Roof Box Boat - Roof top Nirvana - Missions Boat

Discussion in 'Boat Design' started by Skint For Life, Jun 21, 2014.

  1. Skint For Life
    Joined: Jul 2010
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    Skint For Life Junior Member

    RWatson, Thanks for that although I think the Sealegs would be far too heavy for the roof of my 4x4. Also I bet they cost far more than I'm looking at spending.

    BobE, Thanks for all those details, The aluminium Jon Boat is far longer and heavier than I'm looking for. Being Sea Worthy is a requirement for me, I should have put that in the original post. Seeing as overloading is a possibility I think I'll need something with plenty of freeboard. I had wondered about how light a boat with few panels could be made, it seems to me (no experience) that a boat with more chines and angles would be stiffer for given weight than a boat with fewer panels such as a jon boat. In this respect perhaps a catamaran or tri-hull design may be sensible. Can anyone shed light on this? I'm guessing that: More chines = More stiffness. Less chines = Less wetted surface area. ? Thanks to the link for the Meyers Canoe, I don't think a canoe is going to be very suitable for the criteria I have.

    Ike, regarding the canoe with bumpers on the outside made of poly foam, you say as they become immersed they add buoyancy and stability, something I've wondered is, surely it wouldn't matter if the bumpers were poly or the same material as the hull, as long as they displace water with the minimum amount of weight they'll both be just as effective? Along this line of thinking, what about a Mono hull with large bumpers or wings above the normal water line? Sort of like: http://www.walkerbay.com/dinghies-sailkits/rigid-inflatable-dinghy/ but with the bumpers larger and part of the hull material. This way it would have a low wetted surface area when motoring but be more stable to enter and exit over the side for scuba. Another advantage would be that the more heavily loaded it becomes the more the bumpers are submerged giving more buoyancy (If that makes sense).

    PhilSweet, Thanks for that info. 225lb = 102kg which is enough to load a wet 73kg diver without catch bag or tank. Yes I figure with a boat of the size I'd like that the position of the second person in the boat will be far more critical than the small RIB dive boats I've been in before.

    Portacruise, Yes I mentioned Lightweight twice as it's a relative term and I wanted to specify the ways in which it needs to be "Lightweight" i.e. One person drag, Two person lift. Could a Cat, Tri or flat bottom boat also be considered for shallow draft and the rest of the criteria? I believe inflatable does not allow enough internal volume for the two scuba crew. I think a folding boat is cool but I don't like the lack of built in buoyancy, added setup time or the fact that it can't really perform it's job as a roof box. The scuba ladder did not look very convincing, the vessel didn't look all that stable considering the water is dead calm. Often we pick up divers in 2m swells on reefs. The structure conforming rather than damaging is very appealing.

    TO ALL: I searched some small outboards and found some have the same weight even though the horsepower increases, is this normal or do the sellers have the specs wrong? Perhaps they run the same housing and block and all that changes is piston size. If so surely I'm best to go for the most HP for a given engine weight?

    Thanks to everyone for their help and advice :) I can't wait to find out what the best option will be.
     
    Last edited: Jun 23, 2014
  2. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Comments in your message copy below.

    Hope this helps.

    PC

     
  3. Skint For Life
    Joined: Jul 2010
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    Skint For Life Junior Member

    Devil Cat 2.2m
     

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    Last edited: Jun 26, 2014
  4. Skint For Life
    Joined: Jul 2010
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    Location: CHCH, New Zealand

    Skint For Life Junior Member

    Lamoore 8 Foot Tri-hull

    Livingston catamaran dinghy 7-9 foot

    Stabilized Mono
     

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  5. Skint For Life
    Joined: Jul 2010
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    Skint For Life Junior Member

    Portacruise, thanks for the info on how light the PB is. Good to know that flat bottom is out. We use a 3.5m inflatable with alloy hull, it's quite heavy (4 person lift). For scuba it's good to enter and exit (over the side). I believe a RIB shrunk to my target dimensions 2.2x1.3m would have far too little interior space for two scuba divers. Can you please link me to the video of the Japanese rescue diver? To be clear I didn't mean right on the rocks of the reef, I mean the swells peak up to 2m as there are reefs just under the surface. Thanks for the Disclaimer you seemed like such a big PB fan that I wondered if you had a commercial link. I may have already been in a PB, I went on a holiday to a lake and we sailed a friends folding boat which looked very similar. It had leeboards. I had noticed that 2 stroke were lighter than 4, but I'm talking about a 4hp and 6hp 2 stroke, both 21kg. Trade off being economy? Having to mix fuel? We have a 2 stroke on our 3.5m boat above and it uses very very little fuel. Thank you for your input :)
     
  6. portacruise
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    portacruise Senior Member

    Hope this helps.

    PC
     
  7. Skint For Life
    Joined: Jul 2010
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    Location: CHCH, New Zealand

    Skint For Life Junior Member

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  8. Skint For Life
    Joined: Jul 2010
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    Location: CHCH, New Zealand

    Skint For Life Junior Member

    I went and checked out the Norski 2.3m x 1.2m Dinghy - I think in the photos it says the model is "cuddy" I notice it's advertised as 2-3HP on Trademe but in the store it said "Max 4HP".

    It looks like a good boat, I don't know how easy it'd be to lift and carry for one person, although it's only 26kg it's probably cumbersome and holding it while walking up a rocky shore with shore winds could be interesting. For two people I'd say it'd be a breeze and you'd be able to carry it a fair way.

    It's so hard to grasp how capable it would be for my intended purpose, it looked big in the store but I'm sure it would feel pretty small in the ocean. I don't have any load capacity info on it so I don't know if my goal of 2 scuba divers is achievable. I wonder if it would have the freeboard to deal with ocean conditions.

    I'd really like to try one in the ocean but I don't know of anyone with one, I don't know how many there are in circulation, I wondered if I should call the local sailing club to see if anyone has one as a tender to their yacht. Another thought I had is that I could make a cheap stitch and glue cat or tri similar to the designs I've shown to test out the suitability of this type of boat for what I want, I'd use cheap cheap plywood, epoxy I already have and not worry about finish or durability as it'd only be for testing. I could use it to learn what works and what doesn't about the boat, it seems the cheaper option. Rather than spend $1300 on the Norski only to find out it doesn't have enough freeboard, stability, load capacity or whatever.

    For use when scuba diving I'd consider having long fenders that strap to the side of the boat like Richard Woods's Duo: http://sailingcatamarans.com/index....ats-and-dinghies-/420-duo-10ft-sailrow-dinghy Although I haven't seen fenders that long and narrow in NZ.

    I've wondered about built in buoyancy, boats have to have it now apparently, but what are the requirements? Just that the boat shouldn't sink? Or should it not sink with a certain payload? What about the engine?

    Thanks in advance :)
     

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  9. Jim Caldwell
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    Jim Caldwell Senior Member


  10. Ike
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    Ike Senior Member

    The areas under the seats are more than likely filled with foam. On a small boat like that it may be enough to make it float level. Small boats like this are required to have enough flotation to float the boat in an upright fairly level attitude when swamped. The amount of flotation has to be based on the boat weight, the engine weight, and the persons weight. The engine weight would be about 30 lbs for a 2 or 3 hp engine. However a 4 would bring that up to about 50 lbs. The amount for the boat depends on the material tha boat is made of. Some materials are naturally buoyant, such as wood. Persons weigh significantly less in water and have some natural buoyancy so there would be a small amount of flotation for the persons. See http://newboatbuilders.com/pages/flot2.html if you want to see the exact requirements for the USA. The requirements are almost exactly the same for Europe, Canada, and anyone who uses ISO standards.
     
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