Convert sailboat to power cruiser.

Discussion in 'Boat Design' started by whitepointer23, Aug 21, 2015.

  1. whitepointer23

    whitepointer23 Previous Member

    Has anyone on the forum converted a deep keel sail boat to a full power cruiser. Does the keel need to cut down and have internal ballast added. I have seen some sailboats roll excessively when the mast has been removed for maintenance. Does the rig have a counter effect against rolling.
     
  2. whitepointer23

    whitepointer23 Previous Member

    The style of these 2 with mizzen steady sail is the sort of look i like
     

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  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You can cut down the keel and move the ballast weight inside the bilge, which will also help motion too. Yes, the rig tends to dampen roll motion, so if it's removed, the roll moment will be quicker, but this can be compensated for too, to a degree. You have to remember, sailboats are designed to roll easily compared to most powerboats, so the motion is going to be more dramatic.
     
  4. whitepointer23

    whitepointer23 Previous Member

    Thanks par. Its just an idea that i may look at one day . My boat has a bolt on keel which should make it possible to alter. Whats the average draft for a 35 ft disp launch. Would it be about 3 or 4ft.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There's no such thing as an average draft, you could have just shy of 12" (which my woman is fond of) or a few feet.
     
  6. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    Par, is your wife a fanatic of the BBC? More seriously, Yes a sail boat can be transformed in displacement motorboat. But it will become a roller without the mast, unless a damping system is added. Mono sail boats specially the heavy ones are designed to be stable when heeling, lets say a 5 or 15 degrees. That gives a roll not damped of around 20 degrees when the mast is gone.
    The CG has to be to recalculated to get a sufficient metacenter, knowing that the weight of the mast, and the efforts induced by the sail have been suppressed. That means that you can keep the same stability with far less ballast, and for improving the motions, specially rolling, the CG has to go up. In other words you change the frequency of the rolling but keeping a sufficient stability for a motor boat (who has not the same requisites as a sail boat). But the sail hulls have not a strong shape stability, so it will the remain the penalty of a serious amount of ballast. That asks for serious calculations.
    It may be interesting to keep a small part of the keel (if it's a modern separate keel, the old long keels it's without hope) as local reinforcement. That may simplify the work. After the interior lest must be calculated in function of the new engine.
    After probably it will be necessary to add 2 "damper" similar in function as those used of cruising boats. Other possibility as on a lot of small trawler is to add a litle rear mast with a sail, so the wind will stop the rolling. That was a common problem on old trawler designs, as their hulls descended from work sail boats.
    The engine must be changed for a more powerful one, most of the sail boats have just the engine for an occasional use so they lack of power, it will serve also as lest with the bigger fuel tanks.
    The rudder has to be changed also, with the shaft and the propeller.
    That's a lot of work...
     
  7. whitepointer23

    whitepointer23 Previous Member

    Thanks ilan. The boat already has a 4 cylinder perkins which has plenty of power. I would keep the mizzen mast for a steady sail. I am replacing the cabin top and decks anyway ao it is really only the keel that needs looking at. What do you mean by 2 inch damper.
     
  8. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Hi Brendan,

    I think Ilan may be referencing "Stabilising paravanes" http://www.powerandmotoryacht.com/maintenance/paravanes-stabilizer-option#.VdkBAxx5if8

    Jeff.
     
  9. whitepointer23

    whitepointer23 Previous Member

  10. whitepointer23

    whitepointer23 Previous Member

    How do i calculate the influence of the main mast on the rolling motion i mean its not just the weight but also the leverage of the mast height.
     
  11. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Weight a moment, you're half way there.
    But all aspects interrelate .... vertical center of gravity also affected at a similar change.
    This is not my field, I'm a fixer- put it back how it was & a builder to plans/design information.
    This might help with links to other areas
    http://www.kastenmarine.com/beam_vs_ballast.htm


    All the best from Jeff.
     
  12. Ilan Voyager
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    Ilan Voyager Senior Member

    I meant 2 dampers, ie small lateral keels like on cruising ships, or paravanes, or adding chines to the hull from a bit forward the main beam to the stern (there are molded rubber chines that can be added with just screws) chines can be very effective with little penalty. All that depends of the shape of the hull and must be studied. With a little luck the hull will behave nicely with the mizzen and a good CG.
    As you want a displacement boat, with a speed around 2/3 of the speed limit of the hull (which varies with a lot of factors Gerr has a good formula), after 2/3 of the limit speed the need of power rises drastically compared to gain of speed. The big engine is not needed but it must have enough power to go against a strong head wind and currents. It's a big factor of security; a rule of thumb is around 1.5 to 2 times the power needed for the speed you want. You can start also from the power of the Perkins, define your cruising economical speed and use the good propeller, at least three blades with large cups, the very classic "slow boat" propeller. You can get a better speed as power boat compared to the sail boat as you suppress the weight and windage of the mast, the drag of the keel, the drag of the big rudder, and less total ballast. That means also that the hull will have less draft and less wetted surface, it's not obligatory a blessing as some hull shapes will be more unstable when "light", specially at mooring or cross wind. The wine cup shapes so nice on paper are terrible when light, less stability, bad hydrodynamics. Maybe you'll have to keep a certain displacement to get a sweet slow motion, fastly dampened and accept the penalty of weight.
    You'll have to recalculate the stability curves without mast and sails. Leaving the original ballast you'll see that the CG is now too low with an hyperstable boat, a true poussah. The problem is that induces a roll that nothing dampens or stop if the hull is rounded like a classical sail boat with sweet shapes which are designed for heeling upwind. A sail boat is mainly designed for stability by ballast, a motor boat by shape, in reality there is a mix of the two.
    So you have to calculate an acceptable curve of stability for a motor boat using a sail boat hull, and modify the ballast as needed. If the CG is kept at same place le rolling will change only in frequency, as you have less inertia and no more hydrodynamic brake by the keel and big rudder. In fact the boat will roll at the minimal solicitation. To stop that you have to raise the CG, it's very counter-intuitive at first sight (two examples; to stop the rolling on the cutters the best way was to lift the anchors on the masts, and some old fashion slim fast warships had an inverse metacenter and became stable only with a heeling of 2-3 degrees, that stopped the rolling, in certain conditions of sea these ships were total nuisances).
    Unhappily a lot of sail hulls become rollers when light and with no damping you have to keep them close to the designed displacement. Happily a lot of motorsailors destined to sail flat and using a lot the engines have been designed with hull shapes not inducing rolling (the Dutch ere very good in that exercise) you can gain on the displacement. So it's the balance between stability (shape and displacement), CG, rolling frequency and amplitude (ie the hull shape), and dampers (hull shape, lateral keels, chines) that gives an acceptable boat. The paravanes are used when nothing other works.
    All that looks complicated but acceptable compromises can be found.

    The extreme sail boat tweaked to power boat is Ilan Voyager by Nigel Irens. This trimaran cruises at 23-24 knots with about 150 HP, the remaining 100 HP are in reserve, and has very smooth movements without the penalty of ballasts.
     
  13. whitepointer23

    whitepointer23 Previous Member

    So a higher superstructure should help dampen the roll as it will keep some weight up high. And internal ballast to sit on its original marks.
     
  14. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    It's probably an approach, with the penalty of windage (nothing is free...), aerodynamics have to be taken in account, not because in this case of the (low) speed of the boat, but because of the effects of a strong wind on the total drag (the famous Cx) and worst the stability of the boat. It's a matter of compromise and calculations of the stability.
    An all internal ballast will take a lot of volume, probably keeping a part of the keel, or another external "flat" keel bolted on is a better approach with internal ballasts fo keep the boat in his designed lines.
    You will need a protection on the hull when the boat is on its belly when grounded (mooring at low tide for example and staying in an acceptable atttitude without destroying the rudder and the propeller), you'll need also a protection for the propeller. Sail boats are generally designed to ground on the hard metal keel, non on the soft belly.
    The penalty on hydrodynamics is not a drama at such low speeds. Also you need some "grab" in the water or the boat will be erratic, you'll have to calculate the stability of route, thus the position of the lateral centre of effort. You have no more a deep keel which serves as pivot.
    Boats are global concepts, when you change one thing, you have to modify everything; finally you are transforming a sail boat whose hull shape, repartition of weight, hydrodynamics and balance were designed for using sails and sometimes an engine. Now you want to have a motor boat whose requisites are different.
    You can obtain a good compromise as long as you stay in the "natural" speeds of the hull, that means you won't get a higher top speed, but you'll have a better mean speed closer to the top speed.
    Having the constrain of keeping the hull shape, you have to modify the appendages for getting a good balance and correct sea worthiness.
    Take also in account that you are limited by the actual design of the hull on the diameter of the propeller, you will only play on the number and shape of the blades, and the pitch. It's very probable that keeping the actual propeller won't be be the best solution for global efficiency.
     

  15. philSweet
    Joined: May 2008
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    philSweet Senior Member

    I think you guys are going way over the top with this. Practical hull mods are pretty hard to come by, and most of the work would be inside the boat, at least at first.

    I'd keep the main mast for the stabilizing sail. The mizzen is not really going to be effective. Cut about 10 feet off the bottom of the main mast, reset the spreaders and have the shrouds and stays reworked. Set the boom about 7 feet over the deck, and recut the mainsail. The mizzen can be tossed, or repurposed and a gin pole if you like the salty look. Keep a small jib - 35sqft or so on a club. It is totally self tending and also flys 7 feet off the deck. It is useless for damping but wonderful for balance and does pull some too.

    The keel mod is doubtful, but if you plan to re-engine the boat and push her to 7 knots, then its worth a look as part of the re engine work. You will have to rework the steering and engine shafting system and rear third of the hull as well, which is why I doubt this makes sense. The other plan would be to see how much keel you can loose without needing to redo the rudder and shafting and engine. Probably not much, but recycling the lead and using the top of the old keel to make a mold isn't out of the question. Raising the ballast some, and saving 20-30% for internal trim, will go a long way to helping with the motions.

    My main concern would be promoting the engine space, or finding a boat that has an acceptable engine space to begin with (and I don't think that's going to happen on a 35'er unless you find an old Colin Archer). On most sailboats, you can't even find the bloody engine unless you know where to look. The cooling systems, ventilation, maintenance provisions and fuel tanks would all be inadequate, although motor-sailors might at least have a fuel tank. So gutting the aft third of the hull inside and sorting out the systems would be my first priority. You can rebuild it over time.
     
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