spoon design for jetboats wanted

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brianb2247

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look some of you guys have great ideas on this forum and when a question is asked you usually redirect to someones build could some one pose me a pic or two for spooning the underside of an inboard jet if you could provide measurements or little tricks to install one please need help with cavitation .
 
I have just starred to make a spoon for my jet jon after having some pretty bad cavitation problems. I don't know If my design is the right way of doing it or not
 

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I started with a 4ft × 2ft .90 thick pice of aluminum made the top a hair wider then my intake 6 1/4in. Then I clamped one side of sheet all the across on a table and bent 3 3/4in to slope my sides down to a angle I was happy with
 
I still need to finish the front ramp part of my spoon. Having a hard time finding a perfect way to get a nice gradual front slope along with having being a spoon shape in the front
 

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looks good i was kind of thinking the same thing just wasnt sure how long to make it
 
I too had cavitation problems with jet ski insert flush with jon boat hull.

I dropped the jet ski insert to about 1-1/2 inches below hull. It is level along it's length.

For the transition, I used the material that was cut out of jon boat (for insert).

It is about 4 foot long, so it has a gentle taper. Since my old mig welder hates feeding aluminum wire, I just tack welded it in (at least 100 tacks!)). I used epoxy resin & fiberglass cloth to seal it up. So far, leak free.

Performance wise, I can run in 2 inch chop & no cavitation, so it was worth all the work.

Edit: I also re-shimmed the pump mount. so as to keep the thrust angle similar to before. (this was no small task either).

new bottom 002.JPG
 
I was able to some work making my spoon I was wondering what you guys thinks so far if I'm heading in the right direction
 

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I only have one side of intake tac in right now still have a lot of work with smoothing everything out
 

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Bx - Your spoon looks much like the one I built. Should work great. Hope you have a good welder, those are long weld joints!

BTW, the steering response on my JJ is now heavier than before. Probably shouldn't be a surprise. It's not terrible, but definitely not as fast of response compared with the flush intake.
 
I have a 200 miller spool gun welds pretty nice I was definitely going to weld the whole front down and grind it smooth for a nice transition I was thinking about laying a couple 3inch welds down the sides and flex sealing rest of the sides less welding less chance of having leaks also I'm worryed about the angle of intake seems to be a little steep ( not angle of the spoon intake ) I was never able to get my boat to go over a quarter throttle before it spin the pump or lose prime which I found my jet unit was leaking outside boat which means it must been sucking air sence it wasn't totally sealed
 
I made my aluminum intake almost exactly like the fiberglass tigershark intake
 

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Kinda hard to see in this picture my jet is level coming out of the boat and I also have a trim nozzle
 

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I se what your intake up-angle concern is & I think it valid. If not too late, i would start the transition much earlier. If you could put a radius on the spoon (sideways) that would be a bonus. The water wants to follow a curve.

I did most of the forming of my spoon with stacked/screwed together lumber & bouncing on the sheet to get it to form. I also fab'd a brake press, but it was limited to short section.
 
Now if my boat runs and moves at like a 1/8 thottle with out cavaition does that still mean my intake ramp my be a little to much of a angle or is that all air bubbles getting into pump related. It loses all prime when it seems to get ready to get on plain
 
I have some more time to work on my spoon design
and I almost got done
 

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Tired to get the nicest transition I could but it's kinda hard leaving the center joint in
 

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By looking at all these 'spooning' hulls I've been looking at all over, doesn't the end of the 'spoon' shape
have to end right at the intake of the pump?
Or from what I'm seeing here is that the Spoon sticks out a bit all the way to the input.?

This isn't sounding right.
When I look at the hull of my 'ski', it curves down and then up slightly and flattens out.
So when the water follows the curve (or hull) downward it follows rearward and actually pushes up a bit into the impeller
opening?
I'm at work and can't add any pics or renditions.

thanks E
 
Hey Jon - You are right, a true spoon is convex & wraps back up to feed the pump.

Mine doesn't follow a curve, it just is a straight taper, but it only grows 2 inches over 4 foot.

I think the most important factor is not to have any 'steps' that create turbulence.

Smooth shape all the way to the pump!
 
Older thread, I know, but had to speak up. What I am seeing for "spoons" here resemble deflector more than spoons. A spoon is very shallow and is not always the answer. The angle if the leading edge on your intake is very inportant. Too sudden or too steep will cause loss of suction and cavitation. You should not be able to feel any transitions in the hull forward of the intake. These will make turbulance. A keel is normally ok, as long as it tapers out smoothly at least a foot before the intake. The key here is to make it smooth and gentle. If you can taper it out over about a foot, great. If you are making your own intake, be prepared to spent a lot of time getting it right. You will be working with progressive duplex curves. This is not easy to do and get it right. The trailing edge of the intake needs to be smooth/rounded. This can be a bigger problem then the leading edge when trying to get out of the hole because the impeller is pulling lots of water and the boat is not going very fast to push water into the intake to the suction can pull air under the transom from the rear in some cases.

A deeper keel will improve steering, but will tend to cavitate more in turns.
 
I can’t count the number of people asking for spoon or ventilation problems on the internet.

The best answer is to build your boat differently. I use the jet ski hull as a “pad” when I build.
It works great and is not too hard to do.
 

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