X-18 X-dimension

[The Hedgehog]

Just wanting to make sure I get this right before I go cutting holes in that pretty new transom!

Am I to understand that stock X on a Bravo should be where the cave plate is 3/4 on an inch above the lowest point on the keel?

Any other measurements I need to consider?

[MOP]

I don't think 3/4" is high enough! On my 22 the bottom of the transom Bravo shield is 7-1/4" up from the bottom of the keel, my cave plate is higher then 1" can't measure for a few more weeks.

I think you should check with Steve Mars, I got the right drive for the back of that boat:wrench: :superman: Worth about 11MPH!!!

[Lenny]

14 1/2" from bottom of hull radius, mark that UP from that point on the transom. That is the "X" dimension. Now go to MERC and get a template (paper) and carbon paper it out and follow the 60 degree angle specs.

This will basically make you STOCK for a Bravo or Alpha from the factory. Ted is probably (top secret) UP about 3" from that number. Steve Marr has those numbers readily available BUT you will NEVER be able to run anything else BUT the BLACKHAWK at that location. Short of a mile of spacers.

If you want it easy that is the way to go. Then bolt on a BRAVO, and play the spacer game (up) from there and take advantage of the low C of G . (can you say IMCO shorties's)

:)

[MOP]

What some do not realize is the X on the 16 and 18 is determined by the engine compartment height, the engine will not fit with the proper X it is much lower then what it should be. I just went out and did a careful measurement of my 22 which obviously has a taller compartment it is 17" inches stock from the factory, At 17" I don not blow the wheel in turns and have a great hole shot. With a low X you increase the drag considerably and also increase the lever arm which in turn contributes to porpoising. To get the boat to run at its full potential the engine hatch must be modified to allow the higher X, I would bet my bottom dollar that several miles per hour are scrubbed due to a low stock X. Sure you can spend a few thousand and go to a shorty later but why not start out right in the first place. I heard a rumor over the Dora weekend that the sudden and mysterious porpoising experienced in the late 22's is due to the fact that Donzi lowered the engines down. You are spending a lot of time and $$ and want it run the best it can raise the X to at least 16-17".

Edit: Read this post you will see several boat are running the higher X where the transom shield bottom is 7-1/4" up from the keel which equates to a 17" X!
http://www.performanceboat.us/forum/showthread.php?t=401

You want to run raise it!!!!

Phil

[The Hedgehog]

We need to consider that I am not looking for max speed. I want a decent hole shot, great handling and no blow out on turns. I want my wife to be able to drive this boat. I will be willing to give up a few mph now and work with a shorty later if I want to pick up an extra 5 mph. That being said, I don't want the porpusing issue either.

In talking to Forrest, I learned that I need to be careful of hatch clearance issues as well

[MOP]

The porpoising would only be cruising speeds, a set of trim tabs will handle that issue no sweat.

[BigGrizzly]

Gee willicers I remember a thread before I got sick with how and where to measure from. First there are so many opinions I almost firgot the real answer. First it is always measured from prop shaft center line to the bottom of the boat. This is where the problem lies. The round bottom and Barrel back hulls vary. So it becomes the theoretical bottom. We need to talk the same language. tememberthe thread on the hook in bottom of a classic. One guy though because it had several not flush pockets it had a hook. that is exact ally what is happening. Look up that old thread I believe Cdma did it. then we are on the same playing field we can again talk. As for the X on a Donzi being too low that is only the blackhawk drives! Chris Allard raised his and I don't beleave anything changed. Look that thread up it is several years old. but it was a good one and will help ypu make your derision. All mine have the cav plat 3/4 inches above the round bottom of the boat when the plate is parallel with the bottom. If you ask me would I raise it I would say NO! The inco is temp not a permanent change, a hole in the transom is!

[The Hedgehog]

Gee willicers I remember a thread before I got sick with how and where to measure from. First there are so many opinions I almost firgot the real answer. First it is always measured from prop shaft center line to the bottom of the boat. This is where the problem lies. The round bottom and Barrel back hulls vary. So it becomes the theoretical bottom. We need to talk the same language. tememberthe thread on the hook in bottom of a classic. One guy though because it had several not flush pockets it had a hook. that is exact ally what is happening. Look up that old thread I believe Cdma did it. then we are on the same playing field we can again talk. As for the X on a Donzi being too low that is only the blackhawk drives! Chris Allard raised his and I don't beleave anything changed. Look that thread up it is several years old. but it was a good one and will help ypu make your derision. All mine have the cav plat 3/4 inches above the round bottom of the boat when the plate is parallel with the bottom. If you ask me would I raise it I would say NO! The inco is temp not a permanent change, a hole in the transom is!

Thanks Grizz. I will search the threads. Like I said, don't mind leaving a few mph on the plate. I want good holeshot, no blowout and would prefer for my wife not to have to learn how to use tabs. I understand that a properly setup 18 needs no tabs. If I want to wring out some mph later, I have not prob with the shorty. I also like the idea of the lower CG and not having to modify the hatch. I would really hate having to modify the hatch, have a higher CG and then have to space down a stock Bravo so it will have good handling characteristics!

If I was going for max speed then I would probably go higher. I know how to coax up a boat with a high X and use tabs. But then I would also probably put on a blower. By then I would be pissed that I did not by Geo's boat!

[mphatc]

Big Green ZX,

I agree with Randy . .

I have a Bravo on my Corsican, same hull as your boat. I followed the Merc template to the tee, and only raised it 1/4" IRC. This aligned the drive shaft from the Bravo precisely with the original engine crankshaft location! The boat had a Volvo before.

The boat is real easy to drive, jumps out the hole, and is great in all kinds of water. Sometime I'd like to run a shorty on it, but the boat reaches close to 70 ph, plenty fast as is, and in calm water with a light wind ripple I get a bit of chine walk at WOT.

I suspect I can eliminate a portion of that by adding power steering, as the cable actuated spool valve PS set up leaves some slop in the system. Solution is only More Money . . :eek!: which is what I should name the boat!:yes:

I'll be working on my boat tonight and Saturday, so I can easily measure my location and post it here.

Try to borrow beg, steal the Mercruiser drilling jig, it makes life much easier.


Mario L.

[Rootsy]

hmmm this seems vaguely familiar....
http://www.donzi.net/forums/showthread.php?t=46839

[The Hedgehog]

That is a huge help!

I will say that mulling over all of the different options did net my head turning. :wink: Maybe I should grow a sack and jack it up a little. Fabricating a custom hatch has got to be cheaper than a shorty!

We shall see. I have always wanted to get a shorty anyway. Be fun for swapping around to the big boat and playing around.

[BigGrizzly]

Green Poodle has an X with a trs and a 383 that runs the high 70 with X in stock location. However the IMCO is a lot cheaper than redoing the stern and making a hatch and raising the engine and re engineering the boat. Remember balance my boy balance. Remember this is your wifes boat. As far as I am concerned and having driven that hull style for over 40 years with almost all different drives and engine configurations, and I like the way the boat is in stock form. Where is my X dimension. how does my boat handle. this is not a test bead for experimentation, a fun toy for the wife and you.

[The Hedgehog]

Green Poodle has an X with a trs and a 383 that runs the high 70 with X in stock location. However the IMCO is a lot cheaper than redoing the stern and making a hatch and raising the engine and re engineering the boat. Remember balance my boy balance. Remember this is your wifes boat. As far as I am concerned and having driven that hull style for over 40 years with almost all different drives and engine configurations, and I like the way the boat is in stock form. Where is my X dimension. how does my boat handle. this is not a test bead for experimentation, a fun toy for the wife and you.

Trust me. I have taken what you said totally to heart. I like to mull things over and look at all sides of the equation but if I want to play around later, would rather go the shorty route. I will go stock for now. Want a nice driver for the wife and some water sports. I will probably pick up an IMCO lower for a number of reasons anyway. 14.5 it is

[Lenny]

14.5 it is

:D:D:D

That is what you want.

[BigGrizzly]

Green, I do know one thing about you and that is you test all the waters before you jump in, "Yo mama didn't raise no fool".

[The Hedgehog]

Green, I do know one thing about you and that is you test all the waters before you jump in, "Yo mama didn't raise no fool".
That probably comes from jumping in the waters too many times without testing:bonk: But I have learned a few things as I have gotten older and that is to use knowledgable resources. With the help of you guys, I think that I will end up with one cool boat. The pretty new transom will have a nice 383/Bravo package in it by the end of the week. I will supply plenty of pics.

[MR MAGOO]

Thanks MOP for the link to this thread. I had called the Donzi factory and was told the standard X-Dimention for the 18 2+3 is 14.5" and can be as high as 16.5"

Does anyone know how high one can raise the X without interference with the hatch?

Thanks guys, great info here!

Kurt

[bobwpe]

My boat is a '72 2+3 with a 2004 Merc. 350 MAG/MPI Bravo. The X dim is 14 3/8" and as you can see from the picture it would not have fit if my hatch didn't have a scoop. This is with a throttle body so if you have a carb it might be a little higher or lower, perhaps someone who knows will chime in. :crossfing: This engine also has a remote oil filter and the top of it gets a small shiny spot where it just barely rubs the hatch.:eek:

Hope this helps.
Bob

[MOP]

Hmmmm wonder if a late hump hatch is in order!

[fogducker III]

As Phil pointed out, one of these would do the trick if clearance was an issue.


PS. My Bravo with the Scorpion is exactly 14 1/2" and with the -2 Imco I have no problems with hole shots, blow outs or porpoising......good luck with the install......:popcorn:

[rocksmann]

i am also cutting a new transom this week in my 73 x 18 I was going to cut the x at 14 3/8 I was hoping the stock hatch would clear my injected 5.7 with a brovo is this going to work. I have also been mulling around with changing the lower to a bravo 3 to eliminate torque steer what do you think

[VetteLT193]

I think the BIII will slow you down a good bit but offer a great cruise speed.

not sure on the X. I'd probably stick the engine in the boat and measure to see how high you can go with the crank shaft with your exact boat and engine.

[rocksmann]

thanks I don't have the engine fully assembled yet do you know the hight from engine mount to spark arrester

[VetteLT193]

thanks I don't have the engine fully assembled yet do you know the hight from engine mount to spark arrester

That varies on intake EFI or carb, height of intake, air cleaner/arrester, carb or throttle body height, etc.

Someone around here might have general specs but I think with that bout you'll want to get as accurate of a number as you can.

[realbold]

What I was looking for are numbers I can use now, meaning who would want to install the gimbal housing and engine and then measure the "X"?
George said on his boat the distance from the keel bottom to the bottom of his cutout is 7.375". I measured my housing the best I could and looks like the shaft to cutout is about 7.75" which would put his X at approx 15.125". Mop said the bottom of his gimbal housing is 7.25" from the bottom of his keel which would put his at about 16.25".
Am I in the ballpark with these #'s?