Some cavitation problems.

[tzambo]

Finally got my new prop today. had to go with a 13 pitch when I was looking for a 12. They dont make a prop for my motor according to the 2 local shops.

My cav plate is about 2 inches higher than my stern. and when I go WOT i have cavitation problem and cannot get fully on plane.

Is there anything I can do? I am thinking about cutting down the transom 2 inches and re-glassing it as a last resort. I also do have a "whale tail?" would that help? It's a Merc 500 on a 15 1/2 foot glass runabout.

Any help would be greatly apprciated.

 

[sho3boater]

Re: Some cavitation problems.

There are a couple things...you could go with longer leg motor or swap legs. Could try to get a chopper style prop like a laser2 or something that fit that motor that will surface some, you may have to search for an old performance type prop. The blades will be more pointed rather than round, but they tend to work best on light high speed boats. You can get a jack for it so you can lower the motor. Bobs has a convertible one that is very short and nice for things like that. You could get simple (aluminum angle) brackets made to lower it for a motor that light, and bolt it to the height you want. Put a pair of angles together like a U, and a set for each side makes a simple jack plate.

The tail things only make the boat come up on plane faster by forcing water down and thus lifting the stern....and should only do that off plane and be out of water on plane. They should be used on motors with power trim so you can trim up to lift them out of water at speed.

If your new prop is a stock style of prop with round blades and the boat is not a really light fast speed boat, odds are you should lower the motor with a jack or brackets to solve your problem or longer leg. You could cut the transom, but I tend to not like doing that. A stock type prop will never want to surface much, they are made to be in the water and do work best on slower craft. I'd guess that hull is made for a long shaft, or hull was modified.

 

[tzambo]

Re: Some cavitation problems.

Thank you for the advice. Prop selection is pretty slim with this motor. I had a helluva time finding the prop I have. The old one is being rebuilt. Companies dont even make a 11 spline hub kit for them from what the local prop places have said.

The motor is a clamp on style no bolts so I think a jack plate would not work.
But as luck would have it I have done glass work before and my cousin runs a top notch body shop about 7 blocks away. so I think I will trim the transom down about 2 inches.

Also just wanted to say The Prop Doctor in Duluth MN is the best. He did not have the prop or the hub I needed, But was one of the nicest guys I have dealt with. He took quite a bit of time looking through all his books and inventory to try and help me. You gotta love the mom and pop type shops where they believe in customer service.

 

[tzambo]

Re: Some cavitation problems.

Took 2 1/2 inches out of the transom. The motor was about 3 1/2 inches above the stern. So now I think I should be right in the ballpark. The cav plate is about 1 inch above the lowest part of the stern.

Just wondering where I should trim the motor for the first run? Any idea's?

 

[sho3boater]

Re: Some cavitation problems.

Up until it will not go faster.

 

[tzambo]

Re: Some cavitation problems.

Took it out today it almost gets on plane but I don't have WOT right now. My linkage is not pulling the throttle far enough so tommorow I will fix that and I'm gonna try the fin on it as well. Other than that she runs smooth pulls pretty strong out of the hole for have a 13 pitch on a heavier boat. Also it does not cavitate even when going into a hard turn so I think I may have that problem solved.

 

[tzambo]

Re: Some cavitation problems.

Out again today with the fin on. Got up to plane hole shot kinda sucks but I expected that with the 13 pitch. Top speed only 25mph on gps motor sounds like its reaching the proper rpm gotta find a tach.

 

[hwsiii]

Re: Some cavitation problems.

Tzambo, if you will fill this out I will try to help. You have not given near enough information for anyone to help you out at this point, and I am not blaming you, because I know you just don't know you haven't given us enough information.
Here is a place you an buy a combination tachometer and hour meter for $ 35 including shipping.
http://cgi.ebay.com/ebaymotors/Tachometer-Hour-Meter-Outboard-Jet-Ski-Quad-Go-Kart_W0QQcmdZViewItemQQ_trksidZp4634Q2ec0Q2em14Q2el1262QQhashZitem27ab0dee04QQitemZ170373541380QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the motor and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and stiking out to the anti ventilation plate for a referene
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for $ 50
RPM ___________ Speed ___________ No. of people ____________ Gas ________
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H