Captain Nemo wrote:Guys,
My forward side rakers are flat rather than upswept (a result of the poor quality plans I had available when I built the boat) but I think that supports my theory.
Upswept rakers would induce a lifting moment to the bow, not a dive. In fact, I once thought the upswept bow and spur was to facilitate the ram penetrating a curved ship hull; but someone (Will or Harry?) told me the designer's real intent was to give the basic hull a built-in ascent characteristic that would bring her back to the surface in the event the dive planes failed.
Looking at the camber, one might think the "lift" would be on the bottom side of the curve: but hydrodynamics and aerodynamics differ there, especially at the slow speeds we achieve.
What we experience is mostly what's called "impact lift" resulting from molecules attacking an inclined surface in motion, and pushing it up (as opposed to aerodynamic lift resulting from reduced pressure generated by accelerated fluid flow velocities, in accord with Bernouli's Theorum). So in the case of the upswept NAUTILUS bow, that characteristic will lift the boat's nose if anything.
If there was some other characteristic of the basic NAUTILUS design that was producing the "auto-dive" phenomenon, the lack of an upswept bow on my NM would enhance that effect, if anything. But no: she's as tame as an old mare.
I still come back to the differences between a NAUTILUS that's essentially dry inside; versus one that's a cylindrical pressure vessel inside a larger free-flooding hull. All that water inside doesn't actually weigh the boat down when the whole thing is submerged; but its MASS does figure into the equation when underway. It's a factor with enclosed wet subs like the REEF RANGER, too.
The inertia of all that water weight has to be overcome when initiating a maneuver, and the momentum it imparts once underway must be countered to reverse the effect. Sounds to me like, when speaking of a free-flooding NAUTILUS replica, if a diving trend get's started as speed increases, it will be enhanced by the mass of water in the bow of the boat.
As Bjorn points out, this is not a problem if it enhances the boat's "divability"; but it might be if, as someone else said, the trend continues until the sub is pointing it's tailfins at the sky. That's nice if that's what you want to happen; but if it's happening on it's own, you have a serious lack of control.
Another thing to consider is the open scupper drains that lead into your hull, and the open ballast vents in the bottom: these all let water flow into the hull interior. Underway, all kinds of gremlins could be joining the party that don't meet the eye at first glance. But again, these conditions would effect a free flooding replica, but not one with a dry pressure hull taking up most of it's interior space.
The jury is still out, but I'm inclined to stand by my original assessment that this is somehow related to the characteristics of a cylinderical pressure vessel inside a larger free-flooding hull; because that seems probable to me, and it also seems supported by the differences we observe between the performance of dry versus wet replicas.
Mi dos centavos, amigos...
Carcharadon wrote:I was able to post a picture of two unfinished Nautilus hulls upside down showing the curvature in profile. The hulls are unfinished models of a 7 ft. and 4 ft. The picture illustrates how much the bottom hull resembles the cross-section of an aircraft wing. Now making some broad assumptions here that hydrodynamics and aerodynamics are somewhat similar except of course for density it is reasonable to intuitively assume that the bottom hull shape of the Nautilus can, to some degree act as a inverted airplane wing providing a slight downward thrust (or suction) in relation to increased speed. This seems to make sense to me. However, I could be entirely wrong in this.
Andy I think they mention something about the 22 ft. model being towed by a truck on the 20K DVD. I think scale wise it was in excess of 300 mph. I could be wrong I'll see if I can find it again.
the original intent of Harper Goff Nautilus as he stated was to be simply this......."if for some reason the dive and trim planes failed to operate, the Nautilus would eventually surface under it's own power,this is why the upward sweep...something to that effect anyhoo!!!...Nautilus builders forget one thing it's not just the side rakers that have the approx3 degree upward sweep,it's the entire foward hull..my guess then is that either these R/C boats do not have this shape or that they aren't trimmed properly..Ray Mason sub is an example of what,if anything should be going on...his planes if going with speed lol
Carcharadon wrote:Whatever scale speed they said it was they also said it was excessive, but the director said he liked the effect anyway and he said to go with it. I think its on the DVD in the bonus material some place. I looked but couldn't find it.
177ft/300mph = 22ft/37mph
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