Do Sailboat Outboards Have To Be So Heavy?

by arlyn stewart


The last fifteen years have experienced outboard technology advances with perhaps the greatest changes made in the conversion from two to four stroke with subsequent better fuel economy and less pollution.  These changes have brought about new models and new manufacturer faces.  Sailboats traditionally used designs built for power boating, though some effort was made to produce deeper foot models for saiboats such as  the Johnson Sailmaster.  As time went on, it was also noted that the propeller thrust range for these motors was wrong for the slower moving sailboats.   Recent efforts have responded and produced high thrust sailboat oriented designs with larger diameter propellers with less pitch, deeper foots, electric start, high output charging, exhaust diverting to prevent loss of reverse thrust, flushing ports, and remote control capabilities.  It would seem that our dreams have come true.... or have they?

The price paid for these changes has come in the form of weight... hernia producing kinds of weight.  These motors have all grown to the 108-113 lb range making them difficult to manage.  Is all the weight necessary?  Most of these designs are for a horse power rating of 9.9 a figure that suited the small lake covenant restricting often imposed.    The plentiful supply saw them often used on sailboats in the mid twenty foot length and they became somewhat an industry standard.  That happened in a day when they were 60-80 lbs in weight and 2 cycle in design.  More recent efforts to provide sail boat designs, adopted that 9.9 standard, adding the features with a resulting 20-40 lbs gain.

Sometimes insights are gained in unusual ways.  My observations about this come because I kept the 8 HP 4 stroke from a previous boat to save some money as it fit the minimum requirement of the new boat.  Thinking that its power might be marginal, I was surprised that it performed very well and even cruising the Great Lakes, never suffered for power.  One third throttle provided a good cruising speed and two thirds throttle would drive the boat to its hull speed,   More throttle would reduce the speed as the prop lost traction.  It was then that I made an interesting observation. 

The  9.9 and 15 hp models were equipped with the same prop as the 8hp. My simple conclusion was if full performance was reached with an 8 hp at a reasonable throttle setting, what good were more horses?  The would lose traction at the same revolutions and the extra horses would be useless.  The 8 hp was smooth and quiet running, easy to start, provided great fuel economy and was every thing needed except... it was a long shaft and occasionally spun out in rough conditions.  An extra long shaft was needed, but my dealer told me extra long shafts were only available in 9.9 motors.  If I was willing to wait, an extra long shaft 8hp was in the works and would be released within a year or so.  I waited.

In December of 2000, the dealer called and informed me that the motor I was waiting for was available.   Having waited , it was disappointing to learn that the new motor was indeed a 9.9 detuned to an 8hp and it was very heavy, 29 lbs heavier.  Disappointed with the news, I looked at the other choices.  No better... all were heavy.  While resenting having to buy a 9.9 size motor to get the deep foot I needed, I caved in and bought the 8 hp version of the 9.9 motor to achieve the safety I wanted.

Later to my chagrin, I learned later that Honda had in fact marketed and sold an extra long shaft motor in the same design as my first 8hp.  Its sales were so limited however that evidently not many knew about it including my dealer.  As it was never listed on the Honda web site, I believe the dealer was probably honest about not knowing about it.   It weighed 81 lbs, only two more than the long shaft version I had.  It was the motor I really wanted and never knew existed. When the new 8hp model came out in December of 2000, the old 8hp was dropped but re-released again in 2003 as the Honda 8 Classic but unfortunately not with a extra long shaft option. Why was it not re-offered in an extra long shaft,  was I out of step with the market ?  The simple answer is yes. 

The sailboat market which has the strongest demand for extra long shaft models was enamored with the 9.9 and electric start, something that the Honda 8 Classic didn't have as an option.

From this experience, my perspective is that we have shot ourselves in the foot.  We are now suffering too much weight because of our past sin of locking onto a perception that 9.9 is a necessity.   A 9.9 size motor is rather foolish when 6-8 hp will drive almost all sailboats designed for outboards to their hull speed.  The extra horses add weight astern, something almost all sailboats try to avoid for performance reasons.  One of the proponent arguments for inboards is that they do not add stern weight like an outboard.

Many stories and accounts are offered from sailors making their way through a tide or current claiming they needed every horse possible and wouldn't be satisfied with any thing less than a 9.9 or sometimes even a 15.  Many of these sailors fail to grasp that a sailboat is limited to its hull speed and how that limitation holds true for running against a tide or current, or that a particular prop has a given thrust limitation at a given speed, that increasing the rpms, does lno good and in fact can produce turbulence and drag which will actually slow the boat.    Its tempting, no doubt,  to conclude that when speed over ground as registered by gps or visual observation is slow, that more horse power is needed, when in fact the hull is moving at its maximum speed.

In  my opinion, the issue is not horse power amounts, but rather application of power to movement.... thrust.  Slow moving sailboats require a motor and prop combination that wont lose prop grip at slow boat speeds.  Once traction is lost... horses are useless.  Its almost like spinning tires on a slippery road or ramp.  Like letting air from the tires to get better traction,  a high thrust prop keeps its grip during slower boat speeds. 

To give an example of what I'm saying, an examination of the model airplane thrust issue is relevant.  Several years ago, a trend emerged towards larger models.  Some moved towards chain saw motors with more power while others diverged to gear reduction units.  Both worked, the 2 stroke motors would swing a larger prop for a slower flying plane if they were geared down to swing it slower.  Unfortunately, the two stroke scream of the motor didn't seem realistic and often was obnoxious.  Four stroke model airplane engines were birthed and their lower rpm power band proved ideal.  Four strokes also also muzzle the exhaust much better for a more pleasing noise level.

The same is true for a slow moving sailboats... a prop spinning at a high rate of speed tends to try to bite off a chunk of water that isn't incrementing because the boat won't go faster and the result is loss of grip.  

One way to illustrated what I'm saying is to use some arbitrary numbers.  The numbers will be from anecdotal observations that are probably not far away from how things are for a 25 foot sailboat. 

		6 HP	6 hp HT		8 HP	8 hp HT		10 HP	10 hp HT
Effective Horse Power at the Prop		3	4.5		4	6		5	7.5
based on 50% for a planing prop and 25% better
for high thrust (HT) .
Throttle setting needed for hull speed		WOT	66%		75%	50%		60%	40%
Reserve power beyond hull speed requirement		0	1.5		1	3		2	4.5
based on estimation from observations that
hull speed requires 3 effective horse power
Ability to maintain hull speed with a head wind in knots		0	12		10	17		15	22

As can be seen,  high thrust motors are superior for sailing craft.  The smaller motors have more reserve power than larger versions with a standard prop.  A 6hp HT for example should outperform a 8hp standard and will come close to the performance of a 10 hp standard.    

The problem is that no one has produced a six horse power, high thrust motor with an extra long shaft.  It would be interesting to know just how well it would perform and at what weight one could be built.

If your results are different from those I have tabled above... I'd be interested in hearing them and adjusting the figures if it appears I have missed the mark.

Send to aa5by@suddenlink.net