Zen and the art of sailboat maintenance

Comment

Comment by Dave Skolnick on January 23, 2009 at 1:09pm

Speed over ground (SOG) is the total of boat speed (course over ground (COG) is the aggregate direction) through the water, accounting for leeway ("side slip"), and set and drift (the effects of current). So if your boat heading is N at 5 kts through the water with winds 12 kts from the SW and current 1/2 kt from the S your SOG could be just under 6 kts and COG somewhere between N and ENE. (All this is just grossly estimated, I didn't do any calculations.)

Now lets add a destination that is to the NW. If COG is between N and ENE and the bearing to your destination (usually bearing to waypoint or BTW) then for every mile you move forward you are only getting a little over 1/2 mile closer to your actual destination. So if your SOG is a bit under 6 kts your velocity made good (speed toward your destination or waypoint) is around 3-1/4 kts.

For most folks the concept is more important than the calculation, but if you want I can post the example with numbers and all the calculations.

Comment by Dave Skolnick on January 23, 2009 at 10:46am

DDW = dead downwind
VMG = velocity made good (of the speed the boat is moving, how much is actually getting you toward your destination)

Most riggers should be able to make up synthetic lifelines from Amsteel or spectra. If your nearby West Marine has a rigging shop (many do) they might be able to make them up for you.

Once you cobble something together with line to make sure you are content with any sail conflicts, the least expensive approach is probably to reterminate the existing steel lifelines for whatever fittings you end up with on your pulpit.

Comment by Dave Skolnick on January 23, 2009 at 8:56am

Paul's mechanism for avoiding a trip forward is a good one. It's a natural evolution of the single-line preventer, trading a small amount of additional complexity for added safety (trip forward to the mast instead of trip to the bow) and convenience.

He is more tactful than I about boom brakes. In my opinion, they are overly expensive, overly complicated, and fitted in an inappropriate place. The inherent risks (bending or breaking the boom if the end goes in the water) are not apparent to the uninitiated.

It is worth noting that heading up a bit reduces the chance for an accidental gybe and often increases VMG. As one can see, on my boat in lighter air, my boat will reach a destination dead-downwind more quickly by gybing the boat ("tacking downwind") 40 degrees to either side of DDW:

Comment by PCarrico on January 22, 2009 at 11:22pm

Nice seeing a lively discussion about safety.
Great topic title I might add...
Preventers are great.Dave Scolinick gives a great description.
The preventer that I like installing is one where there is one or two internal boom preventers that will connect to a pair of port/starboard preventer pennants pre-rigged to the bow. I actually prefer the single in-boom preventer control since we usually have lots of crew to manipulate the strings. Either way the line is control goes from the cockpit, forward to a foot block near the mast, up to the gooseneck at a turning block, inside the boom, out near the aft end of the boom through an exit block with a snap shackle on the end of the preventer control line.
Forward there are two lines pre-rigged from a hitch on the toe rail or cleat that are held in place with a hook and short piece of shock cord when not being used. The stowage position for the preventer shackle is somewhere up near the vang or gooseneck. When it's time to hook up the preventer, just shackle the control preventer to the forward pennant. I've experimented with using nylon for the forward end, but usually we have a piece of spectra (low stretch). The theory of having nylon on the front end is to act like a shock absorber to reduce shock load. I seemed to prefer a short bit of nylon attached to some spectra to limit the elasticity. On one boat I put a 5 foot piece of nylon inside the spectra like a bungee to help improve the shock loading capabilities of the system.
Even while racing in the ocean, preventers can calm the slatting of the mainsail in light choppy conditions.
Boom breaks seem ok. Not for race boats. I have used them, and have evaluated them for a cruising world article. I recommended that one of both ends have a tension control for when the wind gets gusty, that when the vang is eased that the boom brake control lines don't get tight. And again when conditions change that the excessive slack can be removed. On a race boat the boom brake is a nuisance, as there are two lines that go across the boat that hinder crew movement. Another issue is that the logical attachment point for a boom break is near the vang. Booms generally don't like side torque, especially if the booms are elliptical in shape.
I hope the above isn't too confusing. What a tangled web we weave...

Comment by h on January 20, 2009 at 8:38pm

Terri,

For short sails, a boom brake may be more suitable than a preventer. This will allow you to gybe when needed without too much de-rigging.

http://www.defender.com/product.jsp?path=-1|118|319697|311639|199&id=911760

Comment by Dave Skolnick on January 20, 2009 at 3:41pm

Hmmm - heated, indoor pool at YMCA or community pool? Could be a good winter project.

Comment by Dave Skolnick on January 20, 2009 at 3:17pm

*grin* I was a Red Cross lifeguard. That's a long darn way from being a SEAL.

I'll do everything I can to save a MOB, but that isn't likely to include going over the side. I know my limitations, and for me thinking through something (quickly) and acting on it is better than jumping in.

Regardless you have given me food for thought and I'm going to be practicing some different MOB recovery scenarios when the water warms up. Training and practice are among the best investments we can make.

Comment by Dave Skolnick on January 20, 2009 at 12:40pm

I disagree with Bill about getting in the water, but I sail in colder places then he does. As in my docking comment, I don't want to leave the boat without someone aboard either.

I don't want to watch my boat sail away without me.

Next summer I'll have to see if I can get a Lifesling around a non-helpful volunteer from the deck with a boat hook ... interesting thoughts. Thanks.

Comment by Dave Skolnick on January 20, 2009 at 10:10am

Preventers are much simpler than they are often made out to be. The most important factor to consider is the location of attachment points. The use of a vang or vang-like tackle clipped to the toe-rail at the shrouds is inefficient, thus the need for mechanical advantage.

A preventer should run from the very end of the boom to the bow. I run mine through the (rather large) bow cleat and back along the side deck to a stern cleat and make it fast there.

This approach makes the preventer a very simple line about twice the length of the boat. It does mean a trip forward (or keeping two preventers rigged) when gybing. I haven't found that to be much of an issue but your mileage may vary.

With respect to using the Lifesling sailors generally use a halyard. You may or may not need some additional mechanical advantage. It's easy to figure out - a 25 gallon trash can full of water will weigh about 200 lbs. Rig a halyard to the trash can with some kind of lashing (don't use the handles - you'll tear them off) and see if you can lift the full trash can using just the halyard winch, or by running the halyard tail back to the sheet winch. If you can't you'll need to do something else. Remember that using 4:1 tackle will require a long line: 4 times the distance from the water to the top of your lifelines plus 10 feet or so. Depending on who you sail with you might need a bigger trash can for your test. *grin* Wet clothing will add a good bit of weight.

Please don't let my thoughts discourage you from getting a Lifesling. It is about the best piece of safety gear available for recovering a MOB. I have a Lifesling III on Auspicious.

Comment by h on January 20, 2009 at 8:11am

Terri,

The boom fang is normaly made out of 2 fiddles to make 4:1 ratio lifting system.

http://www.harken.com/CatalogPDF/020-ref-boom_vangs-outhaul.pdf

To make your preventer, you can use this 2 fiddle arrangement. But you need an anchor point roughly behind your aft shrouds.

If you have a midship cleat you can use that too.

To use the 4:1 fiddle arrangement to pick up someone, you need to wind the loose end of the sheet to your winch. Then you can winch up someone in addition to the 4:1. Make it a light work of it.

Good Luck,

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