Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(21nov97) This issue includes some observations about the Bolger Birdwatcher. There are some line images in there that you will need to get the full story. This particular essay is supposed to be one in a series that will hopefully lead to a new design. A design done live on the net, so to speak. Look for the next issue about the first of December. It should have an essay about a small power cruiser put together by Kilburn Adams of St. Louis.
I built my Birdwatcher in the early part of 1989, launching it in May of that year. The boat was designed by Phil Bolger. My boat was built from the first release of plans and was actually the second Birdwatcher. The first was built by Ron Mueller who lived near Denver at the time. I had a few long phone conversations with Ron before starting mine. Ron took all the chances. By the time I started mine, Ron had proven the concept and I knew it.
Birdwatcher is about 24' long and 6' wide. The main cabin is 3-1/2' deep. The entire boat is enclosed in the cabin area and is open to the point where you can see both stem post and stern post from anywhere in the boat. The cabin is covered with a deck with a centerline walkway 24" wide going from front to rear. So you can stand there securely with the deck coaming (cabin roof?) about belly button high. While camping I cover the walk slot with a fabric cover or an insect net. You walk around inside stooped over then.
The hull itself is a very simple double ended skiff. It's bottom is dead flat with no skids or stiffeners. For the most part it has a skin of 1/4" plywood with a bottom laminated to 1" thick from two plies of 1/2" plywood. The thick bottom is more or less bulletproof and adds to the ballast effect. There is no actual ballast in the usual sense.
My boat was built very closely to the plans, with no epoxy sheathing, and weighs 1100 pounds with its trailer. Marc Smith (Bloomington, Indiana) made his with epoxy sheathing and said his was 1250 pounds including the trailer. We never weighed our trailers. But I'd think the actual boat weighs 800 or 900 pounds complete with rig and a minimum of gear aboard.
My boat/trailer rig is 30' long from the hitch to the end of the raised rudder.
The sail rig on my boat was the first one designed, a simple 126 square foot sharpie sprit sail on a mast about 24' long. The sail has no halyard. It is tied to the mast and is normally furled to the mast in storage. The mast is free standing and is about all an ordinary guy would care to put up by himself. When stepping the mast you need to tie the heel end down to the step area. Then when you start walking mast upward, it won't see-saw on you as you past the balance point. I trailer my boat with rudder attached and tied upward. Once I launch the boat I can rig it in maybe five minutes. Excellent for any sailboat and especially so for one this large.
Later Phil designed a larger rig but I never converted mine and have no intentions of doing so. I mostly sail solo and the simplicity of the first rig suits me.
Birdwatcher was designed to be rowed in conditions too light for sailing. There are oar ports towards the stern with excellent rowing geometry. I recall timing Birdwatcher at 2.7mph over a mile on one windless day. Not deadly fast but useful under those conditions. There are no provisions for a motor and Phil would get upset at the mention of it.
...SECOND TIME AROUND...
My boat sat in my shed from 1993 until 1996. Nothing wrong with the boat. The old pickup truck I used to haul it was kaput at the time and I had to let it go.
But just before I had laid the boat up I had made two changes to it in an effort to make it tack more quickly. In particular I made a shorter tiller (a very simple thing - just a 1-1/2" square stick tapered to fit the socket in the steering gear.), and I cut away some of the centerboard.
Birdwatcher tacked very slowly and could hang up in stays if things didn't go your way. The rudder set up limited rudder deflection to about 20 degrees, I think. I thought that was the main source of the troubles. The tiller is confined to the narrow inside of the pointy stern boat and there simply isn't room there for much deflection. Making the tiller shorter allowed greater angular deflection in those confines and thus rudder deflection increased to about 30 degrees. I'd like still more but that's an improvement.
(While I'm on the rudder subject, there was another mod I made during initial construction that was beneficial, I think. The blueprint rudder is mounted at a hefty pivot angle that almost matches the rake of the sternpost. The rudder itself is vertical. The mismatch in the two means the area of the rudder is well behind the pivot axis of the rudder. Thus the helm loads needed to balance the rudder loads are larger than they have to be. As I recall Mueller said he rerigged his prototype to give less mast rake and ease the helm forces. On my boat I padded out the rudder pivot axis to make it more vertical. That brings the axis closer to the rudder area and reduces the leverage the rudder has on the helm, reducing helm loads. My boat feels OK but the helm gets heavy if the boat is really heeled and honking. Not an unusual problem. I think the general rudder and linkage set up could be improved some more by someone willing to tinker a bit with his boat)
As far as cutting away a part of the centerboard, here is what I did. The raised BW centerboard protrudes well out of the case, which is about 2' deep. In fact the raised board goes all the way to the deck, cutting off easy access to the storage bin on the starboard side. (The mast and centerboard are offset to the side to keep the walkway clear.) The board is weighted to keep it down. On my boat I have a lanyard rigged high on its leading edge to pull it up. The lanyard runs through a pulley to a cleat in the steering area. I cut away all of the centerboard that protruded above the case except for a bit that connected to the lanyard. When the board is raised I have access to the bin.
But the main idea was to improve tacking by narrowing the part of the board that was underwater. In effect the trimming I did accomplished to a degree. The cutting certainly did no harm to performance. When I started sailing the boat again in 1996 I thought it was tacking better. I haven't been caught in stays since then although that may be dumb luck.
....ENTER THE MOTOR....
Phil would never discuss putting a motor on Birdwatcher. Same with Dovekie. Even though I had operated the boat several years without a motor, it was clear there were times when one would be beneficial.
Launching under oars could certainly be difficult to impossible for a solo skipper in crowded areas, especially when docks are used. Also the small rig boats like mine seem to have a "hole" in their weather window. When the wind is about 5 mph or slightly more you have the situation where it's too windy to row and it's not windy enough to sail anywhere. You can still get around but it takes patience. Also, rowing when there are power boats about is nearly impossible. Any wave will stop you dead.
Mueller used a motor from the beginning. He built the boat with a simple plank sticking out the side to which he clamped a 2 horse motor. The motor can mount on the side of this narrow sterned boat like some canoe motors. Ron said 2 horse was plenty and that she was a good motorboat!
Marc Smith added a motor after being caught too many times at the dock on a leeside boatramp. Power boaters waiting to launch were in no mood to wait for him to row to windward out of the launch cove. I think Marc had a 3 horse Johnson mounted on a clip-on steel mount.
In my case I had an old Clinton outboard saved from the dump. I got lucky in 1996 and found a man who knew that some Tecumseh ignition parts were interchangeable with the old Clinton. We got it going again. The mount I made is shown in the next figure. It was quite simple and actually only took about two hours to make. All it is is a 2X6 bolted to the aft bulkhead, the bulkhead which is at the end of the glass area and just forward of the steering gear. A slot was cut through the side just aft of the bulkhead, the 2X6 cut to fit and then bolted in. It worked perfectly from the start but my motor is quite small physically. If you have a BW and want to adapt such a thing, you might make it too long and trim it off after you've played with it. No doubt Mueller's mount was about the same.
....PERFORMANCE WITH THE CLINTON....
Here are the performances I got in clam conditions with the Clinton on the Birdwatcher. The speeds were judged by timing runs past landmarks which measure one mile apart on the Corps of Engineer maps. At 1/3 throttle the boat ran 4.7 knots. At half throttle the boat ran 5.2 knots. At full throttle the boat ran 5.8 knots.
(The boat handles pretty well under power provided the centerboard and rudder are used. This boat draws only 3" or 4" with its boards and rudder up. Having no skeg or skids at all on its bottom, directional stability is quite poor with the boards raised. I've found it steers best with the centerboard about half down and the rudder all the way down. I set up the motor to be pretty stiff on its pivot. Then I set the motor such that the boat runs straight with the rudder centered. Then I steer it with the rudder. I'm sure the rudder and centerboard slow the boat a bit, but steering without them is pretty hairy.)
What do the performance numbers mean?
Look at the figure which plots speed against pounds per horsepower for various lengths of waterline. This plot is based on one which appeared in an essay by Dave Gerr in an issue of Boatbuilder a while back. I don't recall that he stated the source but back in the last century at lot of tank testing was done by scientists to establish this sort of formula.
Engineering data like these are seldom totally correct or totally wrong. They are intended to get you into the right ballpark. If you are lucky they will put you right across home plate. A couple of things are obvious right off the bat. There is no allowance for efficiencies of various hull shapes. And no allowance is shown for effects of windage and wave action. What the plot implies is that the big factors are waterline length, weight and power. The chart here is for lower speed hulls. It's usually titled "semi displacement" but the values with more than about 500 pounds per horsepower are more pure displacement. With weights below 100 pounds per horsepower the boat may be starting to "plane" and you can see things are changing rapidly. Beyond that the ability of the chart to predict things becomes confused. Waterline length loses importance with a planing boat and speed becomes dependant mostly on pounds per horsepower. We'll present that story in a future episode.
If we enter a few values for Birdwatcher we could predict speed. Weight is probably around 1000 pounds with me and the motor and a bit of ordinary gear. Waterline length is probably about 20'. The motor is rated at 3.5 hp at 6800 rpm. So we've got about 286 pounds per rated horsepower. At 20' waterline the Birdwatcher should go about 7.2 knots.
But the boat went 5.8 knots at full throttle. What happened?
Use the chart a different way. Knowing we went 5.8 knots with a 20' waterline length, we can see we would need about 500 pounds per horsepower. That would mean the Clinton is putting out about 2 hp at full bore. Assuming half throttle is 1 horse, we might enter the chart at 1000 pounds per horsepower, go to the 20' waterline length, at predict a speed of 4.8 knots. That compares with the measured speed of 5.2 knots at half throttle. I've used this motor with other boats and get pretty good agreement with the charts as long as I assume the Clinton is putting out 2 hp at full throttle.
One might ask what happened to the power? For one thing I've been told that motors older than about 10 years old were rated at the power head and not at the prop. The loss due to the gearing is supposed to be about 10%. Although my motor is maybe 25 years old, it's not evident that it ever saw a lot of use. That is to say it isn't worn out mechanically. The big factor in the missing power may be that the motor is turning about half the revs of its rated power speed. It's set up for a very light faster boat. For my boat a prop with maybe half the pitch of the given one might produce the full predicted speed.
But on this boat it doesn't matter. The advice of Mueller from ten years ago is remembered: 2 hp is plenty for a Birdwatcher. In round numbers you might think that, for a displacement boat, 1000 pounds per horsepower will push you pretty well, and 500 pounds per horsepower is more than enough. At least in good conditions.
For a cheapskate like me the real question is why run at half throttle? Why not back off again to 1/3 throttle to cruise at about 4.7 knots. That's about the speed a Birdwatcher will go on a good reach while sailing.
I've tried to measure fuel consumption but without much success. To do so accurately requires a fairly long run and the ability to measure the small fuel use. In general I thought the motor went through a half gallon an hour at any throttle setting! That's about 10 miles per gallon. It sounds poor but my little 2-1/2 gallon tank might get me through 25 miles in 5 hours, probably a day's effort. I would expect a modern motor to do much better in that respect. Gerr suggested a rough guess of a gallon an hour for each 10 horsepower. So my motor is about 5 times that!
Next time we'll look at a couple of "planing" boats.
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.
This issue's boat is called IMB. It was designed several years ago when International Marine had a design contest going and I entered this one as the "International Marine Beacher", thus the IMB. (I don't enter contests any more.) IMB is still in the prototypes catalog although I heard of one that was built in Ohio. The builder said he had it to the paint stage when someone saw it in his workshop and bought it for use as a powerboat! Never got a photo or test report.
IMB, BEACH SAILBOAT, 13.5' X 5.5', 350 POUNDS EMPTY
IMB features a "Birdwatcher" cabin and I'm going to explain the advantages of this type because I feel they aren't well understood. Birdwatcher cabins are full length with panoramic windows and a center walkway slot in the deck. Everyone rides inside including the skipper.
Around 1985 Phil Bolger showed me a cartoon of his 24' Birdwatcher and I didn't know what to make of it. He presented it in a 1986 issue of SMALL BOAT JOURNAL. Ron Mueller built the prototype and he convinced me that the this cabin really works. I built the second Birdwatcher and here is what I found:
These boats can be self righting without ballast because the crew weight works as ballast. They sit down low in the hull looking out through the windows (although I found standing in normal winds was quite acceptable). In addition, the cabin sides provide lots of buoyancy up high to insure a positive righting moment. (Since I wrote that a few years ago I'd say these small ones may not be as self-rightious as the larger ones.)
These boats are operated totally from within the cabin. No one need ever go on deck, even during rigging. For boating with small children I can see no equal.
These are cool boats. The tinted Plexiglass windows cut the sun's power. The crew can always sit in the shade of the deck. Downdraft from the sail cascades through the walkway slot, acting like a giant windscoop. If you live in the Sun Belt you need one of these.
IMB is a smaller version of the type, actually an 8' long Birdwatcher cabin atop an enlarged Piccup hull. She takes two sheets of 1/2" plywood, 8 sheets of 1/4" plywood, and one sheet of 3/16" Plexiglass. I think she will daysail three adults and camp two in a pinch.
Plans for IMB are $15 until a prototype is built.
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 currency 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 two boats in that category right now. Scram Pram, a 16' multichine Birdwatcher type, is being built near Savannah. The first Scram was completed two years ago in Houston but I think the builder went straight to building another boat and I didn't get a real test report or suitable photo of the completed Scram. Also Skat, a 12' cat boat daysailer, is being built near Phoenix.
BACK ISSUES LISTED BY DATE