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Budweiser
06-19-2002, 07:50 PM
What is an X-dimension? How do I find or calculate it? How do I modify or adjust it? What will modifing the X-dimension do for performance? What about sefety? I'm new to all of this and need a little direction. Thanks guys.

HavasuDreamin'
06-20-2002, 06:27 AM
What kind of boat do you have? Is it an Outboard? I will assume that it is being that you posted in the Outboard forum.
I do not know the technical definition of X-Dimension, but the general idea is that X-Dimension refers to the height of your propshaft relative to the bottom of your boat. For example, most stern drive family boats have the propshaft 4-6 inches below the bottom of the hull. Most high performance outboards run the propshaft even with the bottom of the hull.
Finding the correct X-dimension is a function of what you want the boat to do and the type of boat you have.
If you want to pull skiers, wakeboards, and have an all around good performing boat for family purposes, leave the propshaft below the bottom of the boat at least a couple of inches so that your prop gets good blue water, not white water and subsequent cavitation. If you have a tunnel, or even a V and want more speed, raise your motor until your water pressure starts to fall below 12 lbs. This will create less drag and produce higher speeds. If you do not have a nose cone and low water pickup, you will be limited on how much you can raise your motor. Another item to think about is a jack plate which allows you to raise and lower your X-Dimension at the flick of a button. Good Luck

Budweiser
06-20-2002, 07:40 AM
Havasu- Thank you. I have an old (1970)Stylecraft 16' v hull with a 105hp Chrysler outboard. Does set back have anything to do with the X-dimension? I do not have a nose cone (is one available for this old engine) or a low water pick up. The engine has been raised about 1" from its stock position though. I will be towing skiers every now and then, but my main goals are to go as fast as possible, accelerate as quick as possible, and to remain as safe as possible. I guess I'm looking for a ballance of all three. If anyone knows of a mathmatical formula to find the X-dimension that would be great... Numbers make sense to me.
Thanks again Havasu
[This message has been edited by Budweiser (edited June 20, 2002).]
[This message has been edited by Budweiser (edited June 20, 2002).]

HavasuDreamin'
06-20-2002, 08:03 AM
Originally posted by Budweiser:
Havasu- Thank you. I have an old (1970)Stylecraft 16' v hull with a 105hp Chrysler outboard. Does set back have anything to do with the X-dimension? I do not have a nose cone (is one available for this old engine) or a low water pick up. The engine has been raised about 1" from its stock position though. I will be towing skiers every now and then, but my main goals are to go as fast as possible, accelerate as quick as possible, and to remain as safe as possible. I guess I'm looking for a ballance of all three. If anyone knows of a mathmatical formula to find the X-dimension that would be great... Numbers make sence to me.
Thanks again Havasu
No, set back does not have anything to do with X-dimension as I understand it, although set back does give you better speed on most v-hulls due to better leverage and lower trim angle. There is no formula for X-dimension. Most new boats have the X-Dimension set up from the factory in the optimum position. For older boats, it is trial and error. The higher you run your motor, the less drag and faster you will go.....until you run your prop out of the water and then excessive slip will reduce your speed. I am not sure about a nose cone for a 70's vintage Chrysler. You could try Land and Sea or Bob's Machine Shop out of FL. I don't have either number, but I would think both would advertise in ***boat or perhaps have a web page.
Remember to run atleast 10-12 lbs. of water pressure or you will fry your motor. If you do run a nose cone and/or jack plate, add solid motor mounts for better handling. Also, if you don't run a stainless propeller, you may want to try one. That will give you the most bang for the buck.
If it were me, I would most likely leave your X-dimension alone. If you start raising your motor through the addition of nose cones, jack plates, etc., I believe you will blow your gearcase. None of the Outboards produced in the 70's had gearcases strong enough to withstand the pressure placed on them when running the prop close to the surface. You need a heavy duty bearing carrier, and a strong propshaft. Plus your gears and bearings need to be in good shape.
If it were my boat, I would most likely add set back depending on where the motor is set up right now. I would think 10" or 12" would be good. I would also add solid motor mounts which will reduce the vibration and movement of the motor. I would then add a steel or bronze prop. This will give you more speed and not sacrafice any safety. Good Luck

Budweiser
06-20-2002, 01:39 PM
Once again, thanks HavasuDreamin'. I'll give the set-back a try. Everything you have said makes sense to me. Do you know how I would check water PSI on this engine? Do I do it at idle, at speed or both. I'm almost done doin' a half ass interior, and should be in the water in a couple of weeks. I'll save the big jobs for a winter project(fix the hook,paint,nice interior,bitchen stereo,etc.). I estamate the boat weighs less than 1500lbs., do you have any idea what speeds I should expect, without the mods. Does anyone else have any input? I wouldn't mind hearing more opinions, even if they are the same.

HavasuDreamin'
06-20-2002, 01:54 PM
You need a water pressure gauge. It is an easy install. You run a tube that comes with the gauge from the block to the gauge. My Merc has a specific piece tapped into the block that water pressure tube hooks up to. It has a clip around it to keep it on. I don't know where on your motor the tube would hook up to. Your water pressure will vary depending on speed. Slower speed = less pressure. More speed = higher pressure. You should see 10-12 lbs. at full throttle or your motor will fry.
I do not know what kind of speed you will get....good luck.

Skaterfast
06-20-2002, 10:24 PM
HavasuDreamin has pretty much gotcha covered.You do not have low water pick up on that motor so carefull on raising it.My Merc motors(225 Promax)run about 4-5psi at idle and somewhere between 15 and 20 at speed.Rough guess on your speed is 45mph give or take 3.

Skaterfast
06-20-2002, 10:28 PM
You only need set back if you are trimming the motor up excessively to get the boat more out of the water.Which is what I'm going to be doing next month to my boat.

HavasuDreamin'
06-21-2002, 10:06 AM
Also, make sure your current bracket assembly or any subsequent set-back assembly you decide to put on is bolted through the transom using high grade 1/2" bolts and lock nuts. Make sure that on the inside of the transom you have at least 1/4" thick aluminum plate for support. The bolts should go through the plate. Most high performance rigs also have what are called knee braces which run from the top of the inside transom plate to the stringers. The knee braces are bolted through the stringers. Periodically check all these bolts and make sure they are tight!
Happy Boating!

Budweiser
06-25-2002, 10:04 AM
Thanks guys. I'll let you know how things turn out. I'll also post some PICs as this project comes together.

Budweiser
06-26-2002, 04:18 AM
Hey guys, I had another thought last night, about water pick-ups. Can you custome rig a through hull pick-up to an outboard? It's not something I'm serious about, just curious. Would it solve the water pressure concernes while running the OB higher than stock?

Rickracer
07-21-2002, 04:18 AM
First off, I believe that any more that 5 1/2" on a 16' hull will not work, especially with a Chrysler with a less than optimum prop. There are a few companies that make LWP nosecones that are universal. I have a Land & Sea Universal Nosecone kit that has never been installed I would sell for $50.00 plus shipping. A transom mounted pickup would work, but I don't believe that the gain would be worth the work or the investment on that old motor. For a jackplate, take a look at the Bob's Machine MJ5. It's a 4 - in 1 plate, either 2.5" or 5.5" setback, and will go 0* trim or 6* negative. I sell them for $189.00 + shipping.

killer beazz
07-29-2002, 12:17 PM
i also have a 105 hp chrysler but resently up graded to 150 hp merc fore the simple fact i could not run over 40mph no matter whate i did my boat only ways 750lbs un riged i tried every thing ssprop not enuf improvement fore the money i sugest not to waist any more money on the chrysler save it and buy a merc you will be happer with it i garentee it