Good evening.
The rule in boat trailer design is very simple. The dimension from the transom to the centre of the axle or axles is 5’.
thats as complicated as it gets. In your case it’s from the hub of the centre axle to the transom.
if you set it up with that dimension it will tow like a dream. Tongue weight should be no less than 300lb.
if you call velocity they’ll send you a plan to layout the bunks in the right place.
If you weld the new bunks in place and the new required cross members the trailer will stiffen up. I rebuild and customize
I beam trailers all the time. The 5’ is an industry standard.

The other posters are correct. A fourth axle is not required. Especially for a boat as light as Velocity builds.

thank you.the tongue is way over 300 by far I'll check the 5ft rule. Where are you located

2 hours north of Toronto on the Trent Severn Waterway.
Cheers

It looks like the boat is riding up on the bow blocks and lifting it off of the two keel supports. Try lowering the bow blocks and let the keel supports take on some of weight. This should remove the positive arch in the I beams and reduce the flex you're experiencing. I'd be willing to bet there's very little contact between the hull and the two most forward keel supports.

If that doesn't remove the positive arch in the trailer the axles are going to have to be relocated further aft.

The 2 front supports you see in pic were just keel rollers I moved one forward and put 6ft bunks in place of 2nd roller. (Remember those pics are when i picked up boat). Can't see them in pics. The last pic they are installed. You can also see where I shortened the main bunks at rear. After thne boat was moved forward the bunks were about even with bullet case where prop shafts start