Moderated Discussion Areas
ContinuousWave: Whaler Performance
|Author||Topic: Porpoising Whaler|
posted 12-10-2001 02:36 PM ET (US)
I have a 17'Montauk powered by a 90 hp 4-stroke Merc. Under normal circumstances, with a moderate load and two people in the boat, everything runs as it should, 5200-5400 rpm's, 42 mph, at WOT. However, with a electric trolling motor, approximate weight, 50-60 lbs., mounted over the starboard bow, the boat's rpm's and speed drops substantially because the boat "porpoises" when trimmed out to run. Could it be that the extra weight on the bow restricts the lift? Any comments and/or suggestions would be appreciated.
posted 12-10-2001 03:10 PM ET (US)
Bow weight should stop porpoising, not cause it. What size prop are you running? What are max Rpm's? I am considering a 2001 90 Merc 4 stroke so I am curious.
posted 12-10-2001 07:51 PM ET (US)
If you will read through posts on the subject over only the past 3 or 4 months you will see strong evidence that Montauks don't like more than 300# on the transom. They porpoise.
Even with my DF70 Suzuki (335#) I had to raise it, relocate the battery, and add a hydrofoil to get it to behave. Even so, if a passenger moves, I have to tweak the trim.
I dearly love my 4 stroke, but the advantages don't come without penalty.
Red sky at night. . .
posted 12-10-2001 09:05 PM ET (US)
I only have the motor on the transom, 2 hole up, and a hydrofoil on the cavitation plate. The boat/motor combination seems to run fine with a 16" s/s Vengence, 39 mph @ 5600 rpms, or 43 mph @ 5300 rpms with a 18" Vengence, without porpoising, until that weighty electric motor is on the bow gunnel. The batteries and gas tank are mid-ship. Several of my friends have 2-stroke 90 hp. Evinrudes as well as 9.9 kicker motors, total weight equivalent to one 4-stroke and do not have the porpoising problem. I"m thinking that the hydrodynamics of the Montauk does not allow for such extra weight on the bow as it prevents "bow lift." As discovered, I just cannot trim out as far as to get maximum speed. The rpms are still where they should be. The Montauk with the 90 4-stroke, or with a 70 4-stoke is the only way to go!
posted 12-10-2001 09:26 PM ET (US)
I can't believe that the bow mount motor is the problem. My Montauk doesn't porpoise with the bow mount motor installed and a group 27 deep cycle in the anchor locker. I am only running a 50 hp 4 stroke (224 lbs). On a normal day I have about 600 lbs of fuel, gear and people on board and top out at 33 mph.
Could it be that the higher speeds increase the tendency to porpoise?
posted 12-10-2001 11:10 PM ET (US)
My Montauk (90HP Evinrude 2-stroke) will
porpoise if I put the bow up too much and
go fast, even if it's just me, near empty
tank, no gear.
posted 12-10-2001 11:50 PM ET (US)
Well from what I've seen in any 17 whaler with a 90 hp and up is a promise of porpoising at high speeds, when you trim the motor up to achieve max speed. my whaler max's at 41.2 mph "gps" and porpoises quite abit. when I trim the motor down it disapates. As for fins on the motor, your asking for trouble. when you trim up and the fins are almost at water level, and you begin to turn your fin, depending on which way your turning will leave the water, and then when it bites the water again can or should i say will bite down hard and send you flying. I have heard many say don't put fins on a 17 whaler since they are so bouyant, referring to the stern. just one honest opinion. goodluck
posted 12-11-2001 01:30 AM ET (US)
I am working on what I call my GENERAL THEORY OF PORPOISING. It is still under development, but so far it goes like this:
As a boat goes up onto plane, the wetted surface decreases and moves aft.
The center of gravity (CG) of the boat remains about the same.
The point where engine thrust is applied remains relatively constant, too. Call this Point the Center of Effort (CE).
Along the fore and aft line of the boat there is some point which is in balance, where the boat would tend to rotate ("pitch") up or down equally around this point. Call this the center of pitch resistance, or CPR for short.
When the boat is in the water in displacement mode, the CPR is near amidships.
As the boat comes on plane, the CPR moves aft along with the reducing wetted surface. This distance from the CPR to the CE decreases
The boat trim is determined by the vector that the thrust is applied thru the CE. This determines the bow lift and running angle.
Pretty soon the boat is running at a speed where there is marginal wetted surface, and the CPR is practically at the transom, just inches in front of the CE.
All the weight of the boat in front of the CPR ( a long lever arm of weight) is being balanced by the very short lever arm of the thrust being applied from the CE to the CPR. It takes very little force acting on the bow--a little wave--to lift the bow up and drive the stern down. The forces at the bow have much longer lever arms around the CPR.
The result is unstable pitching or porpoising--oscillations of the bow pitching up and down from small forces being applied at the bow through the long lever arm they have on the CPR. They cannot be controlled well by the engine thrust because it is acting through a very short lever arm.
The cure to suppressing them is one of two choices:
--change the angle of the thrust vector to force the bow down. This increases wetted surface and drives the CPR forward, bring more stability to the equilibrium around the CPR.
--use trim tabs, or what I call "stern lifters", to raise the stern slightly which will bring the bow down and increase the wetted surface and move it forward.
Notice that using an engine bracket should increase the distances between the CPR and the CE, giving the prop a longer lever arm and more control over the bow. This alone should reduce pitching or porpoising.
Thats my GENERAL THEORY OF PORPOISING.
As the Montauk owners have noted, heavy engines in the stern tend to cause porpoising. This is because the wetted surface when planing is driven smaller and farther aft.
The boats that don't have porpoising problems are boats that get some stern lift going. The boats plane with more of the hull in the water measured fore and aft, but less of the hull in the water measured keel to deck.
Both hulls achieve the same buoyancy by having about the same area of hull in the water, but on the boats with good stern lift the wetted surface is more forward than on boats whose sterns are heavy and dug in.
posted 12-11-2001 07:43 AM ET (US)
Dogfish2, check engine mounting for plumbness! If the four mounting holes were drilled very slightly skewed motor will not be "straight up and down" (scientific term!). This may be cause of problem... just grasping at straws here but this one has me puzzled... add weight to bow and porpoising starts...new one on this old Cracker.. Good luck and Happy Whalin'.. Clark .. Spruce Creek Navy
posted 12-11-2001 08:24 AM ET (US)
as noted earlier 41 mph+ IS close to "flying", you literally aren't touching much...water surface wise..it seems to me that engine torque,play in the steering..cocked eng. mounting?all could be factors..and it seems that the engine weight thing( over 300 #) on a 16-17 hull is a wash because that's what the majority of us have.. v-4s!...and they do good and have done so for lots of years ..even when compared to lighter weighing 2 stroke 90 hp yams which i naturally dream of owning one day!...interesting subject..thanks...lm
posted 12-11-2001 10:02 AM ET (US)
Weight definately is a factor. I can not get mine to porpoise even trimmed to the hilt. I have a 27 pate and bat forward. The 90 Yamaha will blow out before it porpoises. My 15 CC with a 70 OMC would not porpoise either.
posted 12-11-2001 10:30 AM ET (US)
DogFish2, try lowering the engine 1 or even 2
holes which will enable you to trim in further,as needed, and with greater effect. Also, Mercury states the Trophy Plus(4 blade stainless) prop provides stern lift( more than the Laser II stainless I'm using now). Lowering my Merc 90 2stk. did it for me. My next step would have been trying a Trophy Plus. A few dealers provide test props. Let us know what you decide & how it works for you, please.
posted 12-11-2001 08:51 PM ET (US)
Whow! What a weath of ideas, and I thank all of you for the contributions. I have experimented with most of your suggestions, but to no avail. Like I mentioned, porpoising is not a problem until I mount the trolling motor on the bow and attempt to reach the same speed, or near it, without the trolling motor. Next time out, I"m going to leave the trolling motor inside the boat and run for a time, and then, mount the motor back on the bow and compare. If that is the cause of the porpoising, then I can either try a lighter trolling motor (only 15-20 lbs.), or get a Xmas gift of Lenco Trim Tabs. Anyone of you knowledgeable fellow Whalerites know how these trim tabs perform on the Montauks?
posted 12-12-2001 08:41 AM ET (US)
Dogfish2, try counterbalancing with same weight as trolling motor. Locate in stern and move around and see if this equation has any effect! Just a thought (don't have too many these days, heh, heh)... Happy Montaukin'... Clark... Spruce Creek Navy
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