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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Windward performance
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Topic Review (Newest First)
04-01-2012 03:42 PM
MedSailor
Re: Windward performance

Quote:
Originally Posted by kulokoo View Post
I am wondering about the hydrodynamic effects. For instance, does pronounced tumblehome produce a lift to leeward? Or maybe it helps the leeward hull side "stick" in the water and reduces leeway?

The boat with pronounced tumblehome I have sailed on is a Catalina 38 which points and goes to windward very well, and buries the rail (and of course the tumblehome) when there is sufficient wind. It seems to like that.

The tumblehome was part of an S&S race design as I understand it, so I am assuming it was done very purposefully. But why?
It may have been for looks, reminiscent of when tumblehome kept the weight of the cannons closer to the center-line and helped deflect cannon-balls. Not all design features are for performance. These links might help:
http://www.sailnet.com/forums/sailbo...rformance.html

Boat Design Forums

From the link above:
"There are several advantages to tumblehome: as already mentioned, it reduces inverse stability; it allows outboard shrouds to be moved in relative to overall beam and thereby reduces sheeting angles; since a curved panel is stronger than a flat one for a given scantling, one can achieve greater strength with less displacement. The disadvantages are reduction in side deck area and a slight increase in tooling costs. Since offshore racing boats do not utilize outboard shrouds, one can only assume that the reduction in side deck area was a safety factor considered more important by the developers of a rule intended for crewed vessels. Brad"
04-01-2012 12:04 PM
kulokoo
Re: Windward performance

Quote:
Originally Posted by AdamLein View Post
Are you wondering about hydrodynamic effects on the leeward side, or aerodynamic effects on the weather side?

In the first case, if you're heeled over that far you have other problems. In the latter, I can't imagine it would have much affect on windage.
I am wondering about the hydrodynamic effects. For instance, does pronounced tumblehome produce a lift to leeward? Or maybe it helps the leeward hull side "stick" in the water and reduces leeway?

The boat with pronounced tumblehome I have sailed on is a Catalina 38 which points and goes to windward very well, and buries the rail (and of course the tumblehome) when there is sufficient wind. It seems to like that.

The tumblehome was part of an S&S race design as I understand it, so I am assuming it was done very purposefully. But why?
04-01-2012 10:16 AM
peterchech
Re: Windward performance

MODERATOR

This should be a sticky it's a great write up, even ten years later
04-01-2012 01:56 AM
AdamLein
Re: Windward performance

Quote:
Originally Posted by kulokoo View Post
Any thoughts on the effect of pronounced tumblehome on windward performance at high angles of heel?
Are you wondering about hydrodynamic effects on the leeward side, or aerodynamic effects on the weather side?

In the first case, if you're heeled over that far you have other problems. In the latter, I can't imagine it would have much affect on windage.
03-31-2012 02:51 AM
kulokoo
Re: Windward performance

Any thoughts on the effect of pronounced tumblehome on windward performance at high angles of heel?
01-28-2010 07:16 PM
landmarine
Great Explanation

It's interesting modifying rudder shapes etc. and seeing the huge difference to the way the boat feels Jeff. We have a 25 foot trailer yacht and the standard rudder blade is approx. 38mm thick but a poor approximation to say a modified NACA section. When I redesigned the blade (with the assistance of a designer I might add) I couldn't believe the difference it made! The new one is around 42mm thick and approximates an 0012 with the concave exits. Upwind now there is way less windward helm and the boat sits way better "in the trim groove" and heels less. Downwind there is less drag too (no longer trying to hold onto 3 dead Buffalos) - We also brought the blade's leading edge closer to the gudgeon pivot axis which means it's no longer acting as a brake when going through turns (old position was 40mm aft). Looking forward to going through the same process on the keel. Need to seal up the keel / hull interface (layup some carbon and form a template that attaches to the bottom of the boat), then look at the shape characteristics. As you correctly point out Jeff, sails & foils are inter-related and are critical to how a boat performs. Now I'm starting to discover just how everything works by playing around with things. Thanks for your article
09-05-2009 09:11 AM
mgiguere Hi Jeff:

What a great piece! The only thing I didn't hear about is "the slot" and its effect on upwind performance. Doesn't that contribute to lift and isn't that the reason the main is the primary sail that lets you go upwind?

Moe
Apache 37
09-04-2009 04:51 PM
jsmithsussex As a very beginner and newbie to sailing but a mechnaical engineer , thanks for all info. Now I see the advantage of not sail to close into the wind.
01-12-2007 02:09 PM
Jeff_H In the days before low stretch sheets and sail cloth, and in the days when most boat's windward performance was limited by their inefficient long keels as much as by thier sails, cutters with their comparatively small headsails were considered quite weatherly since weather performance was less diminished by the smaller amounts of stretch that occured with their proportionately smaller headsails. With modern low stretch sail cloth and modern low stretch line, stretch has become far less of an issue, and so the interference between the forestaysail and the jib becomes far more critical in affecting windward performance.

In other words, when you have two headsails filling the foretriangle there needs to be an adequate slot between the jib and the mainsail and then a second adequate width slot between the forestaysail and the jib. This makes a wider forestaysail sheeting angle and that typically greatly limits how close a cutter can point as compared to a sloop.

Jeff
01-12-2007 12:52 PM
Idiens Thanks, I should check the dates.
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