Motor for O'Day 20

[CHris F]

I purchased a 1976 O'Day 20 In Spetember 2016, which came with a 5hp Tohatsu 2 stroke. I later needed a dinghy to get to and from my mooring, so I found an old 9' Avon with a 3.5 HP Mercury 4 stroke. I no longer will be needing the dinghy to get to my mooring in a medium sized lake in NH. Will the 3.5 HP Mercury 4 stroke be sufficient for the O'Day 20? I prefer the idea of a 4 stroke, cleaner, no oil/gas mixing, but I want to be sure the 3.5 HP Mercury will be sufficient to get to and from the mooring, and get me back to the mooring if the wind dies or any other reasons. Any ideas would be appreciated. Thanks.

 

[Brian S]

The 3.5hp ought to be enough for lake sailing. Where it would struggle would be against strong headwinds, because more HP can push through aerodynamic drag. People talk about "Will this motor push through strong currents?" The fact is, your motor cannot push you very much faster than the hull speed. More application of power only nets very small increases in speed over ground. Most of the additional power generates a taller bow and stern wave, eventually leading to bad handling. A wave's speed through water is dependent on the wavelength. Longer wavelength, faster wave. So, the wave form that a displacement hull creates can only be as long as the waterline length of the boat. Thus, more power applied cannot make the boat generate a longer or faster wave, and thus the wave just gets higher (amplitude. The energy has to go somewhere.) So, if your boat has a hull speed of roughly 5.5 knots, you will not be able to buck a 6 knot current no matter how much power you apply, because even standing still SOG, the boat hull and wave form sees 6 knot speed through water from the current.

All that being said, even with 20 knot (ish) winds on the nose, my 192 has made good headway at roughly 1/2 throttle of my 5hp 4 stroke Nissan with no current.

 

[Sunbird22358]

The one concern that I would have is that I assume that the little 3.5 hp Merc is a short-shaft? It should still work, but a long-shaft will be better overall. A Long-shaft (20") will allow the prop to stay In the water even when someone goes forward (like to pick up/drop the mooring.) The long-shaft can be used with the adjustable bracket potentially set higher to get the powerhead higher above the water. Finally, you may be planning to use this outboard only if the wind dies or for short distances to/from mooring, but if your lake is anything like the typical boating areas........ you will need to deal with powerboat wakes rocking the boat as you try to maneuver...... the long-shaft will help to keep the prop from coming out of the water when a wake hits you.
3.5 Hp should be enough power, we used a 4 HP on our old CAL 21 which was similar to the O'DAY 20, and I occasionally used my 2hp short-shaft Johnson to move the boat from mooring to dock and dock to mooring in the Spring and Fall to save carrying the 4hp out in the dinghy (the larger motor stayed on the bracket all season, but we usually put it on after launching and took it off before haul out at boatyard) The short-shaft just reached as long as I stayed at the aft end of cockpit.