Catamaran Dinghy

Discussion in 'Boat Design' started by Jolly Mon, Jun 30, 2019.

  1. sailhand
    Joined: Jan 2017
    Posts: 95
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    Location: australia

    sailhand Junior Member

    The hulls are solid glass infused in a mold, 3600gm2 of solid glass. Glass to resin ratio is around 3 to 1. I took resin down to 16% of the laminate but it became slightly porous. The bridgedeck is carefully hand laminated in 400gm2 biax. The topsides are 10mm foam, the bridgedeck 15mm, pvc h80. The picture referred to in the previous post is of a very poorly and heavily built dinghy 4 metres long built in thailand by a very questionable expat who then copied the design and tried to sell it. The dinghy in the picture was mainly polycore and chop strand mat and weighed well in excess of 120kg. I have lived on my sailing catamaram since december 2003 and have never been able to find a decent dinghy. My design suits my needs and i am happy with it. It rides unbelievably smoothly in a 2ft chop or a 3ft chop or 6ft breaking surf and I have tried to capsize it on purpose in the surf on many occasions without success. I have driven it directly into dumpers in the surf and almost been washed out of the dinghy, I have turned sideways running down breaking waves and couldnt capsize it. The video was filmed from a friends jetty in a tidal estuary and therefore calm waters. It does not pound at all and anybody that has ever sailed on a small sailing cat will tell you that these types of boats do not pound at all. Jolly mon is correct about the use of tiller extensions in small vessels with limited displacement. With two people aboard the tiller is no longer required is this not similar to two rowing positions in dinghys depending on weight distribution. This limited displacement allows me to do nine knots with 230 kg of people and groceries in my boat, not 4 knots like the small rib we originally had and needed to replace after two years despite paying considerably more than the combined cost of my current dinghy and mold cost to build.
    The displacement is not an issue with reasonable bridgedeck clearance indeed it is actually a huge advantage in many ways. For boarding from deep water whilst diving etc it is brilliant. In the video you can see the transom steps underwater when I am standing on them, this means they are much easier to board than an inflatable when in deep water, it also means your feet stay clean when you board and the self draining deck makes the dinghy easy to clean. A bucket and a broom whilst floating around the ocean and in a few minutes you have clean decks. This means very little to most people however if you live on a boat keeping your decks clean makes life easier. I suppose the land based equivalent is wearing muddy boots into your house, most people dont like the mess it leaves behind. The wheel system is what most people ask about and has been installed on other types of dinghys not just my designs as wheeling your dinghy above the high water mark where there is 7 metre tides is a great thing.
    The original dinghy in this design was built in plywood and as we live in the tropics it soon became a rot box. It was fully encapsulated in glass and epoxy however in the rough environment of the cruising liveaboard it would sustain damage at dinghy docks, from anchors, engine parts, outboards etc etc and freshwater would enter the ply. Ultimately I became allergic to epoxy and decided to abandon ply and use vinylester. Pvc foam doesnt rot although it can delaminate. My dinghy has no bungs and the hulls are sealed. The dinghy is 6 years old now and has suffered untold abuse and never leaked. I am not sure what boatbuilding practises are like overseas but here in oz resin infusing and pvc foam are hardly high tech. There are many production boatbuilders using these techniques and it is common place here. The mold has so far produced 24 hulls and I believe there is a further 8 hulls on order. I think the mold was a reasonable investment given the number of dinghys built. Again I am not familiar with boatbuilding practises in the US or Mexico but using molds for production boats is common practise here.
    The dinghy was simply designed for my personal use and nothing more. The video was made about two years ago as i was sick of people asking where they could get one and my partner and friends suggested that as I retired at 39 maybe I should sell a few dinghys for fun as so many people asked about them. As you can see by the video we are not to serious about it in fact it would be fair to say I regret ever deciding to sell them in the first place and will be stopping soon. Dealing with a constant stream of emails is not my idea of enjoying life and as we try and spend a lot of time travelling overseas and sailing to remote locations we are often unable to access the internet.
    Anyway its definitely not for everybody and is quite specialised and fit for purpose, my purpose, and if it doesnt suit others thats fine. We all drive different cars, wear different clothes, eat different foods etc etc. The world is a wonderful place as a result of this diversity and we get to choose what we like and what makes us happy and hopefully do just that. After all to be happy is what life is all about.
    Cheers
     
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  2. Jolly Mon
    Joined: Jun 2019
    Posts: 16
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    Location: New Bern

    Jolly Mon Junior Member

    Sailhand, thank you for your post and all the information. I’ve been cruising on and off since 1999 and appreciate what you have said here.

    Rob
     
  3. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    A bit dated, but start here if you are new to composite construction. Marine design manual for fiberglass reinforced plastics : Gibbs & Cox : Free Download, Borrow, and Streaming : Internet Archive https://archive.org/details/marinedesignmanu00gibb/page/162
    Sailhand in his Jan 18 2017 post (edit for above update) said he used 2400 gm/m^2 for the un-cored hulls, 4800 gm/m^2 for the keels and cored the bridge deck with 15mm divinycell. Just looking at the hull side assuming a 50:50 glass:resin construction (i.e. no mat or roving) that means each m^2 weighs 4.8 kg or 10.58 lbs. As a square meter = 10.76 square feet that is 0.98 lbs/ft^2. Using your weight above, that gives an equivalent Okume plywood thickness of 0.036 ft = 7/16". Even assuming that the stated 2400 gm/m^2 is for glass and resin, 3/16" Okume is effectively the same weight after sealing and paint.

    Edit: X-post with Sailhand above but I'll leave it here.
     
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  4. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Mostly just agreeing with jhardiman and others. You can't beat plywood covered in a veil of glass for this job. I reinforced mine with foam inside the 6mm ply bottoms. You seem to have scads of freeboard. Mine is 9'6" and 84 pound when first build but I added some abrasion resistance and replaced the foam after several years of hard use. It's probably 90 pounds now. It was built from three sheets of ply. I have a four sheeter version also designed for 5 hp.

    TEOTWAWKI Boat https://www.boatdesign.net/threads/teotwawki-boat.42752/page-2#post-548067
    Laminating Polystyrene to Ply https://www.boatdesign.net/threads/laminating-polystyrene-to-ply.55053/#post-765747
     
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  5. Jolly Mon
    Joined: Jun 2019
    Posts: 16
    Likes: 0, Points: 1
    Location: New Bern

    Jolly Mon Junior Member

    Hi Phil, nice dinghy!
     
  6. Jolly Mon
    Joined: Jun 2019
    Posts: 16
    Likes: 0, Points: 1
    Location: New Bern

    Jolly Mon Junior Member

    Thank you for the reference, and thank you for thinking through the fiberglass vs plywood comparison. There’s a lot of food for thought there.
     

  7. sailhand
    Joined: Jan 2017
    Posts: 95
    Likes: 19, Points: 8, Legacy Rep: 29
    Location: australia

    sailhand Junior Member

    That is correct for laminates in the hulls. In the earlier post on this thread I averaged 2400 and 4800 to 3600. The surface area in the hulls has roughly even surface area between the keel panel and the lower sheer panels when carrying the heavier laminate around the turn of the bilge to provide added protection for the "business end" of the dinghy. This has proved unnecessary as the two pack polyurethane paint on the bottom of the hulls remains largely intact after years of abuse, this is largely due to the wheel system. As it is easier to install the wheels with 6 inches of water under the hulls i tend to not beach the boat very often at all as i generally use the wheels when going ashore. Prior to the use of the wheel system i wore through the thick laminate on our previous dinghy and actually repaired it twice during the time we used it. Dragging boats over sand is very abrasive on glass particularly the keel of v bottom dinghys in my experience. Now we tend to stop the dinghy in shallow water and install the wheels. My partner has a badly damaged hip from a car accident many years ago and i tend to reverse in to the beach nowadays to make it easier for her to disembark with my assistance. To this end I have small aluminium pads sikaflexed to the rear of the hulls on the keels. These serve as skid plates when on land with the wheels in as well as when I am beaching the boat. For a bit of added fun you can "skate launch" with the wheels in. This is done when we encounter high steep muddy ramps or banks in tidal areas at low tide and the dinghy above the high water mark. You align the dinghy with the bows facing down the ramp and load passengers, cargo etc. You then climb aboard and when ready to launch move your weight forward until you reach the balance point and off you go. This has led to some exciting launches at a fair clip. I some times "jokingly" liken it to the emergency lifeboat launchers on large cargo ships. Get in sit down hang on and hope for the best. So far it has been good fun with only one launch where we got stuck in the thigh deep gelatinous mud just before we reached the water, a bit of judicious poling with an oar and away we went.
     
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