The Kayak tender

Discussion in 'Boat Design' started by cthippo, Oct 11, 2010.

  1. cthippo
    Joined: Sep 2010
    Posts: 736
    Likes: 35, Points: 0, Legacy Rep: 465
    Location: Bellingham WA

    cthippo Senior Member

    Due to the fact that I already know I'm going to have tons of design related questions, I figured I'd put them all in one place.

    I'm looking to build a kayak tender (think destroyer tender rather than yacht tender) to live on while on trips to various places to go kayaking. Here's the SOR:

    Must haves:

    Comfortably sleep 4 people in 2 cabins for 7 days

    Trailerable, with maximum dimensions of 40' long, 11' beam, and 15' total height.

    Range of 1000 miles at 8-10 knots.

    Pure displacement hull

    Capable of being safely navigated in rough conditions (6'-8' seas) in an emergency.

    A "real" pilothouse with a centerline wheel.

    Space to carry kayaks on deck

    A minimal galley and a head with shower.

    Would like to have:

    A design in which the pilothouse is not also used as a hallway

    2 heads, 1 with shower

    2 engines

    Hull divided into watertight compartments

    A larger than minimal galley.

    Commentary

    What I'm looking for is a boat that I can go out to someplace remote and use as a base for kayaking from; someplace comfortable to come back to at the end of the day.

    The designs I've been looking at have tended to be the aft cabin trawlers such as the Little Island Trader, Power Cruiser 36 and Power Cruiser 39 from Chesapeake Marine Designs.

    Based on the features I like from these and other designs I'm working on a composite design tailored to my application. In the end I will probably end up buying a commercial plan and making modifications as needed, but I want to go through the design process as a learning experience.

    So, the questions that are already presenting themselves:

    Is there a standard formula for roughly computing the finished weight of the boat based on dimensions?

    How is anticipated waterline calculated?

    Is there a compelling reason not to put fuel tankage in the Lazarette? I see that advantages as being close to the engine room, ability to gravity feed, and that I've yet to see a boat get rear-ended.
     
  2. patiras
    Joined: Jan 2007
    Posts: 27
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    Location: France

    patiras Junior Member

    OK, I'll bite.

    I had pretty much the same design brief a few years ago. This is what I built, and these are reasons I came up with it.
    http://www.boatdesign.net/gallery/showphoto.php/photo/12840
    http://www.picasaweb.google.com/lh/photo/zLuMcadqkTvj8-NvM5Yu4g?feat=directlink
    (Couldn't get the picture links to work; probably me, but if there's an admin that could do the honours?...)

    One of the major considerations I had was the ease of putting a kayak onto the water and getting into it from the boat. Although I can get into a kayak from the side of a boat, having, for want of a better word, a floating pontoon makes this a lot easier. Given that this needs low freeboard for at least part of the boat means either fast draining or sealed deck and transverse stability. I also wanted something that cruised in the 8-10 knot range, economically. To me this said "catamaran". I've now used this boat for a couple of years with no problems, from transporting passengers, to acting as support boat for kayak trips, to carrying supplies. The 2 8hp motors give me about 13 knots flat out, but let me idle alongside the kayaks at about 3 knots. One run I do is a 35 mile leg at about 9 knots to rendezvous with the kayaks, then 35 miles back with them. This I do on 14 litres of petrol.


    Comfortably sleep 4 people in 2 cabins for 7 days. Mine is slightly smaller and I really only use it for overnights, but sleeps 3 quite comfortably; I can rig a tent over the front deck for more sleeping space.
    Trailerable, with maximum dimensions of 40' long, 11' beam, and 15' total height. Mine is 30 foot long. Bear in mind that build time/cost, running costs etc increase exponentially with length, so don't build any bigger than you have to.
    Range of 1000 miles at 8-10 knots. Just doing the rough maths; you want to do 7 day runs, and a 1000 mile range. At 10 knots that's 14 hours motoring a day, add 8 hours for sleep and that leaves you 2 hours a day to paddle. Do you really need that range?
    Pure displacement hull. Sure. I did compromise a bit on the hull shape. I have a 15:1 L:B, where as idealy I'd have pushed for 18-20. The reason for this is that I often carry a load equal to the displacement of the boat, and I didn't want the waterline moving around too much. Also the forward part has more rocker than I would have liked, which lets it run up a beach to embark people over the bow.
    Capable of being safely navigated in rough conditions (6'-8' seas) in an emergency.Part of my design brief was a certain "bulletproofness". Being a completely enclosed hull lets me stuff the front into a wave and the water gets shed in a couple of seconds with no risk of flooding and the cabin stays dry. The other advantage of building the hull structure completely enclosed is that it can be built insanely strong; all up with the motors it displaces about 1 tonne.
    A "real" pilothouse with a centerline wheel. OK.
    Space to carry kayaks on deck Why not put them on the roof? 4 sea kayaks is a lot of (dead) deck space.
    A minimal galley and a head with shower. Make the cabin a bit bigger, perhaps take the hulls to 11 metres.


    A design in which the pilothouse is not also used as a hallway Yep.
    2 heads, 1 with shower Just a case of deciding the minimum amount of space that all this needs, and then building the cabin to fit it.
    2 engines Yes. I wanted redundancy. Also, small engines are easier to get off and service, and weighing only 40 kg its quite possible to carry a 3rd as a spare.
    Hull divided into watertight compartments Having the hulls purely to support the cabin made it very easy to divide up into several compartments. I also have 6 solid bouyancy compartments that should provide a "get you home" capability in the event of several compartments being compromised. I also put kevlar grounding strips on the underside of the hulls forward. In hindsite, I should have been less of a cheapskate and put a couple of layers of kevlar under the whole hull.
    A larger than minimal galley Same as the heads and shower answer.

    Is there a standard formula for roughly computing the finished weight of the boat based on dimensions? No.

    How is anticipated waterline calculated?
    Funny way of asking the question. If its anticipated, then design to it; otherwise design the boat, do the weight estimate and work out the waterline(simpsons rule, or something like that when I was at school)....then change the design....and repeat (design spiral)

    Is there a compelling reason not to put fuel tankage in the Lazarette? I see that advantages as being close to the engine room, ability to gravity feed, and that I've yet to see a boat get rear-ended. Pass. I carry so little gas (2* 12 litre cans plus a spare) it makes no difference.

    All the best, Al

    www.trinitykayaks.com
     
    1 person likes this.
  3. cthippo
    Joined: Sep 2010
    Posts: 736
    Likes: 35, Points: 0, Legacy Rep: 465
    Location: Bellingham WA

    cthippo Senior Member

    Thanks Al,

    That's a very nice little boat you've built there. I honestly had not considered catamarans, so you've given me something else to think about. How long of overnight trips are you making on it? Are you beaching it at night or swinging on the hook? Are you operating on river or salt water?

    On the fuel, you're right, I probably don't need that much. The thought was to put in enough to make an Alaska trip with a 15% reserve, just in case I ever wanted to do that. Perhaps I could install two tanks and only fill the second one when needed.

    The maximum dimensions are what will fit legally. Currently I'm looking at around the 34' mark for an actual design. Seems to be the shortest length I can get everything in.

    Another question for someone out there. Are Kort nozzles practical and advantageous on a displacement cruiser? The reading I've done on them say they are only useful up to 10 kts and work best with heavily loaded props. The design I'm looking at will probably top out at 10 kts, but aside from offering some more protection to the screw, are they worth it? Is there a disadvantage? How close to the blades need to be to the inside of the nozzle for maximum benefit?
     
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