Nomenclature question

[Lodesman]

Mudnut,

Thanks for the history lesson. Have never had a close call like that, but have done plenty of "plane guard" (rescue standby in case of crash/ditching), and ya gotta be on the ball for that. When they say they're coming to the flying course, it means the rudder's over and they're making turns for 40 knots. We're only half a mile astern of the carrier, but she'll turn inside that so you can't follow them around; you have to clear their path and do your best to get back into station on the new course. Even at full speed we can't catch them, but the deck cycle's so quick, that before you know it, they're turning back around and ya gotta get outta their way again. What a ride!

Kevin

[Whimsical]

Originally Posted by Lodesman

Whimsical,

You're comparing apples to oranges - static buoyancy and reserve buoyancy are two different kettles of fish. I agree that a plumb bow will have a more forward distribution of buoyancy that better serves to resist initial pitching moment.
Not apples and oranges. If we compared boats on waterline length I would agree but we don't. A 40' boat is length overall. a 40' slip is length overall not the waterline length, if it was w/l slips could be very cheap .
I suggest you draw out a couple of examples both profile and sections for a given deck length and it will become obvious. In the sections you must use the same deck plan. Only compare the volume above and forward of the point the raked stem meets the water. The same point is projected to the waterline of the plumb bow version. The plumb bow will have more volume both above and below the waterline. Better yet model a couple of front ends in Rhino or some such and check the volumes.

But reserve buoyancy is the watertight volume above the waterline Exactly
a raked bow of the same fineness as a plumb bow will have more volume above the WL, therefore more reserve buoyancy.
You need to look at the area that is missing and not fall into the trap of thinking you are getting something extra.
Induce a forward pitching moment (ie. shove the bow down) with a given amount of force and the bow with less reserve will go down further. True

Flare in the traditional sense is the increase in beam from waterline to the deck. The opposite is "tumblehome". I don't recall terms for convex and concave hullforms, other than what has been mentioned, though I think the term "fuller" is often applied to convex hulls. And yes, a convex hullform would have more reserve buoyancy than a concave one, where all other parameters were equal.
I'm talking only of the most for'd area. Tumble home is more usually associated with the midship area


Kevin

Maybe I should do some diagrams over the weekend.
Mike

[delmarrey]

Originally Posted by Whimsical

I don't see how rake can increase reserve boyancy.

Your trying to beat a dead horse here. To put it plan and simple, it's the boating industry that calls the overhang of boats reserve buoyancy. The reason being if one were to push the boat straight down into the water the waterline (surface area) would increase UNLIKE a straight up transom and/or plum/straight bow. Thus, giving a vessel more buoyancy surface area, especially in on-coming or following seas.
................................_/)

[Lodesman]

Mike,

I understand your argument, but don't get sucked into defining boat performance by slip length. If you take comparable boats of similar performance, displacement, carrying capacity the one with the raked bow will have a greater LOA over the plumb bow version, but very similar LWL. Sitting static, level in the water, the the submerged sections of the bows of both boats displace an amount of water equal to the weight of the bow. If I throw 500Kg of stuff into each forepeak, each will have to displace an additional 500 litres at the bow; the plumb bow is going to go down further than the raked bow. Therefore the plumb bow has less reserve buoyancy.

Kevin