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Aluminum 12' twin sponson boat

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  • #16
    Glad to help.

    On your engine placement- the lower unit has more drag than the hull. The more of it you can get out of the water the better. Ideally, the cavatation plate above the prop should be even with the bottom of the hull for starters. After you get it running, start moving the engine up a little bit at a time until it cavitates on start off, or won't get on plane due to the prop cavitating.

    Not exactly a fair example, but on the hydro, its worth about 5 mph per 1/2 inch I can raise the engine to that maximum point. On mine, it does have a surface drive propeller and the center of the prop shaft is just a fraction below the bottom of the hull. At speed, only the bottom half of the propeller arc is in the water. Raising the engine from the starting point to where it is took the boat from 65 to almost 90 mph.

    I doubt if that sort of speed is what you're shooting for, but the principles are the same.

    .
    "Gone are the days of wooden ships, and Iron men.
    I doubt we'll see either of their likes again".

    Circa 1920.
    Author:
    Unknown US Coast Guard unit Commander.

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    • #17
      That is neat information. I have been designing an adjustable engine mount as I worked on the tubing today. I think I have a general plan. I am only working with a 40 hp merc two stroke so speed will likely be in the 30 - 40 range. Weight will be between 550 and 650 pounds solo. What would you guess for speed ??? Pontoons are flat bottom and 24" wide each. I have seen v-hulls go 30 at higher weights with a 40 so I am hoping for 35 - 40.
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      Grizzly 12x36 lathe 2010
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      • #18
        Originally posted by Doug Doty View Post
        What would you guess for speed ??? Pontoons are flat bottom and 24" wide each. .
        Its hard to guess, but 30-40 is quite realistic. A drawback of flat sponsons is the area in the water, but then again, you'll be running over the top of it, not so much plowing through and pushing it out of the way.

        Something that will help is when you deck it, seal over the tops of the sponsons. You'll sort of approach the effect of a tunnel hull. Tunnel hulls trap & compress air under them***, helping to lift the boat up out of the water--- reducing drag. Water drag is something like 1,200 times more resistant to (forward) motion than air drag.

        ***
        This is the 'ground effect'. Remember the sensation a airplane has just before it lands? That's the ground effect compressing air under the wings and trying to push it back up instead of a continuing a uninterupted glide slope onto the ground/runway. When the pilot feels the ground effect taking place, he has to literally push the plane down onto the runway or it'll ride along a few feet over the ground on the ground effect forever.
        ***

        Using the ground effect is how/why a 'pad bottomed' tunnel hull, or the most extreme case of a hydroplane, can go faster for the same horsepower & weight than a V bottom or common rounded bottom boat.

        The decks of a hydroplane are airfoils. The top of the deck generates lift like a airplane wing, while the bottom of the hull traps air to push it up also.

        The idea is to get as much boat up out of the water as you can, and reduce any drag in the water as much as possible.
        (this is also why properly locating your center of gravity is so important)


        Hey,, how wide is this thing?
        The 90 degree angle on those sponsons will REALLY grip the water in a turn, rather than 'squish' through it like a regular boat. If its not pretty wide, and your body weight sitting fairly low,,,,,,,,, it'll be super easy to turn over.

        .

        .
        Last edited by Winger Ed.; 05-28-2007, 10:34 PM.
        "Gone are the days of wooden ships, and Iron men.
        I doubt we'll see either of their likes again".

        Circa 1920.
        Author:
        Unknown US Coast Guard unit Commander.

        Comment


        • #19
          Beam is 76" and the seats will be low, about 3" above the top of the pontoons. Will this design corner like a typical boat and lean into a turn or want to dig in on the outer edge and lift on the inner edge ??? I have considered making the pontoon mounts adjustable so I can rock the pontoons down in the middle to create a little v-hull effect if cornering is objectionable.
          Miller Dynasty 300 DX TR, 2007
          Miller MM 251 w/30A spool gun, 2005
          Lincoln SP-100, 1990
          Hypertherm PM 45, 2014
          Scotchman 5014CM Ironworker 2005
          Grizzly 12x36 lathe 2010
          Wilton Belt sander and Cutoff Bandsaw 2006

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          • #20
            Originally posted by Doug Doty View Post
            Beam is 76" and the seats will be low, about 3" above the top of the pontoons. Will this design corner like a typical boat and lean into a turn or want to dig in on the outer edge and lift on the inner edge ??? I have considered making the pontoon mounts adjustable so I can rock the pontoons down in the middle to create a little v-hull effect if cornering is objectionable.
            What the heck you doing up so late?
            don't you young guys ever sleep?


            Your width is great.
            Much wider and you'd have to build a sideways tilting trailer like they carry the 30-odd foot long unlimited hydros on. (that's why you see hydros going down the road carried on trailers at a 40-something degree angle.

            My 16 footer is 84 inches wide at the outside edges of the sponsons. Whenever I'm going down the road and see a State Trooper,,,,,,,,,, I hug the shoulder, and pretend not to see all the gravel and rocks being kicked up .... LOL..

            To answer your question,,,,,,,
            With the sponsons having all those sharp/90degree angels---

            Yeah.........
            Your boat will corner like a (choo-choo)train if you're going above about 20mph.


            For example--------
            In a HARD/HI-SPEED Lt hand turn:
            the hull will (try to) tilt to the right.
            That causes the outboard edge of the stb. sponson to dig in,,,,,,, anchoring itself in the water as it tilts. And yes,,, the inboard edge of the Port side sponson will try to help as much as it can.........

            This effect will do two things--- 'put on the brakes' & stop the boat, and----If you're a Baseball fan--- try to throw you into Center Field.


            .
            Last edited by Winger Ed.; 05-29-2007, 02:35 AM.
            "Gone are the days of wooden ships, and Iron men.
            I doubt we'll see either of their likes again".

            Circa 1920.
            Author:
            Unknown US Coast Guard unit Commander.

            Comment


            • #21
              With regard to the prop and the engine position, basicly two style of props. the cleaver or flat back edge (surface running) and the other rounded sweeping blade. As you lift the engine higher you will need more of a cleaver style propeller. Helps reduce the cavitation.

              On my ski boat, (hydrostream viking) I run what is called a high five by merc, it is a 5 blade cleaver style prop. the only way I could get the boat out of the hole. A guy actually told me to lower the engine when I was trying to prop it for skiing. tried about 20 props before it we found the right one. For speed, run a high pitch long sweeping blade. The best one I had, before it threw a blade, had a second gear when the boat got up out of the water. The more blades the more the prop will hold the water. with the 5 on it, backing off the throttle is a kin to slamming on the brakes.

              For what you are building, for stability, you might want to push a little water. You will not have much aerodynamic lift or aerodynamic stability (downforce), the control of the boat will be primarily the contact of the sponses and the water. (my guess) You will have to play with it to get comfortable.

              If you are going to let anyone drive it, keep that in mind when you prop it and balance it. Some folks just dont have much sense when it comes to go fast toys.

              As you get up to higher speeds, turning you could catch a wave and end up swimming. Remember two strokes are compression transfer engines, ie if you pump water through the engine, you blow out the side of the block. We always had alcohol in a squirt bottle to get the water out of the engines, when we dunked them.

              Ran the kneel down hulls for a couple of years and a 45 tunnel.

              Looks like a fun project, keep us posted.
              Tom

              edit: you might want to add an engine kill pull off cord. My ski boat actually has one, if you get bounced off it would prevent a run away.
              Last edited by trstek; 05-29-2007, 07:27 PM.

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              • #22
                That kill switch will be a must.
                Miller Dynasty 300 DX TR, 2007
                Miller MM 251 w/30A spool gun, 2005
                Lincoln SP-100, 1990
                Hypertherm PM 45, 2014
                Scotchman 5014CM Ironworker 2005
                Grizzly 12x36 lathe 2010
                Wilton Belt sander and Cutoff Bandsaw 2006

                Comment


                • #23
                  Originally posted by Doug Doty View Post
                  That kill switch will be a must.

                  Oh yeah,,,,,
                  Depending on how your steering works, a 'no feedback' system that locks itself down when you stop turning the wheel, or the regular kind where you can push on the engine and make the wheel spin, a boat with no driver will do one of two things....

                  The no feedback system; it keeps going however the steering wheel was left/set was when the driver got ejected.

                  The regular kind: the prop torque will push the engine over to one side, and the boat will keep going in circles trying to run you over until it runs out of gas.

                  I snap my kill switch lanyard onto my life jacket first, even before I start the engine.......
                  every time.

                  Something you might want on it is a spring loaded 'hotfoot'. It uses the throttle cable that normally is in your shifter box, and puts the fuel control down on the floor like a car. Plus, it goes back to idle when ya let off of it. They used to be only used for racing, but nowdays most high powered bass boats have them too.
                  If you keep the normal/original shifter, you have F & W like a regular boat, and you don't lose the neutral safety switch.

                  .
                  Last edited by Winger Ed.; 05-30-2007, 09:18 PM.
                  "Gone are the days of wooden ships, and Iron men.
                  I doubt we'll see either of their likes again".

                  Circa 1920.
                  Author:
                  Unknown US Coast Guard unit Commander.

                  Comment


                  • #24
                    Looking at the craig boat, looks like the steering is done in the center with a lever front / back motion to turn the engine? And the throttle is on the right side.

                    I am interested to hear how it does in the water. Seems counter intuitive to push the lever forward and back to steer.

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                    • #25
                      I am equally curious to try it out, I tracked down the maker of the steering system and they have one now with a much higher ratio now. The one in the video's of the craig cat require an almost verticle to horizontal movement to turn fully one direction.

                      I got the tubing bent and the attachment stands all made up for the front of the boat this week. I will weld them tonight and start on the rear this eve. It will be more time consuming as I don't have a complete plan yet and it will include the motor mount.
                      Last edited by Doug Doty; 05-31-2007, 05:13 AM.
                      Miller Dynasty 300 DX TR, 2007
                      Miller MM 251 w/30A spool gun, 2005
                      Lincoln SP-100, 1990
                      Hypertherm PM 45, 2014
                      Scotchman 5014CM Ironworker 2005
                      Grizzly 12x36 lathe 2010
                      Wilton Belt sander and Cutoff Bandsaw 2006

                      Comment


                      • #26
                        Originally posted by trstek View Post
                        . Seems counter intuitive to push the lever forward and back to steer.

                        I've heard it takes a little getting used to, but once ya do-- its fine. Years ago, it seemed all the small, narrow bass boats with a 40-55 hp engine on them had that lever steering system, and the driver's station was mounted real far forward from most other boats. To driver them, you sat right up there with the trolling motor controls.

                        Is my memory getting fuzzy, or do airboats use that lever system to turn thier rudders? I seem to remember seeing a steering lever beside the driver, but not in front like a tiller.


                        .
                        "Gone are the days of wooden ships, and Iron men.
                        I doubt we'll see either of their likes again".

                        Circa 1920.
                        Author:
                        Unknown US Coast Guard unit Commander.

                        Comment


                        • #27
                          How is the boat coming along?

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                          • #28
                            I have the cross tie bars made but need some finishing on the motor mount befoer I can cut out the deck material.

                            I had some surgery 5 weeks ago and am having a real tough time with the healing up side of things and can only be on my feet part of the day without swelling making me miserable so there is no after work progress taking place. I am slowed down for a while here. If I could have 1 good long weekend I would be close to done. I am going for a second opinion tomorrow morning so we'll see how that goes. All for now.
                            Miller Dynasty 300 DX TR, 2007
                            Miller MM 251 w/30A spool gun, 2005
                            Lincoln SP-100, 1990
                            Hypertherm PM 45, 2014
                            Scotchman 5014CM Ironworker 2005
                            Grizzly 12x36 lathe 2010
                            Wilton Belt sander and Cutoff Bandsaw 2006

                            Comment


                            • #29
                              Doug,

                              Been following the boat building project. Great work there.

                              Just a couple of comments, based on racing boats for many years.

                              The flat, sponson hulls will create suction at speed. On a hydroplane sponson, they are shaped so as to create lift. You may find that you have to add a step to the sponson to break the drag effect.

                              With the 90 degree angle on the outer edge of the sponson, the boat will be "very" tender in a high speed turn. This leads to what we call hooking.

                              With a 40 HP engine, I would think she'll run like a "scalded ape". Just be careful in the turns.

                              There are several manual and hydraulic "jack plates" out there which may give you some ideas about an engine mounting system.

                              Don't leave the dock without an operational "kill switch". I was the one who pulled Hank Bowman (former sportswriter for Popular Boating) out of the water during the '66 Orange Bowl Regatta (Marine Stadium-Miami). He was driving an unlimited inboard, I was driving an unlimited outboard (9 Hr Regatta). He got thrown out and run over by his own boat. The vision of 3 prop walks up his left side stay with me to this day.

                              Great work on the boat. From the looks of some of your other "toys", I'm sure it will come out great.

                              Keep the photos coming.
                              Syncrowave 250 DX Tigrunner
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                              • #30
                                Doug,

                                Don't over do it, you can always work on the toys later.

                                I was just wondering about the project. Sounds like that lever steering is common.

                                Sundown added some good points.

                                The tunnel hulls use a 90 deg inside turning edge. The 2 cylender 45 formula lights were pulling about 2.5g in the turns with the v6 formula merc boats pulling 4 - 5g in the turns. Those boats run full 5 pt harnesses and enclosed canopy.

                                The tunnels used a angled bottom instead of a flat like you are running, like Sun said the hydro's use a step, they are an air entrapment hull design.

                                The prop selection and engine height are the key to setting how you want the boat to run, speed and stability.

                                The rule of thumb is after 60mph the water is like hitting pavement, scrapes, bruses etc. One of the guys I used to barefoot with in my misspent youth had a doctor tell him each fall was the equivalent to a pretty good car wreck, as far as the forces put on the body. that was about 35 - 40mph. when you stuck a toe, were lucky to close your mouth before hitting the water.

                                Water is nothing but fun, but, worthy of a high level of respect.

                                Hope you are feeling better soon.
                                Tom

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