Hello, I plan to build the lightest TNT ever!

[Forge]

Hello all,
I live in Huntington Beach California but I spend a week or so in Canada every summer and I want to build a fun little and speedy cruiser to enjoy at the lake every summer. Our family has a ski boat, so we have that covered, but I want something that would make long trips up the lake for picnics more enjoyable and cheaper on boat gas. I also have some nephews who will be at that age where driving a boat by them self would be just about the most exciting thing ever!...

I am an aerospace engineer, and I have plenty of experience driving boats, sailing, rebuilding engines, repairing cars, and building cabinetry etc. So this build is right up my alley.

The plan:
Build the lightest TNT ever in order to take advantage of Canada's law that does not require you to register any boat with less than 10 HP. Since I plan to use the boat for only a few weeks a year, and it will spend nearly all year in storage it is cost prohibitive to have register it each year. It doesn't need to be too pretty but it needs to survive the winter and be easy to launch (Carry by hand from under the cabin, down to the beach).

I will plan to post this build as I make progress, mostly planning and design for the next year or so. but I intend to share my development with the forum for anyone interested. And of course I hope to get as much input from the experienced folks on this forum to help me achieve my goals.

Key Requirements:
-Highest top speed possible with a 9.9 HP outboard.
-Lightest weight Hull possible to keep the speed up, and make it easy to carry and store for the winter
-Must have seating for at least two adults that can be adjusted for the nephews
-Seating and Helm must allow myself to fit and not be too uncomfortable (I am 6'6'' tall)
-Must have navigation lights for night boating
-Must have remote steering and throttle for a "real boat" driving experience for the nephews
-Must optimize the design and configuration for a lake that has large waves and is not always flat when making large trips up the lake.

Any initial thoughts? Any tips on where to start my research?

[Hercdrvr]

Or you could get a 25 hp and buy all new 9.9 hp decals for it.
Matt B

[Forge]

That had crossed my mind, but I wanted to keep it safe (slow) enough for the lil guys not to get airborne... and I will probably build the remote kit so that I can swap in a larger motor later on.

The real challenge for me though, being an aerospace guy, is to make the hull as lightweight as possible. From what I have seen so far online most people tend to beef up the hull a lot to install a larger motor. SO I am going to take the opposite approach and see what level of performance i can squeeze out of a lightweight hull.

[Forge]

yea that thought had crossed my mind. But keeping it under 10 hp is part of keeping it safe and legal for the nephews to drive by themselves.
Anything over 10 hp and they have to wait until they are 12yrs old to drive.

They are currently ages 2 and 5 so by the time I finish this in 2 summers from now they will only be 3 and 7. Which is a bit you g for anything too fast. There will be plenty of time to upgrade later

[kens]

I think the biggest factor in keeping it as light as you say, is cost.
You would use aircraft grade spruce for wood, use the finest Okoume plywood, vacuum bag S-glass skin, and so on and on.
All that stuff cost money where the rest of us use mahogany, or Oak, or some other wood, and few of use vacuum bag the skin.

[Roberta]

I built the Super Spartan without worrying about weight and got about 20 mph with a 9.9 Merc. Grandkids loved it.

Roberta

[Forge]

Thanks Roberta! That is good to know, 20mph should be plenty fast for what it's I tended for.

Ken's, yea I had gotten as far as determining that spruce was much lighter, but I wasn't sure if there was also much benefit to be had in the plywood type. I definitely don't want to spend a ton of extra to save a few ounces, but where it's a good bang for the buck I would like to use lighter materials.

I assume the vacuum bagging is intended to squish down the glass and make the layer thinner to reduce weight?

[TomB]

Forge,

Vacuum bagging uses atmospheric pressure to clamp layers together (pushing unnecessary epoxy volume from between layers) and probably won't be necessary for your lightweight TNT.

Spruce v. white oak --> big weight savings. You've mentioned that one.

Okoume v. Hydrotek or Douglas Fir marine plywood --> big weight savings. 1/4" Okoume is 21#, Hydrotek and Douglas fir 27.5# = 6.5# per sheet. You will have some scrap so you'll probably save less than 19.5# on a 125# hull. Your wallet will be lightened too, about $120. Here is some good reading on marine plywood. https://www.woodenboat.com/marine-plywood


Tom

[Forge]

Thank you Tom!
That is good to know on the plywood, I think that might be enough bang for the buck to justify it.
I will do some more research and come up with a detailed estimate on what the hull weight might come out to...

[hoodman]

There is not THAT much framing in a boat that size. So you have to weigh whether having the lightest possible wood is worth the potential extra cost.

[Forge]

Hello everyone!
So thanks to this crazy pandemic I am trapped inside a lot, which was the perfect excuse to start on my TNT Build.
We are now officially underway, I ordered my plans, a fastener kit, some epoxy, a Japanese pull saw (because why not), and started making cuts on the frame pieces!
I am really excited, and am very thankful to have this as a distraction from the news right now.

Any tips on getting started and how to avoid those typical pitfalls right out of the gate?

[Roberta]

Welcome to the world of boat building. My best suggestions are to thoroughly read and understand the plans and instructions, research other TNT building threads and photos, accurately mark parts on wood before cutting, cut wide and sand to lines. Ask questions and post pictures.

Roberta

[BayouBengal]

Lower weight wood is a good idea, but if you really want to save weight, to me, you need to have no electrical wiring (heavy), no electric starter (heavy) on the motor, and thus, no battery (heavy). For your evening runs, I'm wondering if there's solar lights with small charge batteries that would be bright enough that you could easily attach and detach. If not, I'd locate the battery in a place where it could easily be installed and uninstalled so that you could easily slide it in/out and connect/disconnect for the evening/day cruising. A small lawn tractor battery, starter, and wiring is at least 30 pounds.

You could have a 6 gallon fuel tank, it wouldn't add much weight over the 3 gallon, but you'd generally also want to run without it filled. 8 pounds per gallon.

If you're really, really going for light, no remote steering or controls. Probably another 30 pounds.

Of course, if you're going to cut out all the above, you may as well just build the "utility".

[Forge]

Thanks for the input BayouBengal, so far I have been reviewing the engine MFG websites and the starter generators only seem to be a few lbs, maybe 5 or 6. And I also found these small motorcycle batteries that are made with lithium ion cells Intead of lead acid. The whole battery weighs less than 4 lbs. And it's small enough I could probably make a mounting box inside the knee at the transom.
I agree with you on wiring and controls, I will keep it very spartan, I want remote steering and throttle, but no gauges or other extras. I will need a few switches for nav lights maybe, or I could just wire them to the ignition so whenever it's running they will be on, and if I need headlights for night, then I would make the connection with the pole on the anchor light, so if you put up the pole, the lights come on (mounted to the pole).

I like the idea of certain extras being removable for different missions, leave the battery and lights etc on the dock when cruising around for speed and fun, and then fill up the tank and pile on the extras for all day trips up the lake.

Thanks!

[Forge]

Just looked at the batteries on Amazon again...
The featherweight firepower brand has 105 cold cranking amps and only weight 1.2 lbs.