Interpreting John Winters

I've been reading and re-reading John Winters' The Shape of the Canoe, and there are a few points I'm having trouble interpreting. Was hoping someone here might be able to clarify.

"At the stern, buttocks as straight as possible...reduce eddy making and the size of the stern wave...some designers suggest that the buttock slope should not exceed 18 degrees..."

Does he mean "straight" as in more parallel to the waterline? And if so where would this slope be measured from? From the midship rising upward or from the tangent where the buttock crosses the waterline?

 

"Straight buttocks" usually means straight as in straight lines. The angle is measured from the waterplane/waterline, and is usually where the buttocks are steepest.

18 degrees maximum for the buttocks angle is a common "rule of the thumb" for an efficient hull shape. I believe it came from a series of tests of some type of hull shape where it was observed that the drag starting to increase appreciably when the buttocks maximum angle was greater than 18 degrees. It's not a universal angle, and for some boats it's steeper than optimum. There isn't a dramatic increase in drag as the magic 18 degree angle is crossed. But that hasn't stopped it from the be the magic angle.

 

Thank you for expanding on that, but I'm still a little fuzzy on this. I just made up some buttock lines here for graphic purposes. Is the red line (tangent to the curve where it meets the waterline) what I'm concerned with?

 

No - but - maybe. The "straight buttocks" and "18 deg" guidance were originally intended for shapes which carry substantial beam to the stern. Your design is a "double ender" with a narrow stern so it could be viewed as outside the scope of the guidance.

However, looking at the sections you posted previously, http://www.boatdesign.net/forums/at...houghts-differntial-cp-foreward-aft-lines.jpg, the underwater curvature of the sections appears to tighten towards the stern. That might cause some higher drag than if the aft sections were eased. It could cause a local, messy separation of the flow or more likely larger waves due to low pressure as the water goes around the tight curvature. Just my opinion which isn't based on direct knowledge.

 

DCockey, thanks for commenting on it. I just want to make sure I'm understanding you correctly. The way I drafted this the 'bilge' curvature does become slightly tighter the further away from the midsection you go. Is this what you're referring to?

Also just to clarify...hypothetically if I had a wide or transom stern, would the red line I've drawn off the buttock be the angle in question?

I apologize, this is a new design vocabulary for me.

 

Yes, that's what I'm commenting on. Would make sense if you were doing a planning hull but I have my doubts on a canoe. Any particular reason for it.

The way I understand the "rule" is the 18 degrees is the maximum angle, but it would be applied to buttocks which run to the wide or transom stern. It's more applicable to something like a lobster boat hull. But I didn't make the rule so I could be wrong. Remeber that water follows a different, more fundamental but much harder to fully understand set of rules.

 

Thanks for the assistance here. Yeah, now that I picture the hull form in my mind, it seems more intuitive that the curvature should flatten out (at least somewhat) as it nears the vertical edge of the stem. This is probably more workable from a construction standpoint too.

Also when comparing my lines to other designs, it seems to be missing that sort of 'rotation' in surface from the midship to the stems. I think after flattening those curves a bit I'll start to see that characteristic form more.

Glad you pointed that out.

 

RE: Tighter Curves @ Stern

No. I made up my own drafting and design conventions for this project, and that's just how it worked out in the drawing process. Basically I struck a sort construction-line datum for the control 'handles' (as they're called in Illustrator) on the splines. I just need to revise the construction line so my curves transition to softer/looser as they move out from the midship.

In other words, I didn't really know what I was doing

I think this will also solve the problem of too high of Cp. I'm at 0.57, and want to be somewhere around 0.52-0.54. The ends are a little too full.