I have boat questions...

[52 Ford]

These aren't really welding questions, but I know some of y'all build boats or otherwise know about boat stuff.

I have a sort of goofy, "for ****s and giggles", project I'm going to start on soon - weather, time, and all else allowing... Gas powered kayak. I have an extra kayak, a 9 foot sit-in, that's going to be the starting hull.

I'm going to cut out a section of the... "deck"(?) behind the seat, make an aluminum or SS frame to mount the engine in, bolt that through the hull (and "glue" it with RTV), and I THINK I'm going to go from the crankshaft, then to a U joint, prop shaft, a pillow block bearing, then a shaft seal, then a propeller. The motor I have is an electric start Chinese GX390 knock-off off a spare genset. I figure if that works, I'll get something with a better power/weight ratio (thinking 125cc 2 stroke, ~30hp). As it is, I'm not worried about weight. With a little over 200 pounds (me and a paddle) in the kayak it draws around 2 inches of water and sits level - would sit better with more weight aft to bring the bow up a bit. I'm going to have a foot throttle for now and steer with a paddle. If it works well, I'll switch to a foot operated rudder and a hand throttle OR a steering wheel and keep the foot throttle.

My questions -
Stuffing box or mechanical seal?
Should I run straight pipe wet exhaust, dry exhaust with a muffler, or muffler then wet exhaust.
Oh, and same question for a 125cc 2 stroke - could I even run a 2 stroke like that with a wet exhaust (without a huge hassle trying to tune thr exhaust or whatever)

[Kelvin]

30 hp in a kayak? You're gonna need a bigger seat cushion! Steer 30 hp with a paddle? I want video! (I suspect you're gonna need a rudder.)

If you're dead-set on straight inboard power, you might want to investigate dripless packings. I have a 22' w/ Chevy 350 (310hp) and dripless packing. You've gotta keep them wet/submerged for lubrication/cooling, but otherwise they are maintenance-free. Don't know if they make one small enough for your application, though. I also suspect you're going to have issues with trim with all that weight aft...got trim tabs?

Sounds like a fun project...be sure to keep us updated.

[Kelvin]

Another thought: I know you've got an engine all picked out and all, but if I were gonna try something similar, I'd look around for a wrecked jet-ski with servicable engine and jet pump. The engine from a jet ski is probably too heavy for your kayak, so maybe use your engine and rig it to a jet pump. Then you wouldn't have to jack around with shaft packings and couplings (I also suspect the "flex" of a polyethylene hull is gonna give you fits with the coupling, definitely gonna need a flexible one), and you wouldn't need a rudder, and wouldn't have the danger of running aground and tearing up the wheel (or getting hurt on it)...or figuring out the right pitch for the wheel...you can still get hurt pretty bad by a jet pump but at least nobody's gonna get all chopped up...

I believe there's at least one company that makes jet powered kayaks, maybe take a look at their setups...

One last thought: I've worked on homemade boats, and a lot of states will want an inspector from the boat cops to come out and inspect it and sign off on it...depending on where you live, this could be a deal breaker for some designs....without a sign-off, they may not allow you to register and number it, or may even shut you down on the spot for a manifestly unsafe voyage LOL...

[Kelvin]

The motor I have is an electric start Chinese GX390 knock-off off a spare genset

Skroo that, you want a Lycoming helicopter turbofan!

[52 Ford]

LOL Here's yer video:

Skip to 3 minutes, 20 seconds. I saw this years ago and that's where the idea came from.

I forgot they make dripless packing. I was thinking that if I go with a stuffing box I'd just have to deal with a wet bilge. I just checked McMaster to see what kinds of packing they have and did the math to figure the surface speed of the prop shaft... under 1000FPM at 5K RPM and most of the fancy Kevlar/Graphite and Teflon stuff is rated for way more surface speed than that. I figured I'd just make the stuffing box, since it's going to be in a tight spot with "unconventional" mounting flanges.

I haven't really thought about trim tabs. I just figured I'd shift the battery forward or back to help trim it. Because of the shape of the hull, there isn't really any transom to speak of. I guess what I'd have to do is make a set of "wings" and have them bolted on and try and figure out a way to mount turnbuckles to tune them.

And I think you're right about needing a rudder. I just want to make sure I can get the engine working like I want before I start messing with that. Shouldn't be hard, though. I put one on another kayak last year.

[Kelvin]

LOL Here's yer video:

Skip to 3 minutes, 20 seconds. I saw this years ago and that's where the idea came from.

Interesting, thanks. That video gave me flashbacks to running my old 13' Whaler, which would rattle your teeth out in any kind of chop, but that kayak looks about 100X worse...don't forget your Excedrin!

I think I'd dispense with the battery and make it pull-start. With so small a boat, weight (and trim) is gonna be your biggest challenge, I think.

One other thing I've experienced with homemade boats (and this would really take a boat designer to answer) is that if you want it to plane, you really need to have almost perfectly straight sections lengthwise on the bottom. If there's any kind of longitudinal rocker (or worse, concavity), the hull wants to "stick" to the water and will have trouble climbing over the bow wave to get on plane. Or it will get up on plane, then fall off plane repeatedly in a pitching, "porpoising" kind of way. In a tupperware kayak, with a lack of longitudinal stringers to stiffen the hull, I suspect the bottom is also gonna wibble-wobble (oscillating between concave and convex) due to all the thrust pushing it forward versus the stiction of the water trying to hold it back, and it's gonna behave funny...I think I would be tempted to try to stiffen the bottom fore-and-aft with some kind of stringers... These kayaks are really meant to be operated in displacement mode, and making them into planing hulls is going to be tricky I suspect...

Here's the motorized kayak I mentioned earlier:



Looks like it will plane, or almost plane, but it has enough vee in the bottom to make it a lot stiffer than a regular paddle kayak...

Anyway, just some thoughts worth about what you paid for them. A lot of it will be trial and error, I guess, but it sounds like a fun project. I still like the idea of the jet pump. If it pushes a jet ski, it should push a kayak just fine I would think...might be worth looking at the jet pumps in those Mokai boats. Good luck.

[52 Ford]

I didn't know Whaler made a 13 foot. I knew they made something like a 16 foot skiff and about the same size Vee hull. We had a 23 foot Boston Whaler With a 225 Mercury on it. Nice boat and I don't think it drew more than 18" of water. Think it's what you'd call a "medium vee". Not shallow, not deep. Still cut though waves nice.

You make a good point about hull flex. I could probably add some fiberglass ribbing to the bottom. That'd make it track straighter, too. Comparing the my kayak to the Mokai, that's WAY wider than a paddle kayak. My Ocean Kayak, which I can stand up on, is about 32" wide.

The reason I want to go electric start is I want a small battery on there anyway, so I can have a fish finder and lights. The motor aught to have enough current coming off the charge coil (coils?) to keep the battery topped off. The starter weighs maybe 5 pounds and the gear on the flywheel is probably a pound or two. I aught to be able to get away with a tiny little battery, too. I was just looking at "Go Kart Batteries" on Google and apparently, people are running 12V electric go karts off of 4Ah lead acid batteries. They weigh around 4 or 5 pounds. If a 4Ah battery can push a kid around in the back yard for half an hour, I should be able to get enough cranking time to get the motor started without having to reach for the pull cord. When it was on the genset, the battery tray was smaller than a brick if I remember right. It was on a Chinese 6000ish running watt generator.

The problem with going jet drive, or at least going repurposed jet ski jet drive, is space. I imagine I'd have to lop off the last 10 or 12 inches of hull to get a large enough cross section to fit the nozzle. The biggest advantage I see is being able to get in shallower water. That and I can tell people I have a kayak with a jet drive.

They look fairly simple. I could probably make most of the housing out of big DOM tube. I just need a provision for the wear ring. The inlet can be aluminum or steel welded up to whatever shape I need. The nozzle would take some trial and error getting the right angle, diameter, taper, all that stuff.

[whtbaron]

I don't even like getting my ears wet and I can't swim... but now I want one...

[Lis2323]

I don't even like getting my ears wet and I can't swim... but now I want one...

What was all the chatter ‘bout swim up bars??


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[Willie B]

Don't know whether you are going to be consumed wound around a shaft, or drown.
Get a small boat, or build one. Canoes propel nicely with a leaf blower.

[Kelvin]

I didn't know Whaler made a 13 foot.

That was their first, and for a long time ONLY, hull. Back before "Flipper" was on TV, we had a '58 or '59, and I still have a '66.

[psacustomcreations]

Interesting, thanks. That video gave me flashbacks to running my old 13' Whaler, which would rattle your teeth out in any kind of chop, but that kayak looks about 100X worse...don't forget your Excedrin!

I think I'd dispense with the battery and make it pull-start. With so small a boat, weight (and trim) is gonna be your biggest challenge, I think.

One other thing I've experienced with homemade boats (and this would really take a boat designer to answer) is that if you want it to plane, you really need to have almost perfectly straight sections lengthwise on the bottom. If there's any kind of longitudinal rocker (or worse, concavity), the hull wants to "stick" to the water and will have trouble climbing over the bow wave to get on plane. Or it will get up on plane, then fall off plane repeatedly in a pitching, "porpoising" kind of way. In a tupperware kayak, with a lack of longitudinal stringers to stiffen the hull, I suspect the bottom is also gonna wibble-wobble (oscillating between concave and convex) due to all the thrust pushing it forward versus the stiction of the water trying to hold it back, and it's gonna behave funny...I think I would be tempted to try to stiffen the bottom fore-and-aft with some kind of stringers... These kayaks are really meant to be operated in displacement mode, and making them into planing hulls is going to be tricky I suspect...

Here's the motorized kayak I mentioned earlier:



Looks like it will plane, or almost plane, but it has enough vee in the bottom to make it a lot stiffer than a regular paddle kayak...

Anyway, just some thoughts worth about what you paid for them. A lot of it will be trial and error, I guess, but it sounds like a fun project. I still like the idea of the jet pump. If it pushes a jet ski, it should push a kayak just fine I would think...might be worth looking at the jet pumps in those Mokai boats. Good luck.

We had a Mokai at work for a while.
If I remember right, it was almost like a lawnmower engine that powered it.
It wasn't working when we got it so I just did a quick cleanup of the carb and it ran fine.
It was fun to motor around in.

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[whtbaron]

What was all the chatter ‘bout swim up bars??


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I can still stumble...

[Lis2323]

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[52 Ford]

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LOL!

[52 Ford]

Is there REALLY such thing as too much HP?


Skip to around 14:30 to see it on the water.

[whtbaron]

Must be a heavy boat... put that poor old Chevy right in the weeds...

[52 Ford]

If I remember right, he said the hull was under 400 pounds. For how big those Sonny
Leonard engines are, they're pretty light. I'd guess that whole boat, with the driver (captain if you're pedantic - I guess), engine, jet drive, and whatever else is probably about 2,000 pounds. He's making over 2,000HP, though. So, if my logic is correct, I should be able to get away with 300HP or maybe 400HP. 🤣

[Kelvin]

. So, if my logic is correct, I should be able to get away with 300HP or maybe 400HP. ��

MIght want to stiffen stuff up some. Imagine trying to apply 400hp (or even 30hp) through a mower deck made of Tupperware.

[52 Ford]

https://i.imgur.com/TfPQE70_d.webp?m...idelity=medium

You just gotta BELIEVE you can!

[Kelvin]

https://i.imgur.com/TfPQE70_d.webp?m...idelity=medium

You just gotta BELIEVE you can!

You laugh but we have some lunatic here named Jet Ski Brian who takes his freakin JET SKI out to the canyons 50 nm off Virginia Beach and fishes for tuna out there on it! The man is seriously hard core.

[Tiggatoo]

https://www.google.com/search?q=long...zlCfN6V_u9FOyM

[52 Ford]

Jet Ski Brian has some balls.

I have no problem with "big water", but in something really small like a 'yak or even a jet ski, I want to keep land close enough I could swim to it.

My problem is that if I break down 50 nautical miles out, there's no way I could drink enough of that water to walk back to shore. That's a long way to walk, anyway.

I knew some people that'd paddle kayaks across the bay to the Eastern Shore.. 17 miles, I think.

[smithdoor]

I guess Turbo is not need.

Dave

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[52 Ford]

Turbo not required. Supercharger and/or N2O... a solid MAYBE.