A Quick and Dirty Model (pics)

[Bruce Taylor]

I made a crude model of the camping boat I've been designing w/ a great deal of help from forum members. I still don't what to call it -- Forum 14? Collaboration? Camel (as in "horse disigned by a committee)?

Anyway, here it is, at present. The model is inaccurate in various ways. The true bow is somewhat finer (I didn't bother to cut gains in my little planks) and the sheer on the model is much flatter than the true sheer will be. The model has six strakes per side, instead of the eight I'll use in the final boat. I didn't take much trouble with aesthetic features, since my main purpose in making the model was to get an accurate picture of the shape below the WL, and to make sure that planking was not too difficult (I'm well satisfied on that score).

Finally, the thwarts/mast partners/cb trunk are entirely provisional. I've made no decisions yet as to thwart placement, centerboard location, etc. and would welcome any suggestions regarding rig, interior arrangement etc.

Incidentally, in the first pic the mould closest to the camera is station 13, not the transom (which got incorporated into the model itself).

http://www.imagestation.com/picture/sraid36/pe8f62492203124b6f69979af1a06077a/fd256776.jpg

http://www.imagestation.com/picture/sraid36/pa56198ae2da2d011b7b15f4bbdf388aa/fd256773.jpg

http://www.imagestation.com/picture/sraid36/p7ce01ff1563bed614958f25d794564db/fd256771.jpg

[Jack Heinlen]

I like it!

It reminds me of some of the early Seabright skiffs, minus the boxed garboards, though I could be off base.

I knew next to nothing about the tech. stuff on the last thread about this boat. Are you feeling comfortable with how those aspects are working out?

I think it will be a handsome little boat, and good for its intended purpose--if you get rid of the split, midship thwart that is ;)

Jack

[A. Mason]

Nice looking little model! Anita

[Paul Scheuer]

Nice job Bruce. :cool: To me it's always good to see the 3-D version of a plan. Is there a tank test in the future ? ;)

I didn't see a discussion of the planking plan in the "Sketch" thread. Did I miss it, or are we about to begin ? I'm sort of partial to a uniform width for the sheerstrake. (except for double-enders). It seems to give more finishing options, bright mahogany, contrasting colors, etc.

I'd also like to hear about the planks that run parallel aft underwater. Is this a drag reduction idea ? I'm trying to visualize the flow when heeled.

[NormMessinger]

You do good work. Fast too. Now the question is what weight do you add and at what speed do you tow it to get data you can extrapolate to the full size version. Sam Devlin covers this in his book but at a 3/4" to the foot scale.

--Norm

[Bruce Taylor]

Jack -- The technical stuff (even at this elementary level) has been interesting. I'm certainly in no position to feel "comfortable." I can do the calculations, and compare the results to certain rules of thumb...however, my lack of experience makes it difficult to guess how elastic the various parameters really are.

What can I get away with? What will happen if I stretch the rules?

To take one example, John W. suggests the prismatic coefficient is on the high side for a boat that will be asked to behave as a displacement hull when heavily loaded. Now, I've read somewhere that my Pc/f (~61%) is in the range for a "semi-displacement" hull. However, I think the author who gives those guidelines is talking about power boats. So, I'm inclined to take John W.'s doubts quite seriously. But, if I leave things as they are, what price will I pay? Damned if I know.

The ability to make judgements of that kind is, of course, what separates a real designer from a stubborn amateur.

Paul, the shapes of the lapped planks in my little model don't really correspond to the final lines, except in the most general way. I'll do what I can to make it pretty, but it will be planked in the manner of any lapstrake boat, without much regard for hydrodynamics. Some clinker enthusiasts claim that the laps have hydrodynamically useful properties, but I'm skeptical.

That's assuming I do build in lap-ply. Lately, I've been wondering whether I should strip-build it, like a Bear Mountain canoe. I'm trying to keep weight down, but don't really know whether a Tom Hill-style "ultralight" is any lighter than a Mac McCarthy-style "featherweight".

Norm -- I have to work fast. If I get a project finished before Maggie finds out I've started, she can't really prevent me from doing it, can she?

I haven't towed my little model, but I've pushed it around in the bathtub a bit. It sure can hold a lot of shampoo bottles and rubber ducks.

[ 10-20-2002, 10:21 PM: Message edited by: Bruce Taylor ]

[J. Dillon]

Sure is a nice looking model. :D In the images shown the amount of "sheer" doesn't come through. How much does she have ?

JD

[capt jake]

All I have to say is, the models are very impresive! Looks like you are doing one heck of a good job! I am envious. smile.gif

[john welsford]

I began my design career by drawing the boat and building it, then trying to work out why it did what it did, then I started working out the "stats" on boats that were aready designed, then I worked them out on paper to what it was I had designed before I built one, and then, I got a Mini Transat to design. these little monsters are 20 and a bit long, mine carried a bit over 500 sq ft of ordinary working sail in the jib and main alone, she carried over 2000 sq ft with the bik spinnaker up and cost the builder the equivalent of four years wages plus his labour. I figured I had an obligation to see that his expenditure and work was not wasted so did a whole lot of research to find out what the "stats" would be for a boat with the handling and sailing characteristics that we needed, got Team New Zealands software designers to digitise my rough drawings and massaged the shape to get the shape that I needed to conform to the Stats, and away we went.
That boat finished third in a seriously tough race, singlehanded from France to the Carribeanm, did 7 consecutive days run over 200 miles and survived a full pitchpole at an estimated 25knots when the skipper went to sleep allowing a squall to overtake him with the spinnaker up.
I now design all of my boats by doing that research first, and am much more confident about being able to predict performance in the finished product than I was, I dont need the fancy computer to do it either.
I like your model by the way, dont apologise for it being not museum quality, the finish is consistent with the purpose.
To tow test, you dont have to measure speed, speed length ratio is fine. Good old Froude with his waves is the way, tape a balsa stick to the model parallel to the water, clearly mark the stick with markers indicating 1ft intervals at scale, and tow it with a video cam recording. When you view her running, you will be able to freeze the frame and see where the wave crests are bow and stern in comparison to the markers on the stick and from that calculate your SCALE speed in knots, or your Speed /Length ratio.
Watch my column in www.duckworksmagazine.com (http://www.duckworksmagazine.com) for a more complete explanation of the method, I think Chuck has it lined up for next month.

Yours, JOhn


Originally posted by Bruce Taylor:
Jack -- The technical stuff (even at this elementary level) has been interesting. I'm certainly in no position to feel "comfortable." I can do the calculations, and compare the results to certain rules of thumb...however, my lack of experience makes it difficult to guess how elastic the various parameters really are.

What can I get away with? What will happen if I stretch the rules?

To take one example, John W. suggests the prismatic coefficient is on the high side for a boat that will be asked to behave as a displacement hull when heavily loaded. Now, I've read somewhere that my Pc/f (~61%) is in the range for a "semi-displacement" hull. However, I think the author who gives those guidelines is talking about power boats. So, I'm inclined to take John W.'s doubts quite seriously. But, if I leave things as they are, what price will I pay? Damned if I know.

The ability to make judgements of that kind is, of course, what separates a real designer from a stubborn amateur.

Paul, the shapes of the lapped planks in my little model don't really correspond to the final lines, except in the most general way. I'll do what I can to make it pretty, but it will be planked in the manner of any lapstrake boat, without much regard for hydrodynamics. Some clinker enthusiasts claim that the laps have hydrodynamically useful properties, but I'm skeptical.

That's assuming I do build in lap-ply. Lately, I've been wondering whether I should strip-build it, like a Bear Mountain canoe. I'm trying to keep weight down, but don't really know whether a Tom Hill-style "ultralight" is any lighter than a Mac McCarthy-style "featherweight".

Norm -- I have to work fast. If I get a project finished before Maggie finds out I've started, she can't really prevent me from doing it, can she?

I haven't towed my little model, but I've pushed it around in the bathtub a bit. It sure can hold a lot of shampoo bottles and rubber ducks.

[Scott Rosen]

Sure looks pretty. Sweet lines.

[Tar Devil]

Very nice, Bruce.

I'm dissappointed, though. I figured you'd get back at me by floating that thing on your river and suckering us in.

What kind of wood did you use?

Later,

Phil

[Tar Devil]

Very nice, Bruce.

I'm dissappointed, though. I figured you'd get back at me by floating that thing on your river and suckering us in.

What kind of wood did you use?

Later,

Phil

[Bruce Taylor]

J. Dillon -- The real boat will have a lot more sheer than the model. I threw this together very quickly, and didn't spile any of the planks (I just cut out shapes that looked about right and adjusted them by eye with a palm plane). The fifth strake was oversized, and the sheer strake covered up the marks on the moulds. Rather than spend time fussing over it I just let the sheer run flat.

John W. -- I don't how I'm going to convince my poor wife to come down to the river in the October chill and film me while I pull toy boats back and forth in front of the dock, LOL.

I'll watch for your article at Duckworks.

Phil -- After failing to detect your cunning hoax, I realized I would have to get some younger eyes, some stronger glasses or a new computer monitor. I opted for the monitor. Just try to fool me now!

I cut the stock from a single 24" offcut from a white cedar deck. I just ran a bit of two by six through the bandsaw...took me about five minutes to make enough planks for three models.

I made the moulds from some 1/8" ply that my lutherie supplier uses for packing guitar-tops.

[ 10-21-2002, 06:50 PM: Message edited by: Bruce Taylor ]

[brian.cunningham]

way :cool: model

[John E Hardiman]

Originally posted by john welsford:
I began my design career by drawing the boat and building it, then trying to work out why it did what it did, then I started working out the "stats" on boats that were aready designed, then I worked them out on paper to what it was I had designed before I built one, and then, I got a Mini Transat to design. John W has it right, get as much data from designs that you like, plot trends, and then decide what direction you wish to push the trends. I know bainbridgeisland keeps a well detailed list of such things, of his own designs, and others. Sometimes it is helpful to also know what you shouldn't do, and if you are way off the trends for "good" boats, you had better understand why.

In light of this on going design, I dug out my copy of SAILING YACHT DESIGN, by Henry & Miller(1965; Cornell Maritime Press, LCCC#65-18207). It's a bit dated for ULDBs but covers CCA boats at their zenith, which IMHO, delivered some best overall balanced cruisers produced. Also they looked into small day sailer classes, so their data goes down into the region you are interested in, including sail area to displacement, lateral plane to sail area, centerboard to sail area, and spar scantlings. You might want to find a copy.

The model looks very good, and remember, sheer should be proportional to length. So unless you're looking for a surf boat, I wouldn't cut down the midships or raise the bow too much. I think if you built the sheer to the lines it would look "proper".

I think the hull shape may start playing a role in selecting the building method. The flatish sides forward and bilge radius aft points to a fair amount of taper required in the strakes. There are many more qualified than I in this forum to speak about plank layout, but if I were doing it with a keel as you show (for weight I assume), I'd lay out a pair of fairly flat garboards short of the stem( rather than what appears to be a long garboard with a lot of twist in the photos) and an even sheerplank, then layout my maxmum plank width on my maximum frame to get the strake count, then give a nice proportion to the layout of the visible strakes at bow and transom. This may lead to a lot of "hook" in the lower forward strakes and taper in the turn of the bilge aft. I really can't tell as you only show the stern of the mold. :(

Very nice work.

[ 10-22-2002, 12:09 PM: Message edited by: John E Hardiman ]

[Jack Heinlen]

Bruce,

I assume a skeg is in your mind's eye?

[Bruce Taylor]

John H. -- Thanks for the suggestions. If I plank in ply, the keel & stem will be "false," over a bevelled keelson. Your suggestion concerning the garboards is interesting & might also help w/ the considerable twist in the gar and broadstrakes.

I've found a copy of the Henry & Miller book through Abebooks (at a Canadian bookseller!), and will order a copy this evening.

Jack -- I'm glad you raised that. I have been thinking of a skeg, but don't have a clear idea how adding one would affect performance. Obviously, it would shift the CLR aft a bit, which might be useful if I want to add a good-sized mizzen w/out creating too much weather helm; and I assume it would make tracking better for rowing (should I ever be reduced to that, LOL...I'm under no illusions that this boat will row worth a damn). But wd. a skeg have other effects? For instance, would it make it harder to get up on plane, when sailing for pleasure?

DOes anyone know?

[ishmael]

Bruce,

The boat I see is going to row fairly well, and not plane, 'cept going downhill--a steep hill. Unless I'm blind, or ignorant, or both.

A skeg! You will want a skeg, methinks. Factor that into your CLR.

Jack

[Buddy Sharpton]

Seldom will you see a skeg on a modern planing performance dinghy. It adds surface area whose extra friction works against getting on plane. It slows the boat in tacks. A performance boat needs steering all the time, but it will spin on a dime, actually the centerboard with lots of rocker keeping the ends of the boat from pushing to much water. But if you've designed a boat with too much immersed lateral area forward, compared to the immersed sections aft, the rudder can be sized to compensate, being of less area, but further back from the pivot point ( it has a larger moment) it can do the job with less area, and if of a foil shape, the few degress of deflection from the centerline it takes to steer the boat will be the angle of attack it needs to create lift in the water to pull the boat to weather. But it will need steering and trimming all the time. Look for a performance boat to be tippy and twitchy and need a crew's attention to make it work- hope you find the challenge rewarding.

[Buddy Sharpton]

But as for planing, I think the boat you have is too narrow and it's aft sections to rounded to expect it to plane very often. A planing boat will have wider, flatter aft sections with the maximum beam about 60 to 65 % back from the bow, little aft immersed plane. lower freeboard to eliminate windage and then as a consequence wider side decks to prevent shipping water- and as long as they decks are there, round them for crew comfort in hiking out which you will need to do to get enough power from the sails without capsizing to get on plane- and since you will capsise, make them into air tight flotation chambers underneath- got to keep it light to plane, so composite, even glass fabrications of these enclosed volumes starts looking necessary- is this really where you want to head. How badly do you want to plane. I think the Thistle shows you about where a wooden boat can go, and that a sixty year old design. The Highlander was the next evolution and was made of glass. When you say a mizzen mast and plane in the some paragraph I don't see how you can have both.

[Bruce Taylor]

Thanks, Buddy.

How badly do I want to plane? Not too badly. I just like to know what I'm trading away whenever I make a choice. Since this boat is meant to sail, most of the time, I prefer not to compromise potential sailing performance unless I have to.

I'm interested in your skepticism about planing yawls. It's my understanding (and this is strictly from what I've read, not personal experience) that many of the old sailing canoes were fairly quick to plane, despite their narrow beam and yawl rigs. Of course, these often carried a lot of sail -- but I'm not averse to overpowering my flimsy boat smile.gif .

I've made the aft as flat and wide as I could, within the 4' beam limit I set. The V-bottom is somewhat wider and flatter, for most of the boat's length, than that of Sunfish, which planes readily enough. Rocker is comparable, I think, and the boat ought to be somewhat lighter than Sunfish when, when lightly loaded. So why wouldn't it plane?

I'm not challenging your opinion, Buddy (or yourse, Jack)...I'm just looking for the flaw in my reasoning.

[ishmael]

Could be the model is deceptive.

I only have a seat of the pants appreciation of these matter, and what I see is not going to plane easily.

But, who needs to plane? If you wanted a planing dinghy, you'd build a design by Fox.

Interesting questions you raise about the Sunfish. I don't have answers. Maybe I'm way wrong about your model's planing abilities.

Jack

[Bruce Taylor]

Well, if it helps, here's the sectional view I used for the model:

http://www.imagestation.com/picture/sraid36/pb785410bb74f184dd4ab2ee023425dc7/fd21d2bb.jpg

I'm not preoccupied with speed -- there have to be compromises in a boat like this -- but of course I am curious about it.

[Buddy Sharpton]

The bit about the mizzen is it is more effecient in trms of parasitic drag and turbulance to have one mast, one set of stays- we're trying to eek out the last iota of advantage in order to plane. Your aft sections are very near parallel,no twist or warp as it is called, also very good for a planing body. But you have an inefficient waterline length to beam ratio here. Something along the lines of 2.5 to 1 would be better for planing, or six foot beam.
Look at long slim destroyers and wide planing race boats. PT boats could have been much wider for the same length and fuel use with a much higher payload.

I imagine you could ocasional surf, but pumping the sail trim in repeated to pop on a plane in a gust, I dunno. I have a 14.5 x 50" whitehall that will not plane at all, no way, and its not terrible different in hull shape from yours- it has a very full afterbody and barely wine glass shaped transom -with a rounded down fairbobt skeg of about 7 inches. Tracks beautifully, tacks sluggishly.

[Bruce Taylor]

Buddy, thanks for responding.

Below the waterline, this hull doesn't strike me as very Whitehallish. A rowing boat, it seems to me, should have a lot more slack in the bilge and steeper deadrise in the aft.

And hell...the shape hasn't changed that much since Jack assured me it would row like a pig!

But I could be wrong, I probably am (as that same Jack has been known to say).

Why do the board boats plane so readily?

[ 10-23-2002, 06:49 PM: Message edited by: Bruce Taylor ]

[Buddy Sharpton]

Board boats plan readily because:
1) they have very low deadrise
2) they have very little rocker
3) they have very lightly loaded waterpalne area
4) they have very high sail area to displacement
5) they are of a size ( and configuration) that shifting human ballast can keep them on their feet

[ishmael]

My memory could be faulty, but I think your boat has considerably more rocker than a Sunfish, and much of it aft.

[Buddy Sharpton]

The "whitehall" of mine I'm talking about is a Cape Dory 14 which is much fuller than a true whitehall-I use it as a camping boat hauling tent, sleeping bags, cooler, stove, food and the like for my wife and myself. Great at island hopping down the rivers along the coast- but won't plane. Shape looks more like yours than the Boston Whitehall at Mystic. Just like a size twelve resembles, but won't be confused with, a size eight. Same length/height- one's more filled out. The fourteen foot 470's we use in the Junior program will certainly plane, more so if you use the trapeze and chute, but they don't allow them in collegiate competion trying to keep the racing intensity at a more sociable level- less physical, just as tactical, but less planing.
Full kit, full tilt on these 470's can wear you out and wipeouts can be scary.Try getting untangled from the strings which have snagged your life jacket, which in stead of protecting you is now holding you to the shrouds, with you head about a foot and a half below the water on a turtled boat. Been there, not the kind of boat I'd want to take camping. We're talking wine, cheese, crackers and conversation with m'lady on the way over to the island here. I like her company and just using a little boat is enough excitement/novelty to make it interesting for her.Doesn't need to be twitchy.

[Buddy Sharpton]

Meant to say 420's not 470's.

[Bruce Taylor]

Hmm. All this is just deepening my doubts. I hope you don't mind if I gnaw this bone a bit more.

Deadrise on a Sunfish is steeper than on the hull I drew (9.5 degrees, as compared to 6.5)

Sunfish has substantially more rocker than this hull (the immersed keel comes roughly five inches above the baseline at each end; at comparable loads, mine has about 3" of rocker).

Displacement will be similar; sail area is undecided, but I'm thinking of something in the range of 70-80 sq. ft.

Of course, a major difference is that Sunfish is hard-chined and wall-sided, which makes the planing surface several inches wider amidships (although not, I think, in the aft).

What you say about wine and crackers and m'lady makes a lot of sense. However, what I'm hoping is that this hull will behave respectably as a displacement boat when heavily loaded. I've actually been more concerned about that than its performance when used as a lightweight fun boat.

Your Cape Dory sounds lovely, by the way.

[ 10-24-2002, 12:00 PM: Message edited by: Bruce Taylor ]