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1985 Montauk, 1985 V4 Evinrude.

I added a jack plate in hopes of gaining the same benefits as having the motor mounted higher on the transom as well as gaining a few mph. Also, moving the motor back that few inches allowed me to tilt it up higher when trailering without contacting my stern seat or bimini as it made a nice space for my bimini to fold behind the seat. This did not work out. The boat is too stern heavy and porpoises at speed, it's not able to stay on plane at low speeds like before and the ride quality is obviously worse. Needless to say I will be removing the jack plate and I am just trying to decide whether or not to raise the motor from it's original pre jack plate position.

Does anybody heave any comparative input on this? It seems to me that the only way to "soften the blow" in nasty chop is to trim way down and let the bow cut through the stuff. I know that the general consensus on theses boats is to mount the engines up two or three holes (mine was mounted all the way down on the transom). I understand the benefits of reduced drag and fuel economy but how does raising the engine effect the ability to trim the bow down and what effect does it have on the attitude of the hull while on plane and the boats ability to stay on plane at low speeds?

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Is it a setback or just a jack plate?

I assume the poor ride characteristics occur with passengers in the stern seat?

Where is the engine mounted (height relative to water, not hull?) It could be you have not factored in the setback distance and the need to raise the motor even MORE than if it was on the transom. With 3" of setback from the transom, you would want the motor at least 4 holes up.

When you start to porpoise, is the engine trimmed out? Trimming in help?

What prop is on the engine?

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Having a passenger at the stern makes it porpoise worse, ballast at the bow helps.

I've played with motor height. I've adjusted it to the point of good speed and RPM but where it won't blow out in a turn or ventilate while getting on plane.

It will porpoise no matter where it's trimmed. It's a little better trimmed down but still does it and steering gets heavy.

Prop is an OMC SST 13X19, was a perfect set up prior to jack plate. It had a wicked hole shot and it did 42mph. I do get a bit more speed now.

I know, I know. Why'd you mess with a good thing?

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Moving any motor back is increasing the propensity for a hull to porpoise.

Moving weight to the bow will help. Battery to console?

For giggles, I would take out the stern seat and do a trial. Bet the porpoising goes away.

If you really want the jackplate, I would investigate a different prop that is a known for lifting the stern.

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You are correct Phil, stern weight is the problem. But it would be nice to be able to use the stern seat when I take the family out. I already have the battery in the console.

Any prop suggestions? Do the stiletto props create more stern lift than an SST?

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I put a 2002 90hp BRP Johnson 2 stroke on my 1977 Montauk. It was mounted all the way down. Cheap aluminum prop. Never porpoised.

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I am not a prop guru.

I do know that there are a few prop shops who are knowledgeable of Boston Whalers, Ken @ Prop Gods in FL is one that comes to mind. I would give them a try.

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Personally I would get rid of the jackplate and modify your motor mount bolt hole locations, getting rid of the blindhole bolts, so that the motor sits on the transom with the anti-ventilation plate 1.5 inches above the keel. I'd wager that's about 1 hole from all the way "up"

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Raising an engine and using a modern prop, even an excellent aluminum like the Mercury Black Max line, will generally reduce porpoising, improve ride, lessen steering torque and increase top end speed.

For a Montauk with engine mounted directly to the transom, the "two holes up" postion, (which means there are two open holes exposed above the bolts) is generally optimal and all you need. This dimension does vary somewhat based on the engine's mid-section design length, but with an older OMC or Merc, two holes up is about all you want.

I agree with Phil that the stern seat weight is most likely your problem, especially with people sitting there. But if you want to keep it, the jackplate must be too much offset weight.

I have a 305# Merc in-line six 115 on mine, installed on a 6" setback Detwiler jackplate, and I get no porpoising, period. I do not have a stern seat, although battery is in the stern. See photos on personal website link. The boat accelerates like a rocket, and planes off in seconds with no stern squat at all. The engine is running about 1-1/2" high. Prop is a Mercury Laser II, but this height will also accomodate a Merc Black Max aluminum with no ventilation. I like the jackplate since it gives a super-clean rigging situation when combined with the Baystar steering.

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If I trim too high mine will porpoise and there is a balance I must attain where the steering doesn't pull one way or another and it rides right. Did you try trimming down? If you can't fix this with a tilt or height adjustment it's probably best to go back to how it was.

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On my 1976 Sport 15 following Joe's and others advice, I raised my engine 2 holes (1 hole from all the way up) and switched to a stainless prop. It solved my problem with porpoising and improved overall performance in terms of hole shot, top speed, and steering pull.

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Lucky Dog
I have a 2003 Montauk 170. This year I added a CMC hydraulic jack plate and hydraulic steering.
It moved the motor back 5" and definitely changed the way it runs.
It took me a couple of days of experimenting to find the best combinations.
On-plane I have the engine all the way down and trim it in a little from where I used to run it.
I trim it up to the point where it starts to porpoise and then back off a little.
I suspect trimming it down a little will burn a little more fuel but the ability to get into real skinny water is worth it.

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Like you, I experimented with adding a jackplate (CMC two piece manual one with 4" setback) to raise the 90 hp 1988 Evinrude on my 1988 Montauk 17. I also have a 5 hp Mercury kicker. Adding the jackplate changed the static trim considerably, causing the boat to lean towards the stern. I got rid of the plate and put the motor back the way it was (mounted flush with the top of the transom).