I've been trying to build some conversion of sea vessels from GURPS to ACKS in addition to playing around with GURPS (3rd ed.): Vehicles and the construction rules in GURPS (4th ed.): Low Tech Companion 3. What I noticed is that the SHP of vessels in ACKS seem to be roughly linear with the length of the vessel, but the SHP of fortifications is closer to the volume of the structure. (To complicate things, GURPS 3rd ed. HP scaled with the surface area!) In addition the raft SHP scale with their area.
Which one should be used as a general rule, if for example creating magical flying buildings?
With the possibility of calculating siege weapon damage with the formula on Guns of War it's possible to compare penetration and damage values between systems (and to any real world data) and benchmark these.
Note: Vessels have around 1 SHP per foot of length (and about ½ of GURPS 4th ed. HP) and stone fortifications have around 1 SHP per 12 cubic feet of wall (or ton of mass).
Also AC of both types is based on the material (and possible angling/rounding bonus), and not modified by the size of the construct (unlike monsters). The increased thickness of ships of war and the English naval ship rating may have been the original source of the Gygax/Arneson decreasing AC, but in ACKS this is not the case. Here SHP are the measure of thickness (linear scaling) and thicker walled ships should have more SHP.
The different treatment of SHP could be thought to represent two different things: Enough damage to breach the structure (to sink a ship or allow egress into a fortress) vs. enough damage to completely demolish the structure. Reading some historical anecdotes (no real data or statistics) I was surprised how many hits ships could survive without sinking.
To have an idea on the the amount of SHP for ships of the line, I'll have to calculate some real world gun penetration vs. hull thickness data and compare to ACKS gun damage. I'm suspecting that getting the ships' endurance in line with the size of the guns carried (and required to breach them) will lead to either increasing their SHP, requiring more breaches to sink larger vessels or taking hull thickness into account by increasing AC. Going backwards into increasing ACs for ships would incidentally eliminate the need for a structural saving throw for them. I'll have to think about that when I look into streamlining firing broadsides of tens of guns in naval battles.
As an example "Sovereign of the Seas", a 1st rate ship of the line is 170' long (ca. 170 SHP) has 20 "cannon of 7" (36-pdr cannon), 28 culverin (18-pdr gun) and 54 demi-culverin (8-pdr gun). A broadside will do on average around 1800 SHP damage if all shots hit, 800 SHP for a point blank shot with 0-level gunnery crews (assuming ship profile is larger than scatter and either AC vs. to-hit or Structural save). So two similar ships firing point-blank broadsides at each other will either sink or breach each other more than 4 times. To give chance at a 1-round sinking, but having the ships survive more than one broadside on average to keep things playable (and more interesting) would require one of these:
- Multiply SHP by 10 (basing them on volume/mass may give similar result, as the ships tonnage [not actual structural mass!] is 1500)
- Divide damage to SHP by 10 like mechanical artillery
- Increase AC to 9 vs. to-hit
- Increase AC to 18 vs. structural save
As a 1st rater it's estimated hull thickness is 28" (possibly lower as the rating/thickness table is for a later century), compared to 2"-5" of boats, civilian ships and viking vessels.
Anyways, something needs to be made more clear or changed, but more data is needed to know how much. What to change is then another question.