Polar diagram for top hat's

[Shaun]

From: Karl010203 (Original Message) Sent: 20/12/2008 22:04
Does anyone have a polar diagram for a tophat (or even a similiar vessel)?

Would love to hear from you if you have one.

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From: shaun_--_ Sent: 21/12/2008 14:40
Hi Karl,
Hadn't heard of a Polar Diagram before now, so had a bit of a look on the internet, still not quite sure i understand them..
found this one.. (click on the "j24polar" file)

http://groups.google.com/group/tophatyachts

Is this the sort of thing your looking for?

Shaun



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From: dougalharris Sent: 22/12/2008 14:33
Hi Karl,

I became intestested in the top hat and joined the group because I am thinking of buying one myself. I really like their underwater hull lines and annecdotal sea-worthiness. I am also a naval architect and have access to sail boat VPP software. I would be happy to generate a theoretical polar diagram for the top hat (and am also interested in doing some stability calcs), however I need some information to do that:

1) Hull data: Lines plan (only needs to be rough I can generate the hull in a surface modeler from a rough lines plan that will be good enough for a VPP).

2) Sail data:
Genoa I
J
LP
Spinaker SPL
SL
SMW
Mast MDT1
MDL1
MDL2
HBI
TL
Main P
E
MGU
MGM
BAS

Terms explained below:

Genoa:
I Height from the sheerline to the top of the foretriangle.
J Distance from the headstay base to the front of the mast.
LP Jib clew to the luff taken perpendicular to the luff.

Spinaker:
SPL Spinnaker pole length
SL Spinnaker luff length
SMW Spinnaker maximum width

Mainsail Definitions:
P Mainsail luff length from lower to upper band on the mast.
E Mainsail foot length from the mast to the boom band.
PC/EC The IOR "corrected" vales of P and C
MGU/MGM The upper and middle girth dimensions of the mainsail.
BAS Height of the lower mainsail luff band above the sheerline.

Mast Definitions:
MDT 1 Athwartships (i.e. beam of the mast) measurement near the deck (Below any taper).
MDL 1 Fore and aft dimension of the mast near the deck (Below any taper).
MDT 2/MDL 2 The same measurements as above taken at the upper mainsail band. If there is no mast taper, the upper and lower dimensions will match and the TL will be zero.
HBI Freeboard at the base of the mast.
TL Taper length of the mast

If we cant get the rig information for i could guestimate most of the dimensions from photos, but i do really need the lines plan. If anyone has a line plan please can you post it in the pictures section (or just attach it as a .jpg to this thread) of the forum, that would be ample to get a good enough hull model for the VPP polar diagram.

Cheers,
Dougal

rigParams.JPG
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From: dougalharris Sent: 22/12/2008 15:07
Ok found a rig diagram on the net (see attached). I should be able to calc all Main, genoa and mast dimensions from this so we now just need the spinnaker dimensions (SPL, SL and SMW) and a lines plan and ill be able to generate a polar diagram. Anyone have this info?
Cheers,
Dougal

TopHatSailPlan.JPG
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From: nswsailor Sent: 22/12/2008 21:19
Dougal,

Two options really, you could wait till somebody slips their boat and measure the hull yourself. Where are you?

Or
Contact Brett Diessel 0402442043 who has the moulds for the 25' and 27' versions. I am sure Brett could help you.

Phillip
SEAKA

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From: Karl010203 Sent: 23/12/2008 02:40
Wow Dougal that is sensational !

Excuse my ignorance but what data exactly do you need from the hull... I might be able to get some photos of the last slip that we can get some measurements from. Would this be of any value?

What a great project if we can produce this!!

As big Kev would say "I'm Excited" not sure he says that much these days however.

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From: shaun_--_ Sent: 23/12/2008 14:16
Welcome to the site, Dougal.

It would be great to have that stability info etc for the Top Hats, we know they feel stable but to have some figures on paper would be an excellent reference.

There is this lay out diagram which may help with some figures

cheers,
Shaun


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From: shaun_--_ Sent: 23/12/2008 14:20
above layout plan as an attachment

thmk2layout.jpg
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From: dougalharris Sent: 23/12/2008 15:34
Hi guys,
Thanks for all the encouragement. Im more than happy to do some calcs for polar diagrams and some stability curves. I will however need the hull geometry information. The lines plan I am talking about is a drawing similar to that in the attached .jpg. It basically fully defines the 3d hull shape in 2d. You actually only really need the body plan view (one on the right) and know the station spacing to have the shape fully defined.
I am currently in Geraldton in WA and there are definately no top hats around here so unfortunately i cannot get to a slipping to do measurements myself. However i am moving to Hobart in January and im sure there will be a few there (actually that is why i am looking into small seaworthy yachts to handle the roaring 40s!). Taking enough measurements off a hull to get a digital model when slipped is relatively straight forward and would probably only take a couple of hours. If someone is planning on slipping soon and has the time to make these measurements please contact me and ill outline the procedure you could use.
Cheers,
Dougal

Sequence Lines.JPG
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From: nswsailor Sent: 23/12/2008 20:04
Dougal,

Just a small hammer in the works, I have become aware that the measurements BAS and HBI are probably different for each Mark, in fact the Mk 3 has lower top to the Mk2 and the Mk 2 a lower top to the Mk1. Will this make a great difference?

The proof of this is to be found by measuring the width of the floor at the sink. To allow for the lowered headroom the floor has been lowered so that the curve of the hull is more exposed with a resultant smaller width of floor. I also suspect that the volume of the water tanks vary over the 3 marks, the normally quoted 159lts being for the mk 1 only!

Phillip
SEAKA


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From: dougalharris Sent: 23/12/2008 20:15
No that is not much of a problem at all, we can just change those values for
the calc of the polar diagrams. About an extra 2 minutes work, however if
the underwater hull shape was different between the different Marks that
would be quite a bit of extra work (as id need to generate 3 surface models
of the hulls for input into the software).
Cheers,
Dougal


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From: nswsailor Sent: 25/12/2008 16:56
Dougal,

As far as we know the hull shape is the same in all Marks, from 0 to 3.

Phillip
SEAKA

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From: dougalharris Sent: 27/12/2008 20:18
Hi guys,
Attached is a polar diagram I generated for the top hat. I must emphasis that this is a very rough calculation as i had to make a lot of assumptions not least being the position of the VCG and the actual hull shape (all i had to go on was the 2d plans and some photos)! Anyway when compared with the j24 i think the results for the top hat are roughly what i would have expected. I would be very interested in any seasoned top hat sailors opinions as to how accurate this polar plot appears to be. Also any rough calcs taken while on the water and compared with the polar plot would be interesting.
Note the two sets of curves are for the upwind and downwind sail sets.
cheers,
Dougal

TopHat_PolarPlot.JPG
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From: nswsailor Sent: 28/12/2008 15:40
Dougal,

Many thanks for your efforts.

I've had a look around but have not found any clear instructions on how to use a Polar Plot.

Can you help us?

Phillip
SEAKA

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From: dougalharris Sent: 28/12/2008 17:59
Hi Phillip,
As an example. Say you want to predict the theoretical velocity of your top hat sailing at 60 degrees to the true wind direction (0 degrees is straight into the wind 180 degrees is sailing directly downwinds) with jib and mainsail up and a wind speed of eight knots. In the attached diagram follow the 8 knot wind curve around until it crosses the 60 degrees true wind angle line, this is marked as point A in the diagram. Then follow this point around in the concentric rings to point B to read off the hull speed (approx 4.8 knots).
As i mentioned before the two curves for each of the wind speeds are for the upwind sail set (main and jib) and downwind sail set (main and spinnaker). In this way the polar plot is useful for determining at what wind angle you should change from upwind to downwind sail set (as can be seen from the diagram this angle is different for differing wind strengths).
Kind regards,
Dougal


TopHat_PolarPlot_Explained.JPG
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From: Karl010203 Sent: 31/12/2008 14:31
Great work Dougal - I will print this out and when my brain is functional I will work through this.

For those wanting to learn more about polars the following is a good article...

http://www.sailingworld.com/from-the-ex ... 61206.html

It is not just for the racing set either... I have read a number of practical books that discuss the fact that navigating by rhumbline or great circle is not the fastest way to get between two points offshore but knowing the boats polar.

Dougal might be able to illustrate this through the polar calculation... with the following question - if that is ok?

If we want to sail 50nm in a NE direction with true wind coming from N at 8 knots... what is the optimal course to steer / tack?

THe reason for the polars is that each type of boat has its own particular characteristics hence all the information needed for these calculations.

Happy 2009 all !


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Recommend Delete Message 17 of 17 in Discussion

From: dougalharris Sent: 31/12/2008 16:31
Hi Karl,

It is all about VMG's (velocity made good). I think the best way to explain it is with the use of your example. Using the diagram I have attached. To calc the quickest way to get to the mark 50 NM to the NE in a Northerly wind:

1) draw a line from centre in the mark direction (45 degrees), this is line C->D in the attached diagram.
2) draw a line perpendicular to CD (right angles) and draw it towards the centre of the polar until it just touches the 8 knot polar line, This is line A->B in the attached.
3) Mark the tangent point (T)
4) Draw a line from the centre through T to the outside circle. That is line CE in the attached.
5) the best VMG angle is then read off the circle and in this case is 70 degrees.

Now obviously if you followed this angle without tacking you would miss the mark! In practice the time to tack is based on wind shifts and when you tack again you run through steps 1-5 to calc the new vmg.

With skiffs the polar diagram for the downwind sail sets normally has a large bulge at about 150 degrees. Using the above method it can easily be explained why they gybe all the way down the downwind leg rather than sticking to the rhumb line.

Cheers,
Dougal

KarlsQuestion.JPG

[karl010203]

I have lost this diagram is there any way of retrieving this from archives or previous site... does anyone have a copy saved?

Thanks

Karl

[auriga]

this one?

http://tophatyachts.googlegroups.com/we ... XZc5buSfmx

[karl010203]

Hey Dan

Actually chasing the one labelled TopHat_PolarPlot.JPG

It was fantastic and specific to our vessels

[masquerade]

Hi Dougal and others,

I have a TopHat 25 Mk 1 in Perth, WA, and will most likely be slipping sometime in the not too distant future.
So if you need me to make measurements etc I am more than willing to do so.
On a side note, I am interested in finding out the range of difference between the different Mks (or even within a specific Mk) as I know there is quite a range of rigs.

Cheers,
Derek
Masquerade

[karl010203]

Guys

Dougal has provided the polar plot as attached.

We are very lucky to have this information in my opinion...