kulas44
Guru
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.
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.
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.
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.
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.
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.
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.
They do test the same way in the tanks but there is then Reynolds Number and other variables that can't be controlled or otherwise accommodated. But it's not the same in many ways. Would the molecular dynamics be the same on a 2' model and a 700'ship. I suspect not.
Would you guys be interested in going from destructive to constructive?
I'm taking the ignore option. Bye
I'm taking the ignore option.
Bye
Art you can bring a smile to my face no matter where we are. Don't ever loose that ability.
Most of the time we don't need these guys for their knowledge. We're mostly here for the fun of it anyway and these guys dislike me so much it's anything but fun. I had psneeld on ignore for many months and decided I wanted to hear what he had to say again ..... But it's just not worth it
Your post was a pleasant shock and .. If there's such a thing as a pleasant shock .. I appreciate you and what you say. Thank you Art.
But no .. I'm going to keep them in their rabbit holes, doghouses or wherever they go.