Trouble with jump drive

[formicae (OP)]

I'm having some trouble using a jump-capable tug to tow an engineless ship through a JP: it tells me that "at least one ship is larger than the ship with the highest jump rating", but I'm not seeing it.   For some reason, this 29,000-ton ship with a 29,000-ton commercial jump drive (which I've confirmed can jump on its own) isn't able to jump a 25,750-ton commercial construction ship:

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Liberty class Tug    29 000 tons     235 Crew     848 BP      TCS 580  TH 1920  EM 0
3310 km/s    JR 2-25(C)     Armour 1-84     Shields 0-0     Sensors 8/8/0/0     Damage Control Rating 6     PPV 0
MSP 110    Max Repair 100 MSP
Intended Deployment Time: 3 months    Spare Berths 0   
Tractor Beam     

JC29K Commercial Jump Drive     Max Ship Size 29000 tons    Distance 25k km     Squadron Size 2
240 EP Commercial Ion Drive (8)    Power 240    Fuel Use 3.54%    Signature 240    Exp 4%
Fuel Capacity 685 000 Litres    Range 120.1 billion km   (419 days at full power)

Thermal Sensor TH1-8 (1)     Sensitivity 8     Detect Sig Strength 1000:  8m km
EM Detection Sensor EM1-8 (1)     Sensitivity 8     Detect Sig Strength 1000:  8m km

This design is classed as a Commercial Vessel for maintenance purposes


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Velociraptor class Construction Ship    25 750 tons     44 Crew     726.6 BP      TCS 515  TH 0  EM 0
1 km/s     Armour 1-77     Shields 0-0     Sensors 8/8/0/0     Damage Control Rating 1     PPV 0
MSP 18    Max Repair 8 MSP
Intended Deployment Time: 3 months    Spare Berths 1   
Jump Gate Construction Ship: 360 days


Thermal Sensor TH1-8 (1)     Sensitivity 8     Detect Sig Strength 1000:  8m km
EM Detection Sensor EM1-8 (1)     Sensitivity 8     Detect Sig Strength 1000:  8m km

This design is classed as a Commercial Vessel for maintenance purposes

Is there something I'm missing?

Edit: After playing around in SM mode a bit, I've found that adding a commercial engine makes the combination able to jump normally, without (to my surprise) it complaining about having no fuel:

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Velociraptor class Construction Ship    27 050 tons     56 Crew     770.5 BP      TCS 541  TH 150  EM 0
277 km/s     Armour 1-80     Shields 0-0     Sensors 8/8/0/0     Damage Control Rating 1     PPV 0
MSP 18    Max Repair 37.5 MSP
Intended Deployment Time: 3 months    Spare Berths 2   
Jump Gate Construction Ship: 360 days

150 EP Commercial Ion Drive (1)    Power 150    Fuel Use 10.61%    Signature 150    Exp 5%

Thermal Sensor TH1-8 (1)     Sensitivity 8     Detect Sig Strength 1000:  8m km
EM Detection Sensor EM1-8 (1)     Sensitivity 8     Detect Sig Strength 1000:  8m km

This design is classed as a Commercial Vessel for maintenance purposes


[83athom]

Detach before you jump. Because you are towing it combines the weight so you have a single 55,000 ton ship instead of 2 ~25,000 ones.

[formicae (OP)]

Detach before you jump.  Because you are towing it combines the weight so you have a single 55,000 ton ship instead of 2 ~25,000 ones.

That didn't seem to help.

Since adding a commercial engine did do the trick, and that combination (where the towed ship has a commercial engine--it does need fuel, though) can jump while linked, I suspect the actual check to see if a jump is possible with a commercial jump drive is whether the ship has commercial engines, rather than checking for the absence of military engines like the wiki and technobabble imply.

I've discovered some other interesting towing behavior, though, which I'm also not sure is a bug or works as intended.  I made a ship to have low fuel consumption per unit time, using a 25 HS conventional engine:

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Freight Barge class Freight Barge    28 300 tons     142 Crew     340.4 BP      TCS 566  TH 2  EM 0
3 km/s     Armour 1-82     Shields 0-0     Sensors 1/1/0/0     Damage Control Rating 5     PPV 0
MSP 38    Max Repair 10 MSP
Intended Deployment Time: 3 months    Spare Berths 0   
Cargo 25000    Cargo Handling Multiplier 50   

Stabilizing Engine (1)    Power 1.5    Fuel Use 2.59%    Signature 1.5    Exp 3%
Fuel Capacity 5 000 Litres    Range 1.0 billion km   (4021 days at full power)

This design is classed as a Commercial Vessel for maintenance purposes

I thought this would last for 11 years being towed around, burning just over a liter a day, but instead it lasts for only about a billion km before needing refueling.  It seems like the towed ship uses fuel based on the distance it travels, not how much time it takes.

I think the next step is to try out a tug equipped with a military jump drive (though I'm not looking forward to the cost), attached to an engineless ship, to see whether that works.