Modify the old hull?

[JetSled]

Re: Modify the old hull?

The transition was aluminum flat plate bent to the contour of the underbelly... it is kinda flat.

Hmm... if this is bad.. do you recommend a round (spoon shape) or V-shaped transition, similar to what I had before?

 

[JetSled]

Re: Modify the old hull?

Here is another angle of what I have...

 

[ondarvr]

Re: Modify the old hull?

What you're looking for is a rounded bend into the tunnel, think of the water flowing under the front edge at 40mph and then needing to follow the curve without forming any kind of air pocket. A sharp or sudden contuor change won't allow the water to follow it at speed.

You will also find that as you move the tunnel foward it will typically collect more air, not less.

 

[JetSled]

Re: Modify the old hull?

I'll form it as you suggest... thanks.

Time will tell whether moving this 'recess' forward will load the intake better. There was virtually no chance of water flowing well into the intake previously... at speed.

I have thought about adding a small grill aft of transition, to purge air. This would funnel back through the transom.... flowing air/water at speed. Just a thought for now.

 

[JetSled]

Re: Modify the old hull?

I finished adding bars to the grate...

Will be 'seatrialing' when I get some time....

 

[JetSled]

Re: Modify the old hull?

OK.. so its been sometime since the hull was modified. I launched her today to see if she still floats.

To recap, I repaired a ding in the impeller, added more bars to the grate, and carved out a longer trough immediately preceding the intake grate.

She gets on step awesome... no cavitation from a standing start, nor any while under power through to full throttle. There was a 10 knot wind and a little chop in the river today, but the boat screamed very nice. O - 50mph very quick...

I did notice the rpm's went from 4300 wot, to 4500 wot. This is weird, as I was expecting the jet to load better and drop in RPM. Maybe the turbulent water doesn't load the impeller as well as before, but there is no cavitation.

I also noticed the bow dropped in altitude, from were she used to run on step. I was expecting it to go up, as I removed the section aft of the intake grate, allowing the stern to ride deeper in the water and getting rid of full trim-tab-down type effect. I will trim up the jet nozzle a degree or so and see if I can raise the nose up a touch. She is running almost horizontal at the moment.

I am very happy with the performance, and happy I did it.

 

[ondarvr]

Re: Modify the old hull?

Its always nice when the plans works, good job.