The question of "what is the proper motor for my Rhodes 22" seems to
be a very straight forward and unambiguous question. Unfortunately
the response is not. It gets complicated by personal opinion,
economics, taste, environmental concerns, safety, comfort and
compromises that confront any sailboat decision.
A little theory first:
Hull Speed:
A sailboat, that does not plan, is governed by the theories of a
displacement hull. What this means as the hull passes through the
water, water is displaced around it. As speed increases different
wave patterns are formed until a wave that starts at the bow and
continues until the stern, this is considered hull speed. If the boat
continues to increase speed it will start to climb the bow wave
forcing the boat to travel uphill. The power that is needed is the
will increase dramatically. Primary factor that governs this
phenomenon is length water line (LWL). The equations that predicts
hull is sqrt(LWL)*1.34=Hull Speed in knots or 20**.5*1.34=5.99 knots
in the Rhodes 22 case. In truth the hull speed is closer to 6.25
knots, this is due to the work of the naval architect Philip Rhodes.
Remember the hull speed is only an inflection point on a curve.By
adding more power you can go faster but with greatly diminishing
returns.
There is also two other problems with adding power above hull speed,
the boat will try to become a "planning" boat, it is not. Tracking
and stability will be effected. The second problem is the self-bailing
cockpit, the speed will force the rear to squat as the boat is trying
to climb over the bow wave. The cockpit drain will then be below the
water line, and water will come into the cockpit,this is adorning.
Current:
This is a reminder, a boat floats on the water and unlike a car its
roadway can move under it. If a boat is traveling north at six knots
and there is a current flowing south at seven knots the speed over
ground will be one knot south. Also the boat can only go to hull-speed,
if the current is faster and against you, you loose, no motor will
help. Anchor or change course.
How much does a Rhodes22 weight?
The Rhodes22 has been getting heavier over the years. I will be using
3200 pounds for the boat and 800 pounds for crew and supplies for
total displacement of 4000 pounds.
How Much Horsepower for a Rhodes22?
The sailing magazines have been using a rule of thumb of 1 horsepower
per each 500 pounds of displacement. By applying this rule leads to 8
horsepower. There is one problem here that this information is
centered around much heavier cruising sailboats with other equipment
tied to the engine.
This formula determines the theoretical horsepower required for a
displacementhull shape to reach hull speed. This formula assumes that
there is no current or wind resistance and the hull is clean and
free of objects that could cause excess drag. Horsepower =
Displacement / ((150^2) / (Hull Speed^2) = 4000 / (22500/39.0625)
=6.389 minimum horsepower for hull speed. frankly I am uncertain
of the origins hundred and fifty constant in this formula for the
time being it will simply accept it.
The other method that I found was to calculated the required horse
power is the Speed to Length ratio for the vessel.
S/L = 10.655 / (Displacement/Shaft Horsepower)^. 3334
Boat Speed = (S/L) * (LWL)^0.5
Using the Rhodes22 numbers
5.99= (10.655 / ((4000/7.95)^.3334) * (20^.5)
O.K. which is right 6.4 or 7.95?
| Hp | method 1 |
method 2 |
| | Knots | Knots |
| 0.10 | 0.75 | 1.39 |
| 0.25 | 1.19 | 1.89 |
| 0.50 | 1.68 | 2.38 |
| 0.75 | 2.05 | 2.73 |
| 1.00 | 2.37 | 3.00 |
| 2.00 | 3.35 | 3.78 |
| 3.00 | 4.11 | 4.33 |
| 4.00 | 4.74 | 4.77 |
| 5.00 | 5.30 | 5.13 |
| 6.00 | 5.81 | 5.45 |
| 6.40 | 6.00 | 5.57 |
| 7.00 | 6.27 | 5.74 |
| 7.95 | 6.69 | 5.99 |
| 8.00 | 6.71 | 6.00 |
| 9.00 | 7.12 | 6.24 |
| 10.00 | 7.50 | 6.47 |
| 11.00 | 7.87 | 6.68 |
| 12.00 | 8.22 | 6.87 |
| 13.00 | 8.55 | 7.06 |
| 14.00 | 8.87 | 7.23 |
| 15.00 | 9.19 | 7.40 |
| 16.00 | 9.49 | 7.56 |
| 17.00 | 9.78 | 7.72 |
| 18.00 | 10.06 | 7.87 |
| 19.00 | 10.34 | 8.01 |
| 20.00 | 10.61 | 8.15 |
| 21.00 | 10.87 | 8.28 |
| 22.00 | 11.12 | 8.41 |
| 23.00 | 11.37 | 8.54 |
| 24.00 | 11.62 | 8.66 |
| 25.00 | 11.86 | 8.78 |
I suspect that method 1 is better at determining hull speed for a
given L. W. L. and method 2 approximates how increasing or decreasing
horsepower responds. Also what should be taken away from this table
is that a little horsepower can go along way.
Which Prop;
As a general rule, if you purchase a typical outboard motor for use
on a sailboat, you should instruct the dealer to provide the lowest
pitch and largest diameter propeller possible consistent with the
motor’s designed operating speed and horsepower. The "standard"
propellers that are provided with these motors would just not be
efficient with a small sailboat (they were most likely for
a small planing boat). Either you will be running at half throttle
and therefore never use all the available horsepower, or you will be
running at full throttle and wasting the energy in propeller slip.
As a propeller turns, it’s pitch determines how far it travels
through the water. No propeller is 100% efficient so that in reality
it "slips" by moving water aside rather than straight backward. For
a planing hull shape, this propeller slip can be as low as 10%. For
a displacement hull shape with a highspeed propeller, this slip is
usually around 40-50% by using the largest diameter prop we’re trying
to minimize the slip. I will use 45%. This means that the maximum
vessel speed is a function of the shaft speed in revolutions per
minute, the pitch of the propeller, and the amount of propeller slip.
Boat Speed = (RPM * 60 * Pitch) / 72913.2
And with a 45% slip it will be Boat Speed = ((RPM * 60 * Pitch) /
72913.2 *.55)
Using my Honda 9.9 hp motor with its 2.1:1 gearbox maximum rpm of
4500-5500, and prop of 9.75x6.5 as an example,
0.98 = (700/2.1 * 60 * 6.5) / 72913.2 * .55)
5.60 = (4000/2.1 * 60 * 6.5) / 72913.2 * .55)
6.30 = (4500/2.1 * 60 * 6.5) / 72913.2 * .55)
7.06 = (5000/2.1 * 60 * 6.5) / 72913.2 * .55)
7.70 = (5500/2.1 * 60 * 6.5) / 72913.2 * .55)
the 9.75x6.5 prop (BTW use a 9.5X8 in the 9.9 Honda case)
| RPM | KNOTS |
| 700 | 0.98 |
| 1000 | 1.40 |
| 1500 | 2.10 |
| 2000 | 2.80 |
| 2500 | 3.50 |
| 3000 | 4.20 |
| 3500 | 4.90 |
| 4000 | 5.60 |
| 4500 | 6.30 |
| 5000 | 7.00 |
| 5500 | 7.70 |
the stock 9.5x8.625 prop
| RPM | KNOTS |
| 700 | 1.30 |
| 1000 | 1.86 |
| 1500 | 2.79 |
| 2000 | 3.72 |
| 2500 | 4.65 |
| 3000 | 5.58 |
| 3500 | 6.51 |
| 4000 | 7.44 |
| 4500 | 8.36 |
| 5000 | 9.29 |
| 5500 | 10.22 |
As that can be seen from the example the lower pitch prop permits
slower docking and better matches the motor. There is also the issue
that the lower pitched props grip the water more firmly and have a
better reverse (they change the prop shape, it’s a little more than
simply pitch). The standard prop would never achieve the higher RPMs,
by most likely over loading the motor and the rated motor horse power
will never be developed. Warning placing too small of a prop could
to over revving the motor. Picking the right prop is an art as much
a science. Please contact the manufacturer and/or Michigan wheel for
the right prop www.miwheel.com. Most likely you want largest
diameter/smallest pitch prop available for the motor.
The question of weight;
The motor mount on the Rhodes22 is far to the port side, courtesy of
a Lever effect, its weight in relation to boat trim is magnified.
Maximum size motor that General Boats have used on the mount is the
9.9 Honda long shaft electric starts at 101 pounds. With this size
motor " man handling it " on and off the boat is problematic.
The shaft length question:
Most people use a long shaft motor (20 inch), general boats has used
two types of motor mount over the years. The 6:1 and the newer 7:1.
The 6: 1 will still need the motor tilted up in long shaft. 8:1 will
lift the Honda long shaft clear of the water without tilting. Some
people have used the Honda eight horsepower extra long shaft motor,
but be careful this is only a 23 in shaft motor not 25 inch (most
markers x-long), it and will need tilting in all cases and might
be in the water on a starboard tack. Recently General Boat has
changed the length of the rails for the motor lift, talk to Stan if
you want to use a x-long shaft on a new boat. The plus side is the
extra length might permit the motor to operate in a foot of steeper
waves. In general sailboat’s make poor motor boats, for all practical
purposes, and depending on conditions, the prop will come out of the
water in 4-5 foot seas. The captain who does not mind the throttle
might burn out the motor by over revolutions (some motors like the
Honda 9.9 horsepower are over rev protected). The good news is the
Rhodes22 can be sailed in worse conditions than motored.
Two cycle motors;
Two cycle motors have been the standard for a longtime, they are
cheap and lighter in weight at any given horsepower. They also are
smelling and prone to following fouling their spark plugs. That EPA
has outlawed most corroborated two cycle engines in 2006. The reason
is by their operation one-third to one-half of the fuel is dumped
into the water unburned. The second problem is the MBTE added to
gasoline in 1996 to reduce air pollution. Unfortunately MBTE might
be cancer causing and does not dissipate from water very well. Many
freshwater Lakes are banning them. Also the ban effects all current
PWCs which some people would consider the true reason for the
regulations. At this time it’s probably not a wise move to buy a
new two-stroke engine.
Electric Start:
The is a agreement for not having electric start, it costs more to
buy, the need to install a starting battery, cabling issues, weight
of the battery and "one more thing to break". With all that said I
still recommended getting it. The overriding issue for me is that
boat motors stall and it always seems to be at the worst time. Like
changing from reverse to forward while turning with at strong cross
wind in a docking maneuver. The manual start motors with the rhode22
mount are generally started in a different position then they are
normally run in. A quick restart has prevented boat damage, more
then paying for this feature many times over. The second issue was
my wife, she simply does not have the strength to start the manual
start motor quickly (or maybe at all). If something happened to me
on the boat, I wanted to give her one less excuse forcollecting on
my life insurance ;-)
Electric charging;
Many motors come with a charging coil rated between six and ten amps.
Unfortunately you only get full power at al most full RPMs. At motor
idle it might produce one-quarter to one-third of their rating.
Running a motor for 10 minutes warm up and 10 minutes getting out of
the dock might only provide .25 + 1 = 1.25 amp hours of charging. Not
much it will be a replacement for solar cells or outlet charging. On
the other hand and it’s a worthwhile feature, imagine doing nights
sailing and finding you do not have enough power to run your
navigation lights, Compass light and depth sounder.
Under sized motors:
From the hp charts above you quickly realized the does not take much
to move the boat. Remember these charts assumed a clean hull, no wind
and no waves. These will all be factors in a real world. I can
already hear "real sailors do the any sticking motors" My comment is
you have limited your options, and might need that motor get the boat
into the wind to lower sails or tack in heavy wind. Also remember you
might need that motor to overcome currents/tides, even if you had
enough wind to sail. And to get off of a lee shore it can be a
lifesaver (anchors do drag).
Altitude:
Motors are rated at sea level, with there power reduced three percent
for each thousand feet. As an example a 9.9 will become a 7.5 hp at
six thousand feet in Colorado. The dealer might also have to modify
the carburetor for these conditions (i.e. it’s running too rich). But
please make sure to readjust if brought back to sea level.
Remote controls:
Some people like remote control kits for their throttle/gearshift.
Generally they are mounted on the port gunnel or my favorite the rear
tiller handle. It gives the boat the "big yacht" feeling and can make
finding the control much better. Some other people have slaved the
tiller to the motor so they turn at the same time, this can be very
good, but remember that there are some turning situations where you
want them to moving independently.
Some real if examples:
Electric motors:
They generally in the three-quarter horsepower range drawing about 50
amps. 2 type 27 batteries should provide about two hours of operation
between charges (it is about 4 in a pinch but you never want to
discharge them more then 50%).Their pluses are quiet operation,
slower dock speed, lightweight, low cost and they are not effected
but altitude. The downside is that electric motors are not a
replacement for a real motor and they’re charging requirement should
not be underestimated. They only one that made seen for me that a
Colorado (high Altitude) sailor who was using a smaller lake (little
waves, no current, 2 hour range crossed the lake) and had power at
his dock.
2.7hp cruise and carry:
A fine little lightweight motors that I saw a Rhodes22 at 3-3.5
knots. Completely under sized for the boat.
4.5 hp Johnson's/Merc's/etc(two stroke)
Used by a lot of Rhodes owners on Lakes, with a limited amount of
waves and nocurrent. Overall the owners like the motor but as a
class they think they are a little under powered. I really only makes
sense to by this type of motor if weight (racing) is the most
important consideration otherwise I would avoid the two strokes.
The five horsepower Honda:
It’s a fine motor, is lightweight. Is still a little under powered
for the boat. And has a bad vibration problem being only one
cylinder. (And this goes for most of the one-cylinder motors) Another
consideration is this motor will be run at wide-open throttle most of
the time (wear/noise).
6 and eight hp two strokes:
These are the most common motors on the Rhodes 22. 6 hp is bested
used on lakes. People use them and seem happy with them. The
Mercury/Mariner being the Favorite for their throttle/gear controls
one the handle. With my opinion of two strokes that will be it on
these.
8 hp Honda.
I have had this motor and many Rhodes 22 owners have it. Is a very
nice fit to the boat. It is also the smallest Honda that has a
charging coil. Highly recommended.
The 9.9 Honda:
It weighs about 17 more pounds then the Honda 8hp and you feel every
one of those pounds taking the motor on and off the boat. It is the
first Honda offer electric start. In truth it is a derated 15
horsepower motor (in fact one screw removal will make it a 13.2 hp
motor). The 9.9 are a little too much for the Rhodes 22.
The 9.9 high thrust Yamaha (four strokes).
This is beautiful motor and I and my current one, it has the combined
throttle gear shift and the right gearing and prop, for a sailboat
right out of the box, also it is electric start with an charging coil
producing more power at lower RPM's. I would give this Motor strong
consideration.
15 hp Honda:
Completely over capacity for the boat, also larger then general
boats Limit for the mount, also it has enough torque to damage the
mount particularly In reverse. Its saving grace it's weight is
exactly the same weight as the 9.9. The yamaha gets more thrust into
the water then this motor.
Other considerations:
Honda has been making four stroke motors for the last 30 year (1973).
My shop manuals for the 8hp indicate this motor is on his fourth
generation. And for the 9.9 it's second to third. These are proven
motors. Many manufacturers are now coming out with for stroke
engine's I am not a fan of buying the first one(and a lot are over
weight). Yamaha has also been doing four strokes for since 1984.
Yamaha might also be doing the power heads for the Merc/Mariner.
Another consideration is finding a dealer for service. The decision
on which brand to buy might ultimately come down to a local support
Street Price:
8 hp Honda long shaft mail-order about 1600+-
9.9 Honda long shaft electric start mail-order 2100+-
9.9 high thrust Yamaha (four strokes), long shaft, electric start,
local dealer, 2300-2500
New vs Used:
Buy a new motor, It does not take much abuse to kill a used one, some
real horror story over the years, most leading to buying a new motor.
Outboard mechanics make plumbers look cheap and bank loans are
available for the parts.
The new Boat:
They are a common problem when a person gets a new boat. Several
new owners have not realized how powerful the motors are, or the
stopping distances of 2 tons of boat, or that boat’s does not have
breaks. Or how poor reverse is, or how an overreving props will
shuck air and have no effect, or how the tiller needs water going
over it for steering, or how wind efects the boat or being or how
trying to manhandle 2 tons from a dock even going slow is almost
impossible.
And that a boat going 3 mph can get person someone "fending off" real
hurt and the boat real damaged. The other common problem is being
unfamiliar with the controls setting the motor for the opposite of
what you want, like full power forward instead of idle or full power
forward in stead of full reverse, and the problems do compound.
I recommend for a new owner please leave the motor in idle anywhere
around the docks (like 200 yards) and in neutral the last 100 feet.
You can not go slow enough (but the wind has to be down) and take
your time and do not try to "slow off", pride can hurt here and do
not push a bad hand. Stay home if the wind is over 10 knots. This
statement can be changed based on wind and current but hopeful you
practiced in the open (drop a buoy in an open area and try docking to
it, like teaching a teenager to drive in a parking lot) before
trying to leave or arrive yourself. Learn how the boat response to
the motor, tiller, and wind and stooping distance. It really hurt
taking a boat in for repair (or paying for another’s boats repair)
the first day.
Michael Meltzer
Nov 2, 2000
We sailed for several years with an Evinrude 6 hp long shaft outboard
(21") with cockpit mounted remote engine controls. 6 hp was plenty
of power, when you could couple it to the water. Prop cavitation
begins to be a problem when the waves get to be 3 - 4 feet high.
In 1996, we replaced the Evinrude with a Honda 8 hp extra long shaft
outboard (25"), also with cockpit mounted engine controls. We
considered the Honda, Mercury, Yamaha, & OMC 9.9 hp 4 cycle outboards.
But, we were turned off by the extra weight hung out over the
transom. We went with the Honda 8 for several reasons:
- 1. 4 cycle engine vs. 2 cycle engine - no more fouled spark
plugs, fuel economy nearly doubled from 6 mile/gal to 10 mile/gal,
quieter running, no smoke or foul odor, no more mixing gas & oil,
fuel in the gas tank lasts much longer. I believe the 2-cycle engine
is an endangered species & the sooner it goes extinct the better.
They're filthy machines that have no business being allowed to foul
the water & air anymore.
- 2. The Honda was available with an alternator. Our Evinrude
didn't have one & it would have been very difficult to retrofit a
charging system. At the time, the Honda 8 was the smallest 4 cycle
outboard that was available with an alternator.
- 3. At the time, the Honda 8 was the smallest 4-cycle outboard
that was available with the 25" shaft. The extra 4" of shaft length
delays the onset of prop cavitation until wave heights reach 5 - 6
feet, a useful improvement. However, even with the motor mount fully
raised, the outboard must also be tilted up to keep the skeg & prop
out of the water. In fact, in calm water, it's very convenient to
use the outboard with the motor mount in the raised position - it's
much easier to start & you can easily reach the outboard's tiller
handle in order to turn the motor for close-quarters, low speed
maneuvering. I nearly always start the outboard & warm it up with
the motor mount in the raised position. It's just easier to deal
with. Then, I lower it down for motoring in rough seas. However,
be advised, that the Rhodes 22 can be more easily sailed in rough
seas than it can be motored - don't get to depend upon your engine
too much.
The cockpit mounted remote engine controls will spoil you real fast.
They make the boat seem more like a real "yacht" as opposed to a
daysailer with overnight accommodations. There's just something
very civilized about pulling up to a dock while standing upright at
the tiller with a commanding view all around, shifting into reverse
with your left hand without even looking down, & stopping on a dime.
Roger Pihlaja
S/V Dynamic Equilibrium
