broaching essentials

Discussion in 'Boat Design' started by coopscraft, Dec 25, 2013.

  1. mcollins07
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    mcollins07 Senior Member

    For reduced risk of broaching, you can go with either a dig in approach or an on-top of the water approach. For a beachable boat, you want a boat that rides on top of the water as much as possible. A draft of about 1/10th of the beam, or less. This criteria makes the materials of the boat critical. Strong, abrasion resistant, and light weight.
     
  2. Skyak
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    Skyak Senior Member

    The optimum shape for broach prevention, moving forward through waves would be the shape of a tadpole. A sharp edge can have fluid velocity only along its length, in all other directions it is zero (causing eddies and force).

    A spherical front has no edges to catch. A sharp keel ridge aft will cause the craft to align with forward velocity.

    If you want to plane and control direction in the surf a flatish bottom (area proportional to weight) with significant rocker forward will steer with heel. Be aware this could cause a broach if you fail to control heel.

    I presume you don't want a tadpole shaped boat due to its performance at all times other than returning through surf. It seems you want a 2 man peapod type craft. All you need is to round the bow a bit (2 or 3 inch radius minimum) and put an oarlock on the tail -guide boat style.
     
  3. SukiSolo
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    SukiSolo Senior Member

    Tricky problem as others have outlined. One feature that is generally slightly misunderstood about marginal surfing conditions is it is not necessary to have long flat aft sections. The link below shows a dinghy that planes as well as any flat run one (but carrying more weight) and a side view would show straight rocker for the first 8 to 9 feet, then a distinct turn upwards, combined with a slight V'ing in the section. Remember this is for marginal to full plane and not going to 25Kn + stuff like 18' skiffs, there is a big difference.

    http://www.national12.org/class/Final Chapter.htm

    I would not suggest this shape at all but it is important to understand the subtlety of form for the design requirement. Some sort of compromise will be needed as every shape will broach but some boats are a lot better than others.

    Some of the old double enders are quite sea kindly, but are heavy and will not plane at all or surf. Probably need some of their properties combined with a more modern ability to surf under control. Something fairly moderate all round most likely. Be interested to see what happens here, some good input, please let us know where it goes.
     
  4. messabout
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    messabout Senior Member

    Take a look at the lines of the Monomy surf boat in the above referenced Coast Guard reference. It is a much bigger boat than you contemplate but the general idea is there. If made smaller it will be much like a Peapod which has proven very able in some very nasty conditions.

    Way back in the dark ages I was the coxswain on a Coast Guard Monomoy. I made numerous surf landings along the Jersey coast. The boat behaved admirably but then I always had an experienced crew who knew their business. Some, actually most, of the landings were done stern to. That gives the oarsmen and the cox a considerable advantage to straighten the boat if it begins to broach. (The Monomy is a double banked, 5 station boat with ten oarsmen and a cox with a very long and strong sweep oar.)
     
  5. Skyak
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    Skyak Senior Member

    There isn't much information on that link. What Froude numbers are we talking about here?

    I am a bit confused about the conversation. My experience with surfing small boats and control is that what is in short supply is acceleration to get on the wave. When a wave passes you (high drag boat) it is unlikely you will have control problems in a double ender with a skeg or rudder. Trouble with control is when you have a low drag boat that can surf faster than a big wave and it stuffs it's bow at the bottom. If you have such a boat the solution is simple, control drag off the tail to keep the boat on the face of the wave -high on the face if it is going to break.

    Are we still talking about a two man fishing boat returning to the beach? That is a very different requirement than a surfboard that has no other requirement than control on the face of a wave.
     
  6. sharpii2
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    sharpii2 Senior Member

    There are a few rules that seem consistent.

    1.) the vessel have full bow, so it can't dig in.
    2.) the vessel have an up swept bow, with no forefoot, so the bow will have a tendency to raise up as it moves forward.
    3.) that it have no deep chines, so it's easy to straighten its course, if it is hit by a following breaker.

    There are surf boats that break these rules.

    One is the coble.

    It has a deep forefoot and very shallow stern.

    I gets away with this because the bow is always facing the surf.

    When coming ashore, it is rowed or powered backwards.
     
  7. coopscraft
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    coopscraft Junior Member

    I forgot what the tanh function is. I'll look that up for the reread. Looks like the author basically says "if you try to cross a surf that is too extreme for your boat, you will get wrecked. Words to live by I should think.
     
  8. coopscraft
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    coopscraft Junior Member

    We are definately still talking about a 2 man fishing boat launched and rcovered fom the beach. I gather if we only go out in nice seas then a peapod or swampscotte dory would be managable under oars. The thing about oars is they offer control without movement. I could even reverse coarse to avoid broaching. A few have mentioned dragging a break. Would a 3 sq ft floating wood parachute on a 30 foot line make a big difference on 1000 lbs?
     
  9. coopscraft
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    coopscraft Junior Member

    I plan to update my other thread periodically until launch. Also I plan toshare a lot of detail as I go. Theoretically, someone could copy my design, but I'm not looking to make money so y'all have fun. (Fishing offshore in a rowboat)
     
  10. jonr
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    jonr Senior Member

    A drogue makes sense to me when you can't outrun the waves.

    As the references and videos show, the highest chance of broach occurs when traveling slightly slower than the wave. So either travel faster (preferred) or do something to slow down. A drogue does that while also providing a force to help maintain the boat perpendicular to the waves. See:

    Heavy Weather Tactics Using Sea Anchors & Drogues
    By Earl R. Hinz
     
  11. gonzo
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    gonzo Senior Member

    A drogue in the surf will create a dangerous situation. To start with, it will hamper maneuvering, which has to be very fast. Secondly, it will take one extra crew to deploy and retrieve. Thirdly, if the boat slows down instead of surfing (it is a surf boat) the wave will break over the stern and swamp it. The best lesson is to get in a boat and go play in the surf. We use to do that as kid and it was a lot of fun. Mostly, they capsized or pitchpoled because they were not designed for surf. However, we found that the person on the stern of a pitch-poling canoe can launch himself a long distance.
     
  12. jonr
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    jonr Senior Member

    In the boat I use in the surf, a wave breaking over the stern (or bow) is of no concern. On the other hand, getting sideways to a wave and being rolled is disastrous.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    The word "fraught" was specifically invented to cover the issues involved in rowing in through surf.
     
  14. SukiSolo
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    SukiSolo Senior Member

    I think Messabout's comments are close to what is required. Having done many miles under sail and oar in the old RN Montague whalers (27') in up to Force 8 they are pretty robust in a rough sea. However they don't surf, too heavy, but as long as you can bail them out they take a hell of a battering.

    There was a very good Dutch GRP boat in the early 70s' which is close to your requirements but a little to big ie 4 man, Unfortunately I can't find pics of the design. It was a joy to row, sea kindly and had built in buoyancy tanks. Designed to be rightable as well if it capsized. Shape wise it was a double ender and a cross between a rowing gig and a Dory. Moderate freeboard (for the sea) and designed to be carried/launched by 4.

    With respect to Skyak, the Froude numbers when broaching are not really relevant. The example I gave earlier were to show that it is not a long flat run that is required to initiate surfing necessarily. Yes, the numbers are important with respect to work done/ease of moving the craft, and I agree there. Getting volume in the ends helps lift bow/stern, but I would keep away from a semi circular midship as it is far too 'tippy' even if efficient in smooth water.

    Any boat will broach in the 'wrong' sea. There is a video of a RIB safety boat rolling over at East Lothian Yacht Club (Scotland) in a Force 2-3 because a local feature (sudden shallows) causes exceptional height waves at a certain tide state. Same video also shows a Laser sailing in and beaching perfectly at the same time. Key feature seems to be, keep the bow or stern at right angles (or very close) to the waves. In moderate surf, under 2.5' most small craft launch OK if kept bow on. Crew can normally either push from stern and jump in, or if two, one can get in earlier and get oars ready. By then the blades should be working on the edge of the surf and primarily in solid water with less rotation shorewards. Really big surf and long flat beaches are a bit different.

    Fortunately so far I have rarely been victim to a surf broach either on shore or with breaking offshore waves. Last time was 2010 when on the South coast UK the wave pattern (in one small area) was such that boats were supported in the middle only (ie ends out) at times and it was hard to maintain control in 30+mph winds. 9 masts were bent or broken that day as it caught out even the very best sailors. The solution was to avoid critical manoevering ie gybing within 150-200 meters of that wave pattern. By being more positive with the boat control the possibility of broaching was significantly reduced. There is little difference in approach with keel boats or rowing craft, mostly it is down to 'reading' the situation.
     

  15. coopscraft
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    coopscraft Junior Member

    moderate surf.

    These are calm conditions in my area. Would you grab a boat and go pitch poling in this? Notice the lack of wind and 10 foot waves. I was on the beach yesturday. To me it seemed exceptionally calm but I'm just starting to learn seamanship by the numbers. http://magicseaweed.com/Newport-Surf-Report/318/
     
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