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PostPosted: August 8th, 2007, 3:23 pm 
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Joined: October 26th, 2004, 12:25 pm
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Location: The Netherlands
I've reached the final building stage of my woodstrip Osprey - installing the trim: gunwales & thwarts. I have one question for which I would like to have some advise...

We've build the Osprey staple-less, and encountered that the glassed/epoxied hull is actually a few cm wider than the width of the stations. This must be caused by stresses in the wood and the laminating of the hull - and I expect that this is a common problem in woodstrip canoe building... Is this correct??

In order to have the final hull shape as close as possible to the orginal Osprey design we think it is required to install the thwarts such that the width of the hull at the location of the thwarts is exactely the same as the width of the station moulds at this position. In our case this means we need to reduce the width of the hull at the shearline strip near the thwarts with about 3 cm.... we noticed that this significantly influences the actual hull shape (especially rocker) at the centre line.

Unfortunatly we had already cut our thwart to length, and didn't think enough about above discribed issue. We had focussed of the hull width at the sheerline strip in the centre of the canoe en cut thwarts to meet correct width at centre... when we double checked the sheerline width at thwarts we noticed that the width on this location is now about 10mm less than width on drawing.
Can anyone advise if it is recommended to make new thwarts (to meet hull width at thwart location) or is a difference of 10mm no problem at all... How much difference does this make to the performance of the canoe... any experience on this?

We'de very much appreciate your advise.


Michel, The Netherlands


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PostPosted: August 8th, 2007, 4:35 pm 
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Location: Kitchener, Ontario Canada
Michel, ahhh now you get to look at your piece of art !!!.

Maybe John Winters will join in with advice, but in case he doesn't see this I'll pass along my 2 cents' worth.

I would not worry about the one cm you are out after you cut the thwarths. In my opinion the increased width will no affect on the performance of the canoe. As for the change in the rocker, I'm surprised that it would have a visible change on the bottom when you moved each side in by 1 1/2 cms. Again i would not sweat it.

When my Bob's Special came off the station it was the same story, it bowed out at the sheer line. I have since experimented with about three or four homemade carrying yokes and have not noticed any change in performance. Now mind you I don't race with this canoe, I just go for enjoyable canoe trips.

Cheers and have a great day

moe


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PostPosted: August 8th, 2007, 4:35 pm 
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I'd use the thwart to bring in the hull back to the correct width, if the epoxy and glass was the thing responsible for deforming the original hull shape that was in the strips... in that case the rocker would have been affected as well by the curing of the glass/epoxy, so drawing in the hull and gunwales should restore the rocker back to the proper amount.

Am I reading this right? If I am, this means that the glued strips originally stayed in place, faithful to the design and stations... then when glass/epoxy cured, the strips separated away, which would affect the rocker at the same time.

If this is true, then drawing in the gunwales back to the correct width with the right-sized thwart, should also restore the rocker back to the proper amount.

Is it possible to check to see if the amount of rocker has been affected? If it actually hasn't been, then it's probably best to not alter the hull's shape below the waterline, and to make a new, larger thwart.

My guess is that the rocker has been affected by the glass/epoxy cure because the glued strips throughout the hull wouldn't be all that compressible to begin with and the widening would change the rocker for that reason... difficult to comment without actually seeing what's changed.

You can also change the amount of rocker to affect the performance of the canoe, so if there seems to be too much or too little, a shorter or longer thwart might actually improve things, whether or not rocker has changed from the original plans. Again, my guess is that the amount of change in performance wouldn't be great, if it's only a few centimeters' change in width.

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PostPosted: August 8th, 2007, 5:05 pm 
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Michael

I concur precisely with Frozentripper, but will add explanation.

The Glass/Epoxy process adds moisture which initially gets absorbed
into the wood strip. When you introduce moisture, wood expands.
The Bow/Stern cannot expand longitudinally as easily
as the gunwales can expand laterally... hence the midsection bows.

You may not wish to do so, but you could actually reintroduce moisture
internally (Plastic wrap and injected steam into the cavity) and encourage
the hull to conform (thru gentle periodic clamping) to original dimension.

Clearly, the expansion can be prevented easily, next time round.
Just a matter of eliminating the capacity to spread laterally.

Otherwise, I would follow Frozentrippers recommendations.

Sundown


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PostPosted: August 12th, 2007, 10:56 am 
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Location: Vandorf, Ontario
The situation described is normal, or at lest something that happens frequently when you remove the boat from the forms. Use the correct thwart/yoke dimensions as per the plans/forms and pull the hull back to the correct shape/width. I'm sure everything will turn out well. The Osprey is a great boat. It was next on my list asmy next building project but now I am seriously looking at the freedom 15' 3" solo as well.

Regards,

Moonman.


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PostPosted: August 12th, 2007, 4:53 pm 
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If I were in your situatioin I'd first get some notes on just what you have with the current configuration, particularly rocker. Since rocker affects maneuverability and tracking, it will probably be more noticeable than minor variations in width at the sheer when using the boat.

Then if you find that the boat in the current configuration is highly maneuverable but doesn't track as well as you would like, you can try to remedy the situation.

My approach, never having done such a thing, would be to remove the thwarts, then re-insert the molds for the stations just fore and just aft of the midship mold. (You may need to cut away the mold to make way for the gunwale.) You may need to fit the molds a tad closer to the centerline than originally to compensate for the thickness of the 'glass. But you may not need to if the 'glass is occupying no more space than you removed in sanding. Then put a couple of band clamps around the hull right at the re-inserted molds and draw the hull right up to the molds. If it were my boat I wouldn't push the issue but if it comes together easily I'd draw it tight.

Then I'd measure what affect that had on the rocker. If rocker is reduced, maneuverability will be reduced and tracking increased. If that's what you want, then cut new thwarts and install them.

I repeat, this is what I would do in the absence of advice backed up by actual experience. If you do it, be sure to post the results. Then the next guy with the same problem will at least know whether the procedure worked for you.

bob

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PostPosted: August 12th, 2007, 6:32 pm 
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I have yet to pull a boat off the form that remained totally true to form dimensions. What you are experiencing is normal. Put the inwales on, cut your thwarts to the correct measurement, and make the boat conform. Or just leave it...one centimenter is not much to worry about. I've built an Osprey too, nice canoe, have fun.


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PostPosted: August 12th, 2007, 7:11 pm 
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RHaslam wrote:
I have yet to pull a boat off the form that remained totally true to form dimensions. What you are experiencing is normal. Put the inwales on, cut your thwarts to the correct measurement, and make the boat conform. Or just leave it...one centimenter is not much to worry about. I've built an Osprey too, nice canoe, have fun.


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What The HeLLLLLL are you doin' here??????
git back to your Huney-Moon :P

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PostPosted: August 13th, 2007, 11:31 am 
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Location: Den Haag, The Netherlands
Hi,

I agree with rhaslam.

Michel, are you coming to the 'Openkanotreffen' next month ?
( An opencanoegathering in the Netherlands )

I am curious to see your Osprey. It will be finished by then ?

At the moment i,m building a stitch and glue Osprey II,
and I,m going to take it with me.

Cheers,

Ewout.


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PostPosted: August 13th, 2007, 3:53 pm 
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I also send my question to Martin Step from Greenvalley, and Martin was soo kind to answer me from an internet connection close to his campground (he's out paddling)...

Here's his response:

Yes, you should pull the hull in until it is the original width of the stations. The
shape of the mould, that is, the stations, is the true shape that is intended
for the finished hull.

This outward flaring that you see is caused by two things, one is the stress
from all the strips, and the other is if you allowed the hull to 'get wider'
before you fiberglassed the inside. If the latter case is true, try to be a bit
gentle and gradual as you pull the sides back in - give the stresses a chance
to work themselves out a bit. Don't just apply all the pressure at the center
all and once and just heave.

At least, pulling it back in will relieve the stress on the outside of the
hull, which when you fiberglassed it, and it was still on the mould, had no
stress in it at that moment. It would now be under compressive stress as the
hull widened up, so that will come back to near zero, while the inside skin
compresses.


... So far considering all your opinions I decided to leave the thwarts as they are right now. As mentioned, one cm isn't that much. Just will test paddle first, and maybe next winter I'll try to make 2 new thwarts with exact the correct length...

Quote:
Michel, are you coming to the 'Openkanotreffen' next month ?
( An opencanoegathering in the Netherlands )

I am curious to see your Osprey. It will be finished by then ?


Ewout,
Yes I'm planning to come to the Open Kanotreffen next month, and yes we (my brother and me) will bring our 2 new Ospreys. The first one (my brothers) was finished last sunday and we test paddled her... What a beatifull moment after 2 years of building. Mine will hopefully be finished next week.
If I knew how to post pictures on the forum I would, but don't know how.

Regards,

Michel, The Netherlands


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