Motorsailers - above and below the waterline. It occurs to me that there are a several loosely coupled factors. As Malcolm points out there is hull shape issues that effect high speed performance. However most mono and cat motorsailers also tend toward larger interiors, although usually not approaching the interior of a dedicated power only vessel. We don't see too many power cats here in southern California (or cats at all for that matter). At one of the fall boat shows I was checking out a Nordhavn 47 and then walked over to a Catana 43. I must say as I sat in the cockpit of that Catana, there was an ahhhhh feeling. A comfortable corner seat, feet up, sun shade, breeze blowing, watching the world go by. While in the salon of the N43 I felt like I was sitting in a wooden box. So a couple of questions for the group: A motorsailer below the waterline: Is there a down side to taking the concept of say a 43-47'cat with something like Malcolm's CS hull form to bump up the speed under power. I don't remember the exact number but I think the Catana dealer claimed that on one engine the 43 could do 7 knots at 1gph. If you could have that efficiency with a twin engine sprint speed of 12-15, that would be a nice combination. The difference between this concept and most motorsailers is that one is not enlarging the interior space. I assume there must be some sailing penalty. A motorsailer above the waterline: All of my long distance crossings were by sail many years ago in my 20's. Since then it's been local island trips in my boat and 1-2 week power and sail charters. For those folks who undertake long range cruises and passages does there come a point where the joy of sitting outside in the big cockpit of a sailing cat gets old and you want just be inside all the time? I tend to like the flybridges on the Selenes better than the Nordhavn as they are a better approximation of the cockpit of a cat. But the Selene dealer said that many people like the pilot house to avoid the cold and wind and then turn on the AC when it gets hot. I'd love to hear the experiences of those folks who've been underway for extended periods of time and how that effects their desired configuration living configuration, regardless of hull shape and propulsion approach. If you were going to do a motorsailer or power only cat, would you consider a boat that had the equivalent interior space of a sailing cat? I also find it interesting that many people doing circumnavigation under sail have told me that they end up powering 50-70% of the time. And several have commented that there is another healthy percentage of time they spend reefed in the trades. Maybe one answer is a slightly smaller rig tweaked toward downwind performance. Thanks Mark ----- Original Message ---- From: Malcolm Tennant <malcolm@tennantdesign.co.nz> To: Power Catamaran List <power-catamaran@lists.samurai.com> Sent: Monday, January 8, 2007 11:47:44 AM Subject: Re: [PCW] Motorsailers, was Engine-room access Dear Georgs, Here is an article I wrote on motor sailers.This discusses the catamaran as a motor sailer. I hope it isn't too long! MOTOR SAILERS IT HAS LONG BEEN MY CONTENTION THAT, AS A MOTOR SAILER, THE CATAMARAN HAS NO EQUAL. IT PROVIDES THAT MOST ELUSIVE OF QUALITIES; THE BEST OF ALL POSSIBLE WORLDS I wrote this back in 1983 when we were first developing our motor sailers. However in the intervening 22 years I have not discovered anything to make me change my mind. What has changed in that time is that there have been tremendous developments in construction methods, materials and sail handling techniques. This can be seen in such developments as the in the boom roller furling and captive sheet winches. These, coupled with the continuing computerization of a lot of sailing functions, today make the motor sailer catamaran concept more viable than it has ever been. Of course, in the early days of the transition from sail to power, all vessels were motor sailers. The engines of the time were neither powerful, nor reliable enough, to propel a vessel unaided. The steam required by the engines was produced by coal fired boilers and it was difficult to carry enough coal for an extended voyage and still leave space for a significant amount of cargo, or a reasonable number of passengers. This gradually changed with the increasing efficiency of the steam engine. And then the advent of firstly the naptha engines, and then petrol and diesel further hastened these changes. All these engines gradually became more efficient and light enough to be the sole means of propulsion. But for many years sails were still kept for emergencies. Then there was also the allure of getting something for nothing ie: propulsive energy from the wind. This attribute was, and still is; along with the not to be overlooked romance of sail, one of the major attractions of the motor sailer. The motor sailer usually consisted of a heavy displacement sailing hull with large engines installed to give better performance under power. The heavier engines and the need to be able to carry more fuel necessitated the heavier displacement. But ultimately this approach is doomed by Froudes law. Neither the speed/length ratio nor the displacement/length ratio was high enough and the resulting vessels did not sail particularly well and neither was their motoring performance very efficient either. So generally the motor sailer has, of recent years, slipped from favour and sails are now found almost exclusively on yachts, or occasionally, in a steadying role on long-range trawler yachts. This is unfortunate because there have been some very good pilot house cutters designed that have quite reasonable turns of speed under power and more than adequate sailing ability. But they did, in the main, need to be relatively long to be successful in both roles. So are there other approaches to this compromised performance problem that might give a more satisfactory solution and allow reasonable performance under both power and sail? One approach has been to dynamically alter the underwater shape of the sailing hull to produce more power boat like hull characteristics when motoring. The major problem with the displacement sailing hull is that the longitudinal rocker combined with limited speed/length ratio causes the stern of the boat to squat under power. This severely limits the speed that can be achieved. However if the aft buttock lines can be straightened out to approximate those of a planning power boat then planning, or at least semi planning, may be possible. This would then mean that higher speeds under power were potentially achievable. This approach was tried in the UK on a moderately sized keelboat in the 70s. Retractable planning wedges being fitted down aft. This was reportedly reasonably successful but this hull modification approach does not appear to have been tried again until very recently. Just launched at the end of last year from the Pendennis Ship yard is MITseaAH a 156 luxury yacht with the naval architecture provided by David Pedrick. That this is a very serious attempt at designing the ultimate in contemporary monohull motor sailers is shown by some of its parameters. It has the afore mentioned planning flaps down aft but it also has retractable rudders and keel. The motive power is provided by two 3,500hp engines driving 2 x 4 bladed 1.65m diam controllable pitch propellers that also retract partially into tunnels when sailing. The result of all this is that it will motor at 20 knots. Faster than is usual for a sailing vessel of this type. However MITseaAH is a luxury vessel that is hugely complex and enormously expensive. Given that the 20 knots achieved is only approx 4 to 5 knots more than the boat would have potentially been able to achieve in normal displacement mode with that power, on that waterline length, MITseaAH would not seem to be the answer to achieving the successful dual role that most people would be able to afford. In New Zealand there has recently been unveiled a design proposal for a monohull motor sailer that has a retractable propeller putting 800hp into the water to achieve 20 knots under power. A huge rig promises similar speeds under sail. This design takes an ingenious approach to the squatting problem. On this vessel twin hydraulically canting rudders act as elevators to control the trim and keep the waterline level when under power. However while this does prevent squatting while under way its does not change the fundamental hydrodynamics of the displacement hull. It still has the curved aft buttock lines of the typical heavy displacement sailing boat. Is it possible then to approach this from the other direction? Rather than trying to alter the shape of the sailing hull underway, can we fit sails onto a power boat hull? It is possible to fit sails to a traditional displacement power boat hull? We have seen that this is exactly what was done in the early days of power. Currently this is often done in the guise of a steadying sail on long range cruising power boats. But again this approach runs up against Froudes Law which limits the performance of a displacement hull under power and sail. However the traditional way to go fast in a power boat without it being very long is to make it planning. So why not fit sails to a planning hull form? This has actually been done, and very successfully, with the 26 McGregor Powersailer. This vessel sails well, for a trailer sailer and planes with outboard power. However, this was a small light displacement trailer sailer and it has proven to be much more difficult to emulate this feat with higher displacement vessels. The planning hull form has not, so far, proven to be particularly adaptable to the sailing role. This would appear to be largely a function of the very large immersed transom that is characteristic of this type of hull. At sailing speeds they have very high hull resistance and not enough speed, or power under sail, to get onto the plane. Consequently the sailing performance leaves a lot to be desired. So, is this approach of using a powerboat hull to produce a credible motor sailer worth pursuing? If we look at a completely different power boat hull form then I believe that the answer would appear to be yes. A sailing catamaran relatively easily achieves higher speeds, under both power and sail, with 30 knots not being uncommon. However, although the sailing catamaran has a higher hull speed than a monohull, it is still ultimately governed by Froudes Law and squatting occurs at higher speeds under power. If squatting occurred with the sailing catamarans at these sorts of speeds then it obviously was going to be worse in the case of a power boat using the sailing catamaran hull shape. What is more the power boat did not have the diagonal forward loading from the rig that tended to counteract the squatting to some extent in the sailing vessel. To get around the squatting associated with these higher speeds on a displacement hull form a hull shape that was different to that of the normal sailing catamaran was needed. We early reasoned that we could minimize the squatting by careful shaping, and placement, of the aft buoyancy. By sweeping the hull inwards at the stern to straighten the aft buttock lines, rather than up to the surface in the traditional manner, we created a fully immersed canoe stern hull with no exposed drive train appendages such as shafts and struts. This also resulted in a clean water flow to the propellers. This shape was then enhanced by then placing a flat section of hull over the top of the fully immersed canoe stern. This did result in an immersed transom but one of much smaller dimensions than that found on a planning monohull so while the increase in drag was minimal, while the effect on squatting was maximal. This approach to a high-speed displacement catamaran hull proved to be very successful. So when we started looking at motor sailers in 1983 we essentially took our powerboat hull form and fitted a rig. There was some extra reinforcing needed to take the loads exerted by the rig but there was very little else to do. We knew that this hull form, in the sizes we were looking at, was capable of more than 30 knots under power [ The 17.5m loa Red Diamond II was clocked by NKK at 32 knots over the measured mile] with very little change in level trim at speed. It was really only a question of how well would it sail. Stability was not a problem because of the inherent high initial stability of the catamaran configuration. Our low wave making CS hull form, which we had developed earlier in 1983, had shown exceptional efficiency as a power boat and the parallel aft buttock lines led us to expect that it would have exceptional sailing qualities also. In practice this has proven to be true, as this hull has turned out to be faster than the equivalent sailing catamaran on a reach. Windward performance is not in the racing boat class but is certainly better than that of most cruising catamarans and could be further enhanced by fitting centerboards if one wished. Of course you could always turn one, or more, of the engines on! The first boat to prove the viability of this approach to the motor sailer was Athenea one of our Cordova designs which has now covered more than 100,000 nautical miles in the open ocean. This 18m [60] by 8.7m [28 6] cruising boat does 15 to 18 knots under power from 2 x 150hp and has sailed at more than 20 knots on many occasions. The 16,400kg [36,155lb] displacement encompasses an interior that would not be out of place in a medium sized house. This house like impression is further enhanced by the use of domestic appliances for the fridge, freeze, dishwasher, washing machine, air conditioning and clothes drier etc The helm is totally enclosed and the sailing functions can all be performed from there with the help of a couple of electric winches. All this was achieved on a motor sailer catamaran in 1983! Because the catamaran usually has a twin-engine installation it leads to the possibility of some interesting sail/motor combinations. It is possible to set a target speed and use whichever combination of sails and engines necessary to maintain that speed. The owner of Athena set a target speed of 12 knots. Some 20 years later he had to admit defeat as in all that time he had only managed to average 11.8 knots! The use of feathering propellers minimizes drag when only using one engine or sails only. If very long distances are contemplated then controllable pitch propellers are an option that might be considered. It has been shown [on Wildwind IV] that running on one engine with one propeller feathered and the pitch on the other optimized can result in a 10 to 20% reduction in fuel use at the same speed. Of course, this also halves the engine hours and allows maintenance to be performed on a cold engine. It would also be possible to completely automate the motor sailer systems and leave them entirely under computer control to achieve maximum efficiency at whatever ratio of sail to power was the most efficient at achieving the desired performance. However controllable pitch propellers, which would be a necessary part of such an automated system, are expensive and their use can only be justified if you are traveling very long distances. [Wild Wind IV is currently approaching 30,000 nm and Athena more than 100,000 nm] This general approach of fitting sails to our power catamarans can be applied to all the various styles of our power catamarans. Provided the required extra reinforcing is built into the structure and it has sufficient beam to give the required stability under sail, any of our long-range power cats can be fitted with a rig and achieve very respectable performance under sail. This also means that you can motor around quite happily without the rig if you wish. Until such time as an exorbitant increase in the price of fuel forces a radical rethink and the fitting of a mast and sails. You could also of course have the latest in Eurostyling in a swept up catamaran such as Pacific Harmony that would out perform MITseaAH under sail, or power, and also offer much more interior space and comfort for considerably less complexity and cost. Malcolm Tennant Multihull Design Ltd PO Box 60513 Titirangi, Waitakere 0642 NEW ZEALAND ph +64 9 817 1988 fax +64 9 817 6080 e-mail malcolm@tennantdesign.co.nz www.tennantdesign.co.nz www.catdesigners.com _______________________________________________ Power-Catamaran Mailing List