Space 1889: shipbuilding calculations seem off


On the off chance anyone still likes this world and has the books to refer to:

The calculation for altitude for screwgalleys seems completely off in the Sky Galleons of Mars and the Cloudships and Gunboats supplements.

Each turncrank position, equal to the number of turncrank men I figure, is 10 tons. So the Skyrunner from "Cloudships and Gunboats" expansion has 16 turncranks = 160 tons.
The ship itself is a 300 ton rated vessel.
To figure out the max altitude, you divide that 300 by the total weight of the ship's components that go into the calculation:

160 tons for the turncrank positions
About 120 for the armament: 2 heavy guns (40ea) and a rogue gun (60)
That's 180 tons; so 300 / 300 = 1

Problem is, a quotient of 1 means only a max of 'High' altitude. But the listed max for the Skyrunner is 'Very High', the highest possible.
Other screwgalleys have this same issue in their calculation compared to the listed altitude rating.

What the heck am I missing here?

aramis erak

You have a math error. Working from your (minimalist) notes
Hull Size
Hs = 3​
allows flight with up to 480 tons. Well, technically 500... see below
Turn Cranks
Tc = 16​
Not a multiple of Hs, so a suboptimal choice. optimal would be 12 or 18
Bad idea, but it's your design
Heavy Gun
Rogue Gun
Now, working through the ratings...
LV = 100×Hs / tonnage
LV = 100×3 / 300
LV = 300/300
LV = 1
So High.
Now, it you stripped the rogue... you'd have 300/240≅ 1.25, cross the 1.2 barrier, and hit very high.
Spd= Turncranks / Hs
Spd = 16/3
Spd = 5.33
Applying the reduction for over 4...
Spd = 4 + (5.33-4)/2
Spd = 4 + (1.33)/2
Spd = 4 + 0.66
Spd = 4.66
Applying rounding, speed = 4. (some GM's keep the fraction for chases, but the rules don't)
Hull Hits 3 per altitude, thus 12 hits - first three for high, next three for medium, next three for low, last three for very low.
Gunners: 7 (2 per heavy, 1 per rogue)
Turncranks: 16. (1 per crank)
Bridge: 4 (captain, helmsman, trimsman, signalman)
Dechands 3 (1 per hull size)

On hull size... Hull size is how much liftwood is there, as much as overall size.
You don't (and should not) used the full.
If tonnage is 125 to 166 per hull size, max is low.
100-125 is medium
83 to 100 is high
Under 83 is very high.

So, optimizing your design a bit...
we use 12 turncranks instead of 16... saves 40 tons. 300/260=1.15... not very high. We need to shave down to 300/1.2=250 tons... but we also lose 4 crew and save £400, but still not very high.

Or, you can drop another thousand on liftwood, and be HS 4... so 16 cranks produces speed 4, the total 300 tons is out of 400 capacity, adds one sailor, and has LV=1.333, and so is very high rated.


The listed Skyrunner Hull Size in my Cloudships and Gunboats book is 4, not 3. So it it using 75 tons per hull size. Maybe the tonnage is a typo, and should be 400(?).
Regardless of what the tons/hull size each ship works out to in Cloudships and Gunboats, should I be using 100Hs for all construction calculations? (But then why use a varying range of tons/Hs instead of always using 100, if 100 is all you ever use for calculations? 'Flavor'?) In that case, what you said about Hs being how much liftwood is used, not necessarily raw size, makes sense. Then a Hs 3 ship whose listed tons is 160 still has 200 tons worth of 'liftwood power', but the ship size is more on the scale of a 160 ton ship in raw size. It just has a higher liftwood to non liftwood hull material ratio.
Because I just did the calculations for the default Aphid as well, and it also does not get to 1.2 because its listed at 160 tons; Hull Size 2 (so only 80 tons/Hs). I'm starting to think none of these ships work out unless you use 100 tons/Hs for calculations purposes regardless of what their listed tons and Hs are.
That number '100' is actually in the formula in Sky Galleons, but Cloudships and Gunboats (which is supposed to take precedence) does not include the '100'. It just says "about 100 tons per hull size'. Then that's the book that plays around with varying tons/Hs in all the example ships. Again, why the varying tons/Hs in the ratings if any variation from 100 means nothing anyway. Just 'flavor'?

But, I don't know where you got that max altitude based on tonnage/hull size. Where is that? I have not see that in Cloudships nor Sky Galleons. Or did you just figure that out yourself by comparing the stats?
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aramis erak

Sky Galleons. Altitude page 43. Plus a little algebra.

You're not groking how it works.

It lists it all as (Hull Size) * 100 / (total weight)
Total weight is the sum of installed components.

Per page 43
LV = 100·HS/tons
LV 1.2+ is VHigh
LV 1-1.1999 is high
LV 0.8-0.999 is medium
LV 0.6-0.7999 is low.
LV<0.6 is cannot lift.

Doing a little algebra
LV = 100 · HS/tons
we can solve for tons for a given lift.
0.6 = 100 · HS/tons
Since we want the ratio per HS, we enter that as 1
0.6 = 100 · 1 / tons
now, move the tons across and simplify the multiplication
0.6 · tons = 100 · tons/tons
0.6 · tons = 100
Move the 0.6 across to isolate the variable
tons = 100/0.6
tons = 166.666
Note that the 0.6 was the Lift Value. we can restate the work as
LV=100 · HS/Tons
LV · Tons = 100 · HS
Tons = 100 · HS/LV
so we can work out all of them
tons = HS · (100/LV)
And extracting it as a ratio, we just use HS 1

It's there, you just didn't grasp the math intuitively.

As for the swiftrunner...

It's got the same 300 tons used...
but being HS 4, is £21k.
And so, for LV...
LV = 100 · 4 / 300
LV = 400/300
LV = 1.333
Max altitude is very high.

Now it can carry some cargo... we need LV 1.2 or more
so, we use the ratio we found earlier for very high, under 83 tons per HS,
4 HS, 332 tons maximum load, of which the ship and crew is 300...
So she can carry 32 tons cargo (or added weapons) and still hit very high.