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Author Topic:   Center of Gravity on a 22' Dauntless

MikMal posted 03-27-2014 04:23 PM ET (US)
I'm getting ready to put my 2000 Dauntless 22' outfitter with a 225 HP Optimax on a lift and was wondering if anyone knows where the center of gravity on this boat is. I'd like to have it centered on the lift's bunks and figure it to be somewhere around the helm seat but that's just a guess. Just trying to keep the weight even on the lift cables. Thanks in advance!
Whalrman posted 03-27-2014 06:34 PM ET (US)
I believe what you are looking for is the tipping center, not the center of gravity.
jimh posted 03-27-2014 07:36 PM ET (US)
The tipping center is defined at
http://www.seasources.net/Stability%20and%20Trim.htm#T

quote:
TIPPING CENTER OR LONGITUDINAL CENTER OF FLOTATION (LCF) These two terms are essentially the same and refer to the point around which a vessel "trims".
I don't think the tipping center a good point for a lift which removes the boat from the water. When the boat is lifted out of the water, all the buoyant forces of the hull are lost. The longitudinal center of flotation will no longer be particularly applicable to balancing the boat on the lift. Gravity will take over. The center of gravity will be the point around which the boat will pivot on a lift when in the air.
MikMal posted 03-27-2014 07:49 PM ET (US)
Yes, I stand corrected...it is the tipping center that I'm looking for. Basically, what I'm looking for is how far forward to place the boat on the lift bunks. I figure I want it more forward than centered (fore and aft) since there is a 500+ lb. engine hanging off of the back plus the fuel tank, which can conceivable weigh another 540+ lbs., is in the aft half as well. I thought the best bet would be to position the tipping center midway along the length of the bunks. I was hoping that the Owner's Manual had it marked, but no suck luck.
Chuck Tribolet posted 03-27-2014 08:02 PM ET (US)
No, you are looking for the center of gravity.
Chuck
whalerron posted 03-27-2014 08:06 PM ET (US)
jimh, I just received a Boston Whaler Center-of-Gravity drawing for my 2004 220 Dauntless. If you would like to add it to the reference section, I would gladly send a copy to you.
- ron
whalerron posted 03-27-2014 08:07 PM ET (US)
MikMal, call Boston Whaler and ask them for the Center-of-gravity drawing for your boat and they will send you an electronic copy. The drawing takes into account the motor which was standard for your boat.
Jerry Townsend posted 03-27-2014 10:33 PM ET (US)
MikMal - your guess of around the helm seat is in the ball park - a good guess.
You can calculate the location of the center-of-gravity (C/G) - knowing: 1) the weight of the hull, 2) the weight of the engine, 3) the load in a cable holding up the bow, 4) the load in a cable holding up the stern, and 5) a few dimensions.
If you can get these numbers, I will make the calculation for you or give you the one equation to make the calculation.
Oh - the so-called "tipping point", and Jimh's "longitudional center of flotation" which is the first time I have ever heard these terms - are directly under the well defined, standard terminolgy and accepted center of gravity (C/G). --- Jerry/Idaho
jimh posted 03-27-2014 10:58 PM ET (US)
Jerry--It's not my term. I never heard of tipping center, but I searched for it and found the definition was longitudinal center of flotation. When the boat is buoyed up by water, the longitudenal center of flotation may not coincide with the center of gravity due to the influence of buoyancy force.
Whalrman posted 03-28-2014 04:03 PM ET (US)
Splendid! Yes, quite right, they are one in the same ole boy! I see the frigid winter hasn't rusted up the minds! Yes, sorry Mik, I just couldn't resist this temptation to check on the chaps up north you see.
Ridge Runner posted 03-28-2014 04:23 PM ET (US)
MikMal - I have set many boats on lifts. Typically I call the manufacture and set boat and lift up based on their recommendations - the manufacture recommendations for a lift are basically the same as positioning a boat on a trailer.
I do not believe you can set-up the boat with equal 50/50 pressure on the lifts bow cables and stern cables. A great local resource for in your area that can help set you boat is Charlotte County Seawalls.
MikMal posted 03-28-2014 04:24 PM ET (US)
This from the folks at BW: On the 22' Dauntless, the boat is designed so the center of the reversible pilot seat (looking from the side) is at the center of gravity.
Most of our boats are designed this way as a balance point for the softest ride (pivot point).
Thanks, everyone, for your help.
Chuck Tribolet posted 03-28-2014 05:40 PM ET (US)
Jerry: I think the only things that need to be known are the
the load on a cable holding up the bow and the load on a
cable holding up the stern, and the distance between the
two cables. If Lb and Ls are the two loads, and D is the
distance:
CG = D *(Ls/((Lb+Ls))
Where CG is the distance from the bow cable to the CG.
That assumes the cables are vertical. If they are not, e.g.
on a sling, a little trig will need to be applied to calculate
the vertical load on each cable.
Chuck
Jerry Townsend posted 03-28-2014 11:07 PM ET (US)
Chuck - right on - to give the C/G of the boat/motor combination your equation is correct - which he is after. Factoring in the weight of the engine allows one to also calculate the C/G of just the boat.
Jimh - my apologies as I didn't mean that you had orignated the term - although that is the way I wrote that piece. Gotta take more time and re-read my words. Incidently, I read the referenced material and there are some interesting definitions of many terms that many of us take for granted. -- Jerry/Idaho
jimh posted 03-31-2014 10:26 AM ET (US)
There is a difference between the center of gravity (CG) of a boat hull and the center of longitudinal floatation (CLF).
The CG is relatively fixed in location, unless you shift weight around the hull. The center of longitudinal floatation is more dynamic. Its location shifts to reflect changes in the position of the hull in the water.
When a hull is at rest in calm water, the CG and CLF will be coincident. If you shift weight in the hull, the CG will move. The hull will pivot around the CLF to a new trim, changing the forces of buoyancy. The CLF will move to be coincident with the CG, unless the hull form cannot create enough floatation. In that case, the hull capsizes.
If the hull comes over the top of a wave, it loses buoyancy in the bow, the CLF shifts, and the hull pitches downward toward the bow.
None of this happens on a lift. There is no CLF to worry about because the hull is not in the water. There are no buoyant forces. There is only the CG.
Jerry Townsend posted 03-31-2014 02:19 PM ET (US)
Jim - What does the term "longitudinal floatation" mean or
refer to? Is there a translational floatation or rotational floatation?
The motion (translational and rotational) of a boat is about the C/G. That is, the dynamic attitude of the boat is determined by the location of the C/G relative to the buoyant forces acting on the boat.
You mention the case where a hull capsizes. A boat will capsize when all buoyant forces are on just one side of the C/G. As when a boat takes a big wave broadside, the buoyant forces are on just one side of the C/G with nothing but air to counter the downward force of the C/G - and the boat will capsize - by definition.
Now, as you point out, when a hull comes over the top of a wave, buoyant supporting forces will be lost and the bow will be pitched downward. But this case is different than with taking the wave broadside - in that as the bow is pitched downward, the distribution of the buoyant forces will move to provide buoyant support on both sides of the C/G. But given the right conditions, the bow could dive into the preceeding wave and the boat take on water over the bow.
As such, lonitudinal floatation seems to be an extra and totally unnecssary term or reference. ---- Jerry/Idaho
jimh posted 04-01-2014 08:44 AM ET (US)
Jerry--Yes, I agree. The tipping center or CLF term was not introduced to the discussion by me. I pointed out that when the boat was on a lift, there would be no buoyant forces acting on the hull, only gravity will be acting on the hull, and, therefore, no concern need be given where the tipping center was located; only the center of gravity would need to be considered.

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Author	Topic: Center of Gravity on a 22' Dauntless
MikMal	posted 03-27-2014 04:23 PM ET (US) I'm getting ready to put my 2000 Dauntless 22' outfitter with a 225 HP Optimax on a lift and was wondering if anyone knows where the center of gravity on this boat is. I'd like to have it centered on the lift's bunks and figure it to be somewhere around the helm seat but that's just a guess. Just trying to keep the weight even on the lift cables. Thanks in advance!
Whalrman	posted 03-27-2014 06:34 PM ET (US) I believe what you are looking for is the tipping center, not the center of gravity.
jimh	posted 03-27-2014 07:36 PM ET (US) The tipping center is defined at http://www.seasources.net/Stability%20and%20Trim.htm#T quote: TIPPING CENTER OR LONGITUDINAL CENTER OF FLOTATION (LCF) These two terms are essentially the same and refer to the point around which a vessel "trims". I don't think the tipping center a good point for a lift which removes the boat from the water. When the boat is lifted out of the water, all the buoyant forces of the hull are lost. The longitudinal center of flotation will no longer be particularly applicable to balancing the boat on the lift. Gravity will take over. The center of gravity will be the point around which the boat will pivot on a lift when in the air.
MikMal	posted 03-27-2014 07:49 PM ET (US) Yes, I stand corrected...it is the tipping center that I'm looking for. Basically, what I'm looking for is how far forward to place the boat on the lift bunks. I figure I want it more forward than centered (fore and aft) since there is a 500+ lb. engine hanging off of the back plus the fuel tank, which can conceivable weigh another 540+ lbs., is in the aft half as well. I thought the best bet would be to position the tipping center midway along the length of the bunks. I was hoping that the Owner's Manual had it marked, but no suck luck.
Chuck Tribolet	posted 03-27-2014 08:02 PM ET (US) No, you are looking for the center of gravity. Chuck
whalerron	posted 03-27-2014 08:06 PM ET (US) jimh, I just received a Boston Whaler Center-of-Gravity drawing for my 2004 220 Dauntless. If you would like to add it to the reference section, I would gladly send a copy to you. - ron
whalerron	posted 03-27-2014 08:07 PM ET (US) MikMal, call Boston Whaler and ask them for the Center-of-gravity drawing for your boat and they will send you an electronic copy. The drawing takes into account the motor which was standard for your boat.
Jerry Townsend	posted 03-27-2014 10:33 PM ET (US) MikMal - your guess of around the helm seat is in the ball park - a good guess. You can calculate the location of the center-of-gravity (C/G) - knowing: 1) the weight of the hull, 2) the weight of the engine, 3) the load in a cable holding up the bow, 4) the load in a cable holding up the stern, and 5) a few dimensions. If you can get these numbers, I will make the calculation for you or give you the one equation to make the calculation. Oh - the so-called "tipping point", and Jimh's "longitudional center of flotation" which is the first time I have ever heard these terms - are directly under the well defined, standard terminolgy and accepted center of gravity (C/G). --- Jerry/Idaho
jimh	posted 03-27-2014 10:58 PM ET (US) Jerry--It's not my term. I never heard of tipping center, but I searched for it and found the definition was longitudinal center of flotation. When the boat is buoyed up by water, the longitudenal center of flotation may not coincide with the center of gravity due to the influence of buoyancy force.
Whalrman	posted 03-28-2014 04:03 PM ET (US) Splendid! Yes, quite right, they are one in the same ole boy! I see the frigid winter hasn't rusted up the minds! Yes, sorry Mik, I just couldn't resist this temptation to check on the chaps up north you see.
Ridge Runner	posted 03-28-2014 04:23 PM ET (US) MikMal - I have set many boats on lifts. Typically I call the manufacture and set boat and lift up based on their recommendations - the manufacture recommendations for a lift are basically the same as positioning a boat on a trailer. I do not believe you can set-up the boat with equal 50/50 pressure on the lifts bow cables and stern cables. A great local resource for in your area that can help set you boat is Charlotte County Seawalls.
MikMal	posted 03-28-2014 04:24 PM ET (US) This from the folks at BW: On the 22' Dauntless, the boat is designed so the center of the reversible pilot seat (looking from the side) is at the center of gravity. Most of our boats are designed this way as a balance point for the softest ride (pivot point). Thanks, everyone, for your help.
Chuck Tribolet	posted 03-28-2014 05:40 PM ET (US) Jerry: I think the only things that need to be known are the the load on a cable holding up the bow and the load on a cable holding up the stern, and the distance between the two cables. If Lb and Ls are the two loads, and D is the distance: CG = D *(Ls/((Lb+Ls)) Where CG is the distance from the bow cable to the CG. That assumes the cables are vertical. If they are not, e.g. on a sling, a little trig will need to be applied to calculate the vertical load on each cable. Chuck
Jerry Townsend	posted 03-28-2014 11:07 PM ET (US) Chuck - right on - to give the C/G of the boat/motor combination your equation is correct - which he is after. Factoring in the weight of the engine allows one to also calculate the C/G of just the boat. Jimh - my apologies as I didn't mean that you had orignated the term - although that is the way I wrote that piece. Gotta take more time and re-read my words. Incidently, I read the referenced material and there are some interesting definitions of many terms that many of us take for granted. -- Jerry/Idaho
jimh	posted 03-31-2014 10:26 AM ET (US) There is a difference between the center of gravity (CG) of a boat hull and the center of longitudinal floatation (CLF). The CG is relatively fixed in location, unless you shift weight around the hull. The center of longitudinal floatation is more dynamic. Its location shifts to reflect changes in the position of the hull in the water. When a hull is at rest in calm water, the CG and CLF will be coincident. If you shift weight in the hull, the CG will move. The hull will pivot around the CLF to a new trim, changing the forces of buoyancy. The CLF will move to be coincident with the CG, unless the hull form cannot create enough floatation. In that case, the hull capsizes. If the hull comes over the top of a wave, it loses buoyancy in the bow, the CLF shifts, and the hull pitches downward toward the bow. None of this happens on a lift. There is no CLF to worry about because the hull is not in the water. There are no buoyant forces. There is only the CG.
Jerry Townsend	posted 03-31-2014 02:19 PM ET (US) Jim - What does the term "longitudinal floatation" mean or refer to? Is there a translational floatation or rotational floatation? The motion (translational and rotational) of a boat is about the C/G. That is, the dynamic attitude of the boat is determined by the location of the C/G relative to the buoyant forces acting on the boat. You mention the case where a hull capsizes. A boat will capsize when all buoyant forces are on just one side of the C/G. As when a boat takes a big wave broadside, the buoyant forces are on just one side of the C/G with nothing but air to counter the downward force of the C/G - and the boat will capsize - by definition. Now, as you point out, when a hull comes over the top of a wave, buoyant supporting forces will be lost and the bow will be pitched downward. But this case is different than with taking the wave broadside - in that as the bow is pitched downward, the distribution of the buoyant forces will move to provide buoyant support on both sides of the C/G. But given the right conditions, the bow could dive into the preceeding wave and the boat take on water over the bow. As such, lonitudinal floatation seems to be an extra and totally unnecssary term or reference. ---- Jerry/Idaho
jimh	posted 04-01-2014 08:44 AM ET (US) Jerry--Yes, I agree. The tipping center or CLF term was not introduced to the discussion by me. I pointed out that when the boat was on a lift, there would be no buoyant forces acting on the hull, only gravity will be acting on the hull, and, therefore, no concern need be given where the tipping center was located; only the center of gravity would need to be considered.