Hey everyone,

I'm currently building/designing a scale RC Sailer. It's a complete scratch-build project, I made plans using some CAD computer program, and based my design upon some 2D plans from existing W60 boats (Bruce Farr Designs)

Specs are
LOA: 128cm / 50,4in
Beam: 34cm / 13,4in
Draft: 42cm / 16,54in
Mast Length: 180cm / 70,85in
Sail Area: Aprrox. 80dm² / 1240 sq.in.

Hull will be build in fiberglass using a mould (which I'm still building)
Keel will be a fiberglass / lead structure

Now, I'm currently building the keel itself, but I have *no* clue on how heavy I should make it. Are there any formula's I can use (knowing the length/beam/draft/sail area etc) to calculate the required bulb weight?

Or is gambling to better method, and finetuning during test-runs a better way? Guess not

Thanks!

(edit: some measurement added)

i have an idea or 2 but ill reserve comment till someone with perhaps some REAL knoldge posts

Remember weight x arm = moment... so the deeper your lead, the more efficient it is... and the less you require. This works to a point, then you start getting into heeling moment and designed heel for optimum waterline length, lateral resisitance, pitch oscillation and inherant stability, and the list goes on.

So.. how much do you need? The numbers you provided are for an EXTREMELY powerful rig, highly efficient in light air, but if you get over 10 kts of wind, you are going to be so over powered, you are probably going to break something.

If you want to compare some numbers, you are describing a beamy Marblehead which carries by Class rule 800 sq in, and you are 1200+ (which is what an EC-12 carries) and you are doing it on a hull that is 7 inches shorter.

IMHO, your beam is too large for the project... unless you are trying for a planing hull, essentially a Laser type... and even then, I would think that you are still a little much.

I sail a Bantock "Strad" Marblehead, and I am looking for my "B" rig sails when the winds get over 11 kts (steady) The boat is just on the edge of having enough sail to make control difficult... although it FLIES when you can hold it right. With 1200+ in that wind it would bury the bow and bring the rudder completely out of the water.

If you are looking at sailing in "drifter" conditions which we have here more than 75% of the time, you would be OK, and probably out ghost most other boats, but I am guessing with that much horsepower you are looking for more wind than that. It would be a screamer around 8 kts, but after that... well.. I would not find it enjoyable wondering if I could get my boat back before it sank.

I *highly* recommend a good watertight hatch cover.

PS, my Marblehead has the bulb down around 20" and weighs... I am guessing off the top of my head... around 8 lbs.

Good Luck with it though, hope you get a good one

First I don't know anything about this, but... I have an idea. The Marblehead class is 50.7" give or take, you can look it up to be sure. [link=http://gbmy.com/]GBMY[/link] sells ballast for Marbleheads and it ranges from 6.6 lbs to 7.7 lbs. So with a boat the same size that uses that range of ballast, you can at least narrow the field down. That said, it is all speculation on my part because I have no experience building or sailing yet. I am still building my boat. The ballast is $30.00 or you could just make your own.

Good luck and post some pics.

@ Larry: Intresting explanation. I know the specifications are rather impressive, but I believe a real W60 yacht is impressive as well right Being a scale model.... W60 are indeed designed to carry quiet some sail, and do have this large beam. They do plan however, so I guess it's intentional.

Now concerning structural strength.... I have a natural tendensy to make things stronger than what's actually required, and being a custom build project, I have all the opportunities to increase the strength at crucial points (mast connection, fittings, keel attachment,.....) That's not really something I worry about.

Off-course I'm going to make the hatch waterproof, and I do have some extra ideas that would make it unsinkable *no matter what happens*

As far as overpowering goes... guess I can always make an extra set of sails, a larger one for mild wind conditions and a smaller when for when it's windier...

PS I do have about 20 years of experience with "real" sailboats, both sailing and maintenance, so it's not that I'm getting into something I'm unfamiliar with. I'm just adding the "scale" aspect and some guidelines/calculation would be handy!

PS2 Will post some pictures, however I allready know it will take years to actually finish it. I'll start with some pics of the keel

well aproching the problem from basic physics.... YOu have an area of triangular sail catching wind. (from the side)

so adding up the total force from the wind pressure on the side of the sail, then applying that force at the Centriod of the sail trinagle that will produce a moment about the centerline of the hull/waterline.

In theroy(i think) you would want to ballance that force with keel weight. so as you make the lenght of the keel longer you require less wight at the expense of more material hanging under the boat. less overall weight means the hull rides higher in the water and waterline=drag. soooo more wight at a shorter keel, or less wight with a longer keel???

This is why its kinda an iterative desing thing. (thus the advent of CFD and computer models for opitmaztion of these things on the americas's cup yachts)

Just build the bugger. If it requires a longer keel, so be it. If it needs more keel weight so be it. Larger sails, so be it. Build the hull, that’s the bit that takes time and is difficult to change. Foils & sails can be adjusted, redesigned to suit. If it looks right, it generally tends to be right.

I’m of the “suck it and see” school of engineering.

WARNING Take this post as you will. I admit I have NO experience in one off designs.

An interesting thought to the "scale" anaylisis

there comes a certain point were the model will not behave the same as the fullsize object no matter how accurate the scale representaion. the physics of the world around the object do not change even though its size does. these two factors in the case of a sailboat would be:

-wind speed or force

-water density or resistance

lets look at the fist one. since your boat is clode to 1 meter we wil say that 1" = !2 " (or 1 foot)

since your boat is 1/12 than one could say that every inch travelled is equal to 1 foot of sailing in real life thus if the WIND travels the same distance than you must factor that in as well, im real tired so im not going to do all the math to figure out the wind speed @ 6 miles/hr down to inches/hr but i think u get my meaning.

while at the same time under the waterline the keel is having to contend with the fact that though the water ia not less dense it is however not more dense and u may find that your keel does not provide the resistance it needs and if that makes sense than u need to sleep as much as me.

the best example i can think of is the majority of real sailbots ive seen had long keels and no keel bulb . every rc has a bulb that usually wieghs more than the boat itself.

anyway hope this helps a little. best advice make it so u can adjust your ballast wieght a little trial and error mar be in order

this page has been helpfull for a plan im working on

http://www.sailingusa.info/design_winds.htm

(not being a sailor or anything) I thought this was the case because the long keel itself was usually EXTREAMLY heavy. But in our model world its easyer to form a lead bulb than trying to fill a (scale) reltivlily narrow keel.

but dude 4:35 am! [&:]