Picking a Prop - Effect of Diameter

[TruckDrivingFool]

I'm trying to pick a prop for my 15' Starcraft Jetstar w/ a 50hp Johnson. (sig link below) Until I started reading here I just thought a prop was a prop but now see the need for fine tuning. I now understand the effect of pitch on RPM's but not the effect of diameter.

Right now it has the factory 13 1/4 X 17 prop and @WOT I get 29mph (phone GPS) @ 4800 RPM with my standard load. The motor has manual T&T and have that set as best as possible. I had figured I would go to a 13P and be about the sweet spot. Then I started looking for one and discovered that to get the pitch I would have to go bigger by 1/2" to 3/4".

My service manual lists 5-6000 for WOT throttle w/ peak power @ 5500. It also lists alt. props up to 14" So I've eliminated my fear that my choices wouldn't fit but now I wonder if I need to figure that little bit extra diameter into my choice.

 

[steelespike]

Re: Picking a Prop - Effect of Diameter

Diameter is a secondary consideration and if the prop is intended for your motor
the diameter will work.If you check prop sizes generally as pitch goes down diameter goes up.
It does appear your setup is working pretty good except for the low rpm.
But calculations seem to indicate you may lose speed going down in pitch.
A 13" prop should add 600 to 800 rpm.but at 5600 speed is about 25.
While some al boats are a little hard to push ( all those seams,rivets and convolutions )
I think your 50 should make 29 to 32 mph. on your boat.
Lets see if we can improve the setup.
Does it have a hydrofoil/whale tale on the motor?
Is the anti vent plate(just above the prop) about even with the bottom of the transom?
A picture would help.
Does the throttle open all the way?
Is the motor in excellent tune?
Is the bottom clean and smooth?
Does it seem now that one trim notch is not enough but one more is too much?
I'd like to see a run with a light load.
Any physical changes we make we need to do one at a time and test.

 

[TruckDrivingFool]

Re: Picking a Prop - Effect of Diameter

Thanks for the reply SS not ignoring you just at the lake with just my phone for internet. I got a pic to post but won't be until tomorrow night.

 

[steelespike]

Re: Picking a Prop - Effect of Diameter

No problem.

 

[TruckDrivingFool]

Re: Picking a Prop - Effect of Diameter

No hydrofoil - It had one on it when I bought it but I took it off when I replaced the impeller and never put it back on since it fits in the garage better w/o it. I didn't notice any real change w/o it other than taking just a touch longer to settle onto plane.

The vent plate is roughly 1/2" lower than the bottom of the transom panel and I think it's close to being right since the bottom of the hull and keel curves up just a bit at the back.

I did a bunch of running and testing and the trim is set in the slot where I get the highest RPM's at WOT - If go either way I lose RPM's pretty equally.

I've gone through the motor and done a whole laundry list or repairs for reliability and tuning so I'd say it's in as perfect tune as it's going to get. I have had a starting issue but think I finally have it whipped and it hasn't effected performance. Yes, WOT is being achieved

The bottom well..... it is a 40+ yr old boat with factory paint with a few scratches but no major dings repair or problems I can see by eyeballing. This old girl will get her resto turn in a few years but for now she's going to keep her ugly dress.

As for a light load run it doesn't seem to effect top speed or RPM more just the time it takes to get on plane. I took a few passes by myself and still achieved 29mph but I did see a few blips to 4900 RPM but I think that was just bouncing in the slight chop.

For moving the motor I think I'm as good as it's going to get as it never vents straight but I do get just a bit in a tight turn. I'm truly happy with the way this little girl performs and think I'm down to just fine tuning with the prop as I don't like the low RPM and figured I might gain just a bit of hole shot and stay the same or slight gain on speed.

I'm wishing I had taken a pic of the prop as I wonder a guess of what I'd gain just from going to new prop? The old is in good shape but the blade edges are scuffed from years of use not to mention I'm sure there's some design gains over this one. This is about the best pic of it I have maybe helpfully maybe not.

And of course now that I've been reading more I feel obliged to ask about a 4 blade for this.

 

[TruckDrivingFool]

Re: Picking a Prop - Effect of Diameter

Forgot to mention I surely don't want to lose any speed.

 

[Texasmark]

Re: Picking a Prop - Effect of Diameter

No offense dude, but you are not running an offshore racer. A tweak here and there on your rig will just not amount to that big of a deal. If you are a perfectionist and spend the money for all the "cool" stuff and wind up with a 5 to 10% increase in your current performance for a ton of $$$.

Problem with us boaters is we are NEVER satisfied, NEVER satisfied. It is a disease we all have. Got the $$$$ puke em.

BTDT.

Mark

 

[TruckDrivingFool]

Re: Picking a Prop - Effect of Diameter

Non taken, I understand what you're saying but I would like to get the R's up a bit and hopefuly gain just a bit more pull through a turn when pulling a tube.

 

[BonairII]

Re: Picking a Prop - Effect of Diameter

How heavily loaded is your boat(people, gear, fuel etc)?

If your motor has good compression....and the boat isn't heavy(waterlogged or heavily loaded) then it sounds like a simple case of going down a couple inches in pitch.

Keeping your motor in it's optimal rpm range is more important than not wanting to lose 1-3mph by lowering pitch.

 

[jestor68]

Re: Picking a Prop - Effect of Diameter

It doesn't appear to be getting anywhere close to it's rated rpm.

 

[jimmbo]

Re: Picking a Prop - Effect of Diameter

With the 13 it will take off like a rocket, and you can ditch the whale tail. doelfin. whatever it is.

 

[steelespike]

Re: Picking a Prop - Effect of Diameter

As far as tweaking things its more a matter of being sure your not dragging 2 or 3" too much motor through the water.
From what little I can see of the prop I think a replacement is in order.
I think a 15" 3 blade will do nicely.About 5200 rpm, I think it will hold speed and improver hole shot may grip better in a turn.
If the new prop doesn't vent in the turns you could try a little more height.

 

[Silvertip]

Re: Picking a Prop - Effect of Diameter

If you really have a 13-1/4 x 17 prop it is wrong for that engine and likely, at least in part, the reason for low RPM at WOT. The range of props for that engine run from 13P to about 17P. You should stop trying to buy a prop based on diameter. Locate your engine in an application chart -- such as the one right here on IBoats. You then are presented with a list of props "specifically" for your engine. From my testing on a fairly light 16 foot V-hull I've found the standard OMC cupped 12-1/4 x 15P is an ideal prop. It will let the engine rev to 5800 and provides great hole shot. I've used cupped and uncupped 11-3/4 x 17 and as well as a 12-1/2 x 13P stainless only to always return to the 12-1/4 xd 15. 13P will over rev. Prop diameter is a function of design as you can see ini my examples. As prop PITCH goes up, prop diameter comes down and vice versa. Props are designed for a specific gearcase size and engine HP range. People somehow think that as long as a prop fits it is right for that engine. And by the way -- lose the whale tail. Properly set up the boat doesn't need it. I've experimented with those as well and always ended up removing them.

 

[TruckDrivingFool]

Re: Picking a Prop - Effect of Diameter

Thanks for all your replies when I originally started looking I didn't know what to think about the different pitches having different diameters I see now that it's just the nature of the beast.

The Solas Amita 12.5 X 13 aluminum is your best performance option.

Hopefully that's a typo as I only see the 13.5X13 SA which is what I was looking at before I asked.

I think a 15" 3 blade will do nicely.About 5200 rpm, .

Are you thinking the 15 instead of the 13 just as a protective measure against over revving should I go out with a light load? Or perhaps taking into account the gain I may already see just from replacing my worn prop?

For the record - I already did away with the fin as I said earlier it fits in the garage better w/o it. And according to the manual the prop that's on it now is the style (possibly the one) it shipped from the factory with.

 

[steelespike]

Re: Picking a Prop - Effect of Diameter

I'm thinking a 15 because you didn't want to lose any speed.
it should get you into your rpm range, improve your power/hole shot,may be less touchy at low cruising speed and hopefully
retain your wot speed.

 

[TruckDrivingFool]

Re: Picking a Prop - Effect of Diameter

Thanks, makes sense.

I'll have to wait until I get paid again to make my purchase, but I'll update once it's done and run.

 

[jestor68]

Re: Picking a Prop - Effect of Diameter

Getting to 5200 is still not rated rpm.

 

[TruckDrivingFool]

Re: Picking a Prop - Effect of Diameter

I'll post it one last time. A 2" reduction in pitch is going to net you 300-400 rpm. That will put you at around 5200 rpm.

What is it about 50 hp @ 5500 rpm that you people don't understand?

I do get it but I also get the point SS is making that since I really don't want to lose speed that I maybe better off to just go enough to quit lugging the motor and retain my speed. Even if not the absolute sweet spot a 15 will get me into the recommended RPM range and may just be the trade off I take for not losing speed.

That said I still have a week and a half to kick it around. I'm truly leaning towards the 13.5X13 with the thought/hope that I'll get close to right on the sweet spot and at least retain my 29MPH speed. If it works out that way I'll be happy, if not I'll wait a bit and buy the 15 and see which of the comprises works best for me. In the end I'll either end up with the right prop and an ok spare or I'll end up with a new prop for a spare.

Good discussion guys I appreciate all the input.

 

[jimmbo]

Re: Picking a Prop - Effect of Diameter

In 1971 or 72, which is what your engine looks like it is, the Propeller that was shipped with the engine, but could be exchanged for one more apropriate, was a 17in with no cupping. Oddly in 1973 it was changed to a 15in, still no cupping. Normaly I would say the 15 should be fine but maybe after 40 years the engine is showing some wear and a 13 might be better. I would be checking to see if the tach is accurate too.
As for losing speed, you are looking at the pitch and seeing less inches per rev, but are forgetting with the lower pitch the engine will be less loaded and will also be higher in its powerband, making more hp. You won't lose any speed, but most likey gain some. Alot of people think the engine will always rev up to the rated rpm and they can have a 'power prop' and a 'speed prop'. it doesn't quite work that way. Most of the time the speed differnece is less than 1 mph provided they stay in the rcommended rpm range at WOT. Running outside of that range, high or low, almost always results in a drop in speed.
On fast boats with big engines there are different types of props that will increase a boats speed, but these require engine height changes to allow the props to do their magic.

 

[Mac525]

Re: Picking a Prop - Effect of Diameter

I agree it looks like that motor can come up 3/4 of an inch right off the get go before you start playing with props