Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(15 August 2018) We measure propeller "slip". The 1 September issue will measure prop thrust.
THE BOOK IS OUT!
BOATBUILDING FOR BEGINNERS (AND BEYOND)
is out now, written by me and edited by Garth Battista of Breakaway Books. You might find it at your bookstore. If not check it out at the.... ON LINE CATALOG OF MY PLANS......which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Then Duckworks sends me an email about the order and then I send the plans right from me to you.
Propellor Slip
(Jeeze! I noticed lately that the rest of the world is using "propellErs" while I am using "propellOrs!" I looked in my dictionary and saw that the chancellOr lives in the chancellEry. Luckily that dictionary gives me a break by saying a propellOr is the same thing as a propellEr. Thus we all call it a "prop". I'm reminded that logic is taught by math teachers and not English teachers. And also that I once took a technical writing class where the professOr said that spelling isn't as important as your old teachers told you. He said meaning is never lost by misspelled words but that humor is often added. Or is that humour?)
In the 15March issue I showed a simple gadget that I use to measure the "pitch" of small propellors. Recall that the pitch is the amount a propellor will move forward with each revolution just as a screw has a pitch. In the US the pitch is stated in inches. Thus a prop with a 10" pitch will screw itself forward 10" assuming it does not "slip".
But props do slip for several reasons. First, the pitch has nothing to do with the actual size and area of the blades. So in a case of a powerful boat with tiny props (but of the right pitch) they might indeed spin around in place and do nothing because they are overloaded, sort of like a dragster spinning its tires into smoke. There are ways to calculate the blade area needed to prevent it but this is not a problem with the size of boat I design.
Another cause of pure slip is a damaged blade. This happened to me a few years ago with a Sears7.5 (an aircooled Eska) on AF4. While motoring around I felt the motor touch bottom. After clearing the sand bar the motor revved to high heaven and the boat went nowhere. Below about 5mph it would bite and push the boat but above that it was all noise and no thrust. I was sure the shear pin was gone but on checking found it intact. I worried the boat back to the ramp and decided to put the motor aside until I could check out its clutch, an internal job I wasn't too interested in. Eventually I noticed the leading edges of the blades had burrs. I sanded them off in a couple of minutes and, guess what? It was like new! Once again I got speed consistant with the noise that engine makes.
Perhaps the major element of slip is that on a boat with a lot of drag the prop is moving the water aft more than moving the boat forward. Obviously, if you tied your boat to the dock and fired up the motor you would measure 100% slip even though the motor/prop were functioning totally correct.
And a prop needs a certain "angle of attack" to the water to produce thrust in the first place and that would measure out as a part of total slip. I don't know what that angle is but an indication might be that it is very hard to get slip down below 10% in any case. On a 10" prop, typical of a small outboard that will peak at maybe 20mph, that would be 1" of pitch. I suppose midway out on the typical prop blade that would amount to 2 or 3 degrees of "angle of attack".
MEASURING SLIP
To measure prop slip you need to compare the actual speed of the boat by the theoretical "no slip" forward speed of the prop.
In my tests the speed of the boat was measured with a gps. (All tests were with my AF4 at about 800 pounds total weight.)
The "no slip" speed of the prop, which I will call Vns, takes some calculating. You start by knowing the rev's of the powerhead.
In my tests the rev's of the powerhead were measured with this handy tachometer on loan from Motor Max. It is a handy thing bought for about $40 from www.northerntool.com. One lead goes to a ground on the motor and the other wire wraps around a sparkplug wire. You can set the thing for different ignition designs, for example a spark on each rev, or every other rev, etc. The only problem I had with it is that the lead is so sensative that on a two cylinder motor it can pick up the jolt from the wire several inches away and give a false reading. What I did on a two cylinder engine was to wrap the lead around both sparkplug wires, right where they come out of the ignition plate and are close together and set the gadget to read two jolts per rev. Then all was fine. The gadget also records the operating time of the motor, thus you see the number on the screen below. When the motor is running it reads revs, when the motor is not running it shows total operating time.
If you know the revs of the powerhead you can figure the revs of the prop by knowing the gear ratio of the lower unit. You can guess that ratio by turning the powerhead one turn and noting how much the prop turns, getting all the system's slack out first (it can be considerable). By doing that and by doing some research on the acutal gears we found that the small Clinton type lower units (also used by Eska, etc.) had a .66 gear ratio and that the OMC lower units that I have were all .57. So if the powerhead measured at 3000 rpm then the Clinton prop would be turning 2000 rpm and the OMC prop would be turning 1710 rpm.
Now you can do some calculating. Powerhead rev's times the lower unit gear ratio gives prop rev's. Prop rev's times the prop pitch gives the "no slip speed" Vns in inches per minute. Inches per minute divided by 1056 gives speed in statute miles per hour. It turns out that for the Clinton lower unit, the Vns (in mph) will be the pitch (in inches) times the powerhead rpm all divided by 1600. For the OMC lower units the Vns (in mph) will be the pitch (in inches) times the powerhead rpm all divided by 1850.
ACTUAL TESTING...
The actual tests with AF4 were quite simple. Just mount a motor, remove the cowling so that the tach could be wired up in a minute, head out on the lake and run at several speeds, writing down the rpm at each speed. Then chart that data along with the Vns for that prop and lower unit. Here is the first one with my lowly SeaKing3.5, which is a Clinton made motor from about 1970:
Slip measures out at about 45% across the entire range. I've read that is very typical of low hp/weight boats with small props that won't plane. I got similar results but up to 50% slip with a Sears5 (Eska) using a 6" pitch that gives 6.8mph at 3500rpm full throttle.
Next I ran a 12hp Goodyear (OMC, about 1956) with a 10" semiweedless prop that was standard on motors like this for a long time.
Now we have something different with slip starting at low speed with about 30% maxing out at 35% when the boat is at its "hump" before planing. Once it starts to plane the slip reduces, going down to about 23% at full throttle at 3670 rpm at 15.1mph. The motor is rated at 4000 rpm indicating that a lower pitch prop might be in order.
Next I tried a Johnson10. This one has a 7" prop that is not weedless. It has large straight blades. It was a very windy day but I found a sheltered cove for most of the testing.
The slip of this prop is much lower than that of the weedless prop. It slips about 25% at low speeds, dropping to about 10% at full throttle of about 14mph. At one point it went 15 mph at 4300 rpm for 6% slip, probably wind aided. This motor is also rated at 4000 rpm. I don't know if that should be considered to be a "redline". But in fact I used this motor for a few years when my AF4 was still stripped down and lighter and a lot faster than it is now. We once measured it at 17mph with this motor and that would mean about 5000+ rpm. That seemed to do no harm. The motor is 47 years old!
A BOX OF PROPS...
Motor Max brought over a box of props for the OMC's for one more round of testing, this time with a SeaKing15, which is an OMC motor from the early 60's.
Funny thing about those props. They were from several makers and sizes but on my pitch gadget they all measured 1" less in pitch than the prop markings indicate! I'm pretty sure my gadget measures pitch on the blade's aft face correctly. Max also brought along a copy of Dave Gerr's PROPELLER HANDBOOK. He states that measuring pitch is not all that easy , and he presents a gadget similar to mine with a triangle template to meaure the angle of the aft face. Here is how I account for the difference. All these props have cambered blades, that is to say they have a curved airfoil shape. So even if they were mounted flat (as I would measure it) they would still produce some forward thrust when spun. So I figure the prop makers allow for that by adding a "one inch for the pot" sort of thing. (I notice that prop makers have very elaborate guages for measuring pitch and don't just measure it across the aft face. They can measure it in several places across the blade and might say that my 10" pitch prop actually has 8" pitch on the leading edge, 10" pitch in the center and maybe 12" pitch on the trailing edge.)
Anyway, off to the lake with the SK15 using a 10" standard prop. Here are the test results:
More or less similar to the Goodyear12 results except now slip continues to drop with speed to 15% at 3800rpm full throttle at 17.3 mph. This motor is rated at 4500 rpm indicating that less pitch might be nice. We tried it with several other props but settled on this 9" standard as being the best of the bunch:
Now full throttle gave 18.5 mph at 4300 rpm with 10% slip. That is not a huge increase from the 17mph that the 10" prop gave even though the revs are higher and the slip is less. The thing about changing prop pitch is that if you increase pitch you increase the load on the engine and you lose revs, so the top speed may not change much. And if you decrease pitch you decrease the load on the engine and you gain revs, but again the top speed may not change much. It is horsepower that moves the boat and horsepower is torque times rev's. Depending on the torque/rpm curve for a particular engine you might find that the horsepower is fairly constant over a pretty wide range because the torqe falls off as the motor revs up. And if your motor has say a constant 15 hp between say 4000 and 4500 rpm then the top speed of the boat might be constant too anywhere in that range. I'm thinking that is about true with my Seaking15 and most of the OMC's and that the extra speed I got was due more to a more efficient prop than to raising rev's and tapping into more horsepower.
Cormorant
CORMORANT, CABIN SAILBOAT, 32' X 8', 2500 POUNDS EMPTY
Cormorant is the largest boat I've ever designed. I always warn folks to think twice and three times before building a big boat because you can buy a good used glass boat for less, maybe a lot less. But a homebuilt boat can have features that aren't available in a production boat and so it is with Cormorant. This one is really a 20% enlargement of Caprice.
Straight enlargements rarely work perfectly and so it was with Cormarant from Caprice. (Don't forget that Caprice was an enlargement of Frolic2, etc., etc., right on down to my Toto canoe.) In this case I narrowed it from a straight enlargement to keep the width within simple towing limits since this large boat is supposed to live on its trailer most of the time. The layout is quite similar. The idea is that the adults sleep in the center cabin and the kids sleep in the forward room.
Like Caprice, Cormorant has water ballast, over 1000 pounds of it. Total floating weight with family is going to be up to 4500 pounds. You don't tow a boat this large behind a compact car but I think towing this sort of weight is common today, all done with expensive large trucks I'm afraid.
The sail rig looks pretty modest with a 207 sq foot main. I'll bet it is enought since this shape is easily driven. I don't think you can go any larger and still hope to handle it without extra crew and gear.Tthe lug sail shown is similar in size to Bolger sharpies and they seem to get by OK. Experience will show if it is too big/too little.
Constuction is taped seam, with no jigs or lofting. Unlike smaller designs this one does not come with a plywood panel layout drawing. Over the years I've learned two things about the ply layout page. First is that almost no one uses it. Second is that with a larger boat the work of finding and drawing and fitting all the pieces to the boat on scale plywood sheets overwhelms all the other work. So part of the deal with doing the design was that there would be no plywood layout drawing. However this is still a true "instant boat" in that all of the parts that define the boat are drawn in detail and you can scale them up on plywood, cut it out and fasten together with no need for lofting or a building form.
Garth Battista, who is a book publisher at Breakaway Books where he publishes sporting books including my Boatbuilding For Beginners (And Beyond), is a true boat nut and has worked himself up from dinghies and canoes to the big Cormorant. He took it initially on a quick shakedown run on a lake near his home and shortly later to Long Island Sound for a week with his family. Here are his comments:
"We had an amazing time living aboard Cormorant (christened "Sea Fever") in Provincetown harbor for 5 days. The tide there was rising and falling about 12 feet a day with the full moon. We'd be high and dry up on the beach for breakfast, swimming off the boat at lunchtime, walking the flats again by dinner. It was a blissful time for me and my wife and two girls. We moved around, took little sails here and there across the harbor (West End to Long Point, then to the lighthouse, then to the East End, etc.) anchoring here and there, usually just running it aground as the tide allowed and staying for a while. Many shells were collected, and tidal pools investigated. Of all the harbors I've ever seen, it is the most alive. It's a couple of miles across and fresh sea water flushes the whole place twice a day. The number of snails, clams, crabs, fish of all sizes, mussels, eelgrass, etc. was just mind-boggling. On high tides I'd go spearfishing (many attempts, no luck) where at low tide I'd been walking around.
We rigged a 8' x 15' white tarp with tent poles running crosswise as a canopy over our cockpit and hatch, supported along the mast folded down in the tabernacle, so we could escape the mid-day sun. Most days were hot and humid and mild, with only gentle winds. We rode out a nighttime thunderstorm with no trouble, just stayed up and watched the lightning. We attended a few wedding-related events, just walking ashore for one party, and for the wedding itself we returned late at night and rowed our dinghy out to the boat, our sleepy children just awake enough to get themselves aboard.
For our last two days we gave up the shallow-water life and sailed from P-town down to Wellfleet, about 7 miles, surfing along on gentle 3-foot waves with a following wind. We beached the boat at Great Island, walked the beach, had a picnic dinner, swam and played, spent the night, and left the next morning at 6 a.m. to beat the falling tide. Our weather radio mysteriously quit working that morning, so all we had was the prior day's forecast of 10-15 knot winds from the SW.
The wind had shifted into the west during the night, so we had to beat out of the harbor, and once we turned north to return to Provincetown, huge rollers were coming in off the bay, more or less directly into our port side, lifting us, rolling us, occasionally breaking and spraying water into the boat. We stayed well offshore to avoid the breakers in by the beach -- but with the falling tide it seemed that we needed to be nearly a mile out. It went from exhilarating to worrisome to mildly terrifying as we neared P-town and the wind kept picking up, past 20 knots to 25 and higher in gusts, and the waves just kept growing. The swells were in the 8-10 foot range, with a high percentage of them breaking at their tops, whitecaps everywhere.
But bless this boat! With its 1000 lbs. of water ballast, and the leeboard mostly up, we were able to bob and roll and slide over nearly all the swells. The worst of them were very steep and threw us sideways, maybe tilting us to 40 or 45 degrees briefly. We had two reefs in the main and the mizzen rolled down to about half-size, and still we blasted along on this nasty rollercoaster of a beam reach. It was the sort of trip that would be scary fun if it was just you and a buddy, but it's awful when you have your loved ones aboard, and you wonder who might get thrown overboard, and how you'd managed a rescue in the rough conditions.
Anyway -- the white knuckles got to relax as we finally made it past the P-town breakwater, and with great relief ran her aground out on the flats. The gale (or near-gale) continued to blow all day, kicking up 3 and 4-foot waves even in the protected areas of the harbor. The only boats we saw going out were an 80-foot schooner and a big whale-watch boat. A lobsterman we talked to later said he'd stayed in as it was too rough to check his traps.
We had a hell of a time taking the boat out and getting her on her trailer for the trip home -- but all worked out in the end, with the assistance of some very kind strangers; and I'm left with the memories of incredibly happy days. -- And an incredible boat.
All best, Garth
P.S. Jim -- I should also mention that on Sunday afternoon as we turned the corner from our run down to Wellfleet to the close reach upwind toward the inner harbor, the boat just drove perfectly. It seemed we made 40 degrees off the wind. That maybe wishful thinking, but it was an angle far better than I'd imagined a lugsail could manage. It was a joy to sail, in all conditions. My hat is off to you.
P.P.S. The number of people who came over to admire the boat and exclaim at its uniqueness, its coolness, its obvious functionality -- well, they were beyond count. "
One more thing, Garth sent me this photo of himself working hard on his new sports book:
Plans for Cormorant are $60 when ordered directly from me.
Prototype News
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.
We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...
And the Vole in New York is Garth Battista's of www.breakawaybooks.com, printer of my book and Max's old outboard book and many other fine sports books. Beautiful job! Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.
And a Deansbox seen in Texas:
Another prototype Twister is well along:
A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) This boat has been sailed and is being tested. He has found the sail area a bit much for his area and is putting in serious reef points.
AN INDEX OF PAST ISSUES
THE WAY BACK ISSUES RETURN!
MANY THANKS TO CANADIAN READER GAETAN JETTE WHO NOT ONLY SAVED THEM FROM THE 1997 BEGINNING BUT ALSO PUT TOGETHER AN EXCELLENT INDEX PAGE TO SORT THEM OUT....
THE WAY BACK ISSUES
1sep17, OliveOly Capsize Test, OliveOly
15sep17, Plywood Butt Joints, Philsboat
1oct17, Sailing OliveOyl, Larsboat
15oct17, Water Ballast, Jonsboat
1nov17, Water Ballast Details, Piccup Pram
15nov17, Scram Pram Capsize, Harmonica
1dec17, Sail Area Math, Ladybug
15dec17, Cartopping, Sportdory
1jan18, Trailering, Normsboat
15jan18, AF3 Capsize Test, Robote
1feb18, Bulkhead Bevels, Toto
15feb18, Sail Rig Spars, IMB
1mar18, Sail Rig Trim 1, AF4Breve
15mar18, Sail Rig Trim 2, Harmonica
1apr18, Two Totos, River Runner
15apr18, Capsize Lessons, Mayfly16
1may18, Scarfing Lumber, Blobster
15may18, Rigging Sharpie Sprit Sails, Laguna
1jun18, Rigging Lug Sails, QT Skiff
15jun18, RendLake 2018, Mixer
1jul18, Horse Power, Vireo14
Table of Contents
