1970 16-footer with 90-HP

Optimizing the performance of Boston Whaler boats
Chris70
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1970 16-footer with 90-HP

Postby Chris70 » Sat Feb 09, 2019 9:19 pm

Last year I re-powered my [modified 1970 NAUSET 16] with a 2017 Mercury 90 FOURSTROKE with standard gear case and 2.07:1 gear ratio.

This modified 16-footer lacks all the original furniture. It has:

--a small fiberglass console
-- a flip cooler bench seat
--12 gallon fuel tank mounted in the stern
--a single battery mounted under the console

[The boat weight] is thus relatively light.

I have two propellers:

The initial propeller is a Turning Point 13.75 x 15 Stainless steel propeller which gives great performance. Boat speed is 40.5-MPH at engine speed 6000-RPM; acceleration from a standing start is excellent. Cruise speed is excellent.

I love the boat with the Turning Point propeller. The boat even steers better at speed with it. My gripe with this propeller is due to its weight. The Turning Point propeller causes chatter, and makes a clunk at idle speed and shifting in and out of gear. These noises caused me to purchase another propeller.

I bought a Mercury 13.25 x 15-pitch aluminum propeller. This propeller remedied the idle speed chatters and shift clunks, but the performance suffered drastically, decreasing to a boat speed of 36-MPH at engine speed of 6000-RPM , and the acceleration was poor. The Mercury aluminum propeller tends to [ventilate] if I really punch the throttle hard.

I’m not very concerned with the loss of top speed.

I’ve tried multiple different hubs with the stainless steel propeller to remedy the noise, but with no luck. I have no [objection] sticking with an aluminum propeller to avoid all the noise.

Should I buy a 13.25 x 17 ?

Should I try a four-blade aluminum propeller?

This is my first post and I hope I’ve posted it in the correct areas.

jimh
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Re: 1970 16-footer with 90-HO

Postby jimh » Sun Feb 10, 2019 12:32 pm

Please give more information about what propeller hub kits you have tried. I would assume you are using FLO-TORQ hub kits.

Mercury has several FLO-TORQ hub kits which are specifically designed to reduce the CLUNK that occurs when shifting in and out of gear. But a side effect of reduced CLUNK is that these FLO-TORQ hubs introduce some rattle in the propeller at low speeds.

At the 90-HP power level an aluminum propeller should still be able to deliver decent performance. As you have observed, the lighter weight of an aluminum propeller tends to reduce or eliminate the CLUNK when shifting.

Propeller rattle often indicates the propeller is not being pressed against the thrust bearing. This can occur if there is a mismatch in the hub kit and the engine. I suggest you check carefully to see if there is any play of the propeller forward and aft on the propeller shaft; you can do this with the boat out of the water and the engine not running. Just push and pull on the propeller on the propeller shaft to see if there is any slop.

If you have a standard hub kit, that is, one NOT intended to reduce clunk, there should not be any slop or play in the propeller on the shaft. Also, you should check the fit after the boat has been operated at full throttle with the propeller. The force of the propeller thrust often pushes it harder onto the trust bearing, producing a loose fit from what was initially a solid fit. Use a torque wrench and set the retaining nut to the manufacturer's specified torque.

jimh
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Re: 1970 16-footer with 90-HP

Postby jimh » Sun Feb 10, 2019 1:10 pm

For some further discussion of Mercury FLO-TORQ hub kits, see my post at

viewtopic.php?f=7&t=4122&p=23363#p23363

I give some pointers to excellent illustrations and prior discussions in the post linked above. The information provided therein may be useful for you if working with FLO-TORQ hub kist.

Chris70
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Re: 1970 16-footer with 90-HP

Postby Chris70 » Sun Feb 10, 2019 6:47 pm

Thank you very much for the reply and I appreciate that you cleared up my original post. I’ll be mindful of the layout when posting again.
The hub kits I have tried are as follows:
-Original Turning Point hub (Apearred to be a Flo-Torque 2 copy) this hub was included with Propeller.
- Mercury Flo-Torque 2
-Mercury Flo- Torque 3
Prop chatter was a problem using all 3 hubs with Stainless propeller though it was slightly less using the Flo-Torque 3. The problem occurs at idle speed in both forward and reverse. If I throttle up slightly from idle speed the chatter subsides.

The propeller had the most fore and aft movement with Flo-Torque 3 hub installed. Even with the other 2 Hubs installed, there is movement fore and aft though it is minimal. It is difficult to differentiate because the propeller shaft itself has a slight amount of fore and aft movement itself.

I’m certain that I have installed these hubs correctly and torqued them according to the manufacturer’s spec in the hub’s included instructions. After much research on the topic I’m almost lead to believe this is a very common and almost never resolved problem when using heavy stainless props on fourstroke outboards in the lower horsepower range.

My next move in choosing an appropriate propeller for my needs is possibly choosing a better Aluminum alternative than what I have currently or perhaps A Mercury brand Stainless propeller will work better when using the Mercury brand hubs.

jimh
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Re: 1970 16-footer with 90-HP

Postby jimh » Mon Feb 11, 2019 4:34 pm

Thanks for the further information on the hub kits tested.

One difference in the FLO-TORQ III and IV is the orientation of where the flexing portion of the plastic insert is located. On one of them, it's at the propeller shaft far end; on the other it's at the near end. This might affect rattle. I'd consult with someone with extensive experience with FLO-TORQ III and IV and seek advice.

I have tested a few aluminum propellers on my own boat. One that worked well was the Mercury four-blade aluminum propeller. I cannot recall the model name in use at the time, perhaps it was in their BlackMax line. The propeller gave good performance on my boat, until the engine was throttled up to higher power. Of course, this was with a 225-HP engine. When the propeller was running at lower throttle settings, more like 100-HP, it seemed to work quite well. At full power, it lost performance, as expected, but that was at 225-HP. I would not anticipate that at 90-HP you would have the same problem.

I am not expert on your 90-HP engine and its propeller shaft diameter and gear case bullet diameter. I would expect that there would be an appropriate hub size and hub kit available for a four-blade aluminum propeller.

I don't know any specifics about your 2017 90 FOURSTROKE, but historically all Mercury gear cases tended to shift with a CLUNK, and especially so with a heavier steel propeller.

Chris70
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Re: 1970 16-footer with 90-HP

Postby Chris70 » Mon Feb 11, 2019 7:32 pm

Thanks once again for the replys and info.

I think I’m going to give an Aluminum Mercury 13x15 4 blade Black Max propeller a shot. I’m willing to sacrifice top end speed in order to keep the boat quiet at idle speed and maintain good acceleration.

I rarely ever run the engine to 6000 rpm. I like running it at between 4000-4500rpm which yeilds a 25-28mph speed depending on conditions and trim. The boat is used primarily for fishing. The prop noise at idle speed with the stainless prop bothers me too much to just deal with because when I’m fishing I’m often maneuvering the boat at idle speed in and around certain areas for extended time in efforts to find fish in said area.

I neglected to mention previously that the recommended rpm range for the motor at WOT for my engine is 5-6000krpm.The engine will swing the Stainless prop to 6000rpm just as easily as it will the Aluminum prop.The Stainless prop has IMO a lot of cup on the blades which I believe is why the the prop accelerates so hard without ventilating. The Aluminum 3 blade prop on the other hand gives me the same sensation on acceleration as a car that has an enormous amount of wheel spin as it’s trying to accelerate. This is why I’m considering using the 4 blade Aluminum prop in place of the stainless.

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Phil T
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Re: 1970 16-footer with 90-HP

Postby Phil T » Tue Feb 12, 2019 8:44 am

Propeller sizing is standard but performance is not universal across brands and models of props due to blade design.
(e.g. a prop from Company x in a size of 13x16 will not necessarily perform the same as a prop in that same size/material from Company B.)

4 blade props have superior grip and are mostly installed on pontoon, heavy boats or those pulling a skier or tube that need strong acceleration. The added blade reduces top speed.

For most Boston Whaler hulls, a 3 blade stainless steel prop is recommended. You want to narrow you choice to Brand then model and then work on size. Members report good performance with the Vengeance prop with 16" of pitch on the classic 17' hull with the 2003+ Mercury 90 FourStroke motor (2.33 gear ratio). This is the hull design one generation after yours. I would start there.

Note you want the engine mounted 2 holes up.

Remember - The goal is not to go fast, it is to select the prop that allows your engine to perform correctly at all RPM's. At wide open throttle (WOT), the engine should accelerate to 56-5800 rpms. Whether you go fast or not is not the point.

If you exceed WOT rpms, you need to increase pitch in the same make and model of prop.. If your rpms are too low, you need less pitch. Each 1" change in pitch will affect WOT rpms by 150-200.
Member since 2003
1992 Outrage 17, 1992 Evinrude 115

Chris70
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Re: 1970 16-footer with 90-HP

Postby Chris70 » Tue Feb 12, 2019 6:32 pm

Thanks Phil for your input I appreciate it. I think you may have missed few key problems that I’m having that Were mentioned previously in the thread though.( not trying to be rude at all).

The main concern is that my 2017 Mercury Fourstroke with 2.07 standard gear case is not really agreeing with the stainless prop despite trying many different Flo Torque hub kits. I get really bad prop chatter/knock at idle speed .

The stainless prop I currently own is perfect performance wise in all aspects except for the noise I mentioned. I previously remedied the concern by using an Aluminum 3 blade propeller in the same size and pitch as the stainless but lost a ton in performance across the board. I previously mentioned I’m not concerned with top speed but I am concerned with holeshot and acceleration. This was my reasoning behind considering the 4 blade Aluminum prop.

The engine is currently mounted 1 hole up. The Mercury dealer that installed my engine mentioned that the new Mercury 20” shaft motors are actually a touch shorter than previous Mercury 20” motors and recommended leaving the motor mounted in the 1 hole up position. He stated that the previous Mercury 20” shafts were actually longer than 20”.

Lupi
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Re: 1970 16-footer with 90-HP

Postby Lupi » Wed Feb 13, 2019 4:56 am

Chris--I am not an expert in propeller and engines setup. We have in our family a 1966 16 Sakonnet, the same hull as your 16-footer. Our Sakonnet is currently equipped with a Yamaha 70 four-stroke and reaches about 38-MPH. We own it since 1966, and it has been equipped with:
  • 50 merc
  • 65 Jonhson
  • 60 evinrude
  • 50 Yamaha 4S
  • 70 Yamaha 4S.
In that range of power, top speed was around 35 to 38 MPH

With 90-HP I would expect a top speed just below 45 MPH.

I understand top speed is not the priority for you. From [the maximum boat speed] you are reporting with a 90-HP on this boat, I am a bit surprised.

jimh
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Re: 1970 16-footer with 90-HP

Postby jimh » Wed Feb 13, 2019 8:40 am

For many years an article in REFERENCE in the several articles about the classic 16-foot hull has given the anticipated boat speeds with various engines. See

http://continuouswave.com/whaler/refere ... power.html

That same articles also points to many prior discussions about boat speed with the 16-foot hull.

As mentioned already several times, the concern is not about obtaining a particular boat speed at maximum throttle. The problem being discussed in the thread is the CLUNK and RATTLE that is occurring when a stainless steel propeller is used on a modern Mercury outboard engine, a 2017 production year Mercury 90 FOURSTROKE.

Even with a FLO-TORQ III hub kit, the propeller CLUNK and RATTLE are not reduced sufficiently to meet the requirements of the boat operator.

Chris wrote:The prop noise at idle speed with the stainless prop bothers me too much to just deal with because when I’m fishing I’m often maneuvering the boat at idle speed in and around certain areas for extended time in efforts to find fish in said area.

Chris70
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Re: 1970 16-footer with 90-HP

Postby Chris70 » Wed Feb 13, 2019 6:08 pm

I’ve checked out the the threads with others combos with the classic hull and various motors and props. Most seem to be in the high 30’s low 40’s with a 90hp outboard. When using the stainless 13.75x15 prop my boat was able to hit 40.5mph.The acceleration was outstanding from all speeds and motor ran all the way up to the top of rpm range effortlessly. I could most likely use 17” or even a 19” prop and get close to or around 45mph out of the boat that’s not my goal.

Main goal is to eliminate the idle speed noises I mentioned previously and maintain great acceleration.

The boat is very quiet with no shift clunks or idle speed prop chatterwhen using my Mercury 13.25x15” Aluminum prop but the acceleration ( to my standard) is unsatisfactory.

I’m uncertain if cavitation or ventilation is the problem I am having on acceleration with the Aluminum prop but it feels as though the prop is “losing traction” upon acceleration even with the motor trimmed all the way down.

So long story short I’d like to find myself an Aluminum prop that performs better on acceleration than the on I have currently or figure out why the boat behaves in such a way with the Aluminum prop and not the stainless prop and resolve the concern.

rtk
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Re: 1970 16-footer with 90-HP

Postby rtk » Thu Feb 14, 2019 9:27 am

Chris70 wrote:I bought a Mercury 13.25 x 15-pitch aluminum propeller. This propeller remedied the idle speed chatters and shift clunks, but the performance suffered drastically, decreasing to a boat speed of 36-MPH at engine speed of 6000-RPM , and the acceleration was poor. The Mercury aluminum propeller tends to [ventilate] if I really punch the throttle hard.


You mentioned that the maximum RPM specification for this [2017 Mercury 90 FOURSTROKE] engine is 6,000.

When I was testing propellers for my 1997 21 Outrage with a Mercury 250-HP engine, I started out with a 17-pitch propeller. My tachometer read maximum engine RPM at full throttle so I assumed that the engine would run at higher RPM if the pitch of the propeller was not adequate. I also experienced a lot of propeller ventilation when accelerating.

I switched to a 19-pitch propeller, and there was no more propeller ventilation; the maximum RPM of the engine was about 100 less than the manufacturer's upper limit.

I believe that most modern outboards have a rev-limiter that will not allow the engine to operate at an rpm level that exceeds the manufacturer's specification. Based on your description, the engine was limiting RPM and performance was not poor because it is an aluminum propeller.

It would not hurt to test a larger aluminum propeller in my opinion. I have a 1966 16-footer with a Evinrude E-TEC 60. The 17-pitch Evinrude basic aluminum propeller has been very acceptable to me. I noticed no difference in performance when compared to the stainless steel propeller I tested.

Rich

jimh
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Re: 1970 16-footer with 90-HP

Postby jimh » Thu Feb 14, 2019 11:28 am

The effect of a rev-limiter can usually be detected because they tend to operate in an ON-OFF manner. That is, the rev-limiter circuit just cuts out the spark ignition if the engine speed exceeds the rev-limiter on-threshold. Loss of spark drops the engine speed. When engine speed falls below the rev-limiter off-threshold, the spark returns. If engine accelerates to the higher speed again, the rev-limiter cuts out the ignition, and the cycle repeats. This is sometimes described as the engine "barking" as the ignition cuts in and out. In well designed control systems the on-threshold and off-threshold are not the same. This prevents rapid oscillation of the control system state from on-to-off in a short cycle time.

For example, on my Evinrude E-TEC engine there is a rev-limiter function implemented so that the engine speed cannot exceed 1,500-RPM in neutral. If the throttle is advanced too far, the engine begins to bark, that is, to accelerate to 1,500-RPM, stop running momentarily, coast down below 1,500, resume operation, accelerate above 1,500-RPM, repeating fairly rapidly. This makes a barking sound.

For a rev-limiter to seamlessly and softly just stop the engine from accelerating past a certain engine speed would be a very interesting implementation of the rev-limiter function. People usually remark about the rev-limiting "cutting in" and holding down the engine speed by the rather brute-force method of just cutting off spark.

Also, it would be unusual for a rev-limiter to operate precisely at the recommended upper engine speed range limit. Usually the rev-limiter provides some headroom above the maximum engine speed allowed, so that it does not cut-in all the time when operating the engine near the maximum allowed speed.

Regarding propeller selection, the recommendation to try more propeller pitch with the aluminum propeller is probably a good idea. Aluminum propellers are prone to the blades flexing under load, so a 17-pitch propeller might, under full load of the engine, have some blade flex that would cause it to act like it had less pitch. Using a 19-pitch might give better results under load.

Chris70
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Re: 1970 16-footer with 90-HP

Postby Chris70 » Fri Feb 15, 2019 6:00 pm

Thanks everyone for the feedback. I’m going give another Aluminum prop a try when I launch the boat in April. I’ll most likely try a Mercury black max 13.25x17 and take it from there.