boat designing help!! PLEASE!

Discussion in 'Boat Design' started by ber1023, May 29, 2008.

  1. gschuld
    Joined: Nov 2007
    Posts: 11
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Toms River, NJ

    gschuld Junior Member

    I did a simple yahoo search using... seagull racing outboard ...and came to a simple website from a guy who races seagull outboards in New Zealand. Apparently, these guys race vintage seagull powered boats for several races a year. Some are quite serious about it! It seems that the New Zealand America's Cup team has designed and entered a 24ft carbon fiber boat for this year. These guys have millions of dollars worth of design software at their disposal and their designers have been somewhat "underworked" as the AC debacle has them out of the picture for a while. I would certainly want to see what they have come up with:D . On the website there is also a good picture of a boat that very much describes what a NA here has already suggested. Basically a rowing shell with small outriggers and a mid-mounted seagull outboard. It seems that some of the races are in more protected waters than others, so keep that in mind. The longest race, about twice as long as the Bermuda round de island race down their longest river, is said to get a bit rough where the river gets wider near the end of the race.

    Food for thought...
     
  2. Athlon64M
    Joined: Jun 2008
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United States

    Athlon64M Naval Architect Student

    One thing that wasn't mentioned that all Naval Architects know is that for displacement boats, your maximum hull speed is 1.34√LWL

    So if your Load Water Line is 20 feet long, 1.34√20 = 5.99 knots

    1.34√24 = 6.56 knots

    If you power your boat faster than hull speed, you just cause the bow to trim up a lot, essentially "stalling" it & not to mention for a very tiny increase in speed above hull speed, you will need tons of horsepower.
     
  3. rwatson
    Joined: Aug 2007
    Posts: 5,875
    Likes: 311, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    That is a well known formulae, but there must be some other factors involved as well.
    I remember hearing that on calm inland lakes in africa their long, thin dugout canoes just over 20ft long were getting nearly 20 mph on small outboards.
    I will have to do more research, and try to verify my aging memory.
    In the meantime, attached is a picture of a streamlined long and skinny boat.
    from
    http://quincylooperracing.us/gpage9.html

    This did 32 mph on a 7.5hp, but it looks like it isnt totally displacement design.

    Also, check out http://www.jwboatdesigns.co.nz/plans/clarenceriverdory/index.htm
    for a 20ft design that will do 20knots with a 20hp engine. This *is* pure displacement from the plans.
     

    Attached Files:

    Last edited: Jul 26, 2008
  4. Athlon64M
    Joined: Jun 2008
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United States

    Athlon64M Naval Architect Student

    I looked at the design of the "CLARENCE RIVER DORY" and did some minor calculations.

    LOA Long Version: 6.04 m - 20 ft 10 in
    Beam: 1.27 m - 4 ft 2 in

    My calculations indicated the boat would have a Maximum Recommended Horsepower of 75 HP.

    So Yes, this boat is well capable of achieving 20 kts, but only on calm protected waters such as a lake.

    If you're lucky, you might even hit over 25 kts.

    For the record, a dory is very much a planing hull at 20 kts.
     
  5. rwatson
    Joined: Aug 2007
    Posts: 5,875
    Likes: 311, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Well, *their* calculations are ..

    POWER - 25 hp Outboard (max)
    SPEED - 20 kts

    check out the website.

    oh - and the original poster said
    "the fastest hulls are displacement hulls and some reach almost 20mph with a olld 5hp engine with 2 adults in the boat"

    I am having trouble with the relevance of this formulae to the discission
     
  6. Athlon64M
    Joined: Jun 2008
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United States

    Athlon64M Naval Architect Student

    Naval Architecture has a lot of formulas to give you a general idea of how things will perform before it's actually tested on the water.

    I was just thinking out loud in my previous two posts...

    But lets get back on topic...

    A Planing hull with a V bottom would better suit racing in unprotected waters.
     
  7. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    One thing that wasn't mentioned that all Naval Architects know is that for displacement boats, your maximum hull speed is 1.34√LWL

    So if your Load Water Line is 20 feet long, 1.34√20 = 5.99 knots

    1.34√24 = 6.56 knots

    This formula was for fat heavy boats, with an LB of 3-1 or 4-1.

    Once the LB gets up to 6-1 or better and the boat is lightweight , the formula is useless.

    Long time ago the British AYRS published an advanced formula that did take beam into account ,,but with a DL ratio of 30 or so , even that formula wont work.

    Since the current fleet IS running faster than any formula would predict , the question is how to improve the breed.

    Less weight , probably only with Unobtanium big buck$ construction.

    So less drag as folks are suggesting seems the way.


    "A Planing hull with a V bottom would better suit racing in unprotected waters"

    I dont think so as the rough portion of the race may prove too hard to stay on top of , and the structure needed to stay airborn and leap from wave to wave , and stand 6G + impacts , would surely be far heavier than being IN the water with almost no G load requirements.

    Not sure $6000 Stidd seats would work for these racers to keep their spines sorta intact.

    FF

    FF
     
  8. powerabout
    Joined: Nov 2007
    Posts: 2,925
    Likes: 66, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    Naval architects design boats that come in second coz...
    thay analyse what works and then make a therory of why it works
     
  9. Jimboat
    Joined: Feb 2002
    Posts: 254
    Likes: 17, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    Planing hulls

    If you are getting into planing hulls, I may be able to help. Call or PM me.
     
  10. kengrome
    Joined: Jul 2006
    Posts: 718
    Likes: 25, Points: 0, Legacy Rep: 305
    Location: Gulf Coast USA

    kengrome Senior Member

    Not much has changed here since I last posted ...

    I still believe the fastest hull will be very long (the full length of 4 sheets of plywood or 32 feet) -- with a deep vee bottom extending up and completely out of the water at the sides when the boat is fully loaded -- to insure that the chines are NOT in the water.

    The boat will run at very fast displacement speeds. The addition of wave piercing / skiing outriggers located well above the waterline will insure that the boat doesn't tip sideways too far when unbalanced. These outriggers will create no friction whatsoever until they are actually needed to stabilize the boat since they are already 4 inches above the waterline, and they can be moved up even more if necessary..

    At 7 inches draft this boat has 560 pound displacement -- more than enough for your races based on the weights you've posted previously -- yet the boat can still carry more weight and ride 2 inches deeper before its chines touch the waterline. Keeping the chines out of the water is likely the most important goal for optimal speed in this boat:
     

    Attached Files:

  11. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    As Fred said, this formula is useless in this discussion. The so called "hull speed" formula is not a hull speed formula at all. It is really a wave speed formula that is only useful for boats that create significant waves and get stuck in them. Long and slim hulls don't create large waves, so the formula is useless for them. For other than fat and heavy boats, the science of naval architecture would be much better off to just ignore such crutches. Besides, the hull speed formula is only an imperical approximation based on observations of a restricted kind of boat. It is not a good design tool for any other kind of boat.

    The most useful thing to have for designing a boat for this race would be an analysis of past performance to see how much emphasis can be placed on the rough open ocean part of the race without too much detriment to speed on the inside part, and vice-versa. I go along with the direction of using long slim displacement hulls as very probably the optimum design. Other small ocean powerboats seem to have settled on the trimaran as the best form.

    Light weight is important in all speedy powerboats but is not the first order of business which is to finish the race intact. Crew weight is going to be greater than boat weight anyway so more can be gained by backing off on the twinkies.
     
  12. Athlon64M
    Joined: Jun 2008
    Posts: 8
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United States

    Athlon64M Naval Architect Student

    Interesting, well it goes to say how much you learn outside of your course material.

    kengrome's design brings one question, where would you fit your outboard motor?

    Also, I'm a big fan of trimarans, especially the US Navy's LCS 2
     
  13. kengrome
    Joined: Jul 2006
    Posts: 718
    Likes: 25, Points: 0, Legacy Rep: 305
    Location: Gulf Coast USA

    kengrome Senior Member

    One option is "on the transom" as shown in the attached image.

    But I don't like this engine location because it puts weight too far aft. Better to install the outboard in a well with a faired and sealed opening in the hull immediately behind the aft passenger, then adjust their positions for best balance.
     

    Attached Files:

  14. ber1023
    Joined: May 2008
    Posts: 35
    Likes: 3, Points: 0, Legacy Rep: 47
    Location: bermy

    ber1023 Junior Member

    my freind just made a boat, 24ft long at the widest its 2.5ft, 15hp and it does 30mph, he will be using it for the seagull race

    that formula doesnt work for this
     

  15. ber1023
    Joined: May 2008
    Posts: 35
    Likes: 3, Points: 0, Legacy Rep: 47
    Location: bermy

    ber1023 Junior Member

    the fastest for a old seagull is i think 3 hrs 4 mins around the island (about 42 miles)
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.