Bow dropping at high speeds
 

I just finished a project sonic 35 SS, 725 hp bravo drives,28pitch bravo 4 blade props
At cruising speed the bow rides good and reacts to trim as it should. It I trim too much it porpoises , after 60mph the bow begins to drop and the faster I go the more it pushes down. At 80mph the boat is bow steering .
I checked all the usual things , hook in hull, drive alignment .
My last thing to try will be spinning props In

Any ideas?

Do you have a notched transom on that boat?

So are you saying it no longer reacts to trim after 60? When it starts to porpoise you either give it more gas to go faster till the porpoiseing stops or trim back.

The porpoising isnt a problem, just explaining that under 60 the boat carries the bow and reacts to trim , but if I go faster the bow starts to push down and I can't trim doesn't help

Does the speed continue to climb thru the entire RPM range? IMO, there becomes a point that the bottom of the boat has more more impact on the attitude than the trim...at least on a straight vee where the hull continues to come further out of water as the speed increases. I am not sure about other hull designs.
Ben

Some older V-bottoms love the extra stern lift that 4 blade props provide. However, certain 4 blade props on certain older hulls just don't like them at all. I tried a Spinelli 4 blade on mine a few years ago and the handling was downright spooky with way too much stern lift (leading to bow steer at higher speeds) and a really un-nerving tendency to try to paddle-wheel the stern around in turns. My Trophy 4 blade has no such tendencies though.

The first thing I'd recommend is a call to Brett at Bblades to see what the master thinks. In the mean time, I'd try a set of 3 blades just to see what happens.

There is a 3/16" hook in the boat I'm going to see if bblades can set me up with props to counter act the hook

The props can help, but with a hook you will still be scrubbing speed. The trade off with no hook is you just drag the tabs a little in the mid-range. You could be talking a +2-4 MPH w/o the hook.

My panther did the same thing when I added significant power. It was unsafe with bow steer. I tried spinning the props in and whala, the boat raised up out of the water and handled significantly better.

There's only so much Brett can do to counter act that much hook. Think if it as a full width trim tab - slightly actuated (down). So the faster you go, the more it forces the bow down. You may not have noticed it at lower speeds / with less power. With what you're running right now, it's time to do something about it.

Speaking of which, any time you're running over 65-70 MPH, I always recommend hydraulic steering too.