Porpoising and engine height

[tony_cliffy]

If all factors are the same (RPM, water conditions, trim setting, etc) will a boat tend to porpoise more or less if I raise or lower my engine height? <br /><br />My 115HP 4-stroke Johnson is mounted to the transom using the top most mounting holes. The engine could probably come up one bolt hole to get the anti-cav plate flush with the bottom of the hull. I never did this because it was suggsted I would only gain about 175 RPM's by doing this and this wasn't an incremental enough gain for me to take on this task. But as I learn more about my new boat I am starting to porpoise a lot while trimming the motor to get my RPM's up. I am *almost* at the right RPM's by playing with the trim, but then the boat starts to porpoise. So, just from a motor height perspective, how will raising the engine impact a boat's tendency to porpoise?<br /><br />FYI - I do not have a trim gauge. It is a fairly light boat without a lot of weight up front. I am using the trim to get the bow up to reach maximum speed.<br /><br />Thanks!

 

[DJ]

Re: Porpoising and engine height

tonycliffy,<br /><br />Raising the engine will probably not effect porpoising.<br /><br />Porpoising is usually caused by too much weight, in the stern.

 

[LubeDude]

Re: Porpoising and engine height

Give us a little more info, is your boat rated for more hore power than you have? What is the lenth of your boat?<br /><br />Porposing will happen mostly by trimming out too much for the weight of your boat. If your boat is say rated for 150 HP and you only have a 115, then it will porpose because it doesnt have enough power to hold the nose up!

 

[walleyehed]

Re: Porpoising and engine height

Whoever told you "ONLY" 175RPM, doesn't understand set-ups very well. 175 is a good deal of improvement.<br />We'll be waiting for more info....

 

[tony_cliffy]

Re: Porpoising and engine height

Boat is rated for max 115HP, and this is what I have installed. I agree I likely have a heavy stern realtive to the rest of the boat since I have a fairly light boat to start off with. The boat is a 20' semi-V aluminum center-console jonboat. The factory quotes the boat around 850 lbs. and I have the engine, 2 batteries and 23 gallon fule tank in the rear. I don't have much gear to speak of forward. I would think 115HP is plenty powerful to keep the nose up. Boat gets on plane in a few seconds and max speed is 43 MPH at 5700 RPMs. WOT is supposed to be 6000.<br /><br />This is really an extension of a previous thread I posted awhile back regarding prop size and RPM's over on the Johnson board. I switched from a 19" to 23" prop and brought my RPM's down from rev-limiter to low-mid 5000's. I have gotten better at trimming the motor and since then can achieve about 5700/5800 RPM's but then run into the porpoising. I am thinking that I can raise the motor to lower drag and that would help the boat get closer to 6000 RPM at WOT. But was now worried that raising the motor might exacerbate the porpoising. Boat's top speed of 43 MPH seems low given boat style/weight.

 

[Dhadley]

Re: Porpoising and engine height

The bottom design has a lot to do with the ride attitude. The prop style will come into play also. Raising the motor takes very little time / money. Try it and see how the set up responds.<br /><br />Put a straight edge on the bottom from the back to about 6' or so forward. How much hook is there and where is it?

 

[walleyehed]

Re: Porpoising and engine height

Boat speed is directly proportional to efficiency of your set-up.<br />Raise the engine one hole at a time and test. You will find you have to trim less to achieve the same RPM and boat attitude, and this in effect, should help with porpoising.<br />I don't recall the other thread, but you may have to do some playing with props to find a good bow-lifter that would require less positive trim.<br />EDIT: Sorry DH, we must have been typing at the same time..

 

[tony_cliffy]

Re: Porpoising and engine height

My dealer had to reseal something in the transom/engine area as I had a worrisome leak. Any specific advice on how I should seal up the bolt holes? I have never moved an engine although I read another post about loosening lower bolts first, then remove one top bolt and then the other and moving the engine up with a jack. Any other points to keep in mind? I see a good bit of sealant around the bolts and also generally around where the engine contacts the transom.<br /><br />Oh yes! I forgot. Should I aim for the anti-cav plate to be flush or 1" above the bottom of the boat?

 

[tony_cliffy]

Re: Porpoising and engine height

I plan to move the engine up this week. Still hoping for some input on what type of sealant to use for the boat holes and any other tips for doing this move. I don't want to learn I didn't something wrong the hard way!<br /><br />Thanks!!!

 

[imported_JD__]

Re: Porpoising and engine height

tonycliffy,<br />(Assuming the lower transom bracket holes are slotted) Get the trailer level and secure so it can't roll. Trim your engine so the skeg is level, this will change as you increase the angle of your trailer. Place a padded block of wood under your skeg/fin. It would be good to have some help, my wife did a fine job of aligning the holes while I jacked the trailer. Elevate the front of your trailer by cranking your trailer jack until the skeg touches the block of wood. Remove the 2 top nuts & bolts. Loosen the nuts on the bottom two bolts so you begin to see just a little daylight between the transom and bracket. You want it loose enough to move and not scratch the transom but not so much its flopping around, you can tell when it separates. If your transom has a "cap" across the top which wraps over the top inside and outside edge of the transom, make sure the transom bracket doesn't catch the lip and bend it up when you start jacking. Simply crank the jack until you get the engine raised to where you want it. Re-install the top bolts and tighten all 4 back down. Any good 3M or Permatex silicone should work. The local WalMart should have marine silicone.<br />JD

 

[P.V.]

Re: Porpoising and engine height

You have.... no weight in the front of the boat, hense, all the weight is in the back! I would tell you "Of corse it's porpoising, no weight in the bow!!" Consider two things. Relocating what you can have moved, further towards the bow! Fuel tank/s, batteries, anchors, etc... AND, raise the engine up to the second hole! This will help "performance" but may not stop the porpoising. If your dealer mounted the motor all the way down, and the vent plate is below the bottom, you are authorised to kick him in the groin! He should know better!!! If you mounted it, we'll let you slide this time!

 

[LubeDude]

Re: Porpoising and engine height

"You have.... no weight in the front of the boat, hense, all the weight is in the back! I would tell you "Of corse it's porpoising, no weight in the bow!!" Consider two things. Relocating what you can have moved, further towards the bow! Fuel tank/s, batteries, anchors, etc... AND, raise the engine up to the second hole! This will help "performance" but may not stop the porpoising."<br /><br />************************************************XX<br /><br />Never set up a boat huh???????????????????????????<br /><br />This is the worst possable advice!!!!

 

[Forktail]

Re: Porpoising and engine height

What makes you say that LubeDude? <br /><br />The outboard is mounted as low as it can go, and the cav plate is below the hull. Obviously it needs to go up. I highly doubt moving it up a little will have any effect on the porpoising. <br /><br />As P.V. said, weight distribution is one of the main culprits of porpoising, and it appears this 850 lb boat has 450 lbs of outboard, 150 lbs of batteries, and 200 lbs of fuel sitting in the stearn. Plus it's doing 43 mph. It's no wonder that it's porpoising.<br /><br />IMO, move the outboard up a notch, re-distribute the weight, and try some different propping (rake, cup, etc).

 

[LubeDude]

Re: Porpoising and engine height

The biggest factor in porposing is too much weight in the nose or not enough power to hold the nose up, Its just a given!<br /><br />Porpoising: A constant rhythmic longitudinal pitching action of the boat, caused by a dynamic longitudinal instability. Often results from improper dynamic balance of weight and aero/hydro-dynamic forces, and usually occurs at specific speed for a unique hull setup. Over trimming the engine might cause this, whereby the bow is being held up by prop thrust, but not enough to stabilize the condition. Trimming 'in' can eliminate it at low speeds. The same action from the boat caused by not enough hull lift and subsequent falling of the bow back into the water. Could be caused by too much weight in the bow. On some particular rigs and setups, this could be a transition zone from hydrodynamic lift to aerodynamic lift that needs to be driven through in some manner before the hull stabilizes.<br /><br />Most of the center console boats Ive seen have the console mounted pretty far forward. If the weight in the rear was a problem, ALL Bass boats would be porposing and that is just not the case unless they are underpowered!<br /><br />There is a chance that the angle of the transome is too straight up and down and a wedge would help to tuck the engine under some. Right know the only thing he can do is just not trim so much!

 

[Jdeagro]

Re: Porpoising and engine height

Lube Dude;<br /><br />I agree with the fact that boat balance is one of the major causes of porpoising. The other is the hull design, and until the boat is sold your stuck with it. The problem with trying to change the boat balance is that it will be constantly variable. More or less people, gas, speed, current etc. Most stern loaded boats porpoise, some more than others and some can be corrected by motor trim. <br /><br />This is one of the top three complaints we hear when it comes to stern loaded boats. You know what I think regarding this problem, and how to fix it.

 

[Forktail]

Re: Porpoising and engine height

The biggest factor in porposing is too much weight in the nose or not enough power to hold the nose up, Its just a given!

LubeDude, this is a jon boat, not a bass boat.<br /><br />Unlike a performance-designed bass boat, the jon boat's hull is going to be very light-weight with a hard, if not flat, chine. The maximum 115 HP is plenty of power to hold the nose up, and the hard chine provides plenty of lift. <br /><br />The problem isn't that the bow is falling back into the water because there's too much weight up front. The problem is that under power, the bow rises past a point of hydrodynamic and aerodynamic stabiltiy and balance, in part because of the extra weight and draft at the stearn. <br /><br />It is the excessive weight and draft in the stern (a large 4-stroke, fuel, batteries, console, operator, etc.) that's causing the longitudinal instability you referenced from Scream and Fly.<br /><br />Many things can cause porpoising, and in some cases of higher performance boats, you might be right. But this jon boat is stearn heavy causing the instability and the bow to rise higher than what is stable. <br /><br />I've been around jon boats and this exact scenario all my life. Raise the outboard a notch, re-distribute the weight, and maybe try a prop with a more negative rake (will help lift the stearn).

 

[LubeDude]

Re: Porpoising and engine height

We seem to be arguing the same point to a point! The fact is that he is trying to trim the boat out more than the boat will take in its present configuration!

 

[P.V.]

Re: Porpoising and engine height

I stand by my first post!! OK, Well, right next to it, maybe slightly to the side, but, pretty close anyway!!

 

[LubeDude]

Re: Porpoising and engine height

Ok, I know when to stand down, But if he didnt trim so far it wouldnt porpose, Like he said, It gets close to the right RPMs then starts to porpose, I know its not a Bass boat, I cant help but think that if he got a prop with a little less pitch, he would achive what he is looking for without having to trim so much which causes the porpose. Am I wrong in this assumption? Im willing to learn something here.

 

[Forktail]

Re: Porpoising and engine height

No problem LubeDude. You're right that a prop with less pitch would increase his rpm. Maybe more cup or a different rake would allow more trim. But I think the first thing to do here before propping, would be to raise the cav plate flush with the hull (Tonycliffy indicated it was set up low). This should raise rpm a little without effecting porpoising. Next would be re-distributing the weight. Then propping. JMO, as I haven't seen the set up.