Effect Jackplate setback has on porpoising?

[juicegoose]

I have a Nautic Star 2110 Tunnel. The boat has a 200 optimax on the back that sits on a Bobs 10" jackplate. From the information I've gathered 10" on a smaller bayboat is a lot of setback, 6" being more the norm.
My question is how much more effect toward porpoising will having 10" setback cause compared to 6".

For those that will ask. I have moved everything I can as far forward as I can.
I run a 17p mercury rev 4 prop
Typically run the motor at 3 on the jackplate
I have never been able to unload the motor enough with trim so that the steering feels free. The steering always is easy to the right and requires two hands to the left. If I trim out more then 4-7 clicks on the trim button then the boat will begin a porpoise.

If switching to a 6 or 4" jackplate would help the porpoising then I would do it and sell the 10" to a bassboat guy or something.

 

[Texasmark]

Normally porpoising is angle of the hull vs angle of the thrust vs water velocity. On 10" you will have to have your anti-vent plate higher than with a 6". Have you tried other height positions? The prop, by it's specific design, could be causing your steering problem or it could be your trim tab adjustment. I just pulled up the Merc prop site and had a look at it. Seems to have all the whistles and bells. You didn't mention what size port plugs you are using and how are your rpms and planeout time?

Running a tunnel hull you may not be able to get the bow lift that this prop should give you. That will/should/might prevent you from feeling the "free steering" you are looking for like you would get with a monohull pad boat.

When you got to the porpoise point mentioned, did you try to bump the throttle up to stop it? How much throttle is left at that point? Have you tried to hammer through it with a lot of throttle for a few seconds to see if you could "get on the other side"? Have you had the rig to WOT and if so what were the rpms? My Merc data says it's 1.87 gearbox and tops out at 5800.

Mark

 

[ssobol]

Check the position of the trim tab on the bottom of the anti ventilation plate. This is used to counter the torque effect of the propeller and engine on the steering. If the steering is easy both ways (forces about equal) when the engine is off or at idle but not when at speed then the trim is likely out of whack.

 

[juicegoose]

Thanks Tex
My hull is actually a pocket tunnel so it's a little bit of both, tunnel/mono.
If by port plugs you are referring the plugs in the prop then I have all of them capped.
My planout time is non existent. With 2 persons and full 49 gallons of full I'm on plane in less then 3 seconds. Obviously with 5 grown men the planeout time is a little longer but still not struggling.

WOT throttle rpm with 2 men and 3/4 fuel is 5500 rpm and around 50 mph. I haven't had a clean enough day to test max speed/rpm over a longer run though. I might be able to squeak out a little more.

I have tried to push through the porpoise and it does seem to help. I'm still somewhat learning the boat and its quirks.

I have also noticed since I've installed the new prop that when I'm running on plane the boat wants to drift to the right. I constantly have to make small corrections to the steering. If I let go of the wheel and watch the position of a wheel spoke it will slowly move to the right.
Wouldn't the hydraulic steering not allow torque on the system?

 

[midcarolina]

I totally agree with Teaxasmark........ when the water exits behind the hull it rises up, that rise up allows you to run a motor with a JP much higher.......you didnt mention this but if you have it setup so the AV plate is level with the bottom of the hull the motor is too deep.

Does the boat have a water pressure gauge/ if so go up one mark and test run. if you go to far the water pressure will drop. and prop will likely cavitate at very little trim.

I wouldnt be a bit surprised with 10" set back if you could go 2" higher than you think you can

 

[Texasmark]

No feedback steering like Teleflex-morse makes might help. Send them an email and ask the question. Your hole time is great and you are correct in leaving the ports capped. You don't need them. Curious about the fact that you can WOT at 5500. I am assuming that you can if you keep the trim tucked in. That being the case, you will reduce drag once you can trim out and that will allow mph and rpms to rise and you will be where you want to be with that prop. If your rpms wind up a little over the spec, like a couple hundred rpms, I wouldn't worry about it. I do that all the time and have for years......2 strokes LOVE to run.

Changing your jackplate height can have a significant impact on some of your problems. Why not move the engine up a notch and come back with the answers on this thread; don't start another.

Mark

 

[juicegoose]

Whats curious about WOT at 5500?


to answer the jackplate height questions.
My normal course of action is to get on plane with the engine at 3" on the jackplate scale. I generally run the plate between 3-4". I can however and have ran the jackplate all the way up to 6" and loose no water pressure at all. I do gain rpms obviously. If it's choppy or I know I'm about to start making some turns then I'll lower the plate and trim in.

 

[juicegoose]

Would buying a 6" plate to replace my 10" plate be noticable?

 

[Texasmark]

The curiousity about 5500 is that you are only 300 from max rated yet you said you can get maxed out.......apparently, and not porpoising was the curiousity. But then I said you probably tucked it in somewhat smooth it out which is probably the case.

Just for a data point I went to the go-fast.com BAM prop slip calculator which I use as my reference. Plugging in your 1.87 gear box, 5500 rpm and 17p prop I only get 47 for a theoretical speed; that's the speed if your prop were turning in a solid medium. All screws (props) running in water have some slip which would reduce the speed even further. 10-15% is a common range. Plugging a modest 10% would drop your speed to 42. Have no idea what to expect on a tunnel hull for slip, course the setup and prop are a big part of that too. So your speedometer or tack aren't giving you the correct answer. What difference does this make? Well I was just fishing for any additional information that might help to solve the problem.

Since you have a variable plate then you have the ability to correct the effects of setback (correct engine height) and have gone through that. I don't see how a 6" plate will solve anything; but who knows. You are talking about a 4" difference. However there could be a physical imbalance (resonance) present making for the instability due to the factors involved: Hull shape, length, setback, speed, prop, and etc. On other physical things, away from resonance not much happens. However, when you approach it and get there (width of the spike depending upon what it is), things change dramatically; sometimes producing violent vibrations. I recall the violin and a crystal wine glass. When the musician got to, and remained on, the resonant frequency, in a matter of seconds the glass just shattered.

The boats in the orient and some mud boats we have come to mind where they are built for shallow running and the prop is like 6 feet behind a planing skiff. Not sure it matters how far back you are if the engine is set up right.

I guess I'm out of answers. Wish you luck.

They use a lot of tunnel hulls on the Gulf coast to fish the shallow bays and tidal waters. You might go to the Chamber of Commerce of a city like Corpus Christi, TX. and get a listing of boat dealers, at least one of whom should be able to answer your questions.

Mark