I’ve been trying to understand the mechanics of using a power downhaul on my Hobie 18. From my understanding the downhaul is used to flatten the sail and depower it. So when the wind picks up I’d take in on the downhaul to depower and eliminate excessive heal making the boat sail flatter and faster? The reason I ask is I once heard the downhaul referred to as the “gas pedal on these boats” which seems a little counterintuitive. Thanks

It is a gas pedal, a flatter sail is faster in bigger wind.

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Greenville SC

Offering sails and other go fast parts for A-class catamarans
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bacho

It is a gas pedal, a flatter sail is faster in bigger wind.

Faster because a flatter sail provides better "lift" in high wind or becuase it's depowered and wont heel the boat as much? Also what do you consider bigger wind?

i see no appreciable speed increase when i apply the downhaul,

what i see is:

less healing, allowing me to have a more controlled boat. sailing at the optimal angle to the wind, with the lee hull in the correct position on the water, without getting blasted with gusts and healing to extreems

the mechanics are:
sail makers build curves into sails to produce draft, cord (and luff and leach)
tightening the luff (via downhaul) stretches the material and moved the draft forward
using the downhaul will bend the top of your mast back & down and closer match the curves cut into the sail (and make it flatter)

PS the outhaul does this too

A flatter sail will spill off more air than a full one.

Modern (flat top) sails have more sail area up there for power, but also will easily bend (the top angled batten) and spill off lots of air to combat over-powering (to a degree)

Without getting into the actual changes it makes to the sail (draft amount/location, leech shape, etc.) here is a very general observation with which some will probably disagree....

Within limits, more downhaul (and mainsheet tension) gives less power but more speed if there's sufficient wind. Within limits, less downhaul (and mainsheet tension) gives more power for lighter conditions. It's more like shifting into the proper gear (for power vs. speed) rather than hitting the gas pedal.

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Jerome Vaughan
Hobie 16
Clinton, Mississippi
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what jerome said.

i can add that with my stock 18 sails and mast downhauling doesn't make much difference when sailing, certainly not as described....

the new boats with carbon masts are real "bendy" and react well to massive downhauling, etc....

on my H18 w/6:1 down haul, I can get some sail shape with it, but mostly I just use it to take the wrinkles out... I will tighten in a blow and release downwind... but its NOT the thing to play when you're on the trap.... whereas the carbon and twisty masts guys are always playing that thing....

keep in mind nothing in a vaccum... changes to downhaul will mean changes to mainsheet... outhaul, etc... overall sail shape..the whole rig in tune.. then just minor finesse changes while driving...

just my $.02.



Edited by robpatt on Aug 20, 2012 - 12:58 PM.

Also keep in mind the newer boats have more purchase on the downhaul. My P19 downhaul is 12 to 1, other boats have more. This helps in making the downhaul more effective.

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Steve Fisherkeller
P19MX
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Mainsheet is the gas pedal, downhaul is the transmission.

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Uwe
94 Prindle 19 - "überKat"
DFW, TX
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More than 8:1 purchase just makes it easier to pull the head plate out or do more damage to the sail.

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Ron
Nacra F18
Reservoir Sailing Assn.
Brandon, Mississippi
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stevefisherkeller

Also keep in mind the newer boats have more purchase on the downhaul. My P19 downhaul is 12 to 1, other boats have more. This helps in making the downhaul more effective.

--Steve Fisherkeller
P19MX

Steve has a mylar main on a mast that can handle 12:1 downhaul purchase (P19MX). If you put more than about 8:1 on a dacron mainsail, you will destroy the sail at its weakest point- usually the riveted headplate.

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Sheet In!
Bob
_/)_____/)_/)____/)____/)_____/)/)__________/)__
Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
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