Outboard on board

The fewer the fixed weights aboard a performance trailer sailer the better. We all know there are fixtures aboard such as the rigging and centreboard that can't be moved to windward to keep the hull as flat as possible upwind, but as the auxiliary engine really is a dead weight under sail, it's essential to choose one that does the job without robbing hull performance.

Trailer sailer outboards are usually mounted in a well in the cockpit, directly on the transom or on a transom bracket and all three mounting methods have their pros and cons. Under power, an outboard mounted in a well is easier to operate and closer to the cabin when the time comes to remove it and stow it below, unless the well is large enough for the outboard to tilt up and a fairing plate attached over the hull opening. Being closer to the axis of pitching the propeller tends to ventilate less and the engine delivers more consistent power. Another plus is that normally only a long-shaft (20in or 51cm) outboard is needed and not an extra-long (25in or 64cm), giving it better resale value when traded in because it can then be used on an aluminium dinghy, whereas an extra-long is suited only to auxiliary applications. From my experiences of testing extra long outboards, they are a pain to stow below in a hurry and take up valuable space below!

But because of a lack of clean air to the carburettor intake it tends to suck in exhaust gases that can't escape easily and this affects engine power output, particularly with two-strokes which always emit some oil smoke. Underway in rough seas slop can easily splash up the leg and the saltwater spray mist seriously shortens an engine's lifespan.

Where possible the best method is to mount an outboard directly on the transom because it can easily be removed and stowed below. However, for any TS over seven metres an extra-long is normally needed, again resulting in stowage problems. Alternatively a height-adjustable transom bracket allows for a long-shaft outboard to be used because underway the outboard can be positioned at the right height relative to the water surface for consistent thrust, but removing an outboard from its mounting well away from the transom can lead to back injury. As all current-model outboards have right-hand or clockwise-rotating props when in forward gear, the bracket should only be mounted to port of the rudder to counter prop torque when motoring.

Several brands of adjustable brackets are available on the market but to avoid excessive transom thrust loadings most brackets are limited to 15HP, which in any case is more than adequate for a TS to nine metres.

The disadvantages of transom brackets are that to maintain reasonable thrust in head seas an extra-long outboard must be used on most TS over seven metres. Most adjustable brackets can be mounted so that in the down position the outboard's antiventilation plate is slightly below the hull to ensure clean water (not aerated) reaches the prop, but this gives less than 15 inches or 38cm to the powerhead for a long-shaft outboard and when pitching or motoring in a following sea waves may reach the carburettor intake. This makes an extra-long essential for open-water motoring and not all outboard brands under 15HP are available in extra-long versions.

Two strokes

Two-stroke outboards are light and compact for their output, making them relatively easy to remove from their mounting bracket and stow below. They're also relatively cheap to buy and service compared to four-strokes, normally once a year after the first service at 10 or 20 hours or one to three months. And with the amount of engine oil for used for lubrication (particularly 50:1 models) when operating they have better saltwater corrosion resistance during periods of disuse.

Any two-stroke will continue to run providing the fuel level in the carburettor bowl is not at an excessive angle. So it when motorsailing the hull heels when a gust of wind hits it the outboard will continue to run without any powerhead damage. But with all four-strokes there is a heel point where the oil pump or oil scoop can no longer pick-up oil from the sump, resulting in oil starvation.

Another advantage of two-strokes is that as the revs increase powerhead vibration levels decrease, particularly with single cylinder outboards. Vibration levels actually rise as four-stroke outboard revs increase and from my testing of them single cylinder four-strokes can create annoying harmonic vibration in fibreglass hulls when worked hard.

In the past the main drawback of smaller two-strokes operating on a fixed premix petrol/oil ratio is that after two or three months the oil separates from the petrol and “gummy” deposits can block the carburettor jets. But since the advent of isopropyl alcohol, which absorbs any moisture in the fuel and maintains the petrol and oil in suspension, this problems is all but eliminated.

Tests I've conducted on premix fuel from 25:1 to 100:1 over the past 14 years show it remains as “potent” a year down the track as the day it was originally mixed. However using OMC/Bombardier 2+4 fuel conditioner and stabiliser, for example, adds 10.4 cents to a litre of petrol when mixed at the 500:1 everyday usage ratio and 20.8 cents when used at the 250:1 storage ratio.

Another problem is that it tends to wet spark plugs and if your engine has an electrode gap of less than 0.7mm this additive should not be used as the ignition system is too weak to cope with the added plug wetness. This rules out engines such as Yamaha's 4A.

Apart from their relatively high exhaust emissions (none are rated better than OEDA or Outboard Engine Distributors of Australia “1 Star”) two-strokes have the disadvantage of needing an occasional run at wide open throttle to burn away combustion chamber carbon deposits. Also, they don't idle as steadily as four-strokes.


Four-strokes have incredible fuel efficiency or distance travelled for fuel used for their outputs, which under normal idling and cruising conditions is around twice that of comparable two-strokes. All the four-strokes mentioned in this guide meet OEDA “3 Star” exhaust emission requirements and, for example, only four-strokes can be used on waterways such as Canberra's Lake Burley Griffin.

But like a car engine, four-strokes require regular servicing and demand much for careful treatment than two-strokes. Regardless of how few hours are clocked up between the manufacturer's recommended oil change interval, the oil (and filter where fitted) must be changed every six months, especially if the engine has been primarily used to motor at low revs from a launching ramp to the starting line.

To ensure the lubricating oil reaches the rocker gear all four-strokes need at least three minutes of warming-up at fast idle before engaging gear, whereas (although I don't recommend this abuse) a two-stroke can be operated with only a minute's warming if needed. When stowing a four-stroke it needs to be kept upright or laid only on one side (storage lugs are usually cast into the lower cowl and warning signs provided) whereas a two-stroke having an upfront gearshift can be laid on either side, on its back or upright, increasing its stowage flexibility in a small cabin.
All four-strokes develop their maximum torque higher up the rev range than comparable two-strokes and because of this overpropping the engine the same as its two-stroke counterpart can do enormous damage.

All four-strokes are much bulkier and weigh much more than their two-stroke counterparts. Fine if the outboard is mounted in a well large enough to allow it to be tilted fully within the compartment, but bad news for yachties who need to stow the outboard below. The additional weight of four-strokes rules out any of them over 10HP on any transom brackets rated to 15HP, because the weight ratings are for two-strokes, most of which weigh less than 40kg compared to 45kg for the lightest longshaft four-stroke 15 on the Aussie market.

Finally, in addition to the 25 percent average higher servicing costs for comparable-output engines the retail pricing is about 35 percent greater. To some extent this difference is reduced by not having to allow for oil and fuel additive costs.

Desirable features

An auxiliary outboard should be as user-friendly as possible so that any crew member can operate it. Features should include an upfront gearshift ahead of the lower cowl or on the tiller arm and a similarly-positioned stop button and ignition cut off switch lanyard attachment point, and automatic tilt lock, rounded cowls for ease of carrying and a carry handle positioned so the outboard may be carried in one hand leaving the other free for gripping a handrail. As most outboards will be locked in position and the tiller used for steering, effective adjustable steering and throttle friction systems are essential.

All outboards should have at least a clutch so the engine can be warmed-up in neutral before engaging gear and a reverse gear is useful for boats kept in a marina or when coming alongside a jetty.

Apart from Tohatsu's 3.5B and Yamaha's F4A, all outboards under 15HP are available with an alternator, which through a rectifier provides DC output for charging a battery needed for powering nav and cabin lights. However as none of the DC outputs in this power range have voltage regulation a battery of at least 70 amp/hours is needed to prevent it being “fried” on long runs under power.

Determining the power needed

All auxiliary outboards should provide hull speed (the maximum before the hull starts “squatting” aft) in calm water at WOT to ensure the engine is not overloading. Calculate the waterline length in feet then square root it and multiply by 1.4 to 1.6 to get the hull speed in knots. For example, if a broad-transomed performance TS has a static (stationary) waterline length of 26 feet (7.9 metres) then the hull speed will be 8.2kts, but if the hull is an older design with a narrower transom the hull speed will be around 7.1kts.

Based on my testing of TS hulls over the past 27 years, the 26-foot waterline TS will need an outboard of around 10HP (either two- or four-stroke) to easily achieve hull speed in calm water, though if the hull will be used primarily on sheltered waterways not likely to experience an ocean swell, then 8HP will suffice. Because of their light displacement and high topsides relative to keelboats TS hulls tend to blow around more in strong winds and to maintain way in strong headwinds up to twice as much power may be needed than in calm water.

As waterline length decreases the hull displacement diminishes rapidly and a TS having a waterline length of 20 feet (6.1m) displaces only half as much as the longer hull. Again using the square root formula the hull speed will be between 6.3 and 7.2kts and depending on the waterway an outboard from 4 to 6HP (either two or four-stroke) is needed. Small TS hulls of around 17-18 feet (5.2 to 5.5m) can get away with 3 to 4HP.

Recommended outboards

Although this article is not intended as an outboard model guide (Andrew Norton's Marine and Motor Guide, ISBN 186513029X, has model comparisons and technical specs), the following outboards are my recommendations based on extensive evaluation under controlled conditions.

Among the two-strokes, the combination of light weight and compact dimensions make the 74.6cc Mariner/Mercury 3.3 and Tohatsu 3.5B the best small auxiliary outboard choices for small TS hulls. The long-shaft models weigh only 13.5kg.

For more power and the advantages of a full gearshift and remote fuel tank the 102cc Mariner/Mercury 5 and Tohatsu M5B are again my choice and in long-shaft form weigh 21kg.

From 6 to 8HP the Mariner/Mercury 6LW and 8LW and Tohatsu M6B and M8B are my pick; and up to 10HP Tohatsu's lightweight M9.8B is unbeatable for its compact dimensions and power-to-weight ratio. All of these engines are based on the same twin cylinder 169cc powerhead and weigh 27kg in long-shaft form or 28kg for the extra-long version (Tohatsu 9.8 only).

From 10 to 15HP my choices are the 262cc Mariner/Mercury 15 (not the Super 15) and 246cc Yamaha 15F, all of which have upfront gearshifts, fold-out carry handles, compact dimensions and relatively low weight. The Yammie also scores by operating on a 100:1 premix instead of 50:1 for the other two-strokes, reducing pollution and oil costs. And having a loopcharged powerhead (the Mariner and Mercury are the only engines in this guide to have crossflow powerheads) the Yammie uses around 20 percent less fuel under the same operating conditions. In long-shaft form the Mariner and Mercury weigh 36kg and the Yammie 38kg.

Of the four-strokes, the 85.5cc Mariner/Mercury F3.5 and Tohatsu MFS3.5 weigh only 18.4kg in long-shaft form but lack a carry handle, while for only for only 4.6kg more Yamaha's 112cc F4A is unbeatable under 5HP for its large carry handle and compact dimensions. But for a slightly greater weight (26.5kg) Suzuki's 138cc DF6 returns much better performance and is easier to maintain and service. It has an even larger carry handle and also the advantage of integral and remote fuel tanks.

Although much heavier at 39.5kg for the long-shaft version Yamaha's 197cc twin cylinder F8C provides a lot more torque and features an upfront gearshift, while Tohatsu's lightweight 209cc MFS9.8 is more compact, weighs 38kg and provides useful power and torque increases, but misses out on the upfront gearshift and one-hand tilting of the Yammie. Yamaha's recently-released 212cc F9.9 is worth considering because it is available in long-shaft and extra-long versions but these weigh 40.9 and 41.9kg respectively.

Propping for performance and fuel efficiency

Nothing “kills” any outboard, two- or four-stroke, faster than overpropping it under load and most outboards sourced from dealers will be propped for planing-hulled tinnies, not displacement hulls. All outboards (particularly four-strokes) only develop full power when the revs are well within the manufacturer's recommended WOT range, usually 4500 to 5500 revs for two-strokes and 5000 to 6000 revs for four-strokes.

The only way to ascertain these revs is to borrow or buy a portable high tension lead tachometer such as the OMC/Bombardier Shoptach, or if the engine has an alternator wire-up a detachable tachometer. Under normal operating load (full crew complement, food, water, fuel and gear) the revs should reach the top half of the rev range.

Again based on my testing of TS hulls, most outboard propellers have about 40 percent slippage when pushing a broad-transomed TS at hull speed and about 50 percent with a narrower transomed hull. So in the case of the 26-foot waterline TS powered by a 10HP outboard that has a WOT rev range of 5000 to 6000 and a reduction gear ratio of 2.08:1 (Tohatsu's two and four-stroke 9.8s make a great example), to achieve hull speed of 8.2kts the smallest-pitch prop of 6.5in will be needed. If the hull has a hull speed of 7.1kts then the additional prop slip should still allow the engine to reached the desired WOT rev range, although the four-stroke may be still slightly overpropped. This also applies to the Yamaha F8 and F9.9 when using a prop of this pitch as like the MFS8 and MFS9.8 they need to reach 5500 revs to develop full power.

With the 20-foot waterline TS powered by a 6HP outboard, using the Tohatsu M6B as an example, to reach the recommended 4500 to 5500 rev range, at 6.3kts with 50 percent prop slip and 7.2kts with 40 percent slip a 6.5in pitch prop will do the job nicely. Alternatively, with its taller gear ratio and lesser torque Suzuki's DF6 will need a 6in prop to prevent overloading, particularly as this engine develops full power over 5300 revs.

For TS hulls to 18 feet waterline, Tohatsu's 3.5B has weedless props that slip up to 60 percent under load, so to ensure the engine reaches around 5000 revs at WOT the plastic 5.7in “F6” prop should be used. With the MFS3.5 the little 4.5in plastic prop may be needed to allow the engine to reach close to 6000 revs. As Yamaha's F4A reaches only 5000 revs it needs a 6.5in weedless prop to perform well.

Good stopping power is an essential part of any auxiliary outboard and only the Yamaha F8 and F9.9, Mariner/Mercury 15 (not the “Super” model) and Yamaha 15F have props designed specifically for pushing displacement hulls. When going astern exhaust gases are diverted away from the blades to ensure the prop runs in “clean” water as much as possible and doesn't ventilate as do props designed for planing hulls.


As the pricing consultant for the past 15 years for the Red Book Marine Guide, a quarterly publication that supplies new and used boat, outboard, personal watercraft and trailers to insurance and finance companies, according to my clients the best prevention of theft is to stow the outboard in the cabin when not using the boat, away from public view. And that even when the boat is kept in the backyard behind locked gates.

If this is not possible (such as when an outboard is kept permanently mounted in a cockpit well) then one of the best deterrents is to loop a stainless steel wire through the outboard safety lug to a through-bolted and glassed-in ring in the hull structure and use hardened padlock with a security rating of 4 or 5 through the eyes on stainless steel thimbles. Chain is just not strong enough for permanent mounting as boltcutters can easily break the links.

Alternatively a stainless steel tube can be slid over the transom bracket screws and again padlocked. This device retails for around $70 and most outboard dealers stock them.

Choose your auxiliary carefully

Although always a yachtie at heart, ever since I acquired my first yacht at the age of 17 I've found that an auxiliary engine matched to the hull really makes owning a sailing boat more enjoyable. When buying an engine use the HP requirement, waterline length calculation for hull speed and prop slip percentages mentioned in this guide to make the right choice, one that gets you quickly, efficiently, reliably and safely to and from the race course. If you primarily sail in saltwater a two-stroke will suffice but for non-flushing freshwater lakes I only recommend a four-stroke.

For your nearest Mariner/Mercury dealer contact Mercury Marine Australia on 03 9791 5822, Suzuki contact The Haines Group on 07 3271 4400, Tohatsu call Lakeside Marine on 02 4392 6110 and Yamaha 02 9757 0011.

Cyclops Marine
JPK August 2023
NAV at Home
Cyclops Marine