An Ounce of Prevention

My big project for this past weekend was to install a preventer for the boom.  When we depart next year the places we want to go will have us running before the wind (trade winds) for much of the time.  When you run before the wind the boom is pulled out to one side or the other to catch the wind in the mainsail.  In light winds the mainsail tends to flop around if there's any kind of wave action.  We'll most likely be running with the waves too so the boats motion will be rolley-poley and that boom will want to go all over the place.  If the boom swings too far aft the wind can catch the sail from the opposite side and force it all the way over to the other side of the boat.  This is called a gybe and when uncontrolled can be dangerous for the crew and possibly damage the boat.  A preventer attaches to the boom and gets run forward and tied down to keep it under pressure and stop any movement.  We have a line that we use for this now but it requires that you go up on deck to attach it or to make any adjustments.  It attaches to a point about halfway out the length of the boom and I've found that this can be a bad thing.  There can be tremendous pressure exerted on this preventer as the boat moves around.  Not only that, our mainsail is "loose-footed" and attaches at the mast and the end of the boom but not along the length of the boom.  As the sail flops around it puts pressure on the attachment points.  If the boom is "prevented" half way out it can cause the boom to snap in half.  This could very possibly ruin your day.  To "prevent" this I've come up with a better plan.  Actually I copied it from another guy's boat.

What I plan to do is attach our preventer about 6" in from the end of the boom.  I want a permanent set-up so we'll have a preventer attached on each side of the boom to use depending on which side the boom goes out to, port or starboard.  The preventer will be in two parts for each side of the boat.  Section "A" will be permanently mounted on the sides of the boom.  It'll attach to eye nuts at the back end and pull forward to a cleat at the front of the boom and store there when not in use.  Along each toe rail will be the other half of the preventer, section "B".  One end will have a snap shackle spliced into it which will attach to the eye at the end of section "A".  When not in use the snap shackle will attach to a storage ring on one of the life-line stanchions.  Section "B" runs through a block attached to the toe rail forward of the shrouds, up towards the bow.  From there it runs aft to a rope clutch, then into a turning block and up to a winch in the cockpit.

Sounds confusing but it'll work like this.....

• the boat comes to a new course to run before the wind and we adjust the main sheet to allow the boom to extend out over the starboard side (or the port side, depending on the winds direction).
• one of us will go up on deck and remove section "A" from the cleat on the boom and attach it to the snap shackle on section "B".
• back in the cockpit we'll take the other end of "B" and wrap it around the winch and snug it up to pull the boom forward and put it under tension.
• once the line is adjusted we'll close the line clutch to lock down the line and we'll be able to remove it from the winch so that can be used for other jobs.

There may be times when we'll sail for days without changing course and all adjustments can be made to the mainsail from the cockpit as needed.  This is absolutely gonna be the cat's meow.

So, the weekend went by and despite being pulled off-track by other "necessities" I managed to get started on the preventer.  Earlier in the week I had ordered the parts I needed on-line.  The eye nuts are available in polished form for $40 from Wichard but I opted to get rough castings from McMaster-Carr for $7.  Didn't have to think that one over for very long.  I also got a good price on the snap shackles and 6" cleats from another on-line source and all my parts arrived by Friday.  I made measurements for the line I'd need and had them made up at the West Marine in Annapolis.  Section "A" is 25' long and has an eye in each end.  One side is reinforced with a metal eye to reduce abrasion and is formed on the ring of the eye nut.  Section "B" is 50', has an eye formed on the ring of the snap shackle and the other end is left plain.  All ends are whipped.  I used 7/16" Sta-Set double-braid for both sections, two each since a duplicate preventer will be  mounted on each side of the boom.

With all my parts on hand I pulled the boom out over the finger pier and lashed it down to keep it from moving around.  I pulled the end cap off the boom and chose a spot to drill based on the flattest location with the fewest extrusions.  The holes I drilled are almost 6" in from the end and slightly more than half way up the side of the boom.  The end-profile of my boom is kinda square on the top half and tapered on the bottom so I chose a part that has parallel sides.  I ran the 3/8"-16 all thread through the holes and placed nuts, lock-washer and flat washers on each inside surface.  This is going to be a pressure point on the boom and I wanted to make sure there would be no flexing.  The all thread extends out each side enough to mount a lock washer and the eye nut.  Where it extends through the inside of the boom I plan to cover the threads with a section of 3/8" hose to prevent any chafing of the out haul line.  I don't think it'll ever come in contact with it but I'm doing it just in case.  Next weekend I'll mount the cleats on the forward part of the boom to complete section "A".

Section "B" will require a little more work.   I'll have to mount padeyes on the toe rail to hold the turning blocks.  I could hang them off the rings at the bottom of the stanchions (like I did in the picture) but I'm not sure how bullet-proof those are.  Be a bummer to have one blow out while you're running before 20 knots of wind in 10' seas.  The padeyes and the line clutches will need backing plates and those will be mounted from the inside of the boat so this job will require two people.

Update 08-19-14:

The Mainsail preventer is installed and useable but not yet "finished".  My friend and twice-removed cousin-in-law (?) Simon wrote and suggested I make a few improvements to the system.  The two eye nuts at the end of the boom are attached to a 6" long 3/8-16 length of "all thread".  I placed nuts and washers on both surfaces inside the boom where it passes through and figured this, with the eye nuts on the outside, would be plenty strong enough.  Simon suggested backing plates would be a wise addition.  He's right.  I don't know why I didn't see that in the first place.  Probably in too much of a hurry to get things installed.  So I went back and took another look at what I had.  Inside the boom there is longitudinal bracing about halfway up the inner surface of either side.  There's also inner bracing at the end for the out-haul and topping lift pulleys.  I managed to place the "all-thread" just a washers distance from all of this which worked out well for the washers but leaves no room for internal backing plates.  My only option, other than finding a new location, would be to place the backing plates between the eye nuts and the boom's outer surface.  Even then there's not much room to play with since there are rivet heads nearby.  I ended up making my backing plates from 1/4" aluminum, 2.5" x 2.5", with the eye nut centered on the plate.  I think this will do the job of spreading the load with the nuts and washers still in place on the inside.  I do plan to keep an eye on it though and at the first sign of fatigue I'll have to go with plan B, whatever that is.

The second thing Simon suggested was using rope that had some stretch to it.  This would work like a built-in shock absorber.  With the reinforced eye nut at the back of the boom this will add some additional insurance.  I think I'll replace the 25' length, section A, that's mounted on the boom.  The double braid that I selected has less than 2% stretch. When I go to order the new line I'll get a recommendation for something more suitable.  Thank you Simon.  It's always good to have an extra set of eyes on these projects.

For the second section of the preventer, section B, that runs along the toe rail I needed to come up with a way to keep it off the deck and out from under foot.  I found these cool-man stanchion fairleads made by Allen.  Five on each side mounted on the life-line stanchions about 4" off the deck will be perfect.  No moving parts, just simple guides.  The run from the turning block up by the bow to the rope clutch aft by the cockpit follows the curve of the toe rail but is straight enough so there shouldn't be any binding or noticeable resistance.  I got ten of these from APS (click here) in Annapolis.

Instead of replacing the line on the boom with something that had more stretch I added a snubber to each side.  These are rated plenty high enough for this job and will provide the amount of give needed to prevent any serious shock to the system.  I also bought a pair of heavy duty padeyes for the forward turning blocks and some nice line clutches for back by the cockpit.  I made backing plates for all these since they'll be mounted on the toe rail.