Stern Winglet / Hydro Pad on a Traditional Sloep-style Displacement Boat – Need Feedback

[Olivier Balay]

Hi everyone,

A few years ago, I built a traditional 6-meter displacement boat with my father. It’s inspired by Dutch sloep designs — round stern, shallow V-hull, central diesel engine (13hp), and about 2.10m beam.

The boat is stable and beautiful, but we’ve hit a speed wall: it won’t go beyond ~6.2 knots, no matter the load or prop adjustment — which I understand is due to the waterline and hull shape (classic displacement behavior).

Recently, I’ve been studying ways to extend the effective waterline length and reduce drag at the stern. I came across some modern boats that use what I’d describe as a “stern winglet” or “hydrodynamic pad” — basically a foil-shaped extension at the transom, aligned with the waterline, to support the stern and clean up the wake.

I’m currently building one from 50mm PU foam, shaped and laminated with fiberglass. The idea is to make it removable for testing, and later permanently glassed once performance is confirmed.

So far I haven’t found any detailed projects like this online. I’m looking for feedback from anyone who has tried something similar.

I’ll be happy to share photos and results as I test it. Any experience, sketch, or input is greatly appreciated.

Thanks in advance from Brazil!

 

[seasquirt]

Hi Olivier, nice looking boat. Could you tell us about the weight of the boat, motor type and size, its rated horsepower, working rpm used, and the size and pitch of the propeller, and what you have tried in order to gain more speed. Being a 'traditional' looking hull, it probably was never designed to get to double digit speeds in Km, Kn, or Mph, and it wouldn't look amiss with a mast and sails. Looks like room for a bigger prop, but it may be a waste of effort if the hull is already running at its optimum speed. Increased power may just make it bog down more than going forward faster. The last pic shows wasted energy, with all those bubbles and froth in the quite short wake, and the wake wave being pushed out. Maybe weight loss surgery on the boat is required.

 

[gonzo]

Those pads are called hobbles over here. Many old boats were retrofitted with them in the past when lighter and more powerful engines became available.