17 SSL Suzuki 90-HP Bow Oscillation

Optimizing the performance of Boston Whaler boats
Tony
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17 SSL Suzuki 90-HP Bow Oscillation

Postby Tony » Wed Dec 23, 2020 10:06 am

I completed engine break-in period [on a SUZUKI 90-HP engine of unspecified model designator] on 17 SSL boat.

[The 17 SSL exhibits excessive up-and-down repeated oscillation of the bow] at all planing speeds, at any vertical height, and at any amount of positive trim. [Trimming the engine to] full-under stops [the bow oscillation].

The SUZUKI 90 engine is mounted three-holes-up on jackplate.

On smooth water with just one person aboard at full throttle the engine accelerates to 6,300-RPM [and the boat speed is] 38-MPH.

The propeller is a POWERTECH LNR 3 14 x 18-pitch.

Q: How does a LNR 4 14 x 21-pitch sound?

jimh
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby jimh » Wed Dec 23, 2020 10:26 am

Please give readers more information:

  • the model designator of the engine
  • the gear ratio of the engine
  • the weight of the engine
  • the manufacturers full-throttle operating speed range, and
  • if available, the manufacturer's recommended optimum full-throttle speed range for best power output.

Do you anticipate that increasing the propeller pitch by three inches will have a a beneficial effect of the tendency of the bow to oscillate?

Please explain your goals for propeller optimization.

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Phil T
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Phil T » Wed Dec 23, 2020 10:35 am

In addition to the above information request, please provide the following:

  • Years of boating experience
  • Engine height on the transom. How many empty holes in the bracket above the top bolts?
  • Location of fuel tank (stern, midships, bow)
  • Location of battery (see above)
  • Location of passengers
  • Location of gear.
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1992 Outrage 17, 1992 Evinrude 115

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Phil T
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Phil T » Wed Dec 23, 2020 10:37 am

The Power Tec LNR 4 is a four-bladed stainless steel prop.

See https://www.ptpropeller.com/content-pro ... amaha.html
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Phil T
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Phil T » Wed Dec 23, 2020 10:40 am

[edit - deleted erroneous information]

The recommended props for your model and engine are 3 bladed stainless steel, eg. 14x18 Suzuki SS or a 14x22 Stiletto Advantage I.

Answer the above questions, and then we can discuss propellers.
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1992 Outrage 17, 1992 Evinrude 115

biggiefl
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby biggiefl » Wed Dec 23, 2020 10:51 am

He has a three-bladed prop but is asking about a four-bladed propeller, which is way too much propeller. And four blades do not help with bow oscillation.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

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Phil T
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Phil T » Wed Dec 23, 2020 11:02 am

Well the jackplate is NOT helping you. How much setback?

Where is the anti-ventilatoin plate in relation to the keel? Use a straight edge to measure.

Adding transom weight and moving the engine aft is exacerbating the tendency for the bow to oscillate.
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1992 Outrage 17, 1992 Evinrude 115

jimh
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby jimh » Fri Dec 25, 2020 6:53 am

It is possible on almost any planing-hull boat to induce bow oscillation under certain circumstances.

As mentioned, moving the engine weight farther aft with a set-back bracket and the added weight of the set-back bracket both tend to affect the static trim on a boat, making the boat have a bow-high trim, and also making the bow easier to raise with engine trim.

Generally careful adjustment of engine trim will eliminate bow oscillation.

At certain boat speeds and certain wave heights and wave directions, the tendency for bow oscillation will be much greater than at other conditions, and the only remedy may be to alter boat speed slightly.

I note that in the initial description of the bow oscillation, there is a trim setting that stops the bow from oscillating. This is a very normal outcome. There is no basis to believe that at EVERY engine trim setting the bow will NEVER oscillate. Having found an engine trim setting that stops bow oscillation seems to have resolved the problem.

As others have commented, increasing the propeller pitch by three inches and changing to a four-bladed propeller are not particularly clear paths to preventing bow oscillation.

The choice of a propeller can affect the behavior of the bow. Some propeller are described somewhat incorrectly as "bow lifting" propellers. This is perhaps a simplification. Typical a propeller so-described will have raked blades. Raked blades tend to produce a tighter cone of accelerated water coming off the propeller, and this, in turn, tends to give the propeller better control of the boat hull trim, producing better outcomes for raising the bow.

I don't have any sort of clear information about the nature of the propeller being used now, so to predict if changing to some other propeller of different blade rake will be beneficial or detrimental will be difficult.

To appreciate the design of the propeller in use now, the initial poster should provide an image or a link to an image that shows readers the blade configuration with regard to the rake angle.

Tony
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Tony » Tue Dec 29, 2020 8:46 am

The current propeller is a PowerTech LNR three-bladed 14 x 18-pitch polished stainless steel.

Further tests confirm less oscillation with three more passengers added to bow seats, but still oscillating occurs if much positive trim used.

Ventilation also begins to occur with positive trim at higher speed.

With five adults aboard the WOT engine speed is 6300-RPM, and boat speed is 36-MPH. There is a loss in boat speed of only 1-MPH or 2-MPH compared to with only a single [adult] aboard.

The ventilation is more related to vertical position near max-up on jackplate with more load.

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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby biggiefl » Tue Dec 29, 2020 10:00 am

If you are getting 6300 with both one person and with five people, you need a larger propeller.

You should remove the jack plate.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

Tony
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Tony » Tue Dec 29, 2020 12:37 pm

Thanks for your help. I will try out a larger pitch and then maybe ditch the jackplate. Those shallow flats on the coast are where I really appreciated that thing. Four or five inches really saved the day many times.

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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby biggiefl » Tue Dec 29, 2020 2:19 pm

I agree but let's see if eliminates the problem. Have you tried different heights? Is it hydraulic or manual.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

Tony
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Tony » Tue Dec 29, 2020 7:10 pm

The CMC hydraulic bracket appears to give about 5-inches of setback and 5.5-inches of lift.

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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby biggiefl » Wed Dec 30, 2020 11:47 am

That is a LOT of weight with the CMC.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

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Phil T
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Phil T » Wed Dec 30, 2020 12:24 pm

An additional factor in bow porpoising is an engine being mounted too low.

Given the 5" of setback, the engine height, at any planing speed, should be such that the anti-cavitation plate, located just above the prop, should be 2 to 2-1/2 inches above the keel of the boat.

To measure this properly take a long straight board, lay it up to the keel and slide one end back past the engine. With a measuring tape, measure from the top of the board to the underside of the anti-cavitation plate.
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Tony
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Tony » Wed Dec 30, 2020 12:44 pm

I will put [the boat under discussion, a 17 SSL] on a trailer [in several days or when the weather moderates]. Then measuring the location of cavitation plate, changing propellers, or changing [engine] mounting [height] will be easy.

The boat lives in a slip on straps most of the time.

With the data from prior performance trials, it sounds like a higher pitch propeller might be the next [propeller] to try.

I would like to keep the jack plate if possible, as I explained.

I think the dealer will swap out my propeller.

[may change] to a 14 x 20 three-blade from a 14 x 18 three-blade.

I understand [some unspecified propeller] may be too much [load].

People say [a 17SSL with a 90-HP engine] should run [about 45-MPH at best]. [My boat top speed] now is at 36 to 38-MPH.

I’ve never run a four-bladed propeller. I hear you lose top [boat speed] but gain [acceleration from a standing start].

Tony
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Tony » Wed Dec 30, 2020 12:47 pm

boat.jpg
Fig. 1. Engine.
boat.jpg (11.5 KiB) Viewed 625 times

[Moderator's note: the original image has been cropped to eliminate the surrounding area and parts of the boat not related to the engine mounting heightand the propeller blade arrangement. Note that the forum is pleased to host and include in-line images into posts to the forum, and those images can be be as large as 900 x 900 pixels with a file size as large as 250-kBytes. The image above is only 137 x 186 pixels in size and the 11-kByte file is only a tiny fraction of the allowed file size. This image is not a particularly useful illustration if the intention was to illustrate the engine mounting arrangement or to show the blade rake of the propeller. Generally to illustrate the engine mounting height the most interesting detail is to see the position of the anti-ventilation plate relative to the water line when the boat is underway and the boat is on plane at moderately high speed with correct engine trim. To show the blade rake of a propeller, one needs to only show the propeller.]

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Phil T
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Phil T » Wed Dec 30, 2020 2:59 pm

Tony--given that propellers are very expensive, $300 to $900, here is some background meant to help.

In completing the task of propeller selection and engine rigging, it is important to note the following:
  • When testing for propellers, it is important to keep all variables, other than the propeller, the same between tests. If test-1 is done with one passenger and 10-gallons of gasoline, test-2 needs to be done with one passenger and 10-gallons of gasoline.
  • Runs into the wind and downwind should be done and with or against any ocean or river current.
  • Speed should be measured by GPS receiver; engine speed in RPM should be noted at full-throttle.
  • Changing more than one variable at a time leads to inaccurate results and inaccurate judgements and poor decisions.

With your low [boat] speed and bow oscillation, I would tackle one element at a time: first engine height; second propeller size.

First, correct engine height; then re-test the boat in the same conditions as before.

Second, change the propeller; then re-test boat in the same conditions as before.

Propeller selection is 90-percent science and 10-percent art.

Propeller size and performance is NOT universal across brands or even models of propellers. Each model of propeller has a unique blade geometry. This geometry will result in unique propeller performance. Also if you change to a four-bladed propeller from a three-bladed propeller, or vice-versa, the size can be and often will be different. If you change brands or even models from the same brand, the propeller size will most likely be different.

Many owners (and dealers) mention size without noting brand and model of propeller. Given the above, noting a suggested propeller size without the make and model is incomplete information.

For these reasons, many Boston Whaler boat owners have tested specific boat models with a specific engines and determined which make,model and size propeller works the best overall. Also engine height is also reviewed and evaluated.

I have captured dozen of these recommendations over the years.

With that said, owners of a classic 17-foot hull like a Standard, Montauk, or Newport, with a Suzuki DF90A engine tested propellers. Their recommendations include the 14 x1 8 Suzuki SS or a 14 x 22 Stiletto Advantage I, as noted above. With these propellers the boat should reach 43 to 45-MPH running light and solo.
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Tony
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby Tony » Wed Dec 30, 2020 6:07 pm

PHIL--I will follow your good advice

PHIL--your collected propeller data for my rig is especially good to know.

I am sure I will be able to [select] a new propeller [that will produce some unspecified benefit].

jimh
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Re: 17 SSL Suzuki 90-HP Bow Oscillation

Postby jimh » Fri Jan 01, 2021 9:15 am

What might occur to boat performance if a four-bladed propeller is used to replace a three-bladed propeller is not particularly easy to reduce to a simple paradigm that boat speed will decrease but boat acceleration will increase. There are many factors involved in a propeller that determine how it will perform, and number of blades is just one factor. The influence of the number of blades cannot be isolated to always have a particular effect on performance.

More about bow oscillation in this article:

Bow Oscillation
viewtopic.php?f=7&t=6051