Visigoths hold the straits

Okay the visigothic kingdom was rather weak but I'm curious how can we make it so the Visigoths hold Spain and keep the Arab invasion at bay?

Is this possible?

If not how can we make it possible?

If the Visigoths hold the Pillars of Hercules(perhaps with Byzantine naval support) how will this affect the development of Iberia?

How will the Umayyads react to this?
 
Is this possible?
Not really : the Iberic Sea is really short, and not that difficult to cross quickly with small but numerous ships (as Vandals did in their time, and as Arabs did).

For the same reason Gibraltar is a strategical place, where a relatively limited naval and fortified force can prevent a fleet to go from Atlantic to Mediteranean (or the contrary); namely a really small gap between Europe and Africa (less than 10 km); it's really hard to prevent a transcontinental cross (especially with the currents of the Iberic Sea).
It's why the pre-steam naval powers generally elected to maintain a coastal controls of both banks, as Goths more or less attempted while taking the scraps of Mauretania I from Byzzies.

Remember, eventually, that you had Berber raids in Betica since the VIIth century, even before the Muslim conquest of Maghrib, and that all (when so) where met on land, not on sea.
In fact, according the Chronicle of 754 (which is mostly correct, especially compared to later accounts, regardless Christian or Muslims), you didn't have one big crossing, but several raids and campaigns that devastated several places before the Battle of Guadalete : it's then pretty much impossible to have Goths preventing this to happen altogether, their only chance being a successful enough battle (or series of battle) on land (or to prevent the Arab conquest of Ifriqiya, or at the very least of Maghrib)

If not how can we make it possible?
When I said Goths had the only noticable mediteranean fleet after Romans in Western Med, it doesn't mean it was that strong either (altough enough to repeal a more or less roguish Roman expedition in the early VIIIth century).

It doesn't help that Goths were undergoing a civil war in the same time, with the usurpation of Roderic in western Spain : while Gothic potentes actively searched to prevent the appearance of a strong dynasty that would monopolize honors and fisc at its benefit (most kings managing to be in place thanks to a large redistribution network), Roderic is more representative of the provincial revolts with a military claimant that opposed the Gothic court nobility, with the caveat that they didn't managed to curb it down (hence why some called Arabo-Berbers as hired help IOTL).

I wouldn't see a real naval development, even during the reigns of Egica and Wittiza, on the south : even with strengthening even more than the tradition accounts for (and it's really not a good source) with the Romans or post-Romans rulers of coastal points as Tingis or Septem (Tangiers/Ceuta), it's not going to stop the Arabo-Berbers raids and transmaritime campaigns.
 
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