Index

Outboard HP Required to Push an SJ23
INDEX - 4 or 2 Cycle, HP, Shaft Length, Propeller, Ducted propeller, Remote Control, Q&A, Manufacturers.

THEORY - A planing hull rides on top of the water when it moves at speed.  At slow speed it uses little power but then requires a lot of power to climb up on the "step" to plane.  Once on the step, it requires less power to keep it there.  A planing boat requires a high pitch propeller to match its fast speed. 

A displacement hull like a SJ23 (2800 lbs empty) never gets up on the step as power is applied, unless you apply a ridiculous amount of push to overcome all resistance, which borders on explaining how a bumblebee can fly!  Basically the hull pushes the water aside as it moves forward through the water.  What water can't move away fast enough, piles up at the bow, creating a bow wave.  This may be impressive but it is a waste of power.  As the hull moves forward it also leaves a vacuum behind the transom, evident by the shallow depression behind the wake.  A wake is created by splitting and pushing aside the bow wave.  There is a limit to how fast water can flow from the pile at the bow, around and under the hull, then into the depression at the transom.  
However, if you push an SJ23 slower than its 6 knot theoretical limit, (Tech Tip H05), the water can flow around the hull quite easily which is evident by the fact that it takes very little power to push it through the water.  Pushing it faster than that requires significantly more power because the hull is trying to climb up out of the hole it's in.  It simply requires a lot of power for a boat to climb uphill on water.  Describing water flow can become very complicated but suffice it to say that both forces slow the hull.  The more draft and curve a displacement hull has the worse this problem is.  A narrow displacement hull is easier to drive through the water than a wide one.  Given the same sail force a longer hull has potential for more speed. 

A displacement hull requires a low pitch propeller, similar to first gear in a car.  The optimum depth of the propeller below the bottom of the boat is dependent on many variables.  When operating a displacement hull at low speed or when pushing a heavy load, a lot of blade surface area is required.  So for a cruising sailboat, the prop should be below the bottom of the hull, spinning in clear water. 

It is more fuel efficient to push a displacement hull than a planing hull, but this has its limits too.  If you keep the speed to around 5 knots you will consume less fuel than at 5.5 knots.  Experiment with your throttle versus fuel consumption and you will quickly discover that beyond a certain speed there is no point in more throttle. 

In other words, hull speed refers to a velocity above which the surrounding water is non-laminar (turbulent).  This implies wasted energy, as turbulent conditions dissipate a lot of energy.  Put a 350-HP V8 in your SJ23 and you could likely make that baby plane!  (or at least get home in a big hurry during a dead calm!).

Please keep in mind that this discussion centers around efficient speed through the water, NOT ground speed over the bottom.  Ground speed is required to go upstream or to get the hell away from conflicting traffic.  This is something that a planing hull excels at.  On the other hand, a planing hull also makes a lot of noise and messes up your hair in the breeze, but that's another issue! 

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OUTBOARD TYPE - 4 cycle vs. 2 cycle

• PROS - A 4 cycle outboard has about 20% more thrust than a 2 cycle, burns cleaner to give 20% better fuel economy and runs quieter.  In general a 4 cycle outboard generates more torque at a lower RPM than a 2 cycle outboard and is therefore a better choice for pushing a displacement hull.  Operating a 2 cycle may not be allowed on certain ecologically sensitive waters where a 4 cycle or electric are the only type of outboard allowed. 
• CONS -  A 4 cycle outboard is heavier than a 2 cycle for equivalent HP because it has more moving parts.  It is costlier and usually more expensive to repair but the power head is not much more complicated than a 4 stroke lawnmower outboard.  It MUST stay upright or lie on its side pegs to keep the crankcase oil from flowing into the cylinders.  I like the pegs because it protects the outboard during transport.

If you have to replace your outboard then consider the environmentally friendly choice and buy a four-cycle outboard.  The lower fuel consumption, emissions and quieter operation are not to be discounted.  They burn cleaner, with more thrust, and are therefore more reliable than their 2-stroke equivalent.  On the other hand, fuel injected 2 stroke outboards are at the same efficiencies and low pollution of a 2 stroke (2015).  If you continue to use your older 2 stroke outboard then use Amsoil synthetic two-stroke oil (or equivalent) to keep your spark plugs clean and reduce your oil pollution by at least 50% over mineral oil.  Amsoil 2 stroke oil is mixed at 100:1 for an outboard that normally requires 50:1.  I regularly mix the oil for Panache's Merc 7.5 at 150:1 with no detrimental effects.  In fact this oil is so good that some snow mobile racers around here mix it at 250:1 for their Skidoos.  Don't do this in an outboard though.  All of us appreciate a clean environment.  Just because sailors don't operate their outboard as long or burn as much fuel as a powerboat, doesn’t excuse you from polluting the water.  Every one of us is part of the problem and part of the solution. 

OUTBOARD HORSE POWER - Be careful when making a HP comparison between an older and a newer outboard.  The HP rating for an outboard changed in the mid 1980s.  Some mechanics say it was around 1986 when the rating became universal for outboards since it was necessary to equate the thrust between propeller and jet drive units.  The two had different power conversion factors.  Prior to the change the HP of an outboard was rated at the power head and afterwards the HP was measured at the propeller.  I believe this was also done as a more equitable means of comparing 2 & 4 cycle outboards, especially since the electric trolling motors are rated in thrust.  In the final analysis who cares how much horse power the outboard has (except for a licensing issue).  What you should care about is how much thrust the outboard produces and if the outboard is overloaded at that throttle setting.  I'm sure this factor plays in the various opinions of SJ23 HP requirements listed below.  Two other factors are up for consideration; weight and noise.  80 pounds should be the limit to hang an outboard on the transom and expect decent sailing performance.  The other is noise, not to be discounted when motoring a long distance.  4 stroke are usually quieter than 2 stroke.  I miss that about my Honda 4 stroke.

• 5 to 6-HP (pre 1986) will push an SJ23 at hull speed in still water.  There is minimal reserve power so expect to move slower when punching into weather.  Since a generator and shifting is generally not available until 6-HP or higher, most people opt for at least a 6-HP.  View Tohatsu/Mercury 6HP.  The 5.7" pitch propeller (mid size offered) with extra long shaft works perfectly on an SJ23.
• 7.5-HP  (pre 1986) will push the boat at hull speed in still water with reserve power.  Expect to be able to push the boat at hull speed in 2 knots of opposing current and to be able to maintain hull speed when punching into weather that generates 2 foot waves.  The boat will move slower if you want to punch into weather that generates 3 foot waves.  View Panache's 7.5HP Merc.  It starts first time, every time.  View Merc 8HP. (Tohatsu manufactures this Merc).  The original Honda BF75 (or BF100).
  - Instead of motoring straight into 3' waves, it is easier on the outboard, the hull and the rigging to tack into the waves.  In effect, motor tacking.  However, this requires more sea room which is possible on a lake but not in a restricted waterway.  On the other hand, I suspect you seldom have 3' high waves in a restricted waterway. 
• 9.9-HP  (pre 1986) will push the boat at hull speed in still water with lots of reserve power.  Expect to be able to push the boat at hull speed in 4 knots of opposing current.  This is great if you have to get out of harms way or to move to safety fast.  With the reserve power you will be able to maintain hull speed when punching the boat into weather that generates 4 foot waves.  However, this is brutally hard on the hull and rigging and it's likely that the prop will be out of the water a good portion of the time, so don't demand this type of performance.  With this much wind the boat will handle better and go faster under sail anyway.
• 15-HP  (pre 1986) will push the boat and strain the outboard mounting bracket well beyond its limits in still water.  If you want this much push then consider installing a high pitch displacement propeller on a 7.7 or 9.9-HP outboard and upgrade the outboard bracket.

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• OUTBOARD BRACKET - If you increase the HP (thrust) or weight of the outboard, take into consideration the limits of the outboard bracket.  For still water or calm weather almost any bracket that can support the weight will do.  In rough water or when bucking a current, a bracket that provides good lateral support is essential to offset the pitching and yawing forces of the hull.  See Tech Tip D04 for bracket replacement procedures.
• OUTBOARD WEIGHT - Also consider that lower HP outboards weigh the same as their higher HP brothers.  This is because the manufacturer uses the same block for both but equips the higher HP with a different crankshaft, pistons, gears and carburetor.  But I certainly wouldn't hang anything heavier than 80 lbs on the transom.  There is a limit to how much I'm willing to hoist.  The outboard should be counterbalanced with weight at the bow to keep the boat level on her lines.
• SHAFT LENGTH (LONG & EXTRA LONG) - Always buy a long or extra long shaft outboard so the propeller stays submerged to maintain drive and cooling.  Another reason for a long shaft is to keep the prop submerged when a crew walks to the foredeck which is all too easy to do when using the auto pilot.  It is almost impossible to counter balance a crew weight and a long shaft has a fighting chance to stay submerged.

CHOICE OF LONG SHAFT or EXTRA LONG SHAFT - On a planing hull the cavitation plate should ride even with the bottom of the hull.  On a displacement hull the cavitation plate should ride deeper than the bottom of the hull to keep the prop in the water in steep seas.  The deeper the propeller is in the water, the more dense the water.  A deeper propeller results in more efficient thrust for maintaining push in big or steep waves and reduced fuel consumption.  Additionally a deep prop will usually stay in the water when you work at the bow.  This is another reason why the cavitation plate should be at least 6" below the surface when the hull is level. 

• STEEP WAVES - If the propeller comes out of the water after cresting a steep wave then the outboard will over rev and the water pump may run dry.  If the prop is out of the water often enough and long enough the outboard will overheat. 
• STAND ON BOW - Standing on the bow to hook the mooring, pull up the anchor or to fold a sail are all time consuming jobs.  It is unlikely that the propeller will stay submerged during one of these jobs.  What is almost certain is that the water intake will suck air long enough to damage the impeller as it runs dry.  It takes surprisingly little time for the friction to burn the fine rubber seal off the side of a dry impeller.  Do this a few times and you will have to replace it.  Don't be fooled by the fact that it still pumps a bit of water.  There is insufficient volume there for good cooling when operating under heavy load. 
• NARROW CHANNEL - If you motor sail in really big winds you will likely loose directional control every time the propeller comes out of the water.  This demands some quick corrective action at the tiller.  In a restricted channel you may not restore control quick enough, which could be nasty.  Motoring in a tight channel with current or waves can be  dangerous.
• SHAFT LENGTH - Outboard shaft length is measured from the cavitation plate to the top inside of the mounting clamp bracket on the outboard.  A short shaft outboard is 15", a long shaft is 20" and an extra long shaft is 25".  Don't even consider a short shaft outboard for an SJ23.

In the cases described above, a log shaft 1975 outboard (equivalent to extra long in 2020) will ensure that the prop stays under water, maintains vessel control plus protects the water pump & outboard cooling.  If your prop comes out of the water often then add a long shaft conversion kit to the outboard, or lower the outboard bracket.  To be more specific, Panache's 1974 7.5HP long shaft Merc has a shaft length of 22", measured from inside the mounting clamp to the cavitation plate.  I modified the outboard bracket to lower it further than original so that at the bottom of the pivot the cavitation plate is about 6" below the surface.  But without modifying the outboard bracket I would buy an extra long shaft outboard to deal with the short steep waves on the lake I sail on.  In the photo of the ducted propeller below I've tried to show how deep the prop rides but it is difficult to portray accurately.  (UPDATE) Therefore I'll try to take the following photos this summer to demonstrate outboard operation at the listed speeds.

PROPELLER THRUST - The pitch of the stock propeller for most small outboards is designed to push a light weight aluminum fishing boat at planing speed, not a 3000 pound displacement sail boat at 5 knots.  On a planing hull the outboard can rev freely and cool properly pushing a light load near the top of its designed RPM.  When using the stock propeller to push a displacement sailboat like an SJ23, the outboard will generally operate in the bottom half of the RPM or torque range.  This may overload the outboard resulting in overheating and accelerated wear.  Fortunately a sailboat is relatively easy to push through the water.  Be wary of the throttle setting on your own outboard.  Find the setting that produces maximum speed with minimum throttle and operate at it.  If you throttle up beyond that you will burn extra fuel with very little speed increase.  Don't operate in this range.  To increase the thrust you could change from a 2 bladed propeller to 3 bladed propeller that has more bite.  Failing that you could install a fin pitched propeller, which usually has three blades anyway.  Either change will achieve more push and longer outboard life.  A three bladed propeller for my 7.5HP Merc was difficult to find and turned out to be cost prohibitive.  The fine pitched propeller, especially if it has three or four blades, is excellent for accelerating, towing, punching into a current or waves and really comes into its own when backing up or stopping.  The high thrust blades are generally quite flat and round shaped (like a lily pad) versus the screw shaped propellers designed for speed.  The down side is the outboard will make more noise at the higher RPM.  It should be noted that a 4 cycle outboard generates more torque at a lower RPM than a 2 cycle outboard and is therefore is a better choice for pushing a displacement hull. 

• Mercury Marine - Their Bravo II series propellers have 3 or 4 large blades to increase efficiency, 11" to 25" pitch to match a load and left or right hand turn.
• Michigan Wheel - Manufacture propellers.
• Evinrude & Johnson - Piranha propellers.
• Honda Marine - Complete product info.

DUCTED PROPELLER (2005) - I was looking for a way to keep the rudder from touching the propeller (see Tech Tip B09) when I discovered nozzle technology; a ring around the prop.  In the commercial marine industry this ring is called a Kort nozzle which was originally used on tugboats to improve low speed thrust for moving a heavy mass.  They also mounted the propeller on a rotating pod to direct the thrust in the direction they need without turning the tug. 
An aircraft turbofan engine uses the same ducting phenomenon to ensure the full surface of the rotating blades is used to pull the aircraft more efficiently, albeit at a faster speed than marine.  A nozzle around the propeller makes it more efficient by preventing water pressure from bleeding off the ends of the blades thereby ensuring equal pressure at the top and bottom of the rotation.
A Line Guard is the recreational boat version of protecting a propeller with a duct to improve efficiency.  While it's primary purpose is to prevent swimmer injury or to keep a fishing line from wrapping around the propeller, it also improves the thrust of an open propeller.

I fabricated this duct from 3" wide aluminum sheet.  The 1/8" thick aluminum was persuaded (pounded) to a circle over a 6" diameter pipe to create a ring with 1/2" space around the propeller.  The shaping required lots of "tender tapping" with a short handle 5 lb sledge!  The ring is bolted to the bottom of the cavitation plate and to the skeg by two support brackets, all with SS screws.  The duct MUST be centered around the propeller and should be positioned ~1/3 forward and ~2/3 aft of the blade tips.  The marine and air craft nozzles are equipped with an internal foil to increase the thrust.  This one obviously doesn't have that.  The photo above shows the outboard on the transom in the lowered position with the top of the prop a bit below the water line.  This view is a bit misleading in that the top of the prop in drive mode is actually well below the water line.  The waves have to be really steep and short for this prop to come out of the water.
- In 2024 I replaced the two support brackets with thicker aluminum.  They get abused occasionally as the leg "touches" the pavement when removing the outboard from the bracket.  This is one part of the haul out job that I have not been able to ease.

A ducted propeller is ideal to push a displacement boat at hull speed or to cruise with reduced throttle and save fuel.  I found it difficult to compare the fuel saving with this duct because I never bothered to record the consumption without it.  However, I guesstimate that this duct saves ~40% fuel.  For example, on its first test run at 4 to 5 knots in very lumpy water with light wind pushing the boat forward, the outboard used only 1.25 US gal (1 Imp gallon or 5L) of gas to travel 12 miles.  The same run in flat water with no wind used the same amount of fuel.  In both cases the hull was clean but its interesting how the two weather conditions used the same amount of fuel.  Without the duct the outboard used ~1.75 Imp gallons (calculated).  The lower fuel consumption due to the increased thrust came as quite a surprise.  The nozzle does an excellent job of protecting the rudder from touching the propeller, which was my original reason for this project. 

At right you can see the outboard pushing the hull at 5 knots in flat water.  There is virtually no turbulence and the tiny exhaust bubbles come to the surface about a boat length back, much smaller and further than without the ring. 
Click on the photo to see the duct in action. --->

Click on Line Guard for size criteria for your outboard.  A Line Guard costs about the same as a 3 bladed Mercury propeller and is a lot quicker to install.  Just don't expect the push of a tugboat or the acceleration of a Boeing 737, OK!

PERFORMANCE TEST 2008 - A few observations I made on a 12 mile run with Panache:

• The outboard runs a bit quieter with less vibration. 
  - I suspect this is due to the propeller being uniformly loaded around the 360⁰ of rotation.
• There is very little turbulence pushed to the surface and the tiny bubbles are evenly spread out. 
  - At 5 knots they surface about twice as far back than without the nozzle.
• Thrust in calm water and wave punching performance is definitely improved.
• Top speed is not affected which I expected at the lower speed of a displacement hull. 
  (Apparently a duct can reduce the top speed of a planing hull unless it is tuned exactly to the speed requirements).
• Fuel consumption is greatly reduced.  While I have never measured the exact consumption with a bare propeller, on a 12 mile delivery trip in 2008 it burned 1.5 US gallons of gas.  Considering that I motor sailed into the wind, tacking through significant waves, this is very good.  On the same 12 mile run in 2016 it burned 1.25 US gallons on flat water.  Since a friend burned about double that on his SJ23 I'm guestimating that fuel consumption is ~40% less with the duct than with the open propeller.  That's pretty good!
• The duct creates total protection for the rudder blade.  I never worry about the prop touching the rudder.

REMOTE CONTROL - See Tech Tip D07.

REPLY 1 - Mike I have an 8HP Merc on a 22' Westerly on the Gulf of Mexico and ICW and would not consider an outboard over 9HP.  A Westerly 22 is a twin keel boat that weighs more than a SJ23.  There is no reason or benefit to a big outboard on a displacement hull.  You will never be able to water ski on a sailboat and you will just stress the hull with too much power.  I have powered with those little Johnson canoe kicker outboards that I keep in the well for a spare.  Rather that spending bigger bucks on a bigger outboard, reef down and sail it home when the wind gets too strong.  (Get reef points in your sail and be sure you know how to use them).  I had a 9.9HP on a 20' Balboa and it was overpowered.  I never could use full throttle.  My next outboard will be the best quality used 6 to 9.9HP I can find, purchased by weight and price, and not a lot of regard to HP.   Paul Halenbeck

REPLY 2 - Sailing weight and attainable speed are certainly important considerations in the choice of auxiliary power, but they aren't the only considerations.  Including the outboard mount replacement, the 15HP two-stroke I now use weighs about ten pounds more than the 7.5HP I retired.  I can rationalize the extra weight pretty easily, but then I'm not a racer.  If it bothers you, leave some of your beer at home or get skinnier crew.  Attaining hull speed at half throttle is adequate compensation for the extra weight, due to reduced noise and vibration and generally less stress on the outboard.  I won't ever pull water skiers, but I might need the extra thrust someday. 
Time at maximum power of an aircraft outboard is strictly limited.  It's a matter of safety, because reciprocating outboards are much more likely to fail at high power settings.  Sailboats are not going to crash if the outboard quits on the water, but it can be really inconvenient.  Running a more powerful unit at a lower throttle setting to get the same performance is just good mechanical practice.
The bottom line on outboard power has to be personal preference.  If you have the opportunity, test drive both sizes and pick the one that feels best.  If all I had to do was manoeuvre into and out of my slip, I'm sure the 6HP unit would be great.  However, I spend a lot of time under power because I live in "stink boat" country, so the big outboard is pretty important to me.  Pogy Bait still handles well under sail, even with the extra weight.   Jack Chandler

REPLY 3 - I have a SJ23 Mark II with an 8HP 1991 Suzuki.  The outboard is very reliable, and propels the boat at hull speed at about 3/4 throttle.  I have wondered if it would be better to have a 6HP to reduce weight, but appreciate the wisdom of an earlier message indicating that operating at full throttle may wear out the outboard faster.  As well, my outboard does not perform particularly well in reverse; a smaller outboard would presumably do worse.
The outboard that I have observed doing very well in reverse and forward is the Yamaha 4 stroke.  It has a larger, displacement propeller (but the outboard also weighs more).  Frank Langer, SV Wind Dancer.

REPLY 4 - The prop will make all the difference.  Go for the high thrust four-blade model.
Jeremy Dixon, SV My Tweety, SJ24 (Evinrude 6 HP).

REPLY 5 - I have a brand new 2 cycle 4HP Mercury on my SJ24.  The boat will do hull speed at about 3/4 throttle.  The gas tank is self-contained with the outboard so no fuel tank or hoses to mess with.  What more do you need?  Mike Chaney, SV Pog Mo Thoin.

REPLY 6 - I have used a Honda 7.5HP and other 6HP outboards on various SJ23s on Puget Sound.  They all work fine.  More HP means more weight and more weight means less performance under sail.  The SJ23 has a finite hull speed and overdriving the hull burns more fuel, with not much more speed.  Gene Adams.

REPLY 7 - I use a 7.5HP long shaft Mercury equipped with electric start and generator.  The center of the prop is 27" down from the top of the transom and it usually runs about 12" deep.  It performs very well in lake sailing and achieves about 5 knots in flat water at 1/3 throttle.  If I throttle up, it makes a lot of noise and burns a lot of fuel without going much faster.  In really rough water I once tried punching to weather in 4' peaky waves at 4 knots with 1/2 throttle.  However, the hull took a lot of pounding, the rigging took a lot of shock loading and my teeth did a lot of chattering.  Therefore I now "tack" into the waves to smoothen the ride for the hull and rigging plus reduce the load on the outboard.  This has the advantage of increasing the speed through the water and reducing fuel consumption.  While I like the outboard, a four stroke (quieter, lower fuel consumption & higher thrust) with top mounted controls would be nice.  When I added the duct around the propeller it reduced the fuel consumption by ~40% at 5 knots.  One day I may replace the standard two blade propeller for a three blade to improve the reverse thrust.  This outboard is such a sweet combination that I doubt I will ever replace it.  Bob Schimmel, SV Panache.

REPLY 8 - We had an outboard failure last summer while trying to leave the dock so we rented the marina's outboard to complete our haul out.  Unfortunately it was a short shaft of 8-9HP.  I can't remember the make, not a Johnson or Honda, maybe Nissan.  We mounted it as low as possible and moved all of our gear aft, sat on the stern and almost went crazy trying to keep the water pickup under water while we motored to the travel crane dock.  From this experience I would never consider buying a short shaft.  Our outboard bracket has the hydraulic lift assist and tilt features to make life easier and we never leave any part of the outboard in salt water except when under power.  Betsy Schultz.

REPLY 9 - If you've read the past posts and you've run into mine that tout the Honda 7.5-10HP family of outboards, please forgive.  There is everything to love about this series except lifting them onto the engine bracket, as they weigh a bit over 80 lbs.  Beyond that, you get reliability, relatively quiet operation, and hull speed at a little over 1/2 throttle.  When you need to push hard into a headwind, there is still reserve thrust available.  Then there is the added benefit of having a charging system for the house batteries.  
The newest members of the class have a spin-on changeable oil filter as well.  The "oil filter" in the older units consists of a mesh screen on the pick-up tube, accessible only when you remove the power head from the oil case.  Unfortunately there is no way to know when that screen is fouled or failed.  We manage this issue by frequent oil changes and found one "filter screen" to have failed.  Where did all that metal go?
Anyway, the decision of which outboard is one of balancing needs, cost, safety, serviceability in far-away places, and a happy crew.  Cheers and keep sailing!  Gleno
 

MANUFACTURERS TO CONSIDER.

• All 4 stroke outboards provide more thrust, run quieter and burn less fuel than the same horse power 2 cycle outboard.
• Tohatsu (2017) https://www.tohatsu.com/outboards/all.html  Tohatsu, Nissan and Merc are the same outboard, just a different label on the cowling.
  - The Tohatsu MFS9.8B will push an SJ23 at hull speed in still water with lots of reserve power. 
     25" shaft, high torque prop, electric tilt, electric start, remote shift & throttle, 12A alternator, weighs ~80 lbs.
  - The Tohatsu 6 HP will push an SJ23 at hull speed in still water with 1/2 throttle.
     25" shaft, high torque prop, electric start, remote shift & throttle, 6A alternator, weighs ~80 lbs.
• Yamaha T9.9 XPB (2017) will push an SJ23 at hull speed in still water with lots of reserve power.
     25" shaft, high torque prop, electric tilt, electric start, remote shift & throttle, 6A alternator, weighs 93 lbs.
• Honda 9.9 DK3LRTC (2017) will push an SJ23 at hull speed in still water with lots of reserve power. 
     25" shaft, high torque prop, electric tilt, electric start, remote shift & throttle, 6A alternator, weighs 110 lbs.

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