Hull Design Question #1 - Dihedral Angle

Previously Posted in General Discussion

In Great Moments in Cat History, t2x states:

“The Chris had a constant dihedral at the transom and therefore more aft hydrodynamic lift. That means that it is preferable for big block inboard applications as it supports the extra weight better. The Shadows have a variable dihedral at the stern and have less sponson friction with lighter engine applications and were/are faster with "kickers" on the transom.”

The dihedral angle is, of course, the angle made between the outside plane of the hull and the “water line”. A constant dihedral would be best described as, forgive me, a “true” V. A variable dihedral would be a hinged or folded V with two or more different angles to the hull. In describing the dihedral angle, the strakes are not considered in the “big picture”.

THE QUESTION:
Taking delta pad designs out of the equation for the moment, and considering hull designs capable or intended for operation above 80 mph, how many hull designs have successfully incorporated viable dihedrals and why were they successful? Has anyone ever manufactured a V bottom hull with a variable dihedral which was successful?

[ 10-09-2001: Message edited by: Adivanman ]

[mltdwn12]

Thanks, saved me from looking for that Websters dictionary Curious to see what feedback your post gets.

Craig

[bob]

Would a variable deadrise have the same meaning as a variable dihedral angle?

[cobra marty]

This will be a little tough. This is what my transom looks like and what angle is it?
! __________________
! ! tunnel
! !
!_ _!
! !
! ..--''
!..--'' Try to see this as
straight angled up and not jagged
Is this a variable dihedral angle? Thanks.

[cobra marty]

I tried to draw a picture and it doesn't seem to have come out. Sorry, Marty.

[RLW]

I find this interesting but hard to comprehend. A diagram would be worth seeing.

The dihedral angle is the angle the hull below the deadrise makes relative to a (theoretical) horizon at the absolute base of the hull. Picture it this way - if your boat were sitting on the ground (and was upright as it would be as if floating in the water) the dihedral angle would be the angle between the ground and hull.

......\____/
........\__/ dihderal
______\/_______

On a traditional (offshore) V bottom, the dihedral angle would be a constant 24 degrees. Of course strakes interrupt the V but they are not considered a "variable."

A variable dihedral V could occur in two ways - both with interesting possibilities.

Case A would be, say, a 24 degree angle to the first strake, then a 30 degree angle to the (last) strake and the start of the freeboard (as viewed from the back of the boat). This could have some interesting stability and speed influences.

Case B would be, say, a 24 degree angle measured at the transom which gradually increases or decreases as it moves forward towards the bow. This could have some serious influences on planing and lift.

[ 10-13-2001: Message edited by: Adivanman ]

[cobra marty]

Maybe this worked. This is the view from the transom of my cat and all the way forward. What dihedral angle do I have? Constant or variable? Notice the 'notches'.
....[
....[ _____________________
....[ ] TUNNEL
....[_ _]
......[ ]
......[_ ]
........''--.._ ]
...............''--.._]
ignore these dots
straighten the lines. I hope this prints OK.

[cobra marty]

I give up.

[Payton]

My boat is a 26' 1979 Mirage, same as an Eliminator SS daycruiser. I have a delta pad, basicly in place of where a jet could be installed ( some were built with a jet). Other than my pad I have a 21 degree hull. However, the foreword portion of the V is less of an angle or flatter. Maybe 19-20 degrees, I've never measured. Planeing and lift are excelent but is a rough ride if the waves get bigger than 3', as the hull tends to slap the waves.
Would this be a varible dihedral?