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Author Topic: Nobby - Morecambe Bay Prawner  (Read 113121 times)

rmaddock

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Re: Nobby - Morecambe Bay Prawner
« Reply #350 on: December 24, 2012, 02:22:44 PM »

You should go into supplying your model rigging fittings!

Yeah...just before they lock me up for endless mad cackling  %%


MERRY CHRISTMAS WORLD!
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rmaddock

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Re: Nobby - Morecambe Bay Prawner
« Reply #351 on: December 24, 2012, 03:42:30 PM »

Now, once the blocks were up, I had stopped modelling for Christmas.

But I just couldn't let it lie  :embarrassed:

I've been plotting sail plans:



Thanks to my other favourite Nobby book, "The Lancashire Nobby" by L.J.Lloyd, I've been studying standardised sail plans and trying to match them to the model.  As we have guests due to arrive NOW, I shall hold fire on creating full sized paper/card templates to check for Centres of Effort (and all that).

Merry Christmas AGAIN!!!
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Neil

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Re: Nobby - Morecambe Bay Prawner
« Reply #352 on: December 24, 2012, 05:27:07 PM »

eye, good ol' Len Lloyd, he used to bore the pants off us at Fleetwood club every single winter when we used to have nights where we'd get each other to speak on a topic of interest.........some sharp whit would always say to Len......"now Len................. you haven't told us how to calculate the centre of G and the metacentre of a ship for a while".....he knew they were taking the mick, but it was they who would suffer as he would then go into the process in great deapth......and would string it out for at least two hours, lol......those nearest to the door would get to the pub before closing time......those that weren't would inevitably fall asleep...........god bless him.....he was a character.

on a serious note...........there were no lines ever taken from a nobbie in history and no nobbie was ever built from plans and lines....just the craft master builder who drew them out on a floor........the lines in that book were taken personally by Len over a period of weeks from a specific prawner using his perps, horizontals and levels, and put onto a scale graph in a notebook that he kept.I know as he roped me into standing with the pine perpendicular poles one Saturday on a very cold day......never volunteered again,lol

He then took the measurements and transposed them into the lines you have today, and all the lines ever seen these days in books about the nobbie/prawner, are taken from those original measurements he took.......he was a very bright man, and a good, just and considerate man too.....It was a privaledge and I was glad to have known him for a good number of years.

neil.
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rmaddock

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Re: Nobby - Morecambe Bay Prawner
« Reply #353 on: December 26, 2012, 07:22:05 PM »

In the ongoing spirit of putting the model away for Christmas, I have made progress today {-)

I've made some mock-up card sails for her:



Superb!  :-))

These will do double service.  First, I will use them to calculate the CoE of the rig and then, when I'm happy with that, I'll use them as templates for the real sails.

Their size and shape is, naturally, a compromise.  Having reduced the height of the rig for scale reasons, I can't simply copy the sail plan.  Rather, I've measured from the model and then tried to capture the general line and "feeling" of the sail shape - for example the way the lower edge of the jib goes across the bottom corner of the stay sail.

For now though, the bits of card are staying put as I like looking at them.  :embarrassed:
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mrpenguin

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Re: Nobby - Morecambe Bay Prawner
« Reply #354 on: December 26, 2012, 10:22:21 PM »

.......  Having reduced the height of the rig for scale reasons, I can't simply copy the sail plan.  Rather, I've measured from the model and then tried to capture the general line and "feeling" of the sail shape - for example the way the lower edge of the jib goes across the bottom corner of the stay sail.

For now though, the bits of card are staying put as I like looking at them.  :embarrassed:
I agree that it is looking very nice indeed, well done.
Be wary of the overlapping sails - while it looks "right", you may find it technically quite complex to radio control your overlapping jib. It is for this reason that the common RC sloop rigs generally have a self-tacking jib with a boom instead of an overlapping soft footed genoa. You may however be able to pair up the jib and staysail and radio control them as one, similar to how flying jibs are often done on the RC square riggers.
It may also be that you will need to winch the mainsail separately to the foresail pair (jib & staysail) to enable you to steer........ I understand this is the case with the RC square riggers - the rudder has only a minor effect on the steering and the sails need to be swung to steer....
 
 
 
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rmaddock

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Re: Nobby - Morecambe Bay Prawner
« Reply #355 on: December 27, 2012, 11:38:28 AM »

Mr Penguin,

Thanks for your warning, but you've been a bad boy and not done your revision!  <*<

She's already fitted with a grand total of 3 winches; one main sheet winch, one port foresail winch and one starboard foresail winch.  A cunning bit of computer trickery is already in the tube to allow realistic tacking of the foresails; i.e. there will be a "tacking" state in the system where both winches are fully wound out. On leaving the tacking state, one side will winch in under stick control.  At the moment, this will be proportional to the main sheet but I'm wondering about using something like a flap knob to allow be to alter this...effectively moving the CofE backwards and forwards in flight.

Your point about getting the sails across each other may still be valid though...I shall give it a go and see what happens.  I can always make the sails smaller.
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gondolier88

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Re: Nobby - Morecambe Bay Prawner
« Reply #356 on: December 27, 2012, 01:26:20 PM »

I look forward to seeing this system working, it should look very realistic with the foresail able to let fly with both winches out, then hauling back in on the other side. It will be one of those that all your thinking should pay off nicely, but if it doesn't it's going to be a pig to try and find where it's not working properly.


It just gets better and better this little boat.


Greg


PS. check this out; http://forum.woodenboat.com/showthread.php?88301-Cockle-Replica-Post-1 a lovely bit of inspiration if ever there was.
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mrpenguin

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Re: Nobby - Morecambe Bay Prawner
« Reply #357 on: December 28, 2012, 03:17:20 AM »

Mr Penguin,

..... you've been a bad boy and not done your revision!  <*<

She's already fitted with a grand total of 3 winches..........

Your point about getting the sails across each other may still be valid though...I shall give it a go and see what happens.  I can always make the sails smaller.
Oops, thought I had been right through the thread, missed that section completely....
The two separate winches for the forward sails should be fine. That should also deal with overlapping sails I would imagine. I had assumed you would be working on one winch for main and one for jib & staysail...
This will be a great vessel when completed, you are doing a terrific job...
 
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rmaddock

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Re: Nobby - Morecambe Bay Prawner
« Reply #358 on: December 28, 2012, 08:21:18 AM »

Oops, thought I had been right through the thread, missed that section completely....

I forgive you  ;)

This will be a great vessel when completed, you are doing a terrific job...

Thanks.  Personally, I'll be staggered if any of it actually works.  :D
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rmaddock

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Calculating my Center of Effort
« Reply #359 on: December 29, 2012, 07:34:48 PM »

Having carefully marked the sail templates whilst in situ, I stuck everything together in as minimal a fashion as possible.
I then punched some holes around the perimeter and arranged to hang the whole thing from a pin in the best room wall  :embarrassed:



A plumb weight (aka my multi-tool) dangled and its line marked in pencil on the sails.



Repeat a number of times at different angles.

Then, using the super piece of ali curtain rail that came to light in the cellar, join up all the dots!



It was very gratifying to observe that all the lines crossed in the same place. Phew!  {-)   Now, this is the Centre of Effort (CoE) for this rig.  I need to check the drawings and see if there's a centre of lateral resistance (CLR) marked..otherwise experiment and find it.

As I understand the theory, you optimally want your CoE to be slightly behind the CLR.  This means that the wind tends to push the back of the boat out and thus the bow upwind.  This is called weather helm and a bit of it is a positive safety feature in full sized boats.  If the wind gusts suddenly, it's better to be pushed into the wind and stall than out of it and blow flat.

Of course, all this counts for naught as they're static CoE and static CLR.  All bets are off when the boat's moving  %%
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Kim

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Re: Nobby - Morecambe Bay Prawner
« Reply #360 on: December 29, 2012, 11:01:17 PM »

Interesting, I thought that CoE would be better plced directly above CLR in model boats ..
a bit like the ballast debate in model boats ... fast roll = better stability .. but out of scale ...blah ...blah
Then their is the 6 degrees of freedom ....
 
But hey, regardless you have made a fine job of her and i'm sure she will sail just fine!
Regards,
Kim
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dreadnought72

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Re: Calculating my Center of Effort
« Reply #361 on: December 30, 2012, 09:20:32 AM »

Of course, all this counts for naught as they're static CoE and static CLR.  All bets are off when the boat's moving  %%

And heeling - the CLR can move fore or aft when the hull's not vertical.

But finding the CoE that way is a neat job. Are you going to do it for other sail settings - reefed, no jib, etc.?

Andy
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Lothar

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Re: Nobby - Morecambe Bay Prawner
« Reply #362 on: December 30, 2012, 04:06:27 PM »

Very good shown and explained!
 
But shouldn't be the COE in front of the CLR? When I design my yachts, I put the COE abot 8 percent or more of the waterline-length in front of the CLR. This make stabil sailing boats.
 
Regards
 
Lothar
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dreadnought72

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Re: Nobby - Morecambe Bay Prawner
« Reply #363 on: December 30, 2012, 08:09:31 PM »

In full-size sailing it's unsafe to have lee helm - the CoE infront of CLR. A little weather helm - CoE behind the CLR - is preferred.

With the rudder set to neutral I'd sooner have the boat turn slowly into the wind and the sails stall, than turn away from the wind and ultimately gybe.

Having never sailed a model sailing boat things could be different ... but I'm unsure as to why they would be. There is the possibility that your vessel's CLR moves in front of the C0E when heeled, close-hauled. A bath check may be in order?  :-)

Certainly my dinghy sailing experience made me realise that the Enterprise I had, required severe mast rake aft to move the CoE back far enough, else the rudder was all-but stalling when close-hauled = inefficient.

Andy
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Lothar

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Re: Nobby - Morecambe Bay Prawner
« Reply #364 on: December 30, 2012, 10:41:51 PM »

Yeah Andy,
 
for full size boats You must be right. I haven't much experience on real boats.
 
For a model the circumstances may be different, because the skipper doesn't feel the pressure on the tiller. The model-skipper isn't able to act, if there is coming gusty winds. He can only REact and this is always to late. So You will get a boat, which may sail not to bad upwinds, but very nervous with half wind or downwinds.
 
This all depends on books of the 1970's by F.K.Ries (he was one of the godfathers of German modelsailing) and my own experience in designing model-yachts.
 
I just try to help....
 
Regards and good luck!
 
Lothar
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Kim

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Re: Nobby - Morecambe Bay Prawner
« Reply #365 on: December 31, 2012, 09:11:38 PM »

I mentioned 6 degrees of freedom,
When the hull heels we change the waterline shape and if heeling to lleward the response is to sail to windward ?  so maybe coe ahead of clr is prefered in rc when we can't shift balast?
 
regards
Kim
 
 
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Lothar

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Re: Nobby - Morecambe Bay Prawner
« Reply #366 on: January 03, 2013, 10:42:19 PM »

Well,
 
may be I don't understand English good enough. Sorry for this.
 
Here is the actual sketch of a really good sailing yacht as it can be seen sailing here:
 
http://www.youtube.com/watch?v=dNuCEQ5D2Js
 
Please notice the boat built from oak.
 
The sketch:
 

 
Lothar
 
 
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mrpenguin

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Re: Nobby - Morecambe Bay Prawner
« Reply #367 on: January 03, 2013, 11:22:48 PM »

Lothar,
 
Thank you for the beautiful video - they are lovely boats....

I am not sure if the method rmaddock used for finding COE is actually valid when you have overlapping sails as these genoa-rigged boats have.
While the Nobby has some overlapping sail, it is only a small percentage of the entire sail area, while your boat has a large percentage of the sail area overlapped.

Perhaps it may be more accurate to find the COE of each sail and then combine them taking the comparative sail areas into account.... this works of a simple Bermuda sloop) rig but with these more complex sail plans I am not sure...
I am not an expert, just thinking out loud ok2
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dreadnought72

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Re: Nobby - Morecambe Bay Prawner
« Reply #368 on: January 03, 2013, 11:37:41 PM »

I am not sure if the method rmaddock used for finding COE is actually valid when you have overlapping sails as these genoa-rigged boats have.
While the Nobby has some overlapping sail, it is only a small percentage of the entire sail area, while your boat has a large percentage of the sail area overlapped.

You may have it here, Mr Penguin.

1/ The taller, high-aspect ratio main is more efficient than the foresail. => moves CoE aft.

2/ The overlapping genoa creates lift itself, but is designed to accelerate air over that main, making the main provide more power. => moves CoE aft.

I suspect, Lothar, were we to test your vessel in a wind tunnel, the true CoE is much further aft than basic area calculations would suggest. And therefore your Rule of Thumb for CoE/CLR placement works in practice.

Andy
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Lothar

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Re: Nobby - Morecambe Bay Prawner
« Reply #369 on: January 04, 2013, 06:58:16 AM »

Well,
 
the COE's are are calculated for each sail and afterwords added. I cannot explain in english, but I am working as a plant designer at CAD every day, believe me, I know hat I am doing if I calculate a Center of gravity of a sail area.
 
Please keep us informed, what is the result in sailing with the COE behind CLR theory. Be sure, I am very interested.
 
Tonight I found something on the net. It's in German, but explains all. Maybe somebody is able to translate.
 
http://www.minisail.ch/de/werkkunde/stabilitaet/stabilitaetballast.htm
 
Best regards
 
Lothar
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rmaddock

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Re: Nobby - Morecambe Bay Prawner
« Reply #370 on: January 04, 2013, 12:09:31 PM »

It's making my brain hurt  %%
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Norseman

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Re: Nobby - Morecambe Bay Prawner
« Reply #371 on: January 04, 2013, 07:44:40 PM »

Might I suggest a bottle or two of the Red sir  O0

Dave
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mrpenguin

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Re: Nobby - Morecambe Bay Prawner
« Reply #372 on: January 04, 2013, 09:41:07 PM »

Not sure if red wine will sop your brain hurting {-)
There is a good discussion of this at http://pages.swcp.com/usvmyg/sails/sail1.htm
The image attached (borrowed from that site) shows how the combined COE of two sails is offset to the larger sail. With this simple bermuda rig, the reference point for all measurements is the tach of the jib.

 
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Neil

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Re: Nobby - Morecambe Bay Prawner
« Reply #373 on: January 04, 2013, 11:03:08 PM »

It's making my brain hurt  %%

Mine too............I haven't got a scooby doo what these guys are on about................and I have a mizzen a main and an aft sail to worry about on my lifeboats.........don't think I'll bother calculating anything except how long the receiver battery will last %% %% %% %%
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Jerry C

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Re: Nobby - Morecambe Bay Prawner
« Reply #374 on: January 04, 2013, 11:35:28 PM »

Don't get hung up on the technicalities of cofe and clrs. They are static and approximate starting points. You've built a nobby, they sail, yours will too. Sailing is all about playing with balance. Reef the main, you get lee helm so strike the jib, you get the balance back. Ever lost your rudder and got back without it. You can when you have to. Stay calm, drink the wine.
Jerry.
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