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Rocker/Trim

seedtick

Well-Known Member
Jul 22, 2006
1,161
7
Denham Springs, LA
Ron,
I think you and I are on the same page, even though you may not think so, your loaded boat as shown in the pic, is trimmed exactly the way I would trim it to minimize wind effects, maximize the effect of paddle strokes and track in a straight line, i.e. the bottom of the front stem is submerged (I call that nose down).

On the other hand, if you're looking for maneuverability, having the bottom of the front stem out of the water is what you're looking for. You may not have enough rocker to load your boat and trim it out so the stem is out of the water (nose up to me). This is nose up to me and it doesn't take much of a weight shift to get to nose down.



Our swamp pirogue has more rocker than the marsh pirogue so I can load it up and still trim it nose up for maneuverability.

Our marsh pirogue with little rocker, goes nose down when loaded just like yours

to me, There's not a lot of difference in paddling characteristics when both boats are nose down. That's why i say that in those conditions, rocker is not significant
 

oldsparkey

Well-Known Member
Aug 25, 2003
10,417
109
78
Central , Florida
www.southernpaddler.com
What I can't figure out is he keeps asking questions and as he posted in one post ... He has two similar boats , one with the rocker and one without the rocker. :roll:

I thought my boats with "lots" of rocker could be improved to better suit me, so I did build one with less rocker. For my use it is a much better boat. I am better served by it than a boat that might turn on a dime. For me better tracking trumps ease of turning where I paddle.
In my simple mind it seams like the best way to answer the questions is to take a day and the two boats and see how they act or react in the same water. That should provide all the answers , a simple and easy way to find out , then doing all the speculation on here about it. If it is needed then a partner to take some pictures and then the other to take some pictures to see the difference. :D

I for one can not see what all the hoopla is about the rocker in a boat since it is a personal choice , especially when a person has two different boats with and without the rocker to see how they will answer all of there questions. :?

It sort of reminds me of the draft on a boat and how that discussion when on forever in several other posts , rocker is what is built in the boat and the draft of a boat ... It depends on the boat and the load in it , Pure and simple.
Appears to me me we are rehashing last weeks lunch.......... :mrgreen:
 

jdupre'

Well-Known Member
Sep 9, 2007
2,290
38
South Louisiana
To me, nose down would indicate the bow being lower than the designed trim level at that particular draft. Many, if not most, paddlecraft are designed with the center section of the bottom of the hull approx. parallel to the water's surface. This might be almost flat to very slightly curved but still should be basically parallel to the surface. This position allows for the least amount of surface area exposed to the oncoming water flow. If you increase or decrease the height of the bow, you SLIGHTLY increase the water resistance of the hull.
A slight (1/4" to 1/2" ) decrease in bow height has the effect of increasing the resistance of the bow section. This might be enough to overcome the effect of weathercocking where the the stern gets blown over therefore tending to steer the bow into the wind. It doesn't take much to overcome this phenomenon. Boats turn, not by moving the stern around, but by initiating a turn in the bow where the water resistance builds quckly and continues the turn. That's why tugs PUSH barges instead of PULLING them - right Wannabe?

Joey
 

woodman

Well-Known Member
Oct 31, 2010
346
0
69
Bates city Mo.
tx river rat said:
As normal these kind of discussions bring out some good points and make us think about how a hull shape and the things we do to that shape effect the handling of a boat.
So lets design a boat with rocker to do the job we want it to do.
Draw a straight line (bottom of the boat) 16 ft length. That is a perfectly level trim no rocker
Just as a general rule I like 2 inches of rocker in the front and 1 inch in the rear.
lets put in these rockers 4 ft from each end , bottom line is still level.
Now set the boat into the water and look at it ,you have 4 ft of rocker front and back so the 8 ft flat area in the center is going to hold the boat in the same plane as it was to the line.
Basically you have an 8 ft boat both ends out of the water, both ends are balance front is 2 high back is 1 inch high are still close to it with a 40 lb boat
now I sit down in the boat in water my seat at the center of flotation. What happens.
At my 200 lbs the boat is going to sink a couple of inches.
Now is when the rocker starts coming into play . My weight should sink the boat around 2 inches
depending on the hull width. go back to your line and remember the words center of flotation, that means I am sitting where I exert pressure straight down .
so you hold the same level as you do with an empty boat.
Front is just touching the water , the back is one inch deep.center section should be 2 inches deep.
This is what I want in a boat easy turning long water line so it is picking all the efficiency it can from the hull , good tracking, and a smooth entry and exit point for the water around the boat. add some more weight and it should get even better up to a point.Of course as you add weight you need to try to keep this trim
Nose down to me is anytime the nose of the boat is lower than that two inches from the line we drew to start laying the boat out, the original level line.
This seems to be the best all around trim for my kind of paddling.
As far as an acurate level you are on the best one in the world if it is calm.
WATER you have a 16 ft level your sitting in.
Ron
PS you can do the same thing with a boat that has 2 inches in front and no rocker in the rear ,the 12 ft of straight bottom of the boat will hold the nose up to the rocker height
What about installing a built in level at the time the boat is built then when loading etc. you can adjust to the buble?
 

Kayak Jack

Well-Known Member
Aug 26, 2003
13,976
168
84
Okemos / East Lansing Michigan
tx river rat said:
<SNIP> now I sit down in the boat in water my seat at the center of flotation. What happens. At my 200 lbs the boat is going to sink a couple of inches.
Now is when the rocker starts coming into play . My weight should sink the boat around 2 inches depending on the hull width. go back to your line and remember the words center of flotation, that means I am sitting where I exert pressure straight down .
so you hold the same level as you do with an empty boat.
Our various definitions of "nose down" trim aren't all saying the same thing. And this item that Ron explained brings up another issue. If I understand you right, Ron, you are saying that as you sit in the boat, at the center of buoyancy (flotation), the boat will sink straight down, say, 2". If I did not understand that correctly, please try again so I do? I want to.

If I did understand you correctly, I'm not sure that is a good assumption in all cases. If a boat is symmetrical, then I agree. But, if a boat is asymmetrical either in width (fish or Swede form) or in keel line (more rocker at one end than the other) then as the boat sinks into the water, the center of buoyancy will shift, and an asymmetrical boat will not sink straight. An end that is wider, will produce more buoyancy as it submerges than a narrow end. An end with more rocker will have less displacement hull in the water and therefore produce less buoyancy. The geometry of the hull presented to the water subtly changes as it submerges.

This is why we need to trim our boats. And when we're done trimming, say to a nose up condition, a boat with no built in rocker would present present less rocker (underwater) in the nose than in the stern. Practical effects of any rocker configuration can be easily lost is such practical situations. It can also be preserved by careful attention to trimming.
 

tx river rat

Well-Known Member
Feb 23, 2007
3,043
2
Waco Tx
You understood me well Jack
and the one thing your forgetting is the center of flotation, it will still be the same the seat position will just change between swede form and fish form. .Ok we agree on a boat with equal rocker,wheeeeeeeee
No remember two things Trim and what we are talking about are two different things. you build a boat and locate the seat for the best performance of that hull design the rocker is built in the hull . when you load the boat you try to maintain that same orientation in the water,this is why we build different style ,shape and rocker boats.
go back to my above post and read the deal about the line we start with and then the rocker
and the length of the water line changing as a boat is loaded.
Our conversation started over the statement that a boat was self leveling and unequal rocker didnt matter, which in my opion is incorrect , we agree that equal rocker is going straight down
the same thing will happen if you set a boat up right with no rocker in the back and 2 inches in the front because of your increased leverage of a longer water line and it will work from no rocker up to an equal one . Again forget trim that comes later and remember center of floatation
get you a pen and paper and draw the lines I think it will help you understand
Remember your center of flotation in an asymmetrical boat has you behind center of the boat
so yes to your question it should go straight down when I put my lard butt into the seat.
Ron
PS I dont know how to explain this any better but I do know my boats will stay in the same plane with the water empty or when I am in them regardless of the rocker unless I move away from my center of flotation
My definition of nose down is my boat not sitting in the water the way it was built raising the stern and lowering the bo2w in the water.
Ron
 

Kayak Jack

Well-Known Member
Aug 26, 2003
13,976
168
84
Okemos / East Lansing Michigan
We agree on most of what you say. We don't see eye to eye that an asymmetrical bloat will maintain the same center of buoyancy (flotation) as the hull submerges. We see eye to eye on the important things - wine, beer, and steak.
 

seedtick

Well-Known Member
Jul 22, 2006
1,161
7
Denham Springs, LA
you maintain orientation by the way you load (trim) the boat. Say your boat is level and the front is 1" above the water. You put 300 lb in the boat and maintain that levelness, the boat sinks 4". The front is now 3" below the water. At this point I don't think it matters whether or not you built the boat with 0,1 or 2" of rocker. I think that was the gist of the original question

and you don't have to raise the stern to lower the bow

a boat in the water is not a see=saw stiff board rotating around a pivot point

this thread has gotten nowhere fast - I'm bowing out
 

beekeeper

Well-Known Member
Mar 4, 2009
1,807
49
oldsparkey said:
What I can't figure out is he keeps asking questions and as he posted in one post ... He has two similar boats , one with the rocker and one without the rocker. :roll:

I thought my boats with "lots" of rocker could be improved to better suit me, so I did build one with less rocker. For my use it is a much better boat. I am better served by it than a boat that might turn on a dime. For me better tracking trumps ease of turning where I paddle.
In my simple mind it seams like the best way to answer the questions is to take a day and the two boats and see how they act or react in the same water. That should provide all the answers , a simple and easy way to find out , then doing all the speculation on here about it. If it is needed then a partner to take some pictures and then the other to take some pictures to see the difference. :D

I for one can not see what all the hoopla is about the rocker in a boat since it is a personal choice , especially when a person has two different boats with and without the rocker to see how they will answer all of there questions. :?

It sort of reminds me of the draft on a boat and how that discussion when on forever in several other posts , rocker is what is built in the boat and the draft of a boat ... It depends on the boat and the load in it , Pure and simple.
Appears to me me we are rehashing last weeks lunch.......... :mrgreen:

Chuck :eek: :shock: :?

"he" is asking questions to understand the points put forth by others. I'm sorry you can't "figure out" the purpose of a question. I did not say I have two "similar" boats. Reread your quote. I did ask another question about "similar" boats with different rocker. Maybe that is why you think my questions are repetitive.


Since I don't have boats similar enough to isolate the rocker factor as the only difference. Comparing a skiff to a pirogue would not answer the question. I had already proposed on the "Rendevzous" post for people to bring their boats and compare. I don't know if it will solve anything , but I bet we will still learn something.

These disscussions become apples to oranges, and opinionated. It takes questions and patience to sort out the facts. No "hoopla" was intended.
I apologize because I haven't grasped or endorsed everyone's ideas, and have failed to comunicate mine successfully. :(
 

oldsparkey

Well-Known Member
Aug 25, 2003
10,417
109
78
Central , Florida
www.southernpaddler.com
I thought my boats with "lots" of rocker could be improved to better suit me, so I did build one with less rocker. For my use it is a much better boat. I am better served by it than a boat that might turn on a dime. For me better tracking trumps ease of turning where I paddle.
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Sorry about that but from your above statement a person would assume that when you mentioned .....
" I thought my boats with "lots" of rocker could be improved to better suit me, so I did build one with less rocker."
That you did build two identical or similar boats one with the rocker and one without.
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Then the rest of your statement ( For my use it is a much better boat ) indicates that you have paddled both of them and your choice is the one without the rocker as your favorite boat.
( I am better served by it than a boat that might turn on a dime. For me better tracking trumps ease of turning where I paddle. )
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It's my fault also due to my misunderstanding or lack of knowledge on what you were saying or trying to say and that I read it differently then you intended , I could only take the words at face value and not the intent or suggested understanding behind them.
Hopefully you will receive the answer to your question , if not here then at the Rendezvous when everyone is together. If you have the rain that was there when I attended the one you will be able to float the boats at pipers.
 

Kayak Jack

Well-Known Member
Aug 26, 2003
13,976
168
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Okemos / East Lansing Michigan
Mr, BK, I like your questions, and it's good to get issues out in the open air and discussed. If any/some/all of my comments created confusion for you, I didn't mean to. Sorry 'bout that. Sometimes a little bit of a venomous nature just creeps into us, then grows......damned humans anyway.
 

Kayak Jack

Well-Known Member
Aug 26, 2003
13,976
168
84
Okemos / East Lansing Michigan
No, let me take all that back. BK, I fully intended to confuse and irritate you. I hope I succeeded.

And, if you're ahead of me in line for the bacon explosion, I'll pee in your boat.

There, I feel better now. Going back to pulling flys off of wings.
 

jdupre'

Well-Known Member
Sep 9, 2007
2,290
38
South Louisiana
beekeeper said:
Changing the trim is changing the amount of rocker in the water, correct? "

Bee, rocker is boat design - trim is how the boat sits in the water. Related but not the same.

"Seems to me little changes here have noticeable effects.
Boat evolution indicates trim is not the total answer to changeing a boats handeling. "

Yup. It's a balance of trim and rocker according to the preference of the PADDLER (very important distinction there).


"I don't know a formula or magic number for figuring the best rocker."

Sorry, no formula. Too many variables. Consider this. Even if you could design the absolute best, most effecient pirogue, it would not paddle exactly the same in salt water, where the bouancy is different. Even multimillion dollar racing craft designers don't have all the answers.

beekeeper
Just remember this sobering fact. There is probably as much performance difference in two fairly similar boats with differing amounts of rocker and trim as in two "identical" boats built side by side.

Joey