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Old Jan 26, 2015, 05:26 AM
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New Zealand, Auckland
Joined Apr 2012
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Some questions from first time scratch-builder

Hi all.
I am scratch-building a sailboat.
I normally fly RC helis. I will be controlling this boat with my DX7s.
I have sailed yachts but never owned an RC one.

I am under no illusions about this being a racing boat, I just want to see what parts I can do myself.
Maybe when this is done I'll build another one using what I learn to make a better one.

The boat is just under 1m long.

I made the hull out of balsa and have fibreglassed the bottom.

I decided on a keel length and ballast weight by looking at commercially available models of a similar size.

I will be buying a mast and sails from a kit to complete the rig. Probably from HK.

I have been reading up on CE and CLR and am confident that I can measure and place these accurately.

Next I need to cast my keel bulb.
I have bought lead and made a plasticine blank with the correct mass. (slightly over-weight as I will recess for the keel and drill for the bolt)

My fist questions is: what shape should the bulb be? I was looking at yacht keels today at the maritime museum and they come in a variety of shapes. Symmetrical? Cigar shaped? Flat top and bottom?

My second question is: I have been reading lots about casting lead and I was thinking of making a concrete mould. I have some smooth cement left over from a job that would work well. Is there any reason no-one else uses concrete? I would rather stay away from plaster as I don't want to risk blowing it and I'm not interested in buying expensive mould stuff.

Any opinions welcome.

I haven't been taking photos of the progress unfortunately. Hopefully I will have some to add sooo.
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Old Jan 26, 2015, 06:17 AM
Rusty
Rusty Nail's Avatar
Auckland, New Zealand
Joined Feb 2006
943 Posts
Welcome. You have chosen a difficult road, but that is sometimes more rewarding.
The easier way is to follow a plan particularly one taken from a common class like the IOM (International One Metre). This site has many tips on building that will suit any project.

Bulbs are easiest made with builder's strip lead sheeting. Glued together, filed and faired with epoxy putty. All casting is fraught and somewhat dangerous from the toxicity of the lead to dealing with very hot and potentially splattering metal. Precast suitable bulbs are also commercially available in lead or steel.

The best shape for a bulb is a cylinder as this is non lifting (unlike the usual airfoiled bulbs). Think of an airliner body shape.
Conventional bulb graph attached.

If you want to make the first hull a mold, there is a good tutorial here. On that site you will see many useful contributions from Claudio who is a naval architect and reading those will help with practice and theory in scratch designing and building.
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Last edited by Rusty Nail; Jan 27, 2015 at 06:15 AM. Reason: Added link to Claudio's design thread.
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Old Jan 26, 2015, 06:28 AM
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Joined Aug 2009
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custom keel bulbs available

If You Like.....I HAve New Aluminum Molds And Have Casted 30To40 Bulbs Of Various.Weights. Dean D
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Old Jan 27, 2015, 12:36 AM
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New Zealand, Auckland
Joined Apr 2012
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Quote:
Originally Posted by Rusty Nail View Post
Welcome. You have chosen a difficult road, but that is sometimes more rewarding.
The easier way is to follow a plan particularly one taken from a common class like the IOM (International One Metre). Their site has many tips on building that will suit any project as does this.

Bulbs are easiest made with builder's strip lead sheeting. Glued together, filed and faired with epoxy putty. All casting is fraught and somewhat dangerous from the toxicity of the lead to dealing with very hot and potentially splattering metal. Precast suitable bulbs are also commercially available in lead or steel.

The best shape for a bulb is a cylinder as this is non lifting (unlike the usual airfoiled bulbs). Think of an airliner body shape.
Conventional bulb graph attached.

If you want to make the first hull a mold, there is a good tutorial here. On that site you will see many useful contributions from Claudio who is a naval architect and reading those will help with practice and theory in scratch designing and building.
Thanks for this information. It is very helpful, especially the info on Lester Gilbert's site.
I am pretty invested in pouring a keel bulb myself rather than fabricating one from sheet lead. I'll see how it goes.

I don't want to use this hull as a mould, it's probably not the best section as I built it purely by taste and not at all scientifically. I think the bottom is too flat particularly towards the stern (longitudinally) and the profile of the beam is probably not ideal either. This is really just a project to see "if I can do it."

Quote:
Originally Posted by Derusha View Post
If You Like.....I HAve New Aluminum Molds And Have Casted 30To40 Bulbs Of Various.Weights. Dean D
Thanks for the offer but I'm in New Zealand and shipping a lead bulb will probably prove expensive and I'm also interested in seeing if I can do a good one myself.
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Old Jan 27, 2015, 12:59 AM
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Canada, BC, Greater Vancouver Regional District
Joined Dec 2011
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For $4 you can buy a bulb from HK and fill it with lead shot:
http://www.hobbyking.com/hobbyking/s...strSearch=bulb
For another $4 you can get the fin too.
Since this is your first build, I would suggest you take the path of lesser resistance and get this on the water ASAP to get some stick time.
Find out what works and what doesn't and spend the effort on the second build, if you find you like RC sailing.

...just my 2cents.
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Old Jan 27, 2015, 03:02 AM
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New Zealand, Auckland
Joined Apr 2012
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Originally Posted by Scratchy101 View Post
For $4 you can buy a bulb from HK and fill it with lead shot:
http://www.hobbyking.com/hobbyking/s...strSearch=bulb
For another $4 you can get the fin too.
Since this is your first build, I would suggest you take the path of lesser resistance and get this on the water ASAP to get some stick time.
Find out what works and what doesn't and spend the effort on the second build, if you find you like RC sailing.

...just my 2cents.
The HK bulbs are are nice but this isn't about the money. If I wanted a cheap boat on the water I would buy the Poseidon HK has in scratch&dent.

I'm quite keen on casting the lead for this boat. In fact that was my original inspiration for this project. It was just coincidence that I had a 92cm balsa yacht hull sitting in my shed.

I looked at buying the fin but decided against it as I was fabricating my own from glassed balsa. But I ballsed it up and then saw a lonely 520 heli blade on a shelf left over from a crash. It's the same aspect as the fin I was building and has a better cross section AND it's made of carbon-fibre. So that takes care of the fin part anyway.

I'm not too worried about rushing to get the job done. The more I do myself the more satisfaction I will have out of the finished model. I'm already buzzing pretty hard about how well the hull has come out.

I always liked red
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Old Jan 27, 2015, 03:12 AM
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New Zealand, Auckland
Joined Apr 2012
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And on the note of heli blades I just noticed that a 450 blade has the same aspect as the rudder I was fabricating... Into the bin with it. This one already has a nice symmetrical pattern on it too. Good on CY Radix! Hacksaw... finished.



Will a 9g servo be enough for the rudder? I have a few lying around.
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Old Jan 27, 2015, 04:31 AM
Rusty
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Auckland, New Zealand
Joined Feb 2006
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Should be, with a balanced rudder. Put the hinge line in 20-25% and let the water help the rudder turn. 450 blade is pretty narrow so make it long and as far back on the hull as possible. You can always cut it shorter. Max deflection to minimize excessive drag is 30 degrees. Make it accessible for replacement.

I use heli blades for keels too, no point in making the build harder than it needs to be. Have you worked out the weight of the bulb to get your displacement and the needed length of the keel. Will depend on the winds you are sailing in and the size of the rig. If you make the bulb and/or the keel removable then you can also experiment.
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Old Jan 27, 2015, 05:12 AM
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hi Mstenbo

hull looks great, well done If this hasn't been mentioned before there is lots of good info in the following pdf...

http://www.theamya.org/boats/us1m/pdf/us1mconst.pdf
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Old Jan 28, 2015, 02:26 AM
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New Zealand, Auckland
Joined Apr 2012
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Quote:
Originally Posted by Rusty Nail View Post
Should be, with a balanced rudder. Put the hinge line in 20-25% and let the water help the rudder turn. 450 blade is pretty narrow so make it long and as far back on the hull as possible. You can always cut it shorter. Max deflection to minimize excessive drag is 30 degrees. Make it accessible for replacement.

I use heli blades for keels too, no point in making the build harder than it needs to be. Have you worked out the weight of the bulb to get your displacement and the needed length of the keel. Will depend on the winds you are sailing in and the size of the rig. If you make the bulb and/or the keel removable then you can also experiment.
Should the rudder be raked backwards? How much?
I was going to put the rudder tube right through to the top of the hull (there is no cut-out on the stern) so replacement should be a piece of cake.

I haven't worked out the bulb weight but was just going to copy the monsoon weights as my boat is a similar length and I will be using the rig. My keel will be a little longer than the monsoon.

Quote:
Originally Posted by tim_uk View Post
hi Mstenbo

hull looks great, well done If this hasn't been mentioned before there is lots of good info in the following pdf...

http://www.theamya.org/boats/us1m/pdf/us1mconst.pdf
That is a great document!
Actually it solves one of my problems which was how to do the blocks.
I really like the silicone tubing in a balsa block.

But it got me thinking... I have heaps of very small ball races lying around from my helis so I got out my dremel and put a groove around one of my smallest. If I put it on a BBQ skewer shaft and make a balsa block I should have a very efficient pully for about 30 cents. Anyone got any thoughts on this? I can't be the first to come up with this idea...

For size reference, it is on a skewer.
It's a 1.5x5x2.5 bearing.
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Old Jan 28, 2015, 03:04 AM
Rusty
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Auckland, New Zealand
Joined Feb 2006
943 Posts
If you rake the rudder it will have the effect of pushing the bow down in a turn - not a good idea.
The Chinese boats are all known as tender. It is generally found that the bulbs cannot be made heavy enough for sailing in any moderate wind without a reduced rig. A general rule is the bulb should be at least 60% of the sailing weight, depending on the keel depth. Their rigs are also generally found needing replacement for good sailing. Masts need reinforcing and sheets not suitable material. Lots of threads on these boats so care in choosing may find a usable set.
You might look to a simple unstayed Unarig like the McCormack for a solution.
The set up will depend on sailing conditions and the displacement and mass of your hull and gear, so look to those sailing in the area you live. Onepoto would be a good reference. Mixed sailing on Tuesday and Thursday mornings (not Seawinds or Electrons).

Sheets go slack so you need something to keep them engaged with the block. A simple solution being more used in some classes is to mount a weathered sail servo on the deck. Look at the Stollery Power Lever for a sensible solution.
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Old Jan 28, 2015, 04:14 AM
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New Zealand, Auckland
Joined Apr 2012
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Quote:
Originally Posted by Rusty Nail View Post
If you rake the rudder it will have the effect of pushing the bow down in a turn - not a good idea.
The Chinese boats are all known as tender. It is generally found that the bulbs cannot be made heavy enough for sailing in any moderate wind without a reduced rig. A general rule is the bulb should be at least 60% of the sailing weight, depending on the keel depth. Their rigs are also generally found needing replacement for good sailing. Masts need reinforcing and sheets not suitable material. Lots of threads on these boats so care in choosing may find a usable set.
You might look to a simple unstayed Unarig like the McCormack for a solution.
The set up will depend on sailing conditions and the displacement and mass of your hull and gear, so look to those sailing in the area you live. Onepoto would be a good reference. Mixed sailing on Tuesday and Thursday mornings (not Seawinds or Electrons).

Sheets go slack so you need something to keep them engaged with the block. A simple solution being more used in some classes is to mount a weathered sail servo on the deck. Look at the Stollery Power Lever for a sensible solution.
I saw that powerlever in your bottle build.
I can make a simple balsa block to keep the cord engaged with the tackle.

The monsoon calls for a 1.2kg bulb. I am going for around 1.3 and my keel is longer. But I notice the IOM bulbs are over 2kg.

I started making my keel mould this evening. I decided to go with a plaster mould. Maybe I should start again and go heavier. My plaster box is big enough so I would only have to throw away the 1kg plaster I poured already...
I haven't worked out sailing weight yet, but it's not light...
I haven't ordered the rig yet.

Actually I just weighed my bare hull at 460g. So that would leave me around 500g for rig and electronics. Should be close to 60% bulb.
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Old Jan 28, 2015, 05:37 AM
Rusty
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Auckland, New Zealand
Joined Feb 2006
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Should be OK.
A similar well regarded commercial entry level boat is the Nirvana with a weight of 2.5kg and a bulb of 1kg to bulb bottom with 30cm keel (hull bottom to bulb bottom. Rated at 20 mph winds with 3390 sq cm sails and a big beam.
In contrast a well designed, home built similar length is 2.5kg with 1.75kg bulb on 42 cm keel (attached).
IOM's are minimum 4kg by old class restrictions with 2.5kg bulbs and carry dead weights!
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Old Jan 28, 2015, 08:23 AM
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Canada, ON, St. Catharines
Joined Jan 2013
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Check out the thread that I posted ' Melting and Casting Lead Keel Weights' . I received a lot of good help and advice on this topic from a lot of fellows with their own experiences making molds and melting and pouring the lead. I am sure you will find the help you need to be able to make your own castings.
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Old Jan 28, 2015, 04:44 PM
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Australia, TAS, Penguin
Joined Mar 2012
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Quote:
Originally Posted by mstenbo View Post
The monsoon calls for a 1.2kg bulb. I am going for around 1.3 and my keel is longer. But I notice the IOM bulbs are over .
A Monsoon is hopeless with 1.2 kg despite what the HobbyKing website says. Around 1.7 kg for a Monsoon is good
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