Jet tunnel size and performance?

[NotAGuide]

New member looking for info on a recent project boat I acquired. Its a Texas scooter style aluminum boat with what I consider to be a oversized jet tunnel. I've seen one other hull similar to this but didn't get a look at the tunnel. From what I understand, several were built in the 90's. It currently has a 50hp Yamaha prop motor 25" shaft on it due to the tall transom. With out measuring it, its aprox. 16" wide and 18" long and 3" tall. Most tunnels I see are very narrow and around 2" in heigth? Will the addtional tunnl width be a problem with performance? It's a heavy built 13' hull, no sides, and a deep key slot transom. It currently will cruise 24mph with a top speed around 30mph with a prop. Any idea what the performance would be with the 50hp Yamaha jet? A F60/40 would be my choice but adds 60lbs from what I've read and aren't cheap. Trying to upload photos but having difficulty.

Any free advice will be appreciated and accepted. Thanks for a great forum!

 

[smackdaddy53]

 

[smackdaddy53]

Here is my tunnel, I built it to the exact dimensions as a local outboard jet guru has on the boats Alweld custom fabs for his boat shop.

16" wide at the top, 19" wide at the bottom, 36" long, 24" from the front to the part where it flattens out and 12" of the flat part past my transom.
I extended it 14" past my transom to eliminate too much loss of floatation and to work better with my sponsons.
It also has a little spoon to the larger part under the hull to channel aerated water away from the intake.
It is 2 3/4" deep at the rear.

 

[NotAGuide]

Your tunnel is closer to what I have. The manufactured boats like SeaArk, Lowe, etc. seem to have much smaller tunnels. I'm just concerned why a previous owner would take a jet tunnel hull and change to prop. Makes me think there's a possible issue. Maybe not.

The tunnel picture makes it looks larger than actual size. The trailer frame cross member is 3" for reference. I don't think changing the 50hp to a jet foot is going to perform like I want though.

 

[smackdaddy53]

My boat is going to be used in the bays down here in Texas so I really wanted to build a hull that will run skinny and jump up skinny. River boat tunnels seem to be rigged a little different than mine.

 

[NotAGuide]

This boat is also for the middle Texas coast. I'm pretty sure I figured out why it doesn't have a jet on it now. Ran it across the scales this weekend and it's 1880lbs with trailer. This is with the 50hp Yamaha (187lbs), two battery's, power pole, and 14gal of gas. Even if I shaved the battery weight, a little gas, and misc. items, I'm still at or around 1300-1400lbs for the hull and motor. Add two people (400lb) and I need a 90hp motor? It's built like a tank and a really cool little scooter but might have to scrap the idea of it being a jet.

 

[Rat]

Most bay skiffs run a prop engine. Do you know for sure it had a jet on it in the past?
Jets don't perform in rough seas or heavy chop as well as prop driven boats.
Jets are usually heavier, due to lower performance curves, than the prop driven engines; flats boats are built to be light as possible.

Before everyone jumps on Rat for the above statements let me say this: this is true of production boats! Building a custom hull like Smack's is the exception. His is top notch and, except for the weight difference Vs. HP, looks like it will perform as well as any prop driven engine.

What I'm saying is there may not be an issue with the hull; it may have always been a prop driven hull. Also, don't worry about adding a jet drive and heavier motor; that skiff looks like it could handle a heavy motor. Just check the specs and rig it for that HP. If you do it right, you will LOVE the jet drive; especially since you already have the tunnel.

 

[NotAGuide]

From what I've been told, the guy built a few of these rigs and they were jets. The tunnel is less then 3" high which is not the optimal design for a prop. I've owned and been around a lot tunnel prop boats and currently have a large tunnel hull (Shallow Sport) but know very little about jets. Trial and error can get expensive is why I've posted. I am wanting a jet is for hole shot and the capability to stop in places a prop isn't capable of stopping in. It's a matter of a couple of inches whether your getting out or not. I'll be running in very little open water so cavitation isn't a concern. I've been the airboat route and wanted to try a jet this time.

As for the "specs" on the rig, it's a home built title and not rated. I would love to find a 70hp Yamaha with a pump. Here's a better pic of the tunnel.

 

[NotAGuide]

Here's a another view of the boat. It's a cool little scooter just needs a lot of TLC. The rigging is pretty bad and I'll be removing a lot screws and hardware and welding up holes. I haven't figured out the bottom thickness but its at least .125 or 3/16 with 1 1/2" angle structure. The 50hp has around 95hrs. per the gauge which may be off. It's in really good shape so wouldn't doubt if the hours are right.

Thanks again for the comments and help.

 

[RiverBottomOutdoors]

Cool looking little boat. I like it.

 

[huntinfool]

[No message]

 

[smackdaddy53]

Very cool little scooter