18' G3 tunnel jet Rigging with a prop motor???

[STVmod]

Has anyone set one of these up with a prop gear case instead of the jet lower??


Just got a 18' G3 Tunnel jet Center console. These were designed for Sport jet motors I think. Got a deal on the boat minus motor, going to set it up for a little creek boat and I have a 50 four stroke Johnson laying around. I rigged it out as low as it will go on the transom but the motor seemed high to me. 10 3/4 X13 prop, 20" motor, it also has flotation steps on the rear of transom. Went and ran it this weekend and here are the symptoms.

With any trim at all on the motor prop just blows out as soon as power is applied.
with motor tucked all the way down boat plains well and runs nice can trim a little but if I try to get the nose up at all the prop just starts cavitating.
very little reverse power prop just cavitates.

I think I will be happy with it for what it is... just think I can play with it and get it a little better. Curious if anyone on here has set one of these up with a prop, that could possibly save me some time.


324781324780

[Carter Powell]

You will probably find that a larger diameter four blade prop with a lot of cupping on the trailing edge will work wonders for what you are doing.

Best of luck, Carter Powell

[perfmarine1]

Nope that's a prop tunnel, jet tunnel is only 3" deep.

[STVmod]

Nope that's a prop tunnel, jet tunnel is only 3" deep.

I am running a 20" motor do these need a 25"? Just seems that the motor is to high. It is certainly not keeping the prop submerge.

Trying to locate a larger diameter prop but even with that it will be surfacing a prop that is not designed to. I think

thanks for all the input.

[perfmarine1]

I am running a 20" motor do these need a 25"? Just seems that the motor is to high. It is certainly not keeping the prop submerge.

Trying to locate a larger diameter prop but even with that it will be surfacing a prop that is not designed to. I think

thanks for all the input.

Sorry I forgot to come back to this. A good SS prop is what you will need. High rake and lots of cupping. 20" is correct, just make sure the water intakes are in the tunnel. That alum prop will not cut it. OMC raker or a merc trophy plus should work fine.

[FlatSleek]

Like the others have said a four blade stainless with some extra cup should help and IMO the best thing you can do is add a Large Texas style anti cavitation plate, the problem with that kind of tunnel is they shoot white water at the lower unit and prop which blows apart easily but the Large plate with turned down edges helps keep the water packed around the prop for much better grip. The grey tunnel was the first boat I built one for and wouldn't turn in a 100 ft circle without the plate, with it you have a hard time getting it to blow out no matter how hard you turn or trim up even with the stock beat up 3 blade aluminum prop. The difference is absolutely Amazing. The dimensions of that plate were aprox. 1/8"x 17"x27" with 2 1/2" on each side turned down 55 degrees which leaves 12" on top. I've made them for many boats since that one and it's been a substantial improvement on all of them but a tunnel will definitely benefit the most. If you don't have the tools to cut and bend the aluminum you can make a cardboard template and take it to a local fab shop for a few well spent $$$ they'll make one.

[STVmod]

thanks for the replies... went with a 12.5X13 3 blade aluminum which I had laying around with a 6"setback off the transom and is working Fairly good for me now. I will break up a cavitation plate this week and give it a try. never thought about using one to keep water packed around the prop always used them to plane faster. For Now I will keep my eyes out for a ss equivalent but this is not my go fast ride we use it to bump around the creeks and get to the hard to reach places. When my son gets a little older (he is 6 now) I will probably set it up with a 90 Yamaha which I have sitting in the shop or possibly a 115 if I can come across one by then.

Again thanks for the input!!

326087326088326089326091