Engine Induced Boat Vibration

Optimizing the performance of Boston Whaler boats
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Joined: Sat Sep 16, 2023 7:38 pm

Engine Induced Boat Vibration

Postby ericjacobson » Sat Sep 16, 2023 7:46 pm

My 2017 170 MONTAUK with Mercury 90-HP FOURSTROKE with a VENGEANCE 13.225 x 16-pitch propeller has a whole-boat vibration at low speeds, but there is no vibration at engine-idle speed or at on-plane speed.

If the cause of the vibration was due to the propeller, I would love try a better propeller,

Give me suggestions for an alternate propeller to replace the VENGEANCE 13.225 x 16-pitch on my 2017 170 MONTAUK.

We use boat for general use on freshwater inland lake. Mostly skiing, tubing, tooling around, some fishing.


Q1: Is the propeller shown in Figures 1 and 2 below the cause of the whole-boat vibration?

Fig. 1, Propeller Blade Damage
propellerBladeDamage1.jpg (65.14 KiB) Viewed 165 times

Fig. 2. Propeller Blade Damage
propellerBladeDamage2.jpg (87.04 KiB) Viewed 165 times


Q2: is this whole-boat vibration due to the propeller size being incorrect?

BACKSTORY In July 2022 I bought the boat from original owner. At that time I did not observe any propeller damage or notice any vibration, but I may have just overlooked both due to my excitement.

In the summer of 2022 I may have damaged the propeller by going over a log. The vibration was noticed all of 2023 Summer boating.

Assuming the propeller is causing the vibration, I was planning to replace the propeller when I winterize.

Q3: should I replace the current propeller with the same brand, model, size, and pitch?

Q4: should I try a different propeller?

To me it seems lots of people use 19 pitch for a 2017 170 MONTAUK with 90-HP FOURSTROKE.

I am confused whether my Mercury engine has the command thrust option. The engine gear ratio is [thought to be] 2.33:1 and not 2.08:1. The Original invoice says "L4NA 4 cyl. Model# is 1F904131D. Manufactured 2016."

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Re: Engine Induced Boat Vibration

Postby jimh » Sun Sep 17, 2023 9:43 am

Your Mercury VENGEANCE stainless steel propeller has CERTAINLY had a significant blade strike, and the notion that it was a "log" is perhaps not quite correct. Generally a stainless steel propeller will survive a blade strike with old water-logged wood. The wounds on this propeller suggest something stronger was hit by the blade. Also, if you overlooked this when you bought the boat, you must have been very excited at the purchase.

Also, you should be checking the hull bottom very carefully. If the "log" could deform stainless steel, it could also deform the bottom of a Unibond hull, at least chipping some gel coat.

ericjacobson wrote:Q1: Is the propeller shown in Figures 1 and 2 below the cause of the whole-boat vibration?
The deformation of the blade is sufficient to cause vibration. The tendency for engine vibration to excite the whole boat into vibration will be very frequency dependent. The "frequency" of the vibration is related to speed of rotation of the propeller shaft. The speed of rotation of the propeller shaft is related to engine speed.

It is very common that a particular speed on a particular boat with a particular engine and particular propeller, the propeller vibration--even from an undamaged propeller--can resonate with the boat hull or fittings on the boat such as long length of railing. These vibrations arise because the nature of an outboard engine propeller.

On an outboard engine the propeller operates in an aperture in the outboard engine gear case. As a propeller blade passes behind the gear case or the skeg, the blade is no longer operating with clear water flowing into the blade. This tends to change the loading on the engine, which in turn introduces small instantaneous changes in engine speed--which are perceived as vibrations. The most significant factors in determining the vibration frequency is the propeller rotation speed AND the number of blades on the propeller.

With a classic three-bladed propeller, each revolution of the propeller shaft results in SIX occasions where a blade passes behind the gear case or skeg, but in each case only one blade at a time is shadowed.

With a four-bladed propeller, each revolution of the propeller shaft results in FOUR occasions where a blade passes behind the gear case case or skeg, but in each case TWO blades at a time are showed. This causes a lower frequency vibration (than occurs with the three-bladed propeller), but the amplitude of the vibration may be greater due to two blades shadowed at once.

Whether the boat will more likely to less likely to resonate with a three-bladed or four-bladed propeller depends on the resonance of the boat structure and the items attached to it. How the boat reacts to the engine vibration is also affected by the engine mounting method. Some engines use relatively softer engine mount couplings in order to reduce the transfer of engine vibration to the hull in the hope of dampening engine vibration transmission to the hull. With higher horsepower, generally the coupling has to become stiffer to keep the engine from wobbling too much.

Of course, a damaged propeller with one blade deformed, will ALWAYS tend to produce vibration due to the uneven blade loading. Also, extended operation of the engine with a damaged propeller will tend to damage the propeller shaft bushings and cause more damage to the gear case. Extended operation with a badly damaged propeller is NOT advised.

Another factor in a propeller blade strike is the state of the bushing that connects the propeller to the propeller shaft. Typically a modern propeller like a VENGEANCE will have a plastic coupling that forms the connection. These couplings are designed to deform when a propeller strike occurs to avoid damage to the gear case. The base-model coupling for a Mercury propeller is their FLO-TORQ II coupler. This coupler is not particular known for producing vibration free outcomes. A typical problem with a FLO-TORQ II coupler is propeller rattle.

My comprehensive advice to you:

FIrst step: inspect the FLO-TORQ II coupling.

NOTE: this will require you have the proper tools to remove the propeller, particularly a wrench or ratchet socket that fits the retaining nut--which is alwasy an odd size that you will not already have in your tool kit. If you do not already have these tools, you should obtain them. Being able to remove or install a propeller SHOULD be within your ability and skill.

You should also be able to re-install the propeller. They may require that certain parts NOT be reused, so you will need those parts on hand for installation. The non-reusable parts are typically parts that are bent after installation to prevent the retaining nut from unthreading--which it will be quite happy to do when the propeller is making reverse propulsion and the nut retainer breaks off due to being bent too many times.

If the FLO-TORQ II coupler is deformed, it may be contributing to propeller rattle. Buy a complete new FLO-TORQ replacement kit, which will include new hardware and needed parts. The cost for the whole kit is only marginally more than buying just the coupler plastic part.

Second step: take the damaged propeller to a reliable propeller shop for assessment. A good shop may be able to repair the propeller for only a fraction of the cost of a new propeller. Even if you decide to buy a new propeller as a replacement, repair of the old propeller will give you a spare propeller. At some point, the cost of repair compared to the cost of buying a used propeller in excellent condition may approach each other. You can decide what to do at that point.

Third step: if the cost of repair is too high, look for a used propeller of similar dimension that is in excellent condition. You can often buy used propellers in really good condition for about half the price (or less) of a new propeller.

Fourth Step: contact Ken at PROPGODS for advice on any new propeller. Ken may also advise you to use a different hub adaptor to replace the notorious FLO-TORQ II hub adapter. For example, Ken suggested to me that I discard the FLO-TORQ II hub adaptor and replace it with a SOLAS RBX-style adapter. This was the best $60 I ever spent on my Mercury propeller.

Keep in mind that Boston Whaler had access to any Mercury propeller in their extensive line of propeller, so if choosing the VENGEANCE (which here I assume was the OEM propeller) then Boston Whaler must have thought it was a very good fit with the boat and engine.

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Joined: Sat Sep 16, 2023 7:38 pm

Re: Engine Induced Boat Vibration

Postby ericjacobson » Mon Sep 18, 2023 7:38 pm

Thanks for the thorough reply, jimh. I will reach out to Ken.

Posts: 11350
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

Re: Engine Induced Boat Vibration

Postby jimh » Wed Sep 20, 2023 9:20 am

For more information about the frequencies produced by propeller blade shadowing, see this pinned article at the top of the PERFORMANCE forum:

Calculating Frequency of Propeller Blade Vibration
https://continuouswave.com/forum/viewto ... 925#p45925

Posts: 11350
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

Re: Engine Induced Boat Vibration

Postby jimh » Wed Sep 20, 2023 9:33 am

Regarding using a replacement for the FLO-TORQ II hub, Mercury has alternative models of their FLO-TORQ hub that were developed specifically to reduce propeller "rattle" and reduce vibration. There is an excellent article on these improved FLO-TORQ hub option at

Flo-Torq Adapters

There are also several other discussions of the newer FLO-TORQ hubs:


This next linked archived discussion is particularly informative;
read the comments from Tom Clark regarding his experiment in
trying to reduce wobble that was not successful, but revealed
the true design of the FLO-TORQ III and IV adaptors:

Flo-Torq IV Hub Decreases Loud Clunk, But Has Wobble

The post I refer to begins: "The design of the Flo-Torq III and Flo-Torq IV hub kits has confounded me for a while..."

Based on these first-hand accounts, I think you should invest in a FLO-TORQ III adaptor for your engine.

Or, you could try the SOLAS RBX series rubber hub insert, which should also be helpful in reducing propeller rattle. For the RBX hub, you will need to consult the SOLAS application chart to get the correct adaptor for your Mercury engine.