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Old 02-12-2016, 05:47 AM   #1
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Information on Hull shapes?

We have narrowed down our hunt and I'm trying to sort out a short list of makes and models, some are very helpful and state the design of hull in the adverts, most don't...!!! Googling produces some results but most of the time nothing definitive and sometimes conflicting information.....

Is there any list of what boat has what hull shape, we fancy a displacement hull, it's sometimes obvious from the quoted cruising speed that the hull is a planing hull, some have pictures of the boat on the hard so you can see what it's got but some brokers list them with little or no information....

One more question, I understand the differences between the Displacement, Semi-displacement and Planing hull, where does the Modified "V" hull fit in and what are it's pro's and con's, again googling provides conflicting information??

Thanks
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Old 02-12-2016, 07:23 AM   #2
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The V modified or deep is to smash into waves at High speed and be comfortable in a $7000 Stidd seat and not break your spine.

At usually 1/2 a mile per gallon its not what folks think of when going cruising.

However many higher speed boats like old Sport Fish at low speeds make fine cruisers.

The fuel burn may be a bit higher but the all glass hull removes the horrors of endless deck leaks and rework.
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Old 02-12-2016, 07:34 AM   #3
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We plan to spend as much time as possible on the hook and away from marinas so I wanted the stability of a full displacement, did look at some sport fishing boats to convert but they all have planing hulls and I've read are not so stable, or is that more smoke and mirrors.... Wish I could buy some experience somewhere...
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Old 02-12-2016, 08:05 AM   #4
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I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.
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Old 02-12-2016, 08:05 AM   #5
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One more question, I understand the differences between the Displacement, Semi-displacement and Planing hull, where does the Modified "V" hull fit in and what are it's pro's and con's, again googling provides conflicting information??

You can consider a modified V as a planing hull.

(There may be exceptions, but I'm not aware of any.)


So there are various kinds of planing hulls: V, modified V, deep V, stepped, etc. Pro and cons for each.

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Old 02-12-2016, 08:28 AM   #6
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I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.

These comercial trawler cat designs dont need stab's at anchor , and can cruise at 15 kts on smaller engines.

Maybe worth considering this type of design as you don't need much load carrying capacity on a cruising trawler.





Maybe something like this with catarmaran Hull form....?
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Old 02-12-2016, 10:03 AM   #7
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If you want better stability on the hook, look for a boat with a comparatively flat bottom and hard chines. A pure displacement hull has neither a flat(ish) bottom or hard chines. Here is an example of what I am talking about. The Kady Krogan is a pure displacement hull and the Fleming is a semi-displacement. The displacement hull will role quite a bit more than the semi-displacement hull.

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Old 02-12-2016, 10:36 AM   #8
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I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.
Oh, right I see....

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You can consider a modified V as a planing hull.

(There may be exceptions, but I'm not aware of any.)


So there are various kinds of planing hulls: V, modified V, deep V, stepped, etc. Pro and cons for each.

-Chris
That makes sence, thanks

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These comercial trawler cat designs dont need stab's at anchor , and can cruise at 15 kts on smaller engines.

Maybe worth considering this type of design as you don't need much load carrying capacity on a cruising trawler.
We did look at some but they don't seem to have as much space , and most were outside our budget...

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If you want better stability on the hook, look for a boat with a comparatively flat bottom and hard chines. A pure displacement hull has neither a flat(ish) bottom or hard chines. Here is an example of what I am talking about. The Kady Krogan is a pure displacement hull and the Fleming is a semi-displacement. The displacement hull will role quite a bit more than the semi-displacement hull.

Oh, right, that makes sense, so does that apply to all makes? are semi displacement always more stable??

And a question to those who have experience of them all, how much of a difference is there between the hull shapes, is it a massive amount or is it something that doesn't really make that much difference..??

Thanks all for taking the time to reply to all these basic questions....
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Old 02-12-2016, 12:19 PM   #9
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I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.
Agree. We have owned both and the FD rolled badly at anchor while our SD boats with chines did and do much better.
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Old 02-12-2016, 02:09 PM   #10
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FD hulls are less stable innitially but many FD hulls like my Willard have considerable ballast (2 tons in our case) so in extreme conditions the FD boats are more stable. Look at some small fishing boats. They operate on a regular basis in seas that would scare the stuff out of most of us here.

But to many w trawlers innital stability is more important. But it usually also makes for a stiff boat.
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Old 02-12-2016, 02:32 PM   #11
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Many trawler types spend most of their time when away from docks at anchor and that is when the SD with chines can be significantly more comfortable than many FD rollers. The FD when properly designed with proper distribution of weight can and is often a better ultimate sea boat. The question I raise with modern weather forecasting and modern electronics how often do we need an ultimate sea boat? For those who want to cross oceans I would advise FD. For the typical coastal cruiser its a toss-up. If roll at anchor and in small waves is important SD has an edge. For pack rats who really weigh a boat down FD tolerates that best. With fuel economy either can do the job if designed with that in mind.
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Old 02-12-2016, 03:23 PM   #12
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eyshulman,
I suspect those that get FD boats do so for a combination of fuel economy and general seaworthyness. Not paticularly survivability. Our Willard is very seaworthy .. not tested by me and in 5 -7' confused seas can roll a lot. But w most seas a slight course change will take care of bad rolling. A fairly heavy FD boat can be a joy compared to the snap rolling SD types. One can frequently spend the whole day in lumpy seas that would send a SD boat heading for smoother water.

The PO had a roll dampening device aboard so he must have anchored in somewhat exposed places.
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Old 02-12-2016, 03:41 PM   #13
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[. One can frequently spend the whole day in lumpy seas that would send a SD boat heading for smoother water..[/QUOTE]

Agree and sailboats are a great example of that. Conditions that used to make for great sailing passages across the Georgian Straits are what I try to avoid. With my SD I pick my crossing weather and use that fuel guzzling speed my twins and hull allow to cross the 20k in less than 1.5 hours if need be. I have also found when things get moderately bumpy upping my speed makes for bumps but less roll that was also the case with my previous Lobster type. Once I get to quite water I go to my general cursing speed 9.2K at <5gal/Hr.
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Old 02-12-2016, 04:05 PM   #14
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Thank you all for that, it's beginning to make more sense now....

We plan on island hopping up and down the Bahamas and Caribbean so I take it a SD hull will suit us best??
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Old 02-12-2016, 04:40 PM   #15
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For a good reference Dave Gerr's "The Nature of Boats" is an excellent resource for this type of thing. Gerr being an eminent naval architect, but he writes in layman's terms. Beebe's "Voyaging Under Power" also discusses hull form in some depth, though concentrates on the use of displacement hulls for long range cruising.
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Old 02-12-2016, 04:48 PM   #16
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Science and theory are wonderful and I support their conclusions. However, I trust my eyes also. In eight years in the Eastern Caribbean we have seen one after another of full displacement motor boats, but only one semi-displacement motor boat, and that boat is in the charter fleet in the Virgins.

I am not counting local boats or boats over 54 feet (Flemings), nor the sport fish which show up for the tournaments.

Right now the seas between the Windward islands are averaging eight to nine feet on the beam with higher seas as you approach the island shelves. The last motor boat to pull into Saint Anne (southern Martinique crossed from St. Lucia). She is a Krogen 48 and had an "interesting ride".

The Bahamas are different as the travel is not always north south with trade winds from the east.
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Old 02-12-2016, 05:31 PM   #17
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For a good reference Dave Gerr's "The Nature of Boats" is an excellent resource for this type of thing. Gerr being an eminent naval architect, but he writes in layman's terms. Beebe's "Voyaging Under Power" also discusses hull form in some depth, though concentrates on the use of displacement hulls for long range cruising.
I will have a read, thanks...

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Science and theory are wonderful and I support their conclusions. However, I trust my eyes also. In eight years in the Eastern Caribbean we have seen one after another of full displacement motor boats, but only one semi-displacement motor boat, and that boat is in the charter fleet in the Virgins.

I am not counting local boats or boats over 54 feet (Flemings), nor the sport fish which show up for the tournaments.

Right now the seas between the Windward islands are averaging eight to nine feet on the beam with higher seas as you approach the island shelves. The last motor boat to pull into Saint Anne (southern Martinique crossed from St. Lucia). She is a Krogen 48 and had an "interesting ride".

The Bahamas are different as the travel is not always north south with trade winds from the east.
Sorry I don't fully understand this, does it make a difference when the boat is over 54"..??

And I thought the Krogen had a FD hull, do you mean a SD hull would have had even more trouble??

Please bear with me it's a steep learning curve...
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Old 02-12-2016, 05:42 PM   #18
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If one intends to travel in areas where the seas are 8-9ft a well found sailing craft is almost always a better choice than an under 60 ft. power boat. Yes the sailboat is FD for the most part not sure how we would define high speed multihulls.
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Old 02-12-2016, 05:51 PM   #19
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For a good reference Dave Gerr's "The Nature of Boats" is an excellent resource for this type of thing. Gerr being an eminent naval architect, but he writes in layman's terms. Beebe's "Voyaging Under Power" also discusses hull form in some depth, though concentrates on the use of displacement hulls for long range cruising.
Just ordered that book from Amazon and another by him that looked good.. Boat Mechanical Systems Handbook... Keep me busy for a few hours... Thanks for the info, they look good

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If one intends to travel in areas where the seas are 8-9ft a well found sailing craft is almost always a better choice than an under 60 ft. power boat. Yes the sailboat is FD for the most part not sure how we would define high speed multihulls.
Sailing is not for us I'm afraid, we are looking at a 50"+ (and probably nearer 60") power boat
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Old 02-12-2016, 06:49 PM   #20
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Just ordered that book from Amazon and another by him that looked good.. Boat Mechanical Systems Handbook... Keep me busy for a few hours... Thanks for the info, they look good



Sailing is not for us I'm afraid, we are looking at a 50"+ (and probably nearer 60") power boat
Expect around 1 mpg at 7-8 knots in that size of boat; depending on the sea state.
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