[Rhodes22-list] Critical Standard Mainsail Standing RigDimensionsFor Rhodes 22

Roger Pihlaja cen09402 at centurytel.net
Wed Jul 7 08:17:01 EDT 2004


Saroj,

Dynamic Equilibrium's boom does not have roller reefing.  The mainsail on
Dynamic Equilibrium has 2 jiffy reef points built in to it.  This might be
where the additional length on your boat's boom vs. mine is coming from.
Does your boom also have jiffy reefing, or do you use the roller reefing
feature?

The calculation I made for you would apply to getting the boom caught on
either the port or starboard backstay or in between the two backstays.

It takes a lot of force on the backstays to bend the mast on a Rhodes 22.
So, unless you a very strong lady, I doubt you are bending the mast with
your backstay adjuster enough to cause the boom/backstay interference you
are experiencing!

Just for snicks & grins, I redid the calculation on boom/backstay clearance
if we assume your boom is longer by an amount sufficient to reduce the
clearance to only 1 inch when the boom is horizontal, angle = 0 deg &
everything else is the same as on Dynamic Equilibrium.  Yes, I know, I need
to get a life!  But, I really want to help you out here.  FYI, the way this
is calculated is I establish a coordinate system with the origin at the
boom's gooseneck.  This is the reason I referenced all my dimensions to the
aft face of the mast at the boom gooseneck.  The gooseneck allows the end of
the boom to sweep out a circle centered on the origin, using the length of
the boom as the radius of the circle.  The backstay is modeled as a line in
2-D space running at a known distance and angle from the origin.  Using this
geometry, you solve the equation of the line & the circle simultaneously in
order to obtain the points of intersection.  Theoretically, there are 2
points of intersection; but, only one of them has any physical reality in
the context of this problem.  Then, using trigonometry, you calculate the
angle which the boom would have to rise up to in order to intersect that
point in space on the backstay.

OK, so in the upper boom position, in order to reduce the boom/backstay
clearance to 1 inch, the boom would have to be:

116 inch + 5.5 inch - 1.0 inch clearance = 120.5 inch long from aft mast
face to end of boom

If we redo the algebra & trig with this new boom length; then, the boom will
hit the backstays at an angle = 1.00 deg relative to horizontal
The aft end of the boom must rise up by about 2.1 inches to hit the
backstays

This much boom rise could very easily occur during a gibe.

Comparing this result with my previous calculations; obviously, this
boom/backstay interference problem is highly nonlinear.  Saroj, if your
static boom/backstay clearance is really only 1 or 2 inches; then, boom rise
during a gibe is very likely the source of your interference problem.

Gary & I are leaving on our backpacking/mountain biking trip to the Land
Between The Lakes National Recreation Area in SW Kentucky in a little while.
I'll be out of touch until Tuesday, 07/13/2004.  But, I'll check back with
you then.  I'll be very interested to hear what your boom measurements
turned out to be.

Roger Pihlaja
S/V Dynamic Equilibrium


----- Original Message -----
From: "Saroj" <saroj at pathfind.net>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Tuesday, July 06, 2004 11:17 PM
Subject: Re: [Rhodes22-list] Critical Standard Mainsail Standing
RigDimensionsFor Rhodes 22


> This is just fantastic information, Roger.  In the upper boom position on
> Pathfinder, which is my normal sailing position except where reefing is
> advised, the clearance is just an inch or two with the backstays
tightened.
> That is from memory.  I've got to take measurements this week to get
precise
> about it.  I would think that my boom is standard issue because it has
that
> furling feature wherein if I pull on the boom towards aft, I can rotate
the
> boom and theoretically at least could roll the main sail around it for
> reefing.  I've never heard anyone on the list talk about doing this, but I
> recall reading about this somewhere quite a few years ago.
>
> I wouldn't describe what happens as "getting caught between the
backstays".
> It is more that when tacking, the boom gets caught on either backstay.
>
> Is it possible that I am tightening the backstays too much and decreasing
> the distance between the mast and the stays?  This seems unlikely as the
> mast doesnt' really have that much flex down low at that point.
>
> Again, many thanks.
>
> Saroj
>
> ----- Original Message -----
> From: "Roger Pihlaja" <cen09402 at centurytel.net>
> To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
> Sent: Tuesday, July 06, 2004 8:38 PM
> Subject: [Rhodes22-list] Critical Standard Mainsail Standing Rig
> DimensionsFor Rhodes 22
>
>
> Saroj, Keith, et al,
>
> I just went thru my files & looked up the dimensions & geometry of Dynamic
> Equilibrium's standing rigging and mainsail before I modified  the
masthead
> casting.  First of all, here are the dimensions of a standard Rhodes 22
> mainsail:
>
> Max Luff: 21 feet - 8 inches
> Max Foot: 9 feet - 0 inches
> Mainsail Area: 110 square feet
> Boom Length From Aft Mast Face To End Of Boom: 9 feet - 8 inches
>
> Nominal Backstay Angle Relative To Horizontal When Backstays Are Taut &
Mast
> Bend = 0.00 Chord Inches: 65.68 deg
>
> Horizontal Distance Along Top Of Boom From Aft Mast Face To Backstay:
> With Boom In Upper Position: 10 feet - 1-1/2 inches
> With Boom In Lower Position: 10 feet - 9-1/2 inches
>
> NOTE 1: In the lower boom position, the boom clears the backstays by
13-1/2
> inches when the boom is horizontal, angle = 0 deg
> When I do the algebra & trig, the boom would have to rise to an angle =
67.3
> deg relative to horizontal to hit the backstays
> The aft end of the boom would have to rise up by 107 inches to hit the
> backstays
> This is a very extreme angle for the boom & highly unlikely
>
> NOTE 2: In the upper boom position, the boom clears the backstays by 5-1/2
> inches when the boom is horizontal, angle = 0 deg
> When I do the algebra & trig, the boom would have to rise to an angle =
53.9
> deg relative to horizontal to hit the backstays
> The aft end of the boom would have to rise up by 93-3/4 inches to hit the
> backstays
> This is a very extreme angle for the boom & highly unlikely
>
> Boy, from these calculations, I don't see how you managed to get the boom
> caught between the backstays unless your upper boom position is a lot
higher
> than mine or your boom is longer?
>
> Hopefully, these standard Rhodes 22 rig dimensions will help you resolve
> your boom/backstay interference issues.
>
> Roger Pihlaja
> S/V Dynamic Equilibrium
>
>
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