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The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
I am very aware that I asked for advice, and I appreciate it. It has influenced what I will end up doing, even if it has not altered my view of the physics and mechanics involved here. The aggravation that I was referring to was the self-imposed aggravation due to my over analyzing this.
I was playing loose with terminology by referring to bulkheads. The forward head wall and front of the galley cabinet do provide some stiffening to the hull. A year ago I used a stud finder to identify all the stiffening structures I could locate. I'll do that again to make sure I did not miss some forward of the keel.
I have not been able to find any sling points marked on my hull. If you could measure where they are, perhaps running a tape measure forward from the transom along the rubrail, that would be helpful to me.
Unfortunately, Rick, my boat is an hour away and tucked away for the winter. It's likely I won't see her again until spring. Somebody here besides me should have these markings (they're right under the rubrail) and can measure them for you.
I think these kinds of discussions are the best on the forum. If we all drank the same Kool-Aid, we wouldn't learn very much. Thanks for bringing this up.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by RhythmDoctor</i> <br />[quote]<i>Originally posted by Steve Milby</i> <br />I'll probably move the stands to go "with the crowd," though I have yet to see any argument for it based on engineering principles. When I estimate the loads, the moment arms, the bulkhead locations, etc. everything tells me that FOR THIS PARTICULAR MODEL OF BOAT the stands would be more effective just behind the keel.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">OK, let's talk "engineering principles"... The keel is your fulcrum between the weight forward and the greater weight aft. The stands in question (#3 & #4), whether just forward or just aft of the keel, should bear virtually no load. The keel is bearing it for that area. You said the forward edge of the keel is providing the support--that would be based on how you blocked it. If you block it to be supported toward the aft edge of the keel, there will be more mass forward of the fulcrum to balance the mass aft of it, so the keel will be bearing even more of the total.
If you place the #3 & #4 just barely aft of the fulcrum and adjust them so they are significantly reducing the load on the stands at the stern, the resulting load on #3-#4 will be disproportionately high, due to the extremely small lever arm from the fulcrum. Supporting that mass from the longer lever, #1-#2, will place less stress on the bottom of the hull. Then the #1, #2, and #5 are used to trim the boat fore-and-aft for proper drainage, making sure to leave the majority of the weight on the keel. (Lower #5 as you raise #1 and #2, and vice versa.)
Some boats I've seen where the keel wasn't properly supported so the amidships stands were supporting too much weight suffered from "oil canning" at those stands. Certainly if you have any grid system or stringers toward the stern, place #1 and #2 accordingly, but as far aft as you reasonably can. The transom is your best stiffener, while the cockpit and cabin structures strengthen the lever arm. Then adjust #3 and #4 to be snug to the hull, but <i>not supporting any weight</i>--if they do, they are lifting that weight off the keel more than off #1 and #2.
I think if you ask that yard, this is what they'll tell you.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Stinkpotter</i> <br />A retraction: Looking at the profile drawing, I think I'd keep the keel blocked forward to balance the forces between the keel and the hull. <hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote"> I agree, for these reasons: With a wing keel, and especially a swept back one like on the C250, you need to be careful to avoid torquing the lead. So you need to make sure that the contact point with the blocks is along the centerline of the keel, and not torquing the wings by having one side or the other higher than the center (which can happen easily if the ground is not perfectly level). Also, if the aft part of the keel bears significantly more compression than the front, you are torquing the aft part of the keel (because of the unsupported "cutaway" created by the swept back design), which is not good. For that reason, the point of contact between the block and the keel is toward the front of the keel, but it is well spread out over the forward 75% of the keel's base. It is not concentrated at any one point.
Don't worry, the center stands are not bearing any significant weight. I tightened them enough to make them snug, and make the chain taut. Rather than lifting the boat up, I like to think of it as the boat pinning the legs down onto the plywood so that they don't shift on the ground.
I did have several experienced guys helping me with all of this both last year and this year. They were powerboat guys, so not used to dealing with the specifics of a wing keel, but they were looking things over closely (since they're my neighbors) and were satisfied with my setup. I also have several sailboat buddies on my row, and they've all seen my setup last year and this year. They've all made suggestions that I incorporated (such as the rubber pads). This is very much a do-it-yourself yard (private club), so people check up on each other.
So if you agree that #3 and #4 shouldn't bear some of the load of #1 and #2, then getting back to Steve's point, there's no reason not to have them forward of your horizontal pivot point where they will resist pivoting (which #5 will not do very well).
But it seems we've beaten this into a coma... Sorry.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Stinkpotter</i> <br />So if you agree that #3 and #4 shouldn't bear some of the load of #1 and #2, then getting back to Steve's point, there's no reason not to have them forward of your horizontal pivot point where they will resist pivoting (which #5 will not do very well)...<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote"> I'll kill the comatose patient here, because I think you frame the issue well enough for me to be brief.
The middle stands don't bear a load when the boat is at rest. They will bear a significant load if Mother Nature tries to push the boat over toward the side, though the leverage to resist this is better with the stands slightly behind the keel.
If the boat pivots around the keel, neither location of the middle pads would help much because the motion of the hull would be tangential to the pads (see my graphic). Pads #1 or 2 are the only ones that would resist pivoting around the keel because they are far enough aft to have a significant moment arm - and the proper orientation - to resist it.
Just to close out this thread, today I moved the middle pads in front of the keel prior to pulling off the outboard motor. My desire to conform for purposes of avoiding potential questions from my insurance carrier overrides my assessment of the physics of the whole thing. In other words, it wasn't worth the aggravation.
Now I can focus on replacing the gudgeons, erecting my mast crutch, and dropping the mast.
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.