Bow radius

[Daniel]

The builds so far are showing a noticeable difference between plan-built and CNC kit-built boats, in the bow radius and resulting fineness of entry where the stem meets the shoe (bottom of the stem).

The near-vertical stem has a strong taper across its width from top to bottom. Combined with the hull plan (shoe) narrowing at the bow, this results in significant flare in two dimensions from where the stem meets the shoe, which is the entry point where the hull drives into water. This two dimensional flare means that a larger radius where the near-vertical stem meets the near-horizontal sole results in a significantly blunter entry point. 

To illustrate, the blogs for plan-built (Trekka) globe580.blogspot.com/2020/06/ and sailorturks.wixsite.com/580build/post/bow-radius-and-filling-holes  have a 60mm radius, and the width of the entry point is ~25mm and ~35mm (a bit less and a bit more than the thickness of the chine plywood exposed between the chine ply)

The kit-built blog at that stage (https://petesclass580builds.com/ has a noticeably larger radius and a blunter entry point, perhaps 55mm width. 

Understanding that this is only a small sample of boats, and there will be individual differences between every build, it does seem clear that the kit has a larger designed bow radius that would result in a blunter entry. The kit provides a sole with the front tip trimmed back, stem and inner stem cut short at the bottom to different degrees, and an angle at the front of the knee.

More builds advancing to that stage may provide clarity on the bow radius and resulting fineness of entry for the CNC kit builds.

But I'm wondering what is the design bow radius for the CNC kit?

[peterbrumby]

This is where we find out I did it wrong!

I hope not as I couldn't see how I could have. Look forward in seeing if others have the same as me.

Anyway didn’t look too shabby when I glassed the joints today. The epoxy spillage on the side is a Jackson Pollock tribute......all deliberately placed splashes and spills.

[Daniel]

Hi Peter, it looks like your build is going well, I'm following along the bolgs. I don't think you did anything wrong, but the CNC kit seems to have a larger designed radius in the pre-cut parts, which makes for a blunter entry point.

[captdan]

Daniel,
That is an interesting point that you brought up.
Just so you know, I used B&B Yachts stem layout that was on their "updated" Jig Drawing. The only thing that I did differently than what they had on their layout was that I made the overall length of the stem 1038 long and left the knee more square to meet this length. The tip of my stem was 18 - 20mm instead of the 28mm on their drawing. I suppose that is why I was able to match the 60mm that Piotr had on Trekka II. Thinking back now, I believe that mine was nearly square before I started to draw the radius on the wood.

Now looking at the drawing again, I see that they drew a finished stem profile in red and it appears that the radius might be very close to what Peter Brumby has on his boat. Structurally, I dont think either one is incorrect but I wonder about the merits of which is more efficient when going through the water. Maybe I should be
modifying mine ....

I understand that Toti Gioia is very close to this stage ..... maybe he has some incite on this too.

Captdan 

[Daniel]

Captdan, I would think that a finer entry probably has some hydrodynamic advantage to windward. Off the wind it may be less relevant.
They are small differences anyway, but over a MGR course all the tiny differences could add up. Probably trivial compared to other decisions about loading weight, sail trimming, seamanship, etc. But maybe obsessing over every small details is what helps differentiate race winners.
With the one-design there are only a few places for design/build to influence hull performance. I'm mostly just curious why the CNC cut kit makes that design choice, rather than leaving it to the builder. The material saved doesn't seem to be utilized in the cut layout.
To my eye, the sharper radius and finer entry is better looking.

[Admin]

I sort of get this conversation, but remember the issue is that the "meat"...timber build up at the stem is all there...the kit and plan built should be very similar if not the same...and THEN you radius and taper, take back the sides by removing materiel to create a cool shape, like shaping a surfboard etc...as shown on TREKKA BLOG...yes there will be slight differences but the extra meat is there to use for shaping, creating a smooth entry, as was done on TREKKA and you can see how much shaping was done by Piotr....you need to not weaken the structure but yes make it flow.. so good luck...also Piotr put some extra Ply pieces behind our stem...so quite thick and then I asked for more Glass as described elsewhere...and there is a small SS strip included in the SS pack to place on the knuckle of the stem in case hitting something to hopeful stop cracking the Glass?? ..good luck!

[Daniel]

Thanks Don, good to have confirmed that the bow meat is there for shaping. It does reinforce my original observation, that the CNC kit removes some of the meat pre-emptively, which seems to take away some of the builder's options for shaping.

[Daniel]

#74 Shinichi Sonoda had to tack on an extra bit of wood to the stem cap, to replace the bit the CNC kit pre-emptively removes, to be able to achieve the desired shape.

[shinichsonoda]

Nov 5, 2020 4:45:17 GMT Daniel said:

#74 Shinichi Sonoda had to tack on an extra bit of wood to the stem cap, to replace the bit the CNC kit pre-emptively removes, to be able to achieve the desired shape.

Certainly, the CNC drawing had a radius of 190m / m, and the ARK2 drawing had a radius of 140R.
I happened to pick up my compass and it was 100 mm open, so I attached a piece of wood.

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