i550class.org

..no thilly,,NOT music from th'50's ....the curvature fore aft!!!

....this measurement seems to have very little control in either the build or the rules**,,,,
......but is quite significant in a boat's performance in various conditions.

....not enough is known of the i550's yet to know where the 'hot' rocker's should be,,,but by measureing and comparing these,,a LOT can be learnt about the build and performance as time goes.

...I did a quick comparison of a couple of westcoast boats with a cord,where I held it 8'' below the stern and touched the hull's centre tangentially and measured string-to-bow knuckle,,,,seeing ~5'' variable between two build methods-rather a lot!

....a more accurate and informative method would be to use a spirit-level-hose,,,,level the bow/stern,,,then have a horizontal base-line touching the lowest part of the boat,,,and measure between this line and each of the bow and stern.

...I think there'll be enough boat variables and consistancies in the northwest next year that we'll be able to make some worthy learning in this matter



**...please correct me if I'm wrong on ANY factual details

I always reckoned it was pre-defined by the curve of the hull panels...if the planset is followed exactly, shouldn't there be zero deviation in "rocker?"

admin wrote:I always reckoned it was pre-defined by the curve of the hull panels...if the planset is followed exactly, shouldn't there be zero deviation in "rocker?"

..in a perfect world........................
....I think there's a LOT of variables that can happen in the details of build ,,especially with that very simple jig,,,and placing weights,,,etc.
....fuller control would only really come with a proper full jig-I'd highly recomend this at least for group builds

First, note the measurement method with a string that you describe actually measures twice the actual hull rocker. Use the average of your measurements to get the actual rocker (i.e., with the string touching the low point, add the 8" at the transom to the measurement at the bow, and divide by two).

In this class, the strongest control on rocker comes from these two rules:

2.2 Hull side and bottom panels shape shall be cut to the specifications of the i550 plan set as provided by the copyright holder and assembled to fit within the bounds provided with this section.
2.3 Chines may be rounded in nature, however all hull panels shall make contact along the chine.

Basically, if you cut the hull panels to the shapes in the plans and assemble them so they make contact along the chine, you'll get a hull shape within a half inch of the plan rocker, in my experience. Note I built on a jig that held the bottom to the plan shape, then dropped the side panels on afterward, so I could see what effect a different panel shape or chine gap would have had. For my build, the side panel met the bottom panel within about a 1/4" along the whole chine, and that gap tightened a bunch when a little coercion was applied.

Can you meet the requirements of those two rules and still get significantly more or less rocker? Well, by varying the part of the edge of the panel that makes contact, you have +/- a 1/4" at the ends added to +/- a 1/4" in the middle, for a total of +/- 1/2". Then, if you make allowance for folks cutting those panels maybe a 1/4" different than the shape in the plans (about the limit of tolerance to be considered a good faith attempt to cut "to the plans" imo), you get an additional +/- 1/4" at the ends or middle, again totaling +/- 1/2". Add those, and it's +/- 1". If you then put a lot of pressure on the hull shell as you stitch the joints and frames, you might get another +/- 1/2"...

Can this be enforced? Since there is no rule that directly controls rocker, I don't think it can be except in cases where the hull panels are obviously not cut "to the plan", for instance where use of a Tyvek overlay reveals convincing evidence that the path of the chine does not conform to the designed hull panel.

Will different rockers have an impact on performance? Well, duh, hell yes! If going to one side of a 9mm tolerance is the only way to get a competitive Fireball, then it's logical to believe that the inches of tolerance in this class will eventually leave a lot of boats behind.

Welcome to grand prix plywood homebuilding!

interesting details Chad,,methinks it'll be interesting to watch the rocker as the class(es?) develop....

...back in th'day,,,I remember how the rockers on little 8' sabots would vary,,,coming off the same jig,,depending on how much spring remained in the chine re-enforcements

I think creative exploitation of those pram tolerances is what has lead to the Opti's 3mm tolerance, which homebuilders still achieve. I've long said that the huge design box isn't in the i550's best interest, unless it is to be marketed as a grand prix development class. Which is just silly, imo.

Interesting thread. I am surprised at the length of time it has taken for this subject to be initially explored. I think Notorious' last paragraph sums up my view pretty well, too. I am aware that the Portland mafia has looked at the rocker issue and may have purposefully pushed the hull rocker one way or the other. I don't know. Remember that we are in generation ONE of the i550. The tight tolerances of the fireball performance/nonperformance issue took how many hulls and years to figure out? The Optimist tolerances; again how many years has it taken to determine the faster shape? Both are essentially sheet goods based hulls. Over time we may find one corner of the design tolerances does this or that in this or that wind and surface condition. Or not. CB and I had a conversation in the earliest days of our acquaintance in which this subject was discussed. The upshot was that we knew that we didn't know and no one else did either. A known unknown. The decision at that time was to keep the tolerance to +/- one inch. Not to encourage exploration but rather to encourage AMATEUR builders to be confident that their skill set would yield a compliant boat. Classes which have a long history (Star) of Olympic level competition and development have tightened the tolerances to such an extent as to make the production of a new plug/mold/product to be extremely critical. That's what 100 years will get all of us.
Hulls evolve. Rigs evolve. Blades evolve. With the variables of Human experience We (the larger group of past present and future i550 builders) MUST be aware that some tolerance must be allowed in hull geometry. In one hundred years its gonna be someone else's headache, or not. By that time they will know what we don't yet.
We are in Generation One. Let's behave like it. We, in fact are behaving like it, voicing opinions with conviction and depth. Let's keep perspective on what we are all trying to achieve, in focus.
Tim

yeh,methinks I hear a reasonable consensus of thoughts on this.
...it'd be interesting if builders were to use the simple measure and compare as they build--it'd show how much it varies build-to-build

I like the idea.

The new ENHANCED LeeH plan package has a set of 12" offsets which will give us a truth mold if done right & set up upside down. This would include rocker as per design and support for the ply continuously.

If you build one, build it. If you build two, make a jig.
Bill Cooper, master machinist. 1887 - 1968

M&S wrote:I like the idea.

The new ENHANCED LeeH plan package has a set of 12" offsets which will give us a truth mold if done right & set up upside down. This would include rocker as per design and support for the ply continuously.

If you build one, build it. If you build two, make a jig.
Bill Cooper, master machinist. 1887 - 1968

,,nice,,I didn't realize the new plans include this!
....I'd update bill cooper with.....if you build one i550,,make a jig ,,then pass it on to th'next guy

Perhaps better yet to lend confidence and a foundation for completion of a build I offer a five piece hull for a price, throw in the plans, and get the confidence hump "humped". A five piece hull; stem, bottom panel, 2 sides panels and one transom. Glued up on the build mold, faired and glassed to x? degree and ready to pick up or ship. Proud recipient will need to place frames on the marked dotted lines of the epoxied hull interior.

I am very keen to increase the number of boats. Ideas are welcome.

So we have some builders pushing limits of the plan set, a given because we are by nature a competitive lot and more so because were building our own boats. I'm unaware of the rocker changes within other classes, haven't followed them. Other then the reflex in the aft portion of the hull to try extending the apparent water line, does anyone have a guess what the optimal rocker might be based on other boats who have played with it? We have a star boat on shore at the moorings. Though I haven't measured it I'm sure it's close to the rocker in my build, maybe a bit more, width being the exception. It gets down to more rocker is better or not. I have what I have, no plans on changing the Twist hull for the extra?

I've found in the few races we entered this fall that it's the upwind legs that we get killed in. It's just a matter of waterline when the next smallest boat in the fleet is a j24. Both races were light winds, under 10 and mostly 5s which didn't help much even on the down wind legs. I just found this topic interesting. From what I know right now, anything to help up hill speed or pointing would be great.

There's been some rocker discussion in Micah's Carbon Offset mods thread, but maybe this is a better place to continue the topic...

Here's a pic of the rocker measurement method Micah described, applied to a hull that is exactly as the plans describe:


So holding the string with an 8" fixed offset under the transom, letting the string just kiss the hull bottom somewhere near the keel, and measuring the gap to the bottom of the bow, you should get something pretty close to 5 5/16" (135mm).

I measured mine (starting 2" from the transom since I extended it to 216"), and I get an extra .12" of rocker measurement at the bow beyond the value shown in the pic above.

My only experience with rocker is with surfboards, where the deeper the rocker the harder it is to get it to plane. Deeper rocker was built into boards that were going to be used in larger surf, to keep the nose from digging in as the wave pitches behind you. Is it the same for boats? If it is, then more rocker means you'll be pushing more water in light air, but you'll climb out of chop a little better.

slowpoke wrote:My only experience with rocker is with surfboards, where the deeper the rocker the harder it is to get it to plane. Deeper rocker was built into boards that were going to be used in larger surf, to keep the nose from digging in as the wave pitches behind you. Is it the same for boats? If it is, then more rocker means you'll be pushing more water in light air, but you'll climb out of chop a little better.

..yup slo-po,,,the comparison's okay,,,,though a surfboard probably doesn't have to deal with displacement conditions too much,where more rocker is probably helpful for less wetted surface,and I'd think it's a sailboat's general buoyancy that has it pop through a wave ,,flat gives more potential speed,and as with a surfboard will plane easier,,,,but can become 'sticky' in the light stuff--definitely lots of trade-offs allround

EDIT...I should add some thanks to chad for the drawing above,,,,it'd be interesting for people to compare this fundamental and easy to take measurement.
...it's interesting to note that as the boat's 'hump' flattens by 1'',,the measurement probably changes by over 2''

Good thread, thanks for including the drawing Chad. I think I'll measure mine today. When we flipped the boat last weekend, I was actually surprised how much rocker the profile had, to the naked eye at least.

Tim Ford wrote:Good thread, thanks for including the drawing Chad. I think I'll measure mine today. When we flipped the boat last weekend, I was actually surprised how much rocker the profile had, to the naked eye at least.

...yeh another function of the rocker is to help accommodate the athwartships-flatness of the bottom panel

,,,as more boats hit the water and sail in more variable and stronger wind conditions, ,,I'll bet it's found that sometimes in stronger downwind conditions,the hard-edge of the bow will sometimes want to 'track' or resist steering impetus from the rudder,,to what degree I'm unsure,,,I guess the Aussie owners could make comment to this?

I remember Josh's comments on Shazza wanting to Chine Walk. apparently sailing it flat will reduce the effect. I know similar chined boats steer funny if the angle of heel changes quickly going downwind.
When the boats begin to gather in groups we will begin the long learning curve vis a vis rocker A and rocker B. There is along path before us.

I recall that Josh's comment was along the lines of running with a little heel to bias one 1 chine over the other helps to keep a consistent behavior down wind. The radius of the forward chine should also be a factor in how drastic that it. The sharper the chine is forward, the more it's going to try to steer the boat. Ron made a comment to me about that too. I think the lunatic has a pretty sharp forward chine with maybe a 1/4" radius. I've not been out in heavy enough conditions while running to be able to comment on the effects of my own chine's profile has.

Kevin.

From my experience the chine hulls do have a distinct transition point from keel-rudder dominated tracking input to all of a sudden the chine having more input. Only time in the boat will give results. Eventually some sort of consensus about chine shape, mainsail area and shape and all the other factors including rocker will become more evident. There is a long way to go.

We've been hearing quietly (privately) from individuals that building to the plan's shape with a single transom hung rudder is the healthiest path for the class at this time. I am in favor of this approach. We've many years of i550 sailing ahead of us.
Every single hull is going to have minor differences just as every builder has different expectations of their build. If the intention of the builder is to make the boat to the plan shape then the class will grow as more boats will be built.

The only way that the i550's will ever be exactly the same is IF they come out of a Jello mold. The odds are against it at this time and in this economic climate.

Kevin wrote:I recall that Josh's comment was along the lines of running with a little heel to bias one 1 chine over the other helps to keep a consistent behavior down wind. The radius of the forward chine should also be a factor in how drastic that it. The sharper the chine is forward, the more it's going to try to steer the boat. Ron made a comment to me about that too. I think the lunatic has a pretty sharp forward chine with maybe a 1/4" radius. I've not been out in heavy enough conditions while running to be able to comment on the effects of my own chine's profile has. Kevin.

...I'd think it'll take a 3-4'' chine 'panel' to rectify this properly,,,but only an issue for -extreme- sailing conditions
...probably get a fair bit of 'feedback' on this issue by early august after the gorge event

Are you adding running backs to the tin rig for Double Dammed? mebbe instead of a Whomper you might consider a Whimper.
tongue planted in cheek.

M&S wrote:Are you adding running backs to the tin rig for Double Dammed? mebbe instead of a Whomper you might consider a Whimper.
tongue planted in cheek.

...realistically,I'll be rather surprised to be doing the doubleDammed after the 3 day Cgod
..don't know if there'll be a lotta boat left--gorge is a rough 'ntumble place

...would -wish- to have a fractional kite forsure,,,doubt I'll be that organized

Actually I am sending as much speed build as I can to Carbon Offset and you. There's a lot to get done. Power on.