HELP!!! Adjusting Trim ( porposing )

[butteco]

I have an 85HP Evinrude on the back of my 16.5' fiberglass runabout/ski boat.... It has manual trim (not power trim) and I have a total of 5 settings the motor can be on. Here is a picture of the settings. I started with it on the setting CLOSEST to the transom and the boat planed out fine but the Anti Ventilation plate is still a little under the water. I want it perfect so I moved it back 1 setting at a time. The 2nd setting away from the transom is better, but I still want it to be perfect. THE 3rd or middle setting is PERFECT for the motor, but the boat PORPOSES!!! It does fine in calm water, but as soon as I hit another boats wake or a small wave I porpose for 100+ yards. <br /><br />Where should it be??? Where should I start? <br /><br />I NEED HELP!!!!!!!!!! <br /><br />Thanks!<br /><br />

 

[mattttt25]

Re: HELP!!! Adjusting Trim ( porposing )

you said the second setting was better, and didn't mention porposing, but you want it perfect. what made you move it from that setting?

 

[steelespike]

Re: HELP!!! Adjusting Trim ( porposing )

The position of the anti vent plate is second to the performance of the boat.You want as little motor in the water as possible without ventilation problems.It appears you are pretty close now.If you rock it out too much you get the<br />porpoising problem.You could try that setting and move some weight forward.Tank or battery for instance.What ever is easiest.If it helps then consider moving one permanently.<br /> You might try moving the motor verticaly. There may be an adjustment on the mounting bracket to do this.Or it may be possible to move the whole motor about 1/4 inch at a time intil just before ventilation becomes a problem.This can be tricky with a bolt on bracket.You could also try the 2nd position and move some weight back.<br /> When thinking pure speed the idea is to have as little motor and boat in the water as possible at speed.When trimming out the boat will ride harder but generally go faster.With the motor trimmed in it will plane easier and generally ride better but with less top end.

 

[ondarvr]

Re: HELP!!! Adjusting Trim ( porposing )

In your other posts you were asking about other problems, have you tried any of the things mentioned before. Did you raise the motor yet?

 

[butteco]

Re: HELP!!! Adjusting Trim ( porposing )

I haven't lifted the motor yet because in the other post they suggested trimming the motor first. <br /><br /><br />When I said the 2nd setting was better, I meant better than the first!!! The motor still pushes thrust towards the bottom of the lake (although at a very minimal angle) and I want to trim the motor one click more out to get it pushing the thrust on plane with the water. But when I put it in the middle, or 3 clicks out, it porposes. <br /><br /><br />I WANT THE BEST SETUP FOR TOP SPEED LAKE CRUISING. And I would also like to know what kind of setting I would want for pulling a skier? <br /><br />Thanks!

 

[fireman57]

Re: HELP!!! Adjusting Trim ( porposing )

Sounds like a job for Nautijohn.

 

[ondarvr]

Re: HELP!!! Adjusting Trim ( porposing )

You can move some things around in the boat to see if that helps, but it may change again when you raise the motor. <br />Now that you know how it runs with it in the current holes, raise it up and do it again.

 

[ob15]

Re: HELP!!! Adjusting Trim ( porposing )

I had the same situation on my boat. The third hole and it would porposes. I raise the motor one hole & kept it on the second trim hole. Ran great. You might want try that.

 

[fireman57]

Re: HELP!!! Adjusting Trim ( porposing )

For the porpoising smart tabs will take care of that. That's why I paged Nautijohn. For about 150.00 they are well worth it.

 

[butteco]

Re: HELP!!! Adjusting Trim ( porposing )

Trim tabs are only $150? Everywhere I looked they were $350+ ? <br /><br />ALSO - Don't trim tabs cause drag and slow the boat down?

 

[ondarvr]

Re: HELP!!! Adjusting Trim ( porposing )

There are different types at different price levels.<br /> <br />Yes, they can create drag if they are not needed or incorrectly adjusted, kind of like the fin on the motor. They can also increase speed and improve the ride when used correctly.<br /><br />First you need to see if there is a no cost way to solve the problem by testing different setups for the motor. If that fails then look at other options.

 

[dajohnson53]

Re: HELP!!! Adjusting Trim ( porposing )

In the mean time, try moving some weight forward. I don't know the configuration of your boat or what's feasible, but you might find that a simple change of weight distribution will solve the porpoising problem. Not to say you shouldn't try the other interventions - raising the motor, maybe tabs - but moving weight is easy and quick. A passenger is a good thing to experiment with. Sometimes a passenger along side you will do it. Sometimes the passenger would have to sit further forward. IF that helps, and you have a reasonable place to stow it, you could replace passenger wt. with something heavy like gear, sandbags or such. If a passenger along side you does it, you'll find that a smaller amount of weight, but further forward, might duplicate the effect.

 

[Jdeagro]

Re: HELP!!! Adjusting Trim ( porposing )

Sorry it took so long to get into this post, I missed it!<br /><br />As most of the answers here have indicated the problem with the boat (and most stern drive and OB) is simply balance. The CG or axis point as I like to call it is in the last 1/3 of the boat. This gives the bow a significant mechanical advantage when it comes to movement up and down. Setting the motor trim down puts the prop in a position which lifts the stern, and raising the stern pushes the bow down (axis rotation). This extra lift will minimize or sometimies eliminate the porpoising. This position also helps lift the stern on acceleration to assist the boat to plane. What is the down side? Generally a loss of speed at any rpm level including top speed, and a loss of economy since the prop is running at an angle not perpendicular. Additionally the boat may run bow down at cruising speeds resulting in bow steering, especially in turns.<br /><br />Changing the balance of the boat (weight forward) will help the boat to plane, and you them may be able to trim the motor out a hole or two. However any permanent change in balance will cause some compromise under some conditions. In other words, fix one thing and cause another problem. <br /><br />Changing prop has been the "common solution" for just about everything relating to speed, attitude, hole shot, porpoising, acceleration, etc. Props are for propulsion, and any other use is a compromise to performance. <br /><br />We developed Smart Tabs specifically for boat in the smaller range which are used for a multitude of tasks. Trim tabs free up the prop of it's other tasks so that the overall performance improves. Porpoising, "hole shot", high bow rise, handling, are all a function of the trim tabs. Trim Tabs are an extension of the hull that can be or does adjust to the condition and circumstance. Like flaps on an airplane wing. <br /><br />They will be the most influential change you can make to the performance of this boat. Do some research.

 

[Mark42]

Re: HELP!!! Adjusting Trim ( porposing )

I had trimmed my boat to the best setting for highest speed (a wopping 30mph best speed). But it did porpoise at top speed. The reason my boat bounced around at top speed was because it was light. And being light helped get more of the boat out of the water at speed. That allowed reduced friction and maximized the speed potential of the boat. <br /><br />I was going to order some Smart Tabs to fix the problem, but waited until I finshed installing a 14 gallon permanent gas tank in the bow of my boat. I figure that the full tank and hardware weigh in at around 110 lbs. <br /><br />Removing the weight of the gas from the rear (2 6 gallon portable tanks) and adding it to the front completely eliminated the porposing. The boat rides flatter than it ever did before (even with 1/2 full tank) but now my best speed with full fuel and one passenger is 28 mph and with a full load is 27mph. The better ride is causing more drag because more of the hull is in the water. <br /><br />I still have to play around with the trim to find the maximized speed with the new weight arrangement. I like the way the boat handles much better now, it rides nicer, cuts wake better and still tracks really well at speed. It does tend to wander a bit at idle in the no wake zones. I can see no reason to buy Smart tabs now (I'm not knocking the product) because the porpoising is gone.<br /><br />So if you are able to move some weight to the front give it a try and see if that solves the problem, and if you like the handling. But I bet its a catch-22, more speed means lighter boat, which leads to porpoising. Adding weight or bringing the bow down will reduce porpoising, but causes more drag, which slows you down.<br /><br />I'm not trying to sound like an expert, just speaking from my own personal experience with my 14.5' boat. Let us know how things work out for you, I'm really interested in your outcome.