Tuning the Bowsprit and Boomkin

[wfraser]

I'm new to the forum and am in the process of learning how to tune the rigging on my BCC 28 and have watched Brion's video on the subject.

I believe I understand the basic concepts involved, but am left wondering if the bowsprit and boomkin require any special considerations, given the nature of their interactions with other aspects of the rigging. General advice on tuning would be appreciated. Should the lengthy bowsprit shrouds be tuned to say 10-12% of load, the bobstay to say 15-20%? Would the same apply to the short shrouds on the boomkin? Thanks.

[Stillraining]

Mines solid so I cant help you...

[Ian McColgin]

Ahoy there,

It's more the bobstay and gobline (the boomkin's bobstay) that need attention. You'll see from a simple parallelogram of force that these stays take a great deal more strain than the jib or backstay.

I don't have the Apprentice at hand but I'm sure Brion discusses sizing these stays. So just a couple of thoughts:

Chain looks shippy but is inferior as bobstay or gobline for two reasons: More exchange sites for air-water induced galvanic corrosion; and surprisingly streatch up to catastrophic failure.

Galvanic corrosion? Attach a bit of wire with an ampmeter to each end of a copper bar. Immerse one end of the bar in salt water so it's about half out. Move the bar up and down between almost fully immersed and almost out. You'll see the ampmeter twitch. This is why smart boaters put an extra coat of bottom paint near the waterline. Chain, with all those surfaces bearing link to link, is just that much more vulnerable in an environment of cyclical wetting and drying.

I moved away from bothering with a turnbuckle for either bobstay or gobline. Seemed a needless complexity. Doing without means that hooking up either must be done before attaching the jib or back stay and putting considerable download on the bowsprit or boomkin. That way when you attach and load the stays, you won't just peal the spars up. It also means very accurate measurement but that's what rigging is all about.

A dolphin striker is often useful to open the bobstay angle a bit and reduce the tension required. This should be mounted to a free pivoting socket on the stem at a point behind and above a straight line from one end of the bobstay to the other. Located rightly, the striker will stay in place as it naturally lies along the shortest available path but it can still deflect a litte when an anchor line bears across it, thus preventing fatigue and breakage where it meets the stem.

G'luck

Ian

[wfraser]

Thanks for the information. The two hefty shrouds on the boomkin have turnbuckles and are currently bar tight, I adjusted the backstay dockside to a tension appropriate for its length as per Brion's video. I have a small amount of bend at the top of the wooden mast. Should the shrouds be this tight? I would tend to say yes because they are only 4 feet long and need to be this tight to avoid any flexing in a heavy seaway. There does not appear to be any downward bend in the boomkin.

The 8 foot wire bowsprit bobstay (with turnbuckle) is also bar tight. However, the two shrouds are much looser, approximating the tension on the mast lowers, which I set to 10-15%, again as per the video. Should the bowsprit shrouds be tighter given their shorter 10-12 foot run?

I purchased the boat last summer and had the stick pulled to completely re-varnish and do maintenance. It was professionally re-stepped, but as I wasn't present at the time to ask questions and am new to tuning, I need to learn this stuff from scratch. I haven't had the opportunity to do a final tuning underway.

As the man once said, there are few things better than tinkering around on boats.

[Ian McColgin]

I am not sure that I would explain that lovely analytic tool "the parallelogram of force" correctly but I know that an on-line google or a trip to your library or a chat with any high school science teacher, civil engineer or whatever, will reveal how you can use it to look at the stay loadings. Once you do that exercise, you will understand that the bobstay loading is so much more than the jib stay, and the back stay loading is less than the jib stay and the gobline loading is less than the bobstay but far more than then back stay.

In a "perfect world" one could make the bow sprit and boomkin about as rigid as the hull and, given sufficiently strong bobstays and goblines, you could tune an old schooner the same way you'd tune a Farr's. That's not going to happen.

Sag, bend and jib luff hollow are beautiful things.

G'luck

[wfraser]

Thanks again, Ian. Your description of interaction is certainly helpful for this total newbie. I understand when it comes to this subject, it's best to have a good understanding of the physics involved. For now though, I'm hoping to find good general rules of thumb to follow as I tune my boat for the first time, similar I guess to those provided by Brion in his video.

Cheers