center of gravity and thrust vector?

[whiteworks]

Interesting stuff for sure very curious to see some math or backyard engineering techniques for finding such locations in a boat.

 

[rrrr]

Interesting stuff for sure very curious to see some math or backyard engineering techniques for finding such locations in a boat.

Why do you want to give everyone a headache?

 

[HolyMoly]

Depends on if Lue is behind the wheel or Kim Hanson.

 

[BajaMike]

Interesting stuff for sure very curious to see some math or backyard engineering techniques for finding such locations in a boat.

Put it on a conveyor belt......and see if it will fly. :champagne:

 

[rivermobster]

sounds like a porno

staring River Dave North!

 

[2FORCEFULL]

Interesting stuff for sure very curious to see some math or backyard engineering techniques for finding such locations in a boat.

how much hp to go 80???

 

[2FORCEFULL]

Interesting stuff for sure very curious to see some math or backyard engineering techniques for finding such locations in a boat.

to find center of attention..put a for sale sign...

 

[OCMerrill]

to find center of attention..put a for sale sign...

LMAO! Nice.

 

[whiteworks]

I find it hard to believe that not one person let alone several on this board do not at least have some jive ass theorys on this subject. I had a really interesting conversation with my neighbor about this topic, told me they used to suspend the boats from 2 points on the stringers to find the CG and then would run the thrust through that point creating a perfectly balanced/aligned boat via thrust vector. I know the V-drive crowd has this shit down to a science, but I am really curious about finding the optimum thrust vector on an outboard. Obviously it can change by trimming it positive or negative but there has to be an equilibrium point where things are just in harmony. point in fact, with my Stoker the slightest adjustment of trim or jack would free up the boat not talking getting it loose but more so to a point where there is no tension on the wheel everything would smooth out run like a sewing machine. Now lets assume there is no jack plate, how does one find the correct propshaft height for an OB?

 

[Crazyhippy]

You are assuming that the only lift the hull recieves is from the prop. If that is the case, you want it run thru the cg of the boat.

It's not though, and it will vary with speed (especially in a tunnel like the stoker).

 

[whiteworks]

You are assuming that the only lift the hull recieves is from the prop. If that is the case, you want it run thru the cg of the boat.

It's not though, and it will vary with speed (especially in a tunnel like the stoker).

Lets take air entrapment out of the equation, talking v hulls. BTW without a prop aint shit happening

 

[Crazyhippy]

Even in a v-hull, you have lift from the hull (assuming a planing hull,not displacement) also have to account for different props (cleaver vs chopper) and their lift styles.

There is no way to simplify it down to the vector needs to go thru the cg.

Let's anyways. The ONLY force acting on the boat is from the prop, and it is directed straight thru the boats cg. All is well and good. Then you hit a wake which pushes the bow up. The thrust is still all going thru the cg, making for a really neat wheely down the lake, or does the boat become a plane?

 

[whiteworks]

Even in a v-hull, you have lift from the hull (assuming a planing hull,not displacement) also have to account for different props (cleaver vs chopper) and their lift styles.

There is no way to simplify it down to the vector needs to go thru the cg.

Let's anyways. The ONLY force acting on the boat is from the prop, and it is directed straight thru the boats cg. All is well and good. Then you hit a wake which pushes the bow up. The thrust is still all going thru the cg, making for a really neat wheely down the lake, or does the boat become a plane?

I understand what your getting at but that does not mean that during the intial rigging you just say fuck it there are to many variables. There has to be some science behind what is going on, at least as a starting point.

 

[Crazyhippy]

V-drives move the motor to adjust the cg after the fact, so sure there is some thought about vector vs cg. but it's not set in stone.

Wackers and IO's can change their thrust vector on the fly, and while it might end up running thru the cg, 95% of the time, it won't.

 

[Crazyhippy]

I would assume most 1st time boat set ups are nothing more than educated guesses, and every other boat built learns from the previous itteritions.

 

[rivermobster]

Where is go-fly? He prolly knows!

Ever seen a jack plate on v-hull?? I dont think i ever have.

I would THINK it would have something to do with how fast you wanna run. On the ricky race outdrives, they have to have low water picks up, so at high speed, part of the prop is actually out of the water! Now would you want the prop partially out of the water, when trying to get on plane? Prolly not so much.

I would think the prop shaft center/height, is a dynamic thing, that NEEDS to change as hull speed changes. There is no one ideal spot for all speed/needs.

 

[Crazyhippy]

Ever seen a jack plate on v-hull?? I dont think i ever have.

Fast bass boats run them frequently. High speed v-hulls w/ wackers usually have a jackplate, sometimes just for the setback after setup is perfected.

 

[rivermobster]

Fast bass boats run them frequently. High speed v-hulls w/ wackers usually have a jackplate, sometimes just for the setback after setup is perfected.

Good call.

 

[OCMerrill]

I find it hard to believe that not one person let alone several on this board do not at least have some jive ass theorys on this subject. I had a really interesting conversation with my neighbor about this topic, told me they used to suspend the boats from 2 points on the stringers to find the CG and then would run the thrust through that point creating a perfectly balanced/aligned boat via thrust vector. I know the V-drive crowd has this shit down to a science, but I am really curious about finding the optimum thrust vector on an outboard. Obviously it can change by trimming it positive or negative but there has to be an equilibrium point where things are just in harmony. point in fact, with my Stoker the slightest adjustment of trim or jack would free up the boat not talking getting it loose but more so to a point where there is no tension on the wheel everything would smooth out run like a sewing machine. Now lets assume there is no jack plate, how does one find the correct propshaft height for an OB?

Are you running low on work?

 

[whiteworks]

All the fast outboards I have seen do not have a jackplate, the prop shaft height is fixed. STV's, Allison, DCB 22 w twins, cyclones, bernico, force.

 

[rivermobster]

All the fast outboards I have seen do not have a jackplate, the prop shaft height is fixed. STV's, Allison, DCB 22 w twins, cyclones, bernico, force.

There was one of those DCB's with twins tryin to tie up at Road Runner on Sunday. Damm that thing was bad azz.

I dunno, but I think Stokers are pretty fast too.

 

[Racey]

There are just way too many variables to take into account in the boat itself, we have gotten where we are by experimentation, trial and error, you could figure out all this badass stuff in a computer, but there is still a limitation to how accurately you can take a design from a computer, and built it exactly to spec in glass, or carbon, or the composite of your choice. Even computer designed and CNC cut plugs end up sagging and changing shape in the mold, which is why they all end up having to be flipped and blue printed for any type of extreme performance roles.

 

[Crazyhippy]

My stv ran 117... It had a manual plate.

Lil boats w/ twin wackers need to have all that weight close to the transom to the transom or they end up doing weelies down the lake.

NWSRA does not allow on the fly height adjustments, so ski race boats will run manual if any.

Allison's do run them. They can adjust heights to the 1/16th of an inch, versus the 2" going by the mounting holes.

 

[whiteworks]

There was one of those DCB's with twins tryin to tie up at Road Runner on Sunday. Damm that thing was bad azz.

I dunno, but I think Stokers are pretty fast too.

The 20' Stoker is an awesome boat, however when you have it all wound up and a DCB with twins comes by you like your standing still it kinda makes you think twice about what fast is.

 

[rivermobster]

The 20' Stoker is an awesome boat, however when you have it all wound up and a DCB with twins comes by you like your standing still it kinda makes you think twice about what fast is.

See if they can turn like you do.

And comparing twin 2.5's to a single engine?? Al can build you a motor that fast, and do it with one engine too.

 

[whiteworks]

There are just way too many variables to take into account in the boat itself, we have gotten where we are by experimentation, trial and error, you could figure out all this badass stuff in a computer, but there is still a limitation to how accurately you can take a design from a computer, and built it exactly to spec in glass, or carbon, or the composite of your choice. Even computer designed and CNC cut plugs end up sagging and changing shape in the mold, which is why they all end up having to be flipped and blue printed for any type of extreme performance roles.

fine tuning is to be expected but there has to be some theory behind a starting point, right?

 

[whiteworks]

See if they can turn like you do.

And comparing twin 2.5's to a single engine?? Al can build you a motor that fast, and do it with one engine too.

You get a 20' Stoker over the 105 mark and your playing with fire.

 

[rivermobster]

You get a 20' Stoker over the 105 mark and your playing with fire.

Fine. Get a 22! Then you can run right with the DCB boys.

 

[whiteworks]

Fine. Get a 22! Then you can run right with the DCB boys.

They are cool boats but you wont see me in one (driving or riding)anytime soon.

 

[Outdrive1]

fine tuning is to be expected but there has to be some theory behind a starting point, right?

I'd say that the set up on most boats is trial and error. Al Stoker didn't use a computer to tell him where to put the first motor. He probably went off something similar someone else had done and worked from there. At some point in time Dillon, almost every configuration has been attempted and then there's some that never work.

Look at Kap trying to run a surface drive in that Howard deck. It never worked and he did tons of research and had hours or trial and error into it.

The little Eliminator with big motor and Arneson never worked. Yet had that boat been set up with a standard i/o, or jet, Bob would have been able to tell you exactly where the x dimension needs to be.

Dual jet on the Trident.


Trial and error. The boating industry doesn't have the r and d money car builders have. So if someone wants to figure out something new, they need to be lucky or have deep pockets. If not go with what you know.

You talking about CG in a vee drive and comparing it to an outboard boat with an adjustable x dimension and trim. Plus a hundred different types of props. The variables are infinite at that point.

 

[rivermobster]

Interesting...



Some hulls and props can't perform to their fullest potential locked at one compromised transom height. Obtaining pulling power from your outboard motor meant giving up the thrill of all out performance. On the other hand, all out top end speed meant giving up pulling power.

What does the performance benefits of a jack plate mean? Quite simply, the setting of the engine back from the running surface moves the boat's center of gravity back. Essentially, this means the hull is rebalanced to favor the bow so the engine can devote more effort to driving the boat forward rather than lifting it up.

Secondly, setting the engine back allows the prop to run in the calm swell behind the boat. So you can run your outboard height for less gear case drag without getting prop cavitation. Moving the prop back gets it out of the turbulent, hull-disturbed water. You get a better "Prop Bite" because the prop is running in smoother, more solid water.

Thirdly, setting the engine back gives it more acceleration power. You get on plane quicker (less hull drag) and accelerate faster. Handling is improved. The more effective prop bite makes steering more responsive. And, the engine set back adds stability in rough water because, in effect, you're running a longer, more stable hull.

http://www.hydrodynamics-usa.com/rapid-jack-plate.html

 

[Crazyhippy]

Not infinite, just a lottoesque number.

 

[whiteworks]

I'd say that the set up on most boats is trial and error. Al Stoker didn't use a computer to tell him where to put the first motor. He probably went off something similar someone else had done and worked from there. At some point in time Dillon, almost every configuration has been attempted and then there's some that never work.

Look at Kap trying to run a surface drive in that Howard deck. It never worked and he did tons of research and had hours or trial and error into it.

The little Eliminator with big motor and Arneson never worked. Yet had that boat been set up with a standard i/o, or jet, Bob would have been able to tell you exactly where the x dimension needs to be.

Dual jet on the Trident.


Trial and error. The boating industry doesn't have the r and d money car builders have. So if someone wants to figure out something new, they need to be lucky or have deep pockets. If not go with what you know.

You talking about CG in a vee drive and comparing it to an outboard boat with an adjustable x dimension and trim. Plus a hundred different types of props. The variables are infinite at that point.

Paul you make several vaild points, the one that sticks out as the most solid is go with what you know. So here is what I know, exactly jack shit however on this board and throughout the industry there is an infinant amount of wisdom. I cant help but think that there is some good mathematical theory in the works.

 

[TomD]

Lots of varibles. Its much easier to study other boats to come close then tweak the setup. I would give my .02 but my back still hurts. Some times little bits make a big difference in handling and speed.

 

[rivermobster]

Bottom line is, to get any hull up on plane, and more importantly, to get it to turn properly, the prop and drive have to be deep in the water.

This is just the opisite of what you want when you goin for max speed! Less stuff in the water (prop and drive), the less drag your gonna have, and hence, the faster speeds.

There was some discussion about this awhile back about outdrive Shiadas being faster then v-drive Shiadad, due to less parts under the boat creating drag.

The 2.5 DCB's accomplish this (gettin the drive up and outa the water) by the huge amount of lift they get when they are up to speed. And with dual 2.5's on there, they have some serious grunt to push that hull as fast and as high as it needs to be.

But all that lift is a bad thing when your tryin to turn at speed. The Stoker will flip you outa the boat. The DCB will prolly just flip.

Like everyone else has said, there are too many variables for a one size fits all starting point.

 

[whiteworks]

The Stoker is a different breed, it flys well but also sticks in the turns. There are acutually 4 hooked strakes up inside the tunnels, when you drop the boat back in the water they suck it down and keep it locked until you pack the air back in and it lifts past them. pretty amazing shit cranking turns in one.

 

[robert1050]

I've had my 1981, 21' Daytona for 10 years now, it has an O/B, 6" set-back and Hydraulic steering and jack-plate. It was special ordered as a balsa/kevlar laminate way back then and it is a very light-weight hull. Even after 10 years of steady driving, I still don't have any magic 'return to' settings for the jack or trim. It changes based on fuel load, passengers, weight distribution and phase of the moon.....

The transom broke (long story) about 7 years ago and once Fiberglass Reformations repaired it for me, it was time to hang the O/B back on the virgin transom. I called Bob Leach and asked him where the X dimension should be (after relaying how the boat was set-up) and he just laughed. "Jack-plate huh? Just make sure the bullet is somewhere near the X in it's range of travel". Some answer.

We made an educated guess and after all these years, couldn't be happier. But........I had Tank-It replace the fuel tanks around 5 years ago, going from cylindrical tanks in the gunnals (sp?)to aluminum tanks mounted flush to the tops of the tunnels. That change affected the CG of the boat.

How could I tell? The boat would begin to porpoise at a different speed.

Like I said, I've lived with it for a long time and I don't have any magic answers for you. All I know is that I like it and it works.

 

[Flying_Lavey]

Interesting...



Some hulls and props can't perform to their fullest potential locked at one compromised transom height. Obtaining pulling power from your outboard motor meant giving up the thrill of all out performance. On the other hand, all out top end speed meant giving up pulling power.

What does the performance benefits of a jack plate mean? Quite simply, the setting of the engine back from the running surface moves the boat's center of gravity back. Essentially, this means the hull is rebalanced to favor the bow so the engine can devote more effort to driving the boat forward rather than lifting it up.

Secondly, setting the engine back allows the prop to run in the calm swell behind the boat. So you can run your outboard height for less gear case drag without getting prop cavitation. Moving the prop back gets it out of the turbulent, hull-disturbed water. You get a better "Prop Bite" because the prop is running in smoother, more solid water.

Thirdly, setting the engine back gives it more acceleration power. You get on plane quicker (less hull drag) and accelerate faster. Handling is improved. The more effective prop bite makes steering more responsive. And, the engine set back adds stability in rough water because, in effect, you're running a longer, more stable hull.

http://www.hydrodynamics-usa.com/rapid-jack-plate.html

This is only maybe 50% true for a V-hull. Wana know how I know? When I bought my boat it had a 15" set-back on it. No jack plate. Well, the thing would porpoise horrendously with ANYBODY or gas in it. As soon as I put a bunch of shit under the bow.... it rode a LOT better. So, I cut 5" off the set-back bracket and what do you know.... the boat ran A LOT better, didnt porpoise, and would actually kinda cut through the water instead of bouncing over it.

Also, moving the motor back doesnt help much at all to get on plane cause if there is too much weight back, the bow wont want to drop once the boat is up. Its actually easier to get on plane with the motor closer to the transom. I.E. the CG further forward.

I was once told a general rule of thumb to use to consider the angle of the water coming off the transom at around 60 mph. For the life of me, I cant think of it right now. I was always told, for maximum engine performance (max speed performance) you want the prop shaft just about even with the bottom of the boat. That should put the water line just above the torpedo and leave almost one complete blade out of the water at neutral trim. This is the way my boat is set-up now with the 10" set-back and a low water pick-up with a nose cone and its running the best its ever ran! Now, just to find the perfect prop for it.

 

[Racey]

This is only maybe 50% true for a V-hull. Wana know how I know? When I bought my boat it had a 15" set-back on it. No jack plate. Well, the thing would porpoise horrendously with ANYBODY or gas in it. As soon as I put a bunch of shit under the bow.... it rode a LOT better. So, I cut 5" off the set-back bracket and what do you know.... the boat ran A LOT better, didnt porpoise, and would actually kinda cut through the water instead of bouncing over it.

Also, moving the motor back doesnt help much at all to get on plane cause if there is too much weight back, the bow wont want to drop once the boat is up. Its actually easier to get on plane with the motor closer to the transom. I.E. the CG further forward.

I was once told a general rule of thumb to use to consider the angle of the water coming off the transom at around 60 mph. For the life of me, I cant think of it right now. I was always told, for maximum engine performance (max speed performance) you want the prop shaft just about even with the bottom of the boat. That should put the water line just above the torpedo and leave almost one complete blade out of the water at neutral trim. This is the way my boat is set-up now with the 10" set-back and a low water pick-up with a nose cone and its running the best its ever ran! Now, just to find the perfect prop for it.

Obviously there are extreme's to both sides, too far back is not good, and too far forward is not good.

 

[rivermobster]

This is only maybe 50% true for a V-hull. Wana know how I know? When I bought my boat it had a 15" set-back on it. No jack plate. Well, the thing would porpoise horrendously with ANYBODY or gas in it. As soon as I put a bunch of shit under the bow.... it rode a LOT better. So, I cut 5" off the set-back bracket and what do you know.... the boat ran A LOT better, didnt porpoise, and would actually kinda cut through the water instead of bouncing over it.

Also, moving the motor back doesnt help much at all to get on plane cause if there is too much weight back, the bow wont want to drop once the boat is up. Its actually easier to get on plane with the motor closer to the transom. I.E. the CG further forward.

I was once told a general rule of thumb to use to consider the angle of the water coming off the transom at around 60 mph. For the life of me, I cant think of it right now. I was always told, for maximum engine performance (max speed performance) you want the prop shaft just about even with the bottom of the boat. That should put the water line just above the torpedo and leave almost one complete blade out of the water at neutral trim. This is the way my boat is set-up now with the 10" set-back and a low water pick-up with a nose cone and its running the best its ever ran! Now, just to find the perfect prop for it.

:thumbsup

 

[RiverDave]

I find it hard to believe that not one person let alone several on this board do not at least have some jive ass theorys on this subject. I had a really interesting conversation with my neighbor about this topic, told me they used to suspend the boats from 2 points on the stringers to find the CG and then would run the thrust through that point creating a perfectly balanced/aligned boat via thrust vector. I know the V-drive crowd has this shit down to a science, but I am really curious about finding the optimum thrust vector on an outboard. Obviously it can change by trimming it positive or negative but there has to be an equilibrium point where things are just in harmony. point in fact, with my Stoker the slightest adjustment of trim or jack would free up the boat not talking getting it loose but more so to a point where there is no tension on the wheel everything would smooth out run like a sewing machine. Now lets assume there is no jack plate, how does one find the correct propshaft height for an OB?

A varaible height / setback bracket to test and tune is how I would do it.. And I'd put mounting holes every one inch, up down / forward / back.. and I'd try every single one of them to see which one worked the best.. When I figured which one I liked, I'd do another one in the range that I liked but with slides, and try it again for finer adjustment. After that I'd set it up wherever it ended up. (If I was going to produce boats)

If I was just building 1, I'd probably just make it look right, and dial in the rest with props, rigging, plates, you name it.

I understand what your getting at but that does not mean that during the intial rigging you just say fuck it there are to many variables. There has to be some science behind what is going on, at least as a starting point.

Performance boats / Hotboats & science do not go hand in hand.. There is literally zero engineering into these boats.

Where is go-fly? He prolly knows!

Ever seen a jack plate on v-hull?? I dont think i ever have.

See it all the time.. On most race boats though jack plates aren't legal for a variety of reasons.. In that case they have manual jack plates. some people lower them manually for when the boat is in play mode, and raise them on race day etc.. Some people just use them to find the adjustment point and leave it there. Others find where it needs to be, then replace with a solid mount bracket / DCB style etc..

All the fast outboards I have seen do not have a jackplate, the prop shaft height is fixed. STV's, Allison, DCB 22 w twins, cyclones, bernico, force.

It can go either way.. Some with / some without. I would think in a double throw down outboard setup, the weight of the hydraulic jack plate might offset it's performance gains. I have seen twins with jack plates though & singles.. and I've seen the same boats without.

See if they can turn like you do.

And comparing twin 2.5's to a single engine?? Al can build you a motor that fast, and do it with one engine too.

A DCB Mach 22, or KMG Phantom 22 with twin Drags is gonna run in the 130's somewhere..

RD

 

[Crazyhippy]

Now, just to find the perfect prop for it.

Different props will want more or less setback for ideal handling...

A Cleaver provides stern lift, and will want more setback.
A Chopper lifts the bow, and will need less.

 

[rivermobster]

buddy of mine went for a test drive in the 22 DCB when he was lookin to upgrade his 20ft stoker. They got it up to 116. Pretty impressive, but it didnt turn for shit (so he said). He ended up with the 22 Stoker.

 

[Flying_Lavey]

Different props will want more or less setback for ideal handling...

A Cleaver provides stern lift, and will want more setback.
A Chopper lifts the bow, and will need less.

Yeah. I know that. I have mine set-up right now where it COULD use a cleaver for absolute optimal top end but it would only be good with just me in it and MAYBE one person with light fuel load. Anything more and I think I would need to much positive trim with a cleaver to run at optimal efficiency for speed.