looking to get a new or used outboard for my soon to be purchased O'day 20 . From what i can tell HP somewhere between 5 and 10 . would you recommend a short shaft or long shaft ? Thanks in advance as always
YepI’ve always wondered how more hp would help against current, once you’ve got enough for hull speed. Hull speed is hull speed, isn’t it?
I would agree that the weight of the outboard is a big factor but it’s not the only one. As soon as you engage the shift lever you introduce other forces. The weight is a somewhat vertical vector where the thrust of the prop is more horizontal. I would think the manufacturer “rating” is a reasonable value to move the boat efficiently without causing damage to the transom etc.It depends on your sailing area. High winds and strong currents get a 6HP for calm lake waters a 2 to 4 HP will do. Check the rating on your motor mount. It is probably in HP but in reality it is the weight of a 2 stroke engine for that size horsepower. The actual limiter of engine size is weight. If you can find a 2 stroke engine in good condition it would be the best because the weigh about half of a 4 stroke engine. The sale of 2 stroke small engines was banned in the US some years back but the ones in use were grandfathered in. In 2 stroke you could probably go as high as an 8HP but I'm afraid that in a 4 stroke an 8HP or a 10HP would be too heavy. As far as shaft length a long shaft is always better but if buying used condition and availability are prime. Once again the conditions prevalent in your sailing area would dictate the need. In large sea swells a long shaft will be necessary.
Hull speed can be exceeded. The caveat is that to do so the horsepower needs increase at a faster rate per increased speed than when under the hull speed. Hull speed is not a magic number, it is the result of a formula that was derived long time ago when boats were made of wood and most had a similar hull shape. The formula for hull speed in knots = The Square Root of the length of the waterline in feet times a constant of 1.34. This constant of 1.34 is a trash number developed to represent the ratio of speed to the square root of the wetted length. It just so happens that back then most hulls were shaped and constructed similarly and would fit this ratio. This ratio of speed to the square root of the length varies widely in today's boats. I agree with your concept of speed that real speed is the speed over the water and not the speed over the bottom but many boats can go faster than the formula would today indicate and these boats would react differently to the addition of increased horsepower. Many times it is not current but wind that may slow the forward progress and in those cases a stronger engine will do better.I’ve always wondered how more hp would help against current, once you’ve got enough for hull speed. Hull speed is hull speed, isn’t it?
I would agree that the weight of the outboard is a big factor but it’s not the only one. As soon as you engage the shift lever you introduce other forces. The weight is a somewhat vertical vector where the thrust of the prop is more horizontal. I would think the manufacturer “rating” is a reasonable value to move the boat efficiently without causing damage to the transom etc.
I’m with others in keeping the outboard weight/power low
I just took a quick look online and it appears that many of the bolt on motor mounts are listed with a max HP and WEIGHT.All I meant to say is that manufacturers years ago rated their motor mounts in Horsepower while today motor mounts are rated by engine weight. The explanation is that they figured it would be easier for a new boat owner to match the engine HP to rated HP for the mount. Back then most smaller engines were two stroke so they figured it would be simpler to say 2 horsepower instead of saying 25-30 lbs for the average 2 hp engine. The weight of the engine yields like you say a vertical force but when a boat drops 5 ft from a sea swell in a couple of seconds it imparts an acceleration force that exceeds its weight and the mount should have ample reserve strength to stop dead that acceleration force. The horizontal force imparted by the engine thrust is of minor consequence as there is slipage to dampen the horizontal forces. As far as the motor mount is concerned weight is the primary factor that should be met.
Hull speed can be exceeded. The caveat is that to do so the horsepower needs increase at a faster rate per increased speed than when under the hull speed. Hull speed is not a magic number, it is the result of a formula that was derived long time ago when boats were made of wood and most had a similar hull shape. The formula for hull speed in knots = The Square Root of the length of the waterline in feet times a constant of 1.34. This constant of 1.34 is a trash number developed to represent the ratio of speed to the square root of the wetted length. It just so happens that back then most hulls were shaped and constructed similarly and would fit this ratio. This ratio of speed to the square root of the length varies widely in today's boats. I agree with your concept of speed that real speed is the speed over the water and not the speed over the bottom but many boats can go faster than the formula would today indicate and these boats would react differently to the addition of increased horsepower.