Shallow draft outboard powered dinghy
spambegone@earthlink.net Plugster
I want to power with a standard outboard and want to have as shallow of
draft as possible but still have a good ride in chop. I know jet outboards
are available but want to see what I can do with a standard (15" shaft)
outboard of about 25 hp.
I am thinking about using some sort of tunnel in front of the prop so that
the outboard can be mounted as high as possible but still get unobstructed
flow. Will this work? What is the best shape for the tunnel? How well it
work when turning? Is there any good methods to try to protect the prop?
Should the tunnel start at the front of the boat? Are there any other
features that I should think about including in the design?
The basic design goal is for a 4 or 5 person tender/dinghy that can also
pull a kneeboarder for fun.
Thanks in advance
Mark
Brian
wide beam ... a barge. Aim for 14' in a flat bottom boat or 16' in a
V-bottom boat.
b) 25 hp is very high for a 12-foot-anything. It'd be fine for the 14' or
16' boat if they are designed for it (planing hulls ... displacement hulls
will use less than half this horsepower and go 8-10 knots, only.)
c) Why complicate the boat and re-invent the wheel by using a tunnel hull?
You can, but keep in mind that the tunnel reduces the planing surface area
(lift) ... your 12-foot boat will have to be even wider, a bigger barge. If
you're willing to go with a longer boat, then having a tunnel will
necessitate the boat being longer than if you didn't. If you go without a
tunnel, then you can use a prop and reduce it's diameter by going with more
blades or a higher pitch (get Dave Gerr's book on the topic), or you can go
with a jet drive outboard motor. Yamaha makes shallow water outboards that
have a wider cavitation plate mounted closer to the prop and smaller
diameter, higher pitch, props. These let you raise the motor and will save
a few inches. For such a small boat, do not be tempted to raise the
cavitation plate clear out of the water. You'll lose appropriate
maneuverability under power and will have cavitation problems. The smallest
jet outboard that I've seen (Mercury, Yamaha) is a 40-hp. I haven't checked
the specs, but they may produce closer to 30-hp at the jet. Motors are
required to state horsepower at the prop (rather than shaft) nowadays, but
I'm not sure if that applies to jets. Check their web sites. If you go
with a jet, be prepared to pay several thousand dollars more for the
motor/jet combo than you'd pay for equivalent performance coming from a
prop, and expect the motor/jet combo to weigh far more than the equivalently
performing prop motor combo. In other words ... jets are heavy, expensive,
and require a longer or wider boat. With a jet, you also give up slow speed
maneuverability and they take LOTS of practice to keep the boat going
straight when running them in reverse.
Hope that helps you narrow down your boat design ... good luck and have fun.
Brian
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"Plugster" <plugster spamb...@earthlink.net> wrote in message
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Joe Stafford
tramp. Then mount a motor to the rear of center of it so you can operate it
forward of center to balance the motor. 25 HP will make a real wild machine.
"Plugster" <plugster spamb...@earthlink.net> wrote in message
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spambegone@earthlink.net Plugster
Brian,
Thanks for the information. I know first post was a bit lean on information
and my ideas since I am hopeing to find out what other people think and not
just say what I am thinking. Maybe the best thing for me to do is tell
everyone what my mission requirements are for the boat.
I want a boat that can be used for a tender on a larger power or sale boat
(not that I have one...yet) but still is a fun boat for water sports and a
bit of fishing. I want a tender that can be stored on the deck of a boat
about the size of a Bayliner 45. To store the boat on deck of a typical 45
foot moterboat I think12 foot length is kind of a maximum. I want to be
able to trailer the boat behind a compact car. I also want the tender to be
able to be beached and used in shallow rocky waters.
> a) For a 12-foot boat to hold 4-5 people, it'll have to be a pram with a
> wide beam ... a barge. Aim for 14' in a flat bottom boat or 16' in a
> V-bottom boat.
>
> b) 25 hp is very high for a 12-foot-anything. It'd be fine for the 14' or
> 16' boat if they are designed for it (planing hulls ... displacement hulls
> will use less than half this horsepower and go 8-10 knots, only.)
I was kind of basing the general perimeters on something like the Boston
Whaler Impact 120 (http://www.bostonwhaler.com/REC/spec_120_impact.shtm)
that is 11' 6" long and takes a minimum of 25 HP and a maximum of 40 HP.
About the tunnel hull, I was maybe thinking about something like the
smallest boat from Livingston Boats (http://www.livingston-boats.com/). It
looks like there is a chance that the draft of engine could be reduced a bit
but I don't know. Maybe once you are on plane the prop would start to
ventilate.
I have seen the jet outboards but they tend to have all of the problems that
you mention so I want to avoid them but still want to reduce the risk of
hitting the prop. I think the main reason for me is that the smallest jet
outboards start over $4000 and you can get a 25 HP for about $2200 new and
less used.
Thanks for the help.
Mark
Terry K
sitting foreward with a 3.5 Suzuki that stuck out about 8" below
the bottom. It was not very good in a chop unless you sat at the
back and went slow. It was a pig to row, not slow but capricious.
I was going to add a keelson, but sold it, much to my present
regret. It was some fun on a flat day, with my chin hanging over
the bow, steering by body weight shift at about 20 kts.
It was a heavy glass copy laid over a rotton old wooden boat I
bought new for 150 bucks in a clandestine basement workshop run
by 2 old rummies that produced one a day from cheap plywood, ring
nails and scrap strapping. It never even got a coat of paint, so
it only lasted 10 years, abused and left to rot on the shore near
my mooring in Douglas Harbour.
Terry K
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specifically denied for mass mailing and unrequested
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Sal's Dad
> about the size of a Bayliner 45.
Then budget isn't an issue? (c:
> I was kind of basing the general perimeters on something like the Boston
> Whaler Impact 120 (http://www.bostonwhaler.com/REC/spec_120_impact.shtm)
> that is 11' 6" long and takes a minimum of 25 HP and a maximum of 40 HP.
Whalers are great, but that's a lot of power...
> About the tunnel hull, I was maybe thinking about something like the
> smallest boat from Livingston Boats (http://www.livingston-boats.com/).
It
> looks like there is a chance that the draft of engine could be reduced a
bit
> but I don't know. Maybe once you are on plane the prop would start to
> ventilate.
Why not just get one of these? http://www.livingstonboats.com/spec12t.html
shows the 12T as using a 20" shaft 20 hp. Try it with a short shaft; it
might be perfect! Or, call them, and ask ...
Or, it would be easy to experiment with hull designs in plywood (you
specified glass, but I don't understand why, in a small one-off); the
catamaran approach would probably work great with flat bottom(s).
Another approach is to experiment with a more "traditional" skiff shape.
I've modified my Bolger Diablo (plywood/epoxy/glass, 15' x 5') to run
shallow. It has a transom height of 20", and I have a short shaft 15 hp.
I cut away a wedge from the transom, about 2 1/2 inches high, 8" wide,
tapering into the bottom about 16" . The boat now runs on a plane in 6" of
water (lightly loaded) with no major decrease in performance.
Have fun!
--
Curtis
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spambegone@earthlink.net Plugster
Sal's Dad <Sal...@nospam-Betts-ORourke.net> wrote in message
news:a7ve8b$chi$1...@bob.news.rcn.net...
> > I want a tender that can be stored on the deck of a boat
> > about the size of a Bayliner 45.
>
> Then budget isn't an issue? (c:
> > I was kind of basing the general perimeters on something like the Boston
> > Whaler Impact 120 (http://www.bostonwhaler.com/REC/spec_120_impact.shtm)
> > that is 11' 6" long and takes a minimum of 25 HP and a maximum of 40 HP.
>
> Whalers are great, but that's a lot of power...
>
> > About the tunnel hull, I was maybe thinking about something like the
> > smallest boat from Livingston Boats (http://www.livingston-boats.com/).
> It
> > looks like there is a chance that the draft of engine could be reduced a
> bit
> > but I don't know. Maybe once you are on plane the prop would start to
> > ventilate.
>
> Why not just get one of these?
http://www.livingstonboats.com/spec12t.html
> shows the 12T as using a 20" shaft 20 hp. Try it with a short shaft; it
> might be perfect! Or, call them, and ask ...
I have called them and took a look at one at a boat show. I think it is a
good boat but am thinking it maybe better without the center part sticking
down. I also want to make the interior a bit nicer.
> Or, it would be easy to experiment with hull designs in plywood (you
> specified glass, but I don't understand why, in a small one-off); the
> catamaran approach would probably work great with flat bottom(s).
One off fiberglass is not too hard. A cheap female mold for a dinghy can be
made using masonite and a bunch of clay. The finished boat will still need
some fairing but much less than a plywood/epoxy/glass. I also think it is
cheaper since you can use polyester.
> Another approach is to experiment with a more "traditional" skiff shape.
> I've modified my Bolger Diablo (plywood/epoxy/glass, 15' x 5') to run
> shallow. It has a transom height of 20", and I have a short shaft 15 hp.
> I cut away a wedge from the transom, about 2 1/2 inches high, 8" wide,
> tapering into the bottom about 16" . The boat now runs on a plane in 6"
of
> water (lightly loaded) with no major decrease in performance.
This sounds like something I should try. Do you think another shape other
than a wedge is best for the cut away? How deep of a wedge do you think
will work? Would a wedge (or triangle or 1/2 circle) cut out work with a
slight "V" hull?
Thanks
Mark
Sal's Dad
> > I've modified my Bolger Diablo (plywood/epoxy/glass, 15' x 5') to run
> > shallow. It has a transom height of 20", and I have a short shaft 15
hp.
> > I cut away a wedge from the transom, about 2 1/2 inches high, 8" wide,
> > tapering into the bottom about 16" . The boat now runs on a plane in 6"
> of
> > water (lightly loaded) with no major decrease in performance.
>
> This sounds like something I should try. Do you think another shape other
> than a wedge is best for the cut away? How deep of a wedge do you think
> will work? Would a wedge (or triangle or 1/2 circle) cut out work with a
> slight "V" hull?
I really don't know... You need to leave enough surface area to lift it up
on a plane. I didn't dare cut away more than I did; it left almost 8" of
flat bottom on each side of the cut-out.
Another approach to shallow water performance: Take a look at JetJon
http://jetjon.homestead.com/page1.html .
spambegone@earthlink.net Plugster
>
> I really don't know... You need to leave enough surface area to lift it
up
> on a plane. I didn't dare cut away more than I did; it left almost 8" of
> flat bottom on each side of the cut-out.
>
> Another approach to shallow water performance: Take a look at JetJon
> http://jetjon.homestead.com/page1.html .
>
I will give your method a try. Maybe making the notch a bit deeper and
longer.
At this time I do not want to use a jet but in the future I thinking about
an aluminum flat bottom (used jon boat?) with a jet drive for use in rivers
and that is great link.
Thanks
Mark
Keith
http://www.scavengerbackwater.com/scavenger.html
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spambegone@earthlink.net Plugster
Thanks for the link.
Mark
Keith <klem...@airmail.net> wrote in message
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Trent Hink
"Keith" <klem...@airmail.net> wrote in message
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spambegone@earthlink.net Plugster
> That engine would be great for dismembering kneeboarders.
At first I also thought it was way to dangerous and not usable for my
application then I thought about using it as an inboard with the prop in a
tunnel under the back of the boat. The engine would then be mid way to the
front of the boat. For steering I would then need a rudder.
Still, I think a regular outboard is more cost effective and I like liquid
cooling.
Mark