Hello, I plan to build the lightest TNT ever!

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Forge
Posts: 5
Joined: Fri Sep 20, 2019 11:15 am

Hello, I plan to build the lightest TNT ever!

Post by Forge » Fri Sep 20, 2019 11:33 am

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
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Joined: Mon May 11, 2015 12:52 pm
Location: McKinney TX

Re: Hello, I plan to build the lightest TNT ever!

Post by Hercdrvr » Fri Sep 20, 2019 1:30 pm

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

Forge
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Joined: Fri Sep 20, 2019 11:15 am

Re: Hello, I plan to build the lightest TNT ever!

Post by Forge » Fri Sep 20, 2019 2:25 pm

:lol: 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
Posts: 5
Joined: Fri Sep 20, 2019 11:15 am

Re: Hello, I plan to build the lightest TNT ever!

Post by Forge » Sat Sep 21, 2019 12:06 pm

:lol: 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 :mrgreen:

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kens
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Location: Coastal Georgia

Re: Hello, I plan to build the lightest TNT ever!

Post by kens » Sat Sep 21, 2019 9:08 pm

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.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Roberta
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Re: Hello, I plan to build the lightest TNT ever!

Post by Roberta » Sun Sep 22, 2019 3:45 am

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

Roberta :D
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

Forge
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Joined: Fri Sep 20, 2019 11:15 am

Re: Hello, I plan to build the lightest TNT ever!

Post by Forge » Mon Sep 23, 2019 7:46 am

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
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Location: Holland, MI

Re: Hello, I plan to build the lightest TNT ever!

Post by TomB » Mon Sep 23, 2019 8:30 am

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
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Joined: Fri Sep 20, 2019 11:15 am

Re: Hello, I plan to build the lightest TNT ever!

Post by Forge » Mon Sep 23, 2019 12:37 pm

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...

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hoodman
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Location: Lafayette, IN

Re: Hello, I plan to build the lightest TNT ever!

Post by hoodman » Mon Sep 23, 2019 5:18 pm

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.
Matt

Building a Geronimo......!
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