CG c/w conversion & X theory smaal boat
 

Another done to death topic but i don't remember seeing it on a small boat like a 20 footer for example, say a 20 cig to put everyones brain in the same arena and maybe spark up some info from past projects


To continue, I don't see a problem, i'm not inferring a problem or leaving out the fact that an adult moving from the rear bench to the front or side or fuel burn changes cg specially on a small boat, this is just some of our favorite topics revisited.


Let's say your converting from trs to bravo, keeping the big block and adding an ITS transom assy.

Personally i see a rear shift in cg on this one regardless the weight shedding.


Minus what 80 pounds worth of c72, ok but that puts 800 pounds of engine back 11ish inches onto the transom.

weight dif between trs and bravo is 10-15 pounds (based off a quick oso looksee trs 155- 160 bravo 145)


Throw that 10 15 pounds out the window after you move it back 7 inches on the ITS


On to the crank CL/X/propshaft height dimension

trs is 23 3/4 (i don't want to look up the exact 1/16 of an inch)

bravo is 21 3/4 (same 1/16 deal)

trs were deep way back in the 70's baby, like 7, 7 1/2 inches specially on a small low freeboard boat,

the 2 inch difference could be ideal, 5 or more inches on the bravo and lest we forget the moving back that 11 inches whole exhaust riser / hatch clearance thing.
cuz my bet would be no theoretical x dimension and it's potential speed with 9 blade prop is worth a 200 mph fountain quanset hut style engine hatch on a conversion such as the example given. (i had to go there !)


right where mercury says to put it c/w transom angle, plus no more than 1 and a 1/2 inches above mother mercury's X because of the ITS, splitting it 3/4 up for ups sake and 3/4 up for setback.


What am i missing ? Throw it and anything else out there

I am still struggling to dial my 25’ Checkmate Convincor that I converted from single big block IO to twin OB.

According to my math I was within I think a 100lbs (heavier) but with center of that mass moved aft approx 4’.

TOTALLY different boat.

Maybe since your thread is IO biased, you will get more help than I did 😎

4 foots a lot on a 25. I know you build your own stuff powerheads and all and i know you know your sh-i-t !
I don't skim your threads and just look at the pics, i read what you post. Most of the build threads are worth paying attention to. not all of them are worth reading, take mine for example.

Not only 4 foot back but no longer down near the the bottom, do you notice or think that had any affect?

cg on a conversion like the example probably will be negligable in terms of driving.
kinda hoping someone would chime in on the engine room clearances, how that related to drive hght etc etc.
what they learned, how they'd do it different and why. That kinda thing.

I was talking to someone and it sparked up the fond memories of being down on my knees in the bottom of a gutted boat with a grinder all day in a tyvek suit mask and direct sunlight. putting the boat in the water is ok but putting it together is all the fun.

Did you bond your swim platform on ? I remember a pretty precise build for strength and layup in your garage but not if it's bolted or bonded. Oh, the transom, you can't try it without a sawzall, nevermind.

I asked because I don’t know.

I thought I did, but I don’t.

In my example, I thought the Checkmate would be the same/similar as my two 24’ Sonic twin OBs.

The Convincor is 1’ longer and 6” narrower.
So similar but not the same.

W/the Sonic template applied…, not even close.
which shocked me.

That’s why I started my thread on that subject.

I know there’s guys on here that know that answer (C of G) but I don’t.

Im fine w/changing course mid stream but at this point it’s all me.

A 4’ shift, aft, in motor weight is unworldly.
I get that.

But I also added 50% to fuel capacity which moves the front of fuel tank 3’ forward and also have a water bladder in anchor locker, at 115 lbs.

I dont know the math for this equation.

The smaller and lighter,narrower a boat is...the more it's affecting the cg.. My old 30 ft v didn't run much different with 2 four stroke outboards on a stainless marine bracket (24" setback with jackplates) vs 2 big inboards with Arnesons or the TRS outdrives

Gary your boat being a narrower single I/O original,you may want to try a ballast setup.

At some point I plan to build a cradle and find the balance point of the boat.

Ive had two or three guys w/basically the same set up call me for help in the past.
They assumed, as did I, w/my successes w/my 24 Sonics that I knew twin OB Vee set up.

Id tell them everything I did on them, they’d say they did the same but the boat still hopped/chine walked.

I wrote them off as not knowing how to drive/fly a fast OB powered Vee.

Now, I want their numbers so I can call and apologize!

Cary, read above, re ballast in the anchor locker.

I have 20” mid sections on their way so that will drop 900 lbs of motors 5” so should help the rolling/chine walk.

Going to extend my tabs 4” as well.
Thinking that extreme drag/angle of short tabs, full down, are digging a trough that the port/LH motor did not like???

Hoping to test prop rotation change before changing mids.

BTW, dropped a motor cowling on top of the rear quarter of my cherry MGB.
It now has a 4” dent!!!!
Arghhhhh!

https://cimg2.ibsrv.net/gimg/www.off...679944fe1c.jpg
Thats how much was cut off the ring to stop chinewalk. The 25 liked nuetral trim and needed the ring cut down , the 27 wanted lots of trim and barely had chinewalk, it would never come on plane after that much ring cut off. Calm water like pictured, 25 top was 73.4, calm water like pictured, 27 was over 74
https://cimg3.ibsrv.net/gimg/www.off...25ed2e0b86.jpg
You can see where the water leaves the hull and the k planes are clearly visible.
https://cimg5.ibsrv.net/gimg/www.off...769ecd09aa.jpg
https://cimg6.ibsrv.net/gimg/www.off...32dfd5b0fe.jpg
https://cimg8.ibsrv.net/gimg/www.off...cbd0ac8b4b.jpg
I would attribute the majority of difference to the hull bottom. Could defineately have it backwards and not discounting the 6 inch narrower beam but;

the sonic has a rounded keel

Where the checkmate sharp keel (with i think small 1/2 or less inch vertical edge that run from the transom forward 8-10 feet and blend into the hull, making it like a pad that's what 12 or so inches wide at the transom)

more of a teeter totter balance point than a wider soft edge/transition of the sonics round keel.



once upon a time the drain plug cam got knocked down and recorderd the water between the hull and leading edge of the drive.
Keep in mind this is a gopro knock off mounted to the drain plug and it clearly shown the difference of the water leaving the hull
at speed, green water was maybe 6 inches wide and outboard of that it was all aerated, the side video showed dry k planes and dry hull outboard the outer strakes, water was just touching the end of the inner strakes, the boat would chinewalk until the diffuser got cut down by almost a 1/2 inch. This is on a 26 x 8'2" boat.
got sideview pics of a dry hull i'll dig up.

https://cimg7.ibsrv.net/gimg/www.off...6429804924.jpg
my last post got screwed up after i edited in the pics, point was the round v sharp keel and the pics was about how high up and dry the hull is.
i don't think you have a ring issue it was just an example. Between the 25 and 27 both m plus props was significant and imo shows how touchy the variables are in trying to dial in a set up.
speed pic is just a neener neener !

Outonsafari, THANK YOU so much for your input and sorry for hijacking your thread!

Not in any order but, here’s some replies/comments.

Yes my Sonics had round keels so driving them at speed was also a challenge as you’re trying to keep it on a pad that, it don’t have!

I tried a set of Bravo Ones on it and it hopped.
I cut the lip off the hubs same as you and it cured it.
It still would not carry the bow w/them.

I have a GoPro and that is on my list for fact finding.

From before the GoPro era, here is my first 24 Sonic At WFO.
I blew this shot up then drew lines for keel, chine, transom and water line to approximate wetted surface.

Very little!
Look close and you can see the tabs.
https://cimg6.ibsrv.net/gimg/www.off...5e9b2d0da.jpeg


The Convincor has a, kinda pad.
15” wide but w/a vee.

In my world a “pad” is flat so….


https://cimg3.ibsrv.net/gimg/www.off...e379f8054.jpeg
https://cimg5.ibsrv.net/gimg/www.off...8d69bd400.jpeg
In theory, (a guy that loves flying fast OB vee’s) this should be easier to drive than the Sonic.
NOT!

I owned rigged, set up, a 100+ mph 17’ Hydrostream, Allison, STV, raced/built kneel down hydros etc, thought I knew how to do this.


https://cimg2.ibsrv.net/gimg/www.off...2402cf1f8.jpeg
https://cimg8.ibsrv.net/gimg/www.off...58716125b.jpeg
Cool thing is, after getting the Convincor to be a really good, weekend cuddy cabin, run 70+, I was out of energy and ready to move on.

Then…I took the winter off to work on my MG and now I’m wired for sound!

Totally rejuvenated.

I think w/stock 225’s I should see 80.

My real love is porting/tuning the motors.
I said at the start that I thought 90 was a legit target but we’ll see.

May not have that much energy left for the project.

At this point extra HPs do nothing.

I will tame the beast!

Gary, Too much to get into and type. First I hated spinning in with my surface drives,but had to because of attack angle,stern lift spinning out...getting on plane etc.., boat always wants to rollover in any kind of throttling in a sea...but they say you get more bow lift which we all need..,now I have my first round bottom old school v but back to an XR shorty single I/O. 8ft beam though. I loved my 7ft beam boats but much more tender with tab etc. Always had at least a 280 merc k plane. You just have to play with weight mid to forward and find a good balance.

My boat has a rounded pad like that. I feel like it could use a long centerline fin or strake. Tuffs have them.

I swear when I’m trying to drive straight it sometimes feels like I’m crabbing, but I can’t be sure.

Yours is twins isn’t it?

If so, shouldn’t be crabbing.

My Hydrostream, the yellow boat was crabbing bad at speed.


Had mechanical steering so easy test.

W/the boat on the trailer, I set the motor straight ahead then put a wire tie on the steering wheel at top dead center then made a pass.

As I cranked in steering correction to keep the mass…,moving straight, I watched the wire tie.

It ended up at bottom dead center!

It wasn’t scary until I put it on the trailer, turned the wire tie to BDC, then stood behind the boat and looked at the motor!
Should have been going in circles!

I modded the gear case and reduced the crabbing by 50% but still was not good.

You didn't/aren't hijacking the thread, the discussion is on topic
Man that sonic is way up out of the water. thats the bottom (keel) i thought the checkmate had.
You had those hydrostream pics on other threads and over a 100 was wha t i rememner.
turning the steering wheel that much is insane.
didn't think diffuser rings would induce chinewalk on twins.

About that crabbing i always thought the kplanes were doing it. Wind over/across the strbrd bow was not a factor, maybe beause of a right handed prop.
wind over the port bow caused a wheel turn and port lean, 1 kplane swear to god would push the transom to right in order for boat to run level. Cross a wake and the changing water pressure on the hull would give that 1 kplane double its force and wiggle the boat up bad. But only wind coming over port bow.

Normally, the farther back you go on a OB powered, heavy Vee, the faster it runs.

Here is my 21’ Checkmate Starliner (that I should have never sold!) that I added 18” of set back to.

And was a successful mod.
Although I never got a handle on the motor I built for it, went from feeling like a heavy ski boat, requiring full up trim to get the bow up, to needling only neutral trim to fly the hull like a 500lb Allison!

And back to the Convincor, I ran w/friends in Fla last winter and though they didn’t get pics of it, they said it was on the drain plug at speed.

The problem is getting it there and back!

My 20” mid sections will be here tomorrow and Yesterday I scored a second Lightning gear case.

I have a second one on the project motor from the Starliner and will convert it to LH rotation (as OMC never made one) and install them as well.

Did the same on my last Sonic and looked to be worth +1 - 3 on top.

Plus, that will complete the equation of making it look like a fast boat!

And to your observations above, sometimes, after yrs of doing this, many projects, thinking we know what we’re doing, it’s good to get our asses kicked to remind us there is still much to learn!

On my first 24 Sonic, it ran so good out of the box, I thought, wait till all the tricks I’ve learned!
It should be a screamer!

Rotation, toe in/out, nose cones, etc.

EVERY one I tried, slowed it down!

A couple of points on the OB Vs IO discussion.

If you can make your OBs work on a bracket w/decent set back, you get back the corners of our swim platforms that none of us want to lose.

And this one I’ve not seen mentioned, is jackplates.
For speed yes.
Same as raising your X dimension.

Ive helped a bazillion fast IO boats into dock, in the wind, due to this.

The OBs act identical in the wind.
W/my plates up, in the wind, you better have bumpers out and a good first mate!

W/the plates down, it docks like a fische Bote!

When I go to the 20” mids I will lose a fair amount of that.

https://cimg2.ibsrv.net/gimg/www.off...4e93b0c44.jpeg


https://cimg5.ibsrv.net/gimg/www.off...af8c621bb.jpeg
https://cimg6.ibsrv.net/gimg/www.off...1caa9b3ba.jpeg