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Old 02-09-2014, 04:51 AM   #81
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The big difference for my untrained eyes is that the DD hull bottom (not keel) has a convex curve from bow to stern while on the FRP bottom it is almost flat.
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Old 02-09-2014, 07:02 AM   #82
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>The Halifax Class in the picture below is 440' long and only 53' wide, that's a beam/length ratio of over 8. In a forty footer of similar proportions the beam would be 5'.....not workable.<

To get most of the speed at low power only 6-1 LWL x BWL is required .

A 6+ beam at floor level with the usual flair of hull sides would make plenty of space for engine ,noisemaker and big tankage.

With an aluminum or foam core deck house a DL of under 100 could be had , and a fast cruiser created.

I have done a number of drawings keeping the beam over all under 7.5 ft , to fit inside a std shipping container and the concept works,

BUT ,

I dont think it would sell as the cabin space is mostly below and only the pilot house is up on deck where most marine motorists seem to insist on living.It works for us as most cruising time is spent in the PH , we go below to cook, eat and use the head , and as long as the sleeping cabin has excellent all weather ventilation , its fine .

If I came across a spare $100,000 , I would have metal cut tomorrow!
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Old 02-09-2014, 10:00 AM   #83
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About the only thing I can say about QBBL (and it sure depends on who you think knows what on boatdesign.net)...is that yes the angle is of some importance to displacement efficiency...but by NO means is a big factor in displacement determination...only efficient displacement design (go back to my barge example)....and again there are other very important factors that weigh into efficiency on displacement boats that equal or exceed QBBL...

The only real acceptance of QBBL as a major player from my research is going in one direction.... in that a larger angle at some point will prevent a boat from attaining decent semi-xxxx performance (whichever one you wish to label it). But you most certainly can have zero QBBL angle and still be designed for full displacement. Which is exactly what many here are trying to say about displacement hulls with lessened QBBL and immersed transoms...they are NOT semi anything to either the NAs that designed them or the people who operate them.
Ditto.
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Old 02-09-2014, 10:17 AM   #84
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The big difference for my untrained eyes is that the DD hull bottom (not keel) has a convex curve from bow to stern while on the FRP bottom it is almost flat.
Exactly.
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Old 02-09-2014, 10:30 AM   #85
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Mark's eye ball is correct about the shape's and I would say probably one is full displacement...but the other isn't necessarily SD/SP...just by QBBL....takes more than that.
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Old 02-09-2014, 10:51 AM   #86
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Displacement hull with horseshoe stern.

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Old 02-09-2014, 12:51 PM   #87
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Interesting boat.

It's the opposite of my Willard in that most of it's displacement is fwd whereas the W30 is light fwd and heavy aft. Relatively ... The W30 is heavy overall for a 30' boat. The other obvious difference is of course the hard chine.

But horseshoe stern it has.

Actually the WL will determine if this boat I'd SD or FD. Riding high enough it would be FD. In a way it's a bit like the Bartender and it's a full blown planing hull.

Skidgear I'm considering your PM very carefully. Thanks.
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Old 02-09-2014, 01:59 PM   #88
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Actually the WL will determine if this boat I'd SD or FD.
Real world testing...not a photograph...has established that it is displacement. (Confirming what the manufacturer says it is).
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Old 02-09-2014, 02:04 PM   #89
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A trap.

So what was this real world testing?
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Old 02-09-2014, 02:23 PM   #90
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A trap.

So what was this real world testing?
What, the displacement guru doesn't understand how to gather performance data on a hull/engine combination?
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Old 02-09-2014, 03:26 PM   #91
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I would not at first glance say it was a pure displacement hull. More like a "lobsta" boat hull, with a round stern. And a beautifull boat, if I may add.
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Old 02-09-2014, 03:35 PM   #92
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The "displacement guru" doesn't recall any discussion about performance being the defining line between FD and SD. That is another method that determines this matter by the speed performance of a hull. It goes something like this "if a hull is capable of speeds of 2x the square root of the WLL" it becomes a SD. But who decides it's with that number and not 1.8 x .... . Just as arbitrary as the QBBL angle. Haven't looked at anything like that for a long time and can't remember how power loading got into the picture if at all.

No one would argue that a boat capable of cruising one knot over hull speed is not a FD boat. However given the history here on TF I should know better than to say that.

IMHO .. the difference lies in the way water under the stern returns to the way it was before the boat came along. Significantly submersed transoms need not apply. Too much drag and turbulence as pointed out before. And I still don't see any methods superior to making the call than the QBBL. There are exceptions like the one you posted above but no other yardstick seems to define the deciding line any better or universally.

The first page was good to very good but this is going down hill badly.
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Old 02-09-2014, 03:47 PM   #93
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kulas44,
Interesting .. The connection to the LB. And I think a straight run aft almost always identifies a planing or SD hull. I didn't make the jump to LB probably due to the very prominent hard Chines. But her performance should be very similar if not overloaded. Should be a very nice running boat at 10 or 12 knots. Judging by the prop in the pic this one's probably powered for slower speeds.
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Old 02-09-2014, 03:57 PM   #94
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FD will always displace their weight with water and will always have the same draft (more or less). They will always displace their weight in water.


PH will use lift to rise up using the lift provided by the reaction of water on their hull form to increase speed by reduced water drag. at full plane the wake will reduce as the boat rides on top of the water and at this point will require less power to maintain this condition. At full plane they will displace most of their weight with lift.



Fast non-planing boats (SD?) will have so much power and some lift built in to there hull that they will achieve a partial plane with slight reduction in surface drag and reduction in draft but with a much higher wake production and with no reduction in overall drag (never over the hump) regardless of use of full power. They will displace a small amount of their weight with lift but most of their weight will be displaced at speed by water.



Its like an airplane, the FD has not enough wing (lift) to fly so its really a bus. The SD is like a hover craft lots of power and can fly up to about 3 ft. The planing hull can fly.





post #18 ....posted 2 days ago (page 1)
images 2 and 5 as a reminder about very similar calculations...
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Old 02-09-2014, 04:56 PM   #95
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All this talk about displacement hull or not reminded me of a fellow that came into the marina at Sabine Pass. He had purchased a 65 foot wood ChrisCraft Connie in florida, supposedly Jerry Lewis's old boat. The broker had to him it was a "pure full displacement boat" therefor it was a trawler. Apparently the guy wanted a trawler so he bought it. He was bragging up the fact that the broker told him if he would buy it without a survey he could have it for the deeply discounted price of $120,000. It had been glassed over, quickly and poorly by someone in the very near past. I personally walked thru the boat and watched a river or large stream of water running over the floor timbers on its way to a bilge pump, one of 7. The owner claimed he had over 12000 gallons an hour of pump capacity, no way was this boat gonna sink !! The windows in the wheel house were surounded by mahogany that could literally be taken out by the handfulls. It smelled like wood alcohol. He told me "we lost all hydraulics" meaning a copper steering line had broken. They dropped anchor and when it caught it tore the bow pulpit completely off of the boat, along with all of the rotten decking and support structure. I have never been on a boat in such poor shape that was still floating. The thing that disturbed me more than the owners ignorance was the brokers outright incredible lies. But, full displacement, and trawler, are red letter words for brokers I guess.
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Old 02-09-2014, 05:12 PM   #96
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[QUOTE=manyboats;212127]

Haven't looked at anything like that for a long time and can't remember how power loading got into the picture if at all.

It got into the picture because I for one, think engineering analysis by photograph is nonsense. But I come from the world of engineering flight test.

No one would argue that a boat capable of cruising one knot over hull speed is not a FD boat. However given the history here on TF I should know better than to say that.

Then you reject what Dave Gerr states in his paper.


IMHO .. the difference lies in the way water under the stern returns to the way it was before the boat came along. Significantly submersed transoms need not apply. Too much drag and turbulence as pointed out before. And I still don't see any methods superior to making the call than the QBBL. There are exceptions like the one you posted above but no other yardstick seems to define the deciding line any better or universally.

The first page was good to very good but this is going down hill badly.


Just because you don't agree or wish to dismiss some excellent technical content doesn't mean it's gone down hill. On the other hand, if you want a dock talk discussion, pictures work as well as anything.

Regarding the hull in the photos, the buttock line along the keel is easily a constant 8 degrees all the way to the transom. There's an arch at the bottom edge of the transom which can be seen in the second photo. That arch curves down to meet the chines. So the bottom of the boat does not follow the chine profile inward toward the keel. In other words, it is not flat between the chines and the keel, but curves all the way until it meets that arch on the transom. The "effective flats" are relatively small...and according to the builder, they provide a little lift to limit squat at high power, and also add some stability at rest. The average mid arch buttock line is probably 7 degrees. In practice the chines do what the builder says, but little more. The boat reaches the bow wave in a relatively flat attitude at roughly the calculated hull speed. The one we tested had a very big engine. Adding power...lots of it... added about a knot. We stopped applying power with plenty in hand as the speed was not increasing and wake was humongous. The overall hull showed no evidence of lifting, in fact it appeared to be sinking down in the "hole". At cruise speed the wake emerges virtually calm.
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Old 02-09-2014, 05:39 PM   #97
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Displacement hulls (well at least spoon shaped hulls like large offshore sail racers were a couple of decades ago) can and have submerged themselves when surfing along with large waves.

Push a displacement hull hard enough and it just might sink into a hole...now that's a seaworthiness trait that taken to an extreme will surprise you...
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Old 02-09-2014, 06:51 PM   #98
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Originally Posted by kulas44 View Post
All this talk about displacement hull or not reminded me of a fellow that came into the marina at Sabine Pass. He had purchased a 65 foot wood ChrisCraft Connie in florida, supposedly Jerry Lewis's old boat. The broker had to him it was a "pure full displacement boat" therefor it was a trawler. Apparently the guy wanted a trawler so he bought it. He was bragging up the fact that the broker told him if he would buy it without a survey he could have it for the deeply discounted price of $120,000. It had been glassed over, quickly and poorly by someone in the very near past. I personally walked thru the boat and watched a river or large stream of water running over the floor timbers on its way to a bilge pump, one of 7. The owner claimed he had over 12000 gallons an hour of pump capacity, no way was this boat gonna sink !! The windows in the wheel house were surounded by mahogany that could literally be taken out by the handfulls. It smelled like wood alcohol. He told me "we lost all hydraulics" meaning a copper steering line had broken. They dropped anchor and when it caught it tore the bow pulpit completely off of the boat, along with all of the rotten decking and support structure. I have never been on a boat in such poor shape that was still floating. The thing that disturbed me more than the owners ignorance was the brokers outright incredible lies. But, full displacement, and trawler, are red letter words for brokers I guess.
44 - That brings back recollections to me of a time in late 1990's. My wife and I went to see a late 60's 57' Chris w/ flybridge that had been presented to me as in "remarkable condition" by a broker. He said the only reason the owner (who lived in Mexico at the time) was selling at such a reduced price ($59K) was because he'd just been divorced and desperately needed cash to keep his resort in Mexico open. OMG - The amount of rot and leaking bilge I found was simply astounding! Sounds like the one you account in post and the one we reviewed were "Kissen Cusens"! That event is actually the one that for first time in my life got me to fully swear off purchasing older wood boats again!
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Old 02-09-2014, 07:22 PM   #99
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Psneeld,
I saw that but didn't study it closely. Should have. Yes that's what I remember from reading books about NA and related subjects. I see displacement hulls go only up to 1.4 don't know w ....... Now I see that chart is for large ships.
I see the upper chart w different numbers. All of this is general and applies to some hulls more so than others and that is the way this whole matter is looked at by most that know and by others that follow like all of us. I wonder why SD was called the "transition zone"? Good description for people like Skidgear that tend to think in the planing mode.

Skid,
How did you measure the BL in the photo? I can't even see most of the bottom. But if the BL alongside or at the keel is in fact 8 degrees then the QBBL will be 4 degrees unless the bottom is concave aft. In that case the QBBL would be more like 2 or 3 degrees. If the hull is convex (or arched as many say) the QBBL would be or could be more than 4 degrees but as I say I can't even see the underside of the hull aft.

By my way of thinking this is not a FD hull for another reason. There are flattish portions of the bottom aft that are placed there to control the tendency to squat and go bow high. Those are not features of a FD hull and since it has these features (much like the Spanish boat w the big flat plane added to it's stern for the same reason. The original FD fishing boat was 100% FD but in my opinion became something else w the attachments (hull modifications).

I am definitely not presenting any of the above in the interest of obtaining followers or believers. It's just my opinion.

Panels I think I mentioned that negative aspect of the FD hull. And being sunk by a stern wave in a second or two could indeed be a surprise.
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Old 02-09-2014, 08:20 PM   #100
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[QUOTE=manyboats;212195]

How did you measure the BL in the photo? I can't even see most of the bottom. But if the BL alongside or at the keel is in fact 8 degrees then the QBBL will be 4 degrees unless the bottom is concave aft. In that case the QBBL would be more like 2 or 3 degrees. If the hull is convex (or arched as many say) the QBBL would be or could be more than 4 degrees but as I say I can't even see the underside of the hull aft.

Well whataya know, an engineering analysis from photographs is just full of unknowns. I've seen the bottom of the boat we tested many times.


By my way of thinking this is not a FD hull for another reason. There are flattish portions of the bottom aft that are placed there to control the tendency to squat and go bow high. Those are not features of a FD hull and since it has these features (much like the Spanish boat w the big flat plane added to it's stern for the same reason. The original FD fishing boat was 100% FD but in my opinion became something else w the attachments (hull modifications).

You're entitled to your opinion, but once again it appears that you and Dave Gerr, whose blue water displacement boat has anti-squat surfaces aft, are in disagreement as to the definition of displacement. He says his boat would require hull mods to semi-plane. I'm afraid I'm with Dave Gerr on this one.


I am definitely not presenting any of the above in the interest of obtaining followers or believers. It's just my opinion.

Perish the thought. Your motives are pure as the driven snow.
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