Flat keel - what to do?

While I agree in principle, in his case some additional buoyancy might be a good thing, his boat is 390kg over the design weight, that's 37% of lightship. The increased drag from the fatter keel will hopefully be less then the current drag from the additional submerged hull area.

 

Ok, this would need a sharpened pencil.

 

Greg"
Read about your new rudder in the article Out of the Blue while researching Yachting World Dimond designed by Jack Holt.

Article - Updating Yachting World Diamond | Competition Composites Inc. https://cci.one/site/about-us/out-of-the-blue/?doing_wp_cron=1619497174.2846529483795166015625

 

I think that YW Diamond with a new keel can hardly be called a Diamond any more, about the only thing left of a YW diamond is the skin of the hull..

Ok I'm not a naval Architect nor an expert but I have spent hours researching keels, so here's a few unqualified comments..
Yes the junction of Keel to hull is awful,.

Have a look at , https://www.ericwsponberg.com/wp-content/uploads/keel-and-rudder-design.pdf
and this Design and Construction Of Centerboards and Rudders http://www.boat-links.com/foils.html

Neil Pollock did some research , on flat sided keels, IIRC, the outcome was, take a NACA foil split it at the widest point, then put as much flat as you need in the middle between the ellipse forward and the cone down to the aft end of the fin
The penalty compared to a true NACA foil is not great.. Shaping on the bow and stern of the fin is more important than the flat down the sides..

So looking at your keel, the biggest problem is the lack of taper to the stern, it's way to abrupt.. But thinning your current keel down at the stern will probably lead to problems with Keel bolts and stiffness. I would think a new keel even of the same draft will require a keel bolt arrangement and stronger materials than ply ...

As to future plans, how are you with draft? As standard the YWD is quite shallow drafted, which can be useful, going to deep keel may cause problems trailering and launching, also will restrict the harbours and waters you sail in..
4 foot 6 inches is just about ok where I sail, but 6ft would, at times, make a good plough ..

 

I thought the Diamonds only adopted the trap in the '80s. The pics I have in old mags and my own recollection of the OD fleet at my club when I was a kid are of a four-crew hiking boat.

.917 is, I think, lower than Saltash's rating even with the inboard she carries. If I recall correctly, she rated about .900 before she had the new keel fitted and in the right conditions she was extremely competitive at that rating. She was, however, off the pace around short courses in light airs before she got the new keel, but .917 may not be "shafting" you but merely an example of the fact that if you have a boat that excels in one condition, you won't be competitive all the time.

The Victorian state basic handicap (where the Diamond remains as a small class) used to be .817 according to my ancient records. For comparison, lightweight mid '80s IOR 3/4 tonners (ie Db1, Davidson 34, etc) were .837; Etchells .823; Farr 1104s (considerably quicker than a Sigma) .806; J/24s .743; Carter 33s .756. Those handicaps were merely derived from average performance with no consideration of factors like quality of fitout and crew skill and the Diamond loves the often rough weather in the area, but the IRC handicap for the Diamond looks pretty good from here. I know that many other dayboats/keelboats (ie 5.5s, Etchells, F15s) are very competitive under IRC.