Trimaran - Especially Searunner - Owners

[longjonsilver]

Thanks Roy. Your sailplan (except for the main) is exactly what i had reasoned would be good for a Searunner. Its good for confirmation/
jon

[Roy M]

WILDERNESS update:

The mast has been installed, but there is still a lot to do on it. I'll send photos once I have the boom installed, furlers and halyards completed. I am still waiting for the stays (headstay, staysail stay and insulated backstay) to be finished. In the meantime I've got galvanized holding the mast up fore and aft. The radar is installed, as well as various nav and deck lights. Sequences being critical to progress, I'm simultaneously doing a bunch of smaller details, getting ready to strip the decks of nonskid from the cabinsides to the outermost edges of the deck. I will later sand the hullsides to the waterline in prep for painting the finish in May.

I sent off my deposit for sails and a dodger and Bimini. This is getting VERY exciting.

I have just learned about a modification on the rudder skeg, called a tubercle. These are bumps on the leading edge of the skeg which create bands of vortices (tiny spinning swirls of water) that reduce drag and cavitation on the rudder when it's going really fast. You can see them on the forward fins of whales, dolphins, even on the leading edges of bat ears. Somehow, they reduce drag and noise of bodies zooming through wind and water.

[Roy M]

Tubercles:
https://biekerboats.com/project/rudder-tubercles/

https://en.m.wikipedia.org/wiki/Tubercle_effect

[longjonsilver]

Originally Posted by Roy M

WILDERNESS update: I have just learned about a modification on the rudder skeg, called a tubercle. These are bumps on the leading edge of the skeg which create bands of vortices (tiny spinning swirls of water) that reduce drag and cavitation on the rudder when it's going really fast.

Thanks Roy. Very interesting

Are you planning on digging out of the skeg or adding to the leading edge?

[Roy M]

Several years ago, before I had any GPS or advanced rigging, I was surfing a big swell in strong winds off the Southern California coast aboard WILDERNESS. One swell, in particular, made me accelerate down the long face at a speed I had never experienced before. The German impeller-type cable speedometer actually pegged for several seconds at the gauge's maximum of twenty knots. The sensation was giddy, yet terrifying. The wheel felt spongy, the rudder wasn't solidly responding to my control. I had never felt this phenomenon before or since. Later, more experienced helmsmen explained this condition to me as "cavitation", when the velocity of the rudder speeding through the water actually created a low pressure at the leasing edge of the skeg, so great as to suck air at the transom, leading to a spongy feel of the helm.

The consequence, had I sustained that speed, could easily have developed into a loss of steering control such that we could have actually broached, suddenly turning sideways in the swell, and potentially capsized.

The experience got me to thinking that there might be something I could add to the rudder or skeg to prevent this. I was thinking of a wing tip or a "spoiler" at the air/seawater margin.

The other night I was sitting at my favorite bar, relating the memorable experience and reliving that scary moment. Chris Winnard, a monohull racer of considerable renown, said one word, "tubercles", then went on to explain the concept. In Seattle, where he is from, the Bieker Boatyard has been experimenting with adding these "lumps" on the leading edge of the rudder foil to create a controlled eddy or vortex to interrupt the formation of air bubbles on the rudder surface, whether caused by actual cavitation or by sucking air from the surface. I'd like to learn more about this effect, not necessarily to go faster, twenty knots is MUCH more than I'm comfortable experiencing again. It fascinating that Mother Nature has been using this effect for millions of years on all sorts of critters that move fast in air or water.

[Cadence]

Originally Posted by Roy M

Tubercles:
https://biekerboats.com/project/rudder-tubercles/

https://en.m.wikipedia.org/wiki/Tubercle_effect

Thanks for the links!

[catsketcher]

In my role as a Science teacher, I try to keep up to date with innovative thinking. Biomimicry, was a great book that tells us that we can learn huge amounts from nature, but I am sceptical about tubercles.
Humpbacks are not the fastest or most manouvrable sea creatures and similar large predators, like sperm whales have smooth leading edges on the pectoral fins, as do tuna and mackerel and dolphins. Often specialised adaptations are produced by other factors such as sexual selection and linked genes.
I think putting tubercles on a Searunner is going to be hard. The rudder is skeg mounted, which means the humpback fin is already not equivalent. Also getting the exact tolerances seems impossible when you need a fluid dynamics degree to understand what is happening. How do tubercles work with a skeg?
I would think the best way to improve Searunner steering at speed is to remove the inboard engine's prop and shaft which lies in front of the rudder. Also the minikeel is pierced with a long slot which sucks air down a massive channel and deposits this in front of the rudder too. Also another method would be to extend the stern, and then mount the rudder in a cassette so that she is underhung rather than transom mounted.
My cat is nicely steerable up to very high teens, but she has outboards and daggerboards which are a snug fit in the case. She has balanced rudders.
Putting tubercles on the Searunner will not address the major issues any high speed tri has with highly aerated flow caused by the inboard and large turbulent centreboard slot. A simple fence may help but I think that little will change with tubercles whilst the rest of the boat aerates the rudder's flow.

cheers

Phil

[Cadence]

Originally Posted by catsketcher

In my role as a Science teacher, I try to keep up to date with innovative thinking. Biomimicry, was a great book that tells us that we can learn huge amounts from nature, but I am sceptical about tubercles.
Humpbacks are not the fastest or most manouvrable sea creatures and similar large predators, like sperm whales have smooth leading edges on the pectoral fins, as do tuna and mackerel and dolphins. Often specialised adaptations are produced by other factors such as sexual selection and linked genes.
I think putting tubercles on a Searunner is going to be hard. The rudder is skeg mounted, which means the humpback fin is already not equivalent. Also getting the exact tolerances seems impossible when you need a fluid dynamics degree to understand what is happening. How do tubercles work with a skeg?
I would think the best way to improve Searunner steering at speed is to remove the inboard engine's prop and shaft which lies in front of the rudder. Also the minikeel is pierced with a long slot which sucks air down a massive channel and deposits this in front of the rudder too. Also another method would be to extend the stern, and then mount the rudder in a cassette so that she is underhung rather than transom mounted.
My cat is nicely steerable up to very high teens, but she has outboards and daggerboards which are a snug fit in the case. She has balanced rudders.
Putting tubercles on the Searunner will not address the major issues any high speed tri has with highly aerated flow caused by the inboard and large turbulent centreboard slot. A simple fence may help but I think that little will change with tubercles whilst the rest of the boat aerates the rudder's flow.

cheers

Phil

I totally agree with you. Halving owned an SR. Also it seemed like lift by the pectorals fins. My SR had none.

[catsketcher]

I am not having a dig at Searunners per se. A sailing friend of mine found much the same occurence on a much later designed Aussie trimaran of 10.6m LOA with an inboard diesel. At about 15 knots the helm went spongy and uncooperative. I remember losing control of my Twiggy when she hit high teens. Her daggerboard case was rectangular and used to gurgle a lot but she had no diesel. It would have been interesting to make a nice plug on the board to keep the air from escaping out the case bottom and see if that made things better. It certainly does in dinghies.

In the 90s I used to see two 12 metres (the 1987 defender sisterships - Kookaburra 1 and 2) being used as corporate training boats. One had a diesel with a saildrive and a folding prop and the other had no engine. (The one with the motor would tow the other). To get them to be the same speed for match racing, they bolted a two by four slightly longer than the folding prop diameter, to the back of the keel of the motorless one. Then they sailed the same speed. So a prop shaft and prop cause the same drag (and maybe the same aeration) as a piece of two by four the diameter of a prop - at least for 12 metres. I think cleaning up down under our boats is the very first thing to better control.

[Roy M]

I am pleased to read the many responses to the Searunner discussion. This tells me they have not lost their appeal. Also, informed and thoughtful conversations often lead to productive changes in older technology.

March 21st will be the 44th anniversary of the launch of WILDERNESS in Marina del Rey, Los Angeles. I was one of the earliest Searunners built using West System epoxy resin, incorporating the tricks developed by the Gougeon brothers, Jan and Meade, such as plywood diesel tanks, graphite powder admixtures below the waterline for abrasion resistance on cobble beaches, graphite tow to reinforce the sliding hatches and cabintop, as well as unique details such as t-beam wing decks outboard of the sterncastle to increase stiffness and improve appearance in the forty, which made it more spacious aft. I even played with a substantive change in the centerboard, which I will be installing later this Spring. So, the idea of playing with tubercles is not going to be too radical, and may, or may not produce moderate changes. So, as my favorite pastime and ongoing experiment, we'll see what develops, and I'll pass it along for review.

[longjonsilver]

It would seem to me that tubercles would be used on airplanes and high end racing yachts if they worked. Perhaps the easiest thing is to get a Flex-o-fold prop? And rubber seal the centerboard trunk, as described earlier in this thread.

[Roy M]

Status Update for WILDERNESS:

The new mast is installed, the headstay furler is also installed, as well as the insulated Ham radio antenna/backstay. The staysail furler has arrived and is waiting for me to install the deck padeye and stem cable support. Supply chain chaos has been in full effect as I waited for a couple months before one became available. Same with the Scanmar Auto-Helm self steering wind vane. Scanmar informs me that this was the last remaining unit on the planet. I still have to build the trim tab for the rudder. This is very exciting. There's lots to do before the new sails arrive (more supply chain chaos), including installing halyard clutches, the boom, completing the traveller, mainstream and other control lines. And just to add some ordinary maintenance, I found a patch of dry rot in the decking of the starboard forward wet locker, requiring repairs. I'm trying to finish all the exterior projects before hauling out this summer.

[Randy]

Anyone interested in a Searunner 40? There's one on ebay for probably less than 1 more day. It was discussed here a couple of years ago by Max.Jester.

https://www.ebay.com/itm/29489396815...mis&media=COPY

[Buzzman]

Originally Posted by Randy

Anyone interested in a Searunner 40? There's one on ebay for probably less than 1 more day. It was discussed here a couple of years ago by Max.Jester.

https://www.ebay.com/itm/29489396815...mis&media=COPY

Wonder why it's for sale...???

[Randy]

Since he lives in Pennsylvania and the boat has been kept in Half-moon Bay, maybe a better question might be why he bought it. Dreams vs reality it sounds like