spreaders, shrouds, and trigonometry

[art heyman]

If the spreader should bisect the shroud at equal angles (above and below) , why do I see so many spreaders perpendicular to the mast ?

Obviously the spreaders need to be angled up; (?) otherwise the angle the spreader makes with the shroud will be about 90 degrees below and significantly less above, as it angles up to the mast top ( leading to forces other than direct compression on the spreader as the boat heels )

As I calculate an appropriate spreader angle for my own boat (mast height 35 feet, from spreader to mast top 210 inches, spreader length 41 inches) I get a 5 degree elevation, about 3 and a half inches above the horizontal at the spreader tip. (My spreaders are in the plane of the mast, they don't angle aft or forward; the only angle I'm concerned with is that of up or down)

I was about to set it up like this,with a distinct upward angle, then wandered around some yards looking ( in some cases at very expensive boats of good pedigree ) and saw that perhaps 8 out of 10 larger ( 30 feet and up) boats had spreaders parallel to the deck

What am I missing ?

[Brion Toss]

Hi,
This is one of my pet peeves. It's not as though the geometry behind this is a recent discovery, it's not as though droopy spreaders never lead to spreader failure, and it's not as though droopy spreaders look good. There's even, I understand, some evidence that raising the spreaders to reflect the vector will help reduce drag, and even add a bit of lift. But if the spreaders are not well-secured they will tend to drift down, until the hardware at the inboard end constrains them. This of course puts a strain on the hardware and the spreader.
Also, people just don't take the effort to do this job. To get a good angle, you just about have to go aloft after the mast is stepped, bop them into place, and then seize them.
As for how they're seized, most people use seizing wire, which is inclined to slip, even if put on tight. We either use Double Constrictors, with twine, or seizing wire put on with a Clamptite tool. Some spreaders come with built-in clamps, and these are fine if they fit the wire well. Ideally, you want a spreader tip that can take someone standing on it without slipping.
I take it that you have a single-spreader rig. Those with double spreaders will find that the lower set will be angled up more than the upper set. In any event, please carry on. Maybe you'll start a trend.
Fair leads,
Brion Toss

[Douglas]

Ahoy Brion,,,, on double spreader rigs,,,,, what happened to the "bisect the angle" rule of thumb, for setting the spreader "up" angle ?

Seems to me that the upper spreader has to be set "up" more of an angle, because that cap shroud does such a radical angle back to the mast head once it leaves the upper spreader tip .



BTW ,,,, I have been using the tarred marline that is sold at WM ,,,, and it has been breaking when I draw down on my constrictor knots,,,,, is there another stronger lashing line available, and just how can I find some, way out here in Singapore ?

Also do you still use a electrical friction tape on the shroud, under the constrictor knots, or have you found some other tape better to use ?

Also, just how much lose a fit, or movement at the spreader base, is acceptable ,,,, like when the mast is on the horse's , and the spreaders are attached, but not loaded,,,,, what kind of spreader tip movement is acceptable ?

Douglas

[davidsamuelson]

Hi Douglas,
There's a chandlery just up the road from you in Phuket, Thailand that has whipping twines #2 to #16, Polyester seine twine (#24) and 1.5mm (1/16") polyester-covered Dyneema: info@eastmarineasia.com
In Singapore itself there are a mutitude of chandleries catering to the fishing fleet - you shoud find something there that you cannot break or go to Raffles marina where they used to stock twines when I was there.
Good luck, David

[Brion Toss]

Originally Posted by Douglas

Ahoy Brion,,,, on double spreader rigs,,,,, what happened to the "bisect the angle" rule of thumb, for setting the spreader "up" angle ?

Seems to me that the upper spreader has to be set "up" more of an angle, because that cap shroud does such a radical angle back to the mast head once it leaves the upper spreader tip .



B

Take a look at a double-spreader rig; I don't know that I've ever seen one where the angle from the upper spreader to the mast is greater than the angle from the lower spreader to the mast.
As for looseness at the spreader roots, on the one hand a little bit is not necessarily a problem. On the other hand the looseness might be the result of undersized pins, or holes that have been worn or torqued into the spreader, so examine carefully.
Fair leads,
Brion Toss

[Douglas]

Thank You Brion, for your reply, and I agree with you ,,,,, but, I also did look at page 127 of The Complete Riggers Apprentice, Figure 5-13 , but my Hess BCC rig, does not have the intermediate shroud go out over the lower spreader tip, like that illustration.

The intermediates go straight down to the chainplates, only the port or stb cap shrouds go out over both the spreader tips. Maybe this is not a common rig practice ? Sooo, if I were to bi-sect the angles, the lower spreader tip will have a much shallower angle than the upper spreader tip .

While we are on the subject of spreaders,,,, what is the current feeling about "Thru-bar" mast spreader bases ?

Does such a large hole cut into the mast wall create a weakness in the mast wall at the spreader bases ? or does the thru-bar hole get back strength by the attached spreader base plate on each side ?

Douglas

[Brion Toss]

Hi again,
Aft-led intermediates. Yuck. and still a double-spreader rig. Further yuck. The lower spreaders are almost decorative, as the angle of the uppers over them is, as you note, very shallow. If I were me I'd move the intermediates to the spreaders (chainplates in line with the ones for the uppers), and add one-part runners instead.
As for through-bars, well-designed ones are no problem at all. Sure, you need to make a good-sized hole in the mast, but it is in a region that is disinclined to buckle (supported section of a column).
Fair leads,
Brion Toss