Waterline Rule-of-Thumb ?

Discussion in 'Boat Design' started by SAE140, May 24, 2011.

  1. SAE140
    Joined: Apr 2010
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    SAE140 Junior Member

    Although a 'raggie' at heart, I fancy doing a spot of 'short' long-lining and maybe a crab pot or two, and so I've just bought a project hull - pretty-much a freebie. (attached is a profile photo from the advert)
    Before starting work on the hull, it would be useful to know where the laden waterline is supposed to be - or maybe it isn't that simple, in view of the loads carried ?

    So I hope this isn't a dumb question - but is there such a thing as a LWL rule-of-thumb for a displacement hull ? I know that with sailing hulls, the transom should just be clear of the water - but how about displacement hulls ?
     

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  2. Perm Stress
    Joined: Sep 2009
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    Perm Stress Senior Member

    A wild guess, based on many similar sized boats I see in my area: hull immersion at reasonable load (2 persons, heavily clothed and with fishing gear) would be about 20...30cm, or 2/3...1 feet.
    Wild guess, no guarantees ;).
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A displacement powerboat hull is much like that of a sailboat and you don't want to drag an immersed transom for best efficiency.
     
  4. philSweet
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    philSweet Senior Member

    How about mounting a 10-15 hp outboard in a well in the cuddy and using all the cockpit area for gear. Pots could be hauled up the transom cutout onto a low deck and flopped back in same way. Access over transom for swimming. Otherwise, you are operating over the side; which is something of a nuisance in a boat this small.
     
  5. SAE140
    Joined: Apr 2010
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    SAE140 Junior Member

    Some really good thoughts there - thank you.

    Another member kindly PM'd me to suggest loading-up the boat and then waiting for the LWL to show itself from the watermark.
    Yes indeed - that would be a good idea, except that I'm in a Catch-22 situation in not knowing how far I can safely (from both a safety and efficiency POV) immerse the hull ....

    Perm Stress - a foot is indeed an inspired guess ! I checked out the Orkney 16ft Longliner drawings (an approximate - very approximate - equivalent to my tub), and that (1.04 ft) is exactly what's shown on their drawings. Ten minutes with a tape measure last night confirms that a foot of immersion would result in 18" of freeboard aft (with the transom only an inch underwater), and 24" of freeboard forward - so that sounds about right to me. Certainly a good enough figure to start work with.

    PAR - appreciated your comment - of course, why should any other hull be different ? After all, movement through the water is pretty-much the same. I guess in the back of my mind were those BIG commercial vessels which experience large changes in laden displacement and so have to take on ballast water to force the hull (and it's transom) much deeper into the water.

    philSweet - I suppose such a radical idea should be expected from a 'Boat Design' forum ! Has this ever been done ? It's certainly a novel idea, and I can see some benefits of what you suggest.
    However, my prejudice (as opposed to experience) tells me that it would be more desirable to have the boat pointing into the tidal flow, with the string ascending/descending forward, ahead of the boat - which either means working over the bow, which isn't really practical, or over the side - the latter of which I fully accept is less than desirable in such a small boat.



    The only other issue I'm snookered with is the method of anchoring in this type of boat. There are 6" wide sidedecks, but I really don't fancy going forward using those in a lumpy sea.
    Some boats of this style have a forward-opening hatch to access the anchor/roller, but I have visions of this being ripped-off if opened in a squall. I have thought about a sliding hatch on top of the cuddy, but I think leaning forward from up there would play havac with me back. Any inspired thoughts on this one ?

    Many thanks.
     
  6. SAE140
    Joined: Apr 2010
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    SAE140 Junior Member

    Think I've just 'sussed' it ...

    At the moment, the 2 front windows (and they are 'windows') are 'V-ed' forward. So - if I modify that arrangement to make the forward surface of the cuddy dead flat - and make up a sliding window: with the port-side half of it fixed, and the s/board-side half sliding in front of it - I reckon that should do it. Then feed back the anchor warp along the s/board side-deck to the cockpit.
    Quick sanity check - any obvious whoopsee's with that ?
     
  7. viking north
    Joined: Dec 2010
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    viking north VINLAND

    SAE140-I was a little confused on your waterline question thinking at first you might want to paint on your antifoul but with a trailorable unit I assume it will be launched and removed as you need it's use. If not then simply rig her out--motor, fishing gear, put her in the water on a calm day and mark her lines with a pencil from a dingy. Your only fixed weight that i can see here (being a past Newfoundland open boat fisherman) is the motor so choose one that is typically used on a similar hull. Remember a four stroke is much heavier per HP than a two stroke. Better to be on the lower end weight wise to keep the transom freeboard sufficient when not running or running at a slow speed. Looking at that hull I would guess around 15hp.(two stroke). This should be approx. the weight range,get her up on a plane and give decent fuel consumption. Most motor retailers will loan/rent you a demonstrator or two to determine the proper purchase size. The other point to remember once she's all fitted out and sitting in the water you can move most other gear around to balance things out- stern down bow up or the other way around but once underway it's the tilt of the motor that plays the big role in her longitudional attitude. Experiment with the settings and set it up for max. speed at the trottle level for your average operation. With this type of a power boat there's alot of control over balance unlike thousands of pound of lead permanently fitted off proper location- Just purchasd a salvage case where 1500lbs of internal trim balace had to be added to a 4000lb poured lead keel. OOPS some one messed up --Geo.
     
  8. SAE140
    Joined: Apr 2010
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    SAE140 Junior Member

    Yes - I could have phrased it better: like - "what is the likely carrying capacity of this size of boat ?" Seems like it's a trade-off between windage and dragging it's transom in a hole, so I guess I'll simply add ballast (water containers) until the transom drops just below the W/L with an even trim - then estimate the weight-carrying capacity from that.

    With all the boats I've owned before, they either had the recommended LWL engraved on the mould to show a mark on the hull, or the boat came with old antifouling and boot strip. On this little tub, from the rubbing strake downwards, the hull is (surprisingly) as clean as a whistle.

    I think you're spot-on about a 15HP 2-stroke being about the right size of motor - do you really think that this hull would plane ? I've been considering it as a displacement hull so far (you can see I'm new to this power boat malarky ...) - a planing hull would certainly extend my operating range dramatically: no longer would there be any need to be a slave to the tides.

    Geo - you mention being an open boat fisherman - during the afternoon I've been mulling over this cuddy business: one option available to me is to remove the top completely, fit new gunwhales and use the hull as an open boat. That isn't a decision I have to make this year, but the upper works of this hull really are naff. From your experience, are cuddy's worth-while having ?
     
  9. viking north
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    viking north VINLAND

    She looks fairley flat bottomed so if not to heavy she should get up on a plane or very close to it. Try the 15HP. if not then try the 20HP. If you're travelling distance the ability to get her upon a plane will save you fuel. We got our 600 to 800lb. 18ft flats upon a plane with 15 to 20 HP. Depending how much gear and fish we had on board, however there were times after picking the cod trap we had only 6in. of freeboard, Not good, we were young and foolish.
    Cuddys are nice if large enough to operate from or crawl into in bad weather if you're staying out but on that size of boat (guessing aroung 16ft.) it might be more of a pain than what it's worth. On our 18footers we never had them but did have a small deck level watertight storage area in the forpeak(bow) to keep some of our gear dry. (propane stove, fresh water, grub, flares, extra clothes and so on. ) ---Geo.
     
  10. philSweet
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    philSweet Senior Member

    yup, used to be common in the mullet fisheries until they changed the rules. The boats were also used in the "square grouper" fishery. (transporting bales from the west coast of Florida as far as possible inland to the east where they were transferred to airboats for the final run to Miami. The later bunch developed some extreme characteristics, but the original ones looked like normal boats.
     

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  11. viking north
    Joined: Dec 2010
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    Location: Newfoundland & Nova Scotia

    viking north VINLAND

    Man that looks strange, don't know if all that weight forward would survive surfing in on a wave. However must have served the purpose it evolved to do.--Geo.
     
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