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Advice, Suggestions and General Help New Topic Printer Friendly Version

  Measuring trim
  Posted by: melenas on Nov-23-13 6:57 PM (EST)
   Category: Canoes 

The importance of boat trim is often mentioned, but how do you measure it?

I can make sure the bilge water or a tennis ball are about centered under the yoke for level trim, but that doesn't work when the boat is loaded with gear. Surely noone carries a bubble level when canoe camping?

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Messages in this Topic

 

  some people install a bubble
  Posted by: kayamedic on Nov-23-13 7:03 PM (EST)
level under the gunwale on the inside. Just a mini level.
 
 
  With the boat empty:
  Posted by: Jackl on Nov-24-13 6:03 AM (EST)
sitting on flat water, put a piece of black tape or equal on one side of the bow an inch above the water line, and another one on the stern.
Then after you load the boat, just stand back and look at it, and then adjust the load until both ends are equal above the water line.
Or you can get in it where you will be sitting, and have some one on shore tell you.
That is how we do it just prior to a race or when my wife and I are camping

Jack L
 
 
  Jack nailed it
  Posted by: MCImes on Nov-24-13 11:27 AM (EST)
-- Last Updated: Nov-24-13 11:29 AM EST --

The easiest way is the 'tape and person on shore (or in another boat)' method. In racing canoes, they dont have bulkheads so you can see the tape shadow too, so you dont even need another person.

If you're the only boat, you would probably need a mini level setup.

 
 
  sounds cumbersome
  Posted by: melenas on Nov-24-13 3:05 PM (EST)
but maybe it isn't! Pack all gear, both people get in and then find someone to look at it. Then possibly adjust while the person waits, ask them again...

The level sounds easier.
 
 
  Cool, But.....
  Posted by: CEWilson on Nov-24-13 1:58 PM (EST)
Most modern designs have differential rocker and some Swede Form shape, droping the fuller stern deeper in the water than the bow. Worse, the boat may align stern high without weight in the boat, so tape is difficult to place with accuracy, excepting on symmetrical hulls with equal rocker.
 
 
  Which begs the question
  Posted by: pblanc on Nov-24-13 2:09 PM (EST)
"What is the proper trim for an asymmetrical hull?"

This question comes up when outfitting an asymmetrical canoe. It is easy to determine pedestal or seat placement to achieve neutral trim for a symmetrical hull, but what about an asymmetrical one?
 
 
  that would have been my next question
  Posted by: melenas on Nov-24-13 3:16 PM (EST)
For asymmetrical hulls, I assumed you would need to find the widest part of the hull aft of the center as your "trim spot" to place a lever.
 
 
  Simple. Hull shape irrelevant.
  Posted by: Glenn_MacGrady on Nov-24-13 4:40 PM (EST)
The linked page with its graphs and formulas explains trim with pellucid clarity:

http://hawaii-marine.com/templates/Trim_Article.htm

My interpretation:

The designed-in (and hence "proper"*) trim is that which naturally buoys when the hull is floating empty. What else could proper* trim be, given that the designer has no idea where the paddler will sit, how much the paddler weighs, or how much and where the gear will be placed.

Let's just call this proper* trim the "empty floating trim". This trim can be marked with tape or paint at the water lines, as others have suggested.

Now when the hull is floating empty, there will be point that is inherently designed into the hull called the longitudinal center of buoyancy (LCB). This the the buoyancy "balance point". The buoyancy force aft of the LCB will equal the buoyancy force forward of the LCB when the hull floats empty. This is true whether the hull is symmetrical, asymmetrical, swede form, fish form, elegant form or crappy form.

The key concept: Every hull will have a physical LCB point when it floats empty.

To maintain proper* trim is simple. You simply load the hull such that the longitudinal center of gravity (LCG) of the entire load placed into the hull is directly over the LCB. That is, if you are paddling without any gear, you sit in such a place that the LCG of your body is directly over the LCB of the hull. If you are paddling with gear, you arrange the combined mass of your body and gear so that the total resultant LCG is directly over the LCB.

Again, you can identify proper total load LCG placement pretty well by just eyeballing the trim tapes you put on the empty hull.

***************

Now, why have I put an asterisk on the phrase "proper* trim", which I have defined as empty floating trim?

It's because hulls can be designed well or poorly. The designed-in LCB placement can produce an elegantly paddling hull or a pig. It may very well be that for a pig boat with a crappily designed-in LCB, you get better paddling performance by placing the LCG of your total gear load either forward or aft of the designed-in (= proper*) LCB. And that's why some boats paddle better when you move the seat further aft or further forward than the (lousy) designer suggests.

 
 
  LCB determines fish vs. swede form
  Posted by: Glenn_MacGrady on Nov-26-13 10:42 AM (EST)
If the LCB is not in the geometrical center of the length of the waterline (amidships), then the waterline shape is "asymmetrical".

If the LCB is forward of amidships, the asymmetry is called "fish form". If the LCB is aft of amidships, the asymmetry is called "swede form".

Many people don't understand this. They think swede and fish form relate to the shape of the gunwale line -- whether the maximum beam of the gunwale line is forward or aft of amidships. This is not technically correct.

John Winters, for example, has published studies of Inuit kayaks that are LCB swede form at the waterline although the seam line has more beam forward of the cockpit than aft. And this misapprehension was the reason Patrick Moore made his first internet appearance in a decade on this site two years ago. He was upset that some his hulls were being claimed on various forums to be fish form when they are actually swede form at the waterline.

That thread was the one in which I was explaining the trim argument as to why swede form asymmetry of the waterline is better for a solo canoe or kayak than a symmetrical waterline. Namely, swede form asymmetry of the waterline allows the hull to remain in trim when the solo seat is placed aft of amidships, over the LCB. If the waterline is symmetrical the LCB will be amidships, and if you then put the solo seat aft of amidships as is customary for proper center-of-stroke placement, the hull will then be trimmed bow light because the LCG will be aft of the LCB.
 
 
  and if you even put a little too much
  Posted by: kayamedic on Nov-26-13 9:28 PM (EST)
weight in the bow of a swede form boat , you will be able to know its out of trim..no level needed.

I find more symmetrical waterline boats to be rather tolerant of a little mismatch. Note that no boat is really ever symmetrical in the water.
 
 
  Seat time.
  Posted by: Steve_in_Idaho on Nov-24-13 4:47 PM (EST)

I used to worry about how the trim "looked". But now it's more about "feel". Spend enough time in a particular boat, and you will get a feel for when the trim needs to be adjusted.

So - my answer to "how do you measure trim?", is "get more time in the boat".

 
 
  What Steve said
  Posted by: TommyC1 on Nov-24-13 5:23 PM (EST)
Most paddle boats do not require "perfect" trim to perform reasonably well.
If your bow is digging in and you have to wrestle it back on course then you are very bow heavy and should try to fix it.
Otherwise play around with your trim. See what works for you.
Be aware that your best trim changes with wind and current and what you want the boat to do.
Nice thing about poling, you can really change your trim walking fore and aft in your boat. Bow heavy works great when you are going down stream slower than the current. Stern heavy works great when you are heading upstream against the current.
A little bow heavy works well paddling into the wind. A little stern heavy can be better running with the wind.
So, like Steve said, put in the time, get the feel for your boat(s) in the places and conditions you paddle. With enough time in the boat you will know your trim by the feel of your boat.
 
 
  I just
  Posted by: rpg51 on Nov-24-13 5:53 PM (EST)
-- Last Updated: Nov-24-13 8:41 PM EST --

ask my buddy - "so, how's my trim look?" After years of loading boats I usually, not always, get it pretty good.

 
 
  I've tried to think of a sensor that
  Posted by: ezwater on Nov-24-13 6:41 PM (EST)
would give not only a spot measure of trim in a resting boat, but a continuous measure of fore/aft hull angle while paddling. So far I haven't succeeded.

Particularly in canoeing, and moreso in solo canoeing, the amount of rocking while paddling hard must have some influence on speed and energy expended. It can be observed if someone videotapes the paddler, but I'd like to see something the paddler could use while paddling, so as to reduce excessive rocking.

One sensor I'm familiar with is a small, low friction potentiometer. It would need some sort of lightly damped pendulum. A small strain gauge selected for the right sensitivity is another possibility.

Or, I could swipe the gyro package from a missile.

Also, if anyone has seen analyses of rockinng motion for flatwater sprint c-1s or (less likely) for solo slalom c-1s, I'd like to know about it. It's possible that the sometimes violent rocking actually aids forward progress, or at least doesn't seriously impede it. I'd just like to know.
 
 
  My experience
  Posted by: rblturtle on Nov-26-13 3:44 PM (EST)
When I first got my Swift Osprey with a sliding seat,I installed a bubble level under the gunnel near the center. I spent enough time looking at it to learn that is a good way to dump! Besides,like others said,you learn to go by feel.
turtle
 
 
  Maybe get one of those
  Posted by: rpg51 on Nov-26-13 9:19 PM (EST)
laser construction levels on a tripod and mount it in the boat over the gear? That might work pretty good. You would have to adjust for the downward tilt on the river. You could probably rig up some sort of electric gizmo to move a weight fore and aft to adjust trim. :-).
 
 
  Inclinometer AKA tiltmeter
  Posted by: Glenn_MacGrady on Nov-27-13 1:21 AM (EST)
http://www.tiltmeter.com/basic_models.html
 
 
  Silva Ranger compass
  Posted by: melenas on Nov-27-13 10:08 AM (EST)
has one built in! The only problem would be trying to read the tiny scale without dumping, even more difficult than the level. You can't just lean forward to get closer, but have to look at it from you normal paddling position.
 

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