Full Displacement of semi

Discussion in 'Boat Design' started by tdamico, Jul 31, 2003.

  1. gonzo
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    gonzo Senior Member

    Hey Will, I though you guys down under organized cruises around beer supply. Pease don't disappoint me and tell me it ain't true :)
     
  2. Willallison
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    Willallison Senior Member

    LOL! Well to give you some idea Gonzo, I can well remember a few years back when the water police tied up next to a fairly large group of boats all rafted up. We wondered what was going on until two of them dragged a keg of beer onto the deck and spen the rest of the afternoon teaching us about the foolishness of mixing boating and alcohol!!:D
    Actually, I'm not much of an example as I rarely drink anything stronger than coke!



    At 10 knots we seem to manage about 1.5 nmpg. At 8, our old boat (a 36ft CheoyLee displacement 'trawler' with 2 x 120 hp) did a bit over 2.5 - so consumption was almost 1/2. Also, it's not good for big diesels to run under light load all the time as it can glaze the cylinders.
    As far as the rolling goes, it's more that the motion is different as opposed to more or less. A heavy, round chined displacement hull would probably tend to roll a little further than a hard chined semi-displacement boat, but its motion would probably be more gentle - less snappy. This is what Gonzo and Mike were debating earlier in the thread. In my opinion, it comes down as much to the competency of the designer as it it does to the hullshape.

    Once again, I don't think that any one boat can do all these things well - or rather, I feel you'd be way better off getting to know boating in something that is relatively simple to operate. Then move on to bigger and more complicated boats as the need arises.

    Wish I was coming with you!;)
     
  3. tdamico
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    tdamico Junior Member

    Actually I have some boat handling experience, albiet is a 22' boat that is used on a rather large lake in the Carolinas. But a few thoughts, if we "worked" our way up through several boats as we gained more experience, I would be too old to get much fun out the one I was really after by the time I got there! Also, I want to make sure that I understand what you are saying. I value all opinions. Are you suggesting that trying to learn and competently handle a 40+ foot boat is too much of a challange and that a much smaller boat should be selected first? Also, purely from a handling point of view. Which would be the easier task. Mono hull, full displacement, or multhull? Both power boats, NOT sail.

    Thanks
     
  4. Tom Lathrop
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    Tom Lathrop Junior Member

    tdamico,

    Greetings from the coast.

    In spite of all the great opinions and advice you are getting from everyone here, I am sure that the best one is to charter boats that you think you would like a couple of times to find out for yourself how things work out.

    Looking over boats at a boatshow is fine IF you already know a lot about what you need. Otherwise, you can be seduced by the "next best thing" while overlooking necessities for a good experience.

    I am certainly not in a position to advise you on what will work for you. Inclusion of AC, full and direct communication with the other world and three dogs place you in a different cruising category from me.

    A recent cruise on part of the "great circle route" does bring out a few things though.

    I saw not a single cruising power catamaran on inland waters.

    Regardless of hull design, most of the owners of large power cruisers we met said that they mostly ran at displacement speed due to cost of fuel. Quite a few were well heeled enough to go faster though.

    "Doing the circle" is not a short time cruise. Many take more than one season while leaving their boat on shore for the winter.

    Some owners of round bilge hulls complained about rolling while I never heard this complaint from hard chine owners. Hard chine hulls far outnumbered their round chine cousins.

    In the North Channel, we encountered over 90 boats almost all power) from the Great lakes Cruising Club and about 30 boats from the Trailer Sailors Association. There was quite a contrast in style and substance but both seemed to be enjoying themselves. There is a niche for everyone.
     
  5. tdamico
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    tdamico Junior Member

    Thanks for your opinions. They are welcome. And you are right. My feeling was to learn all I could via books and the net. Make a list of what we want and don't want, keeping in mind that simplfy is the operative word. Visit boat shows to see as many different boats as we can. Crawl around them. Check the lists we have as to what they offer etc. Then step back, eliminate those that we just didn't like or that had too many "don't wants". Then charter for at least 1-2 weeks the final boats we come up with (hope to narrow it to a max of 4, hopefully 2-3) Then make a decsion, get a slot, and let the building begin. While the boat is being built, we continue to sell off household goods, and all our property. Time line for this, 1-2 years. Next step is to find a live aboard slip and put the boat there and move in. I will continue to work, thus the need for high speed internet access, and we will use the boat on weekends and 2-3 months over the course of the first year. After year one of boat ownership, I quit, and we take a full year to to the Great Circle Loop. Visit America and its inland waterways. Learn our boat even more. At the end (5 years from now) we will have two or more solid years experience under our belts, hopefully are good boat handlers. Our next goal would be to cruise the East coast extensivly and the islands. At least 2 years or more. Now we would have 4-5 years under our belts and the next step is through the canal to the West coast and Alaska. Probably spend at least 3-5 years there and now we will have 7-10 years experience. Then Transocean to ???

    Financially we can make it happen if we keep the boat to around 450K-650K and keep living expenses for two under 30-40K per year. We may have to buy used, but we will see.
     
  6. Portager
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    Portager Senior Member

    Tom;

    Sounds like a good plan, although you may find it difficult to charter the boats that you settle on. I would start the charter process earlier and consider all on boat experience useful even if it isn’t the boat you think you might buy.

    As for choosing the boat for the application, Will is right to a point but I can see your point also. In addition every time you go through a buy/sell cycle you pay agents fees and spend time selling the current boat moving and prepating the new boat for cruising. If you opt for an adequate compromise you can spend more time cruising. OTOH I would concentrate on your 3 to 5 year objectives and not let the far distant future plans compromise the near future plans.

    I am familiar with TracNet and I think it is a good system, however if you don’t have access to a landline or cell phone coverage, then you need to use a satellite phone which gets expensive. Cell Phone coverage on the coast is pretty good, but on the inland rivers it is only in cities and town and near freeway bridges.

    In defense of semi-displacement hulls, it is true that most are configured to operate at 2 to 3 times the speed/length ratio (S/L) which requires a great deal of power and sucks fuel very fast. To make matters worse, these large engines when combined with a fixed pitch propeller, are under loaded at displacement speed to the point of being detrimental to the engine. However, it is possible to configure a semi-displacement boat to operate at about 1.6 to 1.7 times S/L without requiring too large an engine or driving fuel consumption too high. In addition, using a controllable pitch propeller allows the engine to operate on its ideal power/speed curve regardless of boat speed. Portager will use a semi-displacement hull and operate at a maximum of 1.75*S/L or 12 knots with 160 HP. Fuel consumption at 12 knots is predicted to be 0.7 gal/NMi at 3,000 RPM. At 8 knots, predicted fuel consumption is 0.27 gal/NMi requiring 40 HP the engine will run at ~1,600 RPM. Portager will be efficient and relatively quiet at displacement speed, but when I can afford the fuel and conditions allow I can increase my speed and increase my daylight passage distance by 50%.

    Regards;
    Mike Schooley
     
  7. Portager
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    Portager Senior Member

    Tom;

    So that you wouldn’t need to take my word for it and to illustrate the value of the Trawler World List and Trawler World Great Loop list, I did a Google search using [ Catamaran site:samurai.com ]. The “site:” restricts the search to the selected web site, i.e. samurai.com. This search produced 304 returns (many of them useless), but by adding additional words with a “+” sign in front, or strings of words inside “” you can further filter the returns.

    One of the better posts that popped up was from Skipper Bob, an experiences Great Loop cruiser and the Author of several self published books on the subject, http://lists.samurai.com/pipermail/trawler-world-great-loop/2001-October/000130.html . Skipper Bob replied to an query regarding Catamarans on the Great Loop as follows, “Cats have the advantage of shallow draft, but the disadvantage of being quite wide. Many marinas cannot handle a 15-18' wide cat in a slip. Unless they have a T pier or alongside dock there may be no room at the inn. On the bright side, you do not have to tie up to a marina most of the time unless you want to. The wide cat also presents a problem at some marinas if you have to be hauled to be worked on. Many marinas cannot lift the wider cats. From a realistic standpoint there are no problems taking a cat around the Great Circle Route when it comes to steering or transiting canals, rivers, lakes, etc., except for height. Most catamaran masts are too high and you would have to step the mast in NY on the Hudson River and leave it down until well past Chicago on the Inland Waters. A mast on a hinge that you could manage yourself might be ideal since you could step and unstep it at will and actually sail on the Great Lakes.”

    I think this addresses the slip width issue. I don’t think it would be an insurmountable problem, just a potential inconvenience.

    Regards;
    Mike Schooley
     
  8. tdamico
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    tdamico Junior Member

    Michael:

    I certainly appreciate the effort. This would, potentially, only leave one issue. That bridge you mentined in Chicago that is fixed and anything 19' or over can't get underneath. Some, not all, of the powercats I am looking at are higher than 19'. I am still checking into that.

    On another note, I asked a question earlier about just running a semi-displacement boat at a slower speed to conserve gas. The answer was that this could potentially be damaging to the engine. But cats are rated at very high speeds. If I ran a cat as cruising speed, say 10 knots, would I not have the same issue?
     
  9. Willallison
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    Willallison Senior Member

    As usual, I'd have to agree with all that the others have said.

    Mike, so with Portager, @ 8 knots you're looking at 3.7 nmpg and at 12 you get 1.4 nmpg. The latter figure is not too dissimilar to that which we get from the Offshore 48 @ 10-11 knots. This, I think is where the cat comes into its own. You could reasonably expect 2 - 3 nmpg running at nearly twice this speed.

    The July / August edition of Passagemaker magazine has two things that me be of interest to you, Tdamico:

    1. Is a story about the PDQ mv34. A 34ft cruising powercat that could be of interest to you.

    2. A lift out order form for an article presented at last year's 'trawlerport' on "how to choose the right boat for you".

    As far as the boat size thing goes, if you've had some boating experience then you have a good head start, but it seems that every 10 foot increase in length also comes with a quantum leap in complexity. All of these systems - watermakers, generators, A/C, radar, toilets, etc, etc, etc - require maintenance. And whilst I would expect that you could find service agaents for most of these things en-route, one of the great pleasures of boating (for me at least - is to be (as far as is possible) self-sufficient and self-reliant. So the size choice also becomes a complexity choice - though less so if go the custom build route....

    As far as ease of handing of the various types goes, they all have their pros and cons. For instance, as cats tend to have low draft they can be subject to windage problems. A heavy, deep drafted trawler will be less subject to windage, but once on the move will take a lot more stopping.

    I may have missed it in one of your posts, but how many permanent and part time crew are you planning on having aboard?
     
  10. tdamico
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    tdamico Junior Member

    There will be two permanent crew on board, myself and my wife. I am presently 52 and she is 46. Both in great shape and very mechanically inclined. As I said, we hope to start to build next spring, do a part time thing at a marina for a year, then full time at a marina for a year, then the great circle route for a year, and then??? We have divided up the classes. I will be taking navagation classes, and a class in maintaining diesel engines. She will be taking advanced classes in boat handling. Then we will cross train each other. I agree that part of the allure is the self sufficiency that comes with maintaining your own enviornment. That is certainly or goal. Are we crazy? Don't care, we're doing it anyway. I actually read that issue of Passagemaker. When we fit our lifestyle into a boat, it keeps coming up, full displacement trawler, but the additional space that we can get in a cat, is alluring. If we go with a traditional trawler, we are looking at 47'. If we go with a cat, we can get the same or more room at 43'. Can't wait for the boatshows this year!! Please keep all the ideas coming.
     
  11. Willallison
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    Willallison Senior Member

    For live-aboard, that's definitely the way I'd go. And 50ft is about as big as I'd choose too. I imagine that with only the two of you aboard the difficulty of navigating such a big boat through the many locks that you'll encounter will prove to be more of a hinderance than the deeper draft and slower speed that you'll have to contend with.
     
  12. Portager
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    Portager Senior Member

    It depends on how far your cruising speed is from your maximum speed, or actually the power ratio between maximum speed and cruising speed. I will have the same problem. My power level at maximum speed is 160 HP at 12 Knots and cruising power is 40 HP at 8 Knots.

    The problem is the power demand curve for a fixed propeller is steeper than the engines optimum power output curve. This means that a fixed propeller can only be optimized for one engine speed. Below the optimum speed the engine in under-loaded and above the optimum the engine is overloaded. Since it is much worse for the engine to be overloaded than under-loaded, the optimum point is always selected for the maximum engine speed. Since the propeller power demand and engine ideal power output curve are at different slopes, the further you operate from the maximum speed, the more under-loaded the engine is.

    The classical approach to this problem is to select your maximum speed to be only slightly higher than your typical cruising speed. The problem with this is you must give up the high speed potential.

    Another approach is to break-up periods at low speed with short periods operating near maximum speed to burn the glazing off the cylinder walls. A typical approach is 1 hour at high speed for each 4 hours at low speed. The problem with this is if you are limited by speed limits or conditions such as swells for long periods of time, you may be unable to burn the glaze off the cylinder walls often enough.

    If you plan a long period at low speed, you could install larger propellers that are optimized for low speed, but this is limited by propeller clearance requirements.

    Another approach, and the one that I prefer, is to use controllable pitch propellers. These propellers have rotating blades so you can change their pitch, and therefore the thrust and power demand versus speed. This allows the engine to run on its ideal power output curve regardless of boat speed (provided the propeller has adequate range of travel). It does not hurt a diesel engine to run at slow speed provided it is adequately loaded. Pyrometers are the preferred sensor to adjust the pitch of the propeller blades to achieve an exhaust temperature near the ideal 850 degrees F.

    The down side of controllable pitch propellers (CPP) is they are more expensive and they require a special shaft with a pitch control shaft in the center, however with a CPP you don't need a transmission which partially off sets the cost difference. Of course you would need two of them.

    Have you asked the Catamaran builders about their engine out handling? Some of them are difficult to steer a straight course with one engine out.

    Regards;
    Mike Schooley
     
  13. Willallison
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    Willallison Senior Member

    Have you determined the cost of variable pitched props Mike? I would have imagined that the efficiency benefits were way outweighed by the cost. Better to either put up with having to blast about for an hour every now and again, or whack an outboard on the back with a low pitch prop. But never having had the need, I really have no idea about the cost of a 'recreational' variable pitch system.....:?:
     
  14. Portager
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    Portager Senior Member

    That is a common misconception Will. People like to make comparisons to commercial grade CPP which exaggerates the cost difference. They also like to compare the cost of a retrofit of an existing drive train which makes the CPP less attractive because there is no transmission savings.

    For new construction with comparable quality and using a single direction gearbox, the cost difference is less than 5% and break even would be about 5 years at my expected usage level and today’s interest rates, however if you compare low quality fixed pitch propellers to CPP then the initial cost savings of the FPP is closer to 40% but breakeven is sooner due to maintenance cost.

    Regards;
    Mike Schooley
     

  15. Willallison
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    Willallison Senior Member

    That is surprising! What are we talking in $ terms?
    Leads one to wonder why more boats aren't equipped this way....
     
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