Catamaran tender to replace rib?

hi dennis this dinghy is 1.85 metres wide and 3.5 long. another that we built is 1.95 metres wide and definitely better. the payload is a good question. I used to have bungs in it on the transom and I have had seven aussie adults in the boat going back after a beach potluck to do the milk run and drop every one off, calm water in barge mode ie bridgedeck in the water. I always water up my boat with the dinghy and thats usually three to four hundred litres of water plus me no problems, without bungs my feet might get a bit wet with that sort of weight but its no dramas. stability is brilliant if youve ever been on a hobie its pretty much the same due to the wide stance of the hulls. there is about a foot of hull below the bridgedeck and with two up it has about 6 inches clearance. I usually run a 4 yammy these days so i dont have to modify the prop on the three, I use a 7 inch pitch off a 4 yammy cut down to about 6 and three quarter inch diameter on the 3 yammy. it is a much better prop than the standard three and by reducing the diameter I increase the slip and let the motor rev to where it produces its max hp. max speed on gps run in opposing directions in the mary river at slack tide and averaged one up was 12.3 knots with the three and 10.1 knots two up with the three. that was in the first one I built completely in foam. the new ones have solid glass hulls and are really tough as they are infused. there are no bungs or inspection ports in the hulls and they have never had any water in them despite three years of really hard use as a tender. the wheels you see in the pic inside the dinghy are the beachwheels 42cm available from beachwheels australia based on the goldcoast. if you look carefully in the pic on the waterline on the port side near the middle lengthwise you will notice a small hole this is where the wheels mount. they just sit there no clips or bolts required. just push in pull out no problems. I can leave my partner in the boat and wheel her up the beach as she simply moves her seat to the centre of gravity above the wheels and away we go (she has a broken hip from a car accident and this makes it easy for her to get in and out) I have attached a pic of the grandson waiting to go for a ride sitting in the prototype dinghy about a 150 metres from the water, I wheeled him all the way no probs at all.

 

a couple more pics the admiral climbing aboard mark1 and me confused about the directions, it does the same speed in reverse.

 

the admiral

 

dinghy is amazingly good in a choppy sea "no bang or slam at all!!!!!!!" it rides like a big soggy carpet as the hulls are fine enough to go through it as they have a reasonably fine beam length ratio similar to my big cat. I have gone past a 17 foot centre console tinny with three guys getting pounded in it to windward half off the plane as they where getting belted whilst seated in my milkcrate unfixed seat tying a new leader on my line with the throttle friction and steering friction done up hands free. their collective jaws hit the deck as I waved and went straight past them with the mighty three yammy and my feet on the gunnels as if I was lounging in a recliner at home but I was in about a two and a half foot chop doing ten knots. you really have to go for a run to appreciate it. it is sort of displaning as peter brady would say except when you go backwards then it planes. not too much cavitation or more accurately ventilation with the current setup maybe once every couple of months but I do tend to anchor in calm waters like hill inlet or island head creek. its achillies heel would be a stiff chop on the beam at exactly the right frequency to cause each hull to be on separate waves and the prop in thin air. it has happened a few times and you just run off the rhumb line about 20 degrees and tack your way home. I have tried to capsize it in the surf whilst wearing a life jacket with friends keeping an eye on me and what I thought was going to be really dangerous activity turned out to be fantastic fun and a great day. I was running across the face of dumpers like I was surfing the tube getting hit chest high with waves whilst standing and getting knocked off my feet but I couldnt flip it, Im sure it could be done but it would be bloody difficult. I ran down the face of breakers and turned 30 degrees and slowed down and it just surfed all the way in I tried every stupid thing I could but to no avail just a really wet fun day. it has a strange action in any sort of a sea state like a wiggle sort of thing but definitely no slam. if you talk to guys that sail off the beach cats they will tell you they dont slam even at speed, they do flip and come to a screaming halt when they punch it in but at speed even to windward they dont slam!!!!. the dinghy generates most of its speed due to the flat keel panels which seem to generate hydrodynamic lift at speed as clearly evidenced in pictures I have taken. I think its a bit of a feed back type loop more speed more lift, more lift more speed and so on it seems to be fairly efficient. I think it starts out displaning , if there is such a thing and gradually just starts to generate lift to the point where it is more planing I think. anyway I dont really care it works for me and that"s all I"m worried about mate. attached a photo of infusing a hull the first guy on the left in the pic is bob oram the guy opposite is mal salisbury, his family owned the infamous peachfarm in sydney where all the early crowthers where built and his father, bill salisbury, who is 92 and still sailing and for that matter scraping and antifouling his own boat, built what is believed to be the first sailing trimaran built in australia. they built and sailed pennant and made the molds for all the early crowthers. I think bill has a couple of brisbane to gladstone wins under his belt. theres multihull history in that family thats for sure. mal owns the shed where we did the latest dinghys.

 

Great boat! do you have any plans available?
What does the bare solid glass hull weight?

 

As the hulls were infused they were reasonably light. There is 2400grms of glass in the hulls and this is overlapped in the bilge giving 4800 grams of glass in the keel. They weighed between 8 and 9 kilos each but could be made much lighter. I made mine particularly tough as it gets abused every day. As it is my main form of transport it gets used as much or more than your average family car. I have lived on my boat for 12 years and use the tender daily. I dont use marinas at all if i can help it. The last time I was in a marina was 13 months ago as i had family visit for a week at Christmas with two toddler's . At this stage i am not selling plans however i may look into it if there is enough interest. To build the boat as a one off i think foam core is the best way to go. We have built one from ply and western red cedar and it is very heavy around 160 pounds. The deck was 16mm western red cedar which is way too heavy. The foam version we built for around a 100 pound if your really scrupulous in your build. If you want the full self draining deck then 100 pound is achievable and you really dont need particularly heavy laminates as you use the wheels all the time on the beach or shore etc. My current dinghy with the infused hulls and foam tub weighs around 45 kilos (100 pounds). It could be another 10 to 15 pounds lighter though with lighter laminates and thinner foam. We did one with infused hulls and a foam tub that had lower sheer panels only about 6 inches above the deck and less glass in the hulls but slightly wider overall. It was around the 40kg mark and you don't really need the higher sheer like mine it just helps with the illusion of greater security. In Australia i could make the hulls available as i obviously have a mold to infuse the hulls. To have the full self draining deck we tried both building the boat and then decking in over the hulls to achieve a flat self draining deck and we also built a flat bottom punt and glassed the hulls onto that. We found the second method to be much quicker.

 

Hi Sailhand. Very interesting your research with seven sets of full size hulls. Are you willing to share your results of hydrodynamic research? As this is very rare here I would like to learn more of your experience.
If you are interested, there have been threads here: http://www.boatdesign.net/forums/boat-design/6m-low-power-10-hp-cat-55956.html
and http://www.boatdesign.net/forums/mu...new-multi-50-trimaran-46659-2.html#post622246
and http://www.boatdesign.net/forums/boat-design/new-hobie-design-49494.html Regards, M.

 

If you sell plans, I am interested.

 

Here's a friend checking out the 3 yammy's performance, he's sitting on about ten knots. Its no outright speed machine due to the hull shape. Its optimized for around the 10 to 12 knot speed mark and with an 8 hp it did 15.4 knots when we ran a 15hp on it she only did 15.7 knots and threw spray everywhere and squatted at the transom. Its really quite amazing how rocker creates an easily driven hull like a sailing boat up to a certain speed and at the same time puts the brakes on once the maximum speed for that hull shape is achieved. Nearly double the horsepower for a 0.3 knot speed increase gives you some idea that your near or at the limit without getting a ridiculous amount of horsepower on the back. It was optimized for 4 horsepower which is what our state laws mandate if you want to use your tender for fishing diving etc. It has other advantages though, the motor is cheap and light and good on fuel and it has 360 degree steering which is handy around the mother ship and dinghy docks. It also rows really fast for a dinghy of that size and it has beaten some super light dinghys designed for rowing under oars. I used to row it daily until an arm injury forced me to stop.

 

Thank you for sharing. Respect!

Looks very good. The position is well balanced. The wake is moderate for the speed. There is no stagnation point at the bow. The bow wave is low as the wave valley between bow and stern and so the suction is low and thus resistance.
There is no sign of wave interfernce under the bridge deck and hardly a rooster tail or a stern down sign. Little spray behind the engine amazingly modest for this length of waterline. Have not seen it better from a small boat before.

 

Thanks manfred for your kind words i am glad you like it

 

Wow, Sailhand!
Very impressive.
More pics would be great, too.
As would details or plans.
Build any bigger or longer versions?

 

Gday Sailhand. You said Bob Oram had a hand in your tender. Well on the weekend I was aboard the Bobs old Mango. Thinking about buying it if I had a little more cash.

The relevant part to this thread is Mangos tender. I think Bob built it, and if he had a hand in your tender design it seems no surprise. Take a look. Its very similar to yours and the current owner says it performs similar to the way you described yours to perform.

 

Hi dennis bob had no hand whatsoever in my tender design.he is one of my best mates and we do discuss boats 98% of the time. The first time he ever saw my dingy was when he came over to check out the infusion project. We cruised in company for a while last year and bob actually took my dinghy for a spin with his/my tohatsu 9.8. It is way overpowered with that motor. How is paul did you buy mango. The dinghy you saw is completely different than mine. It has your typical straight power hulls. My hulls are ametre longer and closer to a sailing hull than a power one. I spoke to paul about a week ago we have known them since they bought mango. If you buy his boat he might buy mine and i can get my canal boat in Europe. That dinghy was built by bob and its your typical 3 sheet ply boat. Cheap as chips and goes ok. We had bob oram his wife and teenage son myself and my partner in that dinghy out at the reef north of musgrave 13 years ago just after i did my master 5 at bundy. It handled it no probs. Actuall it was brian off schools out who told me about his hundred kilo son with a three hp on an off the beach cat that first got me thinking. Ask brian how it handles a chop. He ran it into about twenty knots of breeze with tide against us and full of fishing gear (about 5 rods throw net battery and sounder two tackle boxes etcetc, i know ive got problems ask brian) he still did 9 knots with the three i think. He was in his dinghy and we were running side by side in a chop, he has a twenty hp four stroke on that dinghy and he was getting a pounding. Ask him what he thought of it. He wont build one i dont think but he spoke about it many times. When he used my wheel system for his boat he solved most of his problems but i don't think he would go that slow. 10 knots is too slow for him. You could do 15 in mine in factit was brians 15 we put on it at cid harbour that was funny. I went first and nearly went out the back of the boat when i hit the throttle i went back in and swapped with brian and told him to be careful and use a quarter throttle max. He did the sama as me and we had a good laugh on the beach watching.

 

Actually denis the shot of me standing in the back of the dinghy at the beac is at northwest island and the two cats yo can see out the back are schools out and mine.that was a day or two before they caught their second marlin for the year.