Monohull verses Multihull powersailers / motorsailers

I have seen Z type drives mounted on the centreline, one I saw used a hydraulic motor and simply swung up laterally under the bridge deck to stow. My doubts about hydraulic drives are about the power loss in the transfer system. Manufacturers of hydraulic motors claim from 80 to 100% efficiency but the power lost in moving the hydraulic fluid through valves and hoses is another matter, I know from experience that hydraulic systems get very hot while under continuous load, meaning power loss. Also potentially a major problem with failure in third world places. Always something to consider for world cruising, as you rightly point out.

We both seem to be coming to the same conclusion, that the best solution is always the simplest, and diesel gensets are commonly available and easily serivced. In southeast asia they adapt cheap Chinese made diesels for longtail type drives, using a chain reduction (very efficient) and a steel shaft that runs through a pipe with the propeller bearing on the outboard end. Those 30 horse diesels can be bought new in Indonesia for about $300 US and seem to be pretty reliable, but I'm told that they can't be rebuilt because the cylinder liners and valve guides etc. are not replaceable. In Australia mounting an inboard engine on the bridge deck on a hinged bracket to raise and lower it is a common way of poweriing cats, especially Wharrams. I heard from someone who had that arrangement that the biggest problem was protecting the engine from splash and spray, especially if it's air cooled. Air cooling is noisier and providing it with enough air and keeping water out at the same time presents a challenge but is probably the best idea overall, because then the genset is useable while you're on the beach. And whatever the pros and cons of multihulls might be, it's beachability that sells me on the catamaran concept more than anything else.

Water generating could be done by driving an alternator via the shaft, with a mechanical clutch to disengage the engine. Not as elegant as an electric drive but maybe more efficient. Water generating can be a great system if it's done right, the key seems to be a large enough diameter prop. A guy I know has that on a big ketch and it will start working with as little as 2 knots boat speed, so when anchored in a tidal river he gets enough power from that alone. Very nice for a cruiser.

The best way to run the exhaust on a cat seems to be straight down out the bottom of the bridge deck. I've been alongside cats with generators running and the exhaust routed that way, and heard absolutely no noise from alongside. You only hear it when directly forward or aft, but the enclosed space between the hulls seems to act as a big muffler and from on deck you can't even hear it.

One thing for sure, I am mystified why virtually all yachts use heat exchangers and wet exhausts. Apologies for being a bit evangelical on the subject, but I think one of the reasons all the cruising cats I've had experience with performed poorly in terms of fuel efficiency, is that the only thing worse than a heat exchanger on a cruising boat is two of them. If you contact a dealer for Walters keel coolers they'll show you test data on various boats comparing keel coolers with heat exchanger systems and they show that the negligible drag of the keel cooler is more than compensated for by the elimination of cooling system power loss, even on high speed vessels. I've owned two of them and found that they were absolutely install-and-forget. None of the problems of corroded mixers, corroded heat exchangers, plugged impellers, and no risk of salt water backing into the engine, which seems to the most common cause of marine diesel breakdown. Not to mention the danger of fire when the raw water pump fails. Best of all, it's much cheaper. Why are keel coolers virtually unknown in the yachting world?

Bob

 

Marine exahust placement

Bob,

I appreciate your experience, I have wondered how I could best avoid the desiel fumes when running slow with different wind angles. I have never had a placement anywhere but in the stern. And in a light following wind, where I would like the sails set, if I ran the motor I would choke on the fumes. If you place the exahust under the wing does't it get a bit smelly on the aft deck of the bridge.
Also I totaly agree with the keel cooler but I thought that water injected exhust was much quieter. With the motor mounted above the water line there wouldn't seem much danger of contamination to the engine. Is the maitenance of the dry system actually easier when it is exposed to such high temperatures. I've never had a dry system but on tractors, and those systems would never stand up to exposure to salt water at the temperatures they run at.

Mark

 

dry exhausts

I don't have personal experience with catamaran sailing, but have been around quite a few, and I'm working at a catamaran factory right now. I know lots of them vent exhausts out the bridgedeck, the auxiliary gensets at the company I work for are done that way. While sailing downwind I would expect the gases to be blown forward, I think the only time it might be a nuisance is while anchored in a flat calm, and those times when motoring downwind and the wind happens to be on exactly the same speed and heading as your boat. All exhaust systems other than raised stacks or through-the-mizzen type dry exhausts are bound to be a problem at times. I do have that problem on my own boat from time to time, but fortunately not too often. It's not fun when those gases start making their way down the companionway. All you can do then is stop the engine, or change course.

My fondness for keel cooler/dry exhaust systems is in diesel inboard installations. It's clearly a superior way of cooling a water cooled engine. On a scale of 1 to 10 I rate keel cooler 10, heat exchanger 0. For the cenerline mounted engine we've discussed it might be problematic to use a keel cooler, but possible. It would just mean longer hoses to the cooling pipes.

Dry exhausts can be quieter, it's just a matter of how much you want to spend on a muffler. And you won't have those weird regurgitation noises that wet exhausts often make. Mine is just piped through stainless flex pipe through an insulated thru hull in my stern. It's all inboard so there's no problem with corrosion.

The absolute best way to get rid of exhaust gases is to pipe them up a mast, if the rig type allows it. I've seen quite a few ketch rigged motorsailers with that arrangement. Once I saw one on a wooden mizzen, with a copper exhaust pipe fastened onto the forward edge, with a hinged and counterweighted rain cap on top. You couldn't get the noise and gases any farther away than that.

Bob

 

Well people, we have all lived long enough to meet the most important person in each boat company. The PUBLIC RELATIONS SALES MANAGER. HE can make improvements we have not even thought of needing, without redesigning the boat. May I please shake his hand? I am so blessed to buy his product.

 

Thanks for those links, I do enjoy seeing what the innovators are coming up with. That is another appeal of the multihull concept, that it's still so relatively new that there are endless possiblilities for future development. The Vara rudder system looks like a good solution to the problem of steering a shallow draft boat. I think the best thing about that concept is that it could allow a variable depth of rudder. Under different speeds and conditions there is no perfect rudder, most cats have very small rudders which are best for high speeds but ineffective for low speed maneuvering.

Earthling is a real work of art, beautiful. Boat design is more art than science, I feel. A good boat must have a lot of good qualities but one of the most important is that her crew must love her. Greg Young has done a fantastic job. I've seen few boats that "pushed the envelope" and departed from the traditions of boat styling so far, so successfully.

I like your idea of putting a centerboard in a nacelle on the centerline, but I've only seen that done in smaller cats. Quite a few cats use a center nacelle to break up the convergence of the two bow waves and reduce underwing pounding but for some reason I've never seen it done in larger boats. It makes sense to me for several reasons, one big board should be more effective than two smaller ones with less drag and wetted surface. Also it would uncomplicate the hull construction by eliminating the two slots. The only possible problem I can think of is placing it in an area of interaction between the two hulls, could that disrupt the flow past the board and possibly cause oscillation? Your dual assymetric CB design sounds interesting but I'm having trouble visualising it. I'd like to see a sketch if you have one.

Your point about outboard motors tending to be unreliable when out of use for long periods is a good one. I still think that a small diesel auxiliary combined with outboards is the best solution for a versatile propulsion system which could be both fast and fuel efficient for long passages, but the drawback would be higher maintenance.

Bob

 

Don't have a lot of detail in drawings at this moment, as it is an alternate proposal I am suggesting for consideration, and that may have some resistance to overcome as does my unusual sailing rig.

NOTE: There is a requirement for a transverse bulkhead in the boat structure at this CB location to which diagonal 'wires' can be attached to offset the big side-loads of the CB's

 

It was too early in the morning when I read your description of the dual assymetric centerboard but now it's clear what you're proposing, and I agree completely. I've thought the same way, a centerline board fits in very well with the concept of the motorsailing cat because keels on the hulls would not be necessary for lateral resistance, reducing draft even further and wetted surface while under power, when the board would not be needed. The use of surplus helicopter blades is exactly the sort of idea I love. Why reinvent technologies that are already perfected and available off-the-shelf?

I've been working on a design of my own which is still a long way from anything I would be brave enough to start building, but I think a centerline board will be going into it in the next draft. The boat I sailed in for eight years was much easier to design because it was so conservative, it was really just a matter of borrowing proven ideas that had evolved for generations in traditional boats, and combining as many of the things I liked into something of my own.

The potential of the concept of a motorsailing cat is exciting, but I'm finding it challenging because there doesn't seem to be very much experience to go on. I have a long list of features I want in my ideal liveaboard/cruiser and can't see any reason why they can't all be achieved. I don't see any reason why a cruising multihull can't be shallow draft and beachable, capable of much higher speeds than a monohull, equally economical under power, and a good sailing performer too. Add to that the rest of my wish list such as a fully enclosed pilothouse with unobstructed 360 degree visibility, ample covered deck space for those hot climates, an efficient but easily handled rig etc. and combine it all into a package that looks good. I know it's possible with the materials available today but I'm finding it's not easy, I'm on about the sixth draft now and I'm still not satisfied, and starting again from scratch.

But I do feel that it's the right direction to go and that if there is such a thing as a perfect cruising boat, it will likely be a motorsailing cat. The first boat I built was a Brown trimaran and I know how resistant the nautical world can be to new ideas. Now that the cruising multihull has gained such wide acceptance I'm sure that the motorsailing cat is the next phase in the evolution of ocean sailing craft, it just makes so much sense.

Bob

 

Outboard motor reliability

I know that there are many kinds of outboards. As pointed out in earlier postings, inboard deisels have been converted to outboards. I have also seen designs that shelter the outboard in a hull, and when retracted nearly completly protect the motors. Surely not all outboards are unreliable, what about the new generation of outboard deisel motors ? These should not be any different than a regular inboard. Are there any outboards that utilize a keel cooler ?

 

Outboards seem to be reliable enough when they're used often, as in the dinghy motor while you're living anchored. It's when they're stowed away for long periods that things seem to go wrong. Also their lifespan is shorter in salt water unless they're flushed out after every use, which is impractical while cruising.

There are quite a few diesel outboards available these days and have been used to drive catamarans, and housing them in a well works nicely. If you have a plug that fits fairly close around the leg the problem of water surging up into the well is solved. I've met quite a few people cruising with outboard auxiliaries and there are some good arguments for that, as well as some negatives. On the plus side the cost of installation is much less than an inboard and unless you're planning to motor a lot of miles, the higher fuel cost of an outboard might add up to less than the higher initial cost of the inboard. On the negative side they don't last as long, and will need more maintenance such as replacing impeller pumps and shaft seals. On the plus side, those maintenance jobs on an outboard are much easier to do than on an inboard. Working in the engine room in the tropics is pretty miserable. Also to consider is the lighter weight of outboards. Another plus is that they're so easily replaced. I feel it's a matter of preference, both ways work.

However I don't think the outboard idea really suits the motorsailer concept except as a "booster" for high speed for short distances. The fact that they have small high speed propellers and a 90 degree bevel gear at the base means that they will never be as efficient as a well designed inboard. The weakest point of outboards and all legs that use a bevel gear are the seals that keep water out of the base, they wear out fairly quickly and when water gets into the bottom end, it causes some very expensive damage. The cost of replacing all those bearings, seals and gears can be more than the cost of replacing the entire motor. Long hours of motoring would mean high maintenance cost, and time.

Converting an inboard to an outboard sounds like it might be a good solution but I don't know how that's done, I assume you would adapt the engine to a sail drive or possibly use the leg from an outboard. Using a keel cooler for an outboard is a novel idea, it wouldn't be practical on a normal outboard because the cooling system is not closed and the water pump is down in the leg below the water. But it could certainly be done with any water cooled engine which has a closed cooling system.

I'd like to know how you convert inboards to outboards. Can anybody help here?

To optimise the motorsailing cat concept what we really need is a populsion system that will work efficiently over a wide speed range. Diesel/electric seems to have a lot of potential. It's been in use in locomotives and big ships for a long time already and lately the technology has been trickling down to us small boaters, it would be very interesting to hear from people who've been using those systems and find out how happy they are with them.

Bob

 

Self righting multihulls

I have lots of offshore experience in a heavy monohull but planning to do my next cruise in a cat, which hasn't been built yet. I agree with most of what I've heard in this thread but would like to add a little more: the tendency to broach in heavy weather is mainly because of the modern "superior" rigs being used. I used a gaff rig which is the rig meant to go on those traditional types. It got me through lots of heavy weather, including one tropical cyclone.

A "traditional" boat with a high aspect rig on it is an oxymoron. The top hamper of such rigs while reefed down becomes very dangerous in heavy weather. The sailors of the last few centuries all used low aspect rigs not because they weren't as enlightened as modern yachtsmen, it's because they knew better. What kind of rig did Moitessier have? And Chichester? The fact that 99.9% of all modern cruisers are using high aspect rigs only proves to me what Benjamin Franklin once said: there's nothing common about common sense.

Looking at the number of cats cruising the "milk runs" convinces me that they're quite capable of getting you around but I also agree that they're not the right thing for high latitudes. If downwind cruising in the trades is the bunny hill of cruising, the roaring forties is the giant slalom. If you want to ever go places like that (I don't) the probability of being upside down at some point is very high. Moitessier did one thing right, he built Joshua like a submarine.

I think it's a fact that sailing in a multi will be riskier than in my old heavy double ender, but I also think the positive aspects of a multihull make it worth it.

Has anyone seen any design work lately in the area of self righting multihulls? Jim Brown and his devotees did some work on that idea back in the '70s and came up with an idea that works, since most capsises happen pitchpole-wise it also turns out to be the easiest way to right them again. Cats and tris have been built or retrofitted to do that, and it's fairly simple, at least theoretically. Bulkheads and built-in buoyancy are placed so that the boat will float bow down after a capsize, then you have to have a means to pump out the flooded bow compartments. Some experiments were done and the idea seemed to be feasible.

 

Search Function in these forums

Bob, do a 'search' for that term using the search function while you are logged into the general forums heading. If you've not utilized this function before you might find it quite useful for other subject matter as well.

That search will search all of the forums. A search under the individual headings will search just in those subheadings

 

Disadvantage

Monohulls are hydrodynamically disadvantaged.
Unfortunately not everybody is aware of this fact.
This is especially true in my field of paddlesports boats.

 

They are the IN boat in all circular power boat racing. 40 years ago. If you want a REAL boat, buy the mono hull. All others, please feel free to pass them anytime.

 

Hi Ice trader.

I beg to differ. It is in hydrostatics that multihulls have the advantage. The Center of bouyancey shift on a healed multihull can be up to five times as far as that of a monohull with the same beam (the usual number seems to be about three), but the multihull has more wetted area for the same displacement which is usually made up for by a larger press of sail. It is for this reason that we don't see multihull container ships and oil tankers.

I see two major advantages with multis: speed and lack of cumulative rolling.

It is the second advantage that I find most compelling. It has led me to consider heavy multihulls that don't do much better than displacement speed, but don't require you to weigh everything on a gram scale before bringing it aboard.

Indeed, heavy multihulls have been built in the past. Take the catamaran Kamiloa (built in 1936) for example, she wieghed in with a whopping D/L of 310 Rehua Maona, built almost thirty years later came in at 335. And this was when there were plenty of fast, light weight cats running about. The two boats just mentioned were not fast, but they were said to be comfortable and they made long voyages.

I don't envision anything that heavy. I'm thinking more in the area of around 200. I'm thinking a tri can be built heavier than a cat if you don't extpect super displacement speeds and are willing to use low bouyancey floats. (because full bouyancey floats are so dominate in tri design these days, I would call such a craft a double outrigger rather than a trimaran). Such a boat can have a moderate rig of, say, an SD of 15 to 20. (Most modern multi SD's start a 20 and go up from there). The floats would have a combined total voume that would exceed the total loaded displacement by about 25% and thereby keep the boat afloat if the main hull was holed.

Bob

 

Multihulls hydrodynamically advantaged

Hi Bob,
What I meant to say more specifically is that the multihull design effers the possibility to achieve logarithmic reduction in residual resistance through smaller wetted beams.
The more constraints you put on a design (e.g. size, mode of propulsion) the more advantageous the multihull design seems to be in terms of speed.
For example: Since the shape of canoes and kayaks is constrained by the need to have passengers seated inside it is a priori non-optimal as far as wetted surface goes, which makes a good twinhull design not that lossy in terms of wetted surface: http://www.wavewalk.com/COMPARISON.html

Monohull sailboats require to incorporate large keels and heavy bulbs into their design, which affects wetted surface, and consequently speed.

I suppose that big ships would hardly benefit from the multihull design in terms of speed because they are not constrained by small size proportions or by propulsion mode.

Yoav