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Post Options Post Options   Quote jeffers Quote  Post ReplyReply Direct Link To This Post Topic: rocker
    Posted: 01 May 18 at 6:15am
Originally posted by NickA

I was looking at an RS Aero the other day and realised that it has no-where to sit "up front".  It has a long nose and a mast that is well forward, but the crew is forced to sit quite a way back, behind the centre-board.  I guess it's designed to work well when planing at the expense of being able to hold the nose down.

The D-zero, on the other hand seems to have a better range of seating positions allowing you to get well forward of the centre board ... and also seems to go better up wind. 

I wonder if these things are perchance related!

In the D-Zero I am rarely too far forward unless it is very light (drifting conditions). There are a set of cleats on the deck that are roughly in line with the back of the daggerboard case. I find my front leg just resting against one is about as far forward as you want to go.

The front bit is used by my kids though when I take them out for a ride.
Paul
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NickA View Drop Down
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Post Options Post Options   Quote NickA Quote  Post ReplyReply Direct Link To This Post Posted: 30 Apr 18 at 9:32pm
I was looking at an RS Aero the other day and realised that it has no-where to sit "up front".  It has a long nose and a mast that is well forward, but the crew is forced to sit quite a way back, behind the centre-board.  I guess it's designed to work well when planing at the expense of being able to hold the nose down.

The D-zero, on the other hand seems to have a better range of seating positions allowing you to get well forward of the centre board ... and also seems to go better up wind. 

I wonder if these things are perchance related!
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Post Options Post Options   Quote Daniel Holman Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 6:46pm
Older, generally heavier and slower boats usually have the ends just kissing the water at full load. Hence not much forward trim needed in the light. Also, older boats squat and trim by the stern at speed so less need to get weight back.

Hullforms optimised more for semi displacement (5-12kts) will have their ends immersed at rest at full load as significant amounts of transom immersion is optimal from a resistance perspective, whilst maintaining waterline length.

What this means is that, along with lighter weight some Modern hp boats have a lot less “rocker” which by proxy means higher cp, and manifests itself in less squat / more dynamic lift, thus less drag in semi displacement p. In practice this was also enabled by the designers subsequent to ok etc understanding that sailors would be happy to move fwd and aft to optimise the boats trim to boatspeed.

Modern straight low rocker hp boats actually require a higher moment to change trim (mct) ie are less sensitive in absolutes trim terms to weight placement, but the hydrostatics / transom and chine immersion changes a lot more per degree of trim than say, a firefly.
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 5:05pm
very easy to over simplify... As a very sweeping generalisation older style boats have the mast much further forward, fuller bows and more rocker, which means that the cockpit is more amidships and more movement is required to make the same trim change.
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 4:43pm
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Post Options Post Options   Quote turnturtle Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 4:38pm
Hate to break it to you ... but your one just has a really fat arse.
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 3:53pm
Yes understood. My curiosity was regarding the need/effectiveness of sitting forward in the light stuff. The supposition being that old boats had more rocker and deadrise you don't need to sit as far forward to avoid dragging the transom. 
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 9:35am
You also need to consider structural requirements. The thwart across the middle of an old style plywood boat tends to be a major structural member supporting the centreboard case and the sides. Without that support a boat would bend in all sorts of undesirable ways. A double bottom boat, especially one with bulkheads under the false floor, is much stronger.
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 9:18am
Skiffy boats aren't designed with light winds in mind so I suspect the latter but we take advantage of the space in a drifter, obviously it's irrelevant once you are sitting out. 

Edited by Sam.Spoons - 16 Mar 18 at 9:20am
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 18 at 8:22am
Rocker and deadrise. But is the cockpit then designed like that to allow movement, or is it simply that a double bottom means you don't need decks, thwarts or other bouyancy tanks?
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