Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15Dec99) This issue gives an update on my AF4 design. Next issue , 1Jan00, will be an overview of Ray Clark's Plyboats Demo design software.
If you are interested in free analytical boat design software go to http://www.plyboats.com/> (archived copy) and download the Plyboats Demo. I'll be discussing it next issue if you care to follow along.
It has been two years since I started working on my AF4 and it has been through two seasons of use. I designed it more or less on the internet when I first started this web page. You can access those old issues by going down to the "WAY BACK ISSUES" link towards the end of the page. I think AF4 is a perfect example of how a design evolves. Twelve years ago I built my Birdwatcher (still have it) with its walk through slot top cabin that allowed self righting with an unballasted boat. But it was clear in the first year that the slot top had all sorts of benefits for any small boat. Kilburn Adams showed up at a messabout with a large powered model of a 22' power dory he was thinking of and it had a slot top cuddy. That evolved into his modified Sturdee power dory shown here:
I saw that and came up with AF4, which is slightly larger than the Sturdee Dory. Kilburn saw AF4 and came up with his newest boat shown below. It is 20' long and 6' wide, 25 hp four stroke, with the slot top cuddy cabin. Here you can see the advantage of that slot top as Kilburn walks forward with his feet on the bottom of the boat standing upright. Kiburn's new boat is significantly larger than AF4.
My calender shows I used AF4 25 days this year and I suppose the average use is about three hours on the water and about half of that time actually under power and moving, the rest of the time on shore or at anchor as I swim or beachcomb. That's not a lot of boat use to some but I'll bet it is more than most homebuilt boats get after their first year. (My calender shows also I used the Roar2 16 days, the Toto 12 days, the Piccup Pram 10 days, and the Birdwatcher only two days.)
And it's not bad for a project that cost me about $600 including a used motor (but not the trailer that I had on hand). My AF4 has held up well. You may recall it was built with economy - lumberyard all the way except for $40 worth of epoxy and some fiberglass tape. Looking back at it, a thicker and stiffer bottom would have been wise. Originally I wanted to keep the weight to a minimum mostly to give my Escort a chance to tow it. But I found out a new clutch in an old Escort is at least $500 so I've always towed it with my pickup truck which is in no way affected by the weight of the AF4 and trailer. So if I had to do it over I'd try 1/2" ply on the bottom instead of 3/8" which should be twice as stiff.
Probably related to the bottom flex is a small leak that developed at the end of one bottom butt plate - the only problem in two years. I secured the end of the butt block with small bolts and the leak is stopped.
AF4 could use a bimini in the summer. I sketched a bimini in on the plans but never installed one on my own boat. I'm quite certain the thing to do is buy a cheap one from Iverton's or West Marine for less than $200 complete with bows and hardware. For a boat stored under cover as mine usually is, the cheap material should last a long time. I put it off last year thinking a $200 cover was extavagent on a $600 boat but I'm sure I would thank myself. That is the only change I'm contemplating.
The general arrangement and size has been quite right for me. My boat has no built in seats and I pilot from a simple folding chair (a zero cost "found" chair, the best kind). I can move it about if needed and, best of all, fold it up and have a nice open 4' x 6' cockpit if needed into which I usually put a folding chaise lounge. Tough life!
I never get in as much "testing" as I want to. That usually takes really good weather and a fair amount of time, especially when testing for fuel consumption. But usually I'll go out to the lake and get side tracked - I go to test and end up messing about, which is exactly what this boat was designed for and what it is quite good at. Add to that this year is that my wife actually likes the boat and if she comes along I don't do any fussy testing..
I've always thought AF4 handled quite well, especially considering I put no special features into it's bottom. I'm reminded of Bolger's advice in his Folding Schooner book of "...I say briefly and dogmatically that if a boat is light and powerful enough to plane well, she can't be prevented from planing: all the designer needs to worry about is her attitude and stability. Design features meant to generate exceptional lift always seem to degrade the boat."
It's a smooth water boat, of course, more or less like a jonboat in that respect. Under power her nose rises a bit at about 8 mph and by about 11 or 12 mph she is up on plane with no change in attitude. At higher speeds the bow drops a little bit but not much. Here is a photo of AF4 at about 12 mph.
I tested AF4 a bit with a 3.5 hp Seaking, an aircooled Clinton powerhead motor from about 1970. I'm pretty certain this motor actually peaks at about 2 hp, burning about .5 gph at nearly any throttle setting. Total weight at the time would have been about 550 pounds. The boat would not plane with this motor. Full throttle gave 7.5 mph (6.5 kts) and about 15 mpg.
More extensive tests were done with a Sears 7.5 hp motor, an aircooled Tecumseh powerhead (I think) from perhaps the mid 70's. The photo shown above used the Sears motor and the boat is clearly planing. Again total weight was about 550 pounds. Although the Sears motor is noisy and shakes, it is acutally pretty efficient fuelwise. It has twice the power of the Seaking, I'm guessing it really has 5 hp, but burns no more fuel. Full throttle with the Sears motor gave 12 mph (10 kts). Throttled back to 10 mph (still on plane) she got 14 mpg. Throttled back to 6mph she got 18 mpg. I made no full throttle fuel runs thinking the motor couldn't handle that for long.
I did some brief runs with the Sears motor with another man on board and then she looked like this:
We didn't measure speed but I thought the boat was on plane, but just barely. You can see that the waterline has moved forward in this photo to about the front end of the cabin window, where solo the waterline would be at the aft end of the window.
Tests were also done this year with a 1956 Johnson 10, a $75 find. The reason it was so cheap is that it wouldn't run more than a minute at a time! Last year I delved into it and found the carb float was incorrect and I fixed that. So this year that motor became my main power and here is a photo of the boat with that motor.
This is an old motor but very well made and probably state of the art at the time. It is a watercooled two cylinder. Unlike the other two motors, I think the Johnson puts out its full rater power. Full throttle with the Johnson gives 17 mph. I usually operate it at part throttle, about 14 mph and very much on plane. Actually part throttle with the Johnson gives the same planing picture as full throttle with the Sears motor. Full weight with me, the boat and the Johnson is probably about 600 pounds since this motor with fuel is almost twice as heavy as the Sears.
The Johnson burns a lot of fuel, coming from an era when a family sedan got 10 mpg. At first I thought there was something else wrong with the motor but now I just accept the fuel consumption. I ran it for an hour at 14 mph and burned 1.5 gallons of fuel for 9 mpg. A new 10 horse four cycle motor would go twice as far for the fuel, if not farther. But they cost about $2000 now and someone pointed out I could buy a lot of fuel for that.
And here was something interesting: I droned around at just below planing speed, about 8 mph, for an hour and burned 1.4 gallons for less than 6 mpg! No point in running this baby slow. The old SeaKing behaves about the same - uses the same fuel no matter what. You can't help but think these older engines squirt fuel at the same rate all the time, the throttle just diverts some of the squirt into the crankcase. The obvious question is what fuel rate would the Johnson burn at full throttle - still 1.5 gph? Maybe so but I've been cautioned about operating a 45 year old motor at full throttle for long periods.
OK, let's drag out the mandatory chart. You may recall this from this web page two years ago. It's based on Dave Gerr's writings in Boatbuilder magazine 12 years ago:
To review, this is just a "average" hull sort of chart and some boats will do better and some worse. And the chart in no way assures you that your design will be under control through its speed range - that is very apparent if you try to put big power on a hull shaped for low speed efficiency. In the slower "displacement" portion of the speed range where the hull has more than about 100 pounds per horsepower, performance is dependant on waterline length. Below about 50 pounds per horsepower the boat is planing and static waterline length is not a factor. I've drawn in straight lines between the two ranges although Gerr didn't discuss that region.
AF4 has about 15' of static waterline. The chart indicates the boat should have a top speed in the displacement range of about 8 knots (9 mph) but my own feeling is that the transition starts at about 8 mph. It's a bit hard to measure with accuracy with no speedometer. I measure my speeds with a stopwatch over a known distance between landmarks, usually a mile. The SeaKing is down in this range giving 7.5 mph (6.5 knots) at full throttle. 6.5 knots on the 15' waterline line corresponds to a ratio of over 200 pounds per horse. So I think the SeaKing is putting out about 2 or 2.5 hp at full throttle even though it was sold as a 3.5 horse.
You might argue that the SeaKing is an old worn motor but I don't think so, old yes, but not really worn since I don't believe it got much use. It's worth looking into and I still have the literature that was sold with the motor. The motor is rated at 3.5 hp at 6800 rpm. The prop is geared at "14-21" which I take to mean the prop turns at 2/3 of engine speed. At the rated engine speed that would be 4500 rpm. The prop pitch is 4-1/2" which means that, with no slip, the prop would advance 4-1/2" per rev, or 4.5 x 4500 = 20250" per minute, which is 1688 feet per minute, which is 19 mph! I'm way below that. It might all be prop slip but I don't think so, the motor is simply not revving that high and the full power is not being developed. If we were racing AF4's with SeaKing 3.5's I'd look for a prop with less pitch to get up to full revs and full power.
While on this subject it might be interesting to speculate on what sort of boat is needed to get full revs and power out of this motor. Let's say the prop slip is really about 20% so the boat speed is 15 mph (13 knots). From the chart a boat planing at that speed needs a weight/power ratio of about 70 pounds per hp. At 3.5 hp that would mean a full boat weight of 245 pounds. The motor and fuel would account for maybe 45 of those pounds. That means the boat and crew weight together can't be more than 200 pounds. It might be possible to have a 100 pound hull and a 100 pound skipper and get full speed out of that motor. So the maybe the rating folks didn't "lie" but they "fibbed".
AF4 with the Sears 7.5 went 10 knots which on the chart corresponds to about 95 pounds per horsepower. That would mean the Sears is putting out about 5.8 horsepower for the 550 pound rig. The chart shows the boat should not yet be on plane but I'm quite sure it is.
AF4 with the Johnson 10 goes 17 mph (15 knots) and from the chart that would require a horse for every 55 pounds. The rig then weighs about 600 pounds indicating the Johnson is putting out about 11 hp. Interesting.
I still have a lot of faith in this chart. I'm sort of using it to use my boat as a dynamometer - I'm predicitng horsepower by knowing boat weight and speed. But all the answers are pretty consistant with reality, I think.
If there is one new thing learned from the tests it is that these engines don't follow anybody's fuel consumption theories. Looks like you have to take each one as it comes. If I went cruising the Johnson 10 besides Kilburn's Yamaha 10 we'd go about the same speed but I'd need three times the fuel.
I'll discuss the Plyboats Demo program.
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AF4, CUDDY POWER SKIFF, 18' X 5', 350 POUNDS EMPTY
Here is the AF4 writeup in my current catalog:
AF4 is a simple low powered cuddy cruiser intended for protected waters. Kilburn Adams of St. Louis brought a boat similar in spirit to AF4 to our Midwest Messabouts and it was easy to see what a good idea this is. Kiburn's boat was a modified Sturdee Amesbury power dory witha a small cuddy added and a new 4 stroke Yamaha 10. He thought the empty boat weighed about 600 pounds ready to go. It planed quite well with two men on board at about 13 knots. He made a 60 mile trip on the Mississippi (above the Alton dam where the current is small) burning about 4 gallons of fuel. It's quiet and cheap to operate. It's a good idea now that smaller 4 stroke motors are more common.
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AF4 is supposed to do about the same job. Its simple hull shape won't handle rough going like the dory, but it's very easy to build and roomier. This boat has an open bow well for anchors and junk. The cabin is 8' long and 3' deep and 4' wide for minimal camping. It should be comfy for one and snug for two. it has a slot top to allow stand up boating in good weather. You cover it with a snap on fabric piece in the rain and cold. The cockpit is a full 6' long and 4' wide and totally open. You could lounge there too with a folding chaise. Aft of the cockpit is a draining motor well which will take the standard 6 gallon fuel tank and then some.
For power I would say 10 horsepower maximum. AF4 planes easity at 12 mph with an old Sears 7-1/2 horse (single cylinder, air cooled, for $50 at a yard sale).
AF4 uses the simplest nail and glue construction. It takes five sheets of 1/4" plywood and four sheets of 1/2" plywood.
Plans for AF4 are $30.
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. Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currency that I can admire but not spend.) I'm way too small for credit cards.
Here are the prototypes abuilding that I know of:
Jonsboat: I suspect a few of these have been built but I've never gotten a report. Here are photos of one being built by Chuck Leinweber of Duckworks Magazine I'm told this boat is now complete except for painting. (As I'm writing this I know of two other Jonsboats in the final stages.)
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Mayfly12: A Mayfly12 is going together up in Minnesota. The decks are on and he's into the sailing bits. By the way, the sailing bits on almost any sailboat large or small consume about half of the effort in labor and materials. Just when you thought you were about finished! (Just talked to the builder and the project is on hold until next spring while he moves into his new house.)
AF4B: A builder in Virginia is building AF4Breve, a 15.5' version of the 18' AF4. I tried to talk him into building the 18' version but he had two very good reasons to go shorter - a short trailer on hand and insufficient building space for the larger boat. The AF4B is essentially a "scrunched" version of AF4 but comes from a whole new set of drawings. The prototype is in the final building stages where the sanding and filling and painting seem endless. (Just heard anew from the builder. Seems his property and priorities have been temporarily rearranged by Hurricane Floyd. So progress on AF4B is now on hold.)
AF2: An AF2 has been started in Oklahoma that should be close to the plans. Bottom (double layer of plywood) is on and finished and the hull has been flipped upright by brute strength, no ropes or jacks used. See the flip at Rich's site. As I'm writiting this he is in the sand/paint stage and progressing very rapidly. This boat is far enough south that it could be launched during the winter.
BACK ISSUES LISTED BY DATE
Herb builds AF3 (archived copy)
Hullforms Download (archived copy)
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