sail area vs ballast

very interesting suggestion, scaling up from a model rather than down from a larger boat.

pirate is certainly beautiful; it must have been a real thrill to sail her. and, yes, the hull is similar. i've heard it said that, when scaling down, the keel should become larger. is this true?

this hull shape is almost a keel, itself. i have entertained the idea that, to reduce draft, the keel could be shifted aft to fill in the rise of the hull between the rudder and where the keel is now. internal ballast would still be relatively deep. i'm afraid, however, that this may make the boat hunt and throw her nose off from the effect of wind or wave. what do you think?

 

What was really exciting was we had Norm Blanchard aboard. He was one of Sir Tom's original crew. Sir Tom was Ted Geary's first R-boat (Pirate being the second), and was quite a successful design - it won the Lipton Cup something like 10 years in a row. Pirate and Sir Tom met 4 times, I believe, and basically came out even. Pirate won the Larchmont Race Week regatta in 1929, but then the stock market crashed and she was sent home and never ventured East again.

So if you want a traditional looking boat with good performance, Pirate would be an excellent place to start. You could scale the hull to the size you want, determine the amount of ballast needed to float her on her lines, and draw in a modern keel that is as deep as you can stand. Then calculate the stabilty available and scale the height of the rig accordingly.

 

These links might be of intest:
www.5point5.org
www.sailingsource.com/6metre

The Etchells is also a darn good proven design.
www.etchells.org

I hate to be boorish, but if we're going to talk about scaling I'll reiterate that the right way, in my opinion, is to scale rig, length, and fairbody draft by a scale factor x and scale beam, freeboard, and keel span by x^(2/3). This is true whether scaling up or down. Displacement therefore scales by x^(8/3), and the ballast is whatever is required to achieve this displacement.

Interesting this post should come along just now. Did you see it?
http://boatdesign.net/forums/showthread.php?threadid=1876

 

yes, stephen, that earlier post was my first as a newbie. now that i've been corresponding for awhile i'm really confused. [ ] actually, i'm learning a good bit but it is obvious there is much i don't understand.

earlier, you wrote: " The bottom line is that if you want to maintain the same length to beam ratio as the original your model will need more draft and a lower centrer of gravity than a scaled down version of the origional in order to achieve the same relative stability."

this concerns me because i would prefer to reduce the draft. can a trim tab work in place of a larger keel? http://www.heymanyachtdesign.com/in3a.html of course, this doesn't solve the ballast issue.

i've taken some measurements:
lod 21' 6"
lwl 15' 2"
beam 4'
bwl 40"
hull weight est 600 lbs with rigging

i'll try to post a pic. thanks everyone.

 

another pic. ignore the date, i neglected to set the camera.

 

Very interesting! Your hull shape is now a given, and I'd think you'd want to ballast the boat to float on its designed lines, as Tom has already stated. The question then becomes how much sail will she carry. Do you already own a mast?

I think the first step is to go ahead and float/ballast her, then perform an inclining experiment. You'll need to know how stable the boat is in order to size the mast diam. and rigging as well as determine the sail area. You could hang a weight from a vert. pole to represent your initial estimate of the weight & center of the rig, and use this as your pendulum for measurung heel as you adjust a known weight outboard across the deck.

If you want to do it by the book go to www.astm.org and request "F1321-92: Standard Guide for Conducting a Stability Test (Lightweight Survey and Inclining Experiment) to Determine the Light Ship Displacement and Centers of Gravity of a Vessel". You won't need to be this exacting though, and your local Coast Guard Safety Office can probably provide you with equally good instructions, or you can hire a naval architect in your area.

Once you have the result look up "Dellenbaugh Angle" and "Gimcrack Coefficients" (or "sail force coefficients") in a yacht design book. Use these to work out the sail area. For a crude initial estimate you can assume 1 pound per square foot of sail at 30 degrees heel. Your sailmaker may be able to help you refine this first estimate.

I'm afraid a trim tab will not increase your boat's stability. What will is dense ballast as low as you can get it, and a light weight rig and rigging.

Fair winds,
Stephen

 

On a boat this size, crew weight is as important or more than ballast to displacement ratio. Are you planning on sitting on the rail?

 

stephen, gonzo thanks for the suggestions.

so, if i have this right i should ballast the boat to her lwl, add weight to the gunnel until it is on the waterline, and that amount of weight will tell me how much sail area? how do i determine the ce?

gonzo, i will set up a wheel steering in the cabin and a tiller in the cockpit. this is to be a dual purpose boat, sail and electric power, and i want to be able to power it from inside in bad weather.

the advantage for epower is the very slippery hull shape. that's a reason i'm so concerned about the keel; i'm afraid the wing will have too much drag under power.

for sailing, a trim tab seems logical as the drag is low but one still can point very high. i'm thinking of reducing the depth of the keel and adding a daggerboard for deeper water. maybe, with a trim tab, the dboard won't be necessary?

as one can see, the hull itself is quite deep. maybe, internal ballast is enough? don't forget it is designed to carry a huge sail. maybe a lower sailplan can compensate for the higher ballast?

 

Ah - now im on the correct thread... Mr Ditmore and Mr Speer are right - the hull should be ballasted to its marks, both for technical (especially handling) and aesthetic reasons. Unfortunately, I personally feel you may be asking too much of the hull for its intended purposes. Let me clarify:

You stated that you wanted to be able to sail your boat from inside during bad weather. Narrow, heavy hulls like this with low freeboard go through the waves, not over them, and swamping or even sinking could occur in choppy water. The electric power idea is nice, but you would need significant battery weight to run for any reasonable length of time. This weight could be partially offset by removing some ballast and keeping the batteries low in the boat, but its still not as good as keeping as lead in the bottom of the keel. Also, the hull is significantly distorted to its measurement points (as are all competitive metre boats), and deep hulls like this create much bigger waves than long shallow ones with more even volume distribution, so the hull isnt actually nearly ideal for low power propulsion.

Now, that keel...
You want shallow draft, but good performance. The keel you have was designed to give you exactly that. The wings increase the effective span of the keel, letting you get away with a lot less depth for a realtively small loss of efficiency over a much deeper fin. The wings would be best cast in lead to keep the stability as high as possible. Any reduction in depth will mean a loss of righting moment, only compensated for by increased ballast, which would put the boat below her marks. The boat would become more tender, reducing the effectiveness of the keel, which would already be less efficient due to a reduction in aspect ratio. A carbon rig would help redress the balance slightly, and a shorter mast and less sail would help also, but you would then lose more performance. Indeed, your photos show a hull which to me looks like it would be quite sensitive to changes in configuration, meaning that you could lose a lot by making realtively small changes.

Given all this, I feel you might be disappointed with the result of your modifications. With all the work you seem to be planning, you might just be better building a new boat to the same brief but better suited to its purpose. From what you've said, a catamaran may be the best way to go, but you might try something like this:

18 foot monohull with 4 foot beam.
Light(er) displacement
Maximum LWL for low resistance
Clean hull shape
Twin carbon windsurfer masts with efficient sails (too many options to list!)
Strip plank/glass/epoxy construction
lead ballast bulb on fin of moderate to low depth (or deeper lifting fin and bulb)
Either twin assymetric daggerboards, a single board forward, or fore and aft boards (check interior requirements as well!).
You would end up with something not far off a transom sterned sailing canoe, with greater freeboard and a cabin - a kind of modern day cone yawl. Indeed, many canoes have also been converted for electrical propulsion as the hull form is ideal for this.

All that said, you have a beautiful hull which, although perhaps not ideal for its intended purpose, is the epitome of specialist design and is thus well worth saving. You could paint it up in the colours of your favourite 12 metre, cast some lead ballast wings, add a trim tab, an appropriate rig and a new daysailing deck. You would have one of the most beautiful, fast, elegant and original day sailers afloat. I believe it would be impossible not to enjoy such a boat, especially given the flattering comments she would undoubtedly receive from all who saw or sailed on her. 'A thing of beauty is a joy forever'...

 

thanks for the advice, andy. just about everyone i talk to suggests keeping the keel, more on aesthetic grounds, i think, than because of performance considerations.

so, just how critical to righting moment is the depth of the ballast? could the keel depth be reduced and compensated for by a daggerboard?

and how effective is a wing? i know it's exotic, but is it worth the increased drag under power? i've thought of building one of steel; lead may be a bit soft, although the extra ballast would be nice.

the keel issue seems to be most important. if we can resolve this, the rest of the design will be much easier.

 

hmmm... Why?

I might be stepping on toes here...

I can't really understand why you want to do this?
Did you get it so cheap (free) that you feel you have to do something with it? Converting it from a proper sailboat into a powercruiser with sails won't impress anyone, and it probably wont impress you either.
I think that with all the modifications that you want, she'll become a terrible sailer and an inefficient powerboat.

If you don't want to have a cool looking "mini meter" the way it was intended to sail and handle, I think your best bet would be to sell it to someone that wants to make a "proper mini meter" out of it, and use the money to build a boat that you really want.

Heavy propulsion does not belong on a boat like that IMHO.

Just my twn cents
ErikG

 

Im afraid im with Erik on this one. Im convinced the modifications you propose will turn a beautiful, interesting and fast boat to sail into a poor sailer and even poorer motor boat. Why dont you post a brief for exactly the kind of boat you want, and let the contributors to this site have a wee competition to see what they can come up with as a solution? It would certainly look different to a mini 12 metre... . But PLEASE restore your boat to racing trim - she's fantastic and deserves it!!!! Hope I don't sound too negative, but you wouldn't convert a Formula 1 car to a mobile home and expect it to drive well or be comfortable...

 

well, i hear you fellas and i appreciate the opinions.

erik hit the nail on the head with cost; i paid $51 for the hull. secondly, being a painter/sculptor/writer i'm most interested in designs of a singular nature so, naturally, somethng like this would pique my interest.

however, i'm sensitive to the opinions of those who love sailboats as a reflection of dynamic grace and beauty and i don't wish to offend the sail muses. that's the reason for this thread; to solicit the thoughts of better minds than mine.

so, i guess the question revolves around "just how much can we modify this boat" and retain its character? or should we follow andy's advice and " restore your boat to racing trim?"

i've been doing a little research, mostly designs from the early 20th century. yesterday, i was reading about "M" boats. many of these older designs don't appear to be far removed from 12-meter style boats, and they often had small cabins, etc. some even had gaff rigs.

the electric power i'm contemplating is simply via trolling motor. if you look at the long, aft overhang, it is plausible to insert a motor behind the rudder without being too intrusive to the aft deck. the motor could be removed for sailing.

right now, i'm removing bondo preparatory to adding more epoxy glass to strengthen the hull. otherwise, nothing has been changed. the problem is that, if i leave the keel as deep as it now is, it will limit sailing in my area of the world. and i can't afford to have a hull built specifically to meet my parameters.

so, is this keel sacrosanct or can it be modified? thanks everyone.

 

Well, I'm stretching out my neck here and risking a linching. I think nothing is sacrosant if your intention is to use the boat. I own and have owned through the years different boats. My focus in always on getting the boat in the water. Modifications and materials that don't fit the traditional or original ways don't worry me at all. The bottom line, is that if you take some of the keel off, some of the rigging has to come off too. Can you live with sailing a boat that is slow and doesn't point well? If you install a motor I guess the plan is to motorsail so it won't matter.

 

thanks gonzo. if we follow the line of logic that " nothing is sacrosant," the next question concerns how much compromise we're willing to live with.

this design is at the far end of the performance development scale. if we back up a bit on performance in favor of shallower draft and a low cabin, how much compromise have we effected? will we end with "a boat that is slow and doesn't point well" or will it be acceptable?

andy and erik have given very good reasons to remain purists and stay with the racing configuration. but [just academically speaking] IF we decide to go shallow draft and low cabin, what's the best design? and just how much will it affect performance?