Bob Stay Post Location

[Sunfish1909]

Can the bob stay post location move one inch aft of the mast step? I have a (Goodwin) DN hull on a Super DN with a full hull length bob stay cable. Is the location of the bob stay post directly under the mast step? The International plans show the post at the forward end of the mast step, the Goodwin plans show it to be at the aft end of the mast step. Could the post be just aft of the bulkhead after the mast step? I have not drilled yet and would like to ask this question.

[Geoff Sobering]

I don't remember a specified location for the bobstay post (But I don't have the specs with me right now).

I'm pretty sure that you want the post to be solidly attached to the mast-step structure.  It's really helping resist the downward force from the mast.

I wasn't clear what you meant when you wrote, "Could the post be just aft of the bulkhead after the mast step?".  If you mean mount the bobstay post just aft of the aft mast-step bulkhead (i.e. mounted just to the bottom skin of the boat), I would think that would fail very quickly since the plywood skin isn't very strong.

Cheers,

Geoff S.

[Sunfish1909]

Geoff, I have a Super DN that has a springboard and thus a 4" wide x 3/4" deep keel board from the noseblock to past the footrest bulkhead. I would have that as backup to my skins. I do understand that the bobstay system is to counteract the mast thrust. I will keep my post right where the Goodwin plans note it, under the mast step . If it ain't broke......don't fix (or change) it. Thanks for the confirmation.

[cmgordon]

Jeff Kent (Composite Solutions) makes a strut that is the mast step ball on top and the bob stay post on the bottom. You drop it in and the mast holds it in place. So, I guess it could go directly under the mast step.

Or at least it is what it seems to me.

CMG

[Paul Goodwin - US 46]

Here is my take on the bobstay and bobstay post, sure to draw controversy, comments about my friends and family, and loss of respect and technical cred within the general iceboating community:
--- The bobstay has no direct line of reaction to the mast force ---


Let me elaborate (I love this stuff).

a) The mast step applies all of it's load to the mast ball.
b) Let's assume the bobstay can only react against the vertical component of the mast load, I think this is a pretty fair assumption if you think about it for a minute.
c) The reaction forces to counter the mast vertical load are applied to the fuselage at the steering chock and plank, and completely balance the force applied at the mast ball.
d) Since the mast vertical force is completely balanced by the steering chock and plank forces, there can be no additional force from the bobstay applied to the mast ball.
e) Thus --- the bobstay does not directly contribute to the forces reacting to the mast downforce.

HOWEVER
a) The hull deflects due to the vertical downforce on the mast ball.
b) This deflection causes an elongation of the bottom of the fuselage, resulting in increased tension in the bobstay.
c) The bobstay increases the fuselage stiffness, and for a given mast load this results in lower fuselage deflection and lower tensile stress in the bottom skin. 
d) Also, any bobstay preload puts an initial compressive stress on the bottom skin, resulting in even lower bottom skin tensile stresses under mast load.
e) A curious byproduct of this analysis - increasing the bobstay tension (assuming it was already fairly tight) will not increase the hull stiffness.  This will only decrease the tensile stress in the bottom skin when the hull bends due to mast loading.

As for the placement of the bobstay post, I don't think it matters too much.  Try to keep it pretty far forward in the boat, since the back of the fuselage drops down under load more than the front (at least on a DN) and you don't want to bottom out the bobstay post on bumps.

This has been my long time analysis of the bobstay system for many many years, and as a result I carry very little tension on my bobstay.  Further, I have sometimes sailed without a bobstay without knowing, even for a whole regatta (don't tell the Measuring Committee).