Stated Displacement vs Real Life - Page 5 - SailNet Community
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post #41 of 45 Old 01-12-2018
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Thumbs up Re: Stated Displacement vs Real Life

Originally Posted by bobperry View Post
Again you are asking me to generalize but I'd say the opposite. I think. Lots of variables. The fun part.

As boats approach hull speed the lighter boats will have the ability to climb over hull speed much easier than the heavier boats. Much of this is hull shape, flatter butts, move volume aft etc. The heavy boat with round butts is going to have a hard time climbing that bow wave and a steep butt angle aft can pull up a big quarter wave.

In light air it's mostly about wetted surface and some heavy boats can have low wetted surface while lighter boats can have high wetted surface due to flatter and more board runs. That's why in some light boats it's critical to get the boat heeled over in light air. Get the pointy end in and the fat end out. The resultant change in wetted surface can be dramatic. Not so much for a boat like a Valiant 40.
Back in the heyday of small boat OD racing (and cruising) in PDX - late 70's - , we shared the river on Wednesday evenings with a large Catalina 22 fleet. We would keep weight centered in our Ray Richards-designed Ranger 20's, but the Cat 22 folks would all send a crew person up to the bow pulpit in light air to lift some of the flatter run aft partway out of the water and reduce drag when sailing downwind.
It was a visible lesson in light air performance.

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post #42 of 45 Old 01-12-2018
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Re: Stated Displacement vs Real Life

Perhaps an easier way to look at this is to understand that most cruisers raise their waterline 5" or more, once the vessel is actually out cruising.

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post #43 of 45 Old 01-12-2018
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Re: Stated Displacement vs Real Life

Seems interesting to me that in these days of CAD, ability to computer generate water plane at any heel angle, computor generated sailing polars and computer generate behavior at various loads why builders donít just give you a disc describing your boat. They could throw in the GZ curve and other key info to make even the engineers among us happy campers.
Over time you learn your boat but itís hard. Example Iíve learned unlike some of my prior boats on this one flat is fast. On open boat derivatives not so much. They should heel some. Iím a slow learner so on a day when youíre running the same course and have the same wind you have the time and ability to fool around. On the current boat if we heel too much we go slower. At anything over ~20 degrees think we dig a hole. Also donít point as well. Counterintuitively weíre faster reefed and point higher. Getting the engine off the rail and dinghy on deck=faster. Moving the anchor aft= no noticeable difference. Total amount of fuel/water doesnít matter so much but using up leeward tanks first big difference. Would think all of this is predictable. Believe current builders/NAs are smart and use these tools. Sits on a drive or disc already. Instead of not knowing what the few numbers they share really mean donít understand why they donít just share the information they have already internally generated for their own use.
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post #44 of 45 Old 01-12-2018
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Re: Stated Displacement vs Real Life

Last three comments are why many race crews, be them paid of a bunch of friends doing local beer cans, practice, take notes as to what works, does not work etc. I;ve moved crew back or forward on the rail inches based on how the helm feels. Sailing newyears eve, by myself going down wind, helm was pulling upwind, Shifted myself back 6", balanced helm, gained half a knot! Generally speaking going down wind, move crew forward, or around the mast for best speeds. ABove hull speed when planing, you see folks on the rear to get the boat to plane some. Seen that in pics of Melges 32's.

A lot of people do not play with these options.


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post #45 of 45 Old 01-13-2018
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Re: Stated Displacement vs Real Life

Originally Posted by blt2ski View Post
I noticed in a recent Jeanneau 42DS owners manual last night. It listed three displacements. Light, IE empty base from factory. Half load and full load. My swag, similar to BP's three listed weights.
That has been my finding as well when it comes to the documentation for modern CE-rated monohull production boats. This strange conspiratorial mindset that there is some nefarious marketing aspect to these published numbers is a bit far-fetched. I'm not saying they are perfectly accurate in every case - but they are certainly NOT the sale-determining aspect of these boats for the broader market. "We bought this boat because of its attractive displacement!" As if.

At the end of the day, these published numbers obviously HAVE to be within verifiable ranges under the CE Standards. But as Bob says, one shouldn't assume that a published "full load" ("loaded") displacement is gospel. That number is obviously dependent on each individual skipper/boat - and what gets loaded onto that boat.

So the marketing conspiracy thing is a serious red-herring for which I've never seen any convincing evidence presented.

My personal interest is now more in the multihull side of things and how to think about loading for those boats - and what "displacement" really means in THAT world. For example, the new FP Lucia 40 specs "Approx. 8900 kgs - unloaded". That's a couple of thousand pounds more than the Hunter 40's published number.

So, what does that mean in terms of loading on the FP? That would be an interesting conversation. But that's a different cage of monkeys I suppose.
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