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

[goodmariner85]

I called a couple of boat shops asking for a prop shop, no good answers. They just sell props, no returns of course. I called a couple of prop shops, but they are out of business. Asked several people who have boats, but they don't know where to go. My nephew has the stock prop on a shelf for an OMC 150 two stroke on a fast boat. He can grab it next time he goes out to his barn. But I bet the pitch is way too high. I'll look into a marina.

 

[flyingscott]

why do you think a 150 prop will fit that motor? Stop making this so hard on yourself. you don't need to buy a Mercury or any of the other brand name props. Solas props are relatively cheap and are pretty good props. You are not running a 100 MPH boat here. Call Michigan wheel or Solas to get their recomendation.

 

[havoc_squad]

why do you think a 150 prop will fit that motor? Stop making this so hard on yourself. you don't need to buy a Mercury or any of the other brand name props. Solas props are relatively cheap and are pretty good props. You are not running a 100 MPH boat here. Call Michigan wheel or Solas to get their recomendation.

+1 to this.

13 in diameter with 13 pitch is probably exactly what will be the overall recommendation. Boats less than 140 HP doesn't get much benefit from stainless steel prop, unless the OP wants to spend more to get stainless and not aluminum or composite.

When boats get into the higher HP engines, aluminum deflects too much or will outright fail and stainless becomes the only option.

 

[jimmbo]

When boats get into the higher HP engines, aluminum deflects too much or will outright fail and stainless becomes the only option.

I call BS on the "Aluminum deflects too much" If there is any Flex, and that's a big if, it is less than 1/100". That bit of Marketing Misinformation has sold millions of SS props

 

[flyingscott]

Yeah there are a lot of high hp motors running with aluminum props out there.

 

[goodmariner85]

But I said "But I bet the pitch is way too high" for the 150 HP prop. I meant that I've called several shops, spoken to several people, and the only loaner prop I can get my hands on is from a rocket boat with a 150 HP motor, which is not going to help.

Two boat shops did make a recommendation though. It was the same as I find here: 13 inch pitch. One service tech said "maybe 15, but I'd try 13 first." (Another boat shop said 15 inches, but like I said he didn't bother to find out how the boat/motor was running)

> Call Michigan wheel or Solas

Calling the manufacturer directly is not a bad idea.

The reason I'm making it hard is that I don't want to buy two or three props to find one that is close the my target RPMs.

 

[jimmbo]

But I said "But I bet the pitch is way too high" for the 150 HP prop. I meant that I've called several shops, spoken to several people, and the only loaner prop I can get my hands on is from a rocket boat with a 150 HP motor, which is not going to help.

Two boat shops did make a recommendation though. It was the same as I find here: 13 inch pitch. One service tech said "maybe 15, but I'd try 13 first." (Another boat shop said 15 inches, but like I said he didn't bother to find out how the boat/motor was running)

> Call Michigan wheel or Solas

Calling the manufacturer directly is not a bad idea.

A prop for a 150 is never going to fit on a 90hp. The Diameter is too great, and the Hub is matched to the V6 Gearcase, which is Larger than the one on the 90

 

[tphoyt]

I have heard that Piranha Props can send you different pitched blades for you to try out and you only pay for the ones you keep.
Personally I can’t wrap my head around a plastic prop but they are supposedly tough and if one blade should break and get damaged you only need to replace that blade.
I don’t know just a thought to add to your problems.

 

[goodmariner85]

I'm waiting on delivery of a 13 3/4 x 13 prop. Thanks for the advice.

 

[brodmann]

Guys, you need to read up a little on props. Alluminum props weigh about 1/3 as much as stainless. They spin faster because they're lighter. They flex under hard acceleration. Here's just a couple of prop experts:

I had a friend tell me that an aluminum prop is actually slightly faster than a stainless prop but I wouldn’t know why

I don’t know about speed but RPM’s for sure. Aluminum has more flex so at higher RMP’s it doesn’t cut as much water, less resistance equals higher RMP’s. Stainless has no flex so there is more water hitting the surface which in turn puts more lugging on the motor there for less RPM’s. Most people that switch from Aluminum to SS in the same pitch on the same boat drop top RPM’s by 200-400. I went from pulling 6000+ with Alum to 5600-5700 in SS.

Aluminum Prop vs Stainless Steel Prop​

Take a look at the table to compare the two types of props –

Factors	Aluminum Prop	Stainless Steel prop
Cost	Inexpensive	Expensive
Flexibility	Flex under power	Less flex under power
Material	light	heavy
Damage	Low damage when hit	Can affect the engine when hit
Speed	Lower Top End Speed	Higher Top End Speed
RPM	Higher RPM	Lower RPM

 

[jimmbo]

Anyone can make a Chart...

 

[flyingscott]

Guys, you need to read up a little on props. Alluminum props weigh about 1/3 as much as stainless. They spin faster because they're lighter. They flex under hard acceleration. Here's just a couple of prop experts:



I don’t know about speed but RPM’s for sure. Aluminum has more flex so at higher RMP’s it doesn’t cut as much water, less resistance equals higher RMP’s. Stainless has no flex so there is more water hitting the surface which in turn puts more lugging on the motor there for less RPM’s. Most people that switch from Aluminum to SS in the same pitch on the same boat drop top RPM’s by 200-400. I went from pulling 6000+ with Alum to 5600-5700 in SS.

Aluminum Prop vs Stainless Steel Prop​

Take a look at the table to compare the two types of props –

Factors	Aluminum Prop	Stainless Steel prop
Cost	Inexpensive	Expensive
Flexibility	Flex under power	Less flex under power
Material	light	heavy
Damage	Low damage when hit	Can affect the engine when hit
Speed	Lower Top End Speed	Higher Top End Speed
RPM	Higher RPM	Lower RPM

You really have no clue do you. Tell me how many lower units or engines have been destroyed by rock strikes on SS props.

 

[jimmbo]

I had to replace the Prop Shaft on my VP I/O after a hitting a Rock Ridge at 23mph with a RAKER on it. Blades on the Prop were torn/bent too. Skeg managed to survive, so I likely caught the side of the Ridge, but there were several Impacts

 

[flyingscott]

I had to replace the Prop Shaft on my VP I/O after a hitting a Rock Ridge at 23mph with a RAKER on it. Blades on the Prop were torn/bent too. Skeg managed to survive, so I likely caught the side of the Ridge, but there were several Impacts

I have bent shafts with aluminum props.

 

[jimmbo]

I have bent shafts with aluminum props.

More likely to with SS as AL ones tend to sacrifice themselves, while SS tend to bend and keep hammering.

 

[goodmariner85]

The new prop ordered online arrived: A 100 dollar Al 13 3/4 x 13 prop. I get 4800 RPM and 23 MPH at WOT (hits 24 MPH very occasionally). Same both directions, so I think there is no wind. With the SS 17 inch prop I got 4300 RPM and 22 MPH.

I tried to find more info on exactly which RPM gives max power, but could not find it. I'm wondering if an 11 inch prop would be better. As long as it doesn't damage the motor, I doubt I could tell much difference.

It seems like it takes a bit longer to reach max RPMs than before. Going from 4600 to 4800 takes a while. I think the biggest number I saw was 4810 or so. But usually 4800 exactly.

I chose the one prop I found that fell under the standard free return policy on amazon. Not sure if they would really take it back. It's been run just for a few minutes in fresh water.

> Blades on the Prop were torn/bent too

ouch. My neighbor just told me that pretty much the only invisible hazard in the lake is a wooden post a good 20 or 30 feet off shore. He broke his skeg off. When I bought my boat, it already had the skeg sheared off. So I guess I'm ok.

 

[brodmann]

Fantastic! Increase in RPM's and MPH's! I bet the kiddos love that! Can they "feel" the increase in speed? I'm surprised that nobody has marked that hazard in the lake! I live on the coast in Savannah Georgia. When we find a hazard, we call DNR and report it. Sometimes, we'll set up a temporary marker and report it to DNR or Coast Guard.

 

[flyingscott]

Yes an 11" would be better. You want to be over 5000 for sure. And as close to 5500 as you can get.

 

[goodmariner85]

My kid really thought it was a wilder ride. It was pretty obvious to me too. I trimmed the prop down a lot, too, so it was easier to stop ventilating (or whatever makes the prop lose bite) The speed was 23, sometimes 24. Hard to say what the max speed is because the gps speed has some flukes. The RPMs were mostly lower than 4800. I'll have to record it more carefully next time. Since I'm turning hard, the rpms and speed are changing a lot.

 

[goodmariner85]

I installed a new 100 dollar 14 x 11 prop. I get
* 14 x 11 prop, 5030 rpm, 21-22 mph at WOT
* 13.75 x 13 prop 4800 rpm, 22-23 mph at WOT

I accidentally had my gps on a very low sampling rate for the 14x11 prop. I'll have to test it again later. I'd trust all those numbers better after averaging for a week or so

I also got today
* 14 x 11 prop, a bit less than WOT, 4650 rpm, 20 mph. At WOT, I usually saw 21 mph.

There's a bit of fuel in the water and a lot of white smoke, especially at low rpms and when colder. I may look into this stuff this winter.
Also the motor made a strange beep or ring noise at maybe 1500 rpm. It was not very loud. No sound when idling or at higher rpms. It was coming from the motor or shaft or prop, not the warning horn on the remote control.