Questions about rocker

Discussion in 'Boat Design' started by Bullshipper, Jun 12, 2021.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, now we are getting somewhere. Firstly, even 20 mph is marginal for a typical GRP cat like that to expect to consistently hold plane in choppy water, the boat really should be able to easily maintain 30 mph in a decent chop, and if it can reach 40 mph, then you should not be flogging the engines to cruise at a minimum 25 mph in rough water, if the boat will not hold a clean plane at 25mph in those conditions, I agree it is likely too heavy. But if it feels unsafe at 25mph, then that is a whole different matter.
     
  2. Bullshipper
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    Bullshipper Bullshipper

    Other smaller cats I have owned all started to plane at 12-14 mph and they were 30% slower at wot., So our experience is different.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    They would need to be light and/or with flat, fat hulls to be planing at 12-14 mph, if that is your aim, you are going to be disappointed, the very best you could expect would be to back off to 20mph minimum, even if that boat was 20% lighter. The kind of boats, cats or monos, that can cruise at 25-30 knots in "average" off shore conditions with a tolerable ride, simply can't be backed off to 15mph, 20 mph would be a stretch for many, even with tabs.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    There are good reasons for the above statement, the good rough water boats don't have the same dynamic lift as boats that ride harder, and they don't generate enough lift at low speeds to plane cleanly.
     
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  5. kapnD
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    kapnD Senior Member

    Further flattening of the hull bottoms would help it plane at lower speeds, while a good sharp entry could still be maintained.
    This could help with fuel load/consumption problem as well.
    Save the fancy bottom configurations for the lake boats, the open ocean demands simplicity and a good deal of compromise!
     
  6. Jimboat
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    Jimboat Senior Member

    Hi Bullsipper - sounds like an interesting project. it's not immediately clear what issues that you are trying to solve, with your proposed mod's. "Too heavy" is pretty clear, and less weight can surely improve performance, since cat's are very sensitive to power and to weight. Add rocker? Increase tunnel height? not sure what you're solving with these?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    If the stated width of 34" for the hulls (doesn't look that much) is correct, this boat should be OK so far as being able to back off in rough conditions, but under 20mph would be unrealistic, and if 20 mph seems too fast for the conditions, then you have the wrong boat for the conditions.
     

  8. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    You may be right about maintaining the higher planning speed. In the 3 2hr break in motor tests I ran, I had to cover a lot of other questions, and as most hulls surge when coming on plane, an accurate mph tipping point between 14-20 has to be nailed down better.

    These were very rough conditions, ran at high speeds, and the hull ran very stable considering conditions. She was wet, taking spray over the bow and top of the cabin, and getting her bow up a LITTLE more will help, And frankly, it it was anywhere this rough the boat would have been coming in, not going out, with a lot less spray in a following sea. The previos 2 tests in 2-3's were flawless.

    The hull bottoms are flat, with a 9" wide flat planning pad, and because the sponsons are a little narrower than common, her entry is slim while not knife sharp at the tips of the bow to allow removing all air bubbles in the glass easier during layups.

    So IMO, the problem is to make her 25% lighter to allow to:
    more room between the water and tunnel top in the stern to avoid blocking off the tunnel when I tilt the bow up a little more
    Tilting up move the bow spray back closer to mid ships to make her dryer on board
    Decreasing vessel weight will get more fuel carrying capacity for longer trips offshore
    Get her speeds with this power from 45 to 55 mph and increase her fuel economy.

    Fuel economy is currently using about 190 lts in 5 hrs, key on key off, running hard with twin 225's averaging about 19 mph. I will get the gauges programmed to read these during tests on the next hull, but I am encouraged by the rough estimate so far.

    Some anglers in the east and southern US now want a 500 mile range and to be comfortable during long trips cruising at 40 mph in seas and these are fairly demanding conditions for a day boat. As it is now, I can do this in 2-3's sitting comfortably in a cabin with my hands off the wheel and making high speed turns with lean in, so except for the bow spray and load capacity of this very heavy layup in bad conditions, I am almost where I want to be and less weight and getting the bows up slightly will increase my range and sea worthiness in 4's instead of 3's, markedly, in my experience.

    And as the hull is very solid going fast in rough conditions, I am confident that she will also last with a lighter layup with 7 skins on the sides and 9 on the bottom as opposed to 16 using much heavier roving with larger overlaps and a higher percentage of resin as was employed on this learning experience.

    So thanks again for your tips, they have helped me feel better already.
     
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