# Cavitation problem.....Please Help!



## rmk800

I am puzzeled and was wondering if you all could give me some advise? I am new to duck hunting with a boat. I not so long ago bought a used boat. I put on a larger motor than what originaly came with the boat. I have a 14x36 jon boat with a Kohler 18 hp longtail motor. The problem is that when i was out at the end of last years hunt i would see people just fly through the water with no problem. so me wanting a bit more out of the boat went with a bigger motor. But now i have some serious cavitation with the prop. I can basically go at idle speed with no problem...but the moment i increase the speed...i cavitate. It surges then stalls...surges then stalls...repeat on and on and on.... I think the prop size off the top of my head is 9 inches with a 5 inch pitch...not real sure on the pitch size...just a guess. anyhow what can i do to get the most of this bigger motor?? any ideas?

please 

oh ya and yes i do have a cavitation plate on...just not sure if it is mounted correctly...the longtail motor is a home made job.....


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## muddydogs

From your post I gather that the new motor is a long tail. If so it has to be your cavitation plate is set to low, ie its keeping the prop to high. When at idle speed the weight of the prop and and tail shaft keep it in the water. As rpms increase the prop tork increases along with the increased water flow under the cavitation plate lifts the tail shaft up. So you need to get the cavitation plate set to keep the tail in the water. Go slow with the adjustment as a little gose a long way. If in fact you have a standard outboard then it gets more complicated as the boat has more effect on water flow over the prop.


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## UTH20FOWLER

What Muddydogs said is true. Take a cresent wrench and adjust the end of your plate up so water flows over the plate pushing it down. Another thing is your boat is a little short and narrow for a long shaft motor the best is long and skinny or short and fat. Another problem might be that your motor sits high on the transom and the pitch might be to sharp.


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## BLKFTDN

i had to do the same thing to my rig when it was new. 

sounds like you need to bend the cav plate *away/up* *just a little at a time*. this will push the prop down in the water column. you will know when you have gone too far when you have to push(down) rather hard to keep the prop(up) in the ideal position(with the boat unloaded, this effect will vary greatly with a load depending on how you have stored it)...having said that....my personal preference was to have to pull up on the handle(slightly) to keep it in the sweet spot. --(make the adjustment while unloaded and leave it)


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## Carl Taylor

First, get the measurement of your prop, tip to tip. Also, you need to inspect the prop and determine wether or not your cup is worn, or quite possibly worn right of. 

Second, when you talk about cavitating, at what rpm? To make sure that I understand, is you prop under the surface? 

If you are cavitating and your motor is cutting out on you, then you have two issues.

What Muddydogs has said about tunning your cavitation plate is true, but you have also got to understand that if you are over working your motor with to big of a prop, your motor will start to load up, run hot and start to cut out. All the tuning in the world will not help if you are over proped. 

If you do not have a tac, get yourself one. This will allow you to tune your load to the motor much faster. Question; Will your motor run at full throttle out of the water?

If you are interested, get ahold of Val Richens in Hooper Utah [801-731-9273]. He can remedy all of your motor issues, get you in the right size prop, re-cup your existing prop, and/or set your motor in the tank and run it under a load right there in the shop and get these proplems solved. Then, once you've have the right prop for the load, you can start working towards tuning your cavitation plate.

Hope this helps,
CT


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## rmk800

Thank you sooo much for the replys!

I dont know much in this department but im learing.

To start off...Yes the motor runs like a million bucks out of the water. I have no tach but will put one on to monitor that. The cavitation plate that was put on this longtail before i bought it is rather large, i think. It starts before the prop and ends after the prop. it is also cupped on both sides of the prop so that makes is rather wide as well...i think this is the starting point for me. My cavitating propblem basically starts a little after i leave idle speed. (im still pleased with the motor at that speed because that speed is still faster than my old motor at high speed.) I was wondering if any of you had some photos of your cavitation plates on your rigs...i could then fab one like it and start my adjusting from there! The current cavitation plate sits horizontal to the prop. no angle at all. so from reading your posts i would assume that is probably one of my big problems....


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## Pintail Hunter

Can someone explain what you are referring to when you talk about a "cavitation problem". I've got a new (used) boat this year that is 16X48" with a 25 HP Vanguard long shaft Mudbuddy motor. When I really get moving the motor tends to jump out of the water. I'm not sure if this is because of the shallow water depth, too much vegetation in the water or because of this "cavitation problem". I can get the prop back in the water about half way and keep moving pretty fast but only with some real "strong arming" on the tiller. Basically the motor turns into a Hyper Drive when I hit high RPM's and high speed. Is this problem what you are talking about? It about wears my arm right out to keep my speed up and the prop at least half way in the water. The prop appears to be in good condition.


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## Carl Taylor

Okay, lets break this down...
The Cavitation plate is held directly over the top of the prop. Most all your brands of long tails should have one. This plate may vary slightly in size, but should be in the neighborhood of 4 to 6 inches per side with a slight angle down-ward at the outer side. The idea is that this plate will direct the flow of the water over the top of the prop. This plate also acts as a foil if you will, that when tuned to the load properly, will almost float the prop at the desired depth (water/mud). Hanging directly under the shaft is the skagg. It sounds like you are talking about the skagg. The idea of the skagg is to act as a rock guard as well as a serious tool for steering when your boat breaks away from its plane . If this is too long, your prop will not reach the depth it requires to work. If it's too short, you'll destroy your prop...

So, if your problem is that your prop wants to lift out of the water/mud at full speed, you could be loosing performance. Again we go back to the tac. If you are running at 3800 at full speed with your prop under the surface, man you're dialed. But, you can also tune your prop to lift above the surface, but you had better have some serious cup, or you'll loose your speed. In order to stop your prop from dancing around above the surface, put a cresent wrench on your cavitation plate and lift up slightly(do this on both sides) then take it for a spin. You'll want to do this with your every day load in the boat. If you bend it too much, the prop will drive down to the stop and stay buried. This becomes very costly on the prop, wearing them out faster. If you go the wrong way, it will flip out of the water.

Remember, key tools are the tac, a GPS, consistancy and your boat loaded like you're going to run it hunting. 100 pounds off makes a huge difference.

If you have any questions, let me know...

CT


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## muddydogs

Before you try and recreate the wheel take a big cresent wrench, boat with motor and head for the water. Bend the plate up a little, 1/4 to 1/2 inch and give it a run. If cavitation still happens bend plate another 1/4 inch and run some more. It will not take long and you will have eather fixed the problem or not. If not then a different plate may be in order. I assume this is a used motor and was probably set up to run on a little taller transom or boat that set higher or lower in the water. So give it a kick.


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## muddydogs

Pintail

Depending on water depth you could be bouncing the prop up. If say you are running in deep water and having the problem described with out running the rig you could also have a cavitation adjustment problem. There is a lot of factors that can effect MM performance. Transom height, transom angle, lenght, width of boat ect. The cavitation plate is the easyest place to start. I have seen guys running nasty stuff bump the plate a little hard and have to stop and readjust the angle to get it running right agian. Your cavitation plate might need to be adjusted a little different when you are carrying a hunting load also. Generaly once you get it set you are good to go. So start with the cavitation plate make small adjustment, if the problem still persists then you will need to look deeper. Also don't be afraid to call Mud Buddy with questions about there motors, they are very helpful.


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## Pintail Hunter

Okay. That explains a lot. As you mention, the problem is different if I've got a 250 lb buddy in the front of the boat. I couldn't quite get the picture in my mind of the basics and how you'd adjust the plate, if needed. I could clearly see the need for the skagg but wasn't quite sure of the purpose of the "winged plate" that was above the prop and certainly didn't know that you could adjust it. 

I'll probably leave it as is until I get into the season then see what adjustments are needed. So far I've been running it with just me in the boat and no equipment, decoys, etc. Have taken one guy out with me once and I did notice a difference with him in the boat. As I recall the problem wasn't as severe. 

By the way the transom is about 15" high which is the lower of the two options for this boat model (Lowe Model L1648L).

Thanks again.


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## rjefre

Threads like this are the reason why this site is a great place. Great info.


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## UTJetdog

Amen brutha! Nice posts Jim and Carl!!!!!!!


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## Carl Taylor

If your boat/load isn't tuned properly, to the point where you are running in the low 3000's RPM area, your motor will load up and run hot, to the point of missing and even backfiring. My advise is to take care of these issues now, before the hunt. A problem like this on opening day in the channel, or in the launch seriously effects every one around you.

Take a look at your props, if they are worn down, lets get ya a new cup. If they need to be sent out to have them built up, its getting close, we need to stand on it...


Good luck
CT


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## Pintail Hunter

Carl, couldn't agree with you more. 

I am curious about one thing. With the WMA's closed until a couple of weeks before the season opens, what public waters are open, other than the reservoirs, where you can run your boat? More importantly, what public water is open where can you can test running your boat in shallow water? 

Maybe the water depth doesn't matter but I would think that running it in water that's from a few inches to a couple of feet deep would be best. Although I haven't run my "new" boat in more that a foot or two of water so far (I have access to a local duck club), it seemed to me that my old boat didn't have as much cavitation problem in a deep reservoir as it did in shallow water. Maybe it was just too shallow. 

Anyway, if shallow water is better for testing your boat out, where can the general public go this time of year? Otherwise, I guess anywhere that's close to home will do.


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## UTJetdog

Utah Lake, Cutler, Mantua, etc.


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## Carl Taylor

access is an issue. It is a bit of a drive, but Cutler is a good one. You can pretty much test any condition needed there. 

If you need hand, give me a call..

CT


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## Pintail Hunter

Thanks for the education.


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## rmk800

thanks to all who have given thier advise!!!!! i believe i have a worn out/under proped situation...i should have it fixed in the next little while!! i will ask for more help if i continue to have problems.....this has been great to read/use your advise!!! thanks so much!!!! 

darin


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