Going down the mine ...

Have recently started sailing an 18 ft skiff and can report bow definately lifted by the kite!  Scariest move is bear away (slow speed, no kite up, so big potential for nose dive).  Apparently, biggest crashes are when bow is lifted so much that boat gets fully airbourne and then remembers it can't fly and comes crashing down bow first.  Trick is to flog kite over biggest waves to avoid this.

Originally posted by aardvark_issues Originally posted by Rupert Lifting the bow out looks great for the pics, but is it actually quicker? I dont think its a matter of quickly - its when the conditions mean that you need to get the bow up to avoid stacking it - if you are right at the transom and cant get the bow up you are in trouble. You can always stand further forward with a longer pole...

Bow up is much faster than bow down ...

Perfect trim is of course the goal; it's all about risk management when it gets "full on"

Rick

Originally posted by Jack Sparrow In other words L4000 and boats like them trim slightly bow up with the kite up due to rocker-line / pole length / crew position and that they don't get over the hump often ( they sit on the hump ), so they don't leap ( and then crash ) in the same way as a 12-ft skiff / UK Cherub e.t.c because the longer waterline length doesn't allow the angle to change to such a large degree as a short water line length..

What your talking about is called the 'dory effect' or 'heavy soap dish'. It trims up cos its bow wave is so large, and it doesn't reach for the sky like a cherub, cos it can't go any faster and mount the bow wave.

Errr... Yep!

Originally posted by BBSCFaithfull I would also think that the size of the pole would dictate the amount of burying of the nose. For example a boat like the 4000 would not pitch pole very easily as it's pole is so long it pulls the nose really far out when it's planing.

Thing is when you go up you must come down and that is when this
threads topic happens.

I personally believe that the visual effect of lift from a spinnaker is an
amalgam of many of the factors that have been expressed here. And I
tend to side with Jim on his study that the spinnaker doesn't create
upward lift. But only to a point.

I think the factors that come into play are:

Length of pole.
mast height
kite luff length
kite shape
water line length
wave length
crest height / angle
rocker line
hull weight
mast foot pressure
main sail head shape
kicker tension

AND whether you escape your own wave ( get over the hump )

Without talking loads of technical crap. I think that in general only a small
proportion of lift ( not enogh to give phisical lift ) is generated from the
kite in normal conditions. But combine a short waterline length with a
long pole and a significant attitude change due to wave length and the
potential for lift alters significantly. So I think Jim is right if the models
are for a steady state but wrong if you add in the large variants listed
above.

In other words L4000 and boats like them trim slightly bow up with the
kite up due to rocker-line / pole length / crew position and that they
don't get over the hump often ( they sit on the hump ), so they don't leap
( and then crash ) in the same way as a 12-ft skiff / UK Cherub e.t.c
because the longer waterline length doesn't allow the angle to change to
such a large degree as a short water line length. Combine a short water
line length with a long pole, light weight, masthead kite, large kite area
and chop and a rapid change in angle of attack and the kite generates
significant lift or at least the appearance of it.

Other craft mostly sit on there planing wave giving the impression of lift
form the kite. Because they rarely escape the hump unlike true skiffs.
When you have escaped the hump and you are getting lift then maybe the
factors i have metioned above do create lift from the kite.

I sail a 2005 rules Cherub with a short pole, low rocker and flat sail shape
and I can tell you that she is prone to sticking the beak down. She is
optimised for grunt, with a fat head main. And in a sea she is very fast
but we have to pick our way where other notable boats with longer poles,
deeper kites and less in the head have a noticeable bow up attitude that
we don't. The interesting thing about the two boats I mention is that one
is very flat in the rocker whilst the other has a resonable amount.

So i would say, to sum up, that the kite can lift the bows but only TRUELY
in the right circumstances. And those circunstances don't happen to the
majority of boats. But then others would say if you need other factors
then is it true spinnaker lift? I would say it's like hydrofoiling :- you
need other things to happen before you can get up, but those factors get
you to a point where you can hydrofoil.