On 9/25/2015 1:40 PM, John Schinnerer wrote: Hi John, Propeller pitch is another variable. An outboard motor for a sailboat needs a slow turning large diameter propeller to get maximum thrust. The problem is, that not many manufacturers offer you a sailboat propeller option, and as far as I recall, when they do we are talking about engines in the 6 HP and up range. The old British Seagull engines were slow turning and designed for thrust, not speed; but they were 2 cycle engines. Connie
On 09/23/2015 02:16 PM, James Poulakis wrote:
So can a 2hp Honda push a loaded (say 2500#) M-17 at its 5.5 knot hull speed against a 4 knot current? I’m hoping it will because I’m tired of doing the “clean and jerk” with my 60# 4hp Mercury.
This article: http://www.sailingworld.com/gear/lightweight-outboards-kick
...gives a rule of thumb and also says look at what is known to work: "My rule of thumb here is to start with a two-horsepower engine for small centerboard and keelboats less than 1,000 pounds, and add one horsepower for every 1,000 pounds of displacement....Compare your boats dimensions against what existing classes have found to work; for example, a Melges 24 at 1,650 pounds is typically rigged with a three-horsepower short-shaft engine, while a J/80 at 2,900 pounds can still squeak by with a long-shaft, three-horsepower engine. A 1,790 pound J/22, on the other hand, typically uses a four-horsepower long-shaft engine..."
One formula I've found is for minimum HP to move a boat at hull speed:
HP = (Displacement)/((150^2)/(Hull speed^2))
So for an M17 at ~1600 lbs displacement that becomes:
HP = 1600/(22500/30.25) = 2.15
This is more or less congruent with the above rule of thumb.
So the Honda 2.3 or the various 2.5's from other makers ought to be adequate to get hull speed. And more than adequate for cruising a bit below full hull speed, at less than full throttle.
A 3.5 HP would give slightly more than 1 HP margin over the minimum - which is a fairly big margin, percentage-wise.
There is also the issue of what prop is used - is it optimized for displacement sailboat needs, or for small light planing hulls (fishing boats, dinghys, etc.)? Apparently this can make a big difference in actual/effective thrust from a given HP motor.
From my recent research on small outboards, multiple major makers, the weight drop comes when you go under 4 hp. Most makers' 4-6 HP units are ~58-62 lbs.
Most makers' 3.5's are ~20 lbs less, at ~38-42 lbs.
Some 2.5's (Tohatsu, Mercury, Yamaha for example) are pretty much same weight as 3.5's.
Other 2.5's (Suzuki for example) are ~30 lbs. (but Suzuki 2.5 has only 15" shaft model, current options anyhow). Then there's the Honda 2.3 at 31 lbs. (long shaft model).
My research summary at this point:
* Honda 2.3 long shaft would be the choice on the minimalist end, 31 lbs.
* One of the 3.5 long shafts at around 40 lbs. would be an option for more margin of power.
* Anything 4-6hp is going to weight ~60+ lbs.
* If you want a reverse gear instead of rotate motor 180 for reverse, you have to go to a 4-6hp size and weight motor.
* The prop matters - presumably the "sailboat specific" models come with the right kind of prop for our needs but I wouldn't assume that's true for any model not specifically marketed to sailboats.
This is all from available mfg. info on current models. There are no doubt exceptions worth looking at in older models. If anyone has more info, please add it to the pile!
cheers, John S.