Bow stem angle?

[beekeeper]

My last boat was built without specific plans. I did use my U.J. planes to see how and when to do things. Determining the proper angle of the stem really eluded me. I knew the 26 degrees for the pirogue shouldn't work because my sides flared 22.5 degrees and the bottom is 26" inside. Tried pulling the sides together and then cutting the stem to fit. I couldn't get that to work, because the Luan was too flimsy. Tried guessing and made a couple with wider angles but they didn't seem to fit right either. Used the one like the plans . It looked fine when I fitted it, but after gluing and screwing it together the sides are pulled in and then flare out to the ribs. Anyone build with out plans or know how the angle can be determined for a new design? Would another way to make the stem work better for non standard builds?
Hope this pictures show the problem:

beekeeper

 

[JEM]

I know how to draw it and translate to panel dimensions. But not sure how you'd do that by eye.

 

[beekeeper]

Thanks JEM. I thought there may be an equation. Degree of side flair, side height, width of bottom, or some other factors considered. If I would stop trying to be different, I wouldn't have these problems.

beekeeper

 

[JEM]

There is some correlation. But you'd have to keep something, like the bottom panel size and shape, constant.

 

[seedtick]

We normally cut stem pieces at 33 or 45 degrees, depending what we're building, but like Matt said there's more to it than just the angle of the stempiece.

also we build around a couple of jigs so we can see the shape before installing the ribs. that way you have the option to make adjustments before getting to the point of no return

Probaly 99+% of the folks that look at your boat would never notice it anyway. For those that do notice, just tell them it's an old Chitimacha Indian shape or some such story. Heck you probably would not have noticed it six months ago. That's one of the downsides of building your own, you know where the flaws are, no matter how tiny.

 

[beekeeper]

We normally cut stem pieces at 33 or 45 degrees, depending what we're building, but like Matt said there's more to it than just the angle of the stempiece.

also we build around a couple of jigs so we can see the shape before installing the ribs. that way you have the option to make adjustments before getting to the point of no return

Probaly 99+% of the folks that look at your boat would never notice it anyway. For those that do notice, just tell them it's an old Chitimacha Indian shape or some such story. Heck you probably would not have noticed it six months ago. That's one of the downsides of building your own, you know where the flaws are, no matter how tiny.

Tried the jig build but the Luan was so flexible I didn't notice the problem. The ribs worked like jigs because it took little effort to bend the sides.

http://i600.photobucket.com/albums/tt86/jdsibley/IMG_0346.jpg[/IMG

The fact that I know they are there is what bothers me.

beekeeper

 

[keith]

I don't think the stem is the bad guy, like tick said we make them with different angles and sometimes we don't put one in at all. your first pictures looks like the first rib is to wide and to far back (there is a formula for that). but that is just my humble opinion. later keith

 

[beekeeper]

keith
I see your point. Had the rib been narrower and moved forward it would have caused the spread or flair of the sides to match the angle of the stem. I caused this trying to add as much width to the bottom for capacity and stability as I could get. Making it not as wide and moving it forward would change the shape of the boat (probably for the better), correct? Couldn't the stem angle be increased and correct the flair and not change the shape of the boat? I asked that again, only to try to understand. I see what your saying will work, but in my simple thinking I should have been able to have made a stem that would fit. My hats off to you guys that understand design and function. Thanks for sharing.

beekeeper

 

[Jimmy W]

If the sides ran in a straight line from the first jig to the bow, then the angle could be calculated by the length of the sides and the distance across the jig.
See: http://en.wikipedia.org/wiki/Triangle#Computing_the_sides_and_angles

If I am figuring correctly, if the sides ran straight from the jig to the bow and were 4 feet long in front of the jig and the jig was 3 feet across, then the bow angle would be 44 degrees. Of course, the sides form a curve so that would be a little off. I am also thinking that since the stem is raked, the effective angle is going to be different from the angle that you cut.

 

[seedtick]

You can probably duplicate the shear - top lines - of your boat by cutting two strips of wood say 3/4" by 3/4" and nailing them to a board in a "V" shape where the angle of the "V" is the same angle as your proposed stem piece. Backup away from the point of the "V" the distances that you want the first and second ribs and measure across the distance that the ribs will spread the top of the boat. I think only one of three things will happen:

1. the strips of wood will not be moved in or out, and you will have a straight line from the rib to the stem

2. the strips of wood will have to be moved outward to accomodate the width and you will have the shear line bowed inward as it matches the angle of the "V"

3. the strips of wood will have to be moved inward to accomodate the width and you will have the shear line bowed outward as it matches the angle of the "V"

I think you want option 3 to happen but only to the extent that it's pleasing to the eye

that's essentially the technique we used in building the Lafitte skiff. Note the 3/4" strips of wood at the shear and at the chine. The shape of the boat was determined by shaping the strips and then buiding the ribs to match.

 

[beekeeper]

Jimmy W
Thanks for the link to "things I forgot". I did try to figure the angle by making a scale drawing and measuring the angle. I don't remember what degree it was now. When I made a stem piece to that spec and tried it, the sides didn't appear to fit as well as the U.J one. Another try with a different angle and it looked no better. I used the U.J. one because it looked best. Problem was the boat was bottom up and I didn't notice the problem with the top of the sides flaring in. I was not trying to address the flair only the fit of the sides at the stem piece.

seedtick
I understand what you are saying, and see how to do it the next time. I see two differences in what your saying and what I did. I was trying to determine the stem angle from established rib widths and locations. You are fitting the ribs to the stem angle or at least seeing if the stem angle and rib dimensions are compatible (reasonably close). Yes?/No? As Keith pointed out the boat would have a more symmetrical look if the first rib would have been narrower and closer to the bow. I'll have to work on that . You used 3/4" lumber. I was working with Luan. I did get what I was trying for. A stable, easy to paddle, fit in my truck, loadable by one person, boat.

Thanks again to everyone who participates on this site.

beekeeper