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
O' Day 20 outboard size
- Thread starter skindeepfx
- Start date
Take a look at the Tohatsu 6hp sailpro long shaft (or extra long shaft). You do not need more than 6 hp on this boat to reach hull speed and more horsepower will not make the boat go faster.... it will just burn more gas. I have this O.B. on my Hunter 26. I can get to hull speed at 3/4 throttle. The sailpro also has a 6 Amp alternator so it will keep your batteries topped off.
Also starts on the 1st pull 90% of the time.
Also starts on the 1st pull 90% of the time.
Measure at the exact center of the transom from the very top to the very bottom. If it measures 15" or with in a inch of that you will need a short shaft outboard. If it measures anywhere from 17" to around 22", then your going to want to go with a long shaft motor. Check the motor manual. Post #2 in the thread below indicates the "horizontal fin (Anti Ventilation Plate) above the prop should be at least 1" below the bottom of the hull".
Long shaft or short shaft?
How does one go about determining which motor "shaft" length will do the job, long or short? Currently sailing an O'Day Daysailer II. Even IF a short shaft will do now, what if I change to another brand of boat? Better to invest in a long shaft to start with and keep it when selling the DSII? Barry
forums.sailboatowners.com
according to O'Day Day Sailer - Small Boats Magazine , this review, anything over 2 1/2 hp is total overkill. the longshaft is the only way to go so that the prop stays in the water in bigger waves. the engine is way above the waterline when tilted up so weight stored that high is a big concern having unwanted weight so high. not good
just the facts given for my $0.02
just the facts given for my $0.02
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.
owned a cal20 for 25 years, now daughter owns. used a 2 1/2 horse merc on it. would push it hull speed at half throttle. sail in prolly the breeziest areas of all our venues. (yep, more than S.F. bay. also an area of the largest thunderstorms on the planet earth) i do not have current issues like some in tidal basins, but the OP is sailing in tenn..
the article by small boats mag. shows the same numbers that i got.
i will repete what i said. anything over 2 1/2 hp is total overkill!
side note, i did not give my daughter the engine, instead i gave her a long shaft paddle.
in my not so humble opinion, a bigger engine is not required ever. cost more to run. gives you no advantage what so ever.
there are alot of "wives tales" out there
that's right , i said it
just my $0.02
the article by small boats mag. shows the same numbers that i got.
i will repete what i said. anything over 2 1/2 hp is total overkill!
side note, i did not give my daughter the engine, instead i gave her a long shaft paddle.
in my not so humble opinion, a bigger engine is not required ever. cost more to run. gives you no advantage what so ever.
there are alot of "wives tales" out there
that's right , i said it
just my $0.02
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I found this posting. Note I don’t know how valid the HP rating is as the data isn’t from the manufacturer.
oday.sailboatowners.com
Oday 20 Owner Modifications and Upgrades
Oday 20 Owner Modifications and Upgrades
oday.sailboatowners.com
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?
dad had one of these 38' racers. grew up sailing it. it had a 7 1/2 evinrude outboard that went in a well and the was pulled out during the sailing. pushed that 38' at hull speed at 3/4 throttle.
there allot of people that love to overkill everthing.
yet there is no logic from what i have learned.
new sails, not there is something you can overkill with good results
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I have been in a situation where the current was running at about hull speed and I was trying to go upstream. The thing to do is pull your keel up and hug shore as close as you can. The current is always less near shore.
@jon hansen is right about not needing a larger O.B. The O'day 20 will not go any faster through the water than about 5.6 kts (ever) and you won't need a lot of HP to get that. People talk about bigger engines helping with current but actually it can only really help you when pushing into a headwind. I recommended the Tohatsu 6 because it is the smallest one I could find that had an alternator... but if that is not important to you I'd look at a 4 or smaller. I had a 2-year old Honda 9.9 on my Hunter 26 when I purchased her and quickly sold it and got the 6 HP. I can lift the 6 with one arm. Take it home in the winter.... etc. The 9.9 was heavy and pooped my stern.
If all you need is a 2 -2.5 hp engine then suggest a long shaft Torqeedo electric motor and avoid the gasoline can, heavier weight, smell and maintenance. If you want to maximize its range purchase one with the 915 WH battery.
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 bought a 6hp Tohatsu long shaft for my 18.5’, they are the same motor as the 4 hp with a bigger carb so there no weight penalty.. Pushes my boat to 4 knots at 1/3 throttle.... 5 knots at 1 2 throttle... and 5.75 knots at 5/8 , wide open it will touch 6 knots but I’m never in that big of a hurry and would rather let my motor have an easy life.
We're pushing our Signet 20 (bilge keels, skeg rudder, displaces #2150) with a 5 hp 2-stroke long shaft. Plenty enough power to push into 20 kts of wind, but she does cavitate in 2 ft seas, or any time I run forward to handle sails (20 foot tends to ride high opposite my position), neither situation is helpful in 20 kts of wind, trust me... Get a nice long shaft is my advice, when you really need the length is when she's blowing hard as the squalls are bearing down on you (really common this summer in Ithaca), not when it's all pretty and light.
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.
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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.
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.
I‘m having some difficulty envisioning the acceleration force in your 5 ft sea example, is the motor stored in its position close to the transom or extended out and down? In either position the weight isn’t “jolting” down as the boat would not be on a plane as it crests the wave.
I would think there are times when there are large horizontal forces in play as in when the boat is stationary and gear lever engaged and throttle up, or while docking and abrupt forward/reverse directions are incurred.
Given that many of the boats discussed here on SBO are “mature” extra care should be taken to not over stress the transom with a heavy, high HP engine. After all they are sailboats
I don't think many of the boats typically discussed in this forum can exceed the old historical hull speed limit by enough to make that an important part of outboard sizing. The hull form most likely to be able to exceed that limit by a lot are very narrow for their length, like those found on many catamarans. Of course, planing hulls can dramatically exceed this limit, but that still takes way more power than you're likely to want to put in an outboard on a sailboat.
I do agree that oversizing the outboard by some amount makes sense if you expect to face adverse winds and seas, and to also help make sure that if facing adverse currents you are at least doing the best you can.
I do agree that oversizing the outboard by some amount makes sense if you expect to face adverse winds and seas, and to also help make sure that if facing adverse currents you are at least doing the best you can.