Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(15 July 2018) This issue will be about power and the Coast Guard. The 15 July issue will start propellers.
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.
 | Left:
Skidadl, a new AF4breve. |
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Motors per the Coast Guard
If you were building an outboard powerboat you might ask yourself "How much power can I safely place on my boat?" It's a good question.
The Coast Guard wrote up a pamplet that addresses that question plus other important ones about safety in homebuilt power boats. I think you can search out a copy on the internet, it seems like it was always hard to find. What I have here was written a while back but I think the basics have not changed.
It is not clear to me what sort of force is behind the pamplet. I see boats powered well beyond its recommendations all the time. The placard you see on commercial powerboats pretty well agrees with the limits given in the pamplet. The pamplet gives the reasoning behind some of the limits, always a dangerous thing to do because sometimes the logic leaves you wondering. I think the limits given in the pamplet are based on experience and are pretty reasonable.
HORESPOWER LIMITS ON INBOARD AND I-O BOATS...
The pamplet says specifically that there are none! This is one area where I fail to follow the logic.
HORSEPOWER LIMITS ON OUTBOARD BOATS...
This you can figure for your boat. The factors involved are length, transom width, transom height, steering method (remote or tiller), and chine type. Weight doesn't figure into it although it is the main speed factor in a planing powerboat.
For any boat you first measure the overall length in feet. For example if your boat is 17' 7" long you would convert it to decimal feet to call it 17.58' long
Then you measure the transom width in feet (I think they intend the width at the planing surface). Lets say you measure 5'2" which you would convert to 5.17'.
Next you multiply the length times the transom with to get the FACTOR. In this case it would be 17.58 x 5.17 = 90.88. Then you round off that to the nearest whole number, in this case the factor is 91.
The pamplet next has several catagories of hulls and ways to figure safe power for each. I'm going to try to present the info as two charts, one for smaller boats and one for larger. Here we go...
This is the chart for smaller boats with the factor of 52 and below. To put this in perspective, My Jon Jr design with length of 12' and width of 3' would have a factor of 36 and be limited to 3hp on this chart. (I think the photo below shows it with 4hp.)
The Coast Guard comes down pretty hard on flat bottomed hard chined boats as you see. To quote the pamplet, "Why? Because with a hard chine and a flat bottom you must turn more slowly to maintain stability." OK, I'll buy that while at the same time pointing out that the same flat bottomed hard chined boats with inboard power would have no limit to power by the same pamplet.
Next I'll try to conjure up the chart for boats with factors above 52.
Here we have three selections. The red solid line is "FOR OUTBOARDS WITH REMOTE STEERING AND A TRANSOM HEIGHT OF AT LEAST 20 INCHES". I've limited the factor to 200 which would be a pretty large boat, 25' x 8' for example. But in the pamplet there is no limit to the factor so to power an even larger boat you could extend the straight line as far as needed with the same slope.
The green dashed line is "FOR OUTBOARDS THAT DO NOT HAVE REMOTE STEERING OR TRANSOM HEIGHT IS LESS THAN 20 INCHES".
The black dotted line is "FOR FLAT-BOTTOMED, HARD-CHINED OUTBOARDS THAT DO NOT HAVE REMOTE STEERNG OR TRANSOM HEIGHT IS LESS THAN 20 INCHES".
OK, the obvious question is what about flat bottomed boats with remote steering and 20" transoms? I'm pretty sure the writers didn't mean to place them in the high powered catagory. For example my stock AF4 has a 15" transom with 18' length and 4.5' transom width for a factor of 81. By the third line it would have a max safe hp of 25, and having used AF4 I would agree with that. Here is mine planing at about 12 mph with a 10 hp motor at about 3/4 throttle.
But if I raise the transom to 20" with a long shaft motor and add remote steering do I now jump to the red line and go to 72 hp?? If the problem with the flattie is handling in turns I don't see how the deeper transom or the remote steering makes it safer for triple the power. (No doubt it would be safer though). But a literal reading of the pamplet makes it appear that way.
My Jonsboat design with 16' length and 4' width has a factor of 64 and would be limited to 17 hp. This one by Greg Rinaca has a long shaft 18 and goes about 25 mph as I recall.
And If the purpose of the long shaft motor with its deeper transom is to keep water out of the stern, what about a short shaft motor sitting in a water tight draining well, as with AF4?
Well, the pamplet can't address every case. So as always, use some judgement.
VAMP
VAMP, LIGHT ROWBOAT, 12' X 3.5', 60 POUNDS EMPTY
After the success Frank Kahr had with Robote I designed a smaller version with most of the same features. The idea was to get a fast and seaworthy boat that would cartop so easily that you would want to leave it on the car top more or less all the time, ready to go at a moment's notice. From cartopping smaller boats like Toto I had found that they hardly slow the car down or keep a compact car from getting 30+ miles per gallon on the highway. I thought the trick was to keep the boat small enough that complicated tie downs were not required. I also knew that Piccup, at about 90 pounds, is about the most a fellow would want to cartop. Lighter is better, of course, but once the weight gets below 70 pounds the boat becomes so easy to load that getting it super light is not mandatory. The prototype Vamp that you see here was built by Ken Prims of Layton, Utah. Here is his boat atop his compact car, a very nice package as I had hoped.
Why not stick to a canoe like Toto? It has the size, weight and shape (mild sheer and squared stern) that makes for easy cartopping. But a good rowing boat can be as fast, dryer, and take a passenger with more ease. The only problem I see with Vamp is that her V bottom can make beaching more challenging than a boat with a flat bottom plank.
Construction is with taped seams from three sheets of 1/4" plywood, the same bill of materials as for the larger Robote. Robote will be a lot better if you are rowing with a passenger all the time. Vamp might be a handier impulse boat for a solo boater, although it can take two adults in a pinch.
Vamp plans 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:
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
1aug17, RowingSetup, Oracle
15aug17, Taped Seams, Cormorant
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
Table of Contents
