Technical understanding of x-dimension

[LaughingCat]

I have more than 20 years of boating experience. Never owned a boat with surface drives or shorties, but have spent the last several years in wonder of how 2 boats with identical power can attain very different speeds based on lower and higher x-dimensions. I always figured if you trim the motors out and let the props break free, you get additional speed but also risk of porpoising. Do the surface drives and shorties get more speed because the blades on the top of rotation are not getting resistance from the water? It would seem that the same engine putting out the same RPMs and horsepower wouldn't be able to give you that much more speed.

Can someone explain this concept to me? Anyone have cool graphics that can illustrate the benefits?

[glassdave]

bringing the X dim up gets more case and prop out of the water allowing less drag. Not the same as trimming the drive up, that also vectors the thrust adversly.

[C_Spray]

Dave's right - It's about gearcase and skeg drag, while keeping the direction of thrust reasonably parallel to the waterline.

Technically, "X-dimension" is the crankshaft height, not the propshaft height. Not all drive packages have the same offset between the two.

[Big Time]

Good question. OK, let's take this a step further since LCat has brought this up. How is x demension measured? C Spray, you say it is crankshaft height....is the measurement from the bottom of the transom to cranksaft? Where do the #'s come from.

[MOBILEMERCMAN]

The X number is used when cutting the transom. It is the vertical value at the transom that intersects crankshaft center. The chart in the installation manual shows how different transom angles change prop shaft height. Once the hole is cut and /or the transom assemblies are installed it is very hard to measure. Prop shaft height is an easy measurement.

[Big Time]

The X number is used when cutting the transom. It is the vertical value at the transom that intersects crankshaft center. The chart in the installation manual shows how different transom angles change prop shaft height. Once the hole is cut and /or the transom assemblies are installed it is very hard to measure. Prop shaft height is an easy measurement.

Makes sense. The measurement for propshaft height would be taken with the drive all the way down I suppose?

[AIR TIME]

Makes sense. The measurement for propshaft height would be taken with the drive all the way down I suppose?

no its got to be level, trimed all the way down puts the bullit at a angle. most single vee hulls non step or no notch come from the factory with a prop shaft at 6 to 8'' below keel thats non pad or pad. which is good all around height that I have seen. my baja liked the 2'' shortie which got me at 5'' with a box 4'' is perfect both you need a 4 blade and will pick up better handling. I have seen some step boats at 2 and 3'' with a box. oh to get a level measure you can put a level on the boat adjust the trailer so it reads level boat then put it on the drive adjust it till its level or use a straight edge:sifone:, hope that helped a little, oh and not all boats like boxes.

[LaughingCat]

So should the prop shaft be level with keel? In some cases it looks like it's a little higher. I'm guessing if you have boxes and the water is beginning to rise from the keep edge, you set the prop shaft up to compensate. But then, once you hit some serious speed, may e you risk having part of lower blade out of water. Am I overthinking this?

I'm guessing that while this concept works, there's no benefit to changing your prop shaft height unless dealing with good power. A 13' Whaler with a 35hp will see no differnce. Just seems odd that gettingthe height will HIV much more speed when you are turning more RPMs on engines.

[Ratickle]

So should the prop shaft be level with keel? In some cases it looks like it's a little higher. I'm guessing if you have boxes and the water is beginning to rise from the keep edge, you set the prop shaft up to compensate. But then, once you hit some serious speed, may e you risk having part of lower blade out of water. Am I overthinking this?

I'm guessing that while this concept works, there's no benefit to changing your prop shaft height unless dealing with good power. A 13' Whaler with a 35hp will see no differnce. Just seems odd that gettingthe height will HIV much more speed when you are turning more RPMs on engines.

I picked up 11 mph in the 10 Bananza with a 50 merc by building a jackplate and raising the motor 3".

Had to have a bigger prop built.

[MOBILEMERCMAN]

Well actually any boat can see gains with less drag. The thing is a propeller change may be necessary. A 13 whaler will see a difference if you can find a prop that cant tolerate ventilating. A basic aluminum will see no gain. But if you were to find a high rake prop it most likely will.

The easiest way to go faster in many cases is to raise the drive. Most boats built in the 80's and early 90's had their motors in the basement. Likewise it is not uncommon to add big power and not see significant gains. It is not just how much power you have it is also how much drag you don't have.


To measure prop shaft height Parallel to the bottom surface is more accurate then level.

[Smarty]

So should the prop shaft be level with keel? In some cases it looks like it's a little higher. I'm guessing if you have boxes and the water is beginning to rise from the keep edge, you set the prop shaft up to compensate. But then, once you hit some serious speed, may e you risk having part of lower blade out of water. Am I overthinking this?

I'm guessing that while this concept works, there's no benefit to changing your prop shaft height unless dealing with good power. A 13' Whaler with a 35hp will see no differnce. Just seems odd that gettingthe height will HIV much more speed when you are turning more RPMs on engines.


We used 2"x4" blocks (between the transom and outboard)to raise the outboard on my 13' Whaler when I was a kid, it helped. Used to race all the other 13' Whalers in Brigintine, AC, Ventnor every chance I could.

[Davidmnc]

We used 2"x4" blocks (between the transom and outboard)to raise the outboard on my 13' Whaler when I was a kid, it helped. Used to race all the other 13' Whalers in Brigintine, AC, Ventnor every chance I could.

I loved my 13 Whaler!!! But mine had a 40hp baby!!! More Power!!!! :26::26::26::26:

[MOBILEMERCMAN]

I loved my 13 Whaler!!! But mine had a 40hp baby!!! More Power!!!! :26::26::26::26:

I had a 4 cylinder 40 on mine too. Mercury of course

[LaughingCat]

I actually had a 15' with a Johnson 70. Thought it was the bomb until my friend pulled up with a 16' Wahoo and a 90hp Johnson.

Not going to do anything with my Scarab Sport regarding this matter. But as I surf through listing at Cigs, Nor-Tech's and Apache's, it will help to know that an identical boat with same power can reach different speeds based on drives and prop shaft settings. I assume all #6's will be set for slip drive as opposed to Bravo XR's with normal lower units.

So basically a 38 Top Gun with shorties should run much faster than with regular lowers, correct?

[LaughingCat]

Well actually any boat can see gains with less drag. The thing is a propeller change may be necessary. A 13 whaler will see a difference if you can find a prop that cant tolerate ventilating. A basic aluminum will see no gain. But if you were to find a high rake prop it most likely will.

The easiest way to go faster in many cases is to raise the drive. Most boats built in the 80's and early 90's had their motors in the basement. Likewise it is not uncommon to add big power and not see significant gains. It is not just how much power you have it is also how much drag you don't have.


To measure prop shaft height Parallel to the bottom surface is more accurate then level.


I never even thought about the propellor change. That would make a huge difference. You could turn a much bigger wheel if the bite only occurs on the lower side of the shaft. Now the concept of #6 prop aerators is making sense. You could avoid installing aerators if you change the prop bite, but lose top-end.

I'm actually getting smarter because of this thread.

[MOBILEMERCMAN]

So basically a 38 Top Gun with shorties should run much faster than with regular lowers, correct?

Maybe, In recent years most manufacturers have been raising the drives to get the big speedo numbers. If it is already set up to an ideal height raising them more will not do any good. It will hurt performance.

[Geronimo36]

Maybe, In recent years most manufacturers have been raising the drives to get the big speedo numbers. If it is already set up to an ideal height raising them more will not do any good. It will hurt performance.

I agree with Jimmy and as he said earlier the older boats had the engines in the basement which was the case in my setup. I raised the X up 2" and so far I picked up 3 mph at cruise without re-propping or making any WOT passes yet.

I hope to see even bigger gains on the upper end. Jimmy, what do you think?

[MOBILEMERCMAN]

I think it probably handles better too.

[Geronimo36]

I think it probably handles better too.

I haven't run it in rough water but so far it feels like it has a better running attitude in the flat. Not that it was bad before but it seems much more responsive to trim/throttle now and overall the it feels smoother. :26:

[Big Time]

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?

[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.