Measuring rocker?

So how do you properly measure rocker? Is there a table anywhere online of the rocker values for well-known boats? Thanks.

Forward and aft from lowest point. The measurement of the space is the forward and rear rocker.

Ryan L.

No proper way
Boat builders really don’t have any consistent method for measuring rocker. Boats with upturned ends which are relatively straight-keeled along most of the hull length might claim to have more rocker than a boat on which the keel line turns upward continuously from the center of the hull. The latter boat will paddle as if it had more rocker than the former.

If you just measure the vertical distance to the bottom of the hull at the extreme end of the stem with the boat level, a hull with very proud, “overhanging” stems will have a lot of “rocker” even if the part of the hull that is actually in the water is relatively straight keeled.

For whitewater canoes a quick and dirty way I have used to estimate rocker is to set the boat on a level surface and measure the vertical distance to the top tip of the stem at each end, then sit on each end of the boat in turn and remeasure the distance. The difference in measurements gives a crude estimate of rocker.

I think the best way to determine the amount of rocker is to simply look at the hull carefully.

Since the amount of rocker cannot be changed on rigid hulls and it relates to other aspects of a hull design, why the interest in measuring it? If you think it is going to give you some insight and a measure of performance from one boat to another, it’s probably not going to be very instructive.

That’s Silly

– Last Updated: Nov-03-13 12:12 PM EST –

Why would you measure to the top of the bow or stern, an area that is not in the water?

Most widely used and comparable method is to measure from the deepest point in the boat to the waterline at both ends. However, this still does not take into account the hull shape between these points and that makes all the difference.

Boats with lots of rocker will have most of the submerged part of the hull near the center of e boat and as you look towards the ends, less and less boat is submerged until you get to the waterline point where the submerged part gently disappears above water.

For boats with no rocker, both ends and the center are submerged equally. Think a flat slab in water. Obviously, such designs will track better and turn harder, all else being "equal" (if that's even possible to say).

So here's how you measure: deepest point (say 4" submerged) minus deepest point at the intersection with the waterline at each end. For a totally flat hull that difference would be 0". For a full rocker boat it will be 4". If the end of the boat is somewhat submerged at the waterline, the difference will be between 4" and 0", less for flatter hulls, more for hulls with more rocker. The overhang above water plays no role in this measurement. It is only relevant in very specific conditions when the bow or/and stern are submerged beyond where their normal positions on flat water would be.

As said, rocker alone is kind of meaningless without taking I to account other factors, such as the shape of the hull and underwater (and in some cases, over water) volume distributions.

I think we need a 2-part 3-D measure, that takes into account the above and underwater profiles and volumes. Part one will measure the "rocker" over flat water. Part 2 will take into account conditions where the bow and stern are submerged (as in between two waves as long as the boat and steep enough to submerge) - e.g., how steep a wave can the boat handle without getting totally locked in and uncontrollable (perhaps offering a measure of the lateral resistance to turning and the resurfacing buoyancy at the ends)... Probably never would happen, unless someone comes up with a clever computer model and the manufacturers feed good data into it.

But what do you do with it?
Having rocker extant from other hull design features, like whether it is fish or swede form, placement of cockpit (check out how far back the seat in in the Romany for ex) and the myriad of other considerations in hull design is kind of like judging whether a car will be a good family hauler by how many seats it has. It will tell you how many tushes you can put into in.

But it won’t tell you how it’ll really work for that purpose because you will have ignored things like storage space, cup holders charging ports and other gadgets that will quiet a seat full of kids and its allover reliability. (Getting stranded on the side of the road en famille is a nightmare.)

Hull design is a lot of things. Focusing on just rocker is not going to tell you much about a boat other than that it is probably more maneuverable than a bunch of other boats. Good for sorting boats into large groupings, not particularly useful for anything more than that.


– Last Updated: Nov-03-13 12:35 PM EST –

Maybe, but I have seen a lot of rocker measurements by boat makers that could only have been obtained by measuring to a point of the hull that is usually dry.

I'll give an example: the Dagger Prophet whitewater canoe was given a rocker specification by Dagger of 8" at the bow and 7" at the stern. Do you think that boat draws 8 inches amidships with a normal load?

Your method is not bad but the point at which the hull intersects the water is going to be influenced by load as well as a lot of other factors that go into determining the overall buoyancy of the hull, such as beam and prismatic coefficient, that have nothing whatsoever to do with rocker. Likewise, V-bottomed hulls, regardless of rocker, are going to draw a bit more water than shallow arch hulls amidships.

The fact remains that boat makers have not established any uniform method for determining rocker.

Good method. I’d add . . .
. . . that to get the rocker “for you”, you should measure to the waterline point when the hull is burdened with the weight it will carry with you and your gear in it.

Of course, as you and others have stated, this depth measurement does not take into consideration the shape of the underwater keel rocker or the distribution of the underwater hull volume. No problem. A solid understanding of integral calculus and differential equations, plus a few graduate courses in naval architecture and relativistic fluid dynamics, can calculate these things on any quad-core personal computer.

But computing keel rocker is only the beginning. Since most boats are turned when at heel angles, the “side rocker” at all heel angles to the rail must also be calculated and integrated.

Then, one must compare rocker turns to flat hull spin turns. For this, we must calculate the angular momentum and vortex shedding of the so-called planing hull.

All in all, I recommend: (a) putting the boat on a flat surface and literally rocking it; (b) turning the boat over to look at the shape of the keel line and side rocker lines; and, most importantly, © paddling the sucker.

If you are really interested

– Last Updated: Nov-03-13 2:22 PM EST –

The problem of measuring rocker and an attempt to adopt a uniform method has been discussed a couple of times on another forum. Three boat builders (Richard Guin of Mohawk Canoe, John Kazimierczyk of Millbrook Canoe, and Jeremy Laucks of Blackfly Canoe) and a boat designer (Craig Smerda) chimed in on the discussion with suggestions.

The discussion pertained to canoes and the OP was talking about kayaks, but the water doesn't know whether the boat has a deck or not (until it's upside down).

Thanks, but those discussions …
… don’t add anything to what we already know.

Which is: There is no industry standard for measuring rocker, and any measurement protocol is arbitrary and not necessarily informative.

However, an industry standard rocker protocol consistently applied would be better than the current situation.

Which is: Different rocker measurement protocols inconsistently applied.

if everyone here already knew that, this thread would not exist.

Good info for surf and whitewater boats

– Last Updated: Nov-03-13 4:50 PM EST –

Measured nose and tail rocker gives you a good idea of how a boat or waveski will surf on a wave, and how it will perform in general. Measurement is the height of the tip of the bow or tail off the ground when measured flat. Tail rocker is actually more tricky to measure. Best to look at a side on photo of the boat to judge the tail rocker.

Its the kind of thing
that you recognize when you see. Give is a look from the side and you can pretty much tell what kind of rocker its has and actually it does tell you quite a bit about how the boat will behave.

The Prophet
Sorry for the “silly” remark, that wa silly to say (on my part)!

If there ever was a banana boat, the Prohpet it is:

I would not be surprised if it has 8" of rocker by “my” method of measurement…

Good point…

– Last Updated: Nov-03-13 7:38 PM EST –

I suppose it is a matter of definition, especially where to measure. Many of these surf and short white water kayaks have a flat hull and very upturned noses. It is important to them to have these upturned noses. That makes measuring the rocker necessary the way you say.

But for a long touring/sea kayak type of kayak, such measurement would be meaningless. Because the area above water does not do much most of the time and is not nearly as important for that long boat as the upturned nose on a surf or whitewater kayak is for it.

For a long waterline kayak that is designed to slice rather than to plane, the curvature of the hull below the waterline is much more important.

I think there should be different ways of measuring for surf vs. for sea kayak.

I agree with both of you
There probably is no one protocol for measuring rocker that will make sense for both long boats, intended primarily for flat water, and short boats, intended for whitewater or wave surfing.

On a flat water boat a long, proud, overhanging stem up out of the water usually does nothing but catch wind and measuring rocker at a point above the waterline would not be meaningful in any functional sense.

On the other hand, big, bulbous, upturned bows and sterns become very functional on whitewater and surf boats. I haven’t surfed big ocean waves, but I can well imagine that when surfing down the face of a large wave the keel line of the boat is at an angle well off the horizontal, and an up-swept bow helps prevent the boat from pearling and pitch poling.

Likewise, whitewater boats going over big drops may come down at an angle well above 45 degrees to the horizontal. In this situation, or when plowing through large waves, a big, bent up nose keeps the boat from pitoning, diving deep, or taking on a lot of water and is very functional. Likewise, a highly rockered stern helps the aft end of the boat clear rocky shelves and ledges without hanging up.

The consensus of opinion on the cboats threads I cited seemed to be that comparing rocker figures for different manufacturers boats, even when they are of the same general type, is basically futile, and that the best way to evaluate how much rocker a hull has is to carefully look at it.

It isn’t simple
If you’re looking to compare boats, measuring at the ends doesn’t tell the entire story. You would have to factor in the rate of rise from the center to the ends. For instance, in the recent generation of creekers (Stomper and Recon), you see continuous rocker, with the deviation occurring from end to end, not just near the ends.

Rocker is vertical rise off the keel line at the stems. If we had stable measurements the data would be useful, but only within given hull lengths. I.e. 2" rocker for a 12 ft hull is moderate, 2" for an 18 ft hull is less so.

The symmetry of rocker is also important. Many hulls, think Prospectors, have symmetrical rocker; the same amount at both stems. Many modern hulls have differential rocker, more in the bow than stern, which resists poor paddle skills tendency to turn hulls away from the last stroke, almost universally at less then a vertical angle and always carried behind the body.

Another rocker feature is carry-out. Does the rocker extend towards amidships or is it abrupt? ICF boats have long carry-out, many WW hulls have abruptly rockered stems.

Winters proposed a standard measurement almost two decades ago; vertical rise one foot inboard from the waterline. Nobody uses that, including, maybe, John Winters, and certainly David Yost. Yost claims rocker is a drafting convention unique to individual designers. Just comparing those guys’ 15 ft solo trippers, both with differential rocker, is interesting. DY’s Kee 15 catalogs 2.5" bow rocker, John’s Osprey 1.5", yet Osprey obviously has more rocker into the bow?? So it goes.

SO rocker dimensions are helpful in determining hull performance between similar length hulls and would be more so if standardized. We need someone with a straight twenty foot board, a tape measure and, say, three or four weeks of time free…

My seat of the pants system for
assessing the effect of rocker is to see how it affects a boat’s ability to spin, to turn, and to track. There might be ways of standardizing tests for those, but I think I can usually make a good guess from photos of a hull from different angles.

Of course, rocker (however measured) isn’t the only thing affecting spin, turning, tracking. And so unless other hull characteristics can be held constant while rocker is varied, knowing the rocker doesn’t help a great deal.

Something I noticed when inspecting boats for slalom races was that a lower rocker boat might turn well because the hull was designed to sit lighter on the water. Sometimes people forget that when considering boats.

Hey, I’ve forgotten 5 semesters of
calculus. It was easy!