jet drive on flat bottom

[Rusty Bucket]

I am currently building a 25' alum. dory skiff, it has about 22' of flat bottom 5' wide. The guys at hamilton jet told me that flat bottoms don't work well with conv. jet drives but my experience with jet boats was that most of them were pretty flat. Boat will be used for inshore comm. fishing. Any comment will be appreciated. Rusty

[tri - star]

To Rusty Bucket,

If you hit the search button - put in " jets, jet pumps...."
you will see a lot on jets.
I draw your attention to the THIRD quote down on this post.
Re: the EFFICIENCY of jets. Which I agree with.

The dory is unquestionably, a seaworthy and practical vessel.
However, there are some KEY concerns where dory design and
good practice in jet design don't easily go together.

....'ole Mr. Hamilton would also advise you to have a fairly
CONSTANT geometry for at least 6 ft. ahead of the intake.
This isn't so in a dory.
After all - it gets it's forgiving nature - by the very fact that
the curves in the chines VARY the geometry......

It IS possible to make some kind of - high thrust, low speed
- industrial type jet work in your boat.

I'm up to the challange.....did it with a vessel like yours.
However, dispite my cleverness, the final results will
likely disapoint you.

Cheers !

[Rusty Bucket]

Thanks everybody for the info. I have actually already configured the boat for an outboard and the information you directed me to just confirmed what I have been told by the Hamilton Jet dealer, I appreciate their honesty. This is a classy forum.

[Tim B]

The problem with all flat-bottomed boats (not just waterjets) is directional stability and control. Notably, that on a flat-bottomed boat there isn't any. You'll need to put a fair-sized skeg on the boat to re-gain some of the directional stabilty that would have been present with a V hull.

Flat-bottomed boats tend to slam a lot and slide round corners a bit like a hovercraft. That's why so many boats have deadrise.

From a jet inflow point of view, you don't want any massive change in geometry (steps etc.) ahead of the inlet, but smooth, fair variations in geometry should not pose a problem. You certainly can't get much better than a flat bottom for inflow purposes.

Tim B.

[KawRiverrat]

Rusty, one of the biggest issues with a flat bottom when running a jet is cavitation. A slight bit of dead rise is desired to split any air before it is in jested into the pump. This can also be achieved with a radius design . But much harder for a home builder to achieve. If running shallow water is desired a delta pad design is my favorite

[breammaster]

Rusty,

I have just been through what you are looking to do, and ended up building a false bottom over the previous flat bottom on my flats skiff. Check out nzjetboating.com, and look in the projects section (inside the forums) for "jet powered flats fishing boat". Heaps of photo's of how I went about it, and the result.

Hope this helps.

Cheers

Warren.