How long should the outboard leg be?
Style of boat and application.
When selecting leg length it’s important to consider your type of boat and its application. Choosing a leg for a tender is vastly different to choosing a leg for a sailing boat. Will it operate as a displacement boat or will it plane? Will it be used in shallow waters or mostly navigate on and off a pontoon? Punch through rough water? In this guide we will help you to ask the important questions that can help you make your decision.
Hard boat facts … Transom height?
It’s all pointless if the propeller doesn’t even reach the water to start with. It’s important to know the height of your transom
to the waterline and compare this to the outboard leg. Most outboard manufactures measure their “Leg length” from the shaft axis to the inside top of the mounting bracket (see diagram).
You’ll then need to subtract the radius of the propeller from the leg length to find the distance from transom top to propeller top. This would be considered the absolute minimum distance to the waterline level from the top of the transom and remember that this waterline may change depending on how much weight (yourself included!) you have in the boat, especially relevant for a tender. “Waterline level” is specifically referred to as the base of the transom may sit below (or above) the waterline level. In this position you would have excellent bottom clearance for navigating into shallow beaches, but it may be inefficient in rough seas as the propeller may periodically leave the water. For your convenience here is the Spirit 1 Plus leg range:
You now need to ask yourself what do you want from your outboard? A well submerged propeller that is positioned below the transom line in un-interrupted smooth laminar water flow is vital for planing vessels and optimal for displacement vessels for both thrust and regeneration, but may not be practical when trying to come into shore or storing the outboard onboard after its use. It’s a personal choice.
Real world compromises.
Finally, you’ll need to consider the way your boat was built and what mounting options you have. For the sailing vessel retracting the outboard, it is not ideal if the shoe drags in the water when raised. Most boats either rely on tilting the outboard up, moving up its mounting bracket, or a combination of both. Here are the tilt angles of the Spirit Plus long and short legs to give you an idea of water clearance. Don’t forget to factor in your own margins for sea conditions and lean/heel angle on the hull.
It wouldn’t be right not to mention the advantages of e-Propulsion electric motors on this website. For example, there is no cooling pump so there is no need to worry about ingestion of sand when motoring in shallow water. The Spirit range is cooled by its motor housing and the propeller will sense a jam and shutdown before breaking the propeller or split pin which gives it excellent utility in shallow water.
Our customers have found that the suitable length is somewhere greater than 5cm below waterline in a (loaded) tender. For all boats, the outboard will ideally remain in the water in most sea conditions and sailing heel angles but be clear of the water with a margin when retracted.
Please note only the Extra Short (XS) and Long (L) legs are stocked in Australia. If you need the Short (S) leg it can be specially ordered in but may incur an extra cost and take several months.