"Fair curves"

[howesaero]

Hi,



New to the forum but not new to the BM. I raced BM515 "Eclair", at Medley back in the '70s. Life got in the way, she got stored less than adequately, started to degrade and I gave her away to a chap who got her racing again. She's still listed as active.



Now at the other end of my career (Aerospace, mostly aerodynamics, structures etc) I'm thinking about revisiting the BM as several things about my old boat annoyed me at the time. These were too much deadrise at the transom relative to midships, too much aft rocker and a far too soft curve in the bow profile, all of which served to make her primarily a displacement, rather than a planing hull. She manoeuvred beautifully but was horrible on the plane and was never particularly quick upwind due to the excessive twist (warp?) in the bottom panels from midships aft. I'm therefore contemplating a build, mostly to see if my original issues can be fixed within the measurement rules.



The rules book states (5(a)) that:

"The hull shall be constructed within the tolerances set out on the Measurement Form. The lines of the keel, chine, and sheer shall be a fair curve within these tolerances, with no sudden changes in direction."



This is really ambiguous. Other posts on this forum also discuss the issue without resolution. An engineering definition would be a minimum energy curve through points lying within tolerances and this is usually achieved via a family of cubic splines. Since there are (for example) eight defining points on the measurement form then taking the cubic spline approach can produce any wavy result that you might want due to over-definition of the curve so this is clearly not what the rule means.



The project is still at the viability study phase at the moment, viability also includes "is it worth the effort" so I would like to lay this issue to rest before going further.



Thanks for any help on this.



Jon

[howesaero]

Having made a CAD model of the hull, all within measurement tolerances, another issue has popped up. The Hull measurement checklist states that double curvature fwd of station 8 must be "not concave". The Class Rules, 5(g) state:
"Forward of Station 8, the outer surface of the hull bottom panel shall be outside of any straight line joining the keel to the chine, parallel to the frames."

I therefore made all frames forward of Stn 8 slightly convex in accordance with the class rules. This is not the same as "not concave" as, if a straight edge is placed raked aft from the keel the surface is obviously concave. How do measurers interpret this?

[Graham Pope]

Hi Jon
This is a very tardy responce to your December post but I was impressed that you identify the inherant problem in the BM design,as a builder of 8 BMs and a class measurer I have some experience building and measuring the current boats which have overcome these issues within the rules to save possible wasted time if you have moved on I will close now but will assist with further detail if needed.Cheers Graham

[howesaero]

Thanks for the response Graham. I interpreted the rules as measuring from projected normals from the centreline. This obviously doesn't show any hollows but slight aft raking of the measurement lines certainly does. There is obviously a benefit to getting these "oblique hollows" as the make the pressure distribution on the forebody more even approaching planing speeds so lift earlier for lower drag.

I'm also interested in the 50mm limit for hull appendages as this suggests the over-contracted afterbody could be widened with a couple of chine strakes which would widen the aft planing surface (but do nothing useful upwind of course) I note that many of the modern hulls milk the 50mm appendage to take the hull bottom aft of the transom.

Any guidance would be appreciated, I miss Mothing!