How big is a ship?

Quick question that someone has hopefully already worked out, what kind of size modifiers would you need to use on a herbam spell to affect a wooden ship?

I remembered the Marius character from Magi of Hermes has a boat he enchanted but they left out size mods on that so I don't have a guide to go on.

Herbam base target is a plant a pace in each direction.

So what size mod do we think for a viking style longboat, or a european style cog or even a carrack?

You should be able to look that up in HP, p.53ff 'The Hermetic Shipyard'. A size table is in the top box on p.54. On p. 65 a cog counts as T: Structure for a Creo Ritual as well as for repair Rituals. The table with sizes for enchantment purposes is on p.66.


The easiest way to think about it is that 1 Individual of Herbam is a little more than half a ton.

A small boat is then within a base Herbam Individual.

Large boats are within 10 base Individuals, so +1 magnitude. This is probably just a little smaller than the smallest longships.

Small ships, including most viking longships, galleys and smallish cogs, are within 100 base Individuals, i.e. +2 magnitudes.

Large ships, including the largest cogs, fall within 100 base Individuals, i.e. +3 magnitudes.

Ships larger than that start appearing only in later centuries.

If I were you, I'd start be researching the dimensions of the ship you want to create, and calculate the volume represented by this.

For example, a cog can be about 70 paces in length and 16 paces wide (found those dimensions here). Since it is described as having high sides, I'd calculate the volume by assuming it is as high as it is wide (ignoring the masts for this), so 70x16x16 or about 18,000 square paces. That would correspond to +4 or +5 size modifier (between 10,000 and 100,000 square paces) for an Herbam spell.

A small viking ship (a karvi) would be about 25 paces long and 6 paces wide. As they are made to travel in shallow water, I'd say they about half as high as they are wide, or 3 paces. So 25x6x3 or about 450 square paces. That would be +3 size modifier for an Herbam spell.

So, depending on how big the ship is, it would anywhere from +3 to +6 size modifier.

Smaller boats, for fishing or river travel, could be smaller. Say +2 or even just +1 size modifier.

(Note that I always estimate size based on how big the final object is, not by how much material (wood in this case) is required to make it. A plant that is 1 pace by 1 pace does not represent 1 cubic pace of wood, so I see no reason why a manufactured item like a ship should be any different.)

Ah, here's where we disagree! I'm assuming 1 Individual is 1 cubic pace of wood.
So I account for a ship by the amount of material, rather than the space occupied.
This can lead to significant differences because a ship, like most containers, is mostly "empty space".
While I do not have Hermetic Projects, the ship of Maris of Tytalus from Magi of Hermes matches my definition of small ship and its hull is indeed affected by a spell with a +2 magnitudes modifier for Size.

Yes, it can make a huge difference in how the size is assessed.

Note that Maris' ship in Magi of Hermes is described as similar to a knarr-type ship, which is fairly small: 16 paces long by 5 paces wide. This would amount to about 200 square paces by my calculation, so it could conceivably fit into a +2 size modifier even by my calculation (it's a bit over, but not exceedingly so). If you only use the amount of wood needed to build it, it might actually fall to only a +1 size modifier, IMHO.

But this is a hugely YMMV issue. I tend to rule towards handling big stuff more difficult, rather than less.

Based on mass, according to the box on p. 113.

Which makes the volume-based targets kinda odd :frowning:

Right. But we still need to assess the base target properly. For Herbam (p.135), it is stated that "A base Individual for Herbam is a plant roughly one pace in each direction."

A plant that size is not the same as a cubic pace of solid wood. In fact, it is not much different in density than a ship. So using the dimensions of the ship to determine the target seems fair, IMO.


I was actually complaining - I'd prefer them based on volume :slight_smile:
Sorry i wasn't clear about that

The creo ritual should require a different target, creo spells that create must be group or individual.

Magic in Ars Magica has always struck me as an 'approximately' item. If something's slightly over, or slightly under, it's acceptable by magic, even if Mundane Science would object.

+1. Magic is magic. Ars, not science :slight_smile: Handwaving such details is fine for us.

Also agreed on the "individual" of Herbam beoing full of air (bush) so a ship should be considered as the same size increment. +1 size 10x1x1 paces "bush", +2 size 100x1x1 paces "bush". The +2 size should be enough to affect a small ship easily :slight_smile:


Cheers chaps. I'll peruse the stuff in Hermetic projects but it looks like +3 should be about right for what I am intending.

Thanks all

By HP p.54 smaller ships up to 8 tons freight capacity are Size +3.


I recall a "discussion" with a SG at one point where they ruled that a boat, being constructed of many pieces of wood required group with size mods. This worked fine, but the ruling was revoked when it was applied to chainmail, and we worked out that it was easier to just kill the target than to remove his armour.

As an aside, anyone else find the size increase a little modern? +1 mag => ×10 mass/volume i mean. Medieval England didn't seem to use 10s much, it was more the 12s...


Indeed. It would have been more natural to use x8, because you can essentially say that you are doubling each dimension.

But my guess is that it was felt that x10 made it easier math for modern players.

One Shot, that's interesting! However, one thing is Size + 3, another is +3 magnitudes over the Base Individual for Herbam. Do you mean the former or the latter?

If the former, it seems kind of small -- it would mean that such a ship is only 10 times as large as a man.
If the latter, there seems to be a contradiction with Maris' ship in MoH, which has 30 tons of freight capacity, and is within +2 magnitudes of the Base Individual for Herbam. Though at a more careful reading, it's only the hull that's within +2 magnitudes of the Base Individual of Herbam. However, it seems that the hull really is the bulk of the ship itself...

You can read that up yourself, of course. Anyway, the table in HP refers to general sizes of items, as defined in C&G p. 77. And C&G there relates to the table on ArM5 p. 192. Size +3 is given in ArM5 as the Size of a - mythical - Aurochs (of about the mass of some 27 men of average size).
I thought I mentioned already, that for terms of spell casting, ships like cogs in HP are handled as T: Structure. By ArM5 p.113 a Structure fits some 1000 base ind's, a Herbam Structure hence some 1000 shrubs of one cubic pace each.


Not quite. As I mentioned a few posts back on this same thread, I do not have Hermetic Projects.
So thanks for quoting the information to which I would otherwise not have access!

Ok, so it's Size, rather than "Size modifier" (magnitudes over the Base Individual of Herbam). Actually, I think Size +3 is defined as 10 times Size 0, so it would be about 10 times a man of average size; 27 men of average size would be Size +4.
In either case, it's very roughly 1 ton of weight. It seems strange to me that a 1 ton ship can carry 8 tons of cargo -- I'd have thought the ratio about 1:1 for small medieval ships; but I'm no expert so I may be completely off here.

T:Structure, by the way, is exactly the same magnitude modifier as using T:Individual with a +3 magnitude for size. So it is coherent. A bigger ship might have to be T:Structure with +1 magnitude for size, or T:Ind with +4 magnitudes for size (or more).

The decision to use T:Structure or T:Individual with a +3 size modifier is simply a matter of preference, although by RAW Creo spells that creates an object outright needs to use T:Ind.