[Public-List] Dynamic Loads while sailing

Sat Jan 16 06:44:35 PST 2010

    Trying to figure out and localize forces on boats is always a 
challenge and many just use the resolved center of the force to 
illustrate the mechanical point of action (as in sail plans, COE and 
CLR.). Sailing World did a story of the Aussie America's Cup boat that 
broke and sank off San Diego about 25 years ago now and figured the  
load on the center of that hull that broke it was over 500,000 lbs. 
(upwards not down) where it broke . This was because of the moment of 
the leverage from fore and back stays primarily, because the hull broke 
at about the center of effort transverse to the length. The bottom of 
the hull opened up before the gunwhales let go.
     If one starts with Newtonian laws of motion  and for every force 
there is an equal and opposite reaction, then there is pull down from 
fore and aft stays as well as all side stays. Some of you say these 
should measure 1000 lbs a wire on the Loos guage for an AL30. These are 
all pulling the mast down and compressing the deck beam with just the 
static load. The fore and back stays have the added leverage that the 
pull up on the hull from both ends is multiplied by the distance of the 
lever which in our case totals 30. I am not a designer or engineer but 
if the pivot point is the mast base, then the tension on the backstay is 
multiplied by 19.5 and the tension on the forestay is multiplied by 10.5 
(the J value).and this is the force offset by the mast beam and the 
technology within the design of the beam and its connections to the 
hull  to spread that load to the hull.  If the point of compression is 
the center of effort, those numbers will change a bit but the multiplier 
factors still sum to 30. The side stays have a moment  factor of 4.375  
for each one and that force pivots on the centerline of the hull. Once 
you get into adding to these static forces with dynamic and shock 
loading, you are way beyond what you have suggested in previous posts. 
When one considers how small the mast wall thickness is and the downward 
force per square inch can only be spread over that cross-sectional area, 
then the mast base plate has to be that thick to take that force. 
Because the mast base plate is not centered over the mast beam, it 
becomes obvious that the torque on the mast beam is more that we think 
at first glance, even though the force is now spread over the area of 
the mast base plate, most of which is behind the mast beam. That too 
will strain the connective joints of the beam to the hull even under 
static loading.
    I guess the main thing here is to let your back stay off every time 
you leave the boat if you run with things as tight as you say you do by 
Loos guage tensions.
Don #528

Roger L. Kingsland wrote:
> Jim,
>
> On the subject of mast compression loads I called Mike Kaufman, the navel
> architect/surveyor in Annapolis who surveyed my boat to ask how he might
> quickly get a rough number.  He thinks taking 1.2 times the breaking
> strength of all stays/shrouds under load would work.  It seems the greatest
> cross sectional area of wire would be under load on a broad reach or close
> reach where the windward shroud, main stay and one (windward) baby stay
> would be under load (one could argue that both baby stays would be under
> load as the wind moves toward the beam but there would then be a reduction
> in the load on the shroud which has greater strength).  One 3/16" baby stay
> at 4,000 pounds and one 1/4" stay plus one shroud at 6,900 pounds each total
> 17,800 pounds, 21,360 after the 1.2 mark up.  
>
> On the unlikely chance that all shrouds/stays under load would reach
> breaking strength at the same time, I would be inclined to discount the
> safety multiplier and use more like 18,000 to 20,000 or roughly double the
> weight of the boat; all things considered, fairly close to the 150%
> mentioned below.  I guess the point is, next time you haul an A30, just lift
> it by the mast (just kidding, of course).
>  
> Roger 148
>
>
> -----Original Message-----
> From: public-list-bounces at lists.alberg30.org
> [mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Jim Davis
> Sent: Friday, January 15, 2010 3:24 PM
> To: public-list at lists.alberg30.org
> Subject: Re: [Public-List] Dynamic Loads while sailing
>
> >From L. Francis Herreshoff's "The Common Sense of Yacht Design, Vol 1"
> page 114.
> This indicates there could be a loading in excess of five tons on the deck
> of an A30.
> Just something more for the thought process.
>
> I apologies for the poor OCR.
>
> Jim Davis
> S/V Isa Lei
> CT 35
>
> But our sail is not flat, so that more likely the strains are about as shown
> in Figure 151,  and in this diagram the total length of the lines added
> together would equal the pull downward of the mainsheet so that we see the
> strain are mostly near the masthead and after end of boom. If the strains of
> the mainsail are rather concentrated aloft, the strains of the  jib or
> headsail are all concentrated in one spot, and if the forestay pulls forward
> as much as the mainsail pulls aft then we have an enormous  strain in one
> spot; and as a matter of fact, when the backstays are carried  unnecessarily
> tight the strain at this point is more than most people realize .  So the
> fore and aft strains on the mast are the pull of the mainsheet and backstay
> which has to be counteracted by the pull of the forestay. These strains may
> be about as follows (in strong breezes) as compared to the weight or
> displacement of the yacht;  mainsheet 12 per cent, backstay 13 per cent,
> forestay 25 per cent, depending on the angle of pull. While the side strains
> on the mast are not as 
>
> great as the fore and aft strains, still as the shrouds do not have as
> favorable  leads the strain on the shrouds is great, particularly on narrow
> yachts or where the shrouds' are carried inboard to clear a lappiiig jib. At
> times this shroud strain is as much as 100 percent of the yacht's weight, so
> that with the halyards and headsail sheets pulling downward the mast must at
> times  take a compression strain of over 150 per cent of the yacht's weight.
>
>
>
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