Jet boat gurus

[Creekchub]

I bought a used jon with an outboard jet. The boat gets on plane really quick and very minimal splash at the transom.

But, with a straight edge, it looks like the foot is 1 inch too low. I can raise the motor one hole which should be about an inch.


Should I go ahead and raise it one hole?

 

[FuzzyGrub]

Still learning here. If you are not getting cavitation, Outboard Jets recommends raising 1/4” and test again. When you get cavitation, drop back down 1/4”. You need a jack plate to be able to do 1/4” increments, though. I don’t see any harm in trying to raise it a full hole. They recommend starting at even with bottom of hull or tunnel, anyway. You could also try side fins and splash plate if you get cavitation. You should see an improvement in top speed.

We are getting some cavitation in chop, so adding the side fins and splash plate, for next season. If that tests well, will raise the motor one hole. The holes are 3/4” on center on the Yamaha.

Please post results if you decide to try it.

 

[Creekchub]

Still learning here. If you are not getting cavitation, Outboard Jets recommends raising 1/4” and test again. When you get cavitation, drop back down 1/4”. You need a jack plate to be able to do 1/4” increments, though. I don’t see any harm in trying to raise it a full hole. They recommend starting at even with bottom of hull or tunnel, anyway. You could also try side fins and splash plate if you get cavitation. You should see an improvement in top speed.

We are getting some cavitation in chop, so adding the side fins and splash plate, for next season. If that tests well, will raise the motor one hole. The holes are 3/4” on center on the Yamaha.

Please post results if you decide to try it.

Thanks for the info. I’ll post a follow up once I get it raised and a test run

 

[Bob La Londe]

A true jon boat with a flat bottom runs air down the entire length of the bottom of the hull. A neat effect of that is in a light chop they tend to make a sizzling noise and run faster on a partial cushion of air. However, jets hate air. It is possible that being an inch below the transom may be the highest it can be and stay out of that aerated water. Still, it's worth raising it a quarter inch at a time or lowering it a quarter inch at a time to see if you can get your performance to improve slightly. I tried a jet on a 50 on a Waco 2050-16 which is a flat bottom jon, and I wound up running with the leading edge of the shoe well below the transom. Personally I didn't think it gave me much shallow running capability improvement over my regular outboard lower unit because of that. It also dropped my top speed from 42 to 28 with a massive over rev.

If you look at the website for Outboard Jet company in San Leandro California they suggest a shallow v with a properly designed pocket tunnel for optimum jet performance and shallow water running. The shallow v sheds air bubbles off to the sides instead of funneling them to the back of the boat, and probably design tunnel actually gives you a clean stream of water off the bottom of the transom. I don't remember all the details but if you want to get into it it's all on their website.

 

[FuzzyGrub]

What boat and jet did you end up getting?

FWIW: The intake fins come in medium, large, and x-large, to fit the equivelent sized jet foot. They run around $100.

From OBJ website:

Intake Fins are designed increase water pickup. This will reduce cavitation, particularly in chop. They also act as a makeshift rudder which will improve handling especially on flat bottom boats. Intake fins are a good fit on almost any setup. The only possible downside is that they run the risk on snagging however this is not an issue of tunnel boats.

 

[Lil' Blue Rude]

Yes your motor can come up more. I usually aim to have the straight edge hit the top of the pin in the front of the shoe that holds your grates in. That's a good place to start depending on the condition of the bottom of the boat you may be able to go higher then that. Every little bit you raise to motor will gain you too end.

 

[JL8Jeff]

Mine is a jet tunnel but I kept mounting 1 bolt higher until I got spray back and some aeration and went back down 1 hole. My foot is almost tucked into the top of the tunnel so it looks like you could go up 1/2" but if your holes are too far apart it might not help. It's worth trying though.