Double-Enders vs Transom Sterns
Another opinion by Graham Byrnes, N.A.
One day back in the 1970s, during my double-ender design phase, a beautiful canoe yawl sailed into our local harbor. The skipper turned out to be Nick Clifton, Single-Handed Trans-Atlantic Race competitor and circumnavigator.
After meeting, I proudly showed him one of my double-ender plans and while studying the design he said, “Be very careful when designing a double-ender. I have been pooped many more times in double-enders than transom sterns. I would add some more buoyancy in the stern.”
He went on to explain that as a double-ended hull surfs down a wave, the more steeply curved buttock lines act like a speed brake, even though gravity is doing its best to force the boat beyond its hull speed. This action sucks the stern downward, reducing freeboard aft, sometimes allowing a breaking sea to roll over the deck.
I took Nick’s advice seriously and modified the stern. I voyaged in that boat for more than a decade, crossing several oceans and many seas. I had a lot of time to watch my boat behave and ponder his words of wisdom. I would dearly love to meet up with Nick and thank him.
Another boat that I sailed and had many sea miles aboard (not my design) had a very cute little stern. She was a joy to sail until you pushed her past her hull speed, at which point she got very cranky. I would look at the bend in the tiller in amazement as I struggled to keep her going straight. Of course, the obvious tactic is to slow the boat down; but sometimes it’s hard to slow down enough to regain good handling.
In order to compare speeds of different sized boats we need to consider the speed to length ratio (SLR). The SLR is the speed divided by the square root of the waterline (WL) length. The square root of a 16 foot WL is 4 so a speed of 4 knots is a SLR of 1 which is a medium speed for a displacement hull. Six knots would be a SLR of 1.5 which is fast and above the myth of the max hull speed of 1.34. If the quarter beam buttock at the stern is flat enough, say 4 degrees above horizontal, SLRs above 2 are achievable—which is considered very fast. This is impossible to achieve in a double ender. Note that we are considering flat-water speeds, not momentary flickers on the GPS while surfing. I consider SLRs above 3 to be fully planing and rarely achievable on most sailboats.
For every speed there is an optimum fore-and-aft position for the center of gravity (CG) and an ideal Prismatic Coefficient (CP), which is the fineness of the ends of the boat. Essentially the higher the speed, the farther aft the CG should be and the higher the CP should be. This does increase the wetted surface and frictional drag but the wave-making resistance, which becomes the principal drag at speeds above SLR of 1 increases more slowly to the point that the transom-stern boat will have less total drag.
For SLRs of 1 or below, slender symmetrical hulls with low wetted surface area are more efficient without doubt. Kayaks, canoes and pulling boats are prime examples. However, for a sailboat I believe that is the wrong end of the performance spectrum to design for.
There are other bonuses for the transom-stern boat. As the boat heels, the center of buoyancy moves farther to leeward which increases the righting moment, allowing the boat to carry more sail. The extra sail area usually offsets the wetted surface-area penalty. The transom-stern boat also pitches less in a head sea than her symmetrical rival. The reason is that for the bow to lift, the stern has to depress. If the stern depresses too easily the bow will lift more and will have farther to fall. The greater volume of the stern dampens the pitching motion.
Another myth is that a symmetrical boat becomes better balanced as the boat heels. In fact, the opposite is often the case. Being more tender, she often heels farther, causing the center of effort to move to leeward. This creates a larger turning couple (rounding-up force) with the drag of the hull to windward trying to resist forward progress. It is standard design practice to increase the lead of the center of effort over the center of lateral resistance for tender boats, and to decrease it for stiffer boats.
Because of these attributes, I believe the transom-stern boat to be more seaworthy than its fine-ended counterpart. (Yes, I know that the Sea Pearl and many other double-enders have done a lot of open-water miles under sail, but I think that with a transom they might have done them faster and safer.) I recall one race where I was invited to helm a newly launched Core Sound 17 for the owner. There was a nor’easter blowing down Core Sound, giving us a wonderful wing-and-wing sail with surfing surges to 10 knots. We looked back at the nearest Sea Pearl getting smaller astern. Suddenly, she was lying beam-to and we discussed whether we should go back to check on them but saw that there were a number of boats near them. After the race they reported that they were having difficulty handling the boat and were afraid of broaching as they surged past hull speed. At about the time the Sea Pearl was reefing, the CS-17 owner looked at our speed and asked if we should reef. I looked quizzically at him and demonstrated steering the boat with just my forefinger as we surfed. He never brought it up again.
Because we are talking about small craft, I have sailed many races and miles in my Core Sound series (having full sterns) going head to head with a lot of Sea Pearls. Seven Great Races from Oriental to Beaufort, NC with seven wins. Six Everglades Challenges (ECs) with two overall wins, two course records and four Class-4 wins. My partner Alan Stewart and his father have sailed four ECs in a Core Sound 20 and have two Class-4 wins and hold the current Class-4 course record. This in an event where 50% of entrants fail to finish. I point out the above record only to show that transom-stern boats have a proven record as top finishers in a race where a large variety of conditions test both boats and sailors. •SCA•
www.bandbyachtdesigns.com
As first appeared in issue #103
Terrific article, thank you. Delighted that after nearly 5 decades I am still learning things.
Roger C.
This article convinced me to take the plunge from a Sea Pearl 21 to a Core Sound. My Core Sound is still a work in progress but I am looking forward to comparing them.
One other aspect which goes to usage. A double end is length that is not very usable. What I mean is that it is length in your garage, on the trailer, when you maneuver in tight places, and marina fees (if you're into that sort of thing). But it is not very usable, because even as a short guy there's not much room in the last few feet aft. It's hard to lean over and reach the outboard, for instance, as not only is it narrow, it's also tippy.
My Sea Pearl 21 is, well, 21 feet, and my Core Sound 20 is, well, you get the idea. The actual size difference of them sitting in my driveway is enormous! Yes, the Core Sound has a foot more beam, but that width translated all the way to the back, instead of the Sea Pearl tombstone transom, is a huge difference when sitting at the helm.