Flat Bottom Jet Tunnel

[dick stave]

I am getting ready to make an engine swap on my skiff.The boat in question is 14 ft. loa,60" wide 3/16 "bottom,78"beam ,1/8"sides all welded 5052 by myself "texas dory" style flat bottom.I am currently running a 30 hp. 2 stroke long shaft yamaha on a 5" setback manual jackplate ,aluminum doel fin setup.I have had really good success with this configuration,the engine is elevated approx. 4.5" and cavitates very little with a stock 12 pitch 3 blade aluminum prop.I run the boat in some real skinny water but I cant get everywhere I want to get in the river so I bought a new yamaha 40 hp. powerhead "30 jet drive" 4 stroke. My concept is to mount the new motor on a 5" setback plate, elevate it 2"-2.5" to get the intake foot slightly above the bottom and use the compression wave off the back to supply clean water to the foot .Im planning to fabricate a "step tunnel" if you will that will mount 2.5" above the bottom at the transom back to the leading edge of the intake with side skirts to channel the flow and feed the pump.I have done an exhaustive search online on this subject, and im well aware of the pitfalls of flat bottoms and jet pumps due to ingested air and cavitation.I still think it can be done in the pursuit of the ultimate shallow drafting/running riverboat.
I look forward to your ideas and suggestions.

[dick stave]

hmm. this idea must be right on....or way off

[marshmat]

Well, you could try it... it's very hard to say how well it will work, though, if it will work at all. The 'step tunnel' sounds like it will introduce a lot of turbulence; steps are usually intended to bring air in, and if you try to feed a jet pump with a turbulent aerated stream, the pump won't like you very much. Have you tried running with just the Yamaha jet on the jackplate, no fancy accessories? It might work better than expected in as-is condition.

[Corpus Skipper]

I'd have to agree with Marsh. Commercial fishermen in the back bays around here run flat bottom Vannoy skiffs with jets in some obscenely skinny water.

[dick stave]

Thanks for your replys Marshmat and Corpus Skipper.Good call on mounting up the motor on the jack plate and trying that first K.I.S.S. approach sometimes works best.I had another idea though, what about a "surf board"pad with a slight vee about36" long and 12" wide transitioning to a proper jet tunnel aft and mounting the motor directly to the transom.Kind of the opposite of a delta pad on a vee bottom inboard jet.It wouldnt have to hang below the rub rails with about 6 degree deadrise,and would split off the air bubbles carried aft .I will try your suggestion first ... even though im itching to break out the skill saw and go to work on the bottom(trying new ideas is where its at for me)Next project 17 ft.,66" bottom ,tunneled 200 optimax sportjet !