New guy with jet Jon ?

[Fordracing2105]

Hey guys I have some questions about building a jet ski Jon boat. The boat I have now is a 1660 with a flat bottom. Will this haul work? Second is the boats only rated for a 40hp. Is there gonna be any problems with the dnr with it being over powered? I looked for awhile but didn't find anything about an over powered boat. I'm not doing this for it to be fast. I want to be able to jump up on plane in short distances and run shallow.

 

[PSG-1]

Horsepower ratings for outboards VS horsepower for inboard jet ski engines is comparing apples to oranges. For example, a 50 HP Mercury 2 stroke weighs 205 lbs. A 100 HP 2 stroke Tigershark jet ski engine weighs less than that.

Also, HP ratings for outboards are taking into account that the weight is entirely on the transom, and they must also figure in the lateral forces and torque that would result from running that engine. With an inboard jet, the weight is farther forward, and there is virtually zero torque being applied to the transom.

One other thing, is that since a jet drive is less efficient at converting HP to torque, a to achieve the same amount of thrust/torque as a given HP outboard, with the inboard jet, you almost have to double the HP.

My boat is a Duracraft 1648, with a 160 HP Yamaha MR-1 engine. No problems with DNR, in fact, they think it's an awesome boat. Also, I went through a company in Florida "BUC" and had an evaluation done on the boat, and got it insured.

So, to sum it up, you shouldn't have any issues regarding power.

And if jumping on plane quickly in shallow water is your objective, check out this video:

https://www.youtube.com/watch?v=0510oeVQoLM

 

[Fordracing2105]

That's a pretty cool video. I knew that a prop compared to jet was fair amount different but I had no clue in board to outboard was that much.

 

[PSG-1]

Yeah, it's a considerable amount of difference. I will say that an inboard jet IS more efficient than an outboard jet. The OB jet has to draw the water up, then turn it 90 degrees. Any time you make a 90 degree bend in plumbing, you lose flow, and the OB jet is no different. The inboard jet draws it up at a slight angle, and the exit is in a straight line flow, which means it is somewhat more efficient than the OB jet, but I'm not sure by what percentage.

 

[semojetman]

I seen the specifications on a website.
I dont know if it was set in stone but it was the typical 30% loss for the outboard jet and they stated 10% loss for the inboard jet. Not sure if these numbers are completley acurate but I feel they are close.

 

[Ranchero50]

What is your donor power coming from?

My '95 Seadoo Xp's 717 was rated at 85 hp, the pump is rated at 30 hp. My '92 1448MV hull as an outboard jon boat was rated for 25 hp on the data plate.

Being that you reconfigured the hull for an I/B jet your data plate is null and void so you should be ok with any knowledgeable DNR guy. Of course I also recommend talking to you PA DNR and checking their online regulations. Laws don't always use common sense.

The Coast Guards HP guidelines also make for good reading.

 

[Fordracing2105]

Thanks for the info guys. I was thinking some where in the 1000cc engine size. I guess this isn't gonna have the hp I thought it was gonna have. Does any one know what the hp rating is at the pump on a 1000 or 1200 yamaha.

 

[PSG-1]

I'd say somewhere between 45-60 HP at the prop. No, it won't be the amount of horsepower you were hoping for, but it will definitely be equivalent performance to having the max size outboard for your boat.

 

[Ranchero50]

Ditto, you should be more than fine. I'm stretched out to 17'6" x 48" bottom running mid 40's at around 800 lbs loaded. A 60" bottom is going to take more umph to get it moving but it should also pop onto plane easier.

 

[PSG-1]

And will require less draft at static flow, meaning you can float over shallower areas that you could with a narrower boat of that same length.

 

[Fordracing2105]

Do you think there will be any problem with the haul of the boat being flat and 60" wide?

 

[PSG-1]

Is it a completely flat bottom? Or is there a slight V? My boat has a 10 degree deadrise, and performs well on flat water to small chop, but in moderate to heavy chop, it cavitates. A completely flat bottom would tend to cavitate a lot more.

Ranchero50 built a 'spoon' into the hull of his boat to eliminate the issue of cavitation with a flat bottom, and the spoon does work, if you look at most jetboats, they will have this incorporated into their hull.

 

[Fordracing2105]

There isn't any v. It's completely flat except for the ribs that run front to back.

 

[PSG-1]

Then you will either need to incorporate a spoon into your hull, or form a 'delta pad' to sit the pump lower in the water.



Also, take out the center strake for at least 3 ft in front of the pump, because this will most assuredly cause issues with cavitation.
Cut along either side of that center "V" and replace it with a piece of 1/4" aluminum flat bar, wide, and long enough to cover the cut-out area and weld a continuous seam around that piece.

 

[Ranchero50]

Ditto, you'll need to spoon it or it'll only go about 15 mph before it sucks air.

Post up some pics of you hull and what you want it to do.

Here's mine:

And a shot of the spoon for reference.

 

[Fordracing2105]

Ok I'll get some pics up tomorrow. As for what I want to do with it or lay it out not really sure yet. Still in the planning stage yet I guess.

 

[PSG-1]

It's good that you're in the planning stage, as it's much easier to build something correctly to begin with, rather than having to go back and modify it later, that can really be a PITA. Take it from someone who's dealt with enough PITA, I should be running a bread factory, not a welding shop! LOL

 

[Fordracing2105]

Well I didn't have time to get pics and post and this is the only one I have right now.

 

[PSG-1]

It's definitely a 'blank canvas' so to speak!

I think if you do some reading on Ranchero's build, and some reading on my build, you should be able to gather enough info to help you through your project with minimal headaches.

 

[Fordracing2105]

I have been reading up on Rancheros build and it's pretty awsome. Still not through it yet though. Right now all my boat really has is a side console and a diamond tread floor it's pretty plain and I want to keep it that way. It I change this boat around ill probly go with a center console up against the front deck. The two seats in the back will stay depending in where the motor falls. There will be a hatch or two in the front deck for life vest and what not.