Does shaving weight off your engine change your prop depth???
 

Just wondering as my 99 Donzi 26ZX seems to sit pretty high in the water when looking at the back? I have a 502 with aluminum heads,intake,exhaust so I have shaved about 300lbs off the back of the boat..... Have I effectively created my own homemade version of a shorty drive???........ I was out playing today and it seems that the boat rides pretty flat in the water? I tried trimming the drive up to get the nose up a bit and I was shocked at how much faster the boat went? I was at 3200rpm at 40mph and from trim alone without touching the throttle it went to 3500rpm and 50 mph! I gained 10 mph from trim alone? Now I am a newbie here with only abut 4 hours on this boat and taking it easy for a while....... I am using a stock 26P Bravo 1 prop and my previous experience is with a 7.4L with a Bravo 3 drive and the differences in the two are noticeable......... I have yet to go much more than about 1/2 throttle and this thing seems pretty fast! I am liking this performance boat thing!!! TBF

Can't change the relationship (X-dimension) of your prop to the bottom of the hull by losing weight. You will change the "draft" of the vessel, as it may not sit as deep in the water.

As previously answered- no, changing weight won't change X-dimension.
I'm no scientist or mathematician, but with regard to the change in speed by minor trim angle adjustments, I'm guessing either:
A. The new aluminum goodies also changed the torque curve slightly(different valves, plenum flow, cam grind, etc.) and you found the trim angle & rpm sweet spot for cruising.
B. It's a stepped hull.... You're in the "drag bucket". (High lift, low drag).
C. Both A and B.

FWIW, a boat test put a stock 7.4MPI 26ZX at 46.5mph on GPS @3900rpm.

http://features.boats.com/boat-conte...t-donzi-26-zx/

Those speeds sound about right mine with a stock 24 p will run 45 MPH at 3300 and loaded with people and fuel. With my labbed 26 p it's a little faster. I assume you are running in salt which will be faster than my freshwater numbers. I will say with mine anything below 30 mph the drive trim wants to be all the way down or slip goes up and speed drops with just a little trim. When running hammer down it likes a lot of trim.

Michael

My 26 would blow out the prop getting up on plane.

Just need to get used to pulling back if it does and let it settle.

I took the 125 pound swim platform off in addition to stainless marine exhaust and aluminum intake which was about another 125 pounds out of the back, it was enough that a laser 2 prop couldn't carry the bow. It would blow out at higher speeds. Had to go up in diameter to a mirage plus and it was good again.

Speed is always effected by power vs load. Load = Weight / water resistance (drag)/ wind resistance (drag) / mechanical power losses / etc/etc

Obviously trim setting greatly change load.

It doesn't change the relationship between the prop and hull, but it can definitely change the relationship between the prop and the water. Less weight in rear=boat sits higher=less prop in the water.

I came into the same problem as you... went aluminum exhaust, intake, heads, etc... laser 2 could no longer could carry the bow... went with a mirage plus and all is good now.

This is Exactly what I am talking about here.... The prop will not be as deep in the water and it might change the normal balance or "attitude" of the boat? I did buy this boat with no engine and have no experience with it otherwise..... I do have raised port heads that I massaged a bit and a .600 lift cam and high compression so I am sure I have a lot more oomph than a stocker..... I have heard all the horror stories of step hulls spinning out because the back of the boat is not planted in the water (trim in?) and my lightweight 509 might be contributing to that? I am taking things slow for now and learning how the boat handles in different conditions and trim settings.....