Jet boat build advice on ply thickness and fiberglass weight.

[Mrjake]

Have anyone ever seen a "jet jon" conversion? Im basically doing that but with a v bottom plywood boat with more freebore. Its not as scary as many of the 110hp jet jons out there going 60mp in a 160lb aluminum boat patched together with b5200 and prayers

 

[Mrjake]

I plan to add a sole fore of the pump and engine with buoyancy foam underneath. I will also be building it as a runabout with a dash, small windshield and a foredeck on the bow. Aft of the bench seat will be a small sealed hatch for gear.

 

[comfisherman]

I've seen them.... one of my lovable lunatic friends has had about every manner of mini jet arraignment imaginable over the last decade. His first one was a super jet 760 in a lowe with stick steer with the factory jet system. The last one was a 315 hp supercharged monster that had .5 inch uhmw over a 3/8 plate bottom with minimal ribs an in house made intake grate with an aftermarket reverse bucket.

My wife jokes that my life insurance policy only covers one run a year with them. It certainly is fun.

6 oz of cloth isn't a lot of thickness, nor is 10 oz frankly. If your going to be genuinely hitting hard obstructions its gonna take a bit more glass at least in the critical areas. I added several layers of 10 oz to some core testing for my dingy. Was amazed how much a couple extra layers helped. I could take a chunk with one layer and scrape through by hand, two took some doing and three took more than I had to scrape to core. Granted the right hit and anything is gonna have problems, but its worth a little bit of material to see the difference a few layers make.

 

[Mrjake]

I've seen them.... one of my lovable lunatic friends has had about every manner of mini jet arraignment imaginable over the last decade. His first one was a super jet 760 in a lowe with stick steer with the factory jet system. The last one was a 315 hp supercharged monster that had .5 inch uhmw over a 3/8 plate bottom with minimal ribs an in house made intake grate with an aftermarket reverse bucket.

My wife jokes that my life insurance policy only covers one run a year with them. It certainly is fun.

6 oz of cloth isn't a lot of thickness, nor is 10 oz frankly. If your going to be genuinely hitting hard obstructions its gonna take a bit more glass at least in the critical areas. I added several layers of 10 oz to some core testing for my dingy. Was amazed how much a couple extra layers helped. I could take a chunk with one layer and scrape through by hand, two took some doing and three took more than I had to scrape to core. Granted the right hit and anything is gonna have problems, but its worth a little bit of material to see the difference a few layers make.

That sounds awesome! This guy gets it. Its meant to be fun and its a creative venture. Thats what boat design is about in my opinion.

What do you think a about a layer of 1710 and a layer of 8 ounce in addition to the 3/8" ply bottom? I toyed with the idea of a flattened out piece of 1/8 thick aluminum angle and glassing it under a few layers of fiberglass tape running from just fore of the jet pump (would also act as a sort of spoon to help feed the pump) to the bow up the center of the hull to protect the area where it would most likely impact first in shallow water.

 

[comfisherman]

I dug for a scrap of 1710 but couldn't find any as its been a while since we used any. Only found some 2415 and 6 oz cloth... know some 10 is floating around the shop attic somewhere. Ill try and post up a Pic if I can locate several types all next to each other. Single layer of 6 oz wet out and you can probably read news pring through it.

Im a back of the napkin wild guess guy so dont take it as gospel engineering fact. Seems like the freight canoe guys like 2x layers around 10 oz as a minimum regardless of core for abrasion. Little dingy and mini jets are about the same size but lots more inertia so my hunch was add a layer. Obviously the ply is tougher at single point load like rocks on a beach or in a river than my high density foam dingy so maybe its not needed.

Aluminum is my least favorite wear strip, the plastics are so much better suited for sliding over stuff. My hunch is some thin uhmw would be better suited for cladding the keel. Its what the hard-core river guys use now, it can take a serious beating.

Seems to me jet pumps usually like clear semi flat unencumbered flow, although most the series river guys add larger aftermarket intakes on the varios popular donor ski jets. Usually guys run into the need for a different impeller as a the shape of the hull plus the weight of the seating arrangement is a bit heavier than a jet ski.

 

[skaraborgcraft]

I built a DK17 dinghy with a modified bottom. the 9mm bottom panel with epoxy 6oz sheath would ride like a carpet at speed in a chop with a 25hp, No harm done, but so freaky i added extra stringers on the inside to stiffen it up. Some flexibility is ok, but i would not have wanted to hit a solid fixed object at 40mph. Have you seen what happens to a car at 40mph?

 

[Mrjake]

I would love to see your build do you have a link? I hope to not hit an object at 40mph but i want to build as if I will at some point.

 

[gonzo]

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

The slamming forces are the least of your problems. Dynamic instability, when the boat can roll and capsize without warning, or other unexpected behavior, should be your main concern.

 

[Mrjake]

Found a great deal on 10 yards of 55" 1708. $90 shipped on a roll. Will be buying 6 gallons of Raka epoxy of the slow cure variety.

 

[Mrjake]

The slamming forces are the least of your problems. Dynamic instability, when the boat can roll and capsize without warning, or other unexpected behavior, should be your main concern.

Any suggestions on how to mitigate these issues??

 

[gonzo]

Any suggestions on how to mitigate these issues??

Yes, use a design that is appropriate for the power and speed.

 

[Mrjake]

Yes, use a design that is appropriate for the power and speed.

Nah, that wouldn't be nearly as much fun

Google Image Result for https://media.tenor.com/1d6cMCDcIPYAAAAM/motorboat-jump.gif https://share.google/images/DfDxRqjQrgFYbBhdf

 

[Barry]

In the PWC market installing high horsepower in an 11 foot craft is not unusual. ( 50 - 200 hp) Your 650 is rated around 50HP

BUT, PWC hulls are specifically designed to handle that type of HP.

You Garvey 11, G11. has a 10 degree bottom which is extremely dissimilar to the way that most PWC hulls are built. The G11 is not a PWC designed hull.

For some insight into the bottom profiles Google "bottom of pwc hull pictures"

You will note that most of the hulls have much more than the 10 degree deadrise that the pictures will show. Additionally they feature lift strakes which provide increased cornering ability

What I suspect may happen with the 10 degree, 11 foot boat.
1) The inability to make tight turns without doing a victory roll. On a PWC, they are built for this being fully enclosed and unsinkeable. **
2) The possibility of porpoising, high speed short wide hulls with low deadrise are prone to this**
3) The 10 degree deadrise will pound excessively at even minimal chop. I am thinking at crossing waves over 18 inches will rattle your teeth. Certainly, wake jumping will pound the
hull mercilessly.

** The outlet thrust angle on PWC's are such that they keep the bow down to resist porpoising and increase turning ability.

Shotover Jet video
Note: the thrust of the jets have been set up to keep the bow of the hull down and in contact with the water to improve steering control. Even with most of the keel in contact
with the water it is extremely easy to do a victory roll and these guys have it down to a full 360 degrees

Sprint jet boats, again notice the amount of keel that the boats have in the water. I am not sure if they have a cockpit controlled jet diverter to change the thrust angle during the race
or that the navigator and driver would not have time to make adjustments.

 

[Mrjake]

Seems as if I could add strokes to the sides of the hull with hardwood 2x2 ripped at a 45 degree angle glassed into the hull. Im hoping the driver weight being far forward on the boat will help with the porpoising. Jet conversions seem to work for v bottom and flat bottom jon boats im curious why this hull seems so problematic. I appreciate your post and the visual examples.

 

[Mrjake]

Strakes