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rocker

Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Technique
Forum Discription: 'How to' section for dinghy questions and answers
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=13018
Printed Date: 16 Jul 19 at 9:10am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com


Topic: rocker
Posted By: Sam.Spoons
Subject: rocker
Date Posted: 15 Mar 18 at 11:25pm
I used to sail an OK in my teens, now I sail a Blaze. I've just got to thinking about the way the cockpits are laid out in older boats compare to more recent designs.

The OK and Solo for example have an area limited, usually by the mainsheet traveller/thwart, that keeps the helm in the aft part of the boat (no further forward than the widest mid section and behind the centreboard).

I think that is because older boats have more rocker than more recent designs and, maybe, don't need weight as far forwards to keep the transom out of the water in the light stuff (and nobody can argue that a Solo is slow in the light.....). In the Blaze it definitely pays off to sit next to the mast in a drifter (and I seem to remember sitting on the foredeck facing backwards in the OK but that's nowhere near as far forward as in the Blaze).....




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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"



Replies:
Posted By: zeon
Date Posted: 16 Mar 18 at 12:27am
.


Posted By: Pierre
Date Posted: 16 Mar 18 at 8:04am
.....


Posted By: Rupert
Date 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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Firefly 2324, Lightning 130, Puffin 229, Minisail 3446


Posted By: Sam.Spoons
Date 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. 

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: JimC
Date 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.


Posted By: Sam.Spoons
Date 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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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: turnturtle
Date Posted: 16 Mar 18 at 4:38pm
Hate to break it to you ... but your one just has a really fat arse.


Posted By: Sam.Spoons
Date Posted: 16 Mar 18 at 4:43pm
Embarrassed

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: JimC
Date 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.


Posted By: Daniel Holman
Date 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.


Posted By: NickA
Date 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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3604 ...lapse of reason
Javelin 558


Posted By: jeffers
Date 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.


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Paul
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D-Zero GBR188
Ex Rooster 8.1 '11'
Ex Laser 167534
Ex Blaze 655



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