Know-How

A Tale of Two Props

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Although I have sailed boats fitted with every conceivable make of folding or feathering propeller, I have long-term experience of only three. When we acquired our 1973 Norlin 34, it was equipped with a vintage two-bladed Martec Geartec folding propeller. The boat displaces some 13,000lbs and boasts a rowdy but reliable and economical Yanmar 2GM20 diesel with saildrive. 

A 13,000lb boat would be better off with a 30hp engine, so it was important to make the most of the limited power at our disposal. The 15in x 10in Martec was fine for getting on and off the mooring and in flat water could propel us at a little more than 5 knots flat out. In any kind of weather, though, its limitations were exposed. Faced with a headwind and a bit of chop, it was hard pressed to make 4 knots under power. Also, the discernable lag between engaging reverse gear and any stopping effect was not good for the nerves.

Prop #2 was a Kiwiprop, a three-bladed feathering propeller from New Zealand. These have several unusual features. One is the blade material—Zytel, a resin composite made by Dupont. Another is that the blades are not linked by gears; instead, each blade pivots independently on a stainless steel pin so that each is free to take up its own angle to the water flow. The only metal part is the stainless steel hub, so the propeller is very light, which must be beneficial for the saildrive’s innards.

If ease of installation were the main criterion for choosing a low-drag prop, the Kiwiprop would win hands-down. You can install or remove it in a few minutes, which is handy when it comes to changing the zinc on the saildrive. Speaking of which, where our zinc was completely wasted away after six months in the water with the Martec, it was barely touched after a season with the Kiwiprop. We saildrive owners are paranoid about galvanic corrosion so this can only be a good thing. 

Although it was better in every respect (except drag) than its undersized predecessor, the Kiwiprop did not quite achieve the speed I was hoping for. We managed a cruising speed of 5.8 to 6 knots at 3,200 rpm, which was as close as the little 2GM would get to its 3,600 redline. It did provide noticeably more thrust when powering into wind and waves, which was a good thing. Kiwiprop had recommended a 15 ½ inch prop with 20 degrees of pitch, which I ended up tweaking just a little—this adjustment is very easy and can be done with the boat in the water—to no discernable effect.

In reverse, the blades flop over so their trailing edges become the leading edges when going astern, prevented from going all the way by a trio of stainless pins that limit blade travel. This results in a very coarse pitch. Backing up, the boat stopped quickly and gathered sternway well, at no more than a fast tickover. Prop walk was not an issue, but then again it seldom is with saildrives, as they’re usually located more toward the center of the boat. I kind of miss it, actually. 

In our second season with the Kiwiprop, the engine acquired a habit of stalling almost every time I put it in reverse, which does the heart no good at all when you’re approaching a dock. That winter I stripped the prop down and greased the pivot pins, and had no more trouble, though the little engine clearly struggled with the prop’s coarse pitch when going astern. All in all, though, it was a good propeller.I was still wondering how to make the most of our little Yanmar’s limited horsepower. Would a higher-end folding propeller, with its more efficient blade shape, make a difference? I snapped up an offer to test a three-bladed Gori folding propeller, with its intriguing “overdrive” function. Enter prop #3.

Installing the 16-inch Gori, while not as easy as the Kiwiprop, was by no means difficult (though I suspect getting it off won’t be so easy). Underway it felt different right from the get-go, generating less turbulence under the counter from a standstill and leaving a smoother wake when up to speed. We had gained over half a knot in top speed; in a flat calm (and with a clean bottom) we could easily motor at 6.5 knots, according to both GPS and sailing instruments. The prop also seemed to deal better with headwinds and chop—in one instance we motored into a sustained 35 knots of wind at just under 4 knots with the little Yanmar hammering away at 3,200rpm. 

I had been a little skeptical of the “overdrive” function. This is achieved by stopping the boat, putting it into gear and getting it moving astern, then shifting into forward. Thanks to a patented hinge pin design, the transition from reverse to forward leaves the blades open in the reverse position, so that the normal trailing edge is now the leading edge. That results in a 20 percent increase in pitch, which means cruising speed can be attained at lower engine rpm, with obvious benefits in fuel savings when motoring long distances.

In practice, this worked as it was meant to: in calm conditions, where we would be making 6 knots at 3,000rpm in “normal” mode, we could make that same speed at 2,700rpm in “overdrive.” You have to be aware of this function when maneuvering in marinas, where switching from reverse to forward when backing out of a slip will leave you in overdrive unless you put the engine in neutral momentarily to let the blades close. Using too much throttle at low speed in overdrive resulted in complaints from the engine and drivetrain when they were overloaded by the coarser pitch, though I only made that mistake a couple of times before learning to preempt it; and I suspect it wouldn’t be a problem with a more powerful engine. 

The Gori does not stop the boat as quickly as the Kiwiprop did, requiring a healthier dose of throttle in reverse, though it is far superior to the old Martec, and once you’re used to it, it is very predictable. Once you’re moving astern, it behaves impeccably and prop walk, again, is negligible. Under sail, I can’t fault the prop—it seldom auto-rotates, and whatever drag there is is created by the saildrive leg itself, not the prop. If I had a gripe, it would be with the Gori’s appetite for zincs, which will set you back about $50 a year (although, again, the saildrive’s protective zinc was scarcely touched). The Kiwiprop in contrast needs no zincs but does want stripping and greasing annually.

Martec’s Geartec line is no more, but its Eliptec Mk III sibling costs about half as much as the Kiwiprop, which in turn costs about half as much as an equivalent three-bladed Gori. The Gori is by far the most sophisticated product of the three, and if you are going on a long-term cruise—which invariably will involve long periods of sustained motoring or motorsailing—the potential fuel savings will no doubt be a factor in your choice. But your first choice will be between retaining your fixed propeller and investing in a low-drag prop, whether feathering or folding. If you race your boat, or like it to perform to its potential, that’s no choice at all.

Read Duncan Kent's related article "How to Beat the Drag"

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