Newby questions on sailboat design

Thanks for the referrals. I don't need the computer just to draw a picture. I want to start running some numbers on stability and windage, to find out how much headroom I can put in the cabin without ruining the sailing performance. I'm a software developer myself; I understand about "garbage in, garbage out."

I'd love to start with a proven design. If someone knows of a proven design for an easy-to-build 30-footer with draft under 2 ft. and headroom over 6 ft., please send me a link :^.

Actually, except for the headroom, pretty-much everything I'm looking for is already available in proven designs by Bolger and a few others. If anyone has already created a data set for such a boat, maybe they could send me a copy. Then I can save some time by editing that, instead of creating the data from scratch.

Thanks --

 

Come to think of it, I do know of one design that meets my criteria, although I don't know if we can consider it "proven." Has anyone ever actually built a Bolger "Superbrick?"

I just went back and re-read his article. This is a sailing houseboat/barge just 20 ft. long, with a draft under 1 ft. It has about 6ft. of headroom in the tall part of the cabin; and external coamings make the sides of the boat that high for much of her length. There's a gigantic-looking lateen sail, 184 sq. ft., on top of that high hull; the peak of the sail is about 35 ft. above the waterline.

This design is an exercise, if not an outright joke; Bolger's writing is quite sarcastic in tone. Apparently the boat will be a pretty lousy sailor in anything but very mild conditions. But he does say that it's "self-righting, unsinkable, strong, and stiff." I think "stiff" in this context means that it stands up well to its sail, doesn't heel too easily: rather surprising when you look at that towering sail.

I find this encouraging. Suppose you stretched the hull to 30 ft. or so, and "de-barge" the hull by curving the sides in at the ends; that will make her sail a lot better by keeping the corners out of the water when she heels. Using low-aspect junk sails, I can get almost 400 sq. ft. of sail on that hull, with masts that don't rise more than 30 ft. above the water. Seems like it ought to work, or am I missing something?

Thanks --

 

Hi folks -

OK, it's been a couple of weeks. I've read "The Nature of Boats" to get an intro to the math.

I downloaded Delftship Free; looks like it can do what I need. Pretty good manual, except that it has NOTHING about how to actually start designing something. Literally; there's no explanation of the settings in the New Project dialog.

The default New Project hull is a sweet-looking thing, but it's wildly different from what I'm trying to create. I think if I could just figure out how to draw a rectangular box, I could go on from there to add points and refine the shape. But starting from the default hull and converting it to what I want is clearly going to be very unpleasant if I have to figure it all out by trial and error.

Any advice? Are there any other tutorials or docs available? Can someone create a "design" for a rectangular box and email it to me :^?

Thanks for all the help so far --

 

get a copy of Sketch-up from google. With a little time on the tutorials, you can do a pretty swish "box" with dimensions, 3d and everything.

200% better for beginners than the very specialised packages.

I dont think you will do better than a Bolger design for what you want, so I would get an NA to do the mods for you

 

Use the search function of the Forum, several tutorials pop up. More on Freeship+ than on Delftship.

Sketchup might be nice for fancy drawings, but cannot replace Delftship / Freeship.

Regards
Richard

 

Good start You probably have some idea, then, of what the various coefficients (SA/D, SA/WS, Cm, Cwp, Cp, etc.) are and why they're important. You probably also now have some idea of how to calculate the weight carrying capacity you need.
Of course, additional reference material- especially books with a more technical/design focus- will be helpful.

Start by figuring out what you want. Before hitting the computer, you should have a pretty good idea of the displacement, dimensions and general shape you're thinking of.
There are only a handful of possible operations you can do to a Delftship model, the main ones being:
- Move control points
- Add/remove points and edges
- Crease/uncrease edges (ie, create hard chines)
Just start dragging points around, and you'll get the hang of it fairly quickly. Extra points and edges can be added as necessary.
As easy as it is to draw pretty pictures in this software, it's important to remember that it's your understanding of the underlying hydrodynamic and structural principles, not the program's, that determines whether or not the design will actually work. For example, if you know how Cp is calculated and how it affects the boat's performance, Delftship's ability to spit out something like "Cp = 0.61" in 1/10 second will be very helpful. If you know only that Cp has something to do with how fine the ends are, you could end up with an unbalanced, poor-performing shape by trusting the computer output without fully understanding it.

 

Menu item "tools", command "add box"

Describe required parameters - actually coordinates of diagonally opposite points and you have it !

Read the help carefully, it is one of the best I have seen.

regards.

 

Seems you are looking for a chined boat? If so...start an new default one and put in 3, 3 and leave the rest. Once you have the model go to View, intersections and click the garbage can for all the buttons then the green check. Pick the longitudinal line connecting the points in the middle of the side of resulting default hull, press ctrl and click on it. The whole line should light up yellow. Look for the button that has an angle under an arc and click it or go to edit/edge/crease. This will turn the line from a soft curve to a hard line creating a chine. Go to View/Linesplan and look at your creation.

Oh...and here is your box: http://angelfire.com/ego/lewisboatworks/Stuff/Misc/Box.part

Download it and import into Delftship after file/new then select all and delete to get rid of the default hull.

PS: have fun

PPS: Here is a basic hull you can start with..21 ft long, pram-ish shape. http://angelfire.com/ego/lewisboatworks/Stuff/Misc/basic.fbm

 

Hi folks --

Thanks for the additional tips. I'm starting to like Delftship now :^) . Here's a view of my prototype design (attached). The hydrostatics report gives the displacement around 5 tons, which is what I had in mind. However, the prismatic coefficient is up over .7. According to Gerr, this is quite high for a sailboat, but apparently that's normal for boxy scow-type hulls. What Gerr doesn't mention is, what happens if the coef. is too high? Will the boat be slow, or unstable?

Also, because I started from a rectangular box, I have a surface -- or two surfaces because of symmetry -- running right up the centerline. How do I get rid of that? There doesn't seem to be a delete-surface command.

Thanks again -- have a great 4th!

 

Turn on Faces and highlight it then delete. Depending on how things were put together though...this might wipe out more than just the single face. If this is the case you could try highlighting the face and put it on a new layer then see if you can delete it without taking out anything else. The boat would probably be slow but most likely far from unstable. I would be inclined to lower cabin height...it will have much windage and raises the center of gravity quite a bit. How are you going to sail it and still see where you are going? OTOH...with all that upper displacement...you could probably lay it on its side and never get the inside damp if there is sufficient decking width to keep the cockpit area out of the water.

 

are you looking for a 30ft boat? , or something smaller?

also is it for you to build?

 

Hi folks -- hope everybody got in some water time over the holiday.

How do you "turn on Faces?" I can't find a command for this. I can't find any way to select or highlight a single face.

The boat may be slow upright, but its underwater shape should improve a lot when it heels. Does Delft have the ability to do the math at various angles of heel?

The high deckhouse will not have a significant effect on center of gravity if I follow the rule of not having any storage spaces up there, except little shelves and holders for binoculars and toothrushes. All heavy gear and supplies will be down in the hull; the house is basically just a big box of air. And yes, all that buoyancy should be a safety feature.

Windage is a question mark to me. This design obviously has more than most sailboats. But I suspect that, in moderate conditions, it won't be a problem; and this boat is not intended to win races or claw her way off a lee shore during a storm. But so far I haven't found any math, or even rules of thumb, for figuring windage. Anyone?

The pilot-visibility issue did occur to me. Hey, wouldn't it be funny to build a boat with great stability, performance, and comfort; and then step on board for the first time, and discover that you can't see where you're going?!?!

George Buehler's "Rufus" is similar to my design. He solved this problem with a stand-up pilot station with a wheel. I stand up a lot when sailing anyway. But I could add a deck box with a seat on top, or one of those fishing-boat seats on a pedestal. I could also take a chunk out of the aft stbd. corner of the deckhouse to provide some kind of raised piloting station; the interior plan has the foot end of a berth in that area.

And I might end up lowering the house some, too. The current design has 7ft. headroom along the centerline (3 in the hull, 4 in the house). I'm 6'3", and the shower stall needs a raised floor to make room for a sump. I figure, as long as I'm dreaming, might as well dream big :^) . If the result is impractical for my budget & skills, I can scale it down later.

-- which brings me to Stubby's questions: yes, I hope to build this boat eventually, or something that evolves from this design process. My current design is 32ft. x 8, but as I said, that's just a starting point for the design. It fits nicely on the 1/4" graph paper I'm using for sketches :^) .

Have Fun! --

 

3 buttons to the right of the drop down for resolution is the button to turn on interior edges or faces. It will put a mesh on all the surfaces. If you click on the mesh it will highlight...then you can delete it. It may remove more than just what you highlighted though. If that is the case...click the button to create a new layer, highlight the face then select the new layer in the drop down...it will assign that to a new layer. You MAY be able to delete it then.

Windage can slow you down, cause the boat to drift when you don't want it to, drag anchor, turn in directions you don't want to go and cause a lot of leeway in reaches.

 

Why 8ft wide, I just drew up a model on delftship and that is very thin, but is this so its trailerable?

Also when designing you need to think, do you want it to be easily trailered, or will you basically live in it so therefore trailering ain't a big issue, will it have to be able to dry out? Do you want a twin Bilge keel, or a single keel, lifting or bolted. etc etc etc.

 

They do speak english in Australia, correct?