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Trim/Rocker

beekeeper

Well-Known Member
Mar 4, 2009
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My Bayou Dorchete pirogue floor is 12' long with 1" of rocker at the bow and 3" at the stern. When It is trimed so the bow stem touches the water the stern stem is submerged about 2". This seems to be the best trim I tried and it paddles well like that.
My problem with that is the seat needs to be 6" to 8" closer to the stern. My gear needs to stay where it is and I don't have room or the desire to add balast. When the seat is moved rearward to where it needs to be the bow stem is 1" to 2" out of the water and the stern has sunk to 3" or 4".
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Keeping all other dimensons close as possable would decreasing the stern rocker from 3" to 1" add volume and push the bow down?
 

jdupre'

Well-Known Member
Sep 9, 2007
2,313
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South Louisiana
Yep, you would have 2" more of "boat" to displace water to raise the stern. Also, 2" less of an obstruction for the free, linear flow of water under the floor.
 

beekeeper

Well-Known Member
Mar 4, 2009
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I believe that but I am having a hard time wrapping my head around it. Most information about rocker is vague and only relates to how the boat tracks and/or turns. Homemade designing to specifics is difficult. Each feature is connected and can affect all other aspects.
I have no way of knowing if reducing the rocker a couple of inches will be enough. I think I will also increase the volume (a little) at the stern by increasing the rake angle of the stem or using a square transom like my son's boat has. Overall length of the boat would increase by a few inches but I can live with that.
 

jdupre'

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Sep 9, 2007
2,313
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South Louisiana
Bee, you can never "know" how much a change in design will change the characteristics of a boat. The best you can hope for is to look for tendencies. My MO is to take a design change to a ridiculous level on both ends of the spectrum. Wider is more stable........but a 48" wide boat is too wide.....and an 18" wide boat is too narrow. Well, the optimum width is somewhere in the middle. THAT'S where feel and not numbers come in. You have narrowed the extremes down in your past designs so you have a great grasp on what you need.

As for numbers, brush up on your buoyancy calculations. The amount of water (weight) the boat displaces at different points will tell you how much weight the boat will support at that point. I treat a pirogue like two triagles end to end. Find the square footage of both of them and multiply that by the depth to get the volume. Water weighs 63 lbs per cubic foot. The wider and deeper the boat in that area, the more weight it can support. I routinely guess the draft of my boats to less than a half inch accuracy before I even cut the wood out.
 
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jdupre'

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Sep 9, 2007
2,313
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South Louisiana
Bee, facts and numbers can only get you so far. I look for the reasons WHY certain design elements work the way they do. Once you know that, it's easy to figure what certain changes will do to a boat's performance. I can pretty much walk by a boat now and tell how it will perform.
 

oldbuffpilot

Well-Known Member
May 13, 2014
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Ya’ll have carried this conversation to depths I had not considered :) please keep the information coming. I’m afraid I can’t contribute more.
I did measure some pirogues. 3 commercial made fiberglass and 3 that were built from plans and 2 built by hand and eye. All of the rockers were about(+/- fractions) bow 2 ", stern 1 " ie a 2 to 1 ratio. I don’t know if that means anything, maybe my preference in rocker?

I always learn from these posts, thanks for posting.

Andy
 
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jdupre'

Well-Known Member
Sep 9, 2007
2,313
40
South Louisiana
I've never read anything on the reasoning or usefulness of more rear rocker. I'm actually surprised that there was a difference in all of those boats you measured. I've read the specs on canoes ( closest thing to our piroguess) and they normally have different amounts of rocker between models , but I can't remember seeing any front/rear differences,

More rocker causes the boat to sit very slightly lower in the water, causing a very slightly wider waterline width. Most people wouldn't notice it, but it's there.

People seem to be deathly afraid that their boat won't turn so they go for a higher rocker. My latest pirogue is basically dead flat and it's just SLIGHTLY less willing to turn at speed. Cruising slowly, there's almost no difference.
 

oldsparkey

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Aug 25, 2003
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Central , Florida
www.southernpaddler.com
If memory serves me , from other articles the question ( on the amount of rocker ) was about the amount of water behind the boat having any turbulence ( drag ). Or the boat slipping along and not leaving much or any turbulence behind the boat.
The closest association I can think of .....About the same with vehicles and the amount of drag the air being disturbed causing the drag to the vehicle.
Personally I don't think any of us paddles so fast that drag on the stern of the boat is going to make any difference. Anyway as far as my paddling goes.
 

beekeeper

Well-Known Member
Mar 4, 2009
1,849
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Most common canoe designs seem to have equal rocker for and aft but many show slight differences, (2" + or - bow/1"stern)similar to oldbuffpiolets findings. Like most design features little differences are hard to detect and are easily adjusted to. https://www.google.com/search?q=canoe+plans+with+differential+rocker&source=lmns&bih=617&biw=1366&hl=en&sa=X&ved=2ahUKEwi8ucPG9f74AhUSnGoFHXv2CYMQ_AUoAHoECAEQAA

Rocker is usually measured with the boat sitting on a flat surface. This does somewhat set a slandered for discussing it, assuming the boat is trimmed level/flat. How much/deep the stems are submerged is more relevant than how high off the table they measure. Proper trim adjust the effects of the rocker to the effects one wants. This is an interesting discussion on trim/rocker/effects. https://forums.paddling.com/t/measuring-rocker/61792
 

jdupre'

Well-Known Member
Sep 9, 2007
2,313
40
South Louisiana
I measure mine by sitting the boat on a flat surface with the gunnels as level as I can get them. With a pirogue with a hard angle from the stems to the bottom, you can get a pretty accurate measurement. With a a canoe or kayak with curving lines at the stems, I tend to measure from a place where the bottom just starts to really curve up into the stems. No way that is ever going to be entirely accurate. You can only hope for a general idea of what the rocker is. Plenty good enough in my book.

Trim is another thing entirely. They do work together ...... or at odds if you go too far one way or the other. Crazy example: Say I trim my zero rocker pirogue WAY low in the rear at about 6". The actual surface that the water sees as the boat moves forward is almost the same as if the boat has 6" front rocker. Good maybe for turning, but not so good for cruising speed.
 

beekeeper

Well-Known Member
Mar 4, 2009
1,849
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Trim and rocker have to work together for the best performance from any hull. The boat must also be designed for the intended payload and usage. I would guess for average usages a pirogue would paddle "best" with the bow slightly submerged, 1" or less. The stern down maybe 1" to 2" for somewhat of a keel effect. It would not be hard to trim to something close to this with most pirogues and loads. The water line would likely be parallel to the "flat" surface used to measure the rocker.
The hard part is building the boat so the load produces a water line that takes advantage of the rocker. My boat, like Joey's example, needs more volume aft of center. I suspect it will take more than an adjustment to the rocker.
 

jdupre'

Well-Known Member
Sep 9, 2007
2,313
40
South Louisiana
I wouldn't think rocker makes much of a difference in buoyancy of a certain area of the boat. Probably better to concentrate on width in the area you want to carry more weight. Use rocker to tune the handling characteristics of the boat.
 
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beekeeper

Well-Known Member
Mar 4, 2009
1,849
52
Ya’ll have carried this conversation to depths I had not considered :) please keep the information coming. I’m afraid I can’t contribute more.
I did measure some pirogues. 3 commercial made fiberglass and 3 that were built from plans and 2 built by hand and eye. All of the rockers were about(+/- fractions) bow 2 ", stern 1 " ie a 2 to 1 ratio. I don’t know if that means anything, maybe my preference in rocker?

I always learn from these posts, thanks for posting.

Andy
We all learn from everybody's postings.
I assume these are all somewhat different size and shaped hulls? Have you observed any differences in tracking, turning, or how you have to trim each?
Are they symmetrical or asymmetrical?
 

oldsparkey

Well-Known Member
Aug 25, 2003
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Central , Florida
www.southernpaddler.com
The Sasquatch 14 x 30 by Jem handles differently then the 16 Pirogue from Uncle Johns.
The Pirogue will turn on a dime and give you 9 cents in change.
My Sasquatch will track a lot better and is ideal for cruising down river. As far as turning on the dime , closer to getting 4 cents back. It will turn for me but not as quick as the Pirogue. Similar to the old manual steering on cars of the 50's & 60's compared to today's power steering.
Two different styles of boats two different responses on the water.