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JONSBOAT, POWER SKIFF, 16' X 5', 200
Jonsboat is just a jonboat. But where
I live that says a lot because most of the boats around
here are jonboats and for a good reason. These things
will float on dew if the motor is up. This one shows 640
pounds displacement with only 3" of draft. That should
float the hull and a small motor and two men. The shape
of the hull encourages fast speeds in smooth water and
I'd say this one will plane with 10 hp at that weight,
although "planing" is often in the eye of the
beholder. I'd use a 9.9 hp motor on one of these myself
to allow use on the many beautiful small lakes we have
here that are wisely limited to 10 hp. The prototype was
built by Greg Rinaca of Coldspring, Texas and his boat
is shown above when first launched with a trolling motor.
But here is another one finished about the same time by
Chuck Leinweber of Harper, Texas:
In the photo of Chuck's boat you can
see the wide open center that I prefer in my own personal
boats. To keep the wide open boat structurally stiff I
boxed in the bow, used a wide wale, and braced the aft
I usually study the shapes of commercial
welded aluminum jonboats. It's surprising to see the little
touches the builders have worked into such a simple idea.
I guess they make these things by the thousands and it
is worth while to study the details. Anyway, Jonsboat
is a plywood copy of a livery boat I saw turned upside
down for the winter. What struck me about that hull was
that its bottom was constant width from stem to stern
even though the sides had flare and curvature. When I
got home I figured out how they did it and copied it.
I don't know if it gives a superior shape in any way but
the bottom of this boat is planked with two constant width
sheets of plywood.
Greg Rinaca put a new 18 hp Nissan two
cycle engine on his boat, Here is a photo of it:
The installation presented a few interesting
thoughts. First I've been telling everyone to stick with
10 hp although it's well known that I'm a big chicken
about these things. Greg reported no problems and a top
speed of 26 mph. I think the Coast Guard would limit a
hull like this to about 25 hp, the main factors being
the length, width, flat bottom, and steering location.
Second, if you look closely at the transom of Greg's boat
you will see that he has built up the transom in the motor
mount area about 2". When I designed Jonsboat I really
didn't know much about motors except that there were short
and long shaft motors. I thought the short ones needed
15" of transom depth and didn't really know about
the long shafts. Jonsboat has a natural depth of about
17" so I left the transom on the drawing at 17"
and did some hand waving in the drawing notes about scooping
out or building up the transom to match the requirements
of your motor.
I think the upshot of it all is that
short shaft motors need 15" from the top of the mount
to the bottom of the hull and long shaft motors need 20".
There was a lot of discussion about where the "cavitation"
plate, which is the small flat plate right above the propeller,
should fall with respect to the hull. I asked some expert
mechanics at a local boat dealer and they all swore on
a stack of tech manuals that a high powered boat will
not steer safely if the cavitation plate is below the
bottom of the hull, the correct location being about 1/2"
to 1" above the bottom. But Greg had the Nissan manual
and it said the correct position is about 1" BELOW
the bottom. Kilburn Adams has a new Yamaha and its manual
says the same thing. So I guess small motors are different
from big ones in that respect.
But it seems to be not all that critical,
at least for the small motors. Greg ran his Jonsboat with
the 18 hp Nissan with the original 17" transom for
a while and measured the top speed as 26 mph. Then he
raised the transom over 2" and got the same top speed!
I've gotten several Jonsboat photos
although I imagine I have misplaced several. Here is one
by Jim Hauer:
And another by Barry
And another by Don Graham:
There is nothing to building Jonsboat.
There are five sheets of plywood, and I'm suggesting 1/2"
for the bottom and 1/4" for everything else. It's
all stuck together with glue and nails using no lofting
or jigs. I always suggest glassing the chines for abrasion
resistance but I've never glassed more than that on my
own boats and haven't regretted it. The cost, mess, and
added labor of glassing the hull that is out of the water
is enormous. My pocketbook and patience won't stand it.
Glassing the chines and bottom is a bit different because
it won't show and fussy finishing is not required.