Cavitation and Speed and Picture

[Islandlure]

Hi, I have been running this jon boat I have built for about 3 days now trying to work out minor issues. Looking for some advice now, My flotation pods are in my opinion a success. The boat sits out 3" higher now in the stern and jump up on plane in an instant. Now I have had problems with it cavitating so I purchased a doel-fin plate to try to help this and it did a bit but still has issues at higher speeds. The cav plate sits just below the bottom of the boat now about 1" after I have moved it down to try to get rid of the cavitation. Also when I first put this in the water 3 days ago it would run 30 with now problem and now since moving the motor down an inch and adding the doel-fin it struggles to do 20MPH. Is there something that I am overlooking here? Does the pitch on a prop have much effect and cavitation? I am running a 13p and I already think I need less pitch but I am not sure that that would have any effect anyways. But I just can figure out how I have lost 10MPH with such little adjustment, thats why I think I might be overlooking something. I will also add that my first test run was with 3 guys in the boat and the water was choppy and still managed 30MPH and now it struggles to do 20 in calm water by myself. Anyways and insight would be appreciated. Thanks so much!

 

[tashasdaddy]

Re: Cavitation and Speed and Picture

i hate Doelfins, and they are just adding to the turbulance at the stearn, and you obviously don't need them for lift. check the compression on the engine, a drop of 10 mphs, is not right. the addition looks really nice.

 

[Islandlure]

Re: Cavitation and Speed and Picture

Well, pulled the doelfin off gained 5 MPH don't know how it increased that much. I haven't done a compression check yet but will by this weekend. It is still cavitating I can feel and hear it. I think I am going to also lower the motor a little bit more into the water. I think I need to research some setups on tunnel hulls and maybe set my boat up the same way.

 

[Coors]

Re: Cavitation and Speed and Picture

I just pulled the whale tale off of mine, and the world is better. The boat doesn't lean to the left, anymore.

 

[Islandlure]

Re: Cavitation and Speed and Picture

Yeah, at least on my setup I agree with tashasdaddy, it did cause a lot of turbulance that it didn't have after I removed it. And my boat still jumps right up onto plane.

 

[Boomyal]

Re: Cavitation and Speed and Picture

I vote for lowering the motor. That is an awful high mount bracket. Good looking extensions.

 

[Islandlure]

Re: Cavitation and Speed and Picture

Well, the motor is a long shaft and I had read on here people saying the cav plate needs to sit about an inch above the bottom of the boat and mine is about and inch below right now. But I am guessing that measurement might have been on V hull boats. But I am going to go ahead and lower it another inch this weekend and see how that does. A bit of trial and error and I should eventually find the sweet spot. This boat should be easily running 30 or a bit better.

 

[drewpster]

Re: Cavitation and Speed and Picture

Is the trim of the engine having any effect?

 

[Islandlure]

Re: Cavitation and Speed and Picture

I have tried trimming in many different positions and it only gets worse, so I believe that I have that dialed in pretty good. I really need to spend some time working on all the things everybody suggests but the urge to fish become overwhelming after and hour or so of working but I am making a promise to myself to leave the poles in the house and get something productive done on it this weekend

 

[drewpster]

Re: Cavitation and Speed and Picture

I know nothing about the hydrodynamics of spinning props in water so take that in consideration. But the other day a friend and I were discussing your boat. He owns a Lowe flat bottom and his prop and cav plate sit well below the bottom of the boat. He bought it as it is and has had no problems. I think you may be on the right track adjusting the engine height.

 

[burroak]

Re: Cavitation and Speed and Picture

One thing you may want to check is the bottom of the boat. Anything that is not smooth on the bottom can cause an effect akin to cavitation. Do your pods transition smoothly? Is the corner where the pod and transom meet creating an eddy effect introducing air into the prop stream?

Just my $.02

 

[scoutabout]

Re: Cavitation and Speed and Picture

Your extensions look great and you can see how much they help floatation...however I'm afraid they might be the cause of your cavitation. Now that the engine is sitting between what amounts to pontoons, I'm thinking the water flow coming off them is creating a gully in the middle in which your prop is trying to grab water.

Not sure how to explain it better than that. As the entensions travel through the water they displace it, and in doing so push the water down and away from your prop on either side.

I think the specific culprit is the way the extensions are angled on their inner sides. That's probably so the engine can turn side to side, right? I'll bet, though, that most of the water displacement and disturbance is right at those areas which are...voila...right next to the prop.

To get rid of the condition you might have to mount the engine way down -- much further than one normally would -- just to get a bite of clean water. Not sure, though what the effect of running your cavitation plate that far below the hull would be.

Another option -- and not a fun one -- would be to reform the extensions so that the didn't have that angle. To do that they'd have to be much narrow to avoid engine clearance problems but in the end might still give you all the floation you need.


Then again, I ain't no marine architect.

 

[MikDee]

Re: Cavitation and Speed and Picture

I concur with scoutabout, IMO, and I'm no engineer, the extensions seem to be displacing water towards the prop, and creating turbulent water in the "tunnel" between them causing aerated water, and cavitation at the prop when planing.

I don't know how or why this seems to change, or get better when you had more people in the boat? but maybe it's a prop thrust increase issue, due to the extra weight? I wonder if you'd still have these issues if you shortened the extensions to end just before the prop?

 

[Islandlure]

Re: Cavitation and Speed and Picture

Thanks for the input guys, I do agree that the pods changed the flow and displacement of the water a great deal. The last thing I want to do is change the pods, I have done a beautiful job on them and I really need this extra lift for running on the flats here. But I am heading out now to move the motor down. I am going to go down an inch and start with that and run it around a bit, if I need to I will go down a bit more. From what I can tell the pods have raised the back of my boat 3 to 4 inches and now I can get into real skinny water that I couldn't go before. But if it comes down to it I will change the pods as a last resort because speed it important to me, I like to get out to some of these islands that are a good 10 miles or so up the coast and the faster I get there the more time there is for fishing! But I am heading out the door now to mess around with it a bit and see what I can come up with. I will post again and let everybody know what I find out. Thanks again to everybody on their input!!!

 

[burroak]

Re: Cavitation and Speed and Picture

What's the logic of floating the boat higher and lowering the motor?

 

[Boomyal]

Re: Cavitation and Speed and Picture

What's the logic of floating the boat higher and lowering the motor?

I thought your object was for more freeboard in the rear. The motor depth should not affect your freeboard.

 

[Islandlure]

Re: Cavitation and Speed and Picture

My main goal at this time is to get the boat running properly, I am not too concerned with the height of the motor in the water because not matter what it will still run skinny. Before the back set so long that when I came off the throttle it would backwash and swamp the boat and I could not get up into that skinny water that I wanted to with out the bottom dragging. I am not looking to run the boat at full throttle in 3" of water, if I go that shallow I am usually using a trolling motor or pushpole. The reason my motor is up so high right now is because I have read threads on here to set the cav. plate 1" above the bottom of the boat and for this kind of boat it is obviously wrong so I am now looking for that sweet spot.
Now my update, I have moved the motor down 1/2" as of now and it has helped alot but now the boat is porposeing something fierce. I have tried the tilt pin in 4 or the 6 positions with not too much difference, but the first one is the fastest. So I am on my way back down the the boat, I am going to throw the doel-fin plate back on and give that I try. I will post again with results.

 

[MikDee]

Re: Cavitation and Speed and Picture

I think you're getting some lift now from the cavitation plate being lowered, conflicting with the boat bottom, causing porposing. I think the Doel fin may make it worse

Bear in mind that "as the boat was in stock form" the rear lowest edge of the transom would be below the water an inch, or so, (roughly) when planing at WOT. You might want to consider cutting off, or raising the bottom of your extensions a couple of inches, so that you will still have some extra bouyancy (lift) when the boat is static, or going slow, and they'd still act as trim tabs to help you plane off, But be out of the water when planing, not affecting anything. I think you would just have to find out how high to raise the bottom of the extensions to accomplish this. I am interested, & fascinated, by this whole concept you are trying, and would like to see you tweak it, whatever needs to be, to get it right. Good Luck, Mike

 

[Solittle]

Re: Cavitation and Speed and Picture

What is the rpms at WOT & what is your engine rated for?

 

[Islandlure]

Re: Cavitation and Speed and Picture

I am not sure what the RPM's are, I am looking for a tach but I can't seem to find a small one. I dont have much room to mount one. From the sound, I know it isn't reving as much as it should, but without the actual data that means nothing I have a digital multimeter that I used on cars and I could hook it up to the tach wire and it would read RPM's but I am not sure about using it on an outboard.