Props, so many options

[kenojim]

Got back in to boating after being away for a while and finding with props, like everything else, much has changed. .. Props come in single inch (and some have half inch) increments in pitch,.. PVS holes, progressive rake,. It gets to a point of information overload.
My mid 70s, 15+' Glastron has a new 75Hp 4 stroke outboard.
Currently it has a stainless Spitfire X7 prop, 15" pitch. I'm pretty happy with all around performance, speed and handing. My only concern, and
My question for the more experienced ones here is at WOT.. I'm at 38mph.. With motor flirting with Rev limiter at about 1/4 way trimmed up. Try to trim up anymore and I'll hit limiter.. (I think 5600 RPMs)
Question for experienced ones here is this motor pitched correctly?. Or a pitch to low? I know that "correctly" makes it a loaded question based on what the primary use of the boat would be for. And it's a small boat, any one significant change (pulling a skier or adding a person or 2 on boat makes a big difference) So I realize a one size fits all prop probably isn't realistic... I was just curious and open to others opinions.
Other performance on boat is
0-20 a hair under 3seconds.. 0-30 in 5 seconds.

 

[QBhoy]

Before you I even got as far as you mentioning the rpm limiter, I had a feeling it might be under propped with a 15”. These old glastrons like a fairly high mounted motor and with a good stainless prop too. See if you can borrow a few props to get out. Around the 17/18/19” size. Amazing how differently different props will behave and perform. Even if the same pitch.

 

[jimmbo]

When you trim up and the Rpms increase, does the boat speed also increase? You mention a Rev limiter, have you ever hit it? If not, what is the highest Rpm the engine attains with the 15" prop?

 

[ahicks]

I was thinking 15" might be small as well. You don't mention anything, so no idea where the priorities lie in the most frequent use of the boat? If you aren't frequently pulling a tube or skiing, a big hole shot isn't that important. That being the case, the motor is rated to turn 6k, but at those rpms, they can sound pretty busy. A much more comfortable target is something closer to 5500. Frequently you can even pick up a mile per hour or 2 at that rpm as well.

The 15" prop will likely work well for towing, but for everything else, my bet would be a 17" prop.

And as far as all the specialty props available, I suggest you stay with the basics, at least until you have a specialty need. Stainless look cool, but for a 75, alum works just fine....

 

[kenojim]

When you trim up and the Rpms increase, does the boat speed also increase? You mention a Rev limiter, have you ever hit it? If not, what is the highest Rpm the engine attains with the 15" prop?

Yes.. WOT trimmed up 1/4 of the way it flirts with limiter.. 38mph.. Trim up any more and engine sits on limiter..no more speed, may even drop a mph.. Tach says 5650-5700

 

[kenojim]

I was thinking 15" might be small as well. You don't mention anything, so no idea where the priorities lie in the most frequent use of the boat? If you aren't frequently pulling a tube or skiing, a big hole shot isn't that important. That being the case, the motor is rated to turn 6k, but at those rpms, they can sound pretty busy. A much more comfortable target is something closer to 5500. Frequently you can even pick up a mile per hour or 2 at that rpm as well.

The 15" prop will likely work well for towing, but for everything else, my bet would be a 17" prop.

And as far as all the specialty props available, I suggest you stay with the basics, at least until you have a specialty need. Stainless look cool, but for a 75, alum works just fine....

Don't often pull a skier or tube.. Mostly cruising., the limiter kicks in between 5600 and 5700. The engine came with a 4 blade aluminum spitfire 13", was obvious the pitch was too low from the start, and besides the the boat did not handle well in corners over 30mph, acted like a spooked squirrel...
I do have access to an alum 17" I'm going to try this weekend.
Thanks for the input!

 

[ahicks]

If you're hitting the limiter at 55-5700, betting your tach is misleading you. I work on quite a few of these 40-90hp 4 strokes, and any I've driven will turn 6k+ pretty easily. That's been my experience anyway. As mentioned, I don't like turning them that fast, the motor just sounds too busy, and if the hole shot isn't important, they work just as well at 5500 or so.

If this is a brand new motor, you could be hitting a break in rev. limiter? That being the case, you're going to need to finish up the break in before making any new prop decisions.

4 blade is about low end torque and low speed maneuvering.

 

[Texasmark]

So the real question prompting an answer before you go off in the wrong direction is: As you trim out (when in doubt trim it out....Fazbullet) and the RPMs are increasing, is the boat bow lifting out of the water and the speed increasing, or is the boat already pretty level, bow already out of the water, and as you trim up to hit the rev limiter is the speed falling off?

If no, more pitch. If yes more Rake and Cupping.

 

[kenojim]

Motor now has about 63hrs on it.

So the real question prompting an answer before you go off in the wrong direction is: As you trim out (when in doubt trim it out....Fazbullet) and the RPMs are increasing, is the boat bow lifting out of the water and the speed increasing, or is the boat already pretty level, bow already out of the water, and as you trim up to hit the rev limiter is the speed falling off?

If no, more pitch. If yes more Rake and Cupping.

Bow definitely lifts.. And speed increases. Just hitting the trim button for a second you can feel the bow lift, the spray from the hull recedes from half way up the boat to toward the stern..
WOT speed trimmed down all the way is about 35-36 mph.. Sits right about 5400 RPMs... Trim out 1/4 way, bow rises, speed increases to 38-39..RPMs 5600-5700 and trim out anymore starts to bounce on limiter

 

[jimmbo]

Try the 17, observe and report the results. It will accelerate slower.

 

[kenojim]

I borrowed a 17" spitfire aluminum prop to try. Trimmed down all the way WOT was 4600 RPMs at 33MPH..
Start to trim up and immediate increase in both.
Can run trimmed out all the way at WOT with no limiter.. Top RPMs hit was 5440 at 40MPH.
Performance was just a hair slower both 0-20 and 0-30
Anywhere between 1/5
And 1/2 seconds slower depending on how fast my reaction time was to GO using vessel view app.

 

[ahicks]

So your call, depending on preferences. 15 for towing, 17 for cruising?

There was never any doubt about a slower hole shot.

 

[kenojim]

Yeah, I was surprised it wasn't a bigger difference.
The 15 is stainless, don't know if that makes any difference or not.
I only pull a skier or tube a couple times a year so I think I'll keep this set up.
I like to be able to sit in boat and let someone else drive and not have to explain to them that the motor hitting limiter too.
Thanks for the input from all,
It was helpful.

 

[jimmbo]

Props vary considerably when it comes to blade shape, blade area, rake, cupping, venting, material, even casting/molding techniques. Only when two props are from the Same Lineage/Model/Series are the results truly predictable. Change Brand/Model and it becomes an Apple to Oranges comparison.

 

[QBhoy]

Just think how well she’ll go with a good 3 blade stainless ! Fast boats these wee glastrons. I’ve got a big v6 merc 150 2 stroke on my cvx16. Last time out, she ran out of prop at 66.6 mph and way over revving with a 23”. I also ram out of bravery about there too

 

[kenojim]

Props vary considerably when it comes to blade shape, blade area, rake, cupping, venting, material, even casting/molding techniques. Only when two props are from the Same Lineage/Model/Series are the results truly predictable. Change Brand/Model and it becomes an Apple to Oranges comparison.

Mercury pitches their Mercalloy metal mix or whatever it's called..
Anyone have any info on this?
They say better than standard aluminum...
Sounds like a used car sales pitch to me, but don't know for sure.

 

[QBhoy]

Props vary considerably when it comes to blade shape, blade area, rake, cupping, venting, material, even casting/molding techniques. Only when two props are from the Same Lineage/Model/Series are the results truly predictable. Change Brand/Model and it becomes an Apple to Oranges comparison.

Never a truer word said. Just last week I changed my 19” tempest plus over to the 19” laser 2. Just astonishing the difference in how the boat behaves, performs and difference in rpm too. Sitting cruising at 19 mph, the laser has to be around 300+rpm

 

[Texasmark]

Mercury pitches their Mercalloy metal mix or whatever it's called..
Anyone have any info on this?
They say better than standard aluminum...
Sounds like a used car sales pitch to me, but don't know for sure.

Turning Point brand Hustler commercials shows you the metallic structure cutaways of regular cast aluminum and their proprietary process. I would assume Mercury has a similar process.

 

[Texasmark]

Never a truer word said. Just last week I changed my 19” tempest plus over to the 19” laser 2. Just astonishing the difference in how the boat behaves, performs and difference in rpm too. Sitting cruising at 19 mph, the laser has to be around 300+rpm

I had a 3 blade 20P SS Laser II early edition (square vents) and replaced it with a 4 blade 19P SS Solace. Got the increased hole shot performance, good solid cruising performance (the goal) and only dropped from 49 to 46 at WOT which was a surprise........but speed wasn't the objective.

 

[Texasmark]

Just think how well she’ll go with a good 3 blade stainless ! Fast boats these wee glastrons. I’ve got a big v6 merc 150 2 stroke on my cvx16. Last time out, she ran out of prop at 66.6 mph and way over revving with a 23”. I also ram out of bravery about there too

I have "run out of bravery" on occasions too. You start thinking about "what if"........and the answers aren't good!