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Re: F18ht mast on I-20 [Re: ] #253028
10/03/12 08:32 AM
10/03/12 08:32 AM
Joined: Jan 2005
Posts: 6,046
Sebring, Florida.
Timbo Offline
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Scarecrow, I'm not a designer, or even an engineer, but I did take some engineering classes....many, many, many beers ago.

Please draw me a force/vector type diagram and show me where the additional forces are applied when you step a mast on:

1. a dock
2. a heavy boat
3. a light boat

As I said earlier, if you put the exact same amount of rig tension on it, it has zero idea of what it's stepped to.

What DOES change, is the amount of WIND (generating lift force, sideways to the mast) it takes to flip a light boat, vs. a heavy boat, vs. a dock. In our little beach cat world, we usually flip over sideways long before the mast breaking point is reached.

Those clowns on Wild Thing broke their stick because they went out in too much wind. The added lead just let them keep the boat upright while it broke.

I think we are both saying the same thing, you are calling that force: "increased righting moment", I am calling it too much lift.

The end result is the same. But to answer the original question, will the HT mast work on an Inter 20?

Probably, as long as you don't put your wife and 4 kids and a barbeque on board and go out in 30knots.

;^)


Blade F16
#777
-- Have You Seen This? --
Re: F18ht mast on I-20 [Re: bacho] #253030
10/03/12 08:58 AM
10/03/12 08:58 AM
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Posts: 932
Solomon's Island, MD
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samc99us Offline
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The better question is WHY put the HT mast on the Inter/Nacra 20? Did the stock telephone pole break? Are you thinking the more flexible rig will let you select better sails? Maybe you should order a new spinnaker and see what that does to your boat speed before getting the upwind sails/rig sorted?


Scorpion F18
Re: F18ht mast on I-20 [Re: bacho] #253031
10/03/12 09:20 AM
10/03/12 09:20 AM
Joined: Jan 2005
Posts: 6,046
Sebring, Florida.
Timbo Offline
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I think he said he wanted to save some weight up top.

I once owned a Javelin F18HT and an Inter 20, the Javelin mast was MUCH lighter and easier to step!


Blade F16
#777
Re: F18ht mast on I-20 [Re: samc99us] #253035
10/03/12 09:30 AM
10/03/12 09:30 AM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
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Originally Posted by samc99us
The better question is WHY put the HT mast on the Inter/Nacra 20? Did the stock telephone pole break? Are you thinking the more flexible rig will let you select better sails? Maybe you should order a new spinnaker and see what that does to your boat speed before getting the upwind sails/rig sorted?


It broke. Sad story.



Jake Kohl
Re: F18ht mast on I-20 [Re: Timbo] #253038
10/03/12 09:34 AM
10/03/12 09:34 AM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
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Originally Posted by Timbo
Scarecrow, I'm not a designer, or even an engineer, but I did take some engineering classes....many, many, many beers ago.

Please draw me a force/vector type diagram and show me where the additional forces are applied when you step a mast on:

1. a dock
2. a heavy boat
3. a light boat

As I said earlier, if you put the exact same amount of rig tension on it, it has zero idea of what it's stepped to.

What DOES change, is the amount of WIND (generating lift force, sideways to the mast) it takes to flip a light boat, vs. a heavy boat, vs. a dock. In our little beach cat world, we usually flip over sideways long before the mast breaking point is reached.

Those clowns on Wild Thing broke their stick because they went out in too much wind. The added lead just let them keep the boat upright while it broke.

I think we are both saying the same thing, you are calling that force: "increased righting moment", I am calling it too much lift.

The end result is the same. But to answer the original question, will the HT mast work on an Inter 20?

Probably, as long as you don't put your wife and 4 kids and a barbeque on board and go out in 30knots.

;^)


Rig tension increases beyond your initial settings with wind power. As the mast/sail catches wind and creates lift/force, it pulls on the cables holding it upright and to the platform. These cables have a high degree of down angle to them...so...adding tension to the cables results in considerable downward force (compression) on the mast (pulling it harder toward the mast ball). The more the boat/platform/dock/concrete foundation resists the motion of the sail/mast/cables, more compression force is carried by the mast....until it goes pop.

I'm not drawing a diagram. wink


Jake Kohl
Re: F18ht mast on I-20 [Re: bacho] #253048
10/03/12 10:46 AM
10/03/12 10:46 AM
Joined: Jul 2006
Posts: 164
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I20RI Offline
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I dont understand why this is so complicated. An HT hull weighs like 75 pds? An I20 hull weighs like 125pds or thereabouts. When you fly a hull on the HT the shroud (which is attached to the mast) PICKS UP 75 pds out of the water and into the air. When you fly a hull on the I20 the shroud (which is attached to the mast) PICKS UP 125 pds of hull into the air (where it is not bouyant).

The difference between flying a hull and strapped to a dock is not a useful analogy unless you managed to fly a hull on the dock, which breaks either mast. Just dont fly a hull on the I20 and you'll be fine, but what fun would that be...

Last edited by I20RI; 10/03/12 10:51 AM.
Re: F18ht mast on I-20 [Re: Jake] #253052
10/03/12 10:54 AM
10/03/12 10:54 AM
Joined: Jan 2005
Posts: 6,046
Sebring, Florida.
Timbo Offline
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Jake, in your example...what actually caused the tension to increase, for the mast to break?

More wind.

So we could say you exceded the mast's 'wind limit', what ever that is. Maybe they should come with a stamp on them that says, "This mast was designed to support xxx sq. feet of sail area, at XX knots of wind, on a 500lb. (boat + crew) 8'6" wide catamaran platform" or something like that?


Blade F16
#777
Re: F18ht mast on I-20 [Re: I20RI] #253053
10/03/12 10:56 AM
10/03/12 10:56 AM
Joined: Jan 2005
Posts: 6,046
Sebring, Florida.
Timbo Offline
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Originally Posted by I20RI
I dont understand why this is so complicated. An HT hull weighs like 75 pds? An I20 hull weighs like 125pds or thereabouts. When you fly a hull on the HT the shroud (which is attached to the mast) PICKS UP 75 pds out of the water and into the air. When you fly a hull on the I20 the shroud (which is attached to the mast) PICKS UP 125 pds of hull into the air (where it is not bouyant).

The difference between flying a hull and strapped to a dock is not a useful analogy unless you managed to fly a hull on the dock, which breaks either mast. Just dont fly a hull on the I20 and you'll be fine, but what fun would that be...



It's not complicated, just don't put two big fat guys on the F18HT and go out in 30 knots!


Blade F16
#777
Re: F18ht mast on I-20 [Re: Jake] #253054
10/03/12 10:56 AM
10/03/12 10:56 AM

M
MN3
Unregistered
MN3
Unregistered
M



Originally Posted by Jake
I'm not drawing a diagram. wink


here is one that will not explain much, but still is the best mathematical diagram i ever saw on this site...

[Linked Image]

Re: F18ht mast on I-20 [Re: bacho] #253055
10/03/12 10:58 AM
10/03/12 10:58 AM
Joined: Jan 2005
Posts: 6,046
Sebring, Florida.
Timbo Offline
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Nice ballast!


Blade F16
#777
Re: F18ht mast on I-20 [Re: samc99us] #253057
10/03/12 11:11 AM
10/03/12 11:11 AM
Joined: Aug 2011
Posts: 774
Greenville SC
bacho Offline OP
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Originally Posted by samc99us
The better question is WHY put the HT mast on the Inter/Nacra 20? Did the stock telephone pole break? Are you thinking the more flexible rig will let you select better sails? Maybe you should order a new spinnaker and see what that does to your boat speed before getting the upwind sails/rig sorted?



It's an idea that was sparked when I remembered the ht/a-cat thread. But there is no close HT that I know of for sale. Just exploring options. Unfortunately I have a brand new set of sails that were just built for te 20 mast so it would probably mast more sense to stick with that.

Re: F18ht mast on I-20 [Re: Timbo] #253058
10/03/12 11:14 AM
10/03/12 11:14 AM

M
MN3
Unregistered
MN3
Unregistered
M



Originally Posted by Timbo
Nice ballast!

that should answer all your questions smile

Re: F18ht mast on I-20 [Re: bacho] #253060
10/03/12 11:24 AM
10/03/12 11:24 AM
Joined: Dec 2006
Posts: 2,490
On the Water
P.M. Offline
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On the Water
The I20 mast is thick, especially above the hound and the I20 kite is huge compared to the HT kite. My concern would be what would happen if Bach is out having fun with 3 on board (which he does), flying the kite in sporty winds, stuffs a wave, and the mast is not in the best rotation??

Typically the I20 mast survives this. Not sure the HT mast would. Will he have to re-position the spin hound and mast hound on the HT mast, and what affect will that have?

Great discussion, but looks like he will need to find another carbon I20 stick, to complement his investment in new sails.


Philip
USA #1006
Re: F18ht mast on I-20 [Re: ] #253062
10/03/12 11:27 AM
10/03/12 11:27 AM
Joined: Jan 2005
Posts: 6,046
Sebring, Florida.
Timbo Offline
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Originally Posted by MN3
Originally Posted by Timbo
Nice ballast!

that should answer all your questions smile



Wait...what was the question again?

:^)


Blade F16
#777
Re: F18ht mast on I-20 [Re: P.M.] #253063
10/03/12 11:28 AM
10/03/12 11:28 AM

M
MN3
Unregistered
MN3
Unregistered
M



Originally Posted by mummp
and the mast is not in the best rotation??

what is the best rotation?
is it with the rotator (arm) facing straight back (so the spin pulls on the front of the mast...

or rotated 90% (so the pull is on the side of the mast and supported by the diamonds?)

Last edited by MN3; 10/03/12 11:28 AM.
Re: F18ht mast on I-20 [Re: ] #253068
10/03/12 03:06 PM
10/03/12 03:06 PM
Joined: Dec 2001
Posts: 5,590
Naples, FL
waterbug_wpb Offline
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with the kite up, you want the rotator off (running free)


Jay

Re: F18ht mast on I-20 [Re: ] #253069
10/03/12 03:21 PM
10/03/12 03:21 PM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
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spin pulls sideways...so mast rotated at 90 degrees gets the thickest part of the mast to support the spin loads (particularly since the diamond wires don't go that high).


Jake Kohl
Re: F18ht mast on I-20 [Re: bacho] #253075
10/03/12 04:09 PM
10/03/12 04:09 PM

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MN3
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MN3
Unregistered
M



thanks guys, that's what i thought and how i fly, just making sure.


Re: F18ht mast on I-20 [Re: bacho] #253095
10/03/12 11:29 PM
10/03/12 11:29 PM
Joined: Jul 2005
Posts: 267
Ocean Springs, MS
Capt_Cardiac Offline
enthusiast
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Posts: 267
Ocean Springs, MS
The Bimare Jav2 circa 2002-2005 mast is tougher than a old school Nokia phone. I have punished my 18HT relentlessly over the past 5 or so years and can say confidently that the mast will survive an I20 application. By the way. That mast is totally class legal in the open 20 class.
I believe that version was made by RIBA composites. Do some homework and you'll be impressed with their product.


Capt Cardiac
Ocean Springs Yacht Club
Sailor
Nacra20 - Flight of Ideas #5
Re: F18ht mast on I-20 [Re: bacho] #253201
10/05/12 04:53 PM
10/05/12 04:53 PM
Joined: Sep 2011
Posts: 32
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evansdb78 Offline
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Of course the mast doesn't know what it's attached to; it's just a stupid mast. But the boat beneth the mast matters a lot whether or not it will break. Stepping it to a stationary object is not a good comparison. Also wind speed would be a constant in comparing the two boats. Wind doesn't increase when you put a mast up on a N20. Best case in point is downwind sailing, spin up. As you sail down wind the apparent wind decreases the load on the rig. The highest loads will be during acceleration, and when the boat suddenly slows stuffing into waves. Compound that with waves causing the top of the mast to whip. There are much higher stress loads than mounted on a stationary object. Up wind the apparent wind is increasing the wind velocity, add waves again and you'll have much higher stresses than stationary.

Mainly the this will cause over-stress on the mast: Downwind boatspeed is key, if you can maitain boatspeed close to the true wind speed the rig is not under stress even though the boat is moving fast. As waves slow the boat down, pressure on the rig increases. This will happen on a heavy boat to a greater extent than a light boat.

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