Making a Scale Model of My Boat ( in the 10 year plan)

[John R - Kitenui]

I hope someone can assist with a little advice. I want to build a sailing scale model of Kitenui and I have a question with regard to ballast. I have a copy of drawings published in 1952 and I have photocopier enlarged these to a scale of 1" to a foot.
http:////s2.photobucket.com/albums/y27/JohnRawson/?action=view&current=KitenuiLinescs.jpghttp://www.woodenboat.com/forum/%3Ca%20href=%22http://s2.photobucket.com/albums/y27/JohnRawson/?action=view&current=KitenuiLinescs.jpg%22%20target=%22_blank%2 2%3E%3Cimg%20src=%22http://i2.photobucket.com/albums/y27/JohnRawson/KitenuiLinescs.jpg%22%20border=%220%22%20alt=%22Ph otobucket%22%3E%3C/a%3Ehttp://i2.photobucket.com/albums/y27/JohnRawson/KitenuiLinescs.jpg
These drawings allow me to shape the hull and the attached ballast but as the volume is decreased in a cubic relationship and the sail area will be reduced in a squared relationship how do we correct the righting moment. Any comments on this problem with my 10 year plan would be welcome

[John B]

You could speak to Steve Johnno. He built Little Jim and went into that with Des townson. As I recall it, Des suggested that the hull and ballast was built as the drawing for display, but he recommended the extra ballast you need for equality was then was added by fastening below that.

[John R - Kitenui]

I thought that if I build as drawn she should float pretty much to her lines, which would preclude adding for sailing, but that it may be necessary to lower the ballast on some form of fin ??

[John B]

Thats what he suggested, a clip on fin and the extra ballast to compensate for the scale differences. I forget what the factor was but its a well known figure.

[rbgarr]

Floating on the lines depends on how heavy the wood hull (alone) is. I got a hull (at an auction) that floats below her lines with no deck, rig or ballast at all, so it's only good for display purposes.

[outofthenorm]

I'd say don't worry about it - yet. Build it to the plans, using lead for the ballast keel. That way, when you display the model, it will look like it should. Build the hull and deck as light as possible. This is assuming you are not going to re-create an interior.

When it comes time to sail it, you'll have 2 options. The first is inside ballast, which can be added as needed to achieve stability. The second is to tap the bottom of the lead for machine bolts and attach additional ballast as needed. Trial and error will sort it out pretty quick. You can shape the top of the "addition" to match the bottom of the ballast keel for good flow.

- Norm

[Todd D]

There are a couple of ways of dealing with this if you want to sail the boat. First, you can build a scale model balasted as shown in your drawings and just accept that you canonly sail in light air (< 5 knots). If you want to sail in heavier air you can build the model to scale with minimal ballast for display purposes and then build a bolt on fin with most of the ballast in a bulb at the bottom of the fin. The deeper the fin, the less balast you need. Of course, adding the fin means that you need considerable deeper water to sail the boat.

I would build the hull as light as possible. I generally plank the models I build with 1/16" thick balsa and then cover them with a layer of 6 oz fiberglass cloth set in epoxy. For a sailing model I glass the inside of the hull too. That makes for a stiff, light hull that can carry lots of ballast. Remember that you have to include the weight of the batteries, radio receiver and servos when you build a radiocontrol model.

These links will take you to detailed descriptions of how I have built several sailing models. Both the Schooner and the day sailer are for light air sailing only. The Morris M36 can handle up to about 15 knots.

RC Schooner - http://www.todddunnmicroyachts.com/2008/schooner.html

Custom Day Sailer - http://www.todddunnmicroyachts.com/2007/chabuka.html

Morris M36 - http://www.todddunnmicroyachts.com/2006/morris_m36.html

[John R - Kitenui]

Thanks for the input guys. I've started thinking why add ballast to the hull when used in display mode ? Ballast is really only needed in sailing mode and then who is going to see it ? What do you reckon ?

[rbgarr]

Correct, for a display-only model ballast is unnecessary unless complete accuracy is one of the objectives.

[Ed Armstrong]

Todd,

As I think I've said before (at least I thought it), your models are very impressive! I've been working on a model of a Gartside cutter, though I'm not sure yet if it is for display, or if I'll set it up for RC. I'm just finishing the hull. If it turns out half as nice as your models, I'll be happy.

Ed

[Charles Burgess]

In general, reducing a plan by half results in 16 times reduction in all hydrostatic factors. Thus, a scale model will never perform like the full size boat...the hydrostatics do not scale at all.

The simplest method for a scale model is to build it with the idea that the ballast section of the keel will need to be adjusted later. So build her, then make a mold of the hullform at the waterline: make a plywood box, some plaster of paris, then seal the hull in saranwrap and insert the model in the wet plaster. After it hardens, gently remove the model and fill the plaster mold with water and weigh the water to get a very accurate displacement figure.

You can separately do the same for the lead keel section of the plan.

With this data you can reformulate all the hydrostatic moments of the model at the model's actual scale.

[Todd D]

Alternatively you could just put the model on a scale and weigh it. After that you can use the lines to calculate the displacement required for the model to float on its lines. That is a LOT less trouble than making a mold and filling it with water. The difference between the calculated displacement and the weight is a good first approximation of the ballast weight. I say first approximation because you need to factor in the weight of anything you remove when installing the ballast.

By the way, don't hydrostatics scale as the cube not the 4th power?

[Charles Burgess]

[...snipped...]

By the way, don't hydrostatics scale as the cube not the 4th power? Depends on which way you scale it, and how drastic it is, and exactly what formula you are using. There are numerous variations. All numeric solutions are only rough approximations of what is actually a complex geometric process.

A scale model is linear, hydrostatics/dynamics are not. That is why you cannot take a stable boat/ship design and scale it up or down, even just a little - the result will not be stable and/or it performance will drastically suffer.

When I scale a design up or down, either to make a full size boat or a RC model...I have to use a geometric method that was used over a hundred years ago to accurately scale the hydrostatic/dynamic properties of the design, which is what the hull shape is all about.

[donald branscom]

Build the model as close to the drawing as possible and it will all come out right.

[Todd D]

Charles, I think we are at odds over definitions. Hydrostatics (i.e., displacement, etc.) scale as the cube of the linear dimensions. Dynamics are a different story. What I tell people about scaling drawings down to build a radio control sailing model is the following.

The forces on the sails scale as the square of the change in size since the sail area decreases with that ratio but wind speeds remain constant. On the other hand, the hydrodynamic forces on the keel are impacted by the fact that a scaled down RC model doesn't go as fast as the full sized boat. Hence, to balance the forces on the sails, the keel and rudder need to be increased in size. Anyone with a rudimentary knowledge of fluid dynamics can do the needed calculations to determine how big the keel and rudder need to be in order to generate the required hydrodynamic forces.

Incidentally, while I am not a naval architect, I did work as one many years ago (the the emphasis on the "naval" part). However, I worked mostly on boats where sail has a subtly different meaning - submarines.

[Charles Burgess]

[...snipped...]
By the way, don't hydrostatics scale as the cube not the 4th power?

Here is a more specific followup: These scaling factors should work...


Surface Area varies as the square of the dimensions L x W
Volume ... cube of the dimensions L x W x H
Stability ... fourth power of the dimensions L x W x H x Righting Arm ; varies directly with changes in length only if other dimensions remain the same; varies as the square of changes in beam only.
Speed ... square root of the waterline length
Resistance ... cube of the dimensions
Wetted surface .... square of the dimensions
Sail Area ... square of the dimensions
Heeling Moment .... cube of the dimensions
Wind Pressure ... square of the wind speed
displacement ... cube of the dimensions
GM ... cube of changes in beam only; inversely with changes in displacement only


The above is basically the inverse of the process used to translate tank test data to full scale performance estimates. The actual models used in tank tests are not simply scale models ...the hull lines have to be hydrostatically and hydrodynamically scaled accordingly. An RC scale model needs to be scaled differently than a display scale model.