Long - Skinny Power Boats

Discussion in 'Boat Design' started by SAQuestor, Sep 24, 2004.

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  1. fcfc

    fcfc Guest

    Just some compute...

    65 LWL, 35000 lbs, 12 kts. That gives on my sheet nearly 100 hp.

    3000 nautic at 12 kts = 250 hours.
    100 hp for 250 hours = 1500 gals. No reserve and always good weather.

    1500 gals = about 9000 lbs.

    So the boat, empty @ 25 000 lbs, will start cruise @ 40 000 lbs, and arrive @ 30 000 lbs (some fuel, water, crew and gear still aboard).

    That leave a 15 000 lbs variation for an empty weigh of 25 000 lbs.

    Assuming a waterplane coeff of .7, waterplane is 65*8*.7 = 364 sqft.
    That means 8 inches flotation variation.

    LWL 65, Disp 35000. Cp assumed of .63 That means a immerged cross section (dont know the us:english term for that) of 13.3 sq ft.
    BWL = 8 , S = 13.3. assume section coef of .7 that means 2 ft 4 in canoe draft.

    Now, you want to reach some offshore stability criteria with a boat about 8 ft BWL, and hull depth between 2 ft empty 2ft 8 in fully loaded. How do you plan it ?
     
  2. Portager
    Joined: May 2002
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    Location: Southern California

    Portager Senior Member

    I agree with Tad, there are really two threads here. It might be less confusing for Fast Fred to start a new thread for a fast economical Passagemaker.

    The Dashew FPB could be scaled down to a 12’ beam (66%) and 65’ length (78%) but I’m not sure they have the right hull form for this application. They are operating in the semi-displacement speed range (S/SQRT(L)=1.55) so I think they would benefit from a slightly broader stern, Fast Fred’s Fast Passagemaker (FP) will operate at 1.512 which is also best served by a semi-displacement hull IMHO.

    I ran some numbers for the FP 65 assuming a light ship weight of 25 Klbs. At 12 knots she requires 71.22 HP and consumes 3.561 gph providing 3.37 NMi/gal. To make a passage of 2,200 NMi with a 12% fuel reserve (this is the longest passage required to circumnavigate) I figure she needs 900 gallons of fuel.

    Adding the weight 900 gallons of diesel at 7.11 lbs/gal and 1,600 lbs for crew, gear and supplies. I get an optomistic fully loaded weight of 32,000 lbs. At her fully loaded weight she requires 94 HP and consumes 4.7 gph providing 2.55 NMi/gal.

    Calculating fuel consumption and resultant weight on an hourly basis she achieves a range of 2,208 NMi in 184 hours (7.67 days) at 12 knots having consumed 784.44 gallons or 87.16% of her original 900 gallons.

    I think this is a significant accomplishment. Portager has to slow to 7 knots to achieve this range so she requires 13 days and 528 gallons to make the same voyage. So am I going to stretch Portager to 65’? Probably not. First, at 65’ the FP 65 will be a pain to transport although the fuel efficiency and higher speed potential are hard to pass-up. This is a compromise that requires additional thought. At 32 Klb fully loaded she can not carry enough weight for the accommodations that the Admirable and I want. Fast Fred has a good point that shorter passages allow you to reduce supplies, but what about emergency supplies encase of a breakdown? Meals ready to eat? Also if we are going to live aboard Portager after retirement, the Admirable wants a dishwasher, washer & dryer, air conditioning, … and I want an air compressor for my SCUBA tanks, …

    Regards;
    Mike Schooley
     
  3. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Having lived aboard for over 22 years , I'm delighted to use a sail bag , rather than washer , drier et all.
    The power requirements are far far beyond the amount of gen set noise that would be needed on the hook.And to go voyaging quickly & cheaply , weight has to be a dirty word.

    Air cond (rooftop RV 15,000btu ,14A @120V ,90lbs) off a DC genset with a trace 2440 Inverter would work fine , as would a microwave .

    There is not a problem with sinking 8 or 10- inches into the water at full load , as the boat would STILL be quite light interms of DL ratio..

    Only disadvantage would be the USCG stops boats (with extra wide waterline) as part of their Zero Tolerance policy , so the simple interior might get smashed to bits (at owners expense) on occasion.A very open plan would solve this.

    When doing a work sheet of costs , after the hull ,opening ports or Mfd exterior doors are the next highest item.

    Simple solution would be to select fine (for US$ ) inexpensive commercial vessel Canadian opening ports and cut the holes to fit.
    BUT install lexan over the holes as fixed ports untill the $1000 for a fixture or two can be justified.

    Many systems can be done for very little money, baseboard heat is cheap to install and operate from main eng or genset underway.

    As in most boats the fridge freezer would be biggest long term power draw, fairly EZ with a large volume hull to find 6in for insulation arround the boxes and the fine Danfoss DC units can run from 2 solar panels fairly well. Not cheap stuff , but Salsa dip , chips and cold BEER is a Religion to some.

    FAST FRED
     
  4. Portager
    Joined: May 2002
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    Location: Southern California

    Portager Senior Member

    I think the issue that fcfc is pointing out is that when you are riding 8" high it will be precariously unstable. This is why most long range passagemakers require ballast. i.e. to maintain stability as they expend fuel.

    I think the problem can be solved by using saltwater ballast. Since you have plenty of internal volume, what if you provide salt water ballast tanks below the waterline with a weight capacity equal to your fuel capacity. As you consume fuel you place an equivalent mass of salt water in the ballast tanks, thus maintaining a constant waterline and stability. This allows you to maintain stability as you consume fuel without increasing your fully loaded weight as fixed ballast would require. In favorable conditions you may be able to off load some of the salt water ballast and reduce power requirements, but, since the tanks are below the waterline, you would always have the ability to take on weight as conditions require.

    Regards;
    Mike Schooley
     
  5. fcfc

    fcfc Guest

    I fear that you need to assert stability boat minimum load and maximum load. On minimum load, tanks are empty and ballast are filled per operating manual. On maximul load, from what I have understood, all tanks that are fillable are filled up, including ballast tanks.

    Another point, if you have 6000 lbs of fuel, 6000 lbs of water from a 25000 lbs boat, you already have filled more than half of the space under the waterline. (fuel density < 1). And you need to add also the engine, batteries, fresh water tank, grey tank, black tank etc ...

    Also has FF checked price of prop and shaft ? He needs 100 hp continuous, so he will probably in the 150 -200 hp range. He wants high efficiency, so he will rather have a big prop (in the 40" ) turning slowly (< 500 rpm). That means at least 20 ft shaft, 2.5 - 3 in dia if he wants an engine about weigth centered

    Going 12 kts has a high price.
    Slowing to 10 kts, with same weight, same LWL only needs half the power. You will cross in 12 days instead of 10, but you wil use only 60% of the fuel.

    BTW all theses figures need the same power for 12 kts
    LWL disp
    40 16 000
    45 18 500
    52 22 500
    65 30 000
    90 55 000
    120 98 000

    And my raw formulae for power uses disp and LWL as input, but not the beam.
     
  6. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    Most small boats do nor have balast tanks as ships do,

    The fuel on many passagemakers is ONLY filled for offshore cruising , so with costal work much less weight is carried.
    Todays practice does NOT put much fuel in bilge tanks , as the loss of stability empty/coastal cruising would be unsetteling.

    Instead the fuel is usually carried in twin or more sets of P&S tanks which extend from low in the boat almost up to deck level.Perhaps 6 or 8 ft tall(Located near the CG) but narrow enough that free surface area will not effect the vessels stability.
    If more than one set is required , the unused sets are left full till the first tanks are empty.Full the ultimate stability may be reduced slightly , but the weight makes for a slower roll.

    "He wants high efficiency, so he will rather have a big prop (in the 40" ) turning slowly "

    Although efficency is the name of the game , some sacrifices must be made for a normal draft , so a 30 inch prop would be more desirable than 40 inches.

    A cruising boat MUST be able to "take the ground" as she certainly will on occasion.

    With very lightweight diesel , my engine location preference would be in the first 15ft of lwl.An absolutly horrible place for people , but fine for the iron.

    The shaft would be simple truck drive shafting with intermedate bearings. This can easily run 40 ft a very low cost , little noise or vibration. Trucks regularly run over 500HP thru the same shafts after tranny reduction so they would be very underloaded.

    Of course a Thrust bearing would be required on the actual prop shaft , but the Variable pitch props (Hundstat)or simple surface drives (others have sugested) provide for this .

    "And my raw formulae for power uses disp and LWL as input, but not the beam."

    Using the AYRS drag tables (referenced)my numbers (which are for weight and do include beam) I figgure 2 1/2 gal to 3 gal per hour at 12 to 14K.

    The reason for a long skinney boat (with all the needed compromises) is the low fuel requitements ,needed at what is still simple displacement speeds.

    Most formulas are for 3 or 4-1 LB boats , when you get up to 8-1 or 10-1 , the wavemaking is very reduced.

    FAST FRED
     
  7. fcfc

    fcfc Guest

    It is know for a long time that the foreplace is not a comfortable one, so why very few builders put the engine there ?

    And 2.5 / 3 gph means you will get about 50 to 60 hp. Going 12 - 14 kts in a boat with four people, their gears for 10 days, and 800 gallons of fuel with that power seems IMHO a bit optimistic.
     
  8. 8knots
    Joined: Feb 2002
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    Location: Wasilla Alaska

    8knots A little on the slow side

    My guess would me the long shaft lengths. This to me means a fairly complex set of pillow blocks and modifications to your transverse framing to allow removal/service of a multi piece shaft. Your cabin sole will need to be removable "as it should be" I imagine you will need a thrust bearing to carry the load (prop thrust) on the first length the rest can just "float" Ahhh just dawned on me........It would prove difficult to maintain watertight integrity between bulkheads. fuel vapor and exhaust runs will need to be considered also. I dont think it is a detriment...just costs more for a production builder thats why we dont see it more often. I think a added plus of a long shaft is the reduced shaft angle. I dont know how much...I think I read in Gerr's Propeller handbook that 0-15 degs not much efficiency is lost but logic tells me closer to 0 is better.
    Just some thoughts....8Knots
     
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  9. Portager
    Joined: May 2002
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    Location: Southern California

    Portager Senior Member

    Putting the engine in the forpeak has been tried and for MOST applications it has been discarded. The problem is a combination of the long shaft length and the detremental affect on the pitch moment of inertia.

    Long shafts, especially shafts with joints have a tendency to become unstable in torsion. Think of the shaft as a rotary spring and the drag on the blades as a variable load which has a feedback loop to the engine. The weaker the spring force or the lower the tosional stiffness the lower the natural frequency of the dynamic system. Since an internal combustion engine provides varying torsional loads, when the natural frequency of the drive shaft is equal to the forcing frequency, the system becomes unstable and the results are not pretty. Thus your maximum engine speed determines the stuffness requirements for the drive shaft. If you add joints to the drive shaft then this effectively lowers the spring rate and reduces the natural frequency. As a mechanical engineer I believe the truck drive shaft with multiple U joints will only work at very low shaft speeds (i.e. hundreds of RPM as opposed to thousands of RPM).

    The problem with putting the main engine in the forpeak has to do with the pitch moment of inertia. The moment of inertia about any axis is the sum of each unit of mass times its distance from the center of mass (AKA center of gravity) along the axis squared divided by the total mass. Putting large masses like the engine a long distance from the boats center of mass, i.e. the forpeak, increases the pitch moment of inertia and slows the vessels response to waves. Every vessel in heavy seas will need to slow down for the conditions, however one with a higher pitch moment of inertia will need to slow down more than one with a lower pitch moment of inertia. You could argue that since you are going to use a very small engine the impact on the moment of inertia would be minimal however since it is also a very long vessel and the distance to the center of mass is squared (30^2=900) this factor will overwhelm the reduction in engine mass. If your boat were only intended to operate in sheltered conditions or coastal cruising you could get away with putting the engine in the bow, but for a passage maker it is a bad idea IMHO.

    Regards;
    Mike Schooley
     
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  10. Ross Adams,

    Ross Adams, Guest

    just-a-guy

    wow great info thanks
     
  11. announcer
    Joined: Feb 2005
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    announcer Junior Member

    William Garden My favorite boat designer next to PC Bolger,

    I don’t know why I involve myself in these threads, I was just searching for more options on cylinder molding for long hulls for better passage making with lower fuel costs. But hear is my point. William Garden when he designed Tlingit designed the boat around antique gasoline engine that was not very efficient. Later in life he told me that if a boat such a Tlingit were built to around one hundred feet with a twin vee drives setup (better for maneuvering in small dock areas) with fuel efficient engines the boat would be very fast with no wake and extremely miserly with fuel. Four people could go eight thousand miles on less than six hundred gallons of fuel even considering in bad weather in very comfortable conditions.

    I also had a chance to speak with one of the grandsons of the original owner of Tlingit ( pronounced Klincket for the Eskimo tribe who built the kayaks that Garden used as a base design for Tlingit) and he said that he and his brother enjoyed their grandfathers boat very much and were very sad after his death that the boat was sold and moved to the east coast.

    Had the design process evolved Garden would have shown how a long narrow boat cold have quikly covered long distances with les fuel and a better ride than the pigs that pose as yachts today in modern boating magazines. I have been trying to contact the man who now hs control over all of William Garden’s designs at a well known east coast boating institute. Making contact with this man unless your name is Vanderbilt or Rockefeller is impossible like most east coat institutes. Luckily Garden laid out in his books that the Tlingit design was just formed upon the mount of 2X4 lumber and plywood sheets his friend he was designing the boat for what had available. Although the boat was built at a boatyard that is now a waterfront park. The once great yacht yards are now just condo’s and bicycle paths and finding space to rent to build a boat would cost more than the material used to make the boat.

    Had this been in Portsmouth or Barr Harbor it would have been declared a national treasure. But we must realize that the east coast is the only place where real ships were built. Just ask any snotty so and so that runs one of their maritime museums. Gloucester, Swampscott, New Bedford and Bristol to them are the only places that good boats and their plans should remain. Even if the author of those plans was from the pacific northwest. Sorry about the rant but I cannot see the point in Mr. Garden’s plans residing in an east coast institute that makes common folk like myself jump through fiery hoops to buy a set of his plans.

    One good thing is that Bill Garden left us the formula by telling us that Tlingit was built from 2”X4”’s and plywood. Even if a boat similar we built cold molded and then Laminated all the large dead wood its back would be strong and the waves be Damned. She would carry enough in tankage and room for four people to live in a comfortable manner and be able to cross an ocean. Even George Buehler has a very light version of a boat like this called the Pilgrim but it is only a few feet longer than forty. His version is for a young man filled with wander lust and very little cash. George calls him a Hamburger Flipper but the money saved pushing a light hull over a long distance could make a small grub steak go a long way.

    How I wish I was twenty one again and know what I know now. But a longboat I would have with a narrow beam and a few sails along with my motor to make my fuel last even longer.
     
  12. Yacht designers do not get upset or respond! All ships that can not say "screw it" to the marine insurance companies, have have pretty much picked on hulls of super tankers, air craft carriers, converted WW I & WW II ocean liners. Commerical owners do not believe, short, wide and tall, is the most value or safety for their non-living cargos. The fact is all wealthy yachtsmen have their boats CARRIED across the ocean on large freighters to Cannes, then step aboard .
     
  13. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    Announcer, which "well known East Coast institute are you referring to?
     
    Last edited: Feb 2, 2005
  14. announcer
    Joined: Feb 2005
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    announcer Junior Member

    Mystic Seaport the iron archives of william Garden plans.

    Dear Sail Design,

    Unfortunately I must inform you that the well known East Coast institute that keeps Bill Garden's special plans under lock and key is Mystic Seaport. The Man who is in charge of said plans also works for Wooden Boat Magazine and his name is Maynard Bray and he is the gate keeper of the plans. The Man is very illusive and has proved very hard to contact since I have been unable so far to make contact. All I want is a set of plans to Tlingit or a boat of similar design. I have a large lake I live on now called upper Klamath Lake.

    I would like a long narrow boat to test the abilities of the design since the lake has frequent high winds and is over thirty nine miles long witch corresponds to the town of Klamath falls at the lower end and at the upper end my home at Modoc Point. During times of the year the narrow road along the lake has frequent land slides so the main route into town is blocked and an alternative form of transportation with low cost fuel consumption would be a pleasant alternative to hours of waiting for the road to open.

    My column in a few boatbuilding magazines has gained immensely from my home being so close to such a perfect water supply here in Oregon. A Tlingit design or the one Garden talked about with twin vee drives with up to date power plants and one hundred feet long would be super testing ground until it could be moved too the Ocean and run to Mexico a few times.

    So if anyone has any influence with Mystic Seaport or Maynard Bray of Wooden Boat Magazine who works also at Mystic Seaport please let him know that a writer, columnist and disabled boat builder would like the chance to build one of Gardens best designs. I could wing it and either make a boat only its mother could love and it’s an orphan or really make an inspired boat using all the many drawings I have and the building experience I have to make a very rough duplicate of an inspired design.

    I don’t really have that much faith in myself to build a Garden Design without plans. If Bill were not so old and in need of his privacy in the golden years of his life I would travel to his Island in the San Juan’s and try to get his help. I respect the man and his genius to much to intrude upon him. Lets see what help I can get by this forum in contacting Maynard Bray. I have been on the phone to Mystic Seaport more times than my wife’s telephone bill hiding from her can be worth. That is why I have such a negative attitude about these institutions that control the wealth of designs and always ask for more money but don’t provide the stored knowledge and resources that boat lovers want and need. That is where the disparity comes in.

    In my opinion west coast museums and historic societies have a much more useful and helpful way of dealing with the public. I was born on the East Coast and moved to Oregon where in my town everyone says hello or thank you for opening door. Try to find that in New York or Boston. Well I was harsh and I take back my complaints about Mystic Seaport and Maynard Bray. They probably have a work load that would bury me in seconds. We really do need more access to Bill Garden plans and if there is something I could do to help please use me.

    John Cupp
     

  15. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    A really light boat will have a very variable displacement , but this should NOT be a problem if the tanks are built for the boat.

    Instead of Diesel tanks filling the bilge (where empty./full will vary the stability ) its simple to build the tanks vertically , somewhere near the CG.
    AS the tank empties the stability Increases a bit , so light boat is not a hassle.

    Small diameter and 6 or 7 ft deep will have little free surface area , and 4 tanks could be managed so there either chock full or empty , with only a gravity feed day tank to slosh a bit.

    Remember Dorys can carry & handle loads of huge proportions with little loss in performance.

    Todays engines are so small and light , the proposed Deutz oil cooled engine is 170hp at about 1100lbs, not much for the bulbous bow to hold up.

    Drive shafts are Off The Shelf from any box truck and are cheap and easy to maintain. at the Price of needing a thrust bearing at the stern, no biggie.

    With low enough weight and a tranny that can load the engine well at lower speeds , I think 4 NM per gallon or even 5mpg should be doable.

    The only variable seems to be how fast one can go at that speed.

    AS a long time sailor , we have no need for the complexities of washer driers , bread ovens , and any of the "needs" of the modern water campers.

    Think of a simple interior that would have worked well in 1930 , (but with central heat) and you get the idea , painted ply , wood trim.

    Comments?

    FAST FRED
     
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