An Interview with Dr. Uli Remmlinger - A Modern Study of the DelFt Yacht Series

Dr. Ulrich Remmlinger is a German-born mechanical engineer, fluid dynamicist, sailing charter captain, and lifelong sailor who has over the past 20 years set his sights on creating a new sailboat VPP (velocity prediction program) based on the open-source Delft Series Yacht Resistance Data. Cracking open the data and leveraging the power of modern computers, Dr. Remmlinger used advanced statistical techniques in order to build a better model for the wave-making resistance of sailing yachts. I recently interviewed Dr. Remmlinger about his work, his general thoughts on sailing, and the ever-debated question of what makes a “good” boat.

Dr. Remmlinger’s papers and software programs are located here, for free: http://remmlinger.com/.

Dr. Remmlinger’s UliSail results were recently validated in XFlow with good results. XFlow CFD results (velocity shown here) obtained by Mikko Brummer on a Dehler 33 matched up well with Dr. Remmlinger’s software predictions.

Background on the Delft Series Data:

TU Delft, in the Netherlands, began studying real-world yacht hull resistance data in their test tank in the 1970’s using scaled models of sailboat hulls. The purposes of their tests was to determine the wave-making resistance of different sailing yacht hull geometries, in an effort to understand and predict the forces that will be required to drive a sailboat hull. The tests have been updated several times over the years to reflect developing trends in sailboat hull design. Some of the original research can be read here: Geometry, Resistance, and Stability of the Delft Systematic Yacht Hull Series

1. Where are you from?

Dr. Remmlinger: I am German, born in Heidelberg, and I studied in Germany. From 1996 to 1999 I worked for General Motors in Detroit. After that, I returned to Germany.

2. These questions have to do with your background in sailing:

When, where, and why did you originally start sailing?

Dr. Remmlinger: Even as a little kid, I was attracted by water and used everything that could float. My parents did not have much money, so I needed to wait til I was 13, when I got a kayak and at 16 I got a little sailing dinghy. I sailed with my friends on the river Rhein.

Dr. Remmlinger: Later on, in my first job with Lufthansa, I lived in Hamburg and I learned how to sail a regatta.

You mentioned that you were a charter captain at one point. Can you briefly describe your experiences being a charter captain? What did you learn from being a charter captain?

Dr. Remmlinger: For two years, from 1985 onwards, I tried to make a living as a delivery skipper and charter captain. I worked for an agency in Hamburg. I sailed a 33ft boat from Florida to Hamburg, a 45ft from Taiwan to the Malakka strait, in December from the Baltic Sea to Gran Canaria and from there to the Azores. Besides that, I know practically every island in the western Mediterranean and sailed in Turkey, Greece, and Tunisia. I learned, that the difficult problems are never caused by the yacht, it is always the people. Sailing for an agency, I could not pick the crew, I had to get along with the people that were sent. I had difficult times, when I had to calm down conflicts between crew members. But I had also great times with crews, where we cared for each other in harmony (I would not want to miss these happy times). After the two years, I realized that if you live a life on the water, you get lonely. When you come home after 3 months, "home" has changed and people have carried on with their lives. So I decided to stick with my education as an engineer and get a job with the industry. I still organized charter tours for many years, but I picked the crew by myself.

What is your involvement with sailing now?

Dr. Remmlinger: I am now 74 years old, and I live at lake Constance in the south of Germany. The sailing world has totally changed in the last 30 years. It is less adventure and more money. Here at the lake, it is impossible to get a berth for your boat. I thought about getting a yacht when I retired from work, but I gave up, because of the berthing problem. I am a member of the local sailing club, who exists for 50 years now and secured his berths, when they were still available. I take care of the club's own charter boats, an Albin Express and a H-Boat. These are Scandinavian designs that are robust enough for charter. I can sail these boats single-handed, or with wife and grandchildren, and I am as often on the water, as possible.

3. What did you do in your professional life? From your website, I noticed that in addition to your sailing publications you've published several papers on automotive subjects as well as fluid dynamics and hold several patents. Can you detail what your background and research is?

Dr. Remmlinger: I studied mechanical engineering with a deeper involvement in fluid mechanics. My first employment was with Lufthansa, developing maintenance procedures. I realized then that I would rather work in design and development. I went back to university, did some research and earned a Ph.D. in fluid mechanics and thermodynamics. During those 6 years employment at the university I built among others a wind tunnel. After my two years break, when I came back from sailing, I was looking for a job with the industry. Germany is not a country, where ships or airplanes are built, instead the most exciting jobs were offered in the automotive industry. So I forgot about aerodynamics and joined Mercedes-Benz in Stuttgart.

4. Why did you decide to start studying sailing more formally? Your research is very impressive and the results you've published ultimately led you to developing a VPP (velocity prediction program). What led you down this path?

Dr. Remmlinger: I run my first program in 1971 on a mainframe at the university. Ever since, I thought it must be possible to do the trial and error in yacht design in the computer, instead of doing it on the water. In 1987 when CAD-programs arrived on the market, I tried to write a program, that can automatically design the lines of the hull of a yacht. I could use the Mercedes computing center at that time. In 1995 I bought my own PC. After work, when other people watch movies, I worked on my research and on programming. I still enjoy this today and I find it relaxing. In 2006 I presented my results at the HPYD in Auckland. The paper is on my website. After 2006 a great amount of new tank test data became available. I had to revise my initial assumptions, go step by step through all modules of my initial program and adapt it to the new level of knowledge.

5. On your web page, you mention that you prefer a fast yacht for fast voyages. Many cruisers don't take into account the speed of their boats, or don't speed to be an important quality in a cruising boat. Why do you think having a fast voyaging boat is important?

Dr. Remmlinger: It depends. When you are on a longer voyage, it is important that the crew is in a good mood and the moral is high. If there are favorable winds, and you are directly heading for your destination, it does not matter whether your speed is 7 or 8 knots. Everyone will be happy. Things are different, when you have to tack against strong winds and tide, and you make no progress at all towards your port of call. Or if you have to get away from a lee shore. Or if there is hardly any wind, and you are running low on diesel. Therefore, I recommend a yacht design, that produces a high VMG for the case of little wind as well as for a wind of gale-force. A high VMG (velocity made good) boosts the moral and adds safety.

6. What other qualities do you think makes a boat good for cruising and voyaging?

Dr. Remmlinger: Soft movements and no hard pitching lets people sleep well and is good against exhaustion.

…All material has to be robust and withstand long exposure to salt water. This has to be proven on the water, not just a promise in the catalog.

…On a long passage, everything that can fail, will fail. Do not use fancy constructions that no one can repair. Even without electronics, it must be possible to sail the boat safely. At one time all my instruments failed in the middle of the English Channel with traffic from all sides. I solved the issue after several hours by installing a new wire from the (-) pole of the battery to the ground on the switch panel. The time in between was a challenge.

…It must be possible to sail the boat and enter the harbor, even after a break-down of the engine.

…The attitude "If this happens, I will call the coast guard" is not good seamanship.

…A fast(!) draining cockpit with protective coamings.

…A self steering system that works without electricity.

…Complete and knowledge based maintenance, before(!) you leave the harbor, including new antifouling.

7. In your 2016 report Bare Hull Upright Resistance Prediction Based on the Delft Systematic Yacht Hull Series you published a very interesting diagram (Figure 5) of the degree of correlation between hull geometry parameters and amount of wave-making resistance (in layman's terms - the factors that describe an easily driven hull). Do you think that the results in this table can be used to evaluate how "easily driven" a hull is?

Dr. Remmlinger: In the end, you can not change just one parameter of the hull, without modifying several others. You have to run the program to get the effect on resistance. It might be possible to do a systematic variation of a single parameter and plot it against the calculated resistance. I have never done this. In my 2006 paper I used genetic algorithms for finding the optimum.

8. Again, in reference to Figure 5 from your 2016 report, I was surprised to see that many of the factors that create drag at Froud numbers < 0.4 begin to flip and start to reduce drag when speed increases to numbers > 0.4 Froud number. This seems to suggest that one can optimize a yacht hull for slow speed or high speed. What do you think about these results?

Dr. Remmlinger: You are right. Beyond Fn=0.4 the hull starts planing, or at least it starts climbing the bow wave. An optimal form of a planing craft looks totally different from a hull for slow speeds.

9.  (Related to #9) I've read elsewhere that Cp (prismatic coefficient - fine or full ends) is the most important value for making an "easily driven" hull. Your results suggest that Cp is only a minor contributor to hull drag. Factors such as Beam/Waterline length, maximum draft of the canoe body and where it is located, and hull rocker exit angle are far greater contributors to the drag a hull creates. Moreover, as mentioned earlier, these important factors can produce less drag and high speeds and more drag at low speeds. What do you make of these findings?

Dr. Remmlinger: My answer to #7 applies. It is practically impossible to overlook all complex interactions and predict the outcome just by looking at diagrams. You have to run the program. To find optimal combinations is only possible by trial and error. The best method for this trial and error are the genetic algorithms.

10. Do you feel that the results of your research suggest anything about the trend in modern sailboat hull designs (more beam, shallower canoe bodies, and wide transoms)?

Dr. Remmlinger: As I have mentioned above, the sailing world has changed a lot. The preferences today are extremely diverse. The good thing is, that sailing can offer something suitable to everybody.

There are first the charterers. I have first-hand experience in the club. When I was young, sailing was the one and only passion. Today, it is one of many activities in people's spare time, and it is not the number one. Therefore, they charter maybe twice a year, and they never acquire the routine of an experienced sailor. They do not need all the possibilities to trim the sails and also no fractional rig. A robust mast-head rig will do. Everything has to be absolutely foolproof.

Then there are the boat owners on Lake Constance. They have huge boats (45 ft.), but only sail to the middle of the (lake). Then they go swimming and make coffee, sunbathe and sit in the cockpit. They require boats with lots of space, large windows, the most trendy style, and the boat has to look expensive.

Next we have the regatta sailors. They appreciate all the trimming possibilities, but first of all, the boat has to have a rating, that gives them an advantage over the competitors. The task is, to beat the handicap-rule. I could use my VPP to discover unrealistic ratings in the ORC-rule, but I find this misleading.

Another group likes fast sailing at sea. For them a yacht like a Class 40 (like your description in brackets) is best. They wait for fair weather and enjoy an exciting ride on a reaching course. They can take a video, how they pass all the other slow yachts and can send this video to their friends. In the evening they are back in the harbor. A side note: The VMG of a Class 40 is not significantly better than that of more conventional boats.

And then you have those, who are on a circumnavigation or who live permanently on board. These people do not need any advice. After 3 month, the sea has taught them the important rules. Because these people are a small minority, they have a problem finding suitable boats and equipment.

The market for used boats is huge. I think most people can find proven designs, that suit their needs.

My next step will be, to incorporate my new VPP into a genetic search algorithm. I do not expect a breakthrough new design. Rather a quantitative ranking of good designs.