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Old 10-18-2013, 10:57 AM   #1
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Planing vs Displacement Hull

Stumbled onto this thread not too long ago, actually started from the beginning and was hooked. We are contemplating going from a diesel sport fisherman to trawler style and the 48 Defever is on the short list so following along really gave me a sense of what to expect travel wise (love the short vid clips). Your stat’s from your Florida back to home port were very interest with just over 300 gallons used whereas ours would have been closer to 1500!

One aspect that still holds some reserve for me is the pitching moment of a displacement hull like yours, even at 60klb it sure has a good roll moment of which I’m not sure I would want to live with myself, maybe it’s just something to get use to.

Thanks again for sharing your journey’s, they help people like myself in different ways then you would think. Safe Journey’s to you and your family!
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Old 10-18-2013, 11:43 AM   #2
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One aspect that still holds some reserve for me is the pitching moment of a displacement hull like yours, even at 60klb it sure has a good roll moment of which Iím not sure I would want to live with myself, maybe itís just something to get use to.
This is an interesting consideration of displacement vs planing by someone who has had the resources and used them to try a number of boats.

REFLECTIONS ON DISPLACMENT VS PLANING MOTOR BOATS. | SturiŽr Yachts
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Old 10-18-2013, 03:56 PM   #3
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This is an interesting consideration of displacement vs planing by someone who has had the resources and used them to try a number of boats.

REFLECTIONS ON DISPLACMENT VS PLANING MOTOR BOATS. | SturiŽr Yachts
Totally off the thread, but this is one of the better articles I have read that gives a true real world use comparison.

And a nice take on European boating.

Did anyone notice the 300,000 Euros to buy a berth
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Old 10-18-2013, 04:01 PM   #4
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Originally Posted by Marlinmike View Post
Stumbled onto this thread not too long ago, actually started from the beginning and was hooked. We are contemplating going from a diesel sport fisherman to trawler style and the 48 Defever is on the short list so following along really gave me a sense of what to expect travel wise (love the short vid clips). Your stat’s from your Florida back to home port were very interest with just over 300 gallons used whereas ours would have been closer to 1500!

One aspect that still holds some reserve for me is the pitching moment of a displacement hull like yours, even at 60klb it sure has a good roll moment of which I’m not sure I would want to live with myself, maybe it’s just something to get use to.

Thanks again for sharing your journey’s, they help people like myself in different ways then you would think. Safe Journey’s to you and your family!
The roll moment on the DF48 is actually much better than most as she has a very low center of gravity. All of these boats are going to roll much more then your planing boat.
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Old 10-20-2013, 04:34 PM   #5
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This is an interesting consideration of displacement vs planing by someone who has had the resources and used them to try a number of boats.

REFLECTIONS ON DISPLACMENT VS PLANING MOTOR BOATS. | SturiŽr Yachts
Great link, thanks for sharing

Found this discussion in Daddyo's Underway Today thread and thought it deserved its own thread
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Old 10-20-2013, 07:48 PM   #6
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Them's incredibly nice boats. Sigh...
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Old 10-21-2013, 06:00 AM   #7
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Old 10-21-2013, 06:42 AM   #8
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I wanna part with that guy!!!!
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Old 10-21-2013, 09:21 AM   #9
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I wanna part with that guy!!!!
You wouldn't want all your lovely ladies to find out about this guy or they will really wander why they been hanging with you .
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Old 10-21-2013, 12:24 PM   #10
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Daddyo,

If the your DeFever has a "very low" center of gravity ther'e must be lots more boat below the WL because there's lots of hull and house above the WL.

It's been said before and I think by others other than you so I suspect it's the case but it's hard to imagine w that high freeboard and tall cabin. What's down ther'e in the hull anyway?
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Old 10-21-2013, 02:02 PM   #11
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weight distribution affects center of gravity more than the center of structure....

from - Metacentric height - Wikipedia, the free encyclopedia

The metacentric height (GM) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning. Metacentric height also has implication on the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently high but not excessively high metacentric height is considered ideal for passenger ships...
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Old 10-21-2013, 03:22 PM   #12
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weight distribution affects center of gravity more than the center of structure....
.
There is more there on Wikipedia than I have the energy to learn, but thanks for that.
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Old 10-21-2013, 05:34 PM   #13
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There is more there on Wikipedia than I have the energy to learn, but thanks for that.
You are welcome...

The downside on these forums is there are a few guys that take a look at a picture and profess to know how it performs, rides, whether displacement or not...tthen hrow in a few fancy words that make them an "internet expert" in many reader's eyes....wow...just incredible what some people think they are capable of....

Sure you can guess...but without a few facts along with the pics that show up....it's only a guess...and all too often wrong.
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Old 10-23-2013, 12:50 AM   #14
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weight distribution affects center of gravity more than the center of structure....

from - Metacentric height - Wikipedia, the free encyclopedia

The metacentric height (GM) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric heightimplies greater initial stability against overturning. Metacentric height also has implication on the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently high but not excessively high metacentric height is considered ideal for passenger ships...
"...larger metacentric height >IMPLIES< greater initial stability against overturning."

"...sufficiently high but not excessively high metacentric height is >CONSIDERED< ideal for passenger ships..."

Which brings me to mention this: As a [spring powered] Metronome's swing velocity slows via elevation of its sliding metacenter weight the energy needed for its return to center is increased proportionally (that’s why its swing duration slows – and – if its metacenter weight gets too much increased and/or too elevated compared to its CoG – it tips over!). Therefore, regarding a boat in potential of back to back waves, if the boat's metacenter weight is too high and it reaches a certain tipped angle away from boat’s CoG... well... possible blub, blub, blub! IMHO

Sooo... weights placed in boats to establish its metacentric height should be adjusted to be relevant for seas encountered. In other words – Tis Better to Experience Faster Returns per Roll... Than to Experience Slow Roll Returns with Potential for Capsize/Flounder!
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Old 10-23-2013, 06:18 AM   #15
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Art...it all has to be looked at as a whole...

If my Albin 40 is out in seas big enough to roll it over (and I don't mean bar crossing)...chances are all the windows will be blown out and there will be several other factors causing me to sink or abandon ship rather than absolute stability.

I'd rather have a boat that's comfortable 99 percent of the time than make it so that it would be "more stable" for that one time she probably wouldn't survive for other reasons.
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Old 10-23-2013, 06:44 AM   #16
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if its metacenter weight gets too much increased and/or too elevated compared to its CoG Ė it tips over!). Therefore, regarding a boat in potential of back to back waves, if the boat's metacenter weight is too high and it reaches a certain tipped angle away from boatís CoG... well... possible blub, blub, blub! ...
WTF? Better look up metacenter and sit down with some good diagrams. What is metacenter weight anyway?

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Sooo... weights placed in boats to establish its metacentric height should be adjusted to be relevant for seas encountered.


That is normally called variable ballast and it's used to alter the distance between G and M. GM is what matters, not some arbitrary location of M. The location of M changes when rolling beyond around 10 degrees or so. Where is M when the boat is not rolling? Where is it when the boat is rolled to the rail?

Unless weight can be pumped over the side or flooded onboard, it is kind of hard to adjust GM every time sea conditions change. (spare us the the old sailing ship tricks) And, if you don't know what your are doing and have the results of an inclining experiment to hand, it can be very dangerous.
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In other words Ė Tis Better to Experience Faster Returns per Roll... Than to Experience Slow Roll Returns with Potential for Capsize/Flounder!
That reads like you have never been in seas high enough to change your mind about that concept. The damage to people and structure caused by rapid rolling can be extreme. Antennae, masts, boats, people, furniture, stores, fittings, all are exposed to destructive accelerations when the rate of roll is high. There is a reason ships are built with ballast tanks and cruise ships keep what might seem like a very small GM.
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Old 10-23-2013, 09:42 AM   #17
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Another thing to consider is they keep referring to "initial" stability. In aviation we call that "static" stability. Static stability is the vessel's resistance to be upset. It does not address what happens after the upsetting has occurred. Again, in aviation, dynamic stability is what happens over time. If the upsetting occurs and then the vessel trends back towards a stable state, then it is said to have positive dynamic stability.

It is possible to have positive static stability but negative dynamic stability. IOW, the vessel resists being upset but when it is upset, the motion and condition continue to get worse.

I am just talking outta my ass here. My only point here is they keep referring to "initial" stability which may not mean "overall" stability.
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Old 10-23-2013, 10:27 AM   #18
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cause the one issue I'm talking about is free surface effect...if my decks are full of water and my windows blown out and filling my bilge fast ...all "static" stability is out the window (both long before I would "just capsize" from waves)...

So I'll take a nice ride in uncomfortable seas (which is even more than I usually venture into) before I want a snap roll thinking I may never flip in a CAT 5 hurricane...
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Old 10-23-2013, 10:50 AM   #19
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Another thing to consider is they keep referring to "initial" stability. In aviation we call that "static" stability. Static stability is the vessel's resistance to be upset. It does not address what happens after the upsetting has occurred. Again, in aviation, dynamic stability is what happens over time. If the upsetting occurs and then the vessel trends back towards a stable state, then it is said to have positive dynamic stability.

It is possible to have positive static stability but negative dynamic stability. IOW, the vessel resists being upset but when it is upset, the motion and condition continue to get worse.

I am just talking outta my ass here. My only point here is they keep referring to "initial" stability which may not mean "overall" stability.
and one more just to complicate things....Inherent stability " the tendency of an aircraft to return to straight and level flight, when the controls are released by the pilot" and is the kind of stability I want in my boat
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Old 10-23-2013, 11:12 AM   #20
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dimer2,
Your'e talking about a self righting hull.

And in aircraft you're talking about a two axis AC w pitch positive aerodynamics. That is in a vertical dive it will "pull out" ... not just continue the dive. I flew a hang glider years ago that had that performance feature. I doubt if you'll need a pitch positive boat. Pitch "stable" would be nice though unless it was heavy in the ends and then it would frequently be trying to be a submarine.
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