Now that I've had the boat a few months, another prop question

[maverick974]

A few months ago, I got a 1995 Spectrum Pro Avenger. A 17 footer. Aluminum hull, dry weight of 940 lbs, a 1995 Force engine, 90 hp.
Right now it has a 13X19 aluminum prop on it. With 2 guys, half a tank of fuel and a little fishing gear, I'm getting just shy of 40 mph with the RPM's right at 5400.
The max RPM for the engine is 5250 and if I could, I'd love just a few more mph out of her. I'm guessing I need to go up in pitch an inch or so but there is also the question of prop style. I've been told a stainless prop won't improve things much on such a small engine and if that's true, then we can eliminate a SS prop. So that leaves aluminum or one of the composite props. I installed a comprop on my last bass boat a few years ago and to be honest, I was very pleased with it's performance until my brother snapped all four blades against the rocks because he didn't tilt the engine up. Not the props fault as it was supposed to break away to save the lower unit. I'm sure it would have killed any prop that was on there and no damage to the lower unit.
So I have two choices. A different aluminum or a composite.
Any suggestions on what material? Next is pitch? 20? 21?
If I find the right prop, what differences will it make and will I see any speed improvements? Is it even worth the effort for 1 or 2 mph?
Thanks.

 

[maverick974]

Re: Now that I've had the boat a few months, another prop question

I was calculating different prop pitches and I came up with only a 1 mph gain if I go from a 19 pitch to a 21 pitch prop.
Am I doing it right?
Calculating mph with a given prop.
RPM x Prop Pitch \ 1056 = Pitch speed in mph (Theoretical)
Spectrum is about 8% slip.
1995 Force 90 hp
2.3 lower unit gear ratio. (engine turns 2.3 times for every revolution of the prop)
13x19 aluminum prop.

5400 rpm with current prop means prop is spinning at 2348 rpm's. Theoretical speed is 42.2431028 mph.
Actual speed with 8% slip equals 38.8654576 mph.

With a 21 inch prop spinning at 5000 rpm, the prop RPM is 2174
Theoretical speed is 43.2297726
Actual speed with 8% slip equals 39.771390792 mph.

Do those numbers look about right? 2 inches of pitch equates to just over 1 mph? That just doesn't sound right. lol.

 

[steelespike]

Re: Now that I've had the boat a few months, another prop question

There are a number of prop calculators on the net just search prop calculator the one I use has a green page.
Using your numbers and 40 mph I get about 5% slip.an excellent possibly suspect number. Is your speed by gps?
Sometimes a speedometer isn't as accurate as a gps. Some cells have a gps app.

 

[maverick974]

Re: Now that I've had the boat a few months, another prop question

I have both a speedo and also a speedo on my fish finder that has one of those things that spin down near the transducer off the bottom of the transom. On the boat speedo, it reads 40 at WOT but the fishfinder speedo says 38. I do have a GPS in one of my cars that I'm going to take on the boat to get the exact speed and to see which speedo is more accurate.
My point was that if changing props only gets me one extra mph, is it worth the trouble and expense to change anything?
I guess I need a prop anyway as I have no spare prop so would a comprop be a good spare to keep on board?
If there was a prop that gave me 4 or 5 mph more speed, then I'd be happy to pay for a new prop but if I'm at the top of my performance with my given engine, then maybe all I'm looking for is a back-up prop.
Any thoughts?

 

[jestor68]

Re: Now that I've had the boat a few months, another prop question

Simply replacing your aluminum prop with a good SS prop of the SAME PITCH will sometimes render a slight boost in performance and pull the rpm down a little. Expect a 1-2 mph increase.

 

[mpsyamaha]

Re: Now that I've had the boat a few months, another prop question

I agree with jestor, you might should think about getting a stainless prop in 19p. Most ss props have more cup, which effectively adds a little pitch. Most setups lose a couple hundred rpms going from alum to ss in the same pitch.

 

[jestor68]

Re: Now that I've had the boat a few months, another prop question

I thought you sounded familiar.

Back in November, you said you got 5200 rpm with just you in the boat with the same prop.

Now you get 5400 with two people in the boat.

You using octane boost? :faint2:

 

[maverick974]

Re: Now that I've had the boat a few months, another prop question

I thought you sounded familiar.

Back in November, you said you got 5200 rpm with just you in the boat with the same prop.

Now you get 5400 with two people in the boat.

You using octane boost? :faint2:

Nope. Just paying more attention to the tachometer. It has 5 increments between each thousand RPM instead of 10 like one of my old boats. I was just reading it incorrectly as stupid as that sounds. It was an UNFAMILIAR boat. It's now getting more familiar.
As the heading reads..... Now that I've had the boat a few months, ANOTHER PROP QUESTION.
No, I'm not using octane boost but I am using proper grammar.
Is that OK with you or should I have made that a little clearer?

 

[jestor68]

Re: Now that I've had the boat a few months, another prop question

Nope. Just paying more attention to the tachometer. It has 5 increments between each thousand RPM instead of 10 like one of my old boats. I was just reading it incorrectly as stupid as that sounds. It was an UNFAMILIAR boat. It's now getting more familiar.
As the heading reads..... Now that I've had the boat a few months, ANOTHER PROP QUESTION.
No, I'm not using octane boost but I am using proper grammar.
Is that OK with you or should I have made that a little clearer?

:faint2::grumpy::facepalm::lol::sleeping: Symbolism; worth a thousand words. (Don't have to worry about grammar).

 

[maverick974]

Re: Now that I've had the boat a few months, another prop question

No worries. I didn't have my Wheaties last night. HA!