Bow increase HP the most for its weight, while Keels are being useless in comparison.
Bow increase HP on 4 per 1 weight, Small Keel increase HP on 0.4. Bow is increasing HP ten times better. And it does not cost much more too.
Medium Bow increases HP on 400 with 100 weight for 52$.
Small Keel increases HP on 300 with weight 750 for 40$.
The problem is that even though you can place a Bow everywhere, it can easily make your ship look ridiculous. It becomes a tradeoff: you make your ship stronger by making it ugly.
For now all my tests show that the ships with some Bows attached look silly (even though I limit their placement to not-ridiculous places), but are considerably better practically.
To prevent ugly or weak dilemma I can suggest adding the HP bonus to all filler blocks with the same HP per weight ratio as Bows (4 HP per 1 weight). And adding Light Keel module with absolutely the same stats as Small Bow, but with the size of 1x3 and with no holes. Structural bonus can be small or omitted.
P.S. Steam discussion.