The Boating Forum - Mathematical formula

I want to make my 23 Sea Craft Sceptre sit heaver in the stern; I have the single Yamaha 200 on a jack plate with a 6" set back. I'm wondering if a jack plate with the 10" set back will give the results that I'm wanting. Which brings me to my question of is there a mathematical formula to figure how much weight I'll be transferring to the stern per say 1" of set back?
Thanks Steve

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82 Sea Craft Sceptre, 97 200 Yamaha

As a minimum would need to know where the centre of gravity is for your boat is and the weight of your boat, and that calculation would be an estimate as there are other factors to consider

Not sure what advantage you would gain by having more weight in back.

Would you be better off with larger surface trim tabs?

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yes, I'm "Miss-Be-Haven"
just a little fish in the THT pond

I'm also wondering Why your looking for a "heaver stern":.... Please tell us !!!!....Good Luck ...John

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Triton 2200CC, Honda BF200,

I’m not sure if this is exactly what you’re looking for but it might help. It’s some stuff I learned in Physics class a couple of years ago. Caveat: I’m not a Naval Architect.

Hanging an engine on the back of your boat acts like a lever, with the fulcrum at the center-of-gravity of the boat. The formula for torque (down force at the stern) is Torque = Lever Arm multiplied by Weight, where Lever Arm is the length of the lever (distance from CoG to the motor). So, a 500 pound motor, hung 10 feet aft of the CoG produces 5000 pounds/feet of torque (bear in mind the weight of the boat forward of the CoG is acting opposite the motor).

So here’s the potentially useful part. If you know (or can find out) the CoG, you can measure the distance from CoG to the motor, calculate torque of the current set up, then calculate torque of the motor 4� further back. Subtract the current torque from the torque of the motor 4� back to find the extra torque. You can simulate this calculated extra torque by placing weighed buckets of water at the transom, but you’d have to use the lever arm times length formula since the buckets would be a slightly shorter lever arm.

Simple, huh? Anyway this ought to give you a good idea of what the change will do. Good luck. chip.

hi guys

Chip is right but there is one other stickler . . . Chips formula works for a vessel that has an equal displacement along its entire length . . . like a telephone pole of equal diameter along its total length

What happens is boats have most of their displacement on the back two thirds of the boat so the effect of the torque must must also take into effect the changing displacement (buoyancy) along the length of the boat

I guess I just made things more complicated

HOWEVER, taking Chip's idea, you could add about 75 to 100 pounds onto the transom (while in the water) and you would get an idea of what moving the engine back might do . . . but that is just a guestimate on my part

Again, why do you want to transfer more weight towards the back . . . is the nose of your boat plowing too much into the water?

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yes, I'm "Miss-Be-Haven"
just a little fish in the THT pond

Auguste, I figured I was leaving something out, thanks for clearing it up.

I figured since the only thing changing was the motor position, the unknowns would take care of themselves if he simulated the extra torque with weight, and he could get a pretty close idea of the torque by using T=LxW. You've obviously been down this road before! chip.

Thats not a math formula

here's one

what's 111,111,111 times 111,111,111


Well that would be 12,345,678,987,654,321 http://thehulltruth.com/infopop/emoticons/icon_biggrin.gif

Grunt ....you played B Ball right !!!!!!!!!!!!!!????.john

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Triton 2200CC, Honda BF200,

Thanks guys. The reason why I want to do this is because it is designed for twin engines and with the single it makes it sit bow heavy. This in turn makes for a very good ride, but I don't like not being able to utilize my self-bailing deck. I figured that if I got it to set right in the water and it in turn made the ride rougher then I should be able to use my trim tabs to bring the bow back down to get the smooth ride back. http://thehulltruth.com/infopop/emoticons/icon_smile.gif

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82 Sea Craft Sceptre, 97 200 Yamaha

chip

I hope that my comment was not interpreted negatively http://thehulltruth.com/infopop/emoticons/icon_smile.gif

Yes, I kinda had gone down that road when I was working with my builder and the naval architect during the building of Miss-be-Haven.

The concern was that the bow was very narrow and pointed thus reducing the boyancy up front . . . the effect could be water pouring over the bow in rough weather . . . the naval architect did the computer calculations and suggested lengthening the bow 18 inches . . . tests plowing into 6 ft waves last summer proved the architect was right on, no water over the bow problems . . . yup I have been there http://thehulltruth.com/infopop/emoticons/icon_smile.gif

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yes, I'm "Miss-Be-Haven"
just a little fish in the THT pond

Lost2a6

OK I understand your logic . . . could you lighten the weight at the bow . . . could you move soething else farther back . . . same effect(kind of) without changing the braket

DO YOU KNOW if the transom can handle the twisting weight of the motor on a longer bracket? . . . just thinking out loud

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yes, I'm "Miss-Be-Haven"
just a little fish in the THT pond

You say you want to make your bow lighter. I understand the reason for this, while underway. I would just for the heck of it try jacking your motor down a couple of inches. You did say you have a jack plate. This will enable you to lift the bow more. I had to much lift and raised my motor 1 hole, and it has made my boat handle completely different. One draw back is that I cannot raise the bow as high as before, which would be more helpfull in rough water. If you have dual livewells back aft fill them up and try it. If not enough put some sand bags back aft until you find the weight you need. Contact your boatbuilder when you find the weight, put the question to them. They will know when you tell them haw far aft of the transom you can mount the motor x the distance aft to get your desired lift.

It sounds like you need to put the equivalent of around 700 to 800 lbs on the transom.

Let us know how it goes.

Thanks

Don

Yes jtburf

If that's what you would call it

But this is really not my lifetime scoring totals...

take away about 15 digits....

Your theory of moving the motor back to lift the bow only "holds water" when the boat is at rest. When under way you would achieve the opposite of the desired effect. Your prop generates lift, and moved back would create a larger torque moment of lift, stuffing your bow further.
As others have said, move weight (fuel, batteries) not the motor position. I'm not a naval architect, just my .02.

damn good coastboater

thanks . . . we missed the obvious . . . that's why I ain't no naval architect http://thehulltruth.com/infopop/emoticons/icon_smile.gif

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yes, I'm "Miss-Be-Haven"
just a little fish in the THT pond

quote:Originally posted by coastboater:
Your theory of moving the motor back to lift the bow only "holds water" when the boat is at rest. When under way you would achieve the opposite of the desired effect. Your prop generates lift, and moved back would create a larger torque moment of lift, stuffing your bow further.
As others have said, move weight (fuel, batteries) not the motor position. I'm not a naval architect, just my .02.
I want the boat to sit lower in the stern at rest, when on plane it does just fine. I don't really have any thing in the bow of the boat that can go to the stern. I have allready moved the tank back as much as possible. The batteries are in the very stern of the boat. It just gets aggravating when the boat is in the water and it rains and the water runs to the cabin rather than out of the scuppers. It's not as bad when full of fuel (144 gal).

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82 Sea Craft Sceptre, 97 200 Yamaha

*

Add a kicker motor for trolling and for back up.

Coastboater....you sure confused me! My Parker with the bracketted outboard does not stuff the bow while under way, at all, even with motor trimmed all the way down. In fact, on another post, I made the comment about how badly this boat needs tabs, which I believe is because of the engine being further back. I would think moving the motor back would do exactly what lost2a6 is trying to accomplish.

Lost2a6, maybe instead of a jackplate, how about a bracket/platform to get it even further back. Would be a nice addition to your Seacraft and would imagine can't be too much more than a jackplate. When you ask a bracket company to build a bracket for you, they can build it with different size "flotation chambers" if you tell them what you are trying to accomplish. I would think even if you can't get a custom size "flotation chamber" on a new bracket, or it's an expensive "custom" build, maybe you could fill the chamber with water.

quote:Originally posted by Parker at the bar!:
Coastboater....you sure confused me! My Parker with the bracketted outboard does not stuff the bow while under way, at all, even with motor trimmed all the way down. In fact, on another post, I made the comment about how badly this boat needs tabs, which I believe is because of the engine being further back. I would think moving the motor back would do exactly what lost2a6 is trying to accomplish.

Lost2a6, maybe instead of a jackplate, how about a bracket/platform to get it even further back. Would be a nice addition to your Seacraft and would imagine can't be too much more than a jackplate. When you ask a bracket company to build a bracket for you, they can build it with different size "flotation chambers" if you tell them what you are trying to accomplish. I would think even if you can't get a custom size "flotation chamber" on a new bracket, or it's an expensive "custom" build, maybe you could fill the chamber with water.
I would really like to have a bracket; I was looking for a cheep way out. I can get a manual jack plate with the 10" set back for less than $300.00. The bracket is going to run around $1200.00 plus I'll want to close up the transom. I could kick myself in the a$$ for not enclosing the transom when I replaced it.

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82 Sea Craft Sceptre, 97 200 Yamaha

lost2a6, I don't think you will get any useful results from any math, you will have to figure it out experimentally. Chip's explaination as embellished by others is OK theoretically but determining the center of gravity is problematic. As an alternative, you might try building some totally temporary plywood platforms that clamp or tie to the transom. Make them long enough to sit back as far as the motor set back you are thinking of plus some. Then start putting weight, maybe sandbags from Menards or Home Depot, on them at various distances from the transom. When the weight and back-set gets to be enough to bring your transom where you want it at rest, you will have a very rough idea of the effect you are trying to achieve. Now it won't tell you exactly what to do because your motor is still on the transom and its weight is effecting the situation. But if it takes 200 lbs of sand setting 12 inches back, and your motor weighs 400 lbs, then moving the motor back 6 inches would have a similar effect. Might really be closer to correct to move the motor back by the ratio of total weight to motor weight. In this example the motor is 400 pounds, the extra weight is 200 pounds at 12 inches set-back. So moving the motor back by 600/400 pounds times 12 inches or 1.5 times 12 = 18 inches would be close. Yeah, I know that is not right mathematically as everything needs to be referenced to the COG, but it might get you close. You could take the motor off and do the same thing and get a better answer.

Don

how about trimming the motor up some?