BRACKET OWNERS

[fillet1]

Ok, Ive got some hours on the boat now and have a few questions for you.

I have a 1999 Johnson Ocean Pro 115
I cruise at around 4000 which gives me 22kts.
WOT at 5500 gives 27-28 kts with people, gear and bimini/full curtains.

The speed is fine for me. I fished three full days in the bay, moving from spot to spot, running a couple miles out and back etc and when I filled the tank it took only 17 gallons. That is more efficient then I expected.

If I trim up at all the motor cavitates very easily. If I make a sharp tun at higher speeds, the prop cavitates.

I was thinking of bolting on a hydrofoil, (I ordered one and then cancelled the order) but I cant help but think that I should drop the motor a notch or two to give the wheel a better bite?

I think the problem stems from the fact that I have a light 115 hp 4cyl engine v 6 cyl on the bracket. The heavier motor would weigh down the a$$ end of the boat a little more? (Incidentially, a heavier motor would help my scuppers drain a little better too)

My question is whether the hydrofoil will help the cavitaion problems. It does not cavitate constantly, just occcasionally. I am hamstrung with the trim control in all but flat conditions though. The hydrofoil would be the easy fix. To drop the motor would be a PIA at this point. I dont even know if there is any room to drop it on the motor and I would have to pull the boat etc.

How about I just fill the livewell with sandbags to make the stern heavier ;D ;D ;D

[knightfisher]

I have a 200 mercury on the back of my boat without the bracket But with mine trimed up it will cavitate also in turns. Not all the time but it does.

[CT_V-20]

Fillet, what length is your shaft? With my Sea Drive, which was an XXL shaft (25") I had no cavitation and it was effectively a 115 like yours so the weights should be the same. The new motor I bought is a 25" shaft as well. But I would imagine that if I trimmed my 25" shaft high enough, then I would cavitate with that too - on turns especially.

[fillet1]

Its a 25 inch shaft.

[chumbucket]

fillet1, I don't think a hydrofoil will help that situation. They are designed to lift the stern, not keep it down. I think you may be right with the weight issue or perhaps they mounted the bracket a bit too high when things were changed over. Possibly? ???
One thing you may want to try is a four bladed prop. You'd get better holeshots and better bite in turns. Maybe a bit (negligable) off the top end.

[CT_V-20]

That is odd fillet because you've got a big enough shaft for sure. *??? Can a jack plate be used on a bracket?

[macojoe]

I have the 175 on the bracket, and when I get the thing moving and get into the turns it will do the same as yours.
I run the hydrofoil and it helps get me on plain fast!! I think its great!! But I stii cavatate.

I get 34mph at 5000, And I am also very good on gas!! I think I am close to 3 mpg.

[chesapeake724]

I don't have a bracket, but I'll give my two cents anyway... ;D

Is it possible your motor is mounted too high? I ask this because you shouldn't be getting cavitation when you trim only a little bit up. If it's an option, I might drop the motor one set of bolt holes and see what happens.

The other thing I'm thinking is that you're under-propped. The WOT sounds right, but cruising at 4k rpm sounds a little high. If you get the same diameter prop and go up in pitch by 2", that might cure you. (Yeah, I know, I'm not the one writing the check, so it's easy for me to propose it.) More pitch should give you more bite, as it sounds like whenever you put an extra load on the motor (like carving a turn or lifting the bow out of the water), the prop isn't gripping the water and you're getting cavitation. Going up in pitch will cut down your WOT rpms, but how often do you go WOT? Is 5100rpms still in the recommended range for WOT? It should knock down your cruising rpms to a more comfortable 3600.

FWIW...YMMV

[reelapeelin]

I think that motor's gotta come down some...it just ain't gettin' enough BITE...extra weight may help, but sounds counter-productive and only a band-aide...

F1, I don't have a bracket, but everything you describe spells "prop too high"...

[jaysea]

hi,ithink you should lower the motor ,also i dont know where you have your transducers mounted ,they mite be giving you some air in the path of your prop,just a little some thing to check into,

[macojoe]

If the cavation plate is even with the bottom of the boat then you are fine. Just have to reprop for the best fit.

I have a old prop from my I/O and tryed it and I belive it is best for all around, I think it is a 15??
I have a 17 pitch now and it is great on the top end but when I do a hole shot it spins a little and then in the turns.

One more trip with CB, I will bust it off all toghter and when I replace it I will go with something closer to the other prop.

[buenos_diaz]

Hey,
I don't have a bracket but I have a qustion concerning prop diameter and pitch. I have a 77 200 HP V6 Johnson with a 78 V20 cuddy. It currently has a 15 x 17 prop but I only get 4400 rpm at wot. I am busy refurbishing and I am going to rebuild the fuel pumps which may be the problem but the prop seems to be a bit on the big side. I think the manual calls for 4550 to 5750 at wot which means I am only a little bit shy but it also says the 15 x 17 is the biggest prop I should use.

Any insight woud be appreciated.

Thanks in advance,
Buenos

[chesapeake724]

b_d,

It sounds like you need to prop down, to like a 15x15. This will allow the engine to wind up a bit more. IIRC the ballpark calculation/rule of thumb is for every 2" of pitch you go up, you drop 3-400 rpms at WOT. Conversely, by dropping 2" of pitch, you should gain 3-400 rpms at WOT, putting you in a more optimal operating range.

[buenos_diaz]

Thanks for the quick response. I'll rebuild the fuel pumps since I was going to do it anyway and then go down, so to speak. ;D

Thanks again,
Buenos

[mirage2521]

Just FYI, I think you guys are discussing propeller ventilation, not cavitation. I would not know cuz my propeller does not turn presently :'(


Propeller Ventilation

Propeller ventilation is caused by air from the surface or exhaust gases being drawn into the rotating propeller blades. This results in the propeller slipping more than usual due to the reduced water load on the blades. The obvious symptoms of this are a sudden increase in engine RPM as well as a possible loss of speed. This commonly occurs when trying to turn the boat at high planing speeds or if the outboard or sterndrive is trimmed out too high.

In racing conditions this can also occur when following another boat too closely. The small bubbles in the water created by the leading boat can cause ventilation of the propeller of the following boat, with a subsequent loss of speed. This is why you rarely see high speed race boats following directly in the line of the leading boats, even if they are far enough back to be out of the spray and wake.

Propeller Cavitation
Propeller cavitation is less obvious than ventilation, but can be far more damaging. When the propeller blade passes through the water at an increasing speed, the pressure that holds the water to the sides of the blades is lowered. If the water is sufficiently warm, and depending on the speed of the boat, formation of water vapour (boiling) can occur. These bubbles that are produced then immediately collapse, releasing energy that can cause a cavitation burn on the propeller blades. This is one of the great advantages of stainless steel propellers. Due to their superior strength they can withstand cavitation damage better than aluminium and can also be produced with thinner blades to reduce the occurrence of cavitation.