Bow extensions

Discussion in 'Hydrodynamics and Aerodynamics' started by DennisRB, Jan 14, 2015.

  1. waikikin
    Joined: Jan 2006
    Posts: 2,415
    Likes: 168, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    Hi Dennis,

    The pics are interesting. The Crowther designs I'm familiar with 85 & 95 had a similar but different underwing profile. The difference was that the leading upswept forward end(ski end?) of the bridge deck was consistant from one side to the other( & subsequently high clearance forward)... the "bunk blisters" as we called them developed from zero at the lower tangent of the upswept leading area to a 6-8" depth at the mast bulkhead. I see yours start to develop at the top/deck area & achieves full(?) depth at the same lower point.... the clearance looks low in the pics. I wonder if this is to plan or an adaptation to increase head room over berths. The bow down look seems to be visually a function of the notional sheer & deck edge line & actually some bow down trim at the waterline plus some "weight transfer"/drive to leeward. When I had my cat designed I asked for some aft trim in the sheerline as I didn't like the bow down look & that bow up at rest looks ok to me.... at this point I still have some building to do before testing;)

    The design 85-95 did have bulbs & forward projection, there seemed very little volume within & a topic of discussion about them was that in flat water they could work to potential.... & in broken /rough water that they might give more resistance as they drove in/out of waves.... pure conjecture for me anyway.

    Regards from Jeff.
     
  2. DennisRB
    Joined: Sep 2004
    Posts: 1,270
    Likes: 27, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Thanks Jeff. The "bunk blisters" are part of the design. The forward clearance is about 50-70mm lower at the front under them due to the trim. So its not just the lower freeboard bows which look bow down even at the correct trim. I would prefer the font higher than the back not the other way around. I think its about 55cm at the back (under the blisters) and in the center. Lower at the font due to trim. Its hard to measure anywhere other than a marina, which I dont go to.
     
  3. waikikin
    Joined: Jan 2006
    Posts: 2,415
    Likes: 168, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    Some of the design evolution of these vessels(at least the ones I worked on 85/95) was to increase hull beam aft & sometimes overall centerline beam also, sometimes also bearding line deduction was ignored & foam applied to outside of usual hull lines. This seamed to be to enlarge the vessels interior & match larger engine capacities being installed & increase space that owners wanted, sometimes also underwing core thicknesses increased to improve spanning abilities often from say 20mm to 40 mm & the longti underwing stringers eliminated. Re-engineering was performed for connectives etc by the Office... but sometimes not, at this point it was no longer a Crowther!!
    The increase in hull beam was the point I was getting to, this was a simple adjustment via wedging at the transom to 0 at about 25-33% aft of the bow- this metric was applied to station molds for building, not an actual "insertion of pie/wedge shape" although this would not be unheard of, I know of one that whent from 56' to 86' with hull expansions & 1000s of hp, needless to say although an impressive vessel had a few issues. The transom shape apparent of your vessel might reflect this "wedging"? I'm not sure that the method was ideal.... as the bows stayed the same but the stern didn't, a wild guess might suggest that's the difference or one of them in the two models that you've referenced...

    Jeff.
     
  4. waikikin
    Joined: Jan 2006
    Posts: 2,415
    Likes: 168, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    We had the bunk blisters too but they wedged to 0 at the front lower edge, this "cut away" increased the clearance forward as the vessel drove the bow through waves, of course the headroom lower over the bunk as a result & the steps bigger/higher to get into bed.I'm not suggesting you make this modification:eek: Escape/ingress hatches were fitted into the inner hull sides at this area.

    560mm seams to ring a bell as to underwing clearance under the bunk blister aft & about 760-860 in the middle, sometimes just the cockpit sole area was increased further in clearance so that you could see over the cabin & I suppose following seas had more clearance aft.
    The Grainger vessels at the time used the champher panel method & later Schionnings a large radii in the area.

    Regards from Jeff
     
  5. Barra
    Joined: Feb 2014
    Posts: 94
    Likes: 4, Points: 8, Legacy Rep: 32
    Location: Perth

    Barra Junior Member

    DennisRB, what does the boat actually weigh. have you had a chance to put it on a load cell when its been on the hard.

    Looking at the waterline, it looks heavy.
    Remove all the excess stuff and I reckon you will be suprised how much the trim changes as the boat floats higher.

    Crowther designed performance "SAILING" boats . Many joked that the difference between a crowther racing and cruising boat was the cruiser had the two burner stove.

    Restore it to its designed waterline and yee shall be rewarded.
     

  6. DennisRB
    Joined: Sep 2004
    Posts: 1,270
    Likes: 27, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    I have raised the actual water line by about 50-100mm since ownership. I have removed around five 7x4' box cage trailers worth of stuff from the original owner. I have halved the size of the battery bank. I will get lithiums soon etc etc. Before the stuff was taken out it was weighed at 9 ton. The lift guy said it was out by a lot and it would have been more like 8 ton. The surveyor Jeff Cruse said it did not "look" overweight by the transoms and that to never trust a marina lift for weight unless it has been certified. Design weight is 6500. So the actual weight can only be guessed at this point.

    Like I would go to the extremes of hull modifications if I could just take more stuff out! FFS!! :D At this point weight can only go UP since we are not world cruising in it yet.

    Jeff I do not believe the hull shape was modified. Just the transom was continued on at the slop of the sides, not squared off, so its 1 foot longer. Plus the sides of the decks were rounded rather than a chamfer, which would have been heavier but looks nicer I guess.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.