Kayak lock on roof rack recommendations

Thanks everyone for the suggestions! I'm leaning towards one of those fat chains/shackles like Malibu_Two uses.

Forgot to mention that I live in SF, where car thief/thief in general has been pretty rampant as of late.   

If you really want to freak out the tweakers, you can also attach a "personal alarm" to the chain or kayak.  That way, when they trip the wire, it'll sound the audible alarm. 

https://www.rei.com/product/169718/nathan-saferrun-ripcord-siren-personal-alarm?CAWELAID=120217890009877405&CAGPSPN=pla&CAAGID=97161286701&CATCI=pla-294573402176&cm_mmc=PLA_Google&product_id=1697180001&ad_type=pla&channel=online&partition_id=294573402176&target_id=pla-294573402176&brand_flag=&adgroup_id=97161286701&campaign_id=9893049009&location_physical=9031939&cid=431779568080&network=g&network_type=search&device_type=c&merchant_id=1209243&gclid=EAIaIQobChMIoYHIhM-q6gIVaz6tBh3KrQaXEAYYCSABEgJ2DvD_BwE&gclsrc=aw.ds

It's not as good as a grenade, but at least your kayak won't be blown up.   

If you really want to freak out the tweakers, you can also attach a "personal alarm" to the chain or kayak.  That way, when they trip the wire, it'll sound the audible alarm. 

https://www.rei.com/product/169718/nathan-saferrun-ripcord-siren-personal-alarm?CAWELAID=120217890009877405&CAGPSPN=pla&CAAGID=97161286701&CATCI=pla-294573402176&cm_mmc=PLA_Google&product_id=1697180001&ad_type=pla&channel=online&partition_id=294573402176&target_id=pla-294573402176&brand_flag=&adgroup_id=97161286701&campaign_id=9893049009&location_physical=9031939&cid=431779568080&network=g&network_type=search&device_type=c&merchant_id=1209243&gclid=EAIaIQobChMIoYHIhM-q6gIVaz6tBh3KrQaXEAYYCSABEgJ2DvD_BwE&gclsrc=aw.ds

It's not as good as a grenade, but at least your kayak won't be blown up.   

not a bad investment for $20 , although I can see myself accidentally tripping it off with the neighbors giving me the stink eye

Fro what I hear, SF is the car clout/theft champion, but Santa Cruz is doing pretty well in the bike theft rankings, so I bought one of those alarm locks, but couldn't manage to set it without having it go off, so I sent it back.  Maybe I just had the wrong brand.