Opinions about WOT with a new 4 blade prop

[WillSDQ]

I have a 75 HP Mercury with a WOT range of 5000 to 5500 rpm's. I had a 13X18P SS prop on this engine. It is on a 16 foot Skeeter Starfire 80. The boat weighs 825 pounds and the motor weighs 275 pounds. I was having serious problems with kick out on turns. I needed to constantly trim the motor up and down while driving in bayous. I decided to try a Solas Amita 4 blade aluminum prop (12.5X21P). The idea was it was better to test a $110 prop versus $400 on a stainless steel prop and not like the results.

Bottom line the new prop has completely changed the performance of my boat. No more kick out on turns and it now handles like a Porsche. The hole shot is a little slower than that of the SS prop, but I can live with it. Top end is a little slower, but again, something I can live with. The boat now rides on the very back of the hull while the boat is almost level when driving. The problem is that WOT is now 4900 rpm's and that is with the motor trim all the way up. Should I be worried about the fact that its not with in recommended WOT by 100rpm's or am I good? And if I decide to purchase a SS 4 blade should I stay with a 21P or should I go with a 19P?

 

[jestor68]

Re: Opinions about WOT with a new 4 blade prop

What kind of rpm/speed were you getting with the old prop?

In order to evaluate prop performance, you need rpm and GPS (or some kind of accurate) speed.

You want to prop for at or near max rpm with a light load(one person) in order to maintain decent performance when you take another person along. Meaning it'd be better for all around performance to have the rpm nearer to 5500.

Sounds like you need a 19 pitch if you want to stick with a Solas 4 blade.

Right now, you're not getting full rated power(5250)out of the motor.

 

[WillSDQ]

Re: Opinions about WOT with a new 4 blade prop

The next 4 blade I buy will be a SS. So you're thinking 19P? The old prop, which by the way is SS, ran WOT at about 5600 to 5700 rpm's. I could not run it at that WOT. I don't like to run faster than 5200 rpm's as my motor is an older one and I don't want to stress it. SS 4 blades come in 13 inches and above while the Solas is 12.5 inches. Maybe a 13X19P SS 4 blade would be perfect. Or am I wrong?

 

[jestor68]

Re: Opinions about WOT with a new 4 blade prop

The small difference in dia wouldn't likely make much difference.

Since you were running a little high on rpm with the SS 18" 3 blade, it sounds like a 19" should work out pretty well in a SS 4 blade.

 

[WillSDQ]

Re: Opinions about WOT with a new 4 blade prop

Thanks Jestor

 

[steelespike]

Re: Opinions about WOT with a new 4 blade prop

When you prop a motor to run below its max rated rpm you force it to actually work harder.For best all around performance prop close to the 5500 then simply operate it at what ever rpm you prefer.
The motor will feel peppier,stay on plane easier at reduced throttle, hole shot will be quicker further lightening the load on the motor.
Normally going to a 4 blade in the same pitch will lower rpm by 100 to 200.
It would seem a 19" would lower the rpm another 200.
A 19" isn't wrong it just seems that it puts you too close to the minimum rpm.Not leaving much room for added weight.

 

[WillSDQ]

Re: Opinions about WOT with a new 4 blade prop

Then I need to stay with 18". Say a 13X18P 4 blade? With a 4 blade prop, how many rpm's do you gain with each decrease in pitch?

 

[steelespike]

Re: Opinions about WOT with a new 4 blade prop

When going from a 4 to a 4 you gain or lose about 100-200 rpm per inch. Not carved in stone.It depends on the design and brand.
Typically when going to a 4 blade it is suggested to drop 1" in pitch to maintain rpm.After re reading your present rpm and your preference for 5200 rpm A 19 would;d be closer to the 5200.
It would seem that if you go with a 18 it could put you at 5500 to 5400.In my opinion better for overall performance and just control the rpm with the throttle. You'll get out of the hole faster than the 19 and easier for the motor to respond to added weight.

 

[WillSDQ]

Re: Opinions about WOT with a new 4 blade prop

Thanks Spike. The only problem now is I can't find a 13X18P 4 blade. They only seem to come in odd numbered pitches in 4 blades

 

[WillSDQ]

Re: Opinions about WOT with a new 4 blade prop

I take that back. I found a 13.25x18p Stiletto here on iBoats for $400. Stiletto is the brand of my 3 blade. Think I'm gonna go with that.

 

[steelespike]

Re: Opinions about WOT with a new 4 blade prop

Sounds good.