Yamaha 784cc into welded mod-v hull


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CedarRiverScooter said:
86, I think you are our resident diaphram-carb expert now!

far from expert. it would just be the blind leading the blind <grin>

I have done a bunch of reading over the last several weeks though. The mikuni owner's manual that I found here


has tons of info on how the carbs work, and how to set them up to work properly. Highly recommended reading for anyone who wants to figure out how carburetors work.

I took a couple of videos of popoff testing, but can't seem to upload them here :(

All giddy about getting this thing on the water for testing. Will take my toque off this time so that it won't blow off at speed.

- Brian
Success on the water! Ran great, me and a buddy had fun hooning around for an hour. Started immediately every time, no stalling. Ran good at idle, and accelerating through mid and high speeds. Strava said we hit 75kph (40 knots) so it rips pretty hard. Next step, teardown and measurements, then order materials and start cutting. I wonder if my little SP125 will even penetrate the 1/4" thick pieces of my water inlet. Haha.

Getting excited. Only took 5.5 months since the first water test.
On my 1st iteration, I used a cobbled-up welder (never designed for alum) & in retrospect it tripled the time it took to put boat together. Last time & had a spool gun machine & TIG. Did 10X more welding in 1/3 the time. Plus I can trust the welds.
Welding Aluminum with a MIG is fun, but if you're doing anything thicker than 1/8" you need a bigger welder.

Why? Because aluminum is a big heat sink. The arc is what creates the heat needed to melt the metal. With aluminum, it dissipates the heat so quickly that you need more arc voltage (with MIG) to melt the metal than you would if you was welding mild steel.

Lot of guys say it can't be MIG'd, but it sure can. I've built many projects with an old Lincoln Weld-Pak 100, with a spool of aluminum wire and 100% Argon. 99% of the time the voltage was turned ALL the way up, as was the wire speed.

With Aluminum, it's sorta backwards. Generally speaking if you're burning holes, you're going too slow and need more heat. Sounds odd but that's been my experience. TIG is a little different obviously, but same principles apply...especially with heat needed. 200A TIG unit will be max'd out with 1/4" aluminum and I mean MAX'd out pedal to the metal and feed plenty of filler as fast as possible.

Clean clean clean! Aluminun needs to have EVERY ounce of clean put on it. Why? Aluminum oxidizes quickly (that's the white stuff you see on Al that's been sitting outside for a while). The oxidation...Aluminum Oxide...melts at around 3700 degrees F. The aluminum itself melts at around 1200 degrees. See a problem? Then dirt...grime, grease, etc...all will adversely affect the quality of the weld. The weld in the picture above has a little junk in it, but otherwise looks pretty good. Better than I can do but I'm no pro..FAR from it. My eyes are getting worse and that's affecting my welding skills severely. When I say clean, I mean clean the contact tip (or tungsten if using TIG), clean the filler material (wire/filler rod), clean the base metal, clean all metal to be welded, and everything within 1 inch of the weld. I even clean the table I'm welding on because I've had instances where something was on the table and spattered into the puddle while welding. That's always fun to figure out. If you use a wire brush, use a known clean one made specifically for cleaning aluminum, and use it for NOTHING else until you're done welding. IF TIG, grind your tungsten on a wheel that's fresh and will not be used for anything else but grinding aluminum welding specific tungsten as if you grind something else, it gets embedded into the rock and will make its' way to the tungsten-contaminating the puddle, and throwing you for a loop. Fun stuff. You'll get good if you do it enough.

The jet jon idea has me intrigued, big time. I want to watch some of these threads. Used up/wrecked jet ski's are everywhere out here, and dirt cheap.
Currently the holdup is materials supply. Both of the regular suppliers don't have a sheet of 8x20 in 1/8 available for at least 60-90 days. So I'm in a holding pattern....again. Buddy's miller 211 is on standby as well, so at least that's good news. Hopefully it will push the aluminum wire the same way my little SP125 does, otherwise I'll be shopping for a spool gun. Will wire the 220v outlet when material is secured. That part is easy, I have a panel in the garage.

Ah, the delays in my project. I had envisioned having it on the water by now lol.
Small progress update. 8x20 sheets of 5086 aluminum are still backordered, so I've procured a couple 4x20 sheets of 1/8" 5052. Waited 6 weeks for this stuff to arrive from the manufacturing plant. This week I've spent a bit of time modifying a boat trailer into a semi-flat deck so that I can transport the materials from the supplier to the bending/forming shop.

I cut all the roller tabs off the trailer, then re-spooled my welder for steel. And switch the gas from argon back to co2. Then I welded the old tabs to the crossmembers to gusset and allow securing of my flat deck. Well, not really a flat deck, but rather two 2x12" planks 16ft long so I can secure the aluminum for transport.


The deck beginning to take shape. Pic taken just before front crossmember added.


The 'deck' is secured with 3/8" bolts, and recessed so my materials will be able to sit flat.



This morning I headed to the metal supplier, and their shipping department helped me load and secure the stuff to the trailer. Then off to the bending shop. Their backlog is 18 working days, so I have a few weeks to modify the trailer yet again, so that I can transport the hull-halves back home again.


Quick sketch of what I'm having done. Note, deadrise angle is defintely not to scale. Planned size would be 1654 with 10-deg constant deadrise, mod-v or garvey style hull.

The hull design is mostly based on a Garvey design, but I've added 3" chines to hopefully get some grip in turns, and also stiffen the hull significantly in the chine area without adding material. Not sure if I will need ribs or strakes along the bottom of the hull, I may wait til water testing to find out.


In the meantime, the plan is to pull the trailer decking off, then re-use the 2x12 to build the jig for the hull. Also, time to dismantle the waverunner, and do measurements and layout. I want to build my own water inlet as well, but we shall see what the prototypes look like in the next little while.

I'm getting excited again, now that materials have finally arrived, and dropped off at the bending shop. The fee for 16ft bend with 1/4" radius is about $100/bend. But it would save a lot of welding wire and gas, so should totally be worth it.

See you on the water,

Very cool project!

Do you have a complete plan?

If not, you might want to design out the all details.

I've painted myself into a corner before, as I didn't have subsequent steps figured out ahead of time.

Maybe you can use seating & decking elements to help reinforce the hull.

Definitely make the engine & pump mounting area very stiff.

Hopefully you got pre tariff price on your aluminum.

Keep the pictures coming!
painting myself into a corner is my special skill. I've spent the last 8 months dreaming and scheming.


Dug some stuff out of my stash.


beginning to layout the jig



But running into a small snag, my floor isn't nearly flat enough. But I do have a few weeks to figure this out. Perhaps that's enough for this weekend.


See you on the water

turbotodd said:
Welding Aluminum with a MIG is fun, but if you're doing anything thicker than 1/8" you need a bigger welder

thanks for all the tips! I do have my buddy's 211 on standby. When my stuff comes back from the bending shop I'll start practicing with it. Also, gonna try my hand at TIG and also maybe wire-feed TIG welding.

One step at a time.

Tig is too tedious for me, I just use it to fix the porosity.

Does your donor ski have power trim? if so, great.

The big issue I ran into with my last engine install was pump angle. It is too high & boat porpoises over 30 mph. I finally resorted to trim tabs after trying lesser remedies. Trim tabs are working OK.

If you don't have a way to adjust trim, you may want to have a way to easily shim pump angle. I don't think even trig will predict planing behaviour of a custom hull.
That would be advanced naval architecture. Well beyond my realm of experience. Yes, my waverunner has trim and reverse. Hoping a 3-5 deg down angle will work. I'm planning on 'pods' on the back under the swim platform for both floatation and nerf protection. Basically putting the transom or jet mounting face forwards on the hull for integrated pods. Originally I didn't want pods or swim platform, and was going to recess the jet in a box, but the swim step will be handy for getting back in when the boat isn't beached.

Large rear deck to cover the motor and fuel tanks. Small centre console, and my cooler in front of the console like the 'coffin box' style of the flats skiffs in the southeast as a tandem seat for 1-2 passengers. Additional passengers can sit on the edge of the rear deck, or on the front deck depending on trim on plane.

As for trig, that's just the easy way to do an accurate angle. Rise/run. Roofers and carpenters do it all day. Or at least they have a chart for it. I just need to figure out how to get my jig squared away. Looks like shimming the floor is required.
My little laser level has paid for itself 100 times over . . .

My pump down angle is 3 deg & obviously wasn't enough.

Another big consideration is where to put the fuel tank. It weighs a bunch full & not so much empty, so putting it in a neutral position is key.

Mine is located along side engine but toward front. Would be nice to have it in bow but it has in-tank HP pump, I was able to use the OEM fuel line & fittings so no engineering reqd on my part.
Some stuff I've borrowed from various sources on the internet. Adding a handle to the circular saw made my cutting easier, smoother, and probably safer. Yeah, I'm right handed, and with the blade on the right, parts of my body are potentially in harms way. However, I can SEE the cut line much better than looking over the motor. 10/10 would do it again, and should have done this long ago. Not my idea, it's one from Kevin Morin's bag of tricks.


The handle is the side-handle from the drill in the combo kit. But any knob with a threaded insert can be used for this. In my case, m8x1.25 metric screw was welded to the bottom of the rip guide. With the handle removed, the function of the guide is not compromised. Flip it over, screw on handle, and voila, two handed saw.

The jig/cart I made is a bit heavy, but I had 2x12 laying around from flat-decking my trailer. So I re-used this wood to make the cart. I probably should have used plywood, but I don't have a table saw. Wheels I had laying around because I tend to hoard stuff. Trying to cut back on stuff, but my greedy little heart says NO. But throwing junk away is so therapeutic :eek:

The floor is flat enough, I ended up shimming the corners with blocks and then checking to ensure levelness. There is a spot on the floor that's marked with chalk where the cart will go to help ensure squareness. Need to add a diagonal to the cart to reduce racking. But for now, it will sit in storage until the bending shop is done with my materials (end of Sept.)


Gonna need to borrow a laser line so that I can properly mark out the bow when I actually start working on this thing.

See you on the water.

Today I learned that you can weld aluminum with 110v but you're much better off doing it with 220v. The millermatic 211 is a lot nicer to use than the entry level lincoln sp125. It's like driving a Tercel and trying to pull a big boat, when you KNOW that towing would be effortless with a Tundra. But I had to find out for myself.


I learned that kinking a liner makes for a bad day. If the liner isn't kinked, the aluminum wire feeds fine through the 10ft whip. As long as I don't burn-back to the contact tip, I didn't have feed issues at the lower power settings I was using for 1/8" sheet.


So I spooled the borrowed welder with 5356 and used medium settings and practiced my butt welding. Which is trickier than T welding. But critical for my keel. Tons of fun. Played around for a half hour at a time all weekend. Then this afternoon I re-spooled the SP125 with 5356 and tried, just to confirm. Knobs were at maximum, and beads did look ok. But the little red box was working HARD. And the blue box wasn't working hard at all.


I'm not posting any pics of my butt welds because they're butt ugly. I also found out that when the welding is done on a table, there is much less sagging. Gonna have to sort that out too, because my wooden jig will have the panels in 'space' and not on a steel table.

Since I have to return the blue box on monday, I'm now exploring options, and the green box iMig200 seems to be a good value. But will need to save up a bit.

Other buddy has a miller 252 with a spool gun, but I'm trying to NOT borrow it, so I won't learn the difference. Because I wouldn't have money for all that stuff anyways.

Thanks again for the tips from all the experienced welders. Now I'll have to upgrade my equipment lol. I should rename this thread 'the boat project that hasn't begun' since it's coming up to a year soon.

See you on the water.
If you want to test your weld, bend it backwards, opening up the root.

Use some scraps material for coupons.

Sometimes the weld looks OK but doesn't have any penetration.

When I fed thru a long liner, it bird-nested every other start. More wire went in the trash than on the boat. That was with a nylon liner too.

If you try a spoolgun, you won't go back.
This week I set up the trailer again so that I could drag the hull sides back from the bending shop. Load security had me worried. Thankfully it worked out okay. It certainly would have been easier to rent a u-haul truck and just toss it in the back. I had some highway driving to do, and was worried that flapping would tweak the panels, or worse yet, fall off the trailer. So I built a support for the side, and strapped it down securely.



I also used two screws through the corners for insurance. Plus the 3 straps made it fine at highway speeds. The small ~1/8" holes in the corner of the panels won't affect my build. The screw is *barely* visible in the first pic.

Moving the panels from the trailer to the build cart/jig was fun at best. They're a bit heavy at about 120lbs each, and quite awkward. Lucky for me, wife was home to help move things. Rolling it around was quite loud too, when the panels banged against each other.


I notched the front of the panel using my 6.5" cordless saw, and earplugs. Then pulled up the starboard nose panel using a ratchet strap.


I borrowed a laser level to mark the cut line. Unfortunately the line is difficult to see against a reflective surface. It can only be seen at certain angles.


Once it's laid flat, I'll figure out how to mirror image it for the other side. Might involve cardboard and scissors, plus a 5ft batten to 'clean up' my squiggly sharpie line. Then cut the nose on both sides, and deal with the upward transition of the chine flats. Will likely use CAD (cardboard aided design) for that part as well.

Enough for today, gotta toss some stuff out of the garage to fit a car inside tomorrow.

See you on the water,

That is some nice material!

One thing I ran into is heat distortion from welding a long seam (4 foot). I should have restrained the panel next to the weld with angle iron & c clamps, or something similarly stiff. Tacking didn't stop the bowing that I got. I spent considerable time trying to bend it back & have lots of little dents to prove it.

Maybe your build frame could have stringers next to keel,& cover them with sheetmetal to keep the smoke detector from going off so often.

Keep the pix coming!