Catamaran buyers out there in the world have explicit demands and preferences when it comes to the layout of their catamaran. Things like dagger boards versus keels, Yanmar versus Volvo, flybridge versus bulkhead helms and the list goes on. A hot topic these days is that of the buyer of a straight shaft engine, or the other guy who would like a saildrive unit instead, on his catamaran. Traditionally, there are more straight shaft drive monohulls and more catamaran’s built with saildrives today. This rule tends to go out the window on vessel’s over 50 feet.
Just to gain some perspective, I drove down to the marina by my house, where there are about two hundred vessels tied up in season and about half that being full time slip holders. The marina is known to be the home for many of the Mid-Bays catamarans. It was a beautiful Saturday morning and the marina was already busy at the early morning hour. I decided to speak with ten individuals on catamarans, asking them, if they could choose the drive system of their next boat, what would it be? Saildrive or its contender, the straight-drive that is standard in so many vessels. I was skeptical to find 10 Cat owners to that would be on their boat and I was correct, so 7 of the interviews were done off-site. The results were interesting, but one thing is clear, every owner was firm in their beliefs. Six had saildrives; one 35’, 38’, two 46 footers, 48’ and the biggest out catamaran owner I could find, a 50’, not included in the group, there was one lone wolf, an elderly 28’ with outboards motors.
About half of the conventional shaft owners wish that they had saildrives instead of the existing straight-shat they’re stuck with. Many of the faults and disadvantages of straight-shaft drive are big issues to some but create ease of service as well. Since straight-shafts perform better the less that the prop is angled down, to include spacing for the shaft, the position of the engines is forward of the sterns. This creates an unfortunate design plan that involves boats with straight shafts, to house the engines under the aft bunks. This isn’t necessarily the case in the larger cats bigger than 45 feet in the great many smaller vessels. These straight shafts tend to be very noisy and the bulk heading tends to be very warm, since it puts off a significant amount of heat. Outside of generating noise, vibration and there are much longer exhaust lines that have to be funneled thru the hull aft, and out of the boat. In my opinion this is leaving exponential risk for exhaust leaks into the vessel. Yikes. Of all the boat owners that I spoke to, only two mentioned this happening. Moving on, since the prop is at angle, straight shafts produce less effective horsepower at the prop. According to builders at Volvo-Penta, on a standard 40 foot boats horse power engine, the straight shaft drive looses 3-5 horsepower over catamaran with Volvo-Penta sail drives. Interesting fact! Lastly, straight shafts must be aligned exactly, and put thus puts a lot of pressure on the engine mounts, requiring realignments occasionally.
The general agreement with the advantages to a straight-shaft driven vessel, even with the engines being forward, is that the laws of physics kick in. Anytime weight is moved forward and towards the center in a boat, pitching force is then, lowered. More builders are putting the engines in the very center of their hulls of their race and performance cat’s for this very reason. Balanced low and in the center, allowing the bow to cut through the waves instead of going over. The main reason many get the conventional shaft drive, is due to the cost being much easier on the checkbook. Repairs are easier and more affordable at the same time, since there is more room to access. Lastly, for all the larger boat owners out there, if you want anything over 75 horses, it’s going to be in straight shaft only. Currently there are no sail drives over 75 horsepower that I could find.
Once talking with the saildrive owners, I was intrigued. Of course, with the older saildrive models, there were reports of many repairs, countless hours and dollars spent to repair. Eventually older units were replaced and with much emphasized routine maintenance, kept up and running, smoothly. The older model’s biggest issues are leaking gaskets and faulty shifting issues. Saildrives have seals in them that are designed to prevent saltwater from getting inside to the grease and oil lubricated drive-shaft leg. When compromised, saltwater leads to the fatal deterioration of the oil. It is easy to check and ensure that there is no seawater inside your saildrive housing. Simply inspect the fluid level, if milky-white liquid is seen, it is time to replace your seals with a haul out and change the oil. That’s the result of water mixed with oil. The most influential factor in the care of saildrives’ is the proper zinc protection. It’s critical not to skimp on the zinc, because otherwise, you will corrode the shaft. Make sure to properly inspect all zinc to prevent corrosion. Last, upgrading or conversions of saildrives’ involves intricate work and is expensive.
Every saildrive owner that I spoke to had plenty of good to say in the end about their experiences. Mainly, the engines aft are heavenly, to say the least. There is significantly less noise, heat, vibration and much more room due to compact size engines with a saildrive, can fit in. Most of newer cat’s engine rooms’ are watertight as well, adding extra safety onboard. With the weight of the motors aft, the trend in cats is they sit back quite literally on their haunches like their feline friends. The hulls pierce through the waves, dampening the pitching movement. And alas, with their overall quietness, engine alignment is not as critical as a straight shaft and you are receiving the actual horsepower at the prop.
So, wrapping this article up, is there any difference when running aground, I wondered? It would seem the safer of the vessels are ones with saildrives in their water-tight compartments. Regardless of choice of drive, there is still going to be quite a bit of, either way. (Bent shafts, props, flooding, etc.) The good news is that if you are getting hung up on the decision to choose between a conventional shaft or sail drive as your buying concern, hang it up. If you are sold and change your plan on having a straight-shaft drive, please keep in mind you will live with all the positives and negatives that I talked about above. When I went and shopped around, I found almost 90 % of all the cat’s I looked at, which were modern, from 30-50 feet, were all saildrive. Seems as if, builders have decided that the benefits of saildrive outweigh the concerns of it and therefore seems to be overall, much better for the use in mid-size range of catamarans.