I built my first stripper, a White Guide 18 1/2 footer from Gil Gilpatrick's book. She came out a real beauty, especially for a first effort. One thing I noticed, and have seen on many other boats, including commercially produced boats, is that they seem to have negative rocker. The bow and stern are deeper than the flat hull amidships. This was especially true after adding rope/epoxy skid plates. She tracked true, but obviously was a little slow to turn, a problem on our narrow rivers in MT. I know that the stations were aligned as per the plans, but by the time she got glassed, flipped over and glassed inside, I had a concave surface on the keel.
Has anyone noticed this effect before? Any thoughts? I'm doing the 20 footer next, and think I'll induce about 4 to 5 inches of rocker above what's shown in the plans. It shouldn't hurt anything, except a little more bow to grab the wind. Any comments or ideas?
Greg
PS: I'd post pics, but don't have a websit up yet.
Negative Rocker?!?
-
canoeblderinmt
- Posts: 415
- Joined: Sat Jul 03, 2004 12:04 pm
- Location: Butte, MT
Hi, Greg,
A hull with what you describe is known in some parts as "hogbacked"... this sometimes happens to older flat-bottomed canoes when they're kept upside down in storage for a long time and the center part of the bottom sags down. I've never seen a canoe offered for sale with this designed in - a hogbacked hull is generally thought of as one with poorer performance, since this can increase drag.
Are you sure this is a design feature and not the skid plate being thicker near the bow and stern... maybe the hull is flexible enough to sag a little when upside down. Try flipping it over and see if the keel line is still concave. Some lightweight, flexible hulls can go slightly concave at times when empty on the water, but loading them with packs will restore the proper form.
A hull with what you describe is known in some parts as "hogbacked"... this sometimes happens to older flat-bottomed canoes when they're kept upside down in storage for a long time and the center part of the bottom sags down. I've never seen a canoe offered for sale with this designed in - a hogbacked hull is generally thought of as one with poorer performance, since this can increase drag.
Are you sure this is a design feature and not the skid plate being thicker near the bow and stern... maybe the hull is flexible enough to sag a little when upside down. Try flipping it over and see if the keel line is still concave. Some lightweight, flexible hulls can go slightly concave at times when empty on the water, but loading them with packs will restore the proper form.
-
Juneaudave
- Posts: 522
- Joined: Sat May 08, 2004 12:42 pm
- Location: Juneau, Alaska
- Contact:
Be sure and post some pictures! I'd like to see how that rope and epoxy skid turned out. Regarding the rocker, I have never heard of this happening. If it had rocker upside down on the molds, the finished shape should have been the same unless something(s) happened when it came off. Maybe the cradles were too close combined with the sheers being pushed out? Is the finished width at the center thwart correct?
Also, regarding adding additional rocker on the Laker, you can surely do that. But I wouldn't do that because of a fear that the rocker will mysteriously disappear or diminish once the canoe comes off the forms. Let us know what you find!!
Also, regarding adding additional rocker on the Laker, you can surely do that. But I wouldn't do that because of a fear that the rocker will mysteriously disappear or diminish once the canoe comes off the forms. Let us know what you find!!
-
John B in ID
Negative rocker
Hello Montana!
I, too, have built one of those 18.5 White Guide strippers and love it. I, also, was worried about the lack of rocker. Mine has no skid plates, really has no negative rocker and doesn't like to turn when loaded. If you look at the plans, though, you will notice that it was designed with hardly any rocker. There is only rocker appearing in the last two molds and even then less than 1/2 inch. That doesn't allow much room for skid plates and doesn't encourage turning either.
That 20 footer has also caught my eye. A look at its plans seem to show that it has a lot more rocker. It shows rocker for the last four molds from the end and seems to show about 2 inches of rocker. That would be a lot of difference.
I think adding 4 to 5 inches of additional rocker would be overkill and would affect the lofting of the rest of the hull, but maybe an additional inch or two would be helpful in turning some of those Big Sky bends. If the sheerline seems to get too high at the bow and stern, you could lower them a little when you intall the gunnels and decks. These thoughts have been going through my head lately because the 20 foot White Guide looks mighty attractive. If anyone out there has info or ideas about the 20 footer, I'd sure be interested.
I, too, have built one of those 18.5 White Guide strippers and love it. I, also, was worried about the lack of rocker. Mine has no skid plates, really has no negative rocker and doesn't like to turn when loaded. If you look at the plans, though, you will notice that it was designed with hardly any rocker. There is only rocker appearing in the last two molds and even then less than 1/2 inch. That doesn't allow much room for skid plates and doesn't encourage turning either.
That 20 footer has also caught my eye. A look at its plans seem to show that it has a lot more rocker. It shows rocker for the last four molds from the end and seems to show about 2 inches of rocker. That would be a lot of difference.
I think adding 4 to 5 inches of additional rocker would be overkill and would affect the lofting of the rest of the hull, but maybe an additional inch or two would be helpful in turning some of those Big Sky bends. If the sheerline seems to get too high at the bow and stern, you could lower them a little when you intall the gunnels and decks. These thoughts have been going through my head lately because the 20 foot White Guide looks mighty attractive. If anyone out there has info or ideas about the 20 footer, I'd sure be interested.
-
Jim D
In some of my earlier canoes, I witnessed some negitive rocker. I blamed it on the strips wanting to rise off the molds at the ends... I now shave my forms in this area to compensate, seems to work!.. 4 or 5in. rocker, is a little drastic by my thinking,1 1/2 to 2in. should be plenty... Too much rocker could turn a sleek canoe into a slow canoe!... You're messing with the waterline equation now... Good Luck! Jim
flat keel to hog back
On very straight keeled hulls a slight hog-back can occur during the stripping process. Strips meet at the centerline at the center mold at a nearly flat angle. Toward the ends this angle increases especially in hulls that have V-shaped cross sections. This increases the thickness of the wood profile along a vertical line. On a very flat keeled hull like the E.M. White 18 this is enough to be noticeable. Always check the keel profile before glassing or attaching additional keel pieces. That V cross section may need to be planed down to prevent hog-back.
-
canoeblderinmt
- Posts: 415
- Joined: Sat Jul 03, 2004 12:04 pm
- Location: Butte, MT
Thanks for all the good input!
I really appreciate all of your thoughts. I did notice some flex (like maybe an inch or so) in the middle of the boat as we paddled down river. I had 2 layers of 6 oz on the outside and one inside. I made sure to bring the gunwhales to the width on the plans, though this necessatated spreading them open from where they ended up after glassing. Probably about 5-6". I understand and appreciate not putting too much rocker in, and I think the comments about shaving the forms is probably a good start(thanks, JimD). I guess I'll know what to look for on the 20'er! I was even toying with the idea of putting a couple of ribs into it amidships just to help it retain it's shape under the stresses of running, but after reading other posts I don't think I'll go there.