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Old 09-26-2011
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Hull Rocker

I'm curious to know what effect hull rocker or lack of it has on the motion or ride comfort of a boat. Does it have an effect on slamming in waves? I understand it will affect the boats CP and thus speed potential at different wind speeds, I'm just curious about its effect "on deck" so to speak.

Anybody ? Bob Perry?
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Old 09-26-2011
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JonB:
Yes, I think slamming and pounding have everything to do with rocker and very little to do with sectional shapes in the bow.

The lighter the boat the less rocker it will have and the more it will pound. ULDB's have very little rocker if any and they pound. It's also a fucntion of boat speed. Light bpoats are fast and they pound.

I think the only time the actual sectional shape of the bow may have an effect on pounding is if you are motoring dead into waves. Once you heel the boat over the V sections are not doing you any good at all.

I don't think you can connect rocker with Cp. I know what you are thinking but you have to think in three dimensions. You can have a low Cp boat with very little rocker. My own 65' ICON would be a good example of that.
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Old 09-26-2011
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Quote:
Originally Posted by bobperry View Post
JonB:
Yes, I think slamming and pounding have everything to do with rocker and very little to do with sectional shapes in the bow.

The lighter the boat the less rocker it will have and the more it will pound. ULDB's have very little rocker if any and they pound. It's also a fucntion of boat speed. Light bpoats are fast and they pound.

I think the only time the actual sectional shape of the bow may have an effect on pounding is if you are motoring dead into waves. Once you heel the boat over the V sections are not doing you any good at all.
Bob, could you perhaps expand on this a bit? I can understand the "light boats pounding" part but not the "sectional shapes having little to do with it". Perhaps you were meaning for light-displacement boats only??

eg. here's the sections for a typical heavy displacement boat and at, say, 25degs angle of heel, the v sections are still not "flat"?..

Hull Rocker-sections.jpg

Thanks.
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Last edited by Classic30; 09-26-2011 at 11:20 PM.
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Old 09-26-2011
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Hart:
Heavy boats almost always have lots of rocker. Your example will have lots of rocker and a deep forefoot with very little to distinguish the forefoot from the leading edge of the keel. Your example will not be a fast boat to weather. Your example will not be a pounder under most conditions. Of course the wrong wave at the right time can make just about any boat pound. But your example by design I would think should not be a pounder.

This is far better shown when combined with a profile view.
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Old 09-26-2011
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Quote:
Originally Posted by bobperry View Post
JonB:
Yes, I think slamming and pounding have everything to do with rocker and very little to do with sectional shapes in the bow.

The lighter the boat the less rocker it will have and the more it will pound. ULDB's have very little rocker if any and they pound. It's also a fucntion of boat speed. Light bpoats are fast and they pound.

I think the only time the actual sectional shape of the bow may have an effect on pounding is if you are motoring dead into waves. Once you heel the boat over the V sections are not doing you any good at all.

I don't think you can connect rocker with Cp. I know what you are thinking but you have to think in three dimensions. You can have a low Cp boat with very little rocker. My own 65' ICON would be a good example of that.
Thanks Bob I had a feeling they would ride better but I don't fully understand the last paragraph - I was thinking in 3D (I thought) - thinking back to the design shift in the 70's, where the "Peterson" bow came in - the displacement was pushed into the ends and the mid-body was flattened out - less rocker - did that not INCREASE the CP compared to the earlier, less flat bottomed boats with the Disp. more centralized and more rocker?
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Old 09-27-2011
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No. There have always been high and low Cp boats. The "Peterson bow" was no different from the Carter bow, the Holland bow or the S&S bow of the day. It had nothing to do with Cp and everything to do with girth differences. But due to the IOR's weighting of the forward depth measurements rocker was diminished. At the Carter office we called it the "clipper" profile referring to the rockerless profile of the old clipper ships.

Now, to your example, the old wineglass section hull. The calculation of Cp in that hull depended upon where the the designer said the hull ended and the keel began. It's not so obvious in this example compared to a modern fin keel boat.The question was: Do you include the keel in the Cp calc or do you not include it? But as long as the designer was consistant in determining how he would calculate Cp there was no problem.

Go sit by an entrance to a harbor where there are some steep waves building up. Watch the boats come and go. Note that boats with less rocker when they "buck" will pull a far greater amount of the hull out of the water than boats, say like a Westsail 32, will. Then the rockerless boat will slam the entire forward end of the boat back in all at once, pounding. The boat with lts of rocker will ease the hull back into the water and not pound.

That's about the best I can do without sitting down and drawing illustrations as explanations.

I hate to generalize but here I go: Heavy boats have more rocker and they resist pounding. Light boats have less rocker and they pound. Generally.

I hope that helps.
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Quote:
Originally Posted by bobperry View Post
No. There have always been high and low Cp boats. The "Peterson bow" was no different from the Carter bow, the Holland bow or the S&S bow of the day. It had nothing to do with Cp and everything to do with girth differences. But due to the IOR's weighting of the forward depth measurements rocker was diminished. At the Carter office we called it the "clipper" profile referring to the rockerless profile of the old clipper ships.

Now, to your example, the old wineglass section hull. The calculation of Cp in that hull depended upon where the the designer said the hull ended and the keel began. It's not so obvious in this example compared to a modern fin keel boat.The question was: Do you include the keel in the Cp calc or do you not include it? But as long as the designer was consistant in determining how he would calculate Cp there was no problem.

Go sit by an entrance to a harbor where there are some steep waves building up. Watch the boats come and go. Note that boats with less rocker when they "buck" will pull a far greater amount of the hull out of the water than boats, say like a Westsail 32, will. Then the rockerless boat will slam the entire forward end of the boat back in all at once, pounding. The boat with lts of rocker will ease the hull back into the water and not pound.

That's about the best I can do without sitting down and drawing illustrations as explanations.

I hate to generalize but here I go: Heavy boats have more rocker and they resist pounding. Light boats have less rocker and they pound. Generally.

I hope that helps.
Got it. That's exactly how I expected it would be. I'm going to have to pull out my design books and study CP again - I seem to have missed something in that area.

Thanks.
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Jon:
Cp, i.e. prismatic coeficient is just a measurement of volume in the ends compared to the volume in the middle. That's the simple way of looking at it.
It has the problem of lumping both ends into one so you can have a fine bow and a full stern and have a "normal" Cp. Or vice versa. But none of these numbers tell you much unless you have an extensive, tried and proven data bank of examples. I think trying to combine Cp with a boat's tendancy to pound is a stretch. Cp is not dependant on rocker. Rocker is two dimensional.
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Old 09-27-2011
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Quote:
Originally Posted by bobperry View Post
JonB:
Yes, I think slamming and pounding have everything to do with rocker and very little to do with sectional shapes in the bow.

...
Hi!,

You mean that a mono hull from the last Americas cup that was raced in monohulls (that had little rocker) will slam as much as a Open 60 boat, that also has little rocker?

There is no doubt that a boat with more rocker will slam less but will also not pitch a lot more?

Regards

Paulo
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Old 09-27-2011
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Paulo:
Yes, I think you are correct. Light boats wih minimal rocker are not usually inclined to pitch while heavy boats with lots of rocker and the ends loaded with gear can pitch a lot.

I think the open 60 would slam more than the old AC boat. I don't think those AC monos were all that light compared to open 60's and the AC boats had more rocker.
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