Outboard Conversion & Naval Arch

Discussion in 'Powerboats' started by DEboater, May 19, 2019.

  1. DEboater
    Joined: Feb 2009
    Posts: 19
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Pennsylvania

    DEboater Naval Architect Student

    Hello all,
    I’m fairly new to the site, although frequented it a bit while attending college for Naval Arch at UNO. I’m a NA, but do not work on small boats or yachts. I’ve owned 14’ to 24’ fishing boats for the past ~15yrs, have hand built a fiberglass pilothouse (NOT fancy) and helped others on 20-30’ wooden boat builds. So I know enough to get myself in trouble and probably back out of it.

    I’ve been toying with converting an IO (sterndrive or jackshaft) powered 26-28’ to outboards.
    Hulls of interest are late 1990’s/early 2000; Stamas, Pursuit, Albemarle, Carolina Classic, perhaps others.

    Some questions...
    These are heavy boats (8 to 10k lbs) with sharp entry and 20 to 24 deg deadrise hulls continuing aft. The power is low in the hull and either ~amidships or aft.
    Need to freshen up on general rules of thumb for planning hulls and managing hull characteristics and powering.
    - Centers,
    - weight distribution
    - powering (outboard sizing, location and propeller sizing)

    - This is a big ask...does anyone have general ‘lightship’ hull characteristics for these deep-v planning boats?

    Things that are important to me:
    - A balanced boat that gets up on plane well and can also cruise ~20kts in 2-3’/7-10sec period waves (typical mid-Atlantic decent seas) while loaded with gear (fuel, ice, 3-5 people and gear)
    - Enough low speed torque to get through heavy current inlets, namely Indian River Inlet, DE.
    - A boat that doesn’t rock to sharply while drift bottom fishing (understand this may be unavoidable if I choose a true deep-v hull). Every boat drifts different, would be interested in any observations you have on this.
     
  2. DCockey
    Joined: Oct 2009
    Posts: 4,224
    Likes: 182, Points: 63, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    The best overall reference for someone on planing hull design for someone with a relevant technical background I've found is Performance by Design by Donald L Blount.

    Deep V hulls tend to roll more in a seaway than boats with shallower dead-rise aft.
     
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,343
    Likes: 231, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    One does notice that many conversions from sterndrives to outboards sees the boat sitting higher in the water, which isn't really desirable, especially if a deeper vee. That will exacerbate the tendency to rock and roll at rest.
     
  4. DEboater
    Joined: Feb 2009
    Posts: 19
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Pennsylvania

    DEboater Naval Architect Student

    Thanks for the replies, I did lots of searches but didn’t see the type of discussion I’d like to have...good blend between practical and the physics. I’ve been reading build threads and recording notes. I have about 25 different boat rebuilds and the basic hull characteristics documented. I noticed the lighter condition a lot, along with classic bow up condition. My first two methods of correction will be fuel tank move (these boats need new tank(s) anyways) and the addition of a potable water tank up forward, but low, which can also act as ballast. Also, I will likely build a 3 sided lightweight pilothouse over the cockpit which should add weight as well.

    Another aspect I’m curious about is cutting out a transom and building a motor well, vice bolting on a bracket. Not sure if that may help the boats attitude when on plane or not. I’m not so sure if I like the brackets which also add some flotation while under planning speeds. Have to think about that a bit.

    Simple question...before I buy a hull I’d like to model a typical hull (26’Loa, 9.5’ B, 22deg deadrise or so) and model typical CG and play around with weight additions/movements. Does anyone have typical values for express or walkaround style boats? I can also estimate and calc, but it’s always nice to compare to rules of thumb.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,343
    Likes: 231, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Using fuel tanks as ballast is problematical, as it changes when the tanks drain. Transom hung outboards with wells are de trop, mainly because it is seen as wasted space, but in some cases it does deliver a better balanced boat. However, with a hefty boat in the 26-28 foot range, a couple of big outboards is still a low percentage of the total boat weight, so drastic effects on trim is unlikely, with podded engines. It depends on the boat, if the hull is inclined to porpoise easily, podded engines may worsen it, if the boat is inclined to bog a bit running downhill, pods may improve it. A good guide would be the number of conversions you see, of a particular boat, from inboards to podded outboards.
     
  6. DEboater
    Joined: Feb 2009
    Posts: 19
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Pennsylvania

    DEboater Naval Architect Student

    I’m surprised I thought this would generate some conversation. There are benefits to a transom mounted outboard, less interference for fishing lines, easier to access motors, motors are protected better. Less drastic weight shift. In general these conversions take 500-600lb of engine weight, plus sometimes remove gear weight, and move it anywhere from 5 to 10+ feet AFT and 3-4’ upwards. It can make a difference, especially with the ride quality of the boat, which is why I’m asking about good rules of thumb for planning hull characteristics to compare to calcs I make. I guess I’ll buy the book.
     
  7. bajansailor
    Joined: Oct 2007
    Posts: 181
    Likes: 13, Points: 18, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Have you considered an older power cat perhaps? They would appear to be ideal for what you have in mind re seaworthiness and fishing ability (?)
     
    Last edited: Jun 1, 2019

  8. DEboater
    Joined: Feb 2009
    Posts: 19
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Pennsylvania

    DEboater Naval Architect Student

    I have been looking at glacier bays and world cats closely also. They serve my need for shallower water access to my family’s dock (1.5’-5’) depending on the tide.

    There are some benefits, although I don’t know enough to capture it mathimatically, practically you get some reduced drag, and better control of thrust direction. Torque is a concern though for hole shot and when I’m running inlets against the tide/wind.

    I’ve heard a good rule of thumb for a sterndrive repowering is you can drop the HP by 25hp or so and will get similar numbers. If you stay the same you will get better top end and cruise. (say a 260hp/350CID) to a 250hp outboard. Then you can play with prop pitch to get the right balance of cruise/fuel efficiency and hole shot/low speed torque.
    I’ve been collecting before/after performance numbers and haven’t had the time to analyze the numbers yet to see trends.
     
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.