OK, what are the benefits of rocker?

Rutgers

Rutgers

Well-known member
Going to build a boat this summer, not sure what yet, but one man marsh /small water/fringe edges of big water on the right day type boat. I am strongly considering the ZT Widgeon. Thought about a Gator DH for a while and maybe after the ZTW but for now looking for small one manner.

One thing I read about a lot is the whole it wont get up on plane "problem". My question is two parts and will completely reveal my ignorance on the subject.

1. What is the benefit of having "a lot" of rocker displacement hull. I've always heard the quick answer of "well its more seaworthy" but is this true and what are the principles behind it?

2. Will simply removing the rocker from the rear of a displacement hull make it a planing hull?

Like I said I know nothing about this whole thing so try to respond like you're talking a 12 year old!

Thanks in advance

Paully
 
dude your so stupid! *you said to respond like I was twelve*

Okay so the real answer is you already got it figured out. Taking out the rocker makes it a planning hull by flattening the back third of the hull. Rocker the reverse curve (reverse of the way the bow is curved) makes it a displacement hull. It wont plane because its forces the bow up and the stern down into the water. Its more sea worthy because its bow up cutting deep into the waves instead of planning out on top of and eventually bow into the waves.

How did I do?

All that said, sometimes I really just want my Garvey to plan the heck out and get me there. I know another 10 hp will get me to that point but I am not allowed to buy a 50hp because I bought the surface drive and kept the 40.

I would say unless you are taking this new sneak boat out onto something like Houghton, Black or Lake MI you would be better served in Michigan with a planning hull. My BBIII was a planning hull if I remember correctly and it was safe on Gogebic in normal conditions. Winds over 15 mph though and the waves were to big on the main body of the lake.
 
Rocker is basically a form of how the boat is drafted from stern to bow. Is it flat or is there some curvature in the hull, hence rocker. The more rocker (or draft) you have in a boat the more of the boat surface of the hull is below the water line. A simple principle of friction also known as drag takes place. To go faster you need less hull in the water to be safer you need a better foundation or boat hull in the water.

Rocker will change the performance of the boat in regards to speed, but as you reduce rocker in the boat you also reduce is ability to literally be safe in heavy wave conditions.

I do can not answer you second question. But my assumption would be that as it is designed with less curvature and becomes flatter it should increase speed on the boat.

Regards,
Kristan
 
mvr3a2.jpg

 
Well Paully, they're not stupid questions and you don't have to be a 12 year old to get confused on the subject. If you accept the premise that all boats are compromises, and that the boat owner knows best where he will run the boat and under what conditions, then you can start to narrow in on "how much rocker is too much?" and the closely related question "how much motor are you going to hang on this thing?".

If your hunting spot is more than a mile from where you launch and beating other people to that spot is important, then a planing hull may well be required. If so, a boat with less rocker will be required. Less rocker means the boat won't squat as much as one with more rocker (due to the flatter bottom providing more lift in the stern). Less squatting means a more level ride and potentialy less wake which may be important in your area. It also means you will probably want a motor sized to take advantage of the boats speed potential.

If on the other hand, where you hunt doesn't require a long trip or you are in an area with outboard size restrictions, maybe a boat with more rocker and less motor is the right answer for you. Potentially, a smaller boat, with smaller motor equals a lighter boat, and that could be towed down the road by a smaller vehicle.

As to seaworthyness of a boat with "lots" of rocker, again a trade off. Lets say you are in sitting in the stern of your small boat with lots of rocker, steering the motor. The combined weight of you and motor conspire to depress the stern so that the bottom of the transom is under water. A bigger than avg wave comes along and damn near swamps the boat. All that rocker isin't buying you anything, cause the stern is lower due to all the weight in the rear. On the other hand, if you are in the same boat, sitting more towards the center of the boat, steering with a tiller extension and the gas tank is midship or more forwards, then the boat with lots of rocker may well have the bottom of its transom above the water when the bigger than avg wave comes along. That wave, if it comes from the stern will act on the bottom of the transom first providing a lifting action on the rear of the boat.

If we are talking about ever wanting to row the boat, a boat with enough rocker so that the bottom of the transom is above water, then that boat will row easier due to the reduced drag of not having to tow a squared off transom through the water.

To answer your second question about removing the rocker from the hull? Probably, but will doing that make it an efficient planing hull? Maybe. A lot depends on how much the design of the hull narrows as it proceeds aft. If all you do is extend the bottom part of the freeboard in the aft part of the boat in order to take the rocker out, you will get a planing hull, but it may well take more horsepower to do so. If on the other hand you keep the full width of the boat further aft as well as take the rocker out, you will probably be able to plane with a few less horsepower. Which brings us back to "how much motor do you want to hang on this thing?"

Many books have been written on boat design and all the compromises needed to ensure that a design is successful in what was meant to accomplish. It is not a simple subject and my answer probably causes you to have more questions when you really wanted answers. I would close by saying figure out where you want to hunt and what the conditions are first. Then your choices in boat design get simpler.

John Bourbon
 
What the others said is true about the back of the boat having a curve. You see this is sailboats. That's why most old BBSB are displacement hulls. They were sailed to the hunting spot. The sneakbox I rebuilt last summer was a displacement hull. I have a 15hp on the back. With a full boat and wot I get 17-19mph that's plenty fast for a small one man boat IMO. If the ZTW is a displacement hull I would build it that way.

Your 2nd question the answer is yes. By taking the rocker out of the rear and having it be a straight shot back to the transom it will plane. Good luck with the build.
 
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I built a ZT Widgeon about 15 years ago and have been very satisfied with it. I used a sewn seam-like construction to eliminate all the ribs inside but I made the outside hull dimensions as true to the plans as possible. Although it can handle more water than it might at first appear to be capable of doing, I don't believe it to be a big water boat in either a displacement or a planing hull configuration. It excels in small lakes and marshes. I use a 6 hp motor on it and find it will not win many races. The boat rows well but poles like a dream thanks to the displacement hull. Its shallow draft and smaller size allows me to get into some very remote areas. The boat hunts well and is, I think, pleasing to the eye.

If I were to build another tomorrow, I would consider a couple of modifications to the plans. I would think long and hard about how I could increase the cockpit size versus what the plans specify. I made mine with an oversized cockpit and still find it to be tight quarters especially with a dog. Second, the plans show the transom to be perpendicular to the waterline and that's how I built mine. In hindsight, I think a 15 or 20 degree angle on the transom would improve the boat's performance and would permit the motor to be run at a more optimum angle. I might consider lessening the rocker to bring the bow down while motoring but would not entirely remove the rocker---there is some advantage to having the bow up a little when heading into the wind.
 
This question has been on my mind a lot lately as well. I'm building a modified BBIII at the moment, one of the main mods I have made is to remove all rocker from the stern meaning a dead straight keel line from amidships back, wheras the BBIII as designed has roughly 3/4" of rocker designed into the rear of the sponsons. I used to design, shape and glass surfboards and rocker is a very important part of surfboard design, to make a long story short rocker makes a board easily turnable but the more rocker you have the slower the board will become. My favorite surfboard design used a fairly straight rocker along the stringer (keel) of the board but the rails (chines) of the board had a fair amount of rocker in them, creating a kind of compound rocker that allowed the board to go very fast in a straight line but when it was turned onto the rail it was very turnable due to the rocker in the rail line. I have been wondering if it would be possible to adapt this concept to a boat hull to combine the properties of rockered vs. non-rockered hulls.
 
Thanks for all the replies, gives me a lot to think about, but I'm leaning towards just following the plans. Most everywhere I hunt the marsh isn't conducive to running at any kind of speed and I have a couple other boats that would be used for big water. Not to mention I was a bit intimidated in making alterations to the plan having only built a couple layouts that were pretty cut and dry. Thanks again,

Paully
 
Rutgers, Have you looked at Devlin's Broadbill? I have had a ZTW and a Devlin Broadbill. Both are small boats but the Wigeon is really small. I really like the planing hulls personally. I have had 2 different displacement hulls thru the years and they row well. If I'm hunting and running a motor I will take the planing hull. A Broadbill with a 9.9 to 15 hp will get you there and it is fun to run.
 
I do not know your concealment requirements. The Cackler my be a better choice for a hull design and it is rated for a 30 hp motor and planing. You can contact Devlin about modifications to make the boat more useful for your needs. Dave Clark's new design, that he posted on this site, is very similar to the Cackler with more of a flat bottom to the hull. You may want to a search to find the post of the design.

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