1990 90HP Pontoon, 4300 RPM at WOT, Do I need a new prop?

[goodmariner85]

OMC says rpm at WOT should be 4500 -- 5500. I get 4300 now. Should I try a new prop?

Evinrude 90HP V4 model: E90TLESB
Compression: 122 +- 2 psi on all four cylinders

389948 Evinrude Johnson Stainless Steel Prop (13-3/8 x 17) Thru-Hub Exhaust. Has a small ding on one blade.

24 foot 1994 Crest II

WOT: 4300 rpm, 22 mph, lightly loaded. Maybe 4100-4300 rpm and 19-20 mph pulling one or two 70 pound kids on a doughnut shaped tube. I have not yet tried systematically adjusting trim to maximize speed. But I have not seen much difference when moving trim in the past. I think 22 mph is about the minimum speed to keep the 7-8 year old kids from getting bored.

Uses: puttering around / transportation on a small lake, and pulling kids on tubes.

Not sure what "hole shot" means. But it seems to accelerate pretty fast from neutral to WOT. Don't know about planing either, but the boat does lift up and flatten somehow when near max speed.

Often has problems ventilating (or cavitating) especially in sharp turns at speed and choppy water. (I see this is pretty common)

EDIT: The tach on the dash is inaccurate. I use a spark plug tach that I checked with an optical tach (with reflective tape on flywheel) at 500 to 1500 rpm at dock. Spark plug tach seems to behave well all the way up to 4300 (and higher when ventilating).
The dash tach says 1200 or more when spark plug tach says 500. Dash tach is always high. When spark plug tach is 4300, dash tach is 6200 or so. I'm wondering if there is an rpm limiter that is reading the incorrect rpms off the dash tach. I doubt it, but worth mentioning.

Also I checked the position of the pins (they are vertical) on the axle of the throttle valve at WOT.

 

[Chris1956]

A 15" prop would, in theory, give you 400 more RPM which gets you into the max RPM range, which is good. Will that give you higher speed, who knows? Pontoon boats are not built for speed.

There are some pontoons with really high HP and go 60MPH, but those are specialty and stupid (IMO) items.

So prop selection kinda depends on what you want.

The lower pitch prop gives better acceleration than the higher pitch prop. If you are pulling tubes, the lower pitch may be better, I don't know. Don't overrev, with the lower pitch prop.

Pontoon boats do not really have a hole shot, as they do not really plane out. Pontoon boats are a different animal. They are great for parties, but bad for rough water. and usually do not have good performance.

 

[jimmbo]

You will probably have to drop 4 inches to get close to where the Engine makes its best power.
A bigger motor is what your boat really needs

 

[goodmariner85]

> Pontoon boats do not really have a hole shot, as they do not really plane out.

Yeah it's always got a lot of big pontoon surface making contact. And all the water jetting between the transom and toons at WOT does not look efficient at all.

I wonder if it is hard on the motor, or is inefficient to run the motor below recommended rpms at WOT.

It would be nice to get the best tubing possible with the boat. But I realize you can't ask to cover too many bases with just one boat. It already does ok for carrying 10 people around and also letting the kids have some fun. (As long as the coils aren't broken, and the carbs aren't warped, and the fuel hoses aren't torn off, etc. I think I finally got most of that taken care of.)

 

[goodmariner85]

Is it correct to think that it's lugging now? (Assuming it's not a problem with the motor itself.) Like riding a bicycle up a hill in a gear that's too high. You should shift to a lower gear and increase the rpms

 

[Chris1956]

On a totally unrelated subject. I live on the Barnegat Bay. The good news is that I have the entire Atlantic ocean to work with. We have salt water and big fish, clams, oysters, rough water, tides and the like. Of course the salt takes a toll on the machinery. However, I prefer living on the Bay and Ocean.


However, it would be very cool to live on a large lake. I would have a pontoon boat and a speed/ski boat. Motor out on the pontoon boat in the AM, run the ski boat (hopefully a good one), and ski the kids into relaxing.

 

[jimmbo]

I wonder if it is hard on the motor, or is inefficient to run the motor below recommended rpms at WOT.

Very hard, Engine is strained, and Melted Pistons can occur

 

[goodmariner85]

> Very hard, Engine is strained, and Melted Pistons can occur

I better make this a priority. I bought the boat a couple of months ago. No idea why it has this prop.

> However, it would be very cool to live on a large lake

I'm kind of envious of people who can take long trips and adventure. I'm on a little lake. If the motor died in the middle, I could easily swim home. A lot of family on the lake, so we use it for transportation as well.

 

[goodmariner85]

I need at a minimum to choose diameter and number of blades. It looks like aluminum 3 blades and 14 x 13 is popular for similar boats and HP. And this (as stated above) should get me in the higher end of the manufacturers recommended range.

Does that sound reasonable?

I did read a report from one person (can't find it now) that did something like 16 x 12 or 16 x 13 on a similar boat. Justified the diameter by saying it would prevent ventilating problem that he had.

On the other hand. Going to a prop shop might be worth the premium if it saves me time finding the right prop.

 

[jimmbo]

Larger Diameters are more Prone to Ventilation as the Blade Tips are higher up.
Mercury's Prop Selector was saying 12 or 11 inch pitch, but it is trying to get the Rpms above 5000
A Mercury Black Max and hub Kit to put it on your Evinrude would be okay.

 

[racerone]

Get the revs up to 5300 RPM or risk breaking piston rings.

 

[brodmann]

Switching to an aluminum prop, even with the same pitch, would get your RPM's up and - in my experience - would actually get you a little increas in speed. I'm guessing a 17 pitch same size aluminum prop would get you to about 4600 RPM's and probably get that tube going 22 MPH. It's amazing to hold a stainless steel prop in one hand and the same size aluminum prop in the other. Just glanced at a website on propellers that said aluminum props average 1/3 the weight of the same sized stainless prop. So, lighter weight to spin faster, but same pitch so you don't sacrifice MPH's. I'd try to find and borrow a same sized aluminum prop and see how it works. If you step down to 15 pitch in aluminum, you'll be well into the middle of that WOT range, but I think you'll lose a couple of MPH's. Let us know how it goes. Also, if the lake has logs, stumps and other hazards in it, I would always take the stainless prop as a backup.

 

[ct1762@gmail.com]

a stainless prop is better in almost every single way than an aluminum! a few extra pounds in weight is far far outweighed when you realize how much more efficient the thin, rigid blades of a stainless are. they accelerate harder, cruise faster, and are faster on top end even on pontoons. if his motor is only turning 4300rpm, why would you suggest the same pitch but in aluminum? he clearly needs a 13'' wheel. the extra rpm from an aluminum has nothing to do with weight, but blade flex and thickness, wasting efficiency for the reasons i mentioned above.

 

[goodmariner85]

I used the solas tool, it was saying 11 to 13 inches pitch depending on my guesses for weight and width of pontoons. I don't know what rpms it was targeting. I don't trust the tool enough to buy one thinking it will work well.

> Switching to an aluminum prop, even with the same pitch, would get your RPM's up

I didn't think of that. I do read that changing material and manufacturer can give different performance for the same pitch.

One thing that's becoming clear is that trying props before buying is pretty important. 3 or 4 blades? Al or SS ? 11 inch or 15 inch pitch? Target 5200 rpm or 5800 rpm? But I don't really have many options. There used to be a well regarded prop shop really close, but the owner retired. I'll call boat shops tomorrow.

One person on the phone at a boat shop today said I needed a 15 inch prop. But he didn't ask for my current RPMs at WOT (!) He just thought I shouldn't change by more than 2 inches. Not super encouraging... Maybe he knows more than I do.

 

[jimmbo]

I have rarely seen any real rpm Differences between an AL and a SS Prop, provided they have the same Blade Shape, Area, Contour, Cupping and Rake.

I think the 13 would get you to about 4800- 4900 rpm, an 11 would put you 5200 - 5400 range
The 90 makes its best power, slightly below 5000 rpm

The amount of Flex on an AL Prop, is less than 1/100", if any

 

[ct1762@gmail.com]

I have rarely seen any real rpm Differences between an AL and a SS Prop, provided they have the same Blade Shape, Area, Contour, Cupping and Rake.

I think the 13 would get you to about 4800- 4900 rpm, an 11 would put you 5200 - 5400 range
The 90 makes its best power, slightly below 5000 rpm

The amount of Flex on an AL Prop, is less than 1/100", if any

on this type of boat sure. but yes flex is minimal, its really the thin blades that makes a noticeable difference. obviously the faster the boat the more this low drag effect gets magnified. id say a 15 is still too much, especially when the boat is loaded down. im just paranoid of not lugging a v4 crossflow though.

 

[jimmbo]

on this type of boat sure. but yes flex is minimal, its really the thin blades that makes a noticeable difference. obviously the faster the boat the more this low drag effect gets magnified. id say a 15 is still too much, especially when the boat is loaded down. im just paranoid of not lugging a v4 crossflow though.

We agree that a 15" is too much. A 13 would likely place the motor near its Peak Power, an 11 would raise the Rpms but the Motor will be putting out less than the rated hp

 

[flyingscott]

Switching to an aluminum prop, even with the same pitch, would get your RPM's up and - in my experience - would actually get you a little increas in speed. I'm guessing a 17 pitch same size aluminum prop would get you to about 4600 RPM's and probably get that tube going 22 MPH. It's amazing to hold a stainless steel prop in one hand and the same size aluminum prop in the other. Just glanced at a website on propellers that said aluminum props average 1/3 the weight of the same sized stainless prop. So, lighter weight to spin faster, but same pitch so you don't sacrifice MPH's. I'd try to find and borrow a same sized aluminum prop and see how it works. If you step down to 15 pitch in aluminum, you'll be well into the middle of that WOT range, but I think you'll lose a couple of MPH's. Let us know how it goes. Also, if the lake has logs, stumps and other hazards in it, I would always take the stainless prop as a backup.

Almost none of that statement has any basis in fact. Except the stainless is heavier than aluminum that is true. This just proves how little is known about props.
#1 stainless is better performing pretty much across the board
#2 dropping pitch does not necessarily mean losing speed because the smaller prop is spinning faster so moving more water. It is also running where the motor makes its full HP.

 

[goodmariner85]

> A Mercury Black Max and hub Kit to put it on your Evinrude

I can get an off brand after market Al prop made for the motor for about half the price. Is there an advantage to the Black Max? I suppose it depends on the characteristics of the aftermarket prop. Since the prop has a huge effect on the operation of the boat, I think the 100 dollars would be worth any advantage.

 

[tphoyt]

Do you by chance have a marina close by that will let try different props? My locals do but I’m also on a small island and good customer service word of mouth travels fast so they do their best to provide a good service.
Maybe worth ask around.