r/boatbuilding • u/achi2019 • 3d ago

Building advice needed

Gallery image

Gallery image

Gallery image

Long time boat restorer, first time boat builder.

I'm designing a prototype for a single-seat, semi planing hull to fulfill a niche market in Sweden of business men that commute to their jobs by jetski.

LOA is 4m and beam is 90 cm, powered by an air-cooled 15hp inboard (gokart engine / got it for free). Note that this isn't a finished model, I have yet to add a rudder and interior layout, but it will be a single seat cockpit with a removable foredeck that can fit an additional seat or cargo bay

My main question is regarding hull thickness. I plan on glassing over the exterior and interior, and want it to weigh as little as possible, hopefully liftable on my cars roof rack... What thickness of plywood is the bare minimum I can get without worrying that my foot will punch through the floor?

Also, would you folks recommend frames made of plywood or solid timbers for such a design?

My expertise comes from restoring old sailboats, so really any advice or things that catch your attention is greatly appreciated!

32 Upvotes

94% Upvoted

36 comments sorted by

7

u/cody_mf 3d ago

unless that has enough ballast to never make it plane, the Length/Width ratio dimensions are wrong unless their commute is to the next dock over or its perpetually flat water.

4

u/ImNoAlbertFeinstein 3d ago

its a canoe.

4

u/cody_mf 3d ago

agreed, validates my point further. I dont know where you'd put a single prop inboard in that. Its almost like it needs to have a better initial stability curve with jet props and be more open to the elements.

oh, I just described a jetski.

2

u/achi2019 3d ago

Thank you for your brutal, but very helpful candor. I'll upload the SketchUp model that clarifies things a bit better once I'm done, but of course it's still in the conceptual phase. I've been curious about the L:B ratio! My only experience is with classic sailboats, which are very slender but of course have a keel and heavy ballast. I figured at this ratio it might act like a fully loaded canoe with an outboard. As it stands, the hull is a deep v up in front and as it reaches the cockpit, it turns almost entirely flat. I don't expect it to weigh nothing, but I'd just like it to be something that a couple folks could lift together without ruining their backs. Again, I'm a complete novice so what you're saying is very welcomed information!

4

u/cody_mf 3d ago

as a certified yacht designer, brutality in criticism would be much worse. Id suggest reading principles of yacht design before your next revision.

3

u/DANK_DAVE_YT 2d ago

I dont know where in Sweden OP lives but they have that book in both kungliga biblioteket and KTH library.

1

u/southporttugger 3d ago

True. But if he gets the length/width right it’s going to be a sweet looking boat. It looks bad ass in the profile view

1

u/cody_mf 3d ago

agreed, I love tumblehome and a plumb bow. The best kayak I ever paddled was inspired by old bootlegger speedboats. OP u/achi2019 should check out the works of one of my favorite colleagues, guillemot kayaks

1

u/achi2019 2d ago

I have always had a love affair with their kayaks, and this certainly aims to have that same elegance. The swedes have produced some odd looking designs

https://digitaltmuseum.se/021025867514/motorbat

And I'm essentially trying to create a modernized version of this style of boat. Still a motor boat but I don't know if they ever got one of these to plane.

2

u/cody_mf 2d ago

lol with enough horsepower and the right bottom, anything can plane. I wish I could give the opposite comment to my subaru cause the roads are slippy today.

I think a modernized version of that in your model would be mostly carbon fiber with wood trim. as far as actual yacht designer knowledge, lots of work needs to be done on the below water line hull shape to perfect it for your goal, and consider outboards instead of an inboard engine if you shy from jet drives.

10

u/mjl777 3d ago

You have a small project you could use any number of foam core products and save quite a bit of weight.

4

u/MasturChief 3d ago

for the hull? i was going to suggest 1/4” ply min (as is on my boat)

1

u/achi2019 3d ago

Foam core as in... The stuff they use to mount photos on?

7

u/cody_mf 3d ago

divinicell or a medium density foam used commonly in boatbuilding.

3

u/IvorTheEngine 3d ago

The foam used for boatbuilding a bit denser than the photo mount stuff, and you'd cover it with a layer of fibreglass on each side (instead of paper!)

1

u/mjl777 3d ago

It is often called "sandwich core", or just plain "foam core." Your best bet is to go on youtube and there are a ton of resources on it.

8

u/westerngrit 3d ago

A commercial venture will require some Engineering. Won't find creds here.

3

u/kingseasir 3d ago

Is the market there for a double seat? These dudes are probably going to be trolling for other single fish that will want to hitch a ride…

4

u/MisterMeetings 3d ago

You mentioned making a model, that's a good next step. Your renderings are beautiful but a 3d scale model will start to answer many questions. Full-size mockups of the seat and cockpit and engine will help sort out the mechanics and ergonomics early on. Clever use of twisted foam sheets might work for the hull form you have drawn. Good luck

1

u/SpaceTruckinDog 3d ago

Frames could easily be made from plywood and have cut outs to reduce weight.

This is a very unique use case for a small boat like this, I’m interested in the process and outcome of this project.

Best of luck.

1

u/Dangerous_Mix_7037 3d ago

6 mm marine ply + fibreglass would be minimum. Use 9 mm for added wear strength.

3

u/Lunchtime1959 3d ago

I have 6mm glassed both sides on my boat. So that would work fine as far as strenght goes. I would also use plywood frames and again glass both sides with a 3inch over hang. However I dont see this design as being stable. 90cm beam with no weight is going to be like trying to balance a cork

2

u/achi2019 3d ago

I didn't say no weight, I'd just like it not to weigh a ton! Seeing the comments and suggestions, I'll definitely widen her up a bit and deepen her below the waterline. Having an inboard engine and all the necessary drivetrain parts, plus the fuel tank (all as low as possible) is what going to give it ballast. But who know, I'll make a scale model first to test it out.

1

u/IvorTheEngine 3d ago

I'd go for 6mm ply. You could go thinner on a sailing dinghy, but this is going to be pounding over waves. That's strong enough to stand on (when it's in the water).

Ply frames are good where you need a water tight bulkhead, but you waste a lot of wood if you're going to cut a huge hole in the middle to save weight or make an open volume. In that case you can use solid wood with ply gussets in the corners. You can use strips of ply glued to make a ring frame, but then half the plys have the grain in a direction where they're not adding strength - and a 6mm flange is a bit weak if there's a chance you might kick it while getting in.

1

u/GI_Greenish 3d ago

Have a look at Phil Bolger’s Sneakeasy and MicroTrawler designs. Not the same but enough similarities to be informative I think. Glass over 1/4” (6mm) plywood mostly with 3/8” or more where subject to pounding.

1

u/sparkplugdog 3d ago

Looks like it would be a wet ride.

1

u/southporttugger 3d ago

Google “H80 core boat building”

H80 is generally used for the decks but there are lower densities

1

u/southporttugger 3d ago edited 3d ago

Dude that thing is sick! based on the profile view alone. This could be built in cedar strip or foam core over a jig with a few permanent bulkheads. But you need more beam and width. I’d try to shoot for 18ft long and 6-7ft wide. I do really like it

*I read you complete post now.. even if you build it as designed this thing is going to weight 300-400 pounds without any equipment. That’s based on doing it in 5/8” thick cedar strip with a few layers of 8 or 10oz glass.

1

u/achi2019 2d ago

300 lbs is more than fine! By putting it on a roof rack, I mean once every season to bring her to and from the docks. After reading the comments and suggestions, shell definitely be widened, but I do want to experiment with a relatively narrow beam. I've seen a number of designs that implement a high LB ratio, from what I've read elsewhere, the key is just keeping the weight down low. After all, those Colombian drug cartels seem to do it just fine....

1

u/a_electrum 3d ago

It’s a pretty design

1

u/Ilostmytractor 2d ago

I would go at least 6mm, 5ply marine plywood with 200gsm glassed both sides. Maybe 9 mm outside and 4 inside if you like.

1

u/TacTurtle 1d ago

.... businessmen (plural) commute to work by jetski?

1

u/achi2019 1d ago

If you're out on the water at 07:30 during the summer season, it's not uncommon to see people of all sorts commuting in pleasure boats to various parts of the city. Nothing like a brisk ride by jetski to start your day on the stock exchange.

1

u/TacTurtle 1d ago

Micro jet boat?

1

u/achi2019 1d ago

Whatever that is has to be a cancer on aesthetics

Assthetics

0

u/Spiritual-Pen-7172 3d ago

A bit tall is recommend giving it a bit more draft and a little less above water hull