Planning hull drag at displacement speed?

Discussion in 'Boat Design' started by steveislucky, Nov 17, 2007.

  1. steveislucky
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    steveislucky New Member

    I was thinking of repowering a 35 foot Chris Craft type planning hull, with one or two small out board motors. Would this be eficient enough at close to hull speed? Or is this a dumb idea?
     
  2. PAR
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    PAR Yacht Designer/Builder

    If you're thinking you can motor an old Connie (for example) with a couple of small outboards at displacement speeds, you can, but it will handle sluggishly and the hull isn't well suited for these speeds. A typical 35' powerboat with say a 32' LWL would reach "hull speed" at about 7 and a half knots. You'd have quite a hole dragging behind the boat with her submerged transom, but it certainly would work. Fuel use would be quite high and little engines would be busting their butts, beating the water to a froth, attempting to move enough water through their props.

    Another consideration would be mounting them to a transom not designed to have torque applied this way. Steering and gear shifting issues would also require some thought.
     
  3. Willallison
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    Willallison Senior Member

    As Paul suggests, it takes only modest amounts of hp to trive a boat at displacement speed. If you're prepared to knock a couple more knots off - say back to 5 knots - then you'd probably halve it again.
    (Apart from those already mentioned by Paul) there are a couple of other major things to consider:
    You will need to consider the 'reserve' hp required to drive the boat in adverse conditions
    How will the boat handle in say a following sea with so little lateral plane in the water? I guess you could continue to use the boats existing rudders (possibly along with the outboards - though that would make a pretty comlicated steering set-up!). As they'll no longer be being 'acted upon' by the prop's, the rudders wont be as effective as they used to be either.
    Lastly, planing hulls tend to handle like pigs at displacement speeds - especially in rough weather.

    Of course, if you're only planning on using the boat in reasonably calm conditions, most of these potential problems are allayed - there are many larger houseboats out there being pushed about by quite small o/b's....
     
  4. waikikin
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    waikikin Senior Member

    Luckysteve, maybe if you mounted the outboards to a pod that kinda gave a nicer flow off the original transom perimeter that would give some better low speed hydros, I often see some bayliners/mustangs come up quite cheap with sterndrive/engine issues but still nice boats, when you consider most of these boats are used for marina based weekenders or for posing/postering about the place maybe the slower you go the longer you'll look cool in the groovy boat for(tongue firmly in cheek)! With the loss of original engines & increase in bouyancy aft with the pod you would have to do some ballasting, cos" when you see these boats float without engines they kinda float *** high with the chines on the flop if ya get the picture. Reckon it could be worth it if the boats cheap enough. All the best from Jeff.
     
  5. FAST FRED
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    FAST FRED Senior Member

    The other possibility is to simply install a MUCH smaller pair of engines in the boat.

    The Wisconsin industrial or Onan of 18 to 24 hp are quite reliable and cheap!

    USUALLY ABOUT A $1000 each.

    FF
     
  6. PYSully
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    PYSully New Member

    I was daydreaming about a somewhat similar thing. I was wondering how a planing 24' hull single inboard engine would work with a single diesel in the 30 to 40 HP range with a maximum prop dia of 13". It now has a 260Hp Gas engine. I know my speed would be likely in the 7Kt range. I'm sure it would take many different calculations to match the Hp to the trans, to the prop. The one (I think) good feature is the boat handles magnificently at low speeds.

    The whole idea is not speed but fuel economy for an old man who would like to be able to afford more time on the ocean (not necessarily far offshore) at the current price of fuels. With the current 13d 12p screw 1:1 at 1100 RPM's it runs about 8-9 Kt's (from memory).

    Does this seem too hair-brained?
     
  7. tom28571
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    tom28571 Senior Member

    If your boat were a true displacement hull, I think your numbers are OK. I doubt that a 24' planing boat is going to be economic at 7kts. I think even 6kts is stretching it with such an engine. It depends on the waterline length and some current 24' planing hulls have such a short waterline that it would be silly to even try it, since they would be so slow. If your boat is heavy with a deeply immersed transom, none of this is likely to work well. Since you say the boat handles well at low speed, maybe there is a shot but it looks a bit iffy.

    An educated guess, but still just a guess.
     
  8. Willallison
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    Willallison Senior Member

    No - I'd tend to agree with Tom. Assuming your waterline length is around 20 feet, anything much over 5 knots is going to start to really eat into your fuel economy. Every situation is different of course, but it's usually cheaper and more successful to sell the boat and buy something more suitable, than try to change a boat into something it was never really meant to be.
     
  9. Pericles
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    Pericles Senior Member

  10. PYSully
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    PYSully New Member

    Thanks guys, Common sense usually rules and htat is so in this case. I do think the majority of builders are wrong and there will be a market for moderate sized displacement hulls in the future. I missed one that sold for only a few hundered that was built with a 5 1/2 Hp. put put (19' long?). It was an old wood one and a pig in a poke, but intriguing.

    To be able to get 1/2 -3/4 gal per hour and cruise about easily would be dream like. When I was in the Navy some 50 years ago the French had what we called Bum Boats that came along side the ship and take us to shore. Probably not an option today. They had these little single cylinder engines moved right along, gas and diesel was expensive even then, in France, comparitively speaking. They did the same for the cruise ships that would anchor out as well. I guess I am somewhat trying to relive those days to some extent.

    Thanks for putting this idea to rest in my head.
     
  11. tom28571
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    tom28571 Senior Member

    I think that it is likely that even more people would be interested in a reasonably sized and burdensome cruising powerboat that could economically cruise at a speed/length ratio of 2 or a bit more. This is where the Atkin hull has, in my opinion, the greatest potential.

    Other more extreme examples of the same general idea might be even better. I am referring to the high L/B canoe underbody married to a low deadrise upper body with the canoe underbody taking a high percentage of the of the displacement.

    This must eventually get sorted out if energy costs continue to escalate.
     
  12. Willallison
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    Willallison Senior Member

    hmmm... I'd like to agree with you Tom, and as you know, I've fiddled with just such a hullform, but I'm yet to be convinced. As we are all awar, most owners do so little actual running hours on their boats each year that the fuel costs are (almost) inconsequential. I don't believe that there will be a major shift away from smaller maxi-cube planing hulls until it becomes socially impolite to be seen to be a conspicious over-user....
     

  13. TollyWally
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    TollyWally Senior Member

    Might I add .02?

    I live on Puget Sound and have a 28' boat. I'm out on the water a lot, much more than average. I saw noticebly fewer boats out last summer. It takes a while but fuel prices matter and eventually modify behavior.

    I have a tendency to be a bit of a numbers guy. A flowmeter and gps always let me know how much fuel and horsepower I am burning. I'm also aware of my monthly costs of boat ownership. I'm willing to burn a 1500 plus gallons a year at current prices. I feel that if I can't be willing to spend that I shouldn't own the boat since God knows it's not much of an investment! LOL

    But many people think about it differently. They focus on the last few hundred dollars spent each month rather than the sum of the whole. As I said before, boater numbers were noticebly down last summer.

    Personally the Atkins hull idea fascinates me. I'd be tickled to have a similar hull to mine making 15 knots on 50 or 80 hp. Sadly I know a little bit about the boat business and I can buy a tremendous amount of fuel for what it would cost me to build a new boat.

    Now if the world was a buyers market of nice 20 year old fiberglass Atkin style hulls that I could have my pick of at 10 to 20 % of replacement value...
    I'd be hard pressed to ever show up at work! :)
     
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