kubark42

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About kubark42

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  1. kubark42

    Modern wiring standards

    From experience working with custom brushless motor controllers and other power electronics, if we design the logic correctly it works out fine. What typically happens is that when first energizing a circuit there is an inrush current which can cause voltage to drop outside spec (typically +-10% of nominal voltage). This current is usually associated with either getting things moving-- such as a macerator motor-- or charging up capacitors. The inrush current can be 10-100x the nominal current, so once you make it through this period it's smooth sailing. Modern power electronics are pretty good at this, and are tolerant of inconsistent line voltages. Regarding the discussion of transients, it feels like there are several different understandings of what that means. This is why when discussing power loads, it's usually important to have a graduated scale of timeframes. For instance, with our 1000V @ 1A DC/DC converter we gave ratings for peak continuous load, peak 1 minute load and peak 100ms load. Each was different, and since we were using 400' of 28ga wire it was critically important to understand how each affected the system. So when designing for the windlass/macerator/transfer pump, the transient we want to watch out for is the inrush current. This is on the order of tens or hundreds of ms, which really isn't long enough to substantially affect the conductor temperature.
  2. kubark42

    Modern wiring standards

    This is what I'd like to understand via this discussion. Wiring needs have changed substantially, to the point that in some circumstances 10x less conductor surface area is required for the same end goal. Systems which in the past required a constant DC voltage to perform acceptably can now make up for onset voltage drop with onset current increase. I think we can consider wire as just another component in a design, and overspeccing anything leads to reduced system performance, higher costs, more disruption, etc... Even if we don't frequently treat boats that way, they are sensitive to weight. Trimming a few dozen pounds of superfluous wiring + insulation is a worthwhile goal.
  3. kubark42

    Modern wiring standards

    Is there any reason to suspect this can't be solved by proper design specs? Cables can be manufactured with reinforcement if it's a pulling strength problem. If the strength problem relates to fatigue from vibration, cars and planes are manufactured with 24AWG wiring and that holds up against far worse vibration than a boat is likely to see. With proper crimps, the insulation can become a crucial part of the stress relief at the connector. EDIT: Interesting conversation at stackoverflow about wire thickness vs. vibration. The accepted answer seems to be that thinner wire having a high number of strands can win out over substantially thicker wire.
  4. kubark42

    Modern wiring standards

    I was looking at the marine wiring spec for cabin and nav lights and noticed that it's largely built around spec'ing for incandescent lights. Of course, the modern trend is toward LEDs which consume 10x less and so the required copper is arguably 10x less. Has there been any discussion about updating the specs? How much of them is based on an allowable voltage drop and how much are based on other things, such as corrosion allowance, strength, vibration resistance, etc?
  5. kubark42

    How to properly design a rudder cassette?

    I understand the point about worst-case scenarios, but you'll never be going fast anywhere if you have 1.1G of decceleration. That's only 2 seconds from 17kts to a full stop. That kind of deceleration would throw people off the boat. The assumptions which led to this calculation are probably dependent on things such as the rudder staying fully submersed and the boat speed not changing much due to the drag, like a big heavy keelboat going down a steep wave. Not saying that the forces aren't high, just that IMO the original cassette design likely wasn't for anything approaching 17kN. The aluminum plate -- 3/16" with 100% of the lateral load going through the pivot point-- can only hold 2.5kN or so of shear before failing. There are two plates, so we get to 5kN if the load is perfectly distributed and the aluminum has undergone no corrosion. 5kN doesn't even get to a design speed of 10kts, a speed we exceed on almost every sail. Considering that F-boats have a reputation of being extremely well built and sea-worthy, and that Ian Farrier designed them to sail in coastal waters not deep bluewater, it seems fair to say that the ABS standard is not appropriate for a light trimaran. It also seems fair to say in light of comments here that the aluminum plate was not particularly overbuilt.
  6. kubark42

    How to properly design a rudder cassette?

    No titanium welding required, this is a pure sheet with holes cut via water jet. The allure of this project is that I can experiment with titanium for (almost) the same amount of machining effort as aluminum. I wouldn't call myself a metal or composite guy, I've got some experience with both but not enough experience with either to optimize a design. I've got some materials at hand and am working with them. If after analysis they won't work for the job, then I'll have to find a different solution. Tough to tell without the design requirements. The role of the titanium in this structure is 1) to give me a very flat surface to work against, so I can experiment with CF layups and not have to worry about putting holes in the wrong place and 2) to have something which won't explosively and irrevocably fail. If the titanium gets wrenched out of alignment, I can probably work something out to get back home. I'll read up on the ABS guide @12 metre linked to, and keep in mind that it needs some SF for the fast boats. I'm almost scared to calculate what the forces are for fear of discovering that even the original two aluminum plates are completely underspec'ed! I'm still hazy on whether the principle design feature is to be stiff enough or strong enough. I'm leaning toward strong enough based on what people have said here, but @Jaramazasks a fair question about wobble. UPDATE: I'm sailing tomorrow and there's some good wind so I'll be able to get the boat in stride. I quickly went over the ABS guide and calculated .132*1.33*1.1*1*.3*10^2 ~= 6kN at 10kts, and .132*1.33*1.1*1*.3*17^2 ~=17kN at 17kts. I suspect that these numbers are the worst case scenario for deflection at peak lift. I wonder if it makes sense to design around that, since the boat is very much not going to go that speed if the rudder is deflected over like that. The whole boat weighs something around 1500kg, so a 17kN force would be over 1G of negative acceleration. Ooof.
  7. kubark42

    How to properly design a rudder cassette?

    Done. The rudder is still in good shape, so I just need to rebuild the cassette. Fortunately, I was able to get the original factory dimensions for the cassette cheeks and those are integrated into the CAD. It just seems like a waste of an ME degree to clone when we could challenge past assumptions.
  8. I've been looking around for information about how to (re)build a rudder cassette. I have an F27 trimaran and the aluminum cheek plates are showing their age. Too many years of salt in proximity of stainless have led to the rudder pivot point corroding enough that it needs some fixing. In fact, it's clear that this particular pivot point has already had replacement aluminum brazed on so it's clearly not a question of "it worked for 25 years, so don't fix a design which isn't broken". Plus, where's the fun in just redoing what was already done! I CADed up a replacement made out of titanium, with carbon fiber stiffening reinforcement along principle stress axes. I have aggressively removed sheet metal material where I feel that there is little in the way of stress. The problem is that I have no idea what forces I'm designing against. When I consider how I've sailed the boat, and how little torque is actually transmitted through the rudder, it seems that the original design was massively overbuilt. However, I suspect Ian Farrier designed it for substantial abuse, like sliding down a steep wave in heavy seas, and so I've never come close to pushing the rudder to structural limits. Here's the original (colored lines are for diagramming how the uphaul/downhaul lines run): Here's what I've got so far, with a proposed 0.04" titanium sheet replacing the original 0.19" aluminum sheet: https://cad.onshape.com/documents/1c5a22f10a09417ce492546d/w/6205af308346e6f338843ca6/e/478becd53cd210a595543a50. That's a huge difference in thickness! Of course I'm making up for a lot of it with carbon fiber, but I'm still skeptical that I can just magically get away with using 6x less material. Still, it's easy to imagine that in a day and age of CNC water-jet cutting and (relatively) cheap carbon fiber we can do a lot better than 1989. Does anyone know of a resource for spec'ing design loads? What is the rudder cassette's main job: to be super stiff for sailing feedback or to be super strong for taking that once-in-a-lifetime hit?
  9. kubark42

    Sealing dyneema rope in a thru-hull

    Thanks, that was exactly what I needed! Combining this with the underside picture @Overboredshared, I think I have a very clear idea of how to do this.
  10. kubark42

    Sealing dyneema rope in a thru-hull

    Maybe there are too many assumptions going on here. It's not a thru-hull for a stopcock, it's just a 5/32" hole which is "through the hull" as opposed to "through the deck". Dunno about other boats, but a little bit of weepage through a downhaul mounting plate while heeled to port won't sink a trimaran. It'll just make more of a mess than I want to clean up every few weeks.
  11. kubark42

    Sealing dyneema rope in a thru-hull

    I learned this is called a "soft padeye". There's a bunch of people discussing it, but not much feedback on sealing them. In my system, the soft padeye is mounted on a plate in a daggerboard trunk, and winds up being a few mm above the waterline when the boat isn't sailing, and just a few mm below when it is. I guess I'll play around with butyl, toiletbowl rings, and other flexible sealants and see what works.
  12. I have a dyneema strop (like what Ropeye makes) which is going through a thru-hull. What is a good way to do seal this? Epoxy seems too stiff (it's rope after all) and glues in general won't stick to dyneema. For info, when the boat heels the thru-hull will be below the waterline by an inch or two.
  13. kubark42

    Ultralight windvane selfsteer project

    As a mechanical engineer (learned SolidWorks back in the 90s), that you just did that beautiful and professional assembly on an iPad blows my mind. Next thing I know, we'll be doing symbolic math on iPhones.
  14. kubark42

    Ultralight windvane selfsteer project

    Cool. I'm intensely interested in the results of bike cables after years of exposure to marine environment. Anything which is a consumer product and still survives on a boat is awesome. I wonder if hydraulic bike lines would also function as easily, or perhaps even better, than cables. You get the nice fact that at the low speeds you'd see in a self-steering vane, they are practically frictionless around tight bends. Of course, you might also get some real nastiness around the seals.
  15. kubark42

    Ultralight windvane selfsteer project

    Bike cables aren't made with the marine environment in mind. They might or might not withstand it very well. I'm betting @SySunday isn't the first person to use them, so there's probably a body of evidence as to how well they perform.