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Highlighted
Skipper
Posts: 13
Registered: ‎08-27-2019

Porpoising with RX3 Prop

Hi All;

 

     I have recently purchased a 20' fiberglass bow rider and had a Etec G2 140HP (Boat is rated for 90 to 175HP) installed on it with an RX3 prop.  I am having issues with porpoising through the speed range early into the engine trim.  The dealer told me they started with the engine lower than the current setup, but raise it because they were getting excessive spray and porpoising.  I am not getting spray now, but still getting porpoising.

 

     I tried raising the motor a notch and the porpoising got worse and I started getting ventiilation when I trimmed up to about 60% trim (major porpoising at this trim), so I lowered the motor back down to the notch to where the dealer had it.  This tells me the engine can't go any higher, though I would have thought raising the engine would have helped with porpoising. 

 

     I do have a fair bit of weight in the back with the kicker and kicker and kicker bracket along with this engine, but the fuel tank (55 Gallons) is mid ship and always full.  I tried putting weight forward, and though this did help the porpoising slightly, but not a lot, it also drives the bow down in the water and kills my fuel economy.

 

     The boat I have, Coastal Vokey 198 WT, which is a very common boat here and is not known for porpoising, but there has also never been one of these G2 engines installed on them, I have the only one ever setup with this engine.

 

     My questions is regarding the RX3 Prop.  I don't think weight distribution is my problem, and it doesn't seem like engine not being high enough is the problem (I get comments that the engine is very high), so this leads me to question the RX3 prop.  Is this prop considered to be a high rake prop and could that be causing porpoising?

 

     Anyone have any thoughts?

 

Jeff

Highlighted
Admiral
Posts: 9,387
Registered: ‎07-14-2011

Re: Porpoising with RX3 Prop

Weight distribution and hull design or problems usually contribute to porpoising. Also check the trim in on the motor and the angle of the transom. If either one does not allow the motor to reach a negative trim-in angle, that could also contribute to your issue.

 

Other things could be a rocker in the hull or too much weight in the back. Also try an RX4 prop as various designs affect riding qualities.

 

 

 


"There is never just one thing wrong with a boat";
                    -- Travis McGee, main character in a book series by John D. McDonald 


 



***************

The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.

The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.


Highlighted
Skipper
Posts: 13
Registered: ‎08-27-2019

Re: Porpoising with RX3 Prop

Thanks for the response Bill.  The engine can get negative trim, and there are no porposing issues below 10% positive trim.  The hull design is basically the Grady White SeaV2 hull, likely so close that Grady White would not be happy about it.

 

Weight distribution could definately be an issue, but I don't believe its hull rocker is an issue as there are hundreds of these boats here and I hear very little about porpoising with them. with other props, mine is the only one with an RX3 prop in existance.

 

Is the RX3 prop considered high rake or low rake?

 

Jeff

Highlighted
Admiral
Posts: 9,387
Registered: ‎07-14-2011

Re: Porpoising with RX3 Prop

[ Edited ]

 

 

I remember working on older Gradys that had a "belly" in the hull and that made them porpoise easily and kept them from performing as well as other boats in its class and size. Adding Bennet trim tabs helped immensely  Be sure to try an RX4 prop on it

 

 

 


"There is never just one thing wrong with a boat";
                    -- Travis McGee, main character in a book series by John D. McDonald 


 



***************

The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.

The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.


Highlighted
Skipper
Posts: 13
Registered: ‎08-27-2019

Re: Porpoising with RX3 Prop

Hi Bill;

 

     That's funny, I installed a set of bennett trim tabs yesterday and plan to test them out tomorrow.  I believe as well that this will stop the porpoising, but I am still interested in correcting the actual cause of the porpoising.  I wish I could find detailed information on the physics of porpoising as most of what I read is people's interpretation of it, and there seems to be a lot of variance, and often completely opposite explanations of it.

 

     Trying an RX4 prop may be the solution, but that's a bit of a difficult one here as the dealer I bought the boat from is 500 myles away from me and the local evinrude dealer doesn't not carry these in stock, so I would in effect have to purchase the RX4 Prop to try it, and wouldn't have an avenue to return it if it doesn't help.

 

     Thanks for the suggestions, and if you know of a source of information on the physics of porpoising, the more technical the better, please fell free to pass it on.  

 

Jeff

Highlighted
Skipper
Posts: 13
Registered: ‎08-27-2019

Re: Porpoising with RX3 Prop

If I want to go from an RX3 prop on my 140 Etec G2 back to as SSP prop, is there anything needed besides the prop?  I have read there are different hub setups and would like to know exactly what is needed before starting the swap.

 

On another note, will an SSP prop have less Rake than an RX3?  I have been talking to many other people with the same boat that I have, and a number of dealers, and that all say that with a standard standard stainless steel prop they have not had an issue with porpoising.  They have the exact same boat with G1 etec's, yamaha's, suzuki's and mercury's with standard SS props and have no issues and I have the only boat of this type in existance with a G2 etec and the only one with an RX3 prop. 

 

To add some info here, the dealer that just installed my trim tabs thought my engine was installed very high compared to the engines they install on this same boat, but they are a yamaha dealer so have no experience with this G2 or the RX3 prop.  I tried raising the engine and the porpoising got worse, and the dealer I bought the boat from tried the engine lower but raised if bacause of porpoising.  So it seems like it is porpoising at all engine heights, which I am inferring means it is not an engine height problem.  At the current height I can get to about 75% trim before I experience any ventilation.

 

I have a lot of weight aft with this engine and a 6 hp kicker and kicker bracket, but some of the people I have talked to have a Yamaha 150 hp and a 9.9 hp kicker and kicker bracket, so more weight aft, and have no porpoising with an SS prop.  This tells me that the porpoising may not be a weight distribution problem.

 

I have read that the RX3 has a lot more bight in the water than a standard SSP, because of its higher rake and design, which creates a lot of bow lift.  When I run the numbers in a prop slip calculator for this prop, engine and my top end performance, I am getting a very low prop slip number of 5-6%, which I think is telling me this RX3 is bighting the water very hard, typical of a raker prop.  I  don't know if this is true or not, but my general understanding is that high rake props can arribute to porpoising in some boat designs because of their increased bow lift and that lower rake props, which tend to have more stern lift, can help reduce or eliminate porpoising.

 

I installed trim tabs and tried the boat and they did eliminate the porpoising, but it took a lot of tab down to do so which severly effected fuel economy and speed.

 

I can't keep throwing money at this issues, but I need to fix it.  My OCD won't allow otherwise.  As I said before I am unfortunately not in a position to spend a couple of days testing back at the dealer I purchased the boat from because it is just too far away and I would effectively be taking the better part of a week off work to do that.  I do have dealers nearby that can work on the boat for me, but I didn't buy this boat or motor from them so any trials props would be purchases.

 

My Current theory is that my RX3 prop with this 140 G2 is biting the water hard, which allows it to be mounted high on the transon without ventilating, but is providing too much bow lift and not enough stern lift, and that switching to a lower rake prop will reduce the bow lift and provide more stern lift and reduce or eliminate the porpoising.  I do understand that I will likely have to lower my engine going to an SSP prop, but I am ok with giving up top end speed to solve this issue.

 

I know, long winded, but I want to be thorough with the info.  Any info on hub requirements switching to a SSP prop or thoughts on the rest of the info?  Any info would help.

 

Jeff

Highlighted
Admiral
Posts: 9,387
Registered: ‎07-14-2011

Re: Porpoising with RX3 Prop

Do you have the power steering version or a jack plate on the engine and boat?

 

SSP is not a very efficient prop compared to the RX series.  Try an RX 4 as the RX3 is a bow-lifter style best suited for lighter faster hulls.

 

If you have negative trim then can do some trimming out before porpoising, then you are in good shape as there are too many variables to worry about 

 

 

 


"There is never just one thing wrong with a boat";
                    -- Travis McGee, main character in a book series by John D. McDonald 


 



***************

The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.

The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.


Highlighted
Skipper
Posts: 13
Registered: ‎08-27-2019

Re: Porpoising with RX3 Prop

Hi Bill;

 

I have the power steering version of the motor.

 

You said the RX3 is a bow lifter, but the RX4 prop on evinrude site says "superior bow lifter", wouldn't that make the problem worse?  Don't get me wrong, I love the idea of an RX4 prop, and the better grip of the 4 blade prop.  But when I put my 2 boys in the back, where they typically sit, I do have a lot of stern weight.

 

I guess the question is do I need even more bow lift than the RX3, or do I need more stern lift to account for the extra weight aft.  Thoughts on that? And if I need more stern lift, what would be an evinrude sternlifting prop that would fit this motor?  I'm not going to bug any further on this choice and I am going to try whatever you recommend here, and I do appreciate the help and patience.

 

I completely agree with you on the SSP, but I talked to the dealer last night at length on props and he said they only ever install regular aluminum props or SSPs on these boats, whether they install yamaha or evinrude motors, and noone has even complained about porpoising.  He said they typically install aluminum 13"x19" standard stainless props on yamaha 115 hp motors on this boat, or Stainless 13.25"x17" props on 115 G1 motors on this boat.  They are not likely getting the best performance out of them, but they are not getting porpoising.

 

The dealer is also willing to work with me on trying props, but the only thing he has in stock that would fit my motor is a standard aluminum 17" or a SSP 13.25"x17".  Anything else he would have to order in and ship to me, which will take about 3 weeks for each prop I ask him to order.  He did talk a little about the Viper as well, but he did admit he is no expert on props and that I am the only customer he has ever had that requested anything other than standard aluminum or stainless.

 

If I was to switch to an RX4 prop, what change in RPMs should I see going from an RX3?  I am not concerned with high top speed, but I want the engine in its proper operating range.  I am currently getting ~5600 RPM with typical load out of the RX3 14.5"x17", would I see much of an RPM drop switching to an RX4 14"x18"?

 

Thanks again for helping with this Bill.