Where to locate inner forestay and runners?
 

Hello all. . .

I have Brion's book on order but in the mean time and in case it doesn't answer all my questions, here is my question:

I have a '74 Irwin 37CC. It is a single spreader, sloop rig with single fore and backstay, single upper shrouds, and fore and aft lower shrouds. I do not know who the mast manufacturer is (I believe it was Irwin) but it is very substantial and will not allow for any bending. I have had issues with the mast pumping but for the most part, it is very minimal with it properly tuned.

I would like to add an inner forestay and running backstays to eliminate any hint of pumping and, if for no other reason, additional mast support.

In later versions, Irwin kept the exact same mast but moved the forestay from the stemhead to the end of a 3' bowsprit / anchor roller and offered a cutter rig by running the inner forestay from the stemhead to a point about halfway between the mast head and the spreaders.

Would I be correct in assuming this point between the mast head and spreaders is the best point for an inner forestay and runners on my boat. If that is correct, is it crucial that the inner forestay remain parallel to the main forestay (I will not be flying a sail from the inner) and attach to the deck at whatever point that the mast position dictates? Would it be simpler and more structurally sound to make the inner forestay detachable and when needed, attach it directly to the stemhead?

Thanks for your thoughts. . .

-Jeremy

Two ideas
 

Hello,
Hoping you enjoy the book. As I recall there's a bit in it on this subject, or rather on these subjects, as there are at least two distinct topics here.
Let's talk about mast vibration first. This might have something to do with tuning, but probably not to a significant degree; search under "mast pumping" here to see if your problem is caused by vortices on the leeward side of the mast. If the pumping happens in the marina with the wind more or less abeam, and not underway, the vortices are almost certainly the problem.
Next, you are right in concluding that the forestay typically attaches about 3/4 of the way up the stick, and that the runners would there as well. But it would be extremely unlikely that adding such an assemblage, profitable though they might be for some rigger, would be justified by mast pumping.
The change to a short bowsprit, and the addition of a forestay very likely had to do with feedback on weather helm problems from knockabout owners; how is the helm on your boat? If you add a 'sprit, this would be a good reason, and a forestay would be there to keep the rig in balance when you strike the jib, not to add support to the mast, so a forestay would assume a forestays'l.
The forestay does not have to be parallel to the jibstay, but a good sailmaker is the one to talk to about the ideal angle; from long and wonderful experience with maestra Carol Hasse I understand the importance of head angle, clew height, sheet angle, etc., so please make the sail the determinant of stay angle. This could well mean tacking the stay well aft of the stem.
Another option for you is a Solent Stay, but we can leave that aside until you sort out the vibration issue.
Fair leads,
Brion Toss

Brion,

Thanks for the reply. To address the first issue of the mast vibrations: In the past, the mast would pump significantly in 15+kt winds while in the slip (winds were usually either on an aft or forward corner) as well as sailing when beating into a 15+kt wind. After purchasing a Loos gauge, tuning the rig, and having some friends of the racing persuasion check my work, the pumping is gone while in the slip. Unfortunately I have not been able to sail the boat in higher winds since then so I cannot speak to how it will perform yet. I am somewhat concerned that I am able to induce the same sort of pumping simply by grabbing the mast and shaking it with my bare hands. It's fair to say I'm stronger than most but should I really be able to do that?

My desire for the inner stay and running back stays is simply more mast support. As long as my sails are matched to the wind conditions, the boat balances just fine and has even sailed by herself for quite a few miles with the helm locked. So, I don't really see any need to alter the current sail plan. Am I just being paranoid and should stop trying to fix what isn't broken or would the addition of an inner forestay and running backstays benefit the integrity of the rig?

Thanks for the help.

-Jeremy

Ahh
 

Hi again,
I see. It is possible that the mast is the right size, and was simply mistuned, probably very undertuned. You'll find out when you take it out again; if it no longer pumps, that was the problem. If it does, however, there's something amiss. See, modern sticks are almost always designed to sail as sloops. This is even true for cutters. The idea is that the highest loads will invariably be at the masthead, so the mast is designed to take those loads, without the support of a forestay.
If your mast really needs a forestay, then not having it set up at all times becomes risky. If pumping is still a problem, and maybe even if it isn't, it were a good idea to calculate the moments of inertia that your boat requires, and compare them with the properties of the current mast. It is possible that the stick has always been too light, which is to say too flexy, and with too small a factor of safety.
As for being able to shake it by hand, this is easily doable even with too stiff masts. Run the numbers.
Fair leads,
Brion Toss