Might this work as a backup?

[jlsmith1000]

I know just enough about props to have always bought the exact same one that is on any boat I've owned, though I know there are variations that will provide different benefits (detractions) with pitch, but sort of thinking that the diameter needs to be the same? Also, what number on a prop describes the spline count? Or does the hub-kit take care of that?

Currently have a 15-M / 14.5 Stainless prop (which I believe is 15 pitch, 14.5 diameter) on a Yamaha 200hp outboard, driving a 20' 6" Hurricane Deck boat.

Local seller has an aluminum prop for sale which is a 15.25 diameter, 15 pitch Black Diamond. It might need a hub kit, or do you just use the one on the boat when you swap props? Not sure if they are re-usable or a throw-away when you replace the prop. My boats have always been older, so this idea of a hub kit is a bit foreign to me.

 

[brodmann]

Probably need to stick with the same diameter. You probably don't have room for a larger diameter prop without hitting the something. The pitch of a prop is the number of inches it should travel forward in one complete revolution. Let us know the RPM's and speed at Wide Open Throttle (WOT) and we can tell you what might be a better prop for your boat.

 

[jlsmith1000]

Speed is about 48mph, I don't recall the RPM as I've just been out a couple times since picking it up this past week, but I'm going to say high 4k or better, seems like 4800 rings a bell but not 100%. Will have to check next time I'm out, which will be in the next couple days depending on weather.

 

[jlsmith1000]

I also need to read up on trimming the outboard as I'm used to an I/O and seems like there is a bit of a difference, or I'm just used to my I/O and it's particulars.

 

[airshot]

If you have the clearance to change the diameter, diameter usually effects half what the pitch effects. In other words, an inch of pitch changes rpm about 200 rpm....one inch on the diameter changes rpm about 100 rpm. So....if you have enough clearance another 1/4" in dia will only slow your rpm by 25 rpm in theroy. SS props will be a little different due to thinner blades.
Trimming effects boats differently, however the idea is the same. You need a good tach and a good gps speed to make precise trim adjustments work well.

 

[jlsmith1000]

Trimming effects boats differently, however the idea is the same. You need a good tach and a good gps speed to make precise trim adjustments work well.

Yep, I think I just need a little more time on the water with the boat to figure out where the sweet spot is for the trim at the higher end. I had the older boat for so long I just new where to be and when and didn't have to think much about it.

 

[jlsmith1000]

In other words, an inch of pitch changes rpm about 200 rpm....one inch on the diameter changes rpm about 100 rpm. So....if you have enough clearance another 1/4" in dia will only slow your rpm by 25 rpm in theroy.

So, if I'm reading that correctly, then the increase in diameter by 0.75 should be about 75 rpm difference, which assuming clearance should be good for a backup prop to get me back to shore. The clearance we're discussing is that which I've highlighted below correct?

 

[jimmbo]

Let the Prop Engineers worry about Diameter. However, you do need to know the WOT Range for your engine, and chose a Prop that allows it to run within that Rpm range at WOT, preferably in the upper half. That said, there will be many props with identical/similar Diameter and Pitches, yet will yield considerably different results. That will be due to things like Blade Area, Blade Thickness, Blade Shape, Number of Blades, Rake, Cupping, Material, to mention a few.

 

[jlsmith1000]

Ok, this is what the manual says is WOT RPM.
5000-6000 r/min
I read that as around 5500 rpm is optimal, or a little to the high side of that.

I'll run through WOT RPM Test - How To
next time I'm out on the water this week.

 

[jimmbo]

I would say 5900 as closer to Optimal. What are you currently getting at WOT?

 

[airshot]

So, if I'm reading that correctly, then the increase in diameter by 0.75 should be about 75 rpm difference, which assuming clearance should be good for a backup prop to get me back to shore. The clearance we're discussing is that which I've highlighted below correct?


View attachment 386766

Not from the hub...but from the tip of the prop blade to the cav plate above it !! If there is an anode fin under, thats needs cleared as well.

 

[airshot]

Ok, this is what the manual says is WOT RPM.
5000-6000 r/min
I read that as around 5500 rpm is optimal, or a little to the high side of that.

I'll run through WOT RPM Test - How To
next time I'm out on the water this week.

On my boats, I use a prop that gives me max rpm with the lighest load I will ever run. My motor max is 5500 so with just me and basic gear I can get about 5650 rpm, but I never leave it pegged at that speed, with two on board I am at my 5500 and with three about 5400. This way it doesn't bog down if I take some extra weight, but have to watch long wot runs.

 

[jlsmith1000]

I would say 5900 as closer to Optimal. What are you currently getting at WOT?

I'm just about 5800-5900 and not quit WoT, was afraid to push it farther since the dash shows 6k+ as redzone

 

[jimmbo]

There is a Rev Limiter, but a few moments above 6k isn't going to grenade the engine, it's not like it is a Merc 4 cylinder looper.
If you are almost at WOT, I doubt you are going to see much further Increases, unless you are Underpropped, but you will have your hand on the Go-Go Lever...

 

[jlsmith1000]

Too bad you can't try different props plus or minus a little to see which one's the best. They're too damn expensive to buy just to try and find out they don't perform.

 

[jimmbo]

I've been lucky, I have always gotten along very well with the Dealer(s), and I was always able to just grab a Prop, try it out, not even leaving a Deposit.

 

[jlh3rd]

I know just enough about props to have always bought the exact same one that is on any boat I've owned, though I know there are variations that will provide different benefits (detractions) with pitch, but sort of thinking that the diameter needs to be the same? Also, what number on a prop describes the spline count? Or does the hub-kit take care of that?

Currently have a 15-M / 14.5 Stainless prop (which I believe is 15 pitch, 14.5 diameter) on a Yamaha 200hp outboard, driving a 20' 6" Hurricane Deck boat.

Local seller has an aluminum prop for sale which is a 15.25 diameter, 15 pitch Black Diamond. It might need a hub kit, or do you just use the one on the boat when you swap props? Not sure if they are re-usable or a throw-away when you replace the prop. My boats have always been older, so this idea of a hub kit is a bit foreign to me.

these are figures derived from 3 props I've used on my pontoon.
It is a 24', twin tube (25") suncruiser with a '21 merc. 115 hp CT.
Maybe this can give you an idea of what "propping " is all about.
Diameter, pitch, blades all matter along with cupping, material, etc. as all mentioned above.
It also matters what your trying to achieve.
The 3-16x12 prop gave me the lowest speed but enabled me to pull two tubes with 5 teenagers and 4 adults without boring the kids....or me..
WOT by myself would get over 6100 rpm, over the recommended range, but the " torque" was there when I needed it...I propped my pontoon for max rpm with just me on board.
You can see that a 4-14x14 and a 3-16x13 were almost equal, but the "tubing" rpm was too low for me.

If a prop accepts a hub kit, then you buy the hub kit for that motor which will fit the shaft splines....

I have a solas 3-15.5x13 to try. No hub kit for this prop, it is integral with the prop, unlike my previous mercury props.
I'm curious what the 1/2 " diameter difference will make, if at all compared to the other 16x13.

 

[QBhoy]

Speed is about 48mph, I don't recall the RPM as I've just been out a couple times since picking it up this past week, but I'm going to say high 4k or better, seems like 4800 rings a bell but not 100%. Will have to check next time I'm out, which will be in the next couple days depending on weather.

Hold all thoughts a wee minute perhaps.
Hope you don’t mind me interrupting things…but the prop geek in me has alarm bells ringing here.
If we are thinking that the prop is a 15” pitch…on a Yamaha 200hp, fitted to a 20ft boat doing 50mph…there is absolutely and certainly one or more of the details, definitely not correct. That engine would be screaming through the roof and/or limiting at the very least at or way before that kind of speed.
Excuse the ignorance of me…but what is a deck boat in particular? I’m a uk guy. Just so I’m clear. Is that like an open sports boat. Not a pontoon presumably?
Either way. Easiest to assume, is perhaps that your prop isn’t as low as a 15” pitch. Quite sure (but without checking) that there is no way a 15” prop is getting you to 50 mph or anywhere close, without limiting or blowing things through the side of the crankcase

 

[QBhoy]

these are figures derived from 3 props I've used on my pontoon.
It is a 24', twin tube (25") suncruiser with a '21 merc. 115 hp CT.
Maybe this can give you an idea of what "propping " is all about.
Diameter, pitch, blades all matter along with cupping, material, etc. as all mentioned above.
It also matters what your trying to achieve.
The 3-16x12 prop gave me the lowest speed but enabled me to pull two tubes with 5 teenagers and 4 adults without boring the kids....or me..
WOT by myself would get over 6100 rpm, over the recommended range, but the " torque" was there when I needed it...I propped my pontoon for max rpm with just me on board.
You can see that a 4-14x14 and a 3-16x13 were almost equal, but the "tubing" rpm was too low for me.

If a prop accepts a hub kit, then you buy the hub kit for that motor which will fit the shaft splines....

I have a solas 3-15.5x13 to try. No hub kit for this prop, it is integral with the prop, unlike my previous mercury props.
I'm curious what the 1/2 " diameter difference will make, if at all compared to the other 16x13.

Interesting reading there. I have the same engine on one of my boats. Great engines. Also the CT I have. Some details from the props I run or have ran on mine.
19ft deep v cuddy
19” tempest plus 43mph consistently achieved @6000-6100rpm. Occasionally just touching 44mph
20” enertia 45mph consistently at around 6000rpm. Occasionally a touch quicker
19” laser 2 6100 rpm limiter at 42/43mph
21” laser 2 (terrible) 40 mph approximately at 5900ish rpm
21” high 5 (horrendous) 37 mph @6000 ish rpm.
Tempest plus 19” by far the best all round for efficiency and cruising. But huge diameter and heavy bugger for going into gear. Will cruise at about 23 mph @3700rpm give or take..doing 1.7 miles per litre of fuel.
Sorry to hijack the thread !

 

[flashback]

OP, your asking for a backup right? Anything that fits will do the job.