Rocker Bottoms

[mlp]

Not a professional, just a newbie having fun. I'm looking for some general information concerning rocker bottoms.
To keep the explanation very simple, and to help me to more easily understand, let's stay with a very small boat. For example:
How does one determine the 'shape' or 'profile' of a rocker bottom for a small Flat-bottom skiff?
Is the math formula(s) for determining this complicated?

Ray (mlp@nfo.net)

 

[jehardiman]

Rocker is added for several reasons. It is used to adjust the volume distribution. It is used to minimize wetted surface. It is used to adjust the lateral plane. It is used to adjust yaw inertia to improve maneauverability. It is used to reduce wavemaking. Like most things in boatbuilding, it is interlinked and there are no real hard and fast rules. I generally place my maximum rocker at about the point of maxumum section and have slightly more rocker forward than aft to reduce tripping.

 

[mlp]

Many thanks for the comment. That's what I wanted.
That's a good way of thinking generally about it, still...but how does one (or maybe how -should- one) come up with a more definite plan.

For example, take a scow type of skiff (I'm not planing one.. just for talking). The "maximum section" is uniform every where. Ergo, one can't
just start at the widest section and go forward.

So what is the correct way to think about making the rocker profile. Again, let's just assume a crew of one, and a known waterline.

How does one begin to estimate where to start the forward rise of the rocker --and how much rise?
How does one begin to estimate where to start the aft rise of the rocker...and how much rise?
And maybe .... how much "flat" bottom between the rises?

Are there any good ways to make the first guesses as to these points.

(hey... I told you I was a newbie.)

 

[KCook]

Expected speed range would be some help? Rare to see any rocker in high speed hulls.

Kelly Cook

 

[mlp]

Yes. I should have mentioned that... very low speed. Just for discussion... assume rowing speed... say 3 or 4 knots ?? What elements should one compute to get the first approximation for the profile of the 'rocker'. To keep everything simple, we're just considering some sort of flat bottom skiff or scow sort of thing.

 

[jehardiman]

IMHO, for a rowed single-chine skiff, the rocker should match the rate curvature (but not necessarily the depth of curvature) of the waterline when a straight line is drawn between the stem and the transom chine with the stem and transom just kissing the surface at load condition. This will call for a good knowledge of all up weight to get right. The more rocker, the less directional stability and the larger the skeg will need to be. The more the rates of curvature differ, the more the chine drag will be.

Edit: I just re-read you fist post and I noticed a confusion on your part. When I say section, I mean the area of the immersed transverse section anywere along the immersed body. The intergration of the sectional areas along the hull gives the displacement. Except for a perfect brick, the section begins at zero and changes along the length of the vessel and ends at zero. In a scow or even a barge, the rake of the forward transom and rocker of the hull is used to manipulate the center of volume to reduce resistance.

 

[mlp]

I fear you have lost track of what I am asking.

I have no problem integrating immersed areas from stem to stern.
I have no problem in computing rates of curvature - from a parallel line.
I have no problem computing depth at waterline for differing loads. etc.

Assume a perfect Brick - empty - sitting there in the H2O. At this moment, it ain't going anywhere. Just sitting there.
What math would produce a family of profiles so that one may choose one that will be more suitable for any given situation.

In a perfect brick, we start with the immersed transverse section areas perfectly constant over the whole length.

Specifically, what is the Math used to produce different profiles for different situations?
Or to put it differently, what factors are calculated to produce an increasing or decreasing rate at each immersed or full station.

Are you able to give a simple example. Forinstance. Here's a magnificent boat I just designed this very instant.
with, say, stations 1 foot apart. The verical lines could be 2-3-4 inches each,... or whatever you wish.
So.. all we have here is a very primitive 10 foot scow, sitting in quiet water. (at some uniform depth)



| | | | | | | | | |
| | | | | | | | | |
| | | | | | | | | |
| | | | | | | | | |
| | | | | | | | | |
|---------|--------|---------|---------|---------|----------|--------|---------|---------|-

T 8 7 6 5 4 3 2 1 Bow


Could you show me the math/considerations you would use to plot
1) what part of the bottom could be flat
2) where to start and end the rocker at both the stern and prow.

You are free to add or assume weights to make the calc easier.
You are free to assume any given situation.

What I would like to see, are the numbers.
What I would like to see is just what calculations are involved.

I hope I haven't talked too much, or thrown you off the scent.

mlp mlp@nfo.net

 

[mlp]

Oops. The Vertical lines didn't reproduce correctly.... they were supposed to space out. Sorry.

mlp@nfo.net

 

[lewisboats]

I have some familiarity with box boats....So I'll wade in here a little. If the only curve to the boat is going to be the bottom rocker, I'll want to know where the center of the cargo mass will be and how much the mass is. Within reason, rocker allows an increase in mass while attempting to ease waterflow. The position of the apex of the rocker will dictate the trim of the boat in relation to the location of the mass of the cargo in the boat. I have used the drawings below to illustrate the relationship of rocker to center of mass (and buoyancy). Then you gotta throw in the intended purpose of the boat and the asthetics desired. Shifting the center of mass will alter the trim of the boat and consequently its performance. The boat shown is 8 ft long, 3 ft wide and 14" deep. Rocker in front and back is 6", weight of person is 200 lbs and load waterline is 250 lbs. The amount of flat in the middle of the last picture illustrates only that by increasing the 'belly' of the rocker it can hold more beer...I mean mass. Look closely at the waterline (darker blue). It has shifted from just above the light blue line to just below, indicating an increase in the load carrying capacity of the boat. The squarer the ends, the more the drag tho. The ultimate extension is a brick...maximum carrying capacity with horrendous drag losses.

Steve

 

[mlp]

Steve: Thanks so much. That is exactly the type of analysis that I was looking for. Please let me
digest it and I will get back to you. One of the things on my mind was the center of mass location. In a rocker, that can do strange things, can't it. I will get back to you.

 

[rebar]

I have some familiarity with box boats....So I'll wade in here a little. If the only curve to the boat is going to be the bottom rocker, I'll want to know where the center of the cargo mass will be and how much the mass is. Within reason, rocker allows an increase in mass while attempting to ease waterflow. The position of the apex of the rocker will dictate the trim of the boat in relation to the location of the mass of the cargo in the boat. I have used the drawings below to illustrate the relationship of rocker to center of mass (and buoyancy). Then you gotta throw in the intended purpose of the boat and the asthetics desired. Shifting the center of mass will alter the trim of the boat and consequently its performance. The boat shown is 8 ft long, 3 ft wide and 14" deep. Rocker in front and back is 6", weight of person is 200 lbs and load waterline is 250 lbs. The amount of flat in the middle of the last picture illustrates only that by increasing the 'belly' of the rocker it can hold more beer...I mean mass. Look closely at the waterline (darker blue). It has shifted from just above the light blue line to just below, indicating an increase in the load carrying capacity of the boat. The squarer the ends, the more the drag tho. The ultimate extension is a brick...maximum carrying capacity with horrendous drag losses.

Steve

Ive been looking for plans and noticed the Dory plans I was looking at had rocker.. AI's answer was the designers knew the boat would sag with time so they incorporated rocker in anticipation of the bottom ending up sober, uh flat I mean..

 

[Skip Johnson]

I must say trusting AI is a really really risky business. Hogging was a serious issue with older larger wooden vessels a century ago. The rocker in a dory is for different reasons altogether. For one it's the natural outcome of using fairly straight side panels that are flared out., but the main reason(s) for rocker revolve around hydrodynamic issues.

 

[portacruise]

What are the goals you are trying to achieve using rocker for this boat? Efficiency? speed? maneuverability? occupant space? etc. For some applications using rocker might take you further away from at least some of those goals.

 

[rebar]

What are the goals you are trying to achieve using rocker for this boat? Efficiency? speed? maneuverability? occupant space? etc. For some applications using rocker might take you further away from at least some of those goals.

My goals are a electric outboard Jon20. Possibly a 9.9 hp. Un-rigged around 400 lbs, top speed 10 mph cruise at 4. So basically a 20x7 barge pig. I realize its not a efficient shape, but I need to be able to float on dew when/if I'm rowing it in no motor areas. The rectangle shape also allows space for solar panels. Take a look at True North on YT..

 

[rwatson]

The shape is efficient enough for a powered hull, but you mention rowing. If you are going to spend a significant amount of time rowing, the flat stern will be very "draggy", and slow you down.
Its a function of how much time you expect to be travelling over say 8-12 knots.