Outboard Discussions

[Theis]

The deeper the shaft, and hence the exhaust, the more important it becomes to vent the exhause relief outlets obove the wate line. The deeper shaft increases the back pressure throught the exhaust outlet in the lower unit, particularly when the motor is idling or at low speeds. This will therefore increase the amount of exhaust that exits via the pressure relief holes (so there is no back pressure, regardless of the level of the lower unit exhaust outlet). When operating at higher speeds the through prop exhaust sucks the exhaust out (or is designed to do so - but perhaps not completely) A prop/exhaust that is one foot lower than a shorter version increases the back pressure by 62 lbs/inch.

But still I would opt for a longer prop shaft -and in no case take a standard length because it pops out of the water - in my experience. Other than as mentioned above, I don't see any disadvantage to as long a shaft as you can get.

[Edgewood]

The ultra long shaft(25") that I am considering comes standard with Alternator, which is a plus.

[Mike Goodwin]

According to motor people ,those that fix them and sell them, if your transom is 15" off the water then get a 15 , 20 a 20 and 25 a 25 . They say it is not good to put a 25" shaft on a 15" transom . A 20 comes closest to fitting these boats .

Not my words , my mechanic's .

[Theis]

Ordinarily, I would listen to the OB fixit people, and have done so until some recent inquiries regarding the OB and the Ariel. When the answers are randomlly different, and border on "I don't know but my best guess is.....", then your own best judgement is my recommendation. For example, I no longer take the advice of the OB fixit types with regard for fuel mix. With the Yamaha, it is ONLY 100:1, not 75:1 or 50:1 as most OB types I have contacted have suggested - their reasoning being that a bit more oil is erring on the safe side. The richer oil alternatives do not work.

The problem with the OB fixit types (and generally the manufacturers as well) is that they are generally not familiar with the unique circumstances of powering a sailboat with the motor in an inboard well. Their recommendations for shaft length are based on a conventional boat, and perhaps even a conventional non-planing hull. These boats do not have the problem with cavitation at the lengths they recommend, do not have a keel extending far below the drive shaft, nor do they go out in heavy seas, nor are they asked to run when the boat is heeling at 20 to 30 degrees. In short, the information upon which they base their recommendation is notoriously incomplete.

Summarizing, I would recommend ignoring the OB fixit types and going with the insight of those that have OBs driving the Ariel - which in today's world, is a unique OB driven design. The caveat, however, is if you make it the drive unit lower in the water, the back pressure is greater than the OB design back pressure, and the amount of exhaust exiting the pressure relief ports will be greater. This exhaust must be exited to the outside world (not just the motor well). This may not be mandatory with a shallow draft motor.

[frank durant]

This 'simple' question is getting WAY too complicated for a basic mind like mine. IMHO...I had a 20" shaft and found that even motoring into waves that at times were quite large...I experienced only VERY minimal cavitation. 20" is fine in my books...it works!!

[Theis]

Back to basics. Yesterday I took the critical measurement. I have 15-16 inches from the water surface to the center of the prop. My engine has cavitated a couple times in high seas. If I had a preference, I would put it in the 17-19 inch range.