Model Boat Mayhem

Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: tigertiger on February 13, 2009, 10:27:47 am

Title: Scaling up ballast.
Post by: tigertiger on February 13, 2009, 10:27:47 am
I am thinking about scaling up a model at 1:10 scale to 1:6 scale.

the lengths will be easy to calculate, I am assuming
Y = (X/6)*10     Where Y = new length and X= old length.

But what calculations should I use for ballast?
I assume it is the same for volumes. Is this a fair assumption.

The reason I am talking about ballast is the the original plans suggest 4kg of lead ballast in the keel. So I am thinking about how much lead I need to aquire.
Title: Re: Scaling up ballast.
Post by: dreadnought72 on February 13, 2009, 11:53:36 am
Your assumption is spot-on. Masses scale the same as volume - this is because the density of a substance doesn't change, whether you've a teacup or a mountain of the same "stuff".

So, your 4kg of ballast will rise to 18.6kg - though that could be affected if you build the bigger hull "heavier".

Andy

Title: Re: Scaling up ballast.
Post by: tigertiger on February 13, 2009, 12:52:08 pm
Hi Dreadnought.

Thanks for that.
What formula did you use?
Title: Re: Scaling up ballast.
Post by: dreadnought72 on February 13, 2009, 12:59:11 pm
(10/6)^3

(The linear increase being 10/6, and this figure then cubed, since we're dealing with a volume. Multiply that result by the original mass of 4kg.)

Andy
Title: Re: Scaling up ballast.
Post by: tigertiger on February 13, 2009, 02:07:10 pm
I think if I build the beast it will have to be ballasted with concrete. Or water.
Cannot afford that much lead.
Title: Re: Scaling up ballast.
Post by: dreadnought72 on February 13, 2009, 02:11:13 pm
Tigertiger, what's it a model of? Sailing vessels might still benefit from low-slung lead - but for powered displacement hulls you could well get away with concrete.

Andy
Title: Re: Scaling up ballast.
Post by: tigertiger on February 13, 2009, 02:14:26 pm
It is a sailing vessel.

The question is, for a keel bulb, is it mass or weight that matters.

If I fill a keel bulb with water, it would arguably be neutrally buoyant. But would still require force to swing it.
I could try using steel, but this is also expensive. But I have to pay about 8 quid a kilo for lead.
Title: Re: Scaling up ballast.
Post by: tigertiger on February 13, 2009, 02:18:37 pm
BTW on the 1/10 scale model the ballast is a lead plug in a hollow keel. So no keel bulb.

So if I place 19kg of concrete in the hull would this work.

Consider that the effects of scale (re: heeling forces) will be reduced as the scale is larger. So this is about getting down to the water line.
Title: Re: Scaling up ballast.
Post by: dreadnought72 on February 13, 2009, 02:55:18 pm
Keel bulbs really require mass.

A roll along the boat's axis (for example, when the wind blows sideways across the boat) is countered in two ways:

The keel bulb moves "upwind", while the "downwind" half of the hull takes on the bulk of the buoyancy. So there's a downforce (the mass of the keel bulb) separated from the upforce (the buoyancy). When the hull is not heeled these forces are in a line and matched, but when rolled they form a "moment arm", as the forces are offset, and together they provide a torque which helps right the boat.

The fin is a handy support for the bulb, but it's mainly there to prevent sideslip - reduce leeway - when sailing upwind.

You mention that the model has a hollow keel. If it's a long keel - large, like a traditional working boat - then I would consider dumping as much lead and scrap iron into it as you can, before topping it off with concrete.

You're right in thinking the heeling forces will be reduced - 1/6th scale is big, and less affected by the scaling effects that drive smaller boats to sail with false keels and/or large bulbs.

Andy

Title: Re: Scaling up ballast.
Post by: Damien on February 14, 2009, 12:00:50 pm
TT I source scrap lead from Tyre shops for the scrap price, wheel weights are not used twice, when melting it produces a lot of "xxxxx" and the steel bits that hold the weight to the wheel.
Apart from that lead is still lead, several shops have given me 5kg lots over the years.
A word of warning BE EXTREMELY CAREFUL WHEN SMELTING LEAD THE SLIGHTEST AMOUNT OF MOISTURE  WILL CAUSE IT TO EXPOLDE ALL OVER THE SHED and YOU.
AND WEAR A FACE MASK THE FUMES ARE NASTY.

Damien.
Title: Re: Scaling up ballast.
Post by: dreadnought72 on February 14, 2009, 12:50:28 pm
Agree with Damien's warnings!

Additionally, if you wish to fill a keel with cast lead, you'd need to make a plug of the hole to be filled and then mould this in plaster or similar. Dropping in shot/cut chunks of lead directly into the hull, and pouring in epoxy to seal it would be a whole lot less dangerous. Just don't add too much mass!

Andy
Title: Re: Scaling up ballast.
Post by: derekwarner on February 14, 2009, 12:53:41 pm
Gents.....as a member has suggested.....I am thinking about scaling up a model at 1:10 scale to 1:6 scale

This is a very complex issue....sorry ...work has called after hours @ 1 minute to midnight,,,,.  >:-o.......will come back tomorrow ............ Derek
Title: Re: Scaling up ballast.
Post by: BobF on February 14, 2009, 06:22:55 pm
Hi tigertiger,
What type of sailing model is this please, also what is the length of the hull  and the beam.
Title: Re: Scaling up ballast.
Post by: tigertiger on February 15, 2009, 02:21:41 am
The intended model was LOUIS HELOISE, a small french sailing fishing boat.

The original hull length is about 110cm (+bowsprit), beam 420cm.

I want to scale boats up, because for most sailing conditions where I have sailed, the first 10-30cm above the water, the wind is a problem. And for boats under 1m, that is where most of the sail area is.

I have also experienced problems with choppy water, and as the boat rolls the booms swing and let out the wind from the sail. So even with a light breeze boats can be dead in the water.

I can see that my biggest issues will be ballast, portability, and launching.

I will probably have to put this project on hold now. I know I will be relocating sometime again in the next 18 months.

So for now I will perhaps stick to the original plans.

But thanks for all the advice chaps.
Mark