Cavitation Problem with 25 horse Johnson

[Aquaman-PSD]

I just bought a 74 johnny 25 horse for my duck boat. It purrs like a kitten but seems to lack a little go-go at WOT. I have a feeling it is cavitating a little bit and loosing power that way. I am not sure what the pitch of the prop is but it is a little beat up. Will a hydrofoil help cavitation or not. I would think that with a 25 hp motor and two average people on a 16ft john boat it should pop it up on a plane at least. I am think that maybe shimming the top of the motor to give it a little bit of an angle will help it get on a plane but that won't solve the cavitation problem.

 

[Bondo]

Re: Cavitation Problem with 25 horse Johnson

I just bought a 74 johnny 25 horse for my duck boat. It purrs like a kitten but seems to lack a little go-go at WOT. I have a feeling it is cavitating a little bit and loosing power that way. I am not sure what the pitch of the prop is but it is a little beat up. Will a hydrofoil help cavitation or not. I would think that with a 25 hp motor and two average people on a 16ft john boat it should pop it up on a plane at least. I am think that maybe shimming the top of the motor to give it a little bit of an angle will help it get on a plane but that won't solve the cavitation problem.

Ayuh,.... Before ya do anything, rebuild or replace the prop,....

A Beat-up prop is like drivin' cross country on Flat tires,....

Their Both, where the Horsepower meets it's traction/ load,...

 

[kfa4303]

Re: Cavitation Problem with 25 horse Johnson

Hi Aquaman. Generally speaking, hydrofoils are a bit of a gimmick and a band aid. It's better to take the time to try to dial in the best trim/tilt position for your motor. A lot of your boats ability to plane out will depend on these factors as well as the weight distribution and shape of the hull. In most set ups, you want the anti-vent plate, found just above the prop to be level or slightly higher than the keel (bottom) of the boat. Most folks also find that the 2-3 pin position away from the transom works best. If it feels as if you're "spinning your tires" (i.e. revs, but doesn't get any faster) at high speeds, then you may have a spun hub in your prop as well.
I would avoid any sorts of shims as well. You may find that you need to adjust the height of the motor on the transom, as I did on my '66 20hp, in which case you'll need to build a small transom riser. You can also improve the performance by moving weight around in the boat. Is it tiller drive, or does it have a helm? If it is a tiller you can actually make a simple tiller extender out of a piece of PVC and couple pipe clamps. I use on my motor and it's the best mod I've ever made. It lets me sit, stand at any speed and move around with in the hull. Best of all the extender only costs about $3 and pops off in 5 seconds.
Weight distribution is important as well. Most boats, especially older ones assume and average weigh of each person of around 150lbs. If you're much more than that, then the performance will be effected quite a bit. Excess weight in the rear of the boat can cause it to squat and crate a larger than normal wake which can form air pockets and may cause some cavitation issues as well. You can compensate by placing you and your passenger(s) inline, rather than side-by-side in the hull as well as by moving heavy items like batteries, fuel tanks and anchors around as needed.
As for overall speed and getting on plane, by means of comparison, my '66 20hp on a 14', semi-V hull tinny will push me (160lbs) along at about 20-25mph and hops upon plane in about 5 seconds. Those number go down a bit if I have passengers on board.

 

[NYBo]

Re: Cavitation Problem with 25 horse Johnson

Hi Aquaman. Generally speaking, hydrofoils are a bit of a gimmick and a band aid.

I have never understood this characterization. A bandaid implies a temporary, inadequate fix of some deeper problem. If a 'foil works, it works, and if it doesn't, it doesn't, plain and simple.

But I agree the first step is to get the motor positioned correctly on the transom.

 

[roscoe]

Re: Cavitation Problem with 25 horse Johnson

I highly doubt your prop is boiling off water on a 25 hp motor. Maybe ventilating, but not cavitating.

Replace the prop.
Check the mounting height of the motor; anti ventilation plate should be at or just above the bottom of the hull.

 

[Aquaman-PSD]

Re: Cavitation Problem with 25 horse Johnson

well I found the problem... this is the first time I have bought a small tiller motor... turns out with a 16ft tracker you need a long shaft.... yea I bought a short shaft :facepalm: looks like I'll be selling it and buying another. Its a shame because that motor runs great.

 

[kfa4303]

Re: Cavitation Problem with 25 horse Johnson

I see. Does the boat have a 20" transom? If so, then that is definitely your problem. Most new-ish boats tend to have a standard 20" transom, whereas many vintage hulls tend to have 15" transoms. However, OMC motors of the 50s, 60s, 70s have sort of an odd, 18" leg length. As a result, they usually, have to be raised a bit on 15" transoms, but are too short for many 20" transoms. Glad you got it figured out. Although, it may be cheaper to simply convert your current to a long shaft. All you need is a longer driveshaft, water uptake tube, lower shift rod and housing extender. It sounds like a lot, but it's a pretty easy job and most extensions are sold as complete kits. Check out ebay, aomci.org and other sites for prices. You might be surprised.