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Old 02-12-2019, 05:37 PM   #241
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Old 02-12-2019, 05:37 PM   #242
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By the way Baker, dues this Hobie really look like a FD hull?

https://www.google.com/imgres?imgurl...&iact=c&ictx=1
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Old 02-12-2019, 05:40 PM   #243
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Old 02-12-2019, 05:52 PM   #244
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Astounding. 12 pages, 238 posts about...and no, it isn't dead yet.


What do you expect, we haven't determined who won the pissing contest yet?
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Old 02-12-2019, 06:32 PM   #245
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A boat hull is resting/settled in and moving/pushing through water at hull speed... be it D SD or P


Or, a boat bottom is sliding/gliding on-top-of/over water in planning speeds... be it SD or P


To speak about multi thousand HP pushing or pulling a D hull fast enough for it to at least try to get on top of water is simply silly.


No real boater will ask a real boat to perform other than the hull design qualifies it for.

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Old 02-12-2019, 07:29 PM   #246
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Well, what parameters of a Hobiie 16 are more like SD or planing rather than displacement?


Anyone read up on prismaticcoefficient lately?
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Old 02-12-2019, 08:12 PM   #247
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by the way baker, dues this hobie really look like a fd hull?

https://www.google.com/imgres?imgurl...&iact=c&ictx=1
fd
100% fd
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Old 02-12-2019, 08:21 PM   #248
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fd
!00% fd
Eric, if a Hobie is a full displacement hull, the folks at Nordhvn are going to have to redo their website.

https://www.nordhavn.com/35/overview_hull.php4

What is the difference between a full displacement and semi-displacement hull design? Essentially, the full displacement hull remains completely in the water at all speeds. It virtually "pushes" the water away from its bow. The water then travels down the length of the entire hull, finally leaving a wake behind the stern. This "bow wave" creates a resistance to the hull, from which it can never escape [/I]
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Old 02-12-2019, 08:31 PM   #249
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A Nordhavn marketing blurb hardly makes a convincing technical case, however I'm only here for the comedy anyway.
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Old 02-12-2019, 08:37 PM   #250
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Eric, if a Hobie is a full displacement hull, the folks at Nordhvn are going to have to redo their website.

https://www.nordhavn.com/35/overview_hull.php4

What is the difference between a full displacement and semi-displacement hull design? Essentially, the full displacement hull remains completely in the water at all speeds. It virtually "pushes" the water away from its bow. The water then travels down the length of the entire hull, finally leaving a wake behind the stern. This "bow wave" creates a resistance to the hull, from which it can never escape [/I]
Pushing water at 6.3 knots:
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Old 02-12-2019, 08:43 PM   #251
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[QUOTE=Delfin;740358]Eric, if a Hobie is a full displacement hull, the folks at Nordhvn are going to have to redo their website.

https://www.nordhavn.com/35/overview_hull.php4

What is the difference between a full displacement and semi-displacement hull design? Essentially, the full displacement hull remains completely in the water at all speeds. It virtually "pushes" the water away from its bow. The water then travels down the length of the entire hull, finally leaving a wake behind the stern. This "bow wave" creates a resistance to the hull, from which it can never escape



They could use tweaking as there is so much more to hull design you guys havent begun to address.


Bakers reference to Hobie hulls is but a piece of the pie...but certainly a big one and just the picture IS a thousand words.
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Old 02-12-2019, 08:46 PM   #252
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A Nordhavn marketing blurb hardly makes a convincing technical case, however I'm only here for the comedy anyway.
Actually, Nordhavn's full displacement marketing blurb is closer to the truth than any other boat builders blurb out there. They have backed it up with many hundreds of builds in the past 30 years. Their starting point was to do an update of Beebe's written work and run with it. Trivializing their contribution to safe blue water recreational cruising is off the mark, by far.

But, the beauty of the internet is those that want, can attempt to drag down. Not successful in any sense for this case.
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Old 02-12-2019, 08:48 PM   #253
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A Nordhavn marketing blurb hardly makes a convincing technical case, however I'm only here for the comedy anyway.
So, does that mean that a boat builder who states physical reality with respect to hull design can't be trusted because you can buy their boat? Interesting perspective, Fish.
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Old 02-12-2019, 09:00 PM   #254
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Actually, Nordhavn's full displacement marketing blurb is closer to the truth than any other boat builders blurb out there. They have backed it up with many hundreds of builds in the past 30 years. Their starting point was to do an update of Beebe's written work and run with it. Trivializing their contribution to safe blue water recreational cruising is off the mark, by far.

But, the beauty of the internet is those that want, can attempt to drag down. Not successful in any sense for this case.
Yeah OK I'll genuflect to Nordhavn and beg forgiveness for assuming they had any interest in promoting their product. Does that work for you and Nordhavn? I believe it's quite a stretch to imagine I had any intent to "drag down" Nordhavn with my comment, advertising is not normally where a person goes for detailed technical information. I go to books on the subject in question that address acknowledged facts and how they're arrived at. Sure advertising may actually have valid information however the bias toward selling a product puts the majority of it up for scrutiny.
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Old 02-12-2019, 09:08 PM   #255
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Eric, if a Hobie is a full displacement hull, the folks at Nordhvn are going to have to redo their website.

https://www.nordhavn.com/35/overview_hull.php4

What is the difference between a full displacement and semi-displacement hull design? Essentially, the full displacement hull remains completely in the water at all speeds. It virtually "pushes" the water away from its bow. The water then travels down the length of the entire hull, finally leaving a wake behind the stern. This "bow wave" creates a resistance to the hull, from which it can never escape

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They could use tweaking as there is so much more to hull design you guys havent begun to address.


Bakers reference to Hobie hulls is but a piece of the pie...but certainly a big one and just the picture IS a thousand words.
You are quite right, it is a complicated subject. However, one doesn't need to get too far into the technical weeds to make the observation that there are, in fact, full displacement hulls that have a maximum speed, regardless of how much power you throw at the problem.

Nordhavn's statement pretty much says it all:

"Essentially, the full displacement hull remains completely in the water at all speeds." Simple.

So, if you have a vessel that planes, or lifts up out of the water enough that the water displaced weighs less than the weight of the boat because it can get ahead of its bow wave, be it a catamaran, oil tanker or inner tube you can call it a full displacement hull if you like, but that is effectively the same as saying that all squares are circles. They just are squares with a constant radius.
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Old 02-12-2019, 09:11 PM   #256
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So, does that mean that a boat builder who states physical reality with respect to hull design can't be trusted because you can buy their boat? Interesting perspective, Fish.
You can just share my previous response to this issue.
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Old 02-12-2019, 09:18 PM   #257
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Fish

A read of the voluminous Nordhavn website can prove illuminating, plus the hundreds of available pages of first hand blogs from world cruisers. Don't forget to browse their in house written and then published "update" to Beebe's Voyaging Under Power. Then direct your curiosities and questions to them in well thought out written correspondence that should elicit a cogent reply.
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Old 02-12-2019, 09:23 PM   #258
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What do you expect, we haven't determined who won the pissing contest yet?
As distinct from the "pissing people off" contest. A clear winner there!
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Old 02-12-2019, 09:28 PM   #259
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Eric, if a Hobie is a full displacement hull, the folks at Nordhvn are going to have to redo their website.

https://www.nordhavn.com/35/overview_hull.php4

What is the difference between a full displacement and semi-displacement hull design? Essentially, the full displacement hull remains completely in the water at all speeds. It virtually "pushes" the water away from its bow. The water then travels down the length of the entire hull, finally leaving a wake behind the stern. This "bow wave" creates a resistance to the hull, from which it can never escape [/I]
The laws, rules of thumb and other functions of hull design don’t address extreme aspect ratio hulls.
The Nordhavn description of FD hulls is a little weak but they are pretty busy building boats so I’ve heard. For example FD boats actually start to climb their wave at about a knot below hull speed. Prove it to yourself by slowly increasing your speed until the bow rises a bit and observe your speed. It will be below hull speed and quite a few will start rising long before hull speed is reached. So many variables. Trim, balance and ballast can lead observations astray too. It isn’t black and white. Many boats are FD specifically but very close to SD. They will be way different in hull design than a deep FD design but both will be FD.
And boats that are very close to FD that are SD may look way different than SD boats that are very close to planing boats. Many SD boats have soft chines but many have hard chines.
Some people have great dificulty with things that can’t be positively identified and classified. This is one of those things.
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Old 02-12-2019, 09:35 PM   #260
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While I can't attest that this fellow on boatdesign.net isn't trying to sell me something, so may fail the Fish test, it does seem to be a fairly straightforward description of what characteristics the three type of hull design discussed in this thread, and so useful.

"Displacement hulls are designed to travel in the water at a typical hull speed of 1.34 *(square-root LWL) in knots. This formula applies to the distance between bow and stern wave crests. At faster speeds, wave making resistance increases exponentially because the vessel is trying to climb on top of the bow wave - meanwhile the stern is being sucked down by the dynamic forces from the "hole" created in the water as the vessel moves forward. Displacement hulls tend to have pointed bows and sterns because this form poses the least wave making resistance at "displacement" speeds. It takes a relatively small amount of power to push a displacement hull at its "hull speed."

Semi-displacement hulls tend to have wide, flat aft sections - like a New England lobster boat. These hulls are designed to partially climb on top of the bow wave and separate the transom from the stern wave. Semi-displacement speeds are usually in the area of 1.5 to 2.5 *(square-root LWL) in knots. It takes a *lot* of power to drive a hull in the semi-displacement speed range. The flat wide stern sections help to provide additional lift in the stern to partially overcome this problem.

Planing hulls are designed with straight sections aft. A typical deep-V bottom hull has the same angle to the 'V' (the same "deadrise" angle) from midship to transom. They are designed to climb completely out of the water at high speed and "hydroplane" on top of the water. At planing speeds, water is breaking cleanly from the transom and the hull is riding on its straight aft sections. The greatest resistance at planing speeds is frictional resistance. It takes more power to climb out of the water over the bow wave than it does to maintain planing speed once this is achieved. At very high planing speeds (>25kts) any change in deadrise angle in the aft sections of the hull can adversely affect performance. Hulls with a "variable deadrise" angle in the aft sections (where the angle of the 'V' decreases and flattens toward the transom) are a further modification of the semi-displacement hull form. They are more easily driven at speeds < 25kts, but at higher speeds tend to push the bow down due to the higher dynamic lift in the aft sections. Variable deadrise hulls can actually become unsafe at very high speeds >30kts because of this tendency - it is possible for the bow to dig in and cause the boat to broach at high speed."
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