boom preventer
- Thread starter CapnGL
- Start date
Cutting holes in one's boat is intimidating, I know, so we tend to over-analyze the heck out of their placement. Nothing wrong with that. But all that trepidation goes away when the bit finally digs in.
AFAIK, Mac never offered midship cleats on any of their trailer sailors, so the job falls to the owner, if they want them installed.
thanks for the help... looks like i have a job to do over the winter. Glad I have a shop big enough to store it inside so I can do these things.
I asked about mid-ship cleats for spring lines and such once, and the factory - well, Mike Inman if I remember rightly - basically said they didn't reinforce the hull or deck anywhere along the sides for cleats strong enough for anything on the scale of docking, as I was wanting. He told me that I'd have to reinforce the deck or hull to put something very strong on there. Perhaps a preventer won't require such a strong cleat?
Hey justsomeguy, it just meant that the stresses put on the thinner fiberglass required for holding the boat at dock, such as using spring lines, could rip the fiberglass. The existing cleats are in reinforced areas, or areas with thicker fiberglass cross section. I didn't design the boat, just relaying what he told me.
My interpretation, based on mechanical engineering background and experience with many materials, including fiberglass...
A backing plate helps by spreading the direct load on (in-line with) the bolts across more area, thus less force at any point. This helps with direct pull against the bolts. The fiberglass is also springy that way because the flexing fiberglass is placed into tension. Tension works, even for a thin cross-section. However, backing plates don't stop twist or moment (torsion) created by the pulling ropes. They just relocate it, essentially. So if the fiberglass is too thin, you can still tear (shear) through the fiberglass with a strong twist, or press (punch) through it even with a backing plate. (Think pushing down on one side of the cleat, not pulling up.) Reinforcing the area around a cleat helps spread THAT type of load and transfers them into tension on the flat thin part of the fiberglass. Tension works in fiberglass in a thin cross section, but shear and point loads do not work as well.
That said, we aren't talking about a little load from the evening breeze while docked and sipping a glass of wine. They, presumably, designed the different parts they chose to include - like the few cleats they did - with high winds, rough waters, and shock loads on them. So if you're talking about a small load, it probably isn't an issue. But that really wasn't what I was referring to...I had a bigger load in mind. And someone had said it could be 800 poounds or something, so it sounded pretty hefty to me, like we were talking about some larger loads, and shock loads on top of that.
I don't know how much load a preventer will actually get, shock load being considered. And I'm sure there are some places in the boat that are thicker and will hold more load, take more abuse, than others. The angle of the pull matters, as do other factors. There are many on here with experience that say it isn't a big deal. I would listen to the input of others. I was just passing along a comment from a factory-related source about adding load-bearing cleats to unreinforced parts of the boat.
My interpretation, based on mechanical engineering background and experience with many materials, including fiberglass...
A backing plate helps by spreading the direct load on (in-line with) the bolts across more area, thus less force at any point. This helps with direct pull against the bolts. The fiberglass is also springy that way because the flexing fiberglass is placed into tension. Tension works, even for a thin cross-section. However, backing plates don't stop twist or moment (torsion) created by the pulling ropes. They just relocate it, essentially. So if the fiberglass is too thin, you can still tear (shear) through the fiberglass with a strong twist, or press (punch) through it even with a backing plate. (Think pushing down on one side of the cleat, not pulling up.) Reinforcing the area around a cleat helps spread THAT type of load and transfers them into tension on the flat thin part of the fiberglass. Tension works in fiberglass in a thin cross section, but shear and point loads do not work as well.
That said, we aren't talking about a little load from the evening breeze while docked and sipping a glass of wine. They, presumably, designed the different parts they chose to include - like the few cleats they did - with high winds, rough waters, and shock loads on them. So if you're talking about a small load, it probably isn't an issue. But that really wasn't what I was referring to...I had a bigger load in mind. And someone had said it could be 800 poounds or something, so it sounded pretty hefty to me, like we were talking about some larger loads, and shock loads on top of that.
I don't know how much load a preventer will actually get, shock load being considered. And I'm sure there are some places in the boat that are thicker and will hold more load, take more abuse, than others. The angle of the pull matters, as do other factors. There are many on here with experience that say it isn't a big deal. I would listen to the input of others. I was just passing along a comment from a factory-related source about adding load-bearing cleats to unreinforced parts of the boat.
But to use as a preventer, I can't imagine loads exceeding much over 100 lbs.
"Someone" is overestimating?
See post #35...
I don't presume to know everything, so can't speak with authority to disregard such a comment. I was simply responding based on the input of others.
Perhaps you should provide support for your 100 lb supposition?
vis, if you're applying engineering theory to a 26, I'd offer that you stick your head in the cavity under the front berth and look out either side. If you can't see daylight shining through the glass, you're lucky. The good news in the cleat commentary is that while backing plates are going to help, the boat isn't so heavy that the loads on a cleat at the center's spring line would ever likely damage the boat.
I do believe Mccneo is overestimating the possible loading on a properly set preventer. I appreciate your hesitation, but I think it's unwarranted.
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LOL...I've done this, and I know what you're talking about...
(Really though, that doesn't have much to do with the thickness of the fiberglass, more about the thinness of the gelcoat layers... Even very thick fiberglass, when not covered, painted, gelcoated well, or otherwise tinted/filled, is pretty translucent.)
I dunno... How much load IS there on the midpoint of the boom, going DW with a sail full of wind, and the wind suddenly switches to the other side? What shock loads are created? It's enough to knock a person out, or over the side... But I don't know the numbers.
I'm not really in disagreement with you, and I personally think that it is overkill to stress this point...but I'm enjoying the brainstorming process. Also, it isn't my boat, the person can do what they please. I was simply passing along information from a similar discussion that I had had with factory-ish people, when I asked about adding cleats for additional dock and spring lines in the midships area. If it helps a person who wishes to be conservative, great. If nobody listens, that's great too. I'm not getting paid to do this or anything.
Yah, I've edited my post to reflect that Mccneo was talking about DW stresses, but how far is your boom going to travel, creating shock loading, on an unintended jibe, when it's "prevented"?
You, too?I'm not getting paid to do this or anything.
I added cam cleats to the top of the cabin on both sides (bottom left arrow) for the preventer lines and also horn cleats back on the tops of the coamings but the ones on the cabin top take all the load. They are held on with thru-screws that also have large backing washers. No loading problems from accidental jibes.
I also found that spring lines are the best lines to use when docking as they swing the boat into the dock in a way that keeps the boat parallel to the dock. I used a bow line at first docking and that would swing the bow around into the dock if you had any speed up while securing it to a cleat on the dock. Also docked in a tide area on a sea wall or a fixed, non-floating dock, requires spring lines to properly secure the boat. I added horn cleats to the cabin top in the same area as the winches and just ahead of the preventer jam-cleats and they are backed also. The cabin top in that area is very ridge. I drill all the way through into the cabin and then plug the cabin top holes after putting backing washers/plates on any screws.
Another option is to attach your spring line for docking or lines once docked by putting the loop end of the line over the winch and then adjusting the line with the dock cleat.
I've added a lot of cleats to the mast/boom and sides of the boat and still wish I had more at times
,Sumner
=================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...
Mac Trips to Utah, Idaho, Canada, Florida, Bahamas
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I have sailed the 26S for many years and never had a reason for a preventer.. until recently.
My wife is not into sailing like I am but comes along on some sailing outings which I appreciate. She doesnt expect me to get into quilting in detail, I dont expect her to get into sailing in detail. Recently we were coming back from a long very pleasant down wind sail where I had a whisker pole on the jib sailing wing on wing. We got down near the marina and it was time to drop the sails so first thing I did was ask my lovely wife to keep the boat pointed in the exact direction we were going and I went forward and removed the whisker pole. As I was heading back to the cockpit and was just aft of the mast, the boom swung around and it was all I could do but just hold on as the boom took me out over the lifeline. Since I held onto the boom (loose footed main made this easier), I didnt go overboard and was able to get back on the boat. What had happened.. my wife took focus off where the boat was heading as I guess it was more interesting to watch me up on the deck trying to remove the whisker pole.
So.. looks like I need a preventer now as we tend to do a fair amount of wing on wing downwind sailing.. It is fairly easy on these boats to not even need the preventer but you cant anticipate every possible situation.
My wife is not into sailing like I am but comes along on some sailing outings which I appreciate. She doesnt expect me to get into quilting in detail, I dont expect her to get into sailing in detail. Recently we were coming back from a long very pleasant down wind sail where I had a whisker pole on the jib sailing wing on wing. We got down near the marina and it was time to drop the sails so first thing I did was ask my lovely wife to keep the boat pointed in the exact direction we were going and I went forward and removed the whisker pole. As I was heading back to the cockpit and was just aft of the mast, the boom swung around and it was all I could do but just hold on as the boom took me out over the lifeline. Since I held onto the boom (loose footed main made this easier), I didnt go overboard and was able to get back on the boat. What had happened.. my wife took focus off where the boat was heading as I guess it was more interesting to watch me up on the deck trying to remove the whisker pole.
So.. looks like I need a preventer now as we tend to do a fair amount of wing on wing downwind sailing.. It is fairly easy on these boats to not even need the preventer but you cant anticipate every possible situation.
A preventer on each side is easy to rig so that they are out of the way except when in use and since you have the boat in a slip you don't need to be rigging them all the time. Glad you didn't go overboard but I'll bet the water there is still quite warm,
Sumner
============================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...http://purplesagetradingpost.com/sumner/endeavour-main/endeavour-index.html
Mac Trips to Utah, Idaho, Canada, Florida, Bahamas
With a masthead rig the boom can be extended to almost 90 degrees giving almost a full 90 degree safety margin before the wind gets behind the sail to cause an accidental jibe. Since the boom on our fractional rigs can only go out to about 60 degrees the safety margin is reduced by about 30 degrees making an accidental jibe that much more likely. With this in mind I choose to use a boom-break. I keep it loose most of the time but snug it up whenever I deem it necessary.
Something interesting about preventers.. Some preventers attach to the boom near the vang (or actually use the vang) and some attach near the mid boom sheeting location. When the main sail in extended out for down wind, the farther out you have the attachment to the boom, the farther IN the boom will swing before the preventer kicks in and limits things. Easy to imagine as one end of the preventer (however its implemented) must attach to the boat and if the other end is on the boom and the attachment point is way out to the side over the water, the boom can swing to an equal position the other way before the preventer tightens. So to get a small swing, you have to attach the preventer on the boom closer to the mast.
And.. the other thing about the attachment point is that the torque on the boom from the accidental jibe is the same regardless of where the preventer is attached to the boom. So the force on the preventer is HIGHER for the attachment point closer to the mast (ie, vang location) and LOWER for the attachment point out away from the mast (ie, sheet bail location).. Ie, its a "moment arm".
I am going to use my sheet rope for the preventer line significantly for simplicity and since my isse before was related to going forward for the whisker pole, I dont want to create a new issue of a trip line (I could trade the boom hitting me for a face plant into the cockput).
I found that if I constrain the sheet line to the edge of the coaming, the boom can still swing back to about 15 degrees past the center line before the preventer kicks in. But.. both the shock and the load force will be lower because of the connection farther out on the boom.
FYI, Sumner, the water would have been real cold.. maybe near 60F or less. The river flow on the Colorado comes out from fairly deep on the dams so is cold.
And.. the other thing about the attachment point is that the torque on the boom from the accidental jibe is the same regardless of where the preventer is attached to the boom. So the force on the preventer is HIGHER for the attachment point closer to the mast (ie, vang location) and LOWER for the attachment point out away from the mast (ie, sheet bail location).. Ie, its a "moment arm".
I am going to use my sheet rope for the preventer line significantly for simplicity and since my isse before was related to going forward for the whisker pole, I dont want to create a new issue of a trip line (I could trade the boom hitting me for a face plant into the cockput).
I found that if I constrain the sheet line to the edge of the coaming, the boom can still swing back to about 15 degrees past the center line before the preventer kicks in. But.. both the shock and the load force will be lower because of the connection farther out on the boom.
FYI, Sumner, the water would have been real cold.. maybe near 60F or less. The river flow on the Colorado comes out from fairly deep on the dams so is cold.
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From your discription it sounds like your preventer is attached on the combing by the cockpit. If you move the attachment forward this effect becomes less. A classic preventer is attached to the boat near the bow. Once tensioned it will not allow the boom to swing any significant ammount at all.