TOPIC: Cavitation

I am running my '63 1000 on a 17 foot '62, 17' Thompson and am having problems with prop cavitation (the prop cuts loose). Boat ,equipment (including myself), and motor come in at about 1500 lbs. I'm running an aluminum 3 blade 21 pitch prop. The bottom is slightly hooked about 16 inches ahead of the transom.

When I converted the motor onto a single ram PTT it raised the motor so that now the cav plate (on the motor) is a little less than an inch above the bottom of the boat. It also moved it back couple of inches. I truly cant remember if I had this issue before the conversion, but if I did it certainly was not this bad.

The prop cuts lose easily and I am always having to balance power against a very moderate up trim to get bow lift. I cant just pour on the power either or she just cuts lose. Happens at any speed.

A few questions:
Could the cav plate being an inch above the bottom of the boat make that big a difference, and be the cause?

Could it be I have too much prop? I've not heard of an over pitched prop causing cavitation, but I thought I would ask.

Would a smaller diameter prop help.

I'm exploring options. I really like the PTT and want to keep it. I've considered cutting the transom down or a jack plate to lower the engine, but don't want to do either if there is a better solution.

Any ideas would be greatly appreciated.

I run a '63 850 Merc on a '59 17' Thompson Sea Lancer with the older style power trim. The only time I cavitate is when I try to trim it up too high. The cavitation plate on the motor should be no higher than even with the bottom of the boat when it is straight up and down. If it is higher than that, the prop is breaking the surface of the water and drawing in air (cavitation). You probably will need to notch the transom because the newer style pt&t must raise the motor slightly.
Bill

And your positive the prop isn't spinning on the hub?
To be sure you could make a mark across the hub, rubber, prop to check after a run to see if your mark has misaligned, if so theres slip..

Perhaps raising the motor exacerbated the effect of the hook in the bottom. Some boats will tolerate raising the motor, others won't. Normally a hi-performance boat will like higher transom levels. And the Thompson ain't that!

Seems like 21" pitch is pretty steep for a 100 hp, but I guess it depends on the hull design and boat weight. A flat-bottom hull is certainly easier to plane out.

Does your 3-bladed alum. prop have cupped blades? If not, a cupped prop holds better than non-cupped.

A 4-bladed prop will bite the water even better than a cupped 3-blade. I used a 4-bladed Comprop on my old Merc 1350 and it was almost impossible to blow-out the prop, it had tons of "dig".

Maybe the hull just doesn't "like" that prop with the current motor height/setback.

You're gonna have to experiment, I expect, before settling on a solution. Maybe you've got another prop to slap on there and try out?

HTH.........ed

Thanks all!

I'll check the hub on the prop but I am fairly certain it is solid. I've had a slipped hub before (on a differnt prop) and it was a totally different effect.

I have 17 (and I think a 19) pitch three blade prop and plan to try those out next time out. I have great plans for a flip/strip/repaint on the hull this winter and will see about the hook then. "great plans" of course means "Oh, yeah, right".

Last resort will be notching the transom. Really dont want to cut into the boat.

Maybe as early as this weekend I'll be out and will let you know.

Your 17 pitch prop is probably the correct one for your load and hp. A tach helps in these matters.You do not want to lug these engines as it kills them.The engine jacked 1" is okay and should not have any affect your issue. The hook should be forcing the bow down and I wonder where it is pushing the water in relation to your prop. I also wonder about a blown out hub,you will find out when you throw another prop on her.

The prop that was on the motor when I got it is a 21P two blade brass prop. I have always assumed that was the original. Does two vs three blades make a difference?

In any case I'll be trying her out as soon as this weekend with the three blade 17P.

I am running a 17p 3 blade non-cupped brass prop. It comes out of the hole nice and I get about 38 mph at 5000 rpm. I also have a 2 blade that came with the boat, but I have never run it. The 2 blade will let it wind up quicker and probably to a higher rpm so you might get more speed, but you will lose power out of the hole. I tried one years ago on a 50 hp Merc and was not impressed at all, so I went to a 3 blade Stainless and was very happy with the performance.
Bill

You will need to check full throttle RPM to be sure you have the right pitch. I would wager the 17" will be correct. If RPMs are good and it still cavitates, a stainless prop may be needed. They generally have more cup than aluminum and hold the water better.

If its not the prop Ide say its the hull, I had a issue with a rinker, strait it was fine but I couldnt turn at all without massive cavatation, I replaced the stringers and floor....perfect.

In the end it's likley a comgo of things, and I am sure the hull is part of it. So I will go through the process of elimination.

I need to check my throttle controls. At full throttle (at least as far as indicated by the postion of the control handle) I was only turning 3100 RPM, but that was with a full load of 4 kids and myself and gear. Need to make sure the carbs are wide open with the handle all the way forward. But I suspect it was too much load and too much prop. She didnt sound like she was lugging. When I am out on my own I only run it at 3000 (otherwise it cavitates), and that leaves about a 1/4 throttle movement left (again at least as indicated by the position of the control handle)

If the weather permits, much testing this weekend and I will report back.

You must be sure that all 3 carburetors are functioning properly.----No fuel going through one of them and you will be starving 2 cylinders of oil as well.

Hi,
I would advise the use of a new Mercury 4-blade aluminum propeller. These have a higher rake angle and are cupped. Be careful with the pitch selected, as older lower rake uncupped Mercury propellers do not exchange in the same pitch with the presently offered ones. If someone has a question, please write directly to me (This email address is being protected from spambots. You need JavaScript enabled to view it.), as I am not able to visit here for several days sometimes.

Regards,
Joe
www.fergusonpoolemarine.com
This email address is being protected from spambots. You need JavaScript enabled to view it.

Peter,
Whatever you do,...Please don't cut the transom.
This can be resolved without that.
All the responses have great suggestions.
I agree with the professor when he suggests checking & confirming all three carburetors are functioning properly.
That could be why you are only getting 3100RPMs.
So could running a 21 pitch prop.
Joe Poole's suggestion to change to a late model prop that has more rake & cup would be the first thing I would try after confirming all (3) carburetors are working & all (6) cylinders are firing.
Did you complete your ignition modification to the solid state system that removed the dual points & (2) part ignition? If so,...I would assume all (6) should be firing??
If you decide to get a new prop - consider getting one that uses the Flo-Torque II or III hub system. Then you can buy 'blanks' of different pitch much cheaper.
I would stick with aluminum too.
Thom

Doc!!!

No I haven't yet tried out the Petronix Ignitor conversion - We just had our Family reunion in Pawcatuck CT. Uncle lives on the Pawcatuck river and I can dock her out back and run to the beaches - nudge right up onto the sand on the harbor side. The run back up the river flushes all the salt water out/off with fresh (I flushed and washed it anyway of course). So I was waiting for that event to start back into that project.

Fear not I'll not cut into the transom.

I don't suspect a carburetor problem. I've had that situation where I lost a bank of cylinders and it was very apparent. However, can someone suggest an easy way to test them individually? I was going to simply turn the fuel mix screw in until I detected a change in idle (or of course no change)

Tomorrow going out on the local river and test out the 19 and 17 Pitch props that I have and will report back.

After few tests on water yesterday I’ve more question than answers.

I checked the location of the cav plate to the bottom. Note the attached picture. The cav plate is very close to the bottom of the boat (the ruler is flush against the plate). Still though the boat acts like the prop is too close to the surface.

I switched down from the 21P three bade to a 17P three blade (both un-cupped). I thought I had a 19P but no. She seemed to like the 17 better, but it was more prone to cavitation than the 21 (which I guess makes sense). The 17 could take much less tilt than the 21 before cutting loose.

Next I tested the carbs at idle, in neutral. I believe this test should be done with engine in gear but I was at anchor (not tied to a doc). On the bottom two carbs closing the fuel/air mix caused the motor to all most immediately stumble to a stall. On the top carb it only slowed the idle slightly, even at full closed. However when I reopened it, the motor would rev up slightly for a second or two, then back down.

Question; Before leaving for the water I checked all the plugs and they were all consistent in appearance with a slight oily film. I was running it at 2 full turns open of the fuel air mix. I reduced that to 1.5 as I believe the plugs should show a dry grayish color?

Next steps.
Check compressions, especially on 1&2.
Pop the hood and figure out what is up with the top carb. They were all rebuilt a few years ago and have less than 30 hours on them.
Going to check the fuel pumps since I will be wrenching
Check the diameter of the props I have against spec. Maybe too big a prop (as in diameter).

Hello Peter,
Your last picture is very interresting. It seems to show that your cavaitation plate is a good 1". 1.5" above the bottom of the boat. I do not dought that your prop is surfacing and there is little chance that those old style non cupped props are going to hold. I would suggest that you see if a local dealer will let you try-out a few different style props like some of the other members have suggested (they will usually do this if you commit to buy).

well, I quess that is the plan now.... there is a local dealer that might let me try a few out. I'll have to call and see. Before that though I'll need to sort out the carb and the crappy idle (and the newest issue, the split fuel line at the motor side connector - it got pinched and very small cut was made. This happened within a couple hundred yards of the marina so I am very certain that is not the cause of any of the aformentioned issues. The effect by the way was that the engine pulled enough air in to starve the motor into a stall. (I thought I had run a tank dry, but it wasnt and when I pumped the ball fuel squirted from the line).

Any thoughts anyone on the fuel mix screw (1.5 vs 2 turns) and the oil film vs. dry gray on the plugs?

The mixture screws are adjusted for fuel types and local atmospheric conditions.----There is no set # of turns.

---

Adjust for smooth idle and if motor stumbles on acceleration you open them up 1/8 turn at a time until you have a very slight stumble on hard acceleration.

Not that it will make much difference but cant tell much with the tape on the cav plate. Why dont you use a straight stick 5 or 6 feet long held on bottom of hull and along side of cave plate? Hard to tell in the pic but the edge on the transom looks kinda rounded. I have my cav plate adjusted slightly up from even with keel but its a g3 and might not relate? Good Luck!

apologies - I forgot there was a question asked at the end of this thread.

The pics you see I am using a yellow yard stick. I have it flush to the cav plate with the motor in neutral trim.

I compared the diameter of the bronze two blade prop that came with the motor (and I believe that to be original to the motor, or at least the period correct prop) with the 3 blade alumin. The two blade (marked as 21p) is at least the same if not larger. I cant recall if I tried that prop since I've had the PTT so will do that.

I