Cavatation

Chica · Posted: Sun Sep 12, 2004 1:15 pm Post subject: Cavatation

I have just returned from the northwest coast of Vancouver Island where I went for the maiden voyage of our new 22' angler. I am a dry lander and this was my first time on the big water. The boat has a 90hp Honda and trim tabs. My problem is that I can not find the sweet spot with any combination of trim tab and trim-tilt of the motor. Even when running in a straight line, the slightest wake of another boat or a side swell sends the r.p.m.'s from 4500-5000 to 6000. On calm water, no problem. What combination of trim tabs and trim-tilt of the motor works best. Do you use the trim tabs to just get you up on plane and to level the load or do you use them while running once up on plane. Love the boat!! Was a former Snake-Salmon River jet boater. Chica

Mr. Fisherman · Posted: Sun Sep 12, 2004 2:15 pm Post subject:

I have found that for my '95 22' angler with a 90HP Suzuki that the cavitation issue is nill, nada, zilch, zero compared to my old 75.
This is what I do.

Trim the motor so that it is parallel with the water. This way as much of the prop and thrust is in the direstion you want it.
Also, make sure the pitch and prop are suitable for your load and use.
On my boat I spin a 20 pitch 3 blade factory aluminium prop and I could not be more pleased. I was told that the Honda wouldn't turn that big of a prop so you should check with your local dealer to insure that your prop is right for you and carry a spare.

You might want to check the prop. It is possible that the hub may be spun. It you can throttle down and jump up on plane this is not the case but if you try and it revs up quick withought jumping on plane it is the likely cause.

Never use trim tabs in a following sea. I do not have them yet but have opperated several boats that have them. The trim tabs should be adjusted slowly after you reach your planing speed. Remember to raise them all the way back up before taking off after you stop or you could really surprise yourself when the boat goes a way you may not want to. Raising the tabs after you stop or slow down is a good habit to get into.

You might also want to check the distance that your tabs are from the prop. If they are too close it could cause cavitation.

I hope this helps....

Chuck S · Posted: Sun Sep 12, 2004 2:42 pm Post subject:

Who engined the boat? Is the propeller deep enough in the water? Are you bringing the stern out of the water with the tabs? Cool

-- Chuck

Chica · Posted: Sun Sep 12, 2004 3:16 pm Post subject: Cavitation

The motor and trim-tabs were installed by C-Dory. How do I tell if the motor is deep enough in the water? The motor mounts are not resting on the transom, rather the motor is bolted through the transom. Will check with Honda re. the pitch of the prop. The trim-tabs are 1&1/2 inches from the chine. Thanks for the comeback. Chica

digger · Posted: Mon Sep 13, 2004 12:25 pm Post subject: Cavitation

On Snoopy-C I've never had a problem until just recently. I got to thinking that my fuel consumption might be improved by checking the cavitation plate alignment with the bottom of the boat. It turned out to be 1 1/2 inches below the bottom -- the motor was mounted on existing holes with the mount resting on the transom. To get the plate in alignment with the bottom, with the plate parallel to the bottom of the boat, I needed to raise the engine 2 holes in the engine bracket. Now the cavitation plate was even with the boat bottom. However, now it does cavitate at times. I may need to lower the engine once again. Perhaps Les can give us the straight scoop. Ron

Adeline · Posted: Mon Sep 13, 2004 1:47 pm Post subject:

Chica, On the outboard bracket below the trim/tilt should be a number of holes allowing the insertion of a horizontal rod. This rod limits the amount of down trim possible. It should be in the holes closest to the transom to allow maximum range of motion. If it's not than you wo'nt achieve maximum down trim and you WILL ventilate. Pete

Chuck S · Posted: Mon Sep 13, 2004 9:30 pm Post subject:

Why have that little rod there at all?

-- Chuck

Captains Cat · Posted: Mon Sep 13, 2004 9:59 pm Post subject:

Because if you have the little rod set correctly and keep your engine (s) against it most of the time and use your tabs for trim, the hydraulics doesn't have to hold up the thrust of the engine. Less wear/stress = longer life... Laughing

Charlie

Chuck S · Posted: Tue Sep 14, 2004 11:31 am Post subject:

Ummm, trim tabs on a 16 Cruiser?

-- Chuck

C-Bill · Posted: Tue Sep 14, 2004 7:28 pm Post subject:

If all else fails, it may be your prop. I push my 22ft Cruiser with a 90 hp Yamaha. I started having cavitation problems one day out of the blue. I was convinced the problem had to be the prop hub slipping. After marking the rubber insert with the SS hub, I found that was not the prop slipping. I found I had more of a load than before ( 19 gal. live bait tank) and I needed a prop for the increased load.

I was told to contact Power Tech Propellers and haven't had a problem since. I'm very pleased with the performance and Power Tech is very knowledgeable, even when I thought I needed a second prop at sea-level (I operate at 4,000 ft and above), they said to take it to sea-level and try it first as it would probably be okay - they were right, it works like a charm.

Bill

Sea Wolf · Posted: Tue Sep 14, 2004 8:24 pm Post subject:

Bill- What pitch and diameter prop are you using? Is it a 3 blade? Cupped? Any other specifics? I have a 90 h.p. Yamaha now, so am interested in your set up! What's your WOT rpm? Any other specifics? Thanks!!! Joe.

C-Bill · Posted: Wed Sep 15, 2004 12:22 am Post subject:

Sea Wolf,

I just had a long explanation of how I arrived at my current prop when I timed out and lost my login. I logged in and found my reply and then proceeded to loose it again for good.

This will be much briefer. My current prop is a 14 dia x 11 pitch three bladed thruster series load pushing prop. I expect that I should be able to turn 5,500 rpm's. I do have drag that you probably don't have. I have a thru hull water pickup, a thru hull transducer and a ground bar.

When I get to Shasta, you are welcome to try it, if it will fit your motor and you'll have to put your boat on the trailer to change props. I don't like swimming in muddy water looking prop components.

It may work good. I was considering putting it on the 50 four stroke if I were to buy one. Now that the 90 four stroke is available, that prop would still be a consideration.

Looking forward to Shasta!

Bill

Sea Wolf · Posted: Wed Sep 15, 2004 5:24 am Post subject:

Bill- Currently, I'm running a three-blade 13.25" diameter by 17" pitch cupped aluminum OEM Yamaha prop that I can turn at 5500 rpm and 30.5 mph with a very rough (living room wall equivalent) bottom of anti-fouling paint. I figure I can run a 19" pitch at about 5100 or a 15" at about 6000 rpm, but the 17" is about ideal. A 16" would likely turn about 5700, although they're harder to find. Re-doing the bottom would reduce the drag a lot, and get me somewhere between 150-300 rpm in the process. See you at Shasta! Joe.

C-Bill · Posted: Wed Sep 15, 2004 12:00 pm Post subject:

Joe,

That's interesting! I think your bottom paint could be creating the same effect as shark skin and giving you more speed! On second thought, I think it's the efficiency of your four stroke. I'd have to put on a speed prop to catch up.

Bill

Sea Wolf · Posted: Wed Sep 15, 2004 4:15 pm Post subject:

Bill- I've bought a stainless steel 19" pitch three blade PowerTech prop and a stainless 15" three blade prop which I will be testing between now at the Shasta gig, so should have some more real numbers by then.

I have the disc brakes installed, but still have to tie down the lines properly and bleed the system and connect the back up lock out solenoid to the back up light feed in the factory trailer tow package plug.

We're spending a lot of time with my Mom who's in a rehab/convalesent hospital right now, so progress is not at All Ahead Full power setting on the C-Dory project helm. Joe.