Hobie Cat Forums

It is currently Mon Sep 01, 2014 10:45 pm

All times are UTC - 8 hours [ DST ]




Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: Sun Jun 29, 2014 4:37 pm 
Offline
Site Rank - Deck Hand

Joined: Thu Jul 26, 2012 5:17 am
Posts: 4
Location: Roanoke, VA
The 6:1 downhaul is now considered essential equipment on the H16 but I am wondering if it is as effective a control as it is on more advanced boats. Boats like the Wildcat and A Class catamarans have spreaders angled aft with diamond wires that allow the mast to be pre-bent aft on the major axis. In these boats, downhaul tension induces the mast to bend backwards like a bow string and thereby contribute to flattening of the mainsail. Since the H16 doesn't have any spreaders or diamond wires, can the downhaul really be effective in bending the mast or flattening the mainsail? I am not an engineer but it seems like the effect of the downhaul control on the H16 is limited to simply tightening the luff and moving the sail pocket. What do you engineers think?

Leigh


Top
 Profile  
 
PostPosted: Mon Jun 30, 2014 9:13 am 
Offline
Site Rank - Old Salt

Joined: Mon May 09, 2005 10:25 am
Posts: 2555
Location: Jersey Shore
Diamond wires and spreaders are just another tool which allows you to control mast bend and therefore sail shape. The fact that the H16 does not have them does not negate the effect of the downhaul. A basic understanding of sail shaping principles really helps to understand how the downhaul effects sail shape. Essentially what you have is a mast that "wants" to be straight. If you lay your mainsail spread out on the lawn, you will see that the luff (leading edge) is not straight, it is a curve. So when you hoist your mainsail up the mast, the sail still "wants" to be curved and the mast "wants" to be straight. The two oppose each other and shape is forced into the body of the sail. As you tighten the downhaul further, the sail wins and the mast is forced to bend to more closely match the luff curve. The more the mast is bent, the flatter the sail becomes and if you bend the mast to match the luff curve exactly, you will have pulled virtually all of the shape out of the sail.

Modern windsurfing sails are a prime example of sails that take full use of the effect of downhaul (without the use of spreaders or diamond wires). Windsurfing sails require a huge amount of downhaul to get the sail shaped properly.

sm


Top
 Profile  
 
PostPosted: Mon Jun 30, 2014 9:58 am 
Offline
Site Rank - Deck Hand

Joined: Thu Jul 26, 2012 5:17 am
Posts: 4
Location: Roanoke, VA
SM,

Thanks for the reply. I accept that the H16's downhaul tightens the luff and thereby affects sail shape. What I am questioning is whether the H16 downhaul really induces any aft mast bend to go along that luff tension. Without the benefit of spreaders and diamond wires, which cause a small amount of pre-set aft bowing in the mast, it seems to me like the H16's downhaul tension is straight up the luff track. Even with the mast racked back, it seems like the H16's downhaul tension is still straight up the track.

I am curious about this because I haven't noticed a lot of performance difference when I crank on the downhaul once the wrinkles are out. I am wondering if it is me or just the nature of the control on the H16. I assume it is just me but thought I would ask!

Leigh


Top
 Profile  
 
PostPosted: Mon Jun 30, 2014 1:51 pm 
Offline
Site Admin

Joined: Tue May 27, 2003 12:44 pm
Posts: 8979
Location: Oceanside, California
The H16 6:1 downhaul is really for ease-of-use for crews. The original 3:1 was hard to release at the weather mark and re-tension at the leeward mark. Sailing downwind with downhaul on is not good. The system was specifically set up to limit the ability to adjust while sailing upwind, so is not intended to adjust for puffs etc. This is leverage and quick adjustment.

_________________
Matt Miller
Director of Parts and Accessory Sales
Hobie Cat USA


Top
 Profile  
 
PostPosted: Mon Jun 30, 2014 7:39 pm 
Offline
Hobie Approved Guru

Joined: Mon Jul 14, 2003 7:11 pm
Posts: 4592
Location: Detroit, MI
Increasing the downhaul tension on any mast is going to make the mast want to bend along the axis with the lowest area moment of inertia (least stiffness). The sail acts like a bowstring.

On masts with tight diamond wires and fore/aft pre-bend, the stiffest axis is athwartships, so increasing the downhaul tension increases the fore/aft bend.

On masts without diamond wires, the stiffest axis is fore/aft, so increasing the downhaul tension increases the sideways bend.

This is also why the mast rotation control (power up / depower) is reversed on these two types of masts.

Sideways bending is induced in both bast types by rotation, mainsheet tension, where the gooseneck intersects the mast, and how much forward pressure is transmitted by the boom to the mast.


Top
 Profile  
 
PostPosted: Tue Jul 01, 2014 6:48 am 
Offline
Site Rank - Old Salt

Joined: Wed May 05, 2010 8:28 am
Posts: 685
Location: Clinton Lake, KS
MBounds wrote:
On masts without diamond wires, the stiffest axis is fore/aft, so increasing the downhaul tension increases the sideways bend.



Ok.. So back to the class legal H16. With the stupid comptip how many top sailors actually pull any serious amount of downhaul on?

I used to play with it a lot after doing to much reading and not enough sailing, but this year I have just barely been pulling the wrinkles out of the sail in some conditions, and only just barely more than that when it is really blowing. When I look up all I see happening is the Comptip folding faster than the rest of the sail, and it doesn't seem to create the shape I am looking for, or at least it doesn't feel like pulling hard on the downhaul is doing what I want.

( Although... Sometimes with my only fleeting moments of brilliance, sometimes I ain't sure always "what I want" is exactly the right thing just yet. :? I'll get there soon enough though :D )


Top
 Profile  
 
PostPosted: Tue Jul 01, 2014 8:24 am 
Offline
Hobie Approved Guru

Joined: Mon Jul 14, 2003 7:11 pm
Posts: 4592
Location: Detroit, MI
ronholm wrote:
Ok.. So back to the class legal H16. With the Comptip how many top sailors actually pull any serious amount of downhaul on?
You'd be surprised - the downhaul can depower the sail more consistently than the mainsheet and more efficiently than the traveller. Think of it this way - the downhaul is the coarse depowering adjustment; the mainsheet/traveler is the fine adjustment. Set the downhaul to get it in the range, then the mainsheet/traveler is used to fine tune. Theoretically, the main is optimally trimmed when the mainsheet is two-blocked and the traveler is nearly centered.


Top
 Profile  
 
PostPosted: Tue Jul 01, 2014 11:38 am 
Offline
Site Rank - Old Salt

Joined: Wed May 05, 2010 8:28 am
Posts: 685
Location: Clinton Lake, KS
MBounds wrote:
ronholm wrote:
Ok.. So back to the class legal H16. With the Comptip how many top sailors actually pull any serious amount of downhaul on?
You'd be surprised - the downhaul can depower the sail more consistently than the mainsheet and more efficiently than the traveller. Think of it this way - the downhaul is the coarse depowering adjustment; the mainsheet/traveler is the fine adjustment. Set the downhaul to get it in the range, then the mainsheet/traveler is used to fine tune. Theoretically, the main is optimally trimmed when the mainsheet is two-blocked and the traveler is nearly centered.


That makes perfect sense to me... Yet when on the boat it just looks and feels to me like the comptip is not bending off the same as the rest of the mast, and I just haven't found that sweet spot. (moving targets and all). I think what happens to me most of the time is I get to overthinking stuff on the water, and even though I am telling myself to forget and just sail the boat, I can't let it go.



This weekend I ran the first two races right on Jim Sohn's Rudders ahead of Wessels.. I was sailing way better than I usually do, and was having a great time.. :D ... Well.. The first race anyway.. the second I staged my awesome comeback after a HUGE mistake... but anyway... I was right in the game.. Third race I pitched on a freak gust I didn't see coming (something totally out of character for me while racing... In fact as much as I flip the boat that is the first time ever I can recall flipping at a Hobie Regatta) and cost my crew her glasses and confidence. The weekend was all downhill from there. Sunday watching the tape I am oversheeting.. and doing all kinds of silly stuff.. But besides all that in the lighter air, which I have normally been sailing quite well in, I simply couldn't find the groove. So besides going the wrong way I was down on boat speed ect... I may have just been tinkering to much and not sailing the boat... As I was bouncing back and forth with my theory of using just a wee bit more downhaul in to light stuff to help close off the leech with less main sheet, and doing things by the book and letting off downhaul... Ect.. I just didn't find the right gear.


Wasn't it you Matt who was talking about Enrique sailing around in heavy air not even hardly 'downhualing' the wrinkles out of the sail


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 8 hours [ DST ]


Who is online

Users browsing this forum: Exabot [Bot] and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group