Sundowner Tug Owners - Forum

Thistel Sandwich-Hold the Mayo....
05-May-11 06:56:41 AM UTC
Rick Haverstock

United States, IL

Rick Haverstock:

Hello Everyone,

In an earlier post I had mentioned investigating a rudder mod for the Mary V.. Well, it's completed, installed on the tug, and in a short cold and windy trip around our marina (1500 slips) it seems to have performed beyond my expectations. So, here is the story with pics, on the Thistle project.

I started out researching last winter on the web. You can Google Thistle rudder, fishtail rudder, mid-turn rudder and rudder modifications, and get hits on all. Far and away the best article was by David Gerr and it's downloadable for a very reasonable price. Go here for the article:

Mr. Gerr certainly seems to know his rudders inside and out and has professional experience to back it up. After reading and re-reading, I focused on the Thistle, more formally known as the MacLear Thistle rudder (MacLear was a NA and Gerr at one time had worked for him). The name came from the shape which was reminiscent of the thistle bulb.

I didn't want to replace the existing rudder, just modify it. If you've never taken the rudder off your tug, let me tell you that it's not a fun experience. I originally wanted to go with stainless steel angles, either welded or screwed onto the existing rudder. Cost factors ( boatyard welding, raw material, etc.) and less than optimal performance (see Gerr's article) lead me to toss that idea. I decided to go with the sandwich approach, making the thistle profile in fiberglass and bolting it onto the existing flat plate rudder. This thought was further refined as I went with a hardwood core (poplar), reinforced with glass cloth and many coats of epoxy resin. Since I had to make up a male and female mold to do an all-glass version, it was actually faster to make two male cores from hardwood and coat them with epoxy. If the mod screwed up performance, or made handling worse instead of better, I could always pull the thing off easily. As another bonus, if I ever had to pull the shaft, I could remove the Thistle sandwich and use the hole in the plate rudder ( you did know that's why it's there, didn't you..)

On to the pictures. First one shows the plate rudder and our tug on the hard. For those of you unfamiliar with snow, the background shows the remnents of Chicago's February blizzard. As a cautionary note, it might look like a rectangle, but it's not. The top of the plate actually matches the upward curve of the hull. Big surprise for me when I went back up to test fit but was able to adjust things. Second pic shows what we're after, a small thru section the thistle shape. The darker line in the center is a 1/4 inch piece of scrap ply, used here as a spacer to show the completed cross section (actual rudder thickness is 5/16").

A road trip to my local hardwood dealer and a few bucks later, I returned to the shop with enough board feet to make three thistle sections. Why three you might ask? Keep reading and you'll find out. The wood was jointed and planed to about 1-1/4" thickness and was both square and dead flat. The narrow boards would then be glued up to the final width.

The next pic shows the plots of both the rudder cross section and end plates. These were plotted from the tables in Gerr's article and involved enough math for me to reach for the calculator. They were plotted on 1/4" plywood and would later be sawn apart and used for patterns. The close up shows the "connect the dots" nature of the plot, kind of like plotting an X-Y axis graph if you will.

The thistle shape(or one half of it at least) was traced on the three sections and then run through the tablesaw to kerf it. The saw kerfs provided the controlling depth for the shaping and while taking some time actually saved a lot in the shaping process. Using a screwdriver and a pair of needlenose pliers, I snapped off the thin sections and did a little plane work to shape down close to the line. A 40 grit pass with a random orbital sander brought things down to the finished line. I had a devious reason for using three pieces to end up with two finished sections. My tablesaw is a little cranky on height adjustment and if I were to make a mistake on the height, better to do it on a sample piece rather than the finished version. Yeah, I wasted about six bucks worth of wood, but the anxiety saved was priceless.

The circles on the rudder are actually solid epoxy plugs (or pots), to keep any water intrusion away from the wood core. This is a great technique and in my opinion should always be used when you're bolting something to a boat that will get wet. Endplates were screwed, glued, and filletted with epoxy also. The finished rudder had two layers of 6 ounce cloth and 6 coats of epoxy resin. I thought that they would be heavier, but the whole assembly comes in under ten pounds, including hardware. Finish was the ever-popular Petit bottom paint.

I taped off the plate rudder and clamped on the skins to mark for drilling. Be advised that drilling stainless is not a lot of fun. Use cobalt bits, a lot of cutting fluid, and very slow drill speed for success. After drilling, I bolted on the sandwich, breathing a sigh of relief: the job was complete, everthing fit well, and I didn't have to be on my old knees anymore.

Our tug was launched yesterday, 42 degrees and 15-25 knot winds (we're tough in Chicago), so I didn't do much extended testing. Our marina is huge (1500 slips) and I did play around a little in the fairway on the way from the launch well to the slip. Handling is more crisp, concise, and helm response time is much quicker than the old flat plate rudder. I greased the docking single handed, even after I had overshot the approach. Was able to make slow speed steering corrections coming in straight(with a 15 knot wind on the beam)something that would never have been able to do in the past. Thanks for taking the time to read this very lengthy post and look at the pictures. I do have to make some more sea trials (when the weather improves) but it certainly looks like this one was worth the effort.

Rick Haverstock

Mary V.

Lying Winthrop Harbor, IL

Well, I did try to proof read, just never bothered to proof the title, it's THISTLE.....


10-May-11 08:45:37 PM UTC
Jay L. Butler

United States, Washington

The pictures look great Rick. I think I responded outside of this forum earlier on my thoughts regarding your existing rudder profile. Looking at the pictures again, it seems to show that the existing rudder was an incarnation having very little leading surface ahead of the pivot/rudder shaft. Looking at pictures of my rudder, it projects more forward, but not as much as had thought. For sure, the trailing edge is a lot greater and not tapered toward the bottom. When I get organized, I'll post some of my photos, also showing my stern thruster.


11-May-11 01:11:45 AM UTC
Rick Haverstock

United States, IL


Dave Gerr called it a 20 percent chord, meaning that the leading edge is 1/5th (or 20%) of the width of the rudder. Not althogther uncommon on powerboats as I have found through research. Since many of the pics were done on a wide angle lens setting, I'm sure that there is a lot of perspective distortion that's helping to fool your eyes. Re the rudder shape: the bottom is 90 degrees to the leading and trailing edge, while the top has about a 3/4 inch rise from forward to aft.


11-May-11 02:06:30 AM UTC
Jay L. Butler

United States, Washington

I was thinking about the 20% factor myself Rick, which raised my comment. Your camera wide angle must have been what was throwing me off. Anyway, great photos and good info to absorb.


11-May-11 02:13:09 AM UTC
Doug Robertson

United States, California


I seem to recall your Sundowner is a Sea Tug built by Chauson of Taipei with the Ho Hsing molds purchased at auction after Ho Hsing's bankruptcy. If so, that might account for the rudder difference Lonnie pointed out as his Sundowner was made by Ho Hsing as I recall. Your keel stock is more nicely faired before your rudder than mine, but my SD32 is just larger everywhere and bound to be somewhat different.

In any event, a great project, superb documentation and a tribute to your fine woodworking skills.

SD32/162 SEEKER Doug Robertson

11-May-11 05:18:52 AM UTC
Rick Haverstock

United States, IL

Thank you Doug. I actually spent a lot more time fooling around to develop a good game plan and researching the subject than I did building it. Total work time, excluding glue dry time, epoxy cure time, and install time was probably not more than 15-20 hours. That 5/16 stainless is really tough stuff to drill. I averaged about 20 minutes per hole (7 holes including the zinc not shown in the pictures) and went through 3 drill bits and about a pint of cutting fluid to keep the bits cool. So you could add another three hours to the project there. And before anyone else catches it, I did cut off the bolts to length later as I forgot to bring the moto-tool that day. Can't really comment on the differences between Ho Tsing and Chauson as I've only been on two other early Sundowners and they were fairly quick visits. Seeing everyone's teak framed windows does make me glad that Chauson went with aluminum though.


17-Aug-11 05:27:30 PM UTC
Chip Estabrooks

United States

Any updates on your rudder performance?


20-Aug-11 06:40:52 AM UTC
Rick Haverstock

United States, IL

Hey Chip,

Didn't mean to be a tease and hold off so long before a follow-up. Too much stuff happening this summer and things just got away from me.

Now that I have a reasonable amount of run time/docking time with the new skins, I'll say that they are a definate improvement over the stock flat plate rudder. Overall performance results in a crisp and more immediate feel to helm inputs, less autopilot hunting at cruise speed, and a remarkable improvement in low speed docking manuvers. Even at idle speeds (750 RPM on the mighty Lehman 4-banger) helm response is still good, albiet a little slow. Without the Thistle, there is NO helm response at idle. In short, yeah, I'd do it again in a heartbeat with the results I've had.

I've tried to be very objective on this whole project. It would be easy to imagine the gains based on the time/money/energy put into the project. Objectively, I would have to rate performance increase with the Thistle rudder as a +7.5 to +8.0 on a 1-10 scale. Overall, it could very well be the best $200 or so that I've spend on the tug since buying it.

Finally, a word about backing performance. I initially thought that the oddball shape would degrade backing, To me, it does not seem to do that. I've never been very good at backing this pig, and the Thistle has not improved my marginal skills. However, it does not feel like the new shape is holding me back either. I guess I really need more backing practice and work to improve that particular skill set.

Rick Haverstock