dory catamaran hull

Discussion in 'Multihulls' started by frank smith, Feb 2, 2010.

  1. Milan
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    Milan Senior Member

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  2. bruceb
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    bruceb Senior Member

    More power

    Thanks Wally, I do have pics:cool: Can the power requirements be calculated by estimating the needs for a 4000lbs narrow dory hull and doubling it? My very rough guess seems about 25-35hp per hull for a 4'x32' hull. Bruce
     

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  3. guzzis3
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    guzzis3 Senior Member

    Bought a new forester last september, base model, 2.5 manual. No turbo let alone "interesting" stuff. Boring, but ticks a lot of boxes.

    My sportscar days are probably over, but you never know. It's hard to justify 4 wheeled toys when I've got a shed full of italian bikes.
     
  4. bruceb
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    bruceb Senior Member

    tools

    Right tool for the job- The Forester is nice. I bought my only italian bike in 1972- one "Bruce" power and it get slower every year:mad: Bruce
     
  5. Richard Woods
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    Richard Woods Woods Designs

    In catamaran model tank tests I did as a design student I found there was up to 20% extra drag over and above doubling one hull due to wave interference between the hulls. I don't have the test data with me now, but the model used was about 5ft long and tested to as fast as the tank carriage would go.

    Ecocats in the UK built a 33ft powercat for the London River police powered by twin Yanmar 27hp diesel outboards. It did 15 knots no problem, weight about 3.5T (from memory). Oh and it had hard chine flat bottomed hulls. See very small photo. Sadly Ecocats ceased trading a couple of years ago, although you can still see one of their boats in action if you watch the "Boat Race" as the BBC use it as a filming platform.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

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  6. Chris Ostlind

    Chris Ostlind Previous Member


    Yes, it does, sorta, but from all the images I've seen, the suggested form is only in the very aft sections and is distinctly without hard chines.

    While BP5 did set a hairy record across the North Atlantic, it was easily bested in light air conditions by G3 and almost got beat for total elapsed time at the very end of the route because of that fact.

    http://www.yachtingworld.com/news/416503/banque-populaire-v-groupama-3
     
  7. TollyWally
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    TollyWally Senior Member

    Gentlemen,
    Thanks for the power requirement input. If I am understanding the concensus we are thinking perhaps 50-75 hp moving roughly 8000# at 12 knots. Richard allows that there is perhaps a 20% increase in drag resistance due to the behavior of the water constrained between the 2 hulls.

    That is a fuel performance envelope I would be quite happy with. This is so interesting. All of the interaction between variables.

    The subie I bought was a plain jane forester too. As I said earlier, after a brief internet indoctrination amongst the crazed kids I changed my tire pressure ratios and completely changed the charactor of the vehicle. I was so surprised. I now appreciate that my little bland transportation module that I unsuspectingly drove for several years would have blown the doors off several of the sportscars of my youth that I so fancied. Quite amusing looking back at the follies of youth.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Reminds me of renting a catamaran power boat back in the early 60's. Very comfortable and stable, on a river at least - did not get to take it out to sea - planed effortlessly with a tiny 15 hp outboard. I was struck by the small wake compared to other boats: it was very tempting to ignore the speed limits past the mooring areas' I didn't but I'm sure nobody in the houseboats would have noticed us passing at speed.
     
  9. guzzis3
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    guzzis3 Senior Member


    Hm. Mini cooper s had about 75 hp and a top speed of 95 mph. The TR3A I had wasn't much faster, and the X1/9 was a slug (although you NEVER had to slow for ANY corner). The 2.5 4 cyl forester has about the same real power and torque as a 70's american style 5 liter (308 ci) V8. Mind it's about the same weight also :)

    What did you do to your tyres and how did it change the car ?

    I know this is OT but it wasn't me that started talking about "race" cars...:)
     
  10. TollyWally
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    TollyWally Senior Member

    I was reading through the kids wild *** adventures about doing this and doing that. They were talking about swapping sway bars out and the various benefits of different sizes. Anyhow it sounded cool but you know you'd actually have to get some parts and install them. My forester had a decent stock front bar but a weenie rear bar.

    After reading some rally tuning steps I stumbled across this tuning table about dialing your car in for different tracks etc. Long story short I cranked up the pressure in the fronts and ran the rears about 10#s lower. It really changed the way the car handled, more towards oversteer.

    LOL for a couple days things were flying around inside my rig as I pushed it hard into corners. As I mentioned earlier I bought this thing as a small practical utility rig. The back seats are always folded down and the extended cargo area is always in use. It's the whole point.

    I had driven it for a couple years in a slow sedate fashion. I mellowed out after a week or so but to this day I keep the pressures biased and put it into corners harder than I used to but slow enough to keep things roughly in place. And always grin about the little tiger I never knew I had. I am much fonder of the car now and grin to my self thinking of all my unsuspecting fellow Suby drivers who don't know about the beast lurking within.
     
  11. bruceb
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    bruceb Senior Member

    cat power

    I have installed motor mounts and small outboards on several beach cats and was always amazed at how well they worked, with or with out the rigs up. Even though the hull shape was wrong for a power boat, a Hobie 18 in particular is a decent 12-15kt power boat (although quite wet). I have wanted, and would not mind building a 20' 18-25kt open cat. There are some good "heavy" production 30+kt cats, some "flats" boats, but very little at the light 20' size. Something that would run on 15-25hp. I have a couple of inflatables but they beat me up in 1'-2' chop- the beach cats don't. I am getting to old to like pain:( Bruce
     
  12. dstgean
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    dstgean Senior Member


    Did you see Gary Dierking's outboard powered Tornado? He made a hard deck with a front fairing and it works the treat. You'd have to raise the platform a bit with the Hobie 18 to get above the spray, but look here for some inspiration.
    http://outriggersailingcanoes.blogspot.com/2009/12/what-ive-been-doing-lately.html
    [​IMG]
    [​IMG]

    Notice he got the motor foreward of the aft beam. I think a motor hanging off the back beam is what kills some performance of this type of craft. Looks like fun though.

    Dan
     
  13. rayaldridge
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    rayaldridge Senior Member

    True words. My builder in Malta is motoring his Slider around while he waits on his mainsail. He has an outboard just forward of the aft beam, but unfortunately, it's a 6 horse behemoth. His bows are well out of the water, because he hasn't yet installed the steering lines, so he has to sit aft to steer.
     
  14. Chris Ostlind

    Chris Ostlind Previous Member


    Nicely rounding-up the conversation, please note that the Dierking adaptation riding on a Tornado platform does not have hard chined dory hulls... ;-)

    There's another boat somewhat within a similar genre, the Skeeter by Russell Brown and Paul Bieker. http://www.biekerboats.com/Bieker_Boats/Skeeter.html

    Also a tornado based power cat and also not a hard chined dory hull form.

    I guess that if you wait long enough, the full ride will eventually come 'round on the guitar...
     

  15. dstgean
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    dstgean Senior Member

    So nice of you to come 'round! I know puns aren't funny...but they were in Shakespeare's day, and my dad still insists they are. And I'm teaching The Merchant of Venice, so I can't seem to escape them at present. My point is not hull form though, or even for that matter puns. Rather the need to get the weight foreward. Would the round hull work better? I bet it would. How much better? I suppose you'd have to either have some good computer program or build otherwise identical boats and see. I bet it's a knot or two total unless one throws enough HP to get the boat to plane and then the aft rocker would hurt at low speeds. Nothing's free I suppose.

    Gary's hulls were free or nearly so, and that's why that particular boat came to be with the nice round forms.

    Back to dory hulls and power--get the weight out of the ends and you'll be better off just like Gary's nifty Tornado fish killer!

    Dan
     
    Last edited: Apr 22, 2010
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