Just a couple questions on yet another jet TNT build

Designs for inboard or outboard power

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DSR
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Location: Allegan, Michigan

Just a couple questions on yet another jet TNT build

Postby DSR » Sat Jun 25, 2016 9:13 pm

Hi everybody, i'm new to the forum and currently in the planning stage to build my first boat, a jet TNT, and had a couple questions if i could.
First, i see that the deck beam for frame 3 is listed as a lap joint to the side frames with carriage bolts and screws and i'd really rather do a double gusset instead.
Second, i'm changing the transom angle from 102 deg to 90 deg from the hull bottom and i would like to eliminate the transom cap and put an inner frame and gussets across the bottom of the transom with notches to mount the battens, chine logs and bottom planking.
The boat will be "slightly" overpowered and i wondered if there would be any issues anyone can think of by me doing either of these?

Thanks in advance
Dave
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753

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galamb
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Re: Just a couple questions on yet another jet TNT build

Postby galamb » Sun Jun 26, 2016 9:24 am

Not being a marine engineer, I will give you my take (to some degree) on your questions.

First, I built a much larger boat (Cuddy Sport stretched as allowed by the plans to almost 26 feet long). I went with bulkheads (to sole level) on a couple frames rather than a lapped frame/bolts - figured that would be as strong or stronger then the bolted lap. I (believe) that a double gusset of reasonable thickness would achieve the same degree of strength/stress support.

In "most cases" the transom angle is "vital" if you are mounting an outboard or an I/O unit. Since you plan on using a jet, at least from the motor mounting perspective, the transom angle is of little concern.

I ended up tossing around a number of ideas (mounted an outboard on mine) - considered an Armstrong style bracket etc but ended up going with (essentially) a double transom. I built the transom to spec and then 2' inboard I built a second totally vertical transom and tied the two together with supports members - this allowed me to create a 2' (flat deck/motor well) - so not to spec or design, but it did not detract from the (safety etc) as far as I can tell (5 years running now) and gave me the appearance and functionality I was after. My point is, as long as any of your design changes don't compromise the structure or safety, you are going to be fine.

As to the over-powering. That can create a couple of "potential" issues.

The first might be an "over weight" condition. If the weight of your motor, regardless of whether it's outboard, inboard etc, is "significantly greater" then design you could end up with balance issues, displacement issues (boat sitting lower, reduced carrying capacity etc), reduced top speed and you will "burn more gas" pushing the extra weight, all the time. It's always a trade off somewhere.

Second, a hull is only stable to "X" speed. In general (because there are tons of exceptions), a planing hull (typical hard chine, deep or semi-V, typical of Glen-L designs) of less than about 19 feet (at the water line) is only stable until about 40 mph, with the mid 30 mph range being "ideal".

After that speed, on plane, there just isn't enough hull in contact with the water to maintain stability (think of a hydrofoil racer that cartwheels across the water - it "exceeded" the speed that kept enough of it's hull in contact with the water to allow for control).

(now before guys jump in talking about their 60 mph "bass boats", those are a different hull design, which is why I said "in general" in my above comments)

To figure the max "safe speed" of a given hull you simply calculate the "hull speed" (displacement speed, not on plane - calculations are there if google is your friend) and multiply it by 7. So a 15 foot boat (13 feet touching the water) may have a displacement speed of 4.8 knots, making it's "max safe speed" (4.8 X 7 = 33.6 knots) or about 38 mph.

The point of all my "babbling" IS: do "complete" research on everything you want to do or change. Carefully weigh the perceived benefit against potential issues.

And remember in the end that a JET unit translates into about 30% less power (at the prop) than methods that actually use a prop. So to get the same amount of "forward motion" that you would get with say a 60 horse Outboard, you would require a 90 horse Jet unit.

A Jet is "cool" and on something like a jet-ski, absolutely necessary (there would be hundreds of riders chewed up by props after falling off). They also allow you to run in very shallow water (if the boat floats, you are good to go - nothing "hanging down" to "hang up" on the bottom.

But they are "grossly" inefficient compared to just about every other marine propulsion method. So if you don't need a jet for shallow running or safety, you really have to consider if the "cool factor" is worth 30% more fuel to power it "all the time".
Graham

Yes, Plywood is "real" wood :)

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

DSR
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Re: Just a couple questions on yet another jet TNT build

Postby DSR » Sun Jun 26, 2016 5:26 pm

Hi Graham;
Thanks for responding to my questions. That is exactly the reason i joined this forum.
I apologize that i was negligent in not providing a little background when i posted the questions. I am most certainly not a marine engineer either and this is my first boat build, but I did grow up around boats and working on them with most of my experience and knowledge involving, i guess you could call them "hot boats", such as V-drive flatbottoms and shallow-V jet boats (with a couple fast outboards scattered in there). I guess my issue here is a serious lack of woodworking experience (the closest I've come to woodworking is building a flowbench for my cylinder head work :? ) I've been studying the "Boat Building with Plywood" and from what i could gather, it looks like the double-gusset construction is structurally the better way to go but i wasn't sure if cutting the "landing area" in half for the sheer clamps at frame 3, with the amount of curvature, would create too much local stress when assembling? And i also wasn't sure if mounting the battens into a notched frame at the transom would cause similar issues with the rest of the frames not being notched and sitting on top of the battens? I couldn't find any info regarding doing this so i figured i'd ask the people that have run into issues like this.

Thanks Graham! :D
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753

DSR
Posts: 214
Joined: Mon Mar 21, 2016 8:11 pm
Location: Allegan, Michigan

Re: Just a couple questions on yet another jet TNT build

Postby DSR » Sun Jun 26, 2016 7:11 pm

...........Actually, if i replaced the overlap joint on the deck beam with double gussets, i'd only lose approx. 1/4" for the sheer clamps............there i go thinking again.......
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753


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