Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15DEC)Here we go with the new design, a small cuddy power boat for low power! We'll lay out some guidelines for it - what we want it to do and how we'd like to use it. Then crunch some numbers to see where we go from there. It will be pretty well sketched out. Next issue, maybe at the end of December, we'll go through the final details.
BIRTH OF THE AF4....
I design all my boats for myself. The only exception is the rare occasion when someone pays me to do a custom design.
So in this case I'm thinking I might want to try a low powered cruiser. I've had the experience this past year of motoring around a bit in Birdwatcher and I've seen Kilburn's boat a couple of times and had a nice long ride with him. The boat will be used for poking around in lakes, large and small. Ability to float in dew is very important as most of the privacy is in the shallows no one else can use. Thus you can beach or anchor at night in a hidey hole with little thought of a noisy bunch blowing through in the middle of the night. I want two good places to sleep, at least one under good cover for bad weather, and another in the open or under a bug net for good weather. Not a fuel hog. No real need to go fast. Since I'm coming from small sailboats and rowboats anything over 5 knots is quite fast to me. Has to tow behind my small car
The next thing I need to do is look around and see what I have to work with. I have my old Jinni boat (getting time to rebuild again, expecially the bottom) on a small trailer and that rig can be pulled by my Escort (200,000+ miles). I just weighed the Jinni on the trailer and it weighs 450 pounds with no sail rig or motor. Also out there I've got my old pickup truck that I use to pull the Birdwatcher. In the shed I've got that old "3.5hp" Clinton (that I think really peaks at around 2 hp). Beside it is a "7.5hp" Sears motor that I think really peaks at about 4.5 hp. That motor is on loan, for me to test, with no words as to when it is due back.
One thing that is obvious to me is that weight will be critical on the new boat for two reasons. First, by all the charts we've seen, we need light weight to get the most out of the small motors. The new boat will not plane with either motor but perhaps a cheap 10 horse will materialize some day if I keep an eye out.
More important, if I can't keep the weight down within the range of the Escort's easy towing capability, there is no point in building it for me. The Escort gets 30 mpg even with a Jinni tagging behind. The truck gets 12 mpg when everything is running perfectly which isn't often. I want that 30 mpg. If the new boat is so heavy that the truck is needed to pull it, I might as well stick with the Birdwatcher with lots of room, safety, and a sailing capability.
...WEIGHT, WEIGHT, WEIGHT...
The Escort can pull the Jinni pretty well. The 450 pounds is without sail rig which probably weighs about 50 pounds with the spars, sail, rudder, etc. Also, I've trailered the rig with my Toto lashed on top so perhaps the total then was about 550 pounds. The car is actually rated to pull 1000 pounds although I point out that the folks who rate cars for towing probably aren't concerned with steep boat ramps or your vehicle's longevity. So I recommend towing about half the rated value. I've never weighed the trailer empty. I should. All I would need to do is take a bath scale with me, launch the boat, and weigh the empty trailer quickly by lifting up a wheel and placing the scale beneath. My guess is that this trailer must weigh about 200 pounds empty.
So that means the new boat must weigh no more than 350 pounds. My Clinton weighs 38 pounds and I don't think the Sears motor is much heavier. Two gallons of fuel will weigh another 12 pounds. So the bare hull of the new boat must weigh no more than 300 pounds to be a success.
Having used the 15' x 4' >
hing a bit bigger. Like the Jinni, I want a 2' long open bow well for anchors and junk. Then a cuddy at least 7' long. It needs to be 3' high minimum so I can sit up in it. The main body of the boat ought to be 4-1/2' wide minimum to give perhaps some room for benches. Aft of the cabin I want a cockpit 6' long for lounging length. Aft of the cockpit I want a self draining motor well at least 18" wide to take a 6 gallon fuel tank rescued from the dump by the same man who rescued the Sears motor. The total is 16.5'. I'm going to try for 18' in hopes of getting an 8' cuddy and another 6" in the motor well.
(Experienced plywood builders are groaning, noting that the 4-1/2' width negates making the bottom from a single width of plywood and the length negates making the boat from two sheets butted lengthwise.)
I want to use simple flat bottom construction. As long as I avoid rough water, this shape will go quite well and will be the easiest to build and use. (Kilburn's dory laughs at rough water.) I'm going to copy the conventional styling of the AF3 that I showed in the last issue. I'm calling it the AF4.
So I want a boat 18 feet long, 4.5 feet wide, and say 3 feet deep. Here's a real rough rule I like to use to guess the empty hull weight of a new design to be built lightly in plywood: Take the cubic feet of the new design and multiply by 2 pounds per cubic foot. A quick study of similar existing boats shows 2 pounds per cubic foot to be the approximate density of "boat baloney". It varies a lot. If you are building with 2" oak you use a much bigger number. If you build with microfilm over balsa use a smaller number. The value of 2 would be for a real stripped down hull built with light weight always in mind.
So where does AF4 stand? The cubic footage is about 18' x 4.5' x 3' times a factor of about .7 to allow for one pointy end. That's 170 cubic feet. That's 340 pounds of boat baloney. Clearly I'm pushing the Escort's comfortable towing envelope. Later, once the boat is totally designed we can get a better estimate by weighing up all the plywood sheets needed to build her.
So here's what I'd like to see as a starting point for the AF4:
The drawing is done on a simple CAD thing although I greatly prefer to work with pencil and paper. I used the CAD thing to avoid having to present another fuzzy scan. But I've got a story about my first experience with CAD that goes back to about 1980. At that time I was working for a large aerospace firm and they wanted all of us to play with CAD a bit, even if we weren't designing. So we went to CAD class for a week or so. Their machines had very large monitors in air conditioned darkened rooms, all hooked into a liquid cooled super computer a block away. Drawing was done with a light pen instead of a mouse. I'm sure it was the state of the art back then and is still a light year ahead of the program that drew the above sketch. After a while the instructor asked us to bring in a pet drawing to practice with. At the time I was just getting involved with homebuilt boats and wanted to draw the Bolger Teal skiff on the tube. It's the simplest sort of boat as you know. Straight planks bent around three forms. But I was unable to get the machine to bend the planks around the forms. "It's easy", said the instructor. "Just feed in some offsets and the machine will draw the curve." "But," I protested, "to get the offsets I first need to draw the boat by hand." "That's the only way it will work", he said. "The super computer can't draw your skiff." So the super computer couldn't draw my skiff unless I drew it first by hand. But a boat like Teal can be drawn and built by hand without ever measuring an offset. Was the instructor wrong?
I went through more or less the same conversation about ten years later when I started drawing my own boats and the blueprint man wanted to sell me a CAD program. About that time George Beuhler had written in Boatbuilder about switching over to CAD. As I recall he spent $20,000 and two years of practice before he felt competent at it. For designing $100 rowboats or $1000 sailboats it still makes no sense to me. Perhaps for the big time pros it's great, but for us little guys CAD is an expensive, slow and confining way to draw a boat.
One possible exception to the above is Ray Clark's Plyboats. About five years ago I got to play with an early demo of version 2. It's not so much a CAD program as a "thinking" thing that will feed those offsets to the CAD program. Essentially it works exactly as I like to design plywood boats. You define a midsection, a bow and a stern and it springs electric battens around to connect all. I think this simple method produces a superior boat in most every way. Then it will figure displacement at any draft you choose in a flash. Then it figures panel shapes in a slightly longer flash. (The demo model I tried had the printer function disabled so I can't comment on how well the printer spits out the shapes of everything.) Then in a slow flash it will compute stability data at any heel angle you choose until the sheer line goes under the waterline. For me stability is one of the most tedious calculations in boatdom. The early demo model, however, had a bug in those calculations that was most apparent at high angles of heel. I was told that bug was fixed. And for me there was a more obvious problem in that the designer had no control over the CG location, one of the prime elements in any stability study! But I think a resourceful fellow might be able to scale the CG location used in the calculations off the screen and then adjust the results to the real CG. I didn't buy it because I couldn't afford it but I would think 90% of my designs could have come right off Plyboats. Ray said that his program was worth the money because it would save me a lot of time and that my time was worth at least $30 an hour. I wish!
...WEIGHT AND DRAFT...
Speaking of displacement, lets make a wag at the total weight of AF4 and then the draft to float it. For weight let's start with 350 pounds of hull, 50 pounds of motor and gas, 170 pounds of me right after lunch, and maybe 350 pounds of extra gear like oars, fire extingusher, PFD's chairs, etc. (?) The total is 700 pounds.
Now lets make a wag at the draft of the proposed AF4 at 700 pounds. We'll start with a quick and dirty guess of 3". A look at the line drawing shows that the waterline is 14' long at 3" (that's .25') draft. The max beam is 4.5'. (The hull will have slight flare but so slight that we'll ignore it at this guessing stage.) If the hull were a just rectangular block displacing 14' X 4.5' X .25' of water it would total 15.75 cubic feet of water which is about 1000 pounds of water.
But it's not a rectangular block. It's quite pointy in the bow and mostly rectangular in the stern. So we multiply th 1000 pounds by a "prismatic coefficient" of about .75 for this shape. And we get a displacement of about 750 pounds. Close enough to our guessed at the weight to say the boat will draw 3" of water.
The prismatic coefficient (as I understand it) is just a factor got by dividing the actual displacement of a hull by the displacement the hull would have if it were not at all pointy at the ends - just a prism with the boat's maximum underwater cross section. The pointier the ends, the smaller the coefficient. But it's surprising how consistant the Cp can be for given types of hulls. Barges may have a Cp around .9. Squarish powerboats will be about .8. A hull like a Bolger Brick, slab sided but with upturned bow and stern bottom, might be about .65. A scow with taper in both sides and bottom might be about .6. A regular pointy bow sailing hull might go .55 to .50 and a sleek row boat with bow and stern hollows might go .45. You can usually look at a hull and guess the Cp within 10% without lifting a pencil or mouse. That's good enough for a first wag since nobody can guess a boat's weight that closely.
Folks who are a lot pickier than me spend a lot of time trying to tweak the Cp of a design because of the feeling that there is an optimum that will win every race entered. The idea might be that certain hull shapes make less waves than others and that Cp is a measure of the hull shape. But I think some Cp's do well in some conditions and others do best in other conditions. I thought I read once that a Cp of about .6 was considered to be the fastest for a displacement boat. That means a Bolger Brick is close to optimum. And it's true that in the right conditions a sailing scow is as fast as any monohull. But in rough water it ain't.
The chart below plots speed vs lbs/hp.
Might as well take a wag at the AF4 speed while were here. The chart is the same sort of ballpark thing shown in the previous two issues.
With the Clinton we might expect 1 hp at half throttle. That's 700 pounds per hp. Go up to the 15' waterline (close enough to 14') and we're going almost 5 knots. Full throttle (2hp) and we think we've got 350 pounds per hp and we're going 6 knots. That might be also a part throttle cruise with the lusty Sears motor.
Full throttle with the Sears motor (4.5 hp?) and we're racing along at about 7.5 knots.
If the magical cheap 10 horse ever appears we will have 70 pounds per hp and speed along at maybe 12 knots. Almost as fast as a speeding Kilburn!
I'm making a bit of fun but the speeds are well beyond what any normal sailboat will do under any conditions. And most of the sailboats you will speed by will be carrying a motor on their stern that is at least as big as the Clinton or the Sears.
I hope to have some scans of the finished drawings of the AF4. We'll count up the plywood sheets and see if my weight guess is realistic. I'm getting a little worried!! I'm reminded of the time we worked on the new missile design where it got heavier and heavier to the point where the booster couldn't get it up to flight speed. No suprise to the weights engineer I carpooled with. He always knew what the final weight was going to be. You see he had invented the idea of "missile baloney".
Someday I may get to put my full catalog on the net. For now I'll put one design in each issue. The text will be for the most part the same as what appears in the paper catalog.
We've been looking at all the power/speed charts the past few weeks and it seems like all I've used them for is as evidence that motor manufacturers don't tell the truth about their power ratings. But Petesboat falls directly onto Gerr's "average" planing chart!!!
PETESBOAT, MOTOR/SAILER, 24'X7', 1500 POUNDS EMPTY
This boat was designed for Pete James, brother of Karl James who built the original Jewelbox. Petesboat is an overall enlargement of Jewelbox and I'm guessing a 50% increase in volume and weight over Jewelbox. The hull shape is patterned after my Twixt and thus has a warped V entry to reduce pounding and a wide stern to improve performance under power. There was much discussion about whether the warped V entry was worth the extra work. Converting to a simple flat bottom entry would be easy. On Twixt I'm quite certain the V is worth the work and Ed Heins praised the V entry on his Tween, but perhaps not on these larger boats.
Unlike Jewelbox, Petesboat is supposed to be a planing motorsailer. Picking through the numbers in Dave Gerr's writings, and assuming Petesboat will weigh 2100 pounds ready to go, I figured it takes 40 honest horses to plane out at 17 knots. 15 horses will push it 9 knots and it looks like nothing in between makes much sense. Pete used it with a 4 horse motor the first season and reported 5 knots on the GPS with it. Second season he fitted 30 hp and said it planed at 15 knots full throttle. Then he purchased a new 60 hp outboard which gave 20 knots at full throttle. He said she handles very well although he didn't try any hairpin turns. He's still tinkering with trim tabs to lower the nose a bit under power. Fuel consumption with a 60 hp engine would normally run about 6 gal/hr so you see the down side of trying to "cruise" with a planing boat.
The sail rig is a 200 square foot balanced lug with a single pivoting leeboard. The rudder is offset to one side and the big motor is on centerline. We motor with the rudder retracted and sail with the motor retracted. Pete reports the effect of all the offsets in the rigging is nothing - she handles the same in both tacks. He says she always goes about 8.5 knots on the GPS reaching in a good wind.
The plywood bill looks like six sheets of 1/4", 5 sheets of 3/8", and 14 sheets of 1/2". The tinted Plexiglass for the windows can get expensive and it will often be worthwhile to get the plastic companies to bid against one another. In spite of its size this is still simple jigless construction. This was Pete's first homebuilt. Plans are $50.
Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. (If you order a catalog from an internet page you might state that in your letter so I can get an idea of how effective this medium is.) Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currancy that I can admire but not spend.) I'm way too small for credit cards.
Anyway..... Anytime a design from the Catalog of Prototypes starts getting built I pull it and replace it with another prototype. So that boat goes into limbo until the builder finishes and sends a test report and a photo. There are three boats in that catagory right now.
Scram Pram, a 16' multichine Birdwatcher type, is being built near Savannah. Just got a couple of photos of the nearly done boat and hope to get one on this page soon.
And Skat, a 12' cat boat daysailer, is being built near Phoenix.
And a Fusebox has been started by a Boat Psychologist in Kansas. It occurred to me that he chose the Fusebox because it has two built in couches. I think he said that he has the bulkheads and transoms done.
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