[Low-Tech] Landing Craft at TL2 to TL4

[Icelander]

In a world very different from our own, with supernatural powers, widespread magic, supers on every side in warfare and so on, warfare follows a very different path. Hence, warfare in Dungeon Fantasy or, indeed, any Fantasy world with reliable and reproducable supernatural phenomena useful in warfare never develops along lines familiar to our non-magical Earth.

One thing that is prominent is the heavy emphasis on special operations*, with espionage, covert operations and surgical behind-enemy-lines strikes against the sources of their magical powers often deciding the outcome of battles.

In my TL 1+2, TL1+3, TL 2+1, TL 2+2, TL 3+1, TL 3+2, and TL4 campaign, the PCs are prosecuting a war where control of the oceans and the immense logistical advantage conferred by maritime resupply is key to the strategy of both sides. Accordingly, they want to move past merely raiding the enemy ports to destroy shipping and actually capture the ports that the other side is using to supply their three great field armies that are moving against the last bastions of the side that the PCs are supporting. Thus, hopefully, stranding tens of thousands of enemy soldiers in a hostile land without adequate resupply.

At real world TL 4-5, ships' boats were quite sufficient for amphibious operations. The PCs have the equivalent of TL 4 ships, with a few early TL 5 ships, so they have longboats, cutters and captains' gigs aplenty, manned with doughty Jack Tars and marines armed with smokepowder weapons.

They've had some two months to construct more boats to aid in their amphibious operations, however, and it would probably be effective to construct boats longer and heavier than those routinely carried on sailing ships. They only need to sail a distance of 25 miles, after all, and can be dragged behind sailing ships or galleys if needed.

I was wondering what might be reasonable designs for landing crafts for the proposed amphibious attacks on the two port cities. I considered some designs more efficient** than ships' cutters or longboats, such as chalanas, barges, chasse-marées, feluccas, war canoes or catamarans.

Various local people are partial to galleys, feluccas and catamarans.***

What would be a reasonable design for the PCs to order for their amphibious assaults? What would be the GURPS stats for something that was bigger than a ships' boat, but smaller than an average Age of Sail ship, which could be used as a landing craft in TL4 warfare?

*I. e. stuff PCs do in Mass Combat situations.
**In terms of both man-hours needed to construct it and crew needed to work it, compared to the amount of men and materiel that it could land in a given period of time.
***Local pirates who build extremely fast sailing and rowing boats of that design, with whom the PCs recently allied.