Jim Michalak's Boat Designs

1024 Merrill St, Lebanon, IL 62254

A page of boat designs and essays.

(1August 2020) We look at prop "slip". The 15 August issue will review sharpie sprit sail rigging.



... 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....


...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.


Chuck Leinweber sent this photo of Gordon and Mathew Barcomb in their Mayfly14 all decked out for the Texas200.



Contact info:


Jim Michalak
1024 Merrill St,
Lebanon, IL 62254

Send $1 for info on 20 boats.



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".


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.


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!


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.




Oracle might have been called Roar3. It is a bit longer and lower than Roar2 so it looks sleeker. It is actually closer in shape to my Toto canoe. I'm pretty sure it is a visual improvement over Roar2. Hard to say if it is a practical improvement. Being lower sided it might seem less secure but then again my Roar2 has never taken water over the wale in the ten years I've had it and maybe I've been hauling around excess freeboard. Max Wawrzyniak of St. Louis made the prototype. We rowed our boats side by side on a good day recently. I'd say the Oracle was 1/2 mph faster all around going about 4 mph at cruise and 5+ mph on the gps at full effort. It should float two adults quite nicely.

I'd like to point something about the way I draw the line on many of my boats. I make card models of my designs which help me check the panel expansions and allow me to get a 3D check of the shape. The models are done in the same scale as the blueprint, in this case 1/8 and the model is fairly large. So I knew that Oracle has nice curves all around. But if you look at the end view in the lines drawing you see all straight lines! In a way this is a throwback to the original "instant" boats which had hull panels made from straight saw cuts. It is easy for a designer to make the jump to more flexible shapes, that is hull shapes which can look like about anything you might imagine being made from plywood. Back when I started drawing boats I noticed some of the nicest shapes in 3D had end views made totally of straight line segments. Don't ask me why. Not only does that give a nice shape, the designer's job is simplified. For example, with Oracle I might start by drawing the top view. Then I might sketch in the side view needing nothing more than an idea of what the stem, midsection, and stern will look like. Next I draw the end view which will include those elements connected by straight lines. In normal mechanical drawing it only takes two views to totally define an object. So without even drawing the side view of the boat the shape is defined. It only remains to go back and figure it out and adjust the top view, etc., etc.. Happy is the designer who gets it right the first time through.

Oracle is built with taped seams using four sheets of 1/4" plywood. No jigs, no lofting.

Oracle plans are $20 when ordered direct by mail 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.






15aug19, Rowing1, Cormorant

1sep19, Rowing2, OliveOyl

15sep19, BC Scram Pram, Philsboat

1oct19, Herb's OliveOyl, Larsboat

15oct19, Herb's OliveOyl 2, Jonsboat

1nov19, Herb's OliveOyl 3, Shanteuse

15nov19, Herb's OliveOyl 4, Piccup

1dec19, Taped Seams, Ladybug

15dec19, Plywood Butt Joints, Sportdory

1jan20, Sail Area Math, Normsboat

15jan20, Trailering, Robote

1feb20, Bulkhead Bevels, Toto

15feb20, Cartopping, IMB

1mar20, Small Boat Rudders, AF4Breve

15mar20, Rudder Sink Weights, Scram Pram

1apr20, Two Totos, River Runner

15apr20, Water Ballast, Mayfly16

1may20, Water Ballast Details, Blobster

15may20, Mast Tabernacles, Laguna

1jun20, Underwater Boards, QT Skiff

15jun20, Capsize Lessons, Mixer

1jul20, Scarfing Lumber, Vireo14

15jul20, Lugsail Rigging, Vamp


Mother of All Boat Links

Cheap Pages

Duckworks Magazine

The Boatbuilding Community

Kilburn's Power Skiff

Dave Carnell

JB Builds AF4

JB Builds Sportdory

Hullform Download

Puddle Duck Website

Brian builds Roar2

Herb builds AF3

Herb builds RB42

Barry Builds Toto

Table of Contents