1992 Montauk 17 Optimum Engine Mounting, Porpoising

Optimizing the performance of Boston Whaler boats
Lucas M
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Joined: Fri Jun 29, 2018 2:51 pm

1992 Montauk 17 Optimum Engine Mounting, Porpoising

Postby Lucas M » Fri Sep 14, 2018 9:57 am

Hi, I have searched in the forum and from what I have read, in most instances the recommendation [for the optimum engine mounting on a 1992 Boston Whaler MONTAUK 17] would be to have the [engine mounting height at] two holes up. Before I go ahead and raise my engine, I would like to mention my current layout and boating sea trial observations in order to ask if you also believe that moving to two-holes-up (1.5-inch higher than lowest position) would be my best choice as a first option:

1992 Montauk 17 with 1992 Evinrude 90-HP V4:
  • engine mounting all the way down on transom, lower bolts in slotted tab of motor, upper bolts in fixed holes (appears to be easy job to raise engine), all four bolts are visible in the splashwell
  • propeller is a brand new OMC aluminum 13.25 x 17-pitch
  • sea trial loading underway: one adult at center console, one adult on cooler seat, two six-year-olds on bow cushion, two cooler stern seats weight about 50-lbs total with items inside, battery at stern port side, no auxiliary engine
  • at rest tied to the dock with no people on board, waterline at stern is about 1/4-inch above bottom of splashwell drain tubes when viewed from the astern
  • water conditions for trial: perfectly calm, no waves, fresh water
  • no bottom paint or anti-fouling


Sea trial observations:
  • at speed above approx 25-MPH (SOG per GNSS receiver), boat begins [repeated oscillation of the bow up-and-down or porpoising] with trim up at about 1/4-up from full-down on TRIM gauge
  • at WOT, engine RPM at around 5000, trim max-in (down) to avoid porpoising
  • boat peed and engine RPM increase slightly at WOT with trim up to around 1/4-up-from full down, but then [repeated oscillation of the bow up-and-down or porpoisng] starts again

Goals:
  • eliminate the purpoising so that boat can be run in the 3000 to 5000 rpm range with lighter steering and better fuel economy and no porpoising
  • although I will almost never run at WOT, would like to run motor at its optimum performance which would produce 5500rpm at WOT I have read is the target value for my motor and prop

Questions:
  • Would raising my motor 1.5'' be the best first step?
  • If not, what would recommendations be? For example, would a trim tab be recommended?
  • Could there be an ulterior [concern] given the static water line of boat?
Thank you

jimh
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Location: Michigan, Lower Peninsula
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Re: 1992 Montauk 17 Optimum Engine Mounting, Porpoising

Postby jimh » Fri Sep 14, 2018 12:04 pm

Would raising my motor 1.5'' be the best first step?

On the basis that due to the effects of gravity it is generally easier for heavy items like an outboard engine to be moved down as opposed to move up, I would expect that if:

  • you raise the engine mounting height to be two-holes-up from lowest (the present location), and
  • you find the engine is too high

then it will be easier to lower the heavier engine one hole than if you started at just one-hole and wanted to raise it to two-holes.

Re the problem of porpoising in general: the tendency for the bow of a planing hull to oscillate up and down is very common in small boats with outboard engines, and as you have noted can be controlled with engine trim position.

The peculiarities of a particular combination of hull, engine, propeller, trim, weight distribution, and sea state are highly variable and all influence how likely the boat will be to suffer from porpoising. I don't know that a classic Boston Whaler MONTAUK hull is more prone to porpoising than other hulls, but it most certainly can exhibit that tendency, as evidenced by your narrative.

There are two notions regarding porpoising and engines which have been proposed:

  • an engine mounted too low on the transom can cause porpoising, and as a corollary, raising the engine mounting can mitigate that tendency;
  • an engine heavier than the weight of the original design era engines and a lighter than normal bow create more chance for porpoising

I don't know that either proposition has been proven in scientific testing, but those are the general notions.

jimh
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Re: 1992 Montauk 17 Optimum Engine Mounting, Porpoising

Postby jimh » Fri Sep 14, 2018 12:11 pm

If [raising the engine mounting height is] not [the remedy for porpoising], what would recommendations be? For example, would a trim tab be recommended?


I think trim tabs on a 17-foot boat are a bit excessive, and I cannot recall if anyone has installed trim tabs on a MONTAUK 17. Accordingly, I don't know if trim tabs on a MONTAUK 17 would be a complete and satisfactory remedy for porpoising.

To me trim tabs are most useful to correct side-to-side listing due to weight distribution, engine torque, and wind-induced listing. They are also useful to get some transom lift on boats with too much weight in the stern. Can they mitigate or eliminate porpoising? Perhaps, but it seems like an awfully expensive cure. Adjusting engine trim is much less expense and effort.

Also, regarding trim tabs, they are just one more element of the boat's running gear to be fiddled with. If you like to have a lot of buttons to push and adjust all the time, get trim tabs. I like to just adjust throttle, steering, and perhaps engine trim. Maybe if I had to run for five hours with a big cross wind I would say, "Gee, I wish I had trim tabs." But most of the time I don't miss them, and don't think I need them. And that is on a 24-foot boat with which I have made continuous runs of over 70-miles in open water that lasted several hours.

jimh
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Location: Michigan, Lower Peninsula
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Re: 1992 Montauk 17 Optimum Engine Mounting, Porpoising

Postby jimh » Fri Sep 14, 2018 12:15 pm

Could there be an ulterior [concern] given the static water line of boat?


Ulterior--going beyond what is openly said or shown and especially what is proper.
Cf.: https://www.merriam-webster.com/dictionary/ulterior

The static waterline of a boats always a proper concern. The static water line reflects the loading of the boat. if the static trim of the boat causes too much immersion or draft at the stern, it is a concern. This concern is not concealed nor is it not spoken openly about. It is always there, not hidden.

flymo
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Joined: Thu Oct 22, 2015 2:58 pm

Re: 1992 Montauk 17 Optimum Engine Mounting, Porpoising

Postby flymo » Sat Sep 15, 2018 9:43 pm

Yes, raising the motor those two holes would be the first step. It should lighten the steering, reduce the porpoising, improve the effect of trimming, and increase your top speed and fuel efficiency. It's also easy, and free if you do it yourself. Many, many Montauks have been rigged with those V-4s so there is no reason to be concerned about static trim at this point.