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Old 02-24-2012, 10:56 PM   #21
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Your hull type

Doubt this ship motoring*will reach hull speed with square sails heading into the wind.* (Treasure Island and Yerba Buena [good herb] Island in the background.)


-- Edited by markpierce on Saturday 25th of February 2012 12:00:07 AM
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Old 02-25-2012, 12:08 AM   #22
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Your hull type

*

Our 34'*Tolly Tri Cabin is Planing Hull:

*

6.5 / 6.7*knots at*1900 to 2100 rpm on one engine*= 2 + knpg (hull speed calcs 7.58 knots).* Enjoyable speed*to cruise inland water ways (i.e. SF Bay Delta's slim,*levee lined*canals).* At this relaxing speed... Admiral loves it!* I like that it is easy on drive line and very quite.* Captain (that be me) alternates engine use approximately each hour. And, wake is mellow.*

*

16 /17 knots at 3400 to 3600*rpm on twin screw*= 1+/- knpg.* Full plane and comfortable for a cruise.* Covers fair amount of territory*during multi hour*trip.* Trim tabs set boat at good attitude to a relaxed water surface, she handles well, and drive line has no problem with it!* Synced rpm engines make a powerful combined humming sound that I much enjoy listening to.* Still plenty quiet in main salon and state rooms for normal volume conversations... due to well insulated engine compartment with plenty of outside airflow.* Wake is not too tall.

*

21 + knots at 4300 / 4500 rpm = 1/2 to 3/4 knpg I guess?? (WOT 4600 / 4700 rpm).* I feel it pushes entire drive line too far into top performance levels for extended use.* I use this rpm/speed only in extremely necessary situations and for shortest period possible.* But, it is available if required!* Ive*used this rpm/speed three times in years. wake flattens considerable.

*

None of these numbers are scientific results; all are result of to my observation and experience... but, I bet cha they aint far off!!* LOL

*




-- Edited by Art on Saturday 25th of February 2012 10:21:40 AM
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Old 02-25-2012, 02:58 AM   #23
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RE: Your hull type

"Hull speed" and the rules for hull speed only relate to common boats with a length to beam ratio of 3-1 or so.

Once one gets to at least 6-1 or better 8-1 the boat may stay in displacement mode and yer high speeds are available.

Look at most of the commuters of the 20's , unfortuniatly heavy from their construction and the monster engines of their time , but well over 20K.

Cats and trimirans can use these shinny hulls IF they are operated at high speeds.

At low trawler crawler speeds the cats are less efficient than a similar weight 3-1 boat due to much increased wetted surface.

And of course the wide spaced hulls give the GB floating box snap roll in beam conditions.
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Old 02-25-2012, 06:14 AM   #24
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RE: Your hull type

Quote:
FF wrote:
Simply look at the stern.

The width of the transom at the WL in feet will be close to the speed expected in K.on a 40-50 ft boat.
*Boy I can't wait to go 18knots.

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Old 02-25-2012, 09:33 AM   #25
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RE: Your hull type

Our heavy and fairly-deep-V (19 degrees at the transom) hull seems to be what the boatdiesel prop calculator calls "slow planing".* Cruises nicely at 18 knots, getting about 1.75 nmpg, with 260hp of Volvo diesel and duo-prop sterndrive.* Cuts through the chop beautifully.

Could go a lot faster with a big block gas engine, but the sweet spot for handling is no more than about 24. We've had her up to almost 30 in "light ship" condition, but low 20's felt a lot more controlled.* Max speed fully loaded for a long SE Alaska cruise is 25.

We cruise mostly at 6-6.5 knots, getting 4.5 nmpg.*

Pretty nice combination of higher/lower speed capabilities.
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Old 02-25-2012, 09:53 AM   #26
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RE: Your hull type

Quote:
ksanders wrote:Woodsong wrote:
Actually...I assume our 45 is a planing hull. *She has a small keel, prop pockets, and soft chines. *When I haul her out I will take pics.
Actually I'd venture to say that you have a semi displacement hull. Thats what I think of our 4788 as.

As you indicated she has a full keel, and soft chines.

I've found that mine even at 15 knots is not realy "over the hump" like our planing hull cruisers were.

I could be wrong, but that seems to fit the definition of "semi displacement" *that boat builders and the public have come to use to define a boat that exceeds hull speed but is not as fast as a "cruiser".

That said, I tend to agree with Beebes definition of boat hulls* being either displacement or planing. Using Voyaging Under Power as a guide we own planing hull boats.

Perhaps a better poll would be " What is the maximum speed of your boat"

The link below is to Bruce Roberts site and is his opinion of the differences between hull forms.

http://www.bruceroberts.com/public/H...OAT_DESIGN.htm*



-- Edited by ksanders on Saturday 25th of February 2012 09:20:06 AM

*The way I look at it is if the water is breaking free of the transom, and you can see to the bottom of the hull exposed, you are definitely planing.
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Old 02-25-2012, 10:27 AM   #27
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RE: Your hull type

Quote:
Moonstruck wrote:ksanders wrote:Woodsong wrote:
Actually...I assume our 45 is a planing hull. *She has a small keel, prop pockets, and soft chines. *When I haul her out I will take pics.
Actually I'd venture to say that you have a semi displacement hull. Thats what I think of our 4788 as.

As you indicated she has a full keel, and soft chines.

I've found that mine even at 15 knots is not realy "over the hump" like our planing hull cruisers were.

I could be wrong, but that seems to fit the definition of "semi displacement" *that boat builders and the public have come to use to define a boat that exceeds hull speed but is not as fast as a "cruiser".

That said, I tend to agree with Beebes definition of boat hulls* being either displacement or planing. Using Voyaging Under Power as a guide we own planing hull boats.

Perhaps a better poll would be " What is the maximum speed of your boat"

The link below is to Bruce Roberts site and is his opinion of the differences between hull forms.

http://www.bruceroberts.com/public/H...OAT_DESIGN.htm*



-- Edited by ksanders on Saturday 25th of February 2012 09:20:06 AM

*The way I look at it is if the water is breaking free of the transom, and you can see to the bottom of the hull exposed, you are definitely planing.

*Then we are not planing

Our 4788 seems to "squat" the more power we apply.

Like most other large boats, the original engines in the original design were much smaller than what the manufacturer ended up with at the end of the production run.

Personally I think it is/was a mistake to keep adding larger engines to boats in order to meet a market demographic.

When we re-powered our 4788 I seriously considered going with the Cummins NA engines at about 165 HP (If my memory is correct). The only reason I opted with 330's is because of resale.
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Old 02-25-2012, 01:13 PM   #28
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RE: Your hull type

Ksanders:

If your 4788 has the stock engines, you will never plane. Add 1000 hp and you might. Add 2000 and you will for sure. Don't send me your fuel bill.
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Old 02-25-2012, 01:15 PM   #29
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Your hull type

I think semi-displacement is a totally bogus term thought up by marketing people who wanted to associate their boats with the supposed ruggedness, seaworthiness, etc. that "displacement" conjures up. Same marketing mentality that applied the term "trawler" to what in reality is a cabin cruiser.

Displacement is like dead. You either are or you aren't. You can't be semi-dead, nor can a hull be semi-displacement.

Planing, on the other hand, refers to the ability of the hydrodynamic pressure on the wetted surfaces of the hull generating enough force to lift the hull, reduce the wetted surface, thus reducing the drag, thus enabling the hull to move through/over the water much faster. There are varying degrees of how much the hull can be lifted. Almost all the way up so only a small portion of the hull is needed to contact the water to generate sufficient hydrodynamic force to hold most of the hull actually clear of the water---- the absolute full plane---- to a hull design/power combination that lifts the hull somewhat out of the water, reducing the drag somewhat, and allowing a degree of additional speed. The term semi-planing is an accurate one for this. Because as soon as you start generating sufficient hydrodynamic force against the hull to lift it, you are starting to plane the boat. If it's not on a full plane, it's on a semi-plane.

But displacement is always just that--- displacement. There's no semi- about it, because as soon as you start generating lifting force, you are entering a planing condition, albeit not a very efficient one unless your hull design and power permit it. Semi-planing describes that condition in a meaningful way. Semi-displacement does not, in my (as well as some noted naval architects') opinion.


-- Edited by Marin on Saturday 25th of February 2012 08:03:45 PM
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Old 02-25-2012, 03:11 PM   #30
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Your hull type

My boat is semi-everything...

Semi-efficient
Semi-costly
Semi-done
Semi-used enough (even though I'm a liveaboard
Semi-quick
Semi-planing
Semi-displacement
Semi-manueverable

ooops...the best isn't semi.... because it's ALL-paid for*



-- Edited by psneeld on Saturday 25th of February 2012 04:11:59 PM


-- Edited by psneeld on Saturday 25th of February 2012 04:13:39 PM
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Old 02-25-2012, 04:13 PM   #31
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RE: Your hull type

Quote:
Marin wrote:
I think semi-displacement is a totally bogus term thought up by marketing people who wanted to associate their boats with the supposed ruggedness, seaworthiness, etc. that "displacement" conjures up. Same marketing mentality that applied the term "trawler" what in reality is a cabin cruiser.

Displacement is like dead. You either are or you aren't. You can't be semi-dead, nor can a hull be semi-displacement.
*OK, OK.* Using Bruce's definition, the Coot is a "medium displacement" boat.* Wish we had started with common definitions.
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Old 02-26-2012, 05:35 AM   #32
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RE: Your hull type

" Boy I can't wait to go 18knots."

GE makes a great turbine that would provide the required power.

Your fuel mileage may vary!
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Old 02-26-2012, 07:52 AM   #33
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RE: Your hull type

We've got a full displacement hull on our 58' Hatteras LRC. Hull speed of about 9.5 knots, but we've only seen that on a couple of occasions with a strong helping tide. :-)
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Old 02-26-2012, 09:19 AM   #34
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RE: Your hull type

We have a full displacement Willard. Maybe a little "fuller" than most Willards because ours is the trawler on a 8 ton cutter hull. ;-)

Keith Olive
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Old 02-26-2012, 11:09 AM   #35
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RE: Your hull type

Quote:
kolive wrote:
We have a full displacement ..... because ours is the trawler on a 8 ton cutter hull. ;-)

Keith Olive
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*Should be very fuel efficient. I'll bet a 12 year old can push that 8 tons around with no problem.
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Old 02-28-2012, 09:43 AM   #36
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RE: Your hull type

Quote:
kolive wrote:
We have a full displacement Willard. Maybe a little "fuller" than most Willards because ours is the trawler on a 8 ton cutter hull. ;-)

*I'm sure Eric would like to chime-in here and be counted among the full-displacement group, but I sense that his conservative views with regard to HP and hull design may be only to mask Willy's true capabilities.
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Old 02-28-2012, 11:02 AM   #37
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RE: Your hull type

Quote:
healhustler wrote:kolive wrote:
We have a full displacement Willard. Maybe a little "fuller" than most Willards because ours is the trawler on a 8 ton cutter hull. ;-)

*I'm sure Eric would like to chime-in here and be counted among the full-displacement group, but I sense that his conservative views with regard to HP and hull design may be only to mask Willy's true capabilities.

Wow! And to think Willy can do that with only 36hp!
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Old 03-31-2012, 01:01 PM   #38
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As I think has been said in this thread, semi-displacement is kind of a misnomer. All boats have displacement. The equation for volumetric displacement, which is the symbol for Delta in an inverted display, is the function of Delta, the weird s shaped line, times Delta. The function is substituted with the mass volume for salt or fresh water. Salt is 35 ft cubed over Long Tonnes, and fresh is 36. This sum can then be used to find all of the coefficients-Block, prismatic and waterplane. You can also find the midship section coefficient. Full displacement hulls are flat on the bottom with a radius at the bilge strake. They generally have vertical side shells with a slight flare at the sheer strake. Semi-displacement hulls have chines that help the boat "lift" itself out of the water as it speeds up, thereby having less hull in the water which leads to less volumetric displacement in the water. This is the time when a boat is semi-displacing it's own structural volume. I hope this helps.
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Old 03-31-2012, 02:05 PM   #39
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Quote:
Originally Posted by FF View Post
Simply look at the stern.

The width of the transom at the WL in feet will be close to the speed expected in K.on a 40-50 ft boat.
The waterline of my sailboat, as with most full displacement sailboat hulls, comes to a point at the stern at the waterline. Sailboats are basically canoe shaped at the waterline. So does this mean that since I have no width, I cant expect any speed? Like, just stand still?
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Old 03-31-2012, 07:56 PM   #40
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Quote:
Originally Posted by Tony B View Post
The waterline of my sailboat, as with most full displacement sailboat hulls, comes to a point at the stern at the waterline. Sailboats are basically canoe shaped at the waterline. So does this mean that since I have no width, I cant expect any speed? Like, just stand still?
Actually, those lines at the stern are made to reduce the amount of energy from making waves, thereby reducing the amount of energy needed to propel the boat through the water.
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