Jet boat build advice on ply thickness and fiberglass weight.

[Mrjake]

Good morning, I will be beginning another build this winter. I plan to build a bateau GV11. I will be grafting in a yamaha 650cc jet pump and engine after completing hull to plan specs. Plans call for 1/4" ply for bottom/sides and 3/8 ply for transom and support frames/seats. I would like to overbuild the hull in order to give some reserve safety for the high likely hood of rock or submerged log impact at high speed in shallow water. Here are the options I have. 1. Build with 3/8 ply supports and transom. 1/4" bottom and sides then Sheeth with 6 ounce fiberglass then a layer of xynol for impact and abrasion resistance. ( this is what i did when building my spira "Endurance" boat build)
2. Build with all 3/8 ply and a layer of 6 or 9 ounce fiberglass cloth.
3. Build with 1/4" sides 3/8" transom and supports. And sheeth with 1200 or 1208 or 1700 or 1708.

I will be using marine grade baltic birch found locally. 1/4 is $70 per sheet and 3/8 is $80.

I expect to be moving at around 40mph with the boat weight around 200- 250lbs and the 50ish hp jet engine. The original plan would go 25mph with a 10hp outboard and two passengers when build to 110lbs. I want this boat to be stout and safe.

I would appreciate you guys opinions and any concerns.

 

[Mrjake]

Endurance plywood boat build. https://www.microskiff.com/threads/endurance-plywood-boat-build.96442/
Here is my previous build if anyone is interested.

 

[Mrjake]

https://boatbuildercentral.com/Stud...CYAbC_3Qt3SAOODge33pucj0CpuQ_1gYAn1yQy0bwsh7_

Here are the gv 11 study plans

 

[BlueBell]

Hi Jake,

Welcome to the Forum.

Fun stuff you're into.
I found it difficult to see the final result on your link.

I will answer your question but first would like to stress the relevance of choosing a proven design that meets your needs ( SOR ).
Failing that my recommendation would be to stay as close as possible to the design and simply adding more fg layers to the outside of the hull.
I would discourage mixing layers i.e. just use epoxy fg. If you get excessive wear and damage simply repair it.

I want this boat to be stout and safe.

I'm not sure that's attainable with your current plan / objective / SOR.

 

[Mrjake]

Hi Jake,

Welcome to the Forum.

Fun stuff you're into.
I found it difficult to see the final result on your link.

I will answer your question but first would like to stress the relevance of choosing a proven design that meets your needs ( SOR ).
Failing that my recommendation would be to stay as close as possible to the design and simply adding more fg layers to the outside of the hull.
I would discourage mixing layers i.e. just use epoxy fg. If you get excessive wear and damage simply repair it.


I'm not sure that's attainable with your current plan / objective / SOR.

Relatively speaking

 

[Mrjake]

Thank you blue bell. Not sure why my previous build isn't showing up. If you Google spira endurance build it will show up. Thank you for your imput. Do you have an opinion on the downsides of a heavier glass?

 

[BlueBell]

Not sure why my previous build isn't showing up.

I didn't say it wasn't showing up. I said "I found it difficult to see the final result on your link."

Do you have an opinion on the downsides of a heavier glass?

A heavier boat ( weight is the enemy of speed ).
And a change in the original, proven design.
However, you've already deviated substantially by over powering it.

 

[TANSL]

My main concern is that you seem to want to adopt solutions without having studied the problems the new boat may have. You can't provide a correct solution to a problem that you don't know exists or not. So, in my opinion, you should first study the problems that the desired changes pose and, secondly, once the problems are known, adopt the best solution (based on what you want to achieve).
The increased weight of the engine and the change in its position on board raise other issues to consider, not to mention the overall design of the transom.
Furthermore, and I'm still just expressing my opinion, plywood hulls are designed whit plywood to support all the loads acting on the structure. The layers of FG ensure watertightness, protect against chafing, and improve the appearance. Therefore, in my opinion, it's likely that you didn't need to change the fiberglass layers, but only the thickness of the plywood.

 

[BlueBell]

TANSL raises good points.

There is a considerable lack of information in your posts regarding what you're up to.
Firstly, what is your prime motivation here?
Go fast in a small boat?
Build one that is already a proven design for the task, sticking to the proven design, no significant deviations

 

[gonzo]

The original plan would go 25mph with a 10hp outboard and two passengers when build to 110lbs. I want this boat to be stout and safe.

A design that calls for a 10 HP will likely not be safe with 5 times the power.

 

[TANSL]

A design that calls for a 10 HP will likely not be safe with 5 times the power.

Naturally, the OP already knows this and that's why he wants to make some changes and asks for our opinion. Have you read the first post?.

 

[comfisherman]

6 oz of cloth on plywood seems mighty thin with a high probability of higher speed interactions with river hazards.

Id grab a swatch of 6oz and put it ona scrap of ply and then see what kind of deformation you can do to it by hand scraping against some rocks in the backyard.... might be a decent way to figure out what level of protection you need on the underside.

 

[Mrjake]

Excellent points thank you sir! Ideally I would build directly as the plans dictate. With the extra horsepower and much of the thrust being distributed across the bottom of the boat i may be better off building the entire boat out of 3/8 ply. I then could sheath with 1208 for abrasion and impact resistance when I inevitably impact something in the shallow water.

 

[Mrjake]

Excellent points thank you sir! Ideally I would build directly as the plans dictate. With the extra horsepower and much of the thrust being distributed across the bottom of the boat i may be better off building the entire boat out of 3/8 ply. I then could sheath with 1208 for abrasion and impact resistance when I inevitably impact something in the shallow water.

This was in reply tansyls fordt post.

 

[Mrjake]

For the record I understand im building a over powered boat which has risks. I would like to minimize these risks where possible and that's is why I am here for options and concerns from those more experienced than me. I appreciate all of the input and replies. Great forum