Catamaran tender to replace rib?

Discussion in 'Multihulls' started by DennisRB, Jul 4, 2014.

  1. brian eiland
    Joined: Jun 2002
    Posts: 4,964
    Likes: 188, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    The little Unicat looks interesting for a vessel of her size, but I wonder how he gets those flat top seats and flat floorboards in a 'V' bottomed vessel?
  2. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 171, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    They still do this today. A 4 and 6 are the same engine, a 15 and a 20, a 30 and a 50 etc. But it isn't a quick change from one to other. You have to change gaskets, timing etc. But it does mean that a 6hp costs the same to build as a 4, yet costs us a lot more to buy

    I asked a technical rep at Evinrude why their new 15hp E-tec was so heavy. "Because it is exactly the same engine as our 30" was the reply


    He said that people use that size engine as a "kicker" and think that a 30 is "too big" but a 15 is "just right". Only Evinrude don't make a 15 e-tec, so they rebadged a 30 as a 15. Everyone is happy, even the gullible kicker buyers

    Richard Woods
  3. brian eiland
    Joined: Jun 2002
    Posts: 4,964
    Likes: 188, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

  4. nzclipper
    Joined: Mar 2011
    Posts: 34
    Likes: 3, Points: 8, Legacy Rep: 18
    Location: Auckland

    nzclipper Junior Member

  5. waikikin
    Joined: Jan 2006
    Posts: 2,390
    Likes: 153, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

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

    DennisRB Senior Member

    Waikikin. That first plastic cat is 55kg which is about 20kg heavier than my inflatable. But at 55kg its lighter or comparable to most peoples ribs. I would like to view one.

    Re outboards. There was a reason I specified the tohatsu 8- 9.8 2 stroke. These motors are the most powerful small motors available (I already have an 8 but would like a 9.8) The frame size of a regular 8 hp usually spans from something like 4-8hp. So the 8 is the most powerful for the weight, however tohatsu is the only manufacturer that gets 9.8hp from this frame size. Other OBs of this size are downtuned 15hps. Likewise the tohatsu 18hp is a tuned up 15. So both the tohatsu 9.8 and 18hp are the most powerful motors in their weight class. A tohatsu 9.9 is a tuned down 15 which is the same one available in 18hp! Those guys buying the tuned down 30hp etec might not be so gullible after all as long as they don't care about weight. They are getting a 30hp for the price of a 15!

    When I say fast speed for exploration, I am not sure on exact speed, but however my current 10" rib goes with my tohatsu 8hp. I guess 15K plus?
  7. Manfred.pech
    Joined: Apr 2010
    Posts: 599
    Likes: 82, Points: 28, Legacy Rep: 319
    Location: EU

    Manfred.pech Senior Member

  8. catsketcher
    Joined: Mar 2006
    Posts: 1,315
    Likes: 165, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Cat dinghy

    Gday all

    I have finally got away cruising again for the year. Our old dinghy was looking pretty bad and so I was looking for a replacement. A friend had a catamaran dinghy mould (it was the typical 8ft cat dinghy mould stretched to 10ft). I built the dinghy and we have used it for about 3 months now.

    I was not really a fan of cat dinghies as they are harder to row but by no means impossible. We have long oars and ours rows well enough. I should glue some foot rests into it to help power along. It has a planing shape and so stops pretty quickly when you cease rowing. One major reason I built it was that we are having lots of friends come with us and it is excellent at coping with off centre loads when newcomers don't step where they should. But after a month or two I became a huge fan of the LONG cat dinghy.

    It is also great when collecting anchors up a mangrove creek (I tied Kankama in a spiderweb fashion and pulled all the lines and anchors aboard easily without any stability problems), taking a whole load of people - 4 adults and 2 kids to the beach (everyone on a seat - no-one hanging their bums on the side as in a RIB). We even rescued a turtle for a nature conservation group and myself (85kg) and my 95 kg cousin got a 100kg turtle into the dinghy over the side in the Whitsunday chop with no problems. It took a while to convince Craig we could pull the big reptile in over the side but the dinghy was fine with 285kg mostly on the windward side.

    With just me in the dinghy it goes 8 knots with a 3.3hp on it which is plenty fast enough for checking out channel approaches and scouting around. An 8 or the Toe 9.8 is all the weight I would put on a slightly bigger version. I would measure the back of the cat and make it as long as possible. Ours could be another foot longer and I will do this if I ever have to replace it. (I would consider taking the 3.3 even if you have an 8 as it is easier to use the 3.3 most times and have the 8 for when you need it. I have seen a few boats with two outboards for the tender.

    After building and using this boat I feel the perfect cat dinghy would be made from a foam like extruded polystyrene (or something cheap but thick) about 30 - 40mm with a light 400gm laminate on it. This should give the dinghy heaps of flotation. It should be about 10 ft or longer and have high sides. Keep the tunnel low so that with a load on it it picks up immersion rate quickly. The stern sections should be straight so that it planes easily. Ours just fits our duckboard and to top things off the flat cat dinghy gunwales are perfect for storing our 5.8 and 5.3m metre kayaks on top of it. If I lost it I would build something almost identical.


    1 person likes this.
  9. rogerf
    Joined: Apr 2012
    Posts: 58
    Likes: 3, Points: 8, Legacy Rep: 32
    Location: port stephens

    rogerf Junior Member

    1 person likes this.
  10. waikikin
    Joined: Jan 2006
    Posts: 2,390
    Likes: 153, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    Phils comments reflect my experience cruising with a cat dinghy, ours was alu, the stern sections are important, I traded some rocker/lift in the buttocks to get a sweeter rowing boat... wasn't worth the trade off for powered use though & next time I'd make it strait & maybe make the tunnel sides parallel.

    1 person likes this.
  11. bruceb
    Joined: Nov 2008
    Posts: 1,249
    Likes: 44, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    Cat bow?

    Phil, what shape of bow did you use on your cat dingy, and about how deep/wide are the hulls below the center floor?
  12. DennisRB
    Joined: Sep 2004
    Posts: 1,270
    Likes: 27, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Phil, that sounds like a perfect candidate. How heavy is is? I have a lot of room to fit a huge dingy, so that would not be my limiting factor. I also have a 3.3hp and a Toh 8hp. Its good to know that you can do decent speed with the 3.3 as well. I would use the 3.3 (or planned electric) most of the time and only fit the bigger engine when its actually needed which would probably be less than 15% of the time.
  13. Barra
    Joined: Feb 2014
    Posts: 94
    Likes: 4, Points: 8, Legacy Rep: 32
    Location: Perth

    Barra Junior Member

    why a catamaran

    I cruise a catamaran , but as a tender the catamaran has many down sides.

    for equal LOA and BOA,the catamaran will be heavier (more surface area), deeper draft(the ability to heel a mono onto a chine to drag it over sand/mud bars has proved invaluable over the years, 50 mm makes a big difference), more
    windage/ wave resistance/wetted surface (harder to row) . A pain to move on less than ideal shores because of limited rocker. Front profile will make it nasty to tow., when retrieving anchors from mangrove creeks its necessary at times to use the heeling ability of the dingy to roll out a secondary bower anchor stuck in the mud (ie hook chain over gunnel then move body weight to opposite side of dingy to lever out anchor.

    Quite obviously theres no such thing as the perfect tender, just some suit your cruising better than others. If your intention is to cruise isolated areas far from help then a dingy that can't be rowed any distance in 20 knots of breeze will probably eventually kill you.

  14. sailhand
    Joined: Jan 2017
    Posts: 112
    Likes: 27, Points: 28, Legacy Rep: 29
    Location: australia

    sailhand Senior Member

    cat dinghy for tender

    hi DennisRB are you still looking for plans for a cat dinghy to use as a tender. I might be able to help you out. I have a 3.5 metre self draining cat tender that I use on my 44' cat that I live on mainly in qld but currently in brisbane waters, NSW. ( i know its strange Brisbane waters is near Sydney or Gosford to be more precise ). contrary to a lot of claims on this forum my little dinghy planes quite well thankyou with myself (109 kilos) and my partner aboard with a three horsepower yammy. the hulls are resin infused and based on sailing/displaning (if there is such a thing) hulls that I designed. the bridgedeck is 15mm divinycell and the boat weighs about 45 kilos. I spent about 5 years in development and planning of the dinghy, including running many different designs through various hydrostatic computer modelling software and then built 7 sets of full size hulls which were all tested with a 3 5 and 8 horsepower motor and the best hulls used for the current design. I have lived aboard my cat for 12 years and my dinghy obsession has kept me entertained for much of my spare time. the dinghy has an amphibious feature also with wheels that mount on the centre of gravity via a very simple means and this has proven extremely effective for the cruising lifestyle. access to and from deep water is achieved by simply pushing the transom under the water and sitting on the hull. I then remove fins mask etc and simply stand up and walk in. I drive nearly everywhere standing up and have done for many many years without any major mishaps at all and it gives me a wonderful perspective especially around the reef areas and shallow clear water. my cat stays cleaner because my tender is selfdraining and easy to keep clean and I board over the transom having washed my feet on the submerged hulls at the stern, works great!!

    Attached Files:

    sailhawaii, waikikin and DennisRB like this.

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

    DennisRB Senior Member

    Good work! I love it. Can you please share more details? Beam, payload? Speed with various engines and persons aboard? How many people can you fit on board in choppy conditions etc? I don't even have boat anymore due to separation, but I still find this interesting.

    Are you planning on selling plans?
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.