efficient 10m displacement powercat (build thread)

Discussion in 'Boatbuilding' started by groper, Apr 15, 2012.

  1. groper
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    groper Senior Member

    Ad-hoc - bear with me :)

    I'm off again for a couple days- I'll get the wsa when I get back. I'm surprised you havnt asked for the frontal area for wind drag calcs :) dont know that yet either till I get back. And yes it was naked drag at 3900kgs...
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Only a couple!!...i'd be gone longer in your location and with your boat. Beautiful part of the world...remember it well :)

    No rush.....one step at a time
     
  3. groper
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    groper Senior Member

    Ok im back :) We will be doing a much longer trip a little later in the year Adhoc - we will leave sometime in late May i think...

    total WSA for both hulls is 22.95m^2.

    Total 2D frontal projected area above the DWL is 11.96m^2 for air resistance calc - what coefficient would be reasonable? it was so long ago (4 years?) i cant remember what coefficient i used way back when... from memory it came out around 10% of the water drag...
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    As I said QA is not your thing!..i've had to assume the Lwl (as asked for in my post above) is 10.60m...as that is a value quoted earlier in this thread.

    Also noticed a slight error in my original calc's now corrected.

    Below is power v speed:

    gropers-corrected.jpg

    and now with deckhouse air resistance added:

    gropers-corrected-with air.jpg

    You can see the values you obtained and those I have used from my own data, are not far off each other. Again, not knowing all your details.

    Thus what this suggests, if you are unable to obtain 24 knots, is that the PC, or overall propulsive efficiency with the outboards is over estimated.

    In order to confirm this, you can can run at 3.9tonne, or a known displacement, and obtain the speed and the power at said speed...then we can correlate accordingly.
     
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  5. groper
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    groper Senior Member

    Yes- Lwl is 10.5m - it was 10.6m assumin the topsides intersect at an infinitely small radius at the stem. Practically there needed to be small radius to wrap glass around so the lwl ended up approx 100mm less- is by that a qa problem too? :D

    The max speed we achieved is 27.4kts. That was light ship tho probably a couple hundred kilos less than the 3900kg we are with people and gear and fuel etc. We only get about 26kts now. So the propulsive efficiency is a little better than your estimation...
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Any trials on one engine ?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Agreed...so up to a PC of around 0.65 and it is about right...mid-way between the two predictions, assuming everything else is correct.

    gropers-corrected-with air higher PCs.jpg
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I estimated (well, guesstimated) not much more than 25 knots at 4000 kg. looks like I was not too far wrong. But it is the maximum cruise speed before fuel use goes off the chart that matters, should do 20 knots without losing too much with these engines. Very good result.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    My estimate was 26.5knots at 3900kgs. So close enough

    Of course air drag is a major factor. Try motoring a 35ft sailing cat into a headwind with a 9.9hp outboard!

    But the drag estimates usually assume calm conditions when the air speed = boat speed. That is rarely the case so air drag is normally a lot higher than predicted. Gropers video shows probably 40 knots head wind

    Richard Woods
     
  10. groper
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    groper Senior Member

    Indeed Richard...

    If im cruising in calm water behind a reef, but there is a 20knot wind blowing across the sea, i will setup a course downwind cruising at 20kts. If i then turn 180 degrees so im heading straight into the wind, the boat speed drops to about 16.5kts without touching the throttles... The air drag on a bridgedeck cat is pretty significant...

    The other thing thats difficult to deal with is anchor retrieval when its blowing... very quick and easy to loose the bow as it slightly points off the wind, probably the shallow forefoot is to blame... And the thing tacks back and forth like crazy if you try to anchor it off the bow cleats. Ive learned to anchor it off a bridle which goes between the bow cleat and next cleat further aft. This stops the tacking and keeps it pointed just slightly off the wind. If i move it further aft, the boat wants to sit sideways to the wind and presents a huge windage - so cant do that in a blow but works great when the wind is light and the waves are abeam in an anchorage...
     
  11. waikikin
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    waikikin Senior Member

    I would anchor my beach marine 10m off a stern in light conditions, plenty airflow into the cabin... discovered by accident one morning waking with anchor line around a rudder.. would set a dive weight as a sinker on the line after that.
    Jeff.
     
  12. groper
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    groper Senior Member

    yep we do that too jeff :D i pretty much turn the boat where ever i want it pointed - usually to keep the cockpit in the shade...

    This ones at high tide - hope islands... at low tide we had the transoms touching the sand :)
    [​IMG]

    Oh btw - we named the boat the other day aswell :)

    [​IMG]
     
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  13. DennisRB
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    DennisRB Senior Member

    Finally, a pic of the boat in its natural habitat. Sure looks at home. A sail rig wouldn't look out of place on the hull. Needs a small rigid wing sail so get the LD numbers required for fast motor sailing into the wind. :p

    Also is the boat really all that light compared to other light cats? I thought I saw you take a guess at a balsa Arrow 1200 lightship being about 3500, where this smaller boat is around 3400 or so with no engines. Obviously one is a sailing boat so probably apples to oranges again, but top speed is listed at 20K+. Does the extra weight come from increased scantlings for high horsepower propulsion and expected slamming in adverse conditions under high speed power? Just wondering what a 40 foot sailing cat might weigh constructed in a similar fashion is all.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Interesting data, that. That is a sizeable difference, and your boat doesn't seem excessively high frontal area.
     

  15. groper
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    groper Senior Member

    The oram 39c, built in duflex, is listed as 4800kg lightship for comparison ... we are around 3300 kg in the same condition. 3900kg is loaded up with people and gear and everything so to speak. Not sure what numbers you are quoting - but i have always given numbers including engines and everything the boat needs for basic function including batteries, solar panels, fridge, freezer, etc etc etc... i have almost 400kg in engines alone. Without those, i would be under 2900kg lightship... Id have to check my spreadsheet to give you a number of what the "empty shell only" weighs... but it wouldnt be over 2500kg id say...

    I did put extra laminate in a few places that didnt really need it, i did it for toughness as foam core boats can be a little fragile... for example the entire deck has 1050gsm on it so it doesnt ding as easily. Same goes for the bottoms of the hulls - theres 3000gsm along the bottom and wrapped up onto the bottom edge of the hull sides etc so i can beach her without getting too paranoid about the odd peice of coral putting holes in her... theres a few places i did things like this... could have built her lighter im sure, but i think we have a good balance of lightweight and not too fragile...
     
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