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Twin jet outboards

Posted: Tue Jul 07, 2020 8:17 am
by gandrfab
Just thinking on line.
How about a slight V barge like boat 22' X 8.5' to 9' with enough of a V to give it about a 5" rise from keel to chine.
I'm thinking something along the bottom to help keep it from sliding in turns, be it angle or plate fins on the edges? both I don't know. (I do have airboat and flat bottom skiff experience and know a little about sliding hulls)
About 22" from flat deck to gunwale, can this be built sealed below deck? Possibly open belly with cover plate for rigging a center console set up.
Powered by twin outboard jets, Yamaha 90's off the to top my head.

Possible use's trailer life, shallow waters of Fl shrimping and fishing in salt and speckled perch in fresh.
Gator hunting the Saint Johns river.
Through hull lighting pointing out to the sides below the water line at rest would be the cats meow for shrimping.

Is something like this already drawn, available in cutting files, already in kit form...needing adjustments..

Re: Twin jet outboards

Posted: Tue Jul 07, 2020 8:05 pm
by kmorin
This is for shoal draft "fishing & fiddling" so is the word "barge" used to imply a 'pram' bow- big wide jon boat? No V bow, just a ramped up scow shape?

I've read a bit about flats boats and noticed that now they're featuring a 'jet tunnel' with pretty steep sides. So a pair of these tunnels in the after 1/3 of the hull should allow the jet pumps to keep clean water to their intakes and still stop slewing in a turn due to the steep sides of the two tunnels?

Not sure if that's true? But a few flat bars along the the bottom between two tunnels would surely be enough to keep a big sled from slewing/sliding too much in a turn?

I'd suggest building it as a flat top surfboard" where the deck to keel was done on a rotisserie then the topsides added after the entire below decks is completed and air tested.


here is an image of a 25' offshore saltwater day boat hull bottom built this way. The entire hull, all framing, conduits for controls, structurals of all types and deck were done- and entirely air tested to 3.2psi then.... the topsides were added to the bottom - so the 'hollow surf board' concept could even more easily apply to a hull with less curvature or V.

building on a rotisserie can reduce the crawling around component to a shallow hull build.


not saying these images of a V bottom hull are substitute for a jon boat/scow's build; just all I have to make the point about how such a project could come together from the build perspective.

Kevin Morin
Kenai, AK

Re: Twin jet outboards

Posted: Wed Jul 08, 2020 8:30 am
by gandrfab

Some how I missed this "I'd suggest building it as a flat top surfboard" where the deck to keel was done on a rotisserie then the topsides added after the entire below decks is completed and air tested." Before typing everything else.
Sound promising to build like that, still have to work out the lighting.

That is a neat looking deck over V hull.

As ugly as I see a pram bow that may be the answer, we currently have a glass Carolina skiff Standard 17' that I bought new in 1997 it's still a great boat. The gunwale is important to me, something to keep coolers and stuff on deck or catch our balance.
Not our boat or trailer, same model
zzzb1.PNG (133.31 KiB) Viewed 3604 times
It is a flat bottom with roughly 1 1/4" strakes on the chine and a about a 30° 6" angle to the knuckle. not sure if knuckle is the correct term. The deck to the hull bottom is about 6" thick foam filled, the rigging from CC to motor is above deck with a track cover screwed down (trip track) It is not self bailing and has a glassed bilge recess aft center about 10" x 12" to pump water out of.
Wanting to improve on that in length, width and material. The slight V idea isn't necessary, I think it would be nice at least aesthetically and make a happy builder.
I'm hoping the chine won't have to be that busy as the glass hull but may be necessary for the under water lighting. I would be building without a shear or break at hand.

Tunnels or pockets for twin jets sounds like it could be a nightmare to tune water flow to the intake.
If the tunnel is the way to go maybe a bigger single.


Re: Twin jet outboards

Posted: Wed Jul 08, 2020 9:35 am
by Wantry
Given the power-robbing nature of jet pump outboards (a necessary evil and a lot of the time a worthwhile tradeoff for the shallow water capability) -- I would think a big single would be easier to build, outperform, cheaper, and simpler to rig and maintain.

I also would have a think about running prop OB on a hydraulic jack plate. With or without a little tunnel. P/W ratio is better, performance in grass is better, i like props in heavy sea and I really don't like running a jet in those conditions, etc. it all depends on your draft requirements and what kinds of underwater things you are thinking of hitting, and personal preference. We run 18' and 20' shallow V utility boats rigged with Bob's plates and Yamaha propped outboards and it works out pretty good.

Re: Twin jet outboards

Posted: Wed Jul 08, 2020 12:56 pm
by kmorin
G&R, I don't want to leave you the impression I know about jet tunnels- done some big prop tunnels years ago- and tuning them to run well, singly or in pairs isn't something I can intelligently remark about?

As Wantry says, maybe the big single is the best choice? but all the different conditions mentioned so far are going to have to be considered? Actual water depth of operation both planing and polling; bottom shape (dead flat or V'd to some degree?); chine to topsides configuration Body Section? and deck to hull Body Section.

While the pram bow, scow bow, jon boat bow may not be the most pleasing lines on the water? The scow hull you show isn't bad looking, the bow lines might have swept a little more and might look a little more attractive if the chines didn't turn up so abruptly that close to the bow? But that's in the eyes of the beholder- not the builder!!

I don't know if the boat being discussed would ever see waves come aboard? IF so: I'd want a self bailing deck- personal choice- and IF not: then sealed deck and bilge as flotation wouldn't be as important except for cleaning and washing down considerations?

I don't know how the skiff/scow, shown, performs from experience but it is possible the chine to topsides transition was included to reduce/eliminate chine tripping? If the boat is dead flat and was turned hard over? you've mentioned you'd had experience with a hull sliding or slewing in a turn? Then a chine that wasn't 90 would allow some of the built up wake - to slip under the hull when it was dipped into the surface at speed- like in a turn.

The Boston Whaler shape - to some degree more or less- uses a similar chine on their relatively flat hulls. In the smaller Whalers there's a similar chine ("knuckle" is as good a term as I've heard) that keeps the topsides to bottom (chine area) from being 90.

These surfaces can all be easily developed so there aren't any compound curves to form, meaning they'd all come out of flat sheet goods- NC or hand cut. Adding seams to the hull, means more fitting and welding, but if the surfaces produce improved performance, like a reversed chine deflecting water and helping add to the planing area, then its worth the work to get the improvement in hull shape performance.

My personal view is to make everything at least 20'-1" long to avoid foam filled voids and the resultant headaches I've seen with foam in welded boats.

Kevin Morin
Kenai, AK