prop sizing and changes

[bonz_d]

Generally speaking looking to get an idea of how changing size relates to performance.

What I have right now is a 14' Lund with side console which Lund says weighs 285 lbs. Mounted on it is a 1978 35hp Evinrude with electric start. Total added weight, battery, gas, seats, and gear maybe another 150 lbs. So I figure total weight with 2 adults to be about 850 lbs.

The prop that came on the engine was a worn out 10.5 x 11 that had been ground down to about 9.5". It wouldn't push the boat, it would just blow out and slow down while reving up. I was able to replace it with a new 11 x 9 that a repair shop had for a very good price.

This 11 x 9 seems to work fairly well. Engine sounds like it is up to RPM at WOT and with this prop it is still very fast even with 2 adults and 2 childern onboard. Don't need to go any faster! But this set-up is very stern heavy and doesn't come out of the hole as well as I'd like and at mid throttle the bow rides vey high. At WOT the boat is very flat, on plane and almost too fast.

Surfing the web and the prop manf. sites I see that all seem to recommend either a 10 x 13 or a 10 x 15 for this motor and size boat. While the 11 x 9 is recommended for very heavy boats. So if this engine is up to RPM at WOT, which I'm still uncertain of but assume it's in range, what effect would changing the size and pitch have on this set-up and would going to a 4 blade prop be a better solution? If so what size range should I start looking at?

Hope this makes sense!

 

[junior1113]

Re: prop sizing and changes

error

 

[junior1113]

Re: prop sizing and changes

11-12p 4 blade if it were mine. id also check with in line tach for recomended rpms. need to find one with stern lifting charactistics or smart tabs

 

[hwsiii]

Re: prop sizing and changes

Bonz, my program shows you should be using a 13" pitch prop, I do not believe you need a 4 blade prop as your boat is so light it ought to get on plane very easily.

Prop Picker

But, you may be happier with a 12" Pitch prop so you would lose less of your hole shot.

H

 

[bonz_d]

Re: prop sizing and changes

As for the tach, I just put one in but of course it's not working properly. Time to find another!

I was thinking it would be around a 10-12 pitch for the 4 blade.

So is this thinking correct? To maintain a RPM a decrease in diameter requires an increase in pitch which will also increase speed and reduce low speed torque and hole shot.

 

[hwsiii]

Re: prop sizing and changes

Bonz, in overall theory you are correct more or less, but I will tell you that blade Geometry controls RPM and prop slip much more than pitch or small diameter changes. When i discuss blade geometry I am talking about how much Rake and whether it is flat rake for moderate bow lift, zero rake for stern lift or progressive rake for a lot of bow lift, Pitch and whether it is regressive pitch for better hole shot, progressive pitch for a better hole shot and top speed, or fixed pitch for a narrow speed range at a particular RPM, Cupping and whether it is on the trailing edge for stern lift and less ventilation when running props at high motor heights, or blade tip cup for maximum bow lift and water control, and the actual amount of blade surface area( low blade surface area for light boats and high speeds, high blade surface areas for heavy boats and less prop slip) that are designed into a prop for specific applications.
Most of the recommendations made in this forum are for Pitch only, as the person asking the questions is just trying to get the RPM for his motor in the right manufacturer recommended WOT power range, and they use aluminum props which have very little blade geometry and thus can be exchanged between one brand and another with very little unexpected results, except the Turning Point and Michigan Match series. When changing to a Stainless Steel prop is when all of the above items I mentioned come into play and that is when the Black Art of propeller recommendations matters. When changing out propellers in SS I have seen a 700 RPM difference between two props that have a diameter within a 1/4" of each other and the 16" Pitch prop is the one with the 5,050 RPM versus the 17" Pitch running at 5,750 RPM.
Hopefully this increases your knowledge about props and the differences involved between blade geometry and what it can change in how a boat will react to certain blade configuration changes in geometry.

H

 

[junior1113]

Re: prop sizing and changes

great answer

 

[hwsiii]

Re: prop sizing and changes

Thank you junior.

H

 

[bonz_d]

Re: prop sizing and changes

Thanks for the lesson, "Props 101". You have answered some questions an then created more. Way to go!

So what I'm getting from this is that a 10 x 13 should work better than what I have on now and I understand the meaning of some of the geometry you speak of.

Now more questions.
1. About how much of a difference in diameter do you need before seeing a noticable difference in an aluminum prop? Instance, say 11x9 to 10.5x11 or, 10.5x11 to 10x13. As I am assuming that on the same boat these props should still perform in the same RPM range.

2. What makes the difference between the Turning Point and Michigan Match compared to Solas, Michigan Vortex and OEM of the same size?

3. With SS props that are identical, same part # will a reduction in size of 1/2" make a big difference?

4. That program that you used to come up with the 10x13. Where do you start it? Speed? Where do the numbers come from?

 

[hwsiii]

Re: prop sizing and changes

OK, Question 1, in most cases these numbers would be in the same RANGE, but remember you are trying to compare a 11 x 9 to a 10 x 13
Question 2, The Turning Point props have a LOT of Blade geometry and the Michigan Match props have huge blade area
Question 3, Depends on your meaning of a big difference
Question 4, Yes I do start the Computation with speed and it is run on different mathematical formulas to find the final result.

Hope this helps

H

 

[bonz_d]

Re: prop sizing and changes

Thanks,

One of the reasons I ask about diameter is because I see a wide range of diameters for this size motor. From the 11" I have on now down to 10" with standard sizes like 11x9, 10.5x11 10x13 and 10x15. With th 10's in many different pitches. Then off sizes like 10.25 and 10.125.

Also have a 10.5x11 Stainless that has been cut down to just over 10" that I picked up used. Didn't know at the time that it was ground down that much. Would it be worth putting on and trying it?

I do understand what you state with the stainless option and it's flexablity but I spend much of my time in shallow rocky areas fishing and am afraid of damage to the lower unit. Which brings me back to the problem with lifting the stern and using an aluminum prop. So with all these different sizes how does one figure out what will work w/o spending a ton on trial and error? O ther than from what you stated that the TP or MM might be a better answer.

 

[junior1113]

Re: prop sizing and changes

4x11p or 3x12 thats what u need. dont let all these #'s confuse u. usually lower pitchs have wider diamiter this is because if u were running a 9p on a heavy boat you need the bigger circle (torque) and 15p on a john boat will have smaller diamiter to spin more as it is easyer to spin. people could sit here for a weak explaining every term of a propeller. as it is quit involved

 

[bonz_d]

Re: prop sizing and changes

Thanks for the reply.

I can understand the concepts. Just seems that this boat isn't that heavy that I should have to be using such a large prop. Even though it performs almost as I'd like other than it's not lifting the stern at low speeds.

 

[hwsiii]

Re: prop sizing and changes

Bonz, on my 16' flats boats I have a 40 HP motor at 140# pushing a boat with a base weight of over 800#, when you add in the trolling motor, two batteries, three chairs, 16 gallons of fuel, two ice chests with a total weight of 65#, spare prop with tools and Coast Guard required equipment, tackle box with rod and reels 43#, 3 adults and 3 children I have an allup weight of over 1,800 pounds.
I am using a 10 3/8" x 14" Apollo stainless steel prop with the motor mounted as high as it can go and running Smart Tabs SX to lift the stern, because my Apollo prop is a higher rake progressive pitch bow lifting prop with Cup instead of a stern lifter.
I use this prop because when we go to the islands with just 3 adults onboard I can lift the bow high enough to keep 2 ' to 3' seas from coming over the bow of the boat. When we go up the river with the grandkids it helps very much to have the smart tabs to get on plane much quicker.
As you can see, under most circumstances this would not be the ideal prop for this boat and motor combination because the high rake Apollo tends to make the boat want to porpoise, but with the smart tabs it does not porpoise at all, and yet lets me still lift the bow for the seas on the way to the island.
It depends very much on your intended uses for the boat and what different equipment can be used to overcome basic hull tendencies built into a specific hull form.
I hope this helps you understand better how much time and effort can be spent trying to find the right combination for a multiuse vessel. I have considered ordering the propulse 4 blade composite adjustable pitch prop for this boat, I am just not sure it can handle the loads I carry at times, but I sure would love to be able to change pitch at the dock for the load I am carrying at that time. All the composite props make claims they are stronger than aluminum and have many benefits, but they still break blades very easily if you make a mistake. And since it is a flats boat I do run in very shallow water regularly, and I can guarantee you I will break those blades regularly on the sand bottom, that is why I havent ordered one yet.

H

 

[bonz_d]

Re: prop sizing and changes

Thanks H and all who have responded!

Well all of this has increased my understanding but has also raised more questions which I will have to answer. So it seems there is no perfect aluminum prop to solve my problem.

At this point I think I will shy away from a SS prop as there is just way too much risk of damage with the way I use this boat. I will try to redistribute some of the weight to balance it a bit better. As I can move the battery forward about 2' and relocate the gas tank to the bow which I have tried today and helped some. Then I think I will try either a Match or Rascal prop to see if that will also help improve my hole shot and lower speed stern lift.

Though I have one more question. Is the geometry about the same for the Michigan Match and Vortex models of props?

Thanks again

 

[hwsiii]

Re: prop sizing and changes

I don't know Bonz.

H

 

[bonz_d]

Re: prop sizing and changes

Hi H,

Was on Michigan Wheel website yesterday to research a bit more and from what I've gathered it looks like the Vortex has more rake and a cupped trailing edge so that propbably isn't what I'm looking for either but they do recommend a 10.25 x 14 for this boat and motor size in the Vortex and a 10 x 15 for the Match.

At this point I'm still undecided!

 

[ibrw1]

Re: prop sizing and changes

Hey Commander, I just got a boat, 18 foot 1991 Rinker Mercruiser 3 Liter I/O It has a 14.2" diameter 19" Pitch 15 spline stainless 3 blade propeller.
With 6 people its hard to plane. Engine at 4000 RPM WOT. Was on the river so the speed, 25 to 30 mph is squed

Timing, Dwell set correctly, idle mixture and Idle good too. Has new plugs, cap and rotor. Was told that the engine was manufactured in 1990

Best Guess, engine has issues? or Prop should be resized down to a 17 Pitch? Called Rinker and Mr. "R. Rinker" said an 18 pitch should be ok?

What do you guess, engine or prop? Ideas?