Technical understanding of x-dimension

[LaughingCat]

interesting follow-up.

 

[Geronimo36]

OK, so we went over some of the benefits of a higher x dimension (less drag=higher speeds), what are the down sides? Seems to me that it would be harder to plane and in the rough could hinder speed if the prop is coming out of the water more than it would with a lower drive that would be more likely to stay in the water, is that a reasonable assumption?

I think that's spot-on. There comes a point of diminishing returns... Is it a race-boat or a pleasure boat? etc. etc...

During my project I had discussions with a bunch of racers/builders/riggers/prop masters and most of them told me to set the propshaft 4" below the surface for the peak performance on a Bravo and some were even running higher than that.

My question was, what about all around performance such as planing off, planing speed and using the boat as a pleasure boat with varying loads of fuel, people and supplies but I couldn't get much advice on that so in the end I took a conservative approach to my project.

As it turns out, when the boat was manufactured when the companies were using conservative X Dimension numbers (as Jim said, engines in the basement) so I contacted the builder of my boat (Bob Saccenti) and we spoke in great detail about the X Dimensions and what they were set up in the later years of production with new style props. I wound up going with a conservative X Dimension that put my propshaft about 5" below the surface. I was afraid to go any higher because I didn't want usability issues just to get another 1-2 mph on the top end and I didn't want to spend half the summer testing and messing around with prop styles, blades and/or spacers. Some guys like doing that, but for me I just like to go boating!

I'm no expert in this area but this has been my experience!

 

[Sean H]

remember race boats change their propshaft height depending on water conditions and speeds for that particular race... even some poker run boats change their setups for where they are running.

There is usually no one magical setting for all conditions.

 

[PARADOX]

Couple of thoughts in the " what it's worth" dept.
A higher "X" dim will get the props out of the water a bit so it can spin faster with less HP. In theory the faster the prop spins the faster you will go, up to the point when you actualy lose "bite". It's a fine line and design where is that "sweet spot".
But for some thinking for all you goof hydroengineers.
There is something called "hard water".. meaning undisturbed, no air, water.. the "harder" the water is the more "bite" you will get. Ergo... the "notch " in the boats. The farther you get the prop from the transom. the "harder"... less disturbed or les airated the water will be.. More bite.. faster top end. There is no perfect solution.. Having a step boat will get the hull out of the water,, less friction, better speed, less stability in handling. Getting the "X" dim higher, again.. less friction, some drives can use "surface pearcing" props, better speed, and again less handling stability. In theory.. get your props 10' passed the transom.... just barely submerged,, out of the water to get more spin RPM's.... you will be getting "bite" in "hard water"'.. best % of prop use and HP use.. but it will take you a 1/4 mile to make a turn.

Boating is a compromise in every sence. Speed = Horsepowerdollars.

 

[Geronimo36]

remember race boats change their propshaft height depending on water conditions and speeds for that particular race... even some poker run boats change their setups for where they are running.

There is usually no one magical setting for all conditions.

Sean is correct. My buddy who owned/ran the Team Tellium SCL ran different spacers, had 5 sets of Herrings, swapped prop rotation in rough conditions and added some weights to the bow. The boat was set up to run its fastest with neutral trim/tabs in almost all conditions and they spent a great deal of time balancing the boat and finding the optimal CG. Prop selection, drives, rotation etc was choosen as close to race start as possible... They also had one of the best in the industry up in the copter watching over them and recording data.

Go figure, they won the championship that year! :sifone:

 

[fast fun 2]

I picked up 4-5 mph in the Challenger playing around with diferent x-dimensions. Water pressure was the big issue.