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Daggerboard - use of?

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mike ellis View Drop Down
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Post Options Post Options   Quote mike ellis Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 7:16pm

GRF:

I think one major problem with getting rid of the board on a dinghy would be that suddenly you would find you couldn't steer the boat. A board with only a fin is steered by carving on the rails, I'm not entirely convinced that the chines on a dinghy would be sufficient to provide a similar effect. I'm sure you've noticed in your sailing that tipping a boat creates steering in the opposite direction you get from tipping a board the same way, wether this is to do with hull shape or something i haven't a clue. I know that untill i worked out that I had to equate windsurfing to snowboarding not sailing in terms of steering i couldn't get the board to go the way i wanted it to go, so I'm not sure how you will steer a boat without a board using only the rudder.

I would be very interested to see the results of putting a sliding mast foot on a dinghy and trying your theory but it would be a lot of engineering. The only way i can see of doing it would be to slide mast, stays, and sheet lead positions all together, basically have a sliding frame with all the gubbins on it inside a hull to float the whole lot. If you ever do get round to it I would love to see the results.

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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 7:26pm
Originally posted by JimC

I can't help thinking that one of the key reasons good
boards can break 25 knots so much more readily is that they aren't dumb
enough to try and carry the same rag in 25 knots of wind as in 2.5
knots... Of course very immediate rig control and excellent power to
weight ratios do no harm at all either.

Were it practical, which of course it isn't, it would have been an
interesting comparison to have tried a vertically lifting daggerboard on
your board experiments. The thing about a board that pivots in its case is
that the wetted surface doesn't change very much until its almost
completely up... Take this sketch as an example.



The same board is shown half and three quarters up, both as a pivoting
cb and as a daggerboard. Compare the difference in areas, and also then
think about the span of the foil (= length presented to the water) which is
the main factor leadng to induced drag. I suspect that the one reason you
had to kick the board all the way up to make much difference is that until
its 90% or more of the way up there is very little difference anyway.


Well, at the time I did, we used to have only vertical 'dagger'boards, we
had them made by people like Phil Milanese and lots of other similar foil
builders, there were lots more in those days. They would replace the
pivoting centreboard that production boards came fitted with but were
very inefficient. We also had narrower "high wind" foils built, 'storm' swept
back foils and a variety of shapes designed to try and make those early
single slot sailboards viable in winds above 4-5, most failed.

Like vertical fins on raceboards, foils do not like the bottom of the foil
ahead of the top of the foil and this is what happens as the nose lifts in
any craft with a degree of rocker, the foil resists, keeps the nose down.

Many a 'flappy nosed' sailboard cn be controlled with a more vertical
fin/foil, as often many a 'nose down' board can often be released with a
more swept back fin. It has to be the same with boats. All the time the
foil stays in its vertical axis, every time the nose lifts, gets bumped up by
a wave or the spinnaker, the bottom of the foil will advance ahead of the
top and there will be an opposing reaction to lever itself back into the
vertcial axis.

So sweeping the plate back a bit corrects this tendency.

Then there is the forward facing surface which creates drag, sweeping it
back reduces that element.

So those are the main points for sweeping it back rather than lifting it up
although I do tend to accept the points your sketch illustrates, but it
didn't help much with sailboards.

I will dwell on the final reason sweeping back worked, and that was
steerage, as you know, sailboards are controlled by moving the rig, and
or foot weighting of the rail. With the plate down you weight the
windward rail, much as you would gybing a boat, and the fin interacts
around the plate.

Without the plate, you weight the inner rail, rather like a water skier
slalom turning and with practise in strong winds you turn fast without
dropping off the plane.

The day is yet to dawn when we can do that in the boat, we're close, but
so far we have always gone displacement before we're sheeted in on the
new tack even with a wave helping us.

Edited by G.R.F.
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Post Options Post Options   Quote Black no sugar Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 7:28pm

So, now Captain Morgan is totally confused (he won't install a sliding mast foot or apply so much amount of rake on a Wayfarer), it's time to give a quick update.

If you're sailing in a club where there are other boats of the same class, ask the good guys and do what they're doing. Once you get the hang of it, you can decide to be radical...

 

(Sorry, I'm not far from being a total beginner and I got lost by the end of page 1).

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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 7:38pm
Personally I've never been of the opinion that because a person is labelled
'beginner' they are bereft of basic intelligence. I still consider myself a
beginner, it doesn't mean I don't want to learn everything there is to know
about my new chosen sport.

And if that means information swap and shared similar experiences, why
deny people that opportunity?

You can always skip reading the post.

If that is you're not just on the look out for something to complain about.
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NickA View Drop Down
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Post Options Post Options   Quote NickA Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 8:46pm

Doesn't a windsurfer "carve" when planing due to the rocker of the board rather than due to the lack of a centre board?  Thought you put the board up to reduce the lift rather than change the steering characteristics.

I'd love a dinghy that carved round corners like skis or a snowboard, but I guess it would need a very flat bottom, lots of rocker and the ability to come very high out the water when planing ........... or hydrofoils even.

.

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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 8:57pm
Originally posted by NickA

Doesn't a windsurfer "carve" when planing due to the
rocker of the board rather than due to the lack of a centre board? 

.



Well it carves due more to the introduction of the lee edge than the rocker
per se, given some boards have a straight rocker for example.

But they wouldn't carve with a plate down.

What would happen, the moment you tilted the board over, the foil would
try to climb to the surface a la foil-foil like foilers, if in fact the board was
even still planing, the plate would slow it down on the turn also.
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Post Options Post Options   Quote tmoore Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 10:10pm
i think what GRF is saying is that the turning circle needed for a carve gybe is greater than the distance that occurs when gybing a boarded boat (because the boat pivots around the board). i think thats it in plain english. but i really am not sure myself
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 11:06pm

GRF, I'm sure the effects of introducing sweep or not into a centreboard on a sailboard are just as you claim, but your explanation of why it works doesn't fit in with any aero-, hydro-dynamic theory that I'm aware of.

The fundamental difference between the way sailboards and boats steer when the hull is canted (heeled), is surely that in the case of a dinghy the rig heels too, while in the case of a board the angle of the rig is independent of the heel of the hull.

People always go on about a dinghy hull steering a boat when it heels 'because the chines dig in' or whatever, but at least at low speeds the steering effect is entirely explicable because of the movement of the centre and line of effort of the rig in relation to the location and direction of the drag. Imho, the steering effect of a dinghy hull is very much secondary to the rig effects, at least until very high speeds when the hull forces possibly increase in significance.

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Post Options Post Options   Quote Black no sugar Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 11:30pm

Originally posted by G.R.F.

Personally I've never been of the opinion that because a person is labelled
'beginner' they are bereft of basic intelligence. I still consider myself a
beginner, it doesn't mean I don't want to learn everything there is to know
about my new chosen sport.

And if that means information swap and shared similar experiences, why
deny people that opportunity?

You can always skip reading the post.

If that is you're not just on the look out for something to complain about.

Sorry GRF, I didn't mean to sound as though I was complaining or implying that a beginner was bereft of basic intelligence. I was only trying to put the conversation into perspective and bring an easy solution to Captain Morgan's puzzlement when he's next on the water.

All this debate about friction and wetted area etc. is nothing short of fascinating but I was just being practical. If someone's lacking intelligence, it must be me. Spare me the acerbic comments, I won't understand them anyway.    

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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 08 at 11:59pm
Originally posted by Medway Maniac

GRF, I'm sure the effects of introducing
sweep or not into a centreboard on a sailboard are just as you claim, but
your explanation of why it works doesn't fit in with any aero-, hydro-
dynamic theory that I'm aware of.


The fundamental difference between the way sailboards and boats
steer when the hull is canted (heeled), is surely that in the case of a
dinghy the rig heels too, while in the case of a board the angle of the rig
is independent of the heel of the hull.


People always go on about a dinghy hull steering a boat when it heels
'because the chines dig in' or whatever, but at least at low speeds the
steering effect is entirely explicable because of the movement of the
centre and line of effort of the rig in relation to the location and direction
of the drag. Imho, the steering effect of a dinghy hull is very much
secondary to the rig effects, at least until very high speeds when the hull
forces possibly increase in significance.



Well BNS is right in so far this is a beginner thread and although I have
been trying to illuminate in generalities, once you start on the Aero &
Hydro dynamic theory and a lot of it is still theory, from what I can
deduce from the countless hours of study in this internet place of joy,
vying Bernouilli with Attack Angle thinking, it begins to get complicated.

All i can say with a degree of practically tested veracity and you'll just
have to trust me on this, is that foils need their foil to operate within the
fluid they operate, be that air or water with the tip parallel to or slightly
aft of the head and they will fight hard to remain that way.

Put the tip ahead of the head and they become prone to stall, try it some
time, if you can jiggle your plate so the bottom is further forward than
the top, they don't like it.

As to steering dinghy's, I'm no expert even with the stick thing they give
you that jams against the hull or gets tangled up with rope when you
need it most, so I bow to your greater knowledge although i can't say I've
noticed any great 'hull steering effect'.
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