 July 2018

The tops of the extensions, squared off inside of the diagonal side panels, serve as steps for climbing aboard over the stern. The bright finished squares of decking are hatch covers.

Many small skiffs would benefit from an extended waterline and increased planing surface. My own 14′9″ (4.5m) outboard skiff has deep V-hull shape that makes the ride more comfortable at high speeds in sharp chop, but the hull still weaves from side to side, drags a big wake at in low speeds, and requires a lot of power to get on plane. My boat also has a tendency to hobby-horse at high speeds; while I can stop this by trimming the motor to force the bow down, the remedy reduces top speed and fuel economy.Trim tabs could help get the boat on plane and would probably solve hobby-horsing, but they would also add vulnerable electronics to the boat and are not a visually pleasing solution. When I saw the clever structure of transom extensions in the Tango Skiff 13, I decided to design and add extensions to my boat. Before construction, I tested the theory, temporarily bolting two kitchen-cabinet doors on the transom to extend the bottom surface. Although they didn’t add buoyancy, they surprisingly made a significant improvement in slow-speed behavior and also took care of the hobby-horsing. So the project was a go.

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Comments (5)

Vince Collis says:

July 3, 2018 at 2:30 am

A good idea well done

Reply
tim ferrell says:

July 3, 2018 at 10:33 am

The Bolger Sneakeasy design, among others, developed these some time ago, and they are said to work very well. Glad to see they are still relevant.

Reply
Tim Murfitt says:

July 3, 2018 at 5:33 pm

A good idea and something I will consider. I am guessing that the bottom of the extensions followed the bottom of the hull and if that is the case do you think there would have been a benefit in sloping up the extensions by say 1″ at the back? That way you would still get most of the low speed benefit but have less drag at the top end.

Reply
Nicholas Blake says:

July 6, 2018 at 1:06 am

Strangely enough, my grandfather had a riverboat here on the Mississippi with a flathead V-8 for power. For years it cruised at about 9.5 knots. Then one year he decided to add swim ladders on the back. He built them so they could lock down in place, went up the river and forgot to pull them up. He knew something was wrong only by the speed at which he traveled and the level nature of the boat at cruise. When he tested the boat’s speed, it had increased to 14 knots! All this to say, it is funny what changing what is happening at the transom can do to a boat’s behavior.

Reply
1. Christopher Cunningham says:
  
  July 6, 2018 at 1:52 am
  
  Your grandfather, Nick, missed his chance to patent the idea. If the ladder created some submerged steps, they might have generated some lift to improve the boat’s trim. The idea has been developed (and patented) by Peter van Oossanen, a Dutch naval architect. He calls it a Hull Vane. It can increase fuel efficiency and, in rough water, reduce pitching and rolling. I’m thinking I might carve some foil shapes on my swim ladder.
  Christopher Cunningham, Editor, Small Boats Monthly
  
  Reply