Keelboat news and development: Dehler 31 tuning and sailing

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Dehler 31 tuning and sailing

Printed From: Yachts and Yachting Online
Category: Keelboat classes
Forum Name: Keelboat news and development
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URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=3072
Printed Date: 13 May 25 at 7:02pm
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Topic: Dehler 31 tuning and sailing

Posted By: captainkirk
Subject: Dehler 31 tuning and sailing
Date Posted: 11 Jun 07 at 6:36pm

I'm not sure if this is the right place to asking this but I get the impression
this site has more readers that are into tuning and tweaking than others so
here goes.

We recently bought a Dehler 31 cruiser / racer, the boat is great but it has a
strong tendancy to head up when sailing upwind. This is apparently a
characteristic of the boat but I am sure that with the correct rig set up that
the problem could be made a lot better if not overcome entirely.

Any opinions on mast rake, bend, rig tension, sail twist etc, etc....would be
greatly apprecitated. Add to that helming and sail setting techniques aswell.

Replies:

Posted By: Stefan Lloyd
Date Posted: 12 Jun 07 at 6:36am

Too much rake? But the commonest reason for weather helm on keelboats is not keeping the boat flat enough. So:

Not enough forestay tension (over-deep genoa)?

Old baggy sails? Buy new ones.

Too much sail area in too much wind?

Not enough crew on the rail?

Helm needs to feather slightly?

Posted By: WildWood
Date Posted: 12 Jun 07 at 10:57am

Poor balance in your sail plan could easily explain the boat luffing too much. The mainsail encourages a boat to luff while the head sail makes it bear away. Therefore too much mainsail power relative to the headsail power would make the boat luff. I think the Dehler 31 has a fractional rig and therefore probably has a big main compared to the head sail size. To sort the problem you should try either depowering the mainsail or getting more power from your head sail.

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Posted By: Stefan Lloyd
Date Posted: 12 Jun 07 at 12:45pm

Originally posted by WildWood

The mainsail encourages a boat to luff while the head sail makes it bear away.

That's more true of dinghies than keelboats. On a keelboat, the tendency to luff comes largely from excessive heel, especially on designs with fairly wide sterns, which most modern boats have. As they heel, because there is more volume astern, the stern rises, the bow dips and the centre of lateral resistance therefore shifts forwards. The implication is that too much power from the headsail also contributes to weather helm. This is why forestay tension is a key tuning variable on keelboats with simple fractional runnerless rigs.

Posted By: WildWood
Date Posted: 12 Jun 07 at 1:36pm

Originally posted by Stefan Lloyd

Originally posted by WildWood

The mainsail encourages a boat to luff while the head sail makes it bear away.

So are you saying the boat is overpowered? and does forestay tension relate to power from the headsail?

I agree that too much headsail power makes the boat want to luff if the boat is already over powered, but assuming the boat isn't excessively heeled and still wants to luff, I think that could be linked to poor balance between the main and head sail.

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Posted By: Stefan Lloyd
Date Posted: 12 Jun 07 at 2:23pm

It might well be overpowered. It might be something else. Not enough information to tell.

Forestay tension is a very important tuning variable for keelboats. Too little and the headsail is too deep, lots of power but can't point. Too much and the boat lacks acceleration and is very difficult to keep in the groove.

Posted By: WildWood
Date Posted: 12 Jun 07 at 4:39pm

How much affect has back stay got on the forestay tension then? It seems like it would have an affect but we've primarily used it to a pre bend and flatten the top of the main. Do you adjust forestay tension by adjusting the shrouds?

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Posted By: Stefan Lloyd
Date Posted: 12 Jun 07 at 5:00pm

If the boat has a fractional rig and runners, forestay tension is largely controlled by them. For fractional rigs, swept-back spreaders and no runners, it's a black art. I used to own a boat with a single-spreader swept rig and no backstay. Setting forestay tension involved a combination of caps and lowers tension. If you just tension the caps, you increase forestay tension up to a point, then more caps tension starts to compress the mast and it decreases forestay tension. Adding lowers tension straightens the mast and you get more forestay tension. You need to fiddle about with a gauge and spanners and experiment.

If you have a backstay, then generally more tension = more forestay tension. However the backstay is also tending to compress the mast and the bow effect can push the forestay attachment effect forwards = less tension. Lowers tension counteracts this.

It's paradoxical but these simple rigs are complicated to tune because you get these complex interactions.

Posted By: Pete Cooper
Date Posted: 15 Jun 07 at 1:19pm

Originally posted by WildWood

How much affect has back stay got on the forestay tension then? It seems like it would have an affect but we've primarily used it to a pre bend and flatten the top of the main. Do you adjust forestay tension by adjusting the shrouds?

If it is a fractional rig with raked spreaders then the backstay should not be used to induce prebend. Prebend is set up with the backstay completely slack on this type of rig.