Mechanical Advantage - MA
- Thread starter Hunter216
- Start date
You're probably right. Originally that's how I interpreted it, and it would make more sense to have the single line going to the centerboard at #4. Maybe someone who actually has this type of system can weigh in here.
Yes, you're right, I was thinking that 4 was the load. Think of it as a proxy for the load.
Be aware that you are likely dealing with tensions in the neighborhood of 3,000 to 4,000 lbs in the keel cable (connected to #5) - at least be thinking that way until you measure and analyze the load end of this. 70 lbs tension at lift X 16 is 1,120 lbs tension at #5 for our 150 lb board example. A 500 lb board, rigged the same way, will result in much higher loading. That may be why it has a hydraulic jack on it. Make sure your blocks are rated for the loads they'll carry. Please be careful.
Sorry - your diagram is incorrect. See original rigging drawing above. But, you're very close. @jviss is still correct. Absolutely agree that it has been, and continues to be, interesting and entertaining.
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I appreciate the words of caution, I was thinking 3/8 dyneema (although maybe too slippery to hold by hand) and this line of Harken blocks might be a good choice.
Although the board on the 216 is heavier I believe it is shorter that the 212 with a bulk of the mass near the pivot point. The keel cable is also also attached a fair distance down on the rear of the board so the lever arm (I hope I’m using that term correctly) is fairly effective. I want to inspect the board anyway so my plan is to put the boat in my wet slip slings that I can move over top of dry land, lower the board fully so I can disconnect the Sailtec unit and somehow (haven’t figured out this part) safely measure the “tension”.
Have you tried a local hydraulic cylinder repair shop? They might be able to rebuild your jack. I understand wanting to have something that you can maintain, but if it only needs rebuilding once in several years, is it worth the conversion effort? Would a preventative periodic rebuild be a better overall approach?
Sounds like you have a good plan to go at it. The fact that the attachment/lift point is further down the centerboard is encouraging. I am surprised by your statement that the bulk of the mass is near the pivot point, but your idea of measuring the lift tension should tell you what you're dealing with. Good luck and post progress pics if you go for it.
In the sample system we've been looking at, there are three distinct tensions. The highest tension is between the board and #5. Next the line between #5 and #7. Finally, the line that you actually pull to raise the board. If you are going to design for the highest tension, and just use the same material for the rest of the system, that's good. But, start with determining the max tension during the lift, and go from there to pick your materials/components. Remember that your attachments have to carry these loads too.
Sounds like you have a good plan to go at it. The fact that the attachment/lift point is further down the centerboard is encouraging. I am surprised by your statement that the bulk of the mass is near the pivot point, but your idea of measuring the lift tension should tell you what you're dealing with. Good luck and post progress pics if you go for it.
In the sample system we've been looking at, there are three distinct tensions. The highest tension is between the board and #5. Next the line between #5 and #7. Finally, the line that you actually pull to raise the board. If you are going to design for the highest tension, and just use the same material for the rest of the system, that's good. But, start with determining the max tension during the lift, and go from there to pick your materials/components. Remember that your attachments have to carry these loads too.
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If the original hydraulic system failed, & I owned the boat, I would either -
1) rebuild the hydraulic system, using better parts &/or beef up the original parts.
2) replace the hydraulic system with either an Acme screw or a Buttress screw.
3) replace the hydraulic system with a worm drive, as indicated above.
4) replace the hydraulic system with a rack & pinion, with a ratcheting pawl on the pinion.
...But then, I have easy access to a machine shop, so my best solution may not be the best solution for some other people.
1) rebuild the hydraulic system, using better parts &/or beef up the original parts.
2) replace the hydraulic system with either an Acme screw or a Buttress screw.
3) replace the hydraulic system with a worm drive, as indicated above.
4) replace the hydraulic system with a rack & pinion, with a ratcheting pawl on the pinion.
...But then, I have easy access to a machine shop, so my best solution may not be the best solution for some other people.
Also the skill to utilize all the tools there in!
I’m not stuck on using block and tackle, just stuff I have some experience with. I’m going to research the other options (thank you and @Will Gilmore for the suggestions) as I do have some concern about having spaghetti on the cockpit floor.
The Sailtec was a viable solution for Hunter but the inter web has lots of examples of how difficult and $ they are to maintain so I’m going to put some effort into finding a viable alternative.
I get your logic of a periodic rebuild but there is this. On one occasion I started pumping away and nothing happened, I managed to clear the problem but NOT a good feeling! The point is this can happen at any time with no visual or for that matter operational clues. Board stuck in down position probably isn’t a crisis but it’s not how I like to have things on anything I operate.
From what I read having local hydraulic companies ie NOT Sailtec work on them is dicey. Even newly “Sailtec authorized repair centres” have messed them up.
Don’t get me wrong the ram is a good solution for this boat IF it’s working.
I don’t have a good pic of the board mass but the section that’s always inside the trunk is almost the full width of the slot. The lower section is foiled so much thinner. I think I recall the designer made it that way so still sort of stable board up (draft is about 3.5ft board down)
Thanks for all of your sage advice. I’m thinking maybe you wear an iron ring?
Hard to find anyone competent to work on your stuff these days. Really worries me.
I get it. If the ram isn't reliable, you need to get something that is. A buddy of mine and I ran aground on a falling tide yesterday. He has a heavy dagger board on a winch. If we couldn't get the board up, we were in deep kimchi. It was starting to blow pretty good and getting rough. Would have probably broken the trunk and sunk after a few hours of pounding. All that to say, board stuck down can be an issue depending on where you are and weather.
Iron Ring? Maybe, but I did stay in the Holiday Inn Express... Don't know about "sage advice". I just don't like watching people get hurt or throwing a bunch of time and money into a black hole. The more we exchange, the more I'm sure that you're not one to fall prey to either. Please share what you come up with.
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Not to do this 
to this thread but I came across this. It helped me better understand some of the diagrams and terminology.
https://roperescuetraining.com/physics.php
to this thread but I came across this. It helped me better understand some of the diagrams and terminology.https://roperescuetraining.com/physics.php