Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(1 March 2016) This issue will be about hollow spars. The 15 March issue will be about bulkheads.



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.


Tom Clarke of http://theoarcruising.blogspot.com/ has been dreaming again. Here a Larsboat converted to a row cruiser.



Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



Hollow Spars


At first thought you might think that hollowing out spars is something you work hard at to save a lot of weight in the spars, and that could be true. But I've found that sometimes the hollow spar is the easy one to build and that you seldom save much weight. First a little theory:

elastic bending

The above figure shows the pattern of stresses in a beam that is under pure bending. Here the material is still in the "elastic" range, which is to say it it is not so highly loaded that it won't spring right back to its original shape once the load is removed. What is significant here is that compression and tension stresses develop that resist the bending and that the stresses are highest in the outer fibers of the beam. In the middle, at what is called the "neutral axis" there is no bending stress at all. You might think that the outer fibers are doing all the work, the inner fibers almost nothing. That would be just about correct in the conditon of pure bending but in real life the middle fibers do other important things. So you can't just remove the middle fibers totally but you can skinny them down in a beam meant to take mostly bending loads. The classic way to take advantage of the situation is with a I beam, where the outer fibers are thick and wide while the middle fibers are reduced to a thin web.

Similarly a tube has had its middle fibers removed. It does not have the bending strength of an I beam but has the advantage of being able to take bending loads in all directions because of its symmetry (unlike the I beam which is optimized for bending in just one plane) and is the optimum shape to handle torsion loads.

moments of inertia

It's actually pretty easy to determine the effect of hollowing out a spar, which is to remove a lot of those middle fibers that don't do much, on strength and stiffness. The effects are pretty much related to the "moment of inertia" of the section which is fairly easy to calculate. Some of the sections you might use for a spar are shown above.

The reason the moment of inertia tells you so much is that in a beam under a bending load, the stress and deflection are both directly related to the moment of inertia. For example, all other things being equal, if you double the moment of inertia of a beam's cross section, both the stresses and the deflections will be cut in half.

So the next question becomes, "How does hollowing out a spar affect the moment of inertia of the spars cross section?

The answer is that you can figure the moment of inertia of the hollowed out cross section, provided the cross section is still symmetric, by subtracting the moment of inertia of the hollowed out area that you removed from the full unhollowed moment of inertia. Here are some examples:

hollowed moments

Two things to point out here: First the moments are not reduced in proportion to the amount of material removed. You lose material a lot faster than you lose strength. For the 3" square the tube version reduces the section area by 25% but the reduction in the moment of inertia is but 6% Such a good deal!

Second, the square section is a lot stiffer and stronger than the round section with same width. The 3" square has 26% more area but its moment of inertia is 69% larger. Such a good deal! Some sail types like the lugsail or spritsail or even the sharpie sprit sail can use a square mast with little loss in sail efficiency. If your round mast flexes too much, convert to a square one! Can't do that with a gaff rig though because the gaff jaws insist on a round or octagonal mast.

By the way, another way to look at all of this is to see the effects of going to a smaller diameter mast. You quickly get yourself in trouble. The stiffness is proportional to the fourth power of the diameter or mast thickness. If you reduce the mast from 3" to 2", the mast stiffness is reduced by 80% !!


Some types of masts are hard to make hollow, round masts in particular. Usually the mast is made in two halves and the center of each half hollowed out and then the two are glued back together. Some sections of the mast might not be hollowed out in order to have some beef there for fittings. So it's a bit of work. Indeed making a round mast is a bit of work even when solid.

Let's look at a typical mast to see how much weight is really saved. Let's use this one as an example. It is round and I've shown the diameter at distances along the mast along with the section area at those points.

basic mast

The example shows the solid mast. The method of figuring the weight is to calculate the volume and then figure the weight from the density of wood which is typically about 30 pounds per cubic foot. The figure shows the volume calculated with a quick "curve of areas" where the area at each section is plotted along the length of the mast. The area under that "curve of areas" is the volume of the mast. Add them up and you get the volume of the total. In the example the mast would weigh about 22 pounds if solid.

You can use the same technique to figure the volume of about anything, including a hole. Here we'll use it to figure the volume of the hole in the center of the mast if it were hollow such that it had a 3/4" wall.

hole volume

The proposed hollow amounts to about 4.4 pounds of wood. The hollow mast will weigh about 17.5 pounds instead of 22 pounds assuming it has no beefed up solid sections for fittings. Whether the work involved in hollowing out the mast is worth the trouble is something the individual must decide. But you can see that this mast has its weight reduced about 20%, its stiffness and strength reduced by about 6%.


I've built two hollow masts, one for my Birdwatcher and later another for my Jinni which had been coverted from a sharpie sprit to a balanced lug. I built hollow to allow an easier construction and not to save weight, although the hollow masts were lighter. Both used simple box construction from 3/4" boards like this:


The Birdwatcher mast was 5" across and the Jinni mast 2-1/2". The staves that formed the walls were scarfed to length first and then cut with a circular saw to the proper taper one at a time. Then the masts was assembled with glue and nails like simple boxes, 24' long in the case of the Birdwatcher and 14' long for the Jinni mast. By doing this I avoided having to saw and shape the taper of sticks that would be way to thick to cut with my saw, as it was I never cut any wood more than 3/4" thick.

Both masts worked quite well. The original Jinni mast was square in cross section even though it was a sharpie sprit sail. I doubt that the small square mast disrupted airflow as you might think. The Birdwatcher mast was supposed to be octagonal and hollow - a fairly involved building effort. You might think the 5" square mast that I built would really disrupt the airflow to the sail. To minimize the disruption I turned the mast 45 degrees to the axis of the boat like this:

Birdwatcher mast

I thought it worked out very well. The sail is laced to the mast and is somewhat free to align itself from side to side, unlike a sail that is trapped in a groove or track on the aft face of the mast. (Phil Bolger has written several times that a sail set on the mast with loose mast hoops as in the old gaff days has the best aerodynamics. More modern streamlined masts with sail track or grooves often are supposed to swivel on the deck at each point of sail to allow proper mast/sail alignment. You can see how complex the details can become.) One day I was out sailing in Birdwatcher and snapped a photo looking up the leading edge of the mast. The sail flowed right off the mast as in the above diagram. Alas, I can't find that photo. Here is one in which you can make out the "diamond" mast. (I think you can also see why I've stuck with the slot top cabins and offset masts that characterize the Birdwatcher layout):

Birdwatcher mast

So in this case I felt I had it all: light hollow mast, easy construction, and good aerodynamics. Still, I hardly ever draw hollow spars on plans because the drawings can get really complex and I don't want to deny anyone the challenge of brainstorming a new way to make a hollow mast. There seems to be no end to the ways of making one.


Slam Dink


SLAM DINK, 7-1/2' X 45", 60 POUNDS EMPTY

Slam Dink was presented earlier this year as a prototype design and was quickly built by the Moffitt gang in Stone Mountain, Georgia. Here Sean and Lauren contemplate boatbuilding on a picnic table:


Slam is a pretty straight forward building job from two sheets of 1/4" plywood, which includes the sailing bits like the rudder and leeboard. No jigs needed. It's built by wrapping the sides around three temporary forms and going from there. The only hitch reported in building came when installing the chine logs. They bend in two planes and on a boat this short the bending is pretty sharp. Several chine log candidates broke before getting good ones on. That is a very common problem with short curvy boats, the common solutions are laminating the logs in thinner more plyable layers, steaming the logs to the bend, and bending slowly and carefully with good stock. This is a nail and glue boat with epoxied glass only on the outer chine corners to provide some armor. Yes, it could also be done with taped seams inside and out to eliminate the chine log.


This is the only boat I've ever drawn with a traditional spril sail. I used both a sprit yard and a sprit boom to keep good control of the sail and I was impressed with the photos of the boat sailing. The sail shown is made from a white polytarp sail kit available from Poly Sails. It has most of Poly Sails' technology in that it is taped together (temporarily, eventually you need to sew it together) and uses poly rope edging. But it uses my radial dart shaping technology that you can find in the back issues of this web site. At least in these light wind photos the sail looks to have none of the extreme twist that ruins the set of a lot of sprit sails. I think the key here is stiff spars, stiff sail edging, and the use of two sprits. If you omit the sprit boom you will need a different sheet location for each point of sail and probably will have no way to control it well on downwind.


These little dinks have problems with carrying a lot of weight, they are just too small to do so. Slam will have good flow lines up to a total weight of about 300 pounds. It will weigh close to 100 pounds empty and ready to sail so you aren't going to sail two adults in this little one, or any little one. It will float that much weight easily but the stem and stern will be immersed and it becomes a clumsy "through the water" boat instead of a fast "over the water" boat. Not that anything this short can really be fast. Also any boat this small and light will be dominated by the weight of the skipper and he will need to be somewhat nimble.

(This was written in 2000 and the boys in the photo are full grown men now and great boat adventurers on their own. So this design was a total success.)

Plans for Slam Dink are $20.


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:

And the first D'arcy Bryn is to the point the builder can sit and relax in it and imagine boating. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....

The first Jukebox3 is on the (cold) water. The mast is a bit too short - always make your mast too long. A bit more testing will be nice...

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.) He is doing the windows now...






15mar15, Bulkhead Bevels, Frolic2

1apr15, Capsize Lessons, Riverrunner

15apr15, Hollow Spars, Slam Dink

1may15, Boat Costs, Blobster

15may15, Small Boat Rudders, Roar2

1jun15, Emergency Flotation, RB42

15jun15, Thailand Mixer Cruise, Mixer

1jul15, Rend Lake 2015, Musicbox3

15jul15, Box Boat Stability, Mikesboat

1aug15, Taped Joints, Cormorant

15aug15, Plywood Butt Joints, Paulsboat

1sep15, Navigator Cabins, Vireo

15sep15, Boxboat Stability 2, Philsboat

1oct15, Center of Gravity, Larsboat

15oct15, Hullforms Model, Jonsboat

1nov15, Port Aransas2015, Piccup Pram

15nov15, Hullforms Results, Caprice

1dec15, Sail Area Math, Ladybug

15dec15, Sailing For Nonsailors 1, Roar2

1jan16, Sailing For Nonsailors 2, OliveOyl

15jan16, Sailing For Nonsailors 3, Robote

1feb16, Sharpie Sprit Rigging, Laguna

15feb16, Trailering Plywood Boats, IMB


Mother of All Boat Links

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JB Builds AF4

JB Builds Sportdory

Hullform Download

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Brian builds Roar2

Herb builds AF3

Herb builds RB42

Barry Builds Toto

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