Prop pitch vs length

[Papee]

OK if this is a statement made by the prop manufacturer am I thinking right?

*Increase pitch by one degree for every one inch decrease in diameter.
**A change in pitch of one degree is worth about 200-300 change in engine RPM. Increasing pitch will decrease RPM. Decreasing pitch will increase RPM.


Seeing the above statement, if I cut 1" from the blade am I gaining the 200-300 rpm?

I got my new blades today and at 13 degrees I am only turning the prop at 2200 rpm. The prop is 59" now and top prop speed for the prop is 3300 rpm. I would like to cut the blade some because it is just below my transom.

If I am thinking right, if I wanted 3000 rpm out of this setup I would need to cut the blades down 2", am I thinking correctly? Prop is a three blade carbon fiber.

[chuckitt]

Yes Papee, but if you cut 1 inch from each blade, that is equal to 2 inches in prop diameter. Your 59 inch prop would then be a 57 inch. Can you set the pitch at 11 degrees and try it before you cut the blades.
Thanks, Chuck

[Duane Scarborough]

Papee,

Books could be written on what I do not know about this.

My first thought is that before I started cutting new prop blades, I'd give the prop builder a call.

Balance is very critical, and I "think" it'd be better to decrease the pitch a couple of degrees. I don't think that the 1" below the transom will be a problem. As I understand it, most of the thrust happens about 70% down the blade's length. I don't "think" that the final 1" is doing much.

I'd call the prop builder before I did anything...

Duane

[Papee]

I'll split with you Chuck LOL I'll cut 1" and go to 12 hows that? The prop is a little too close to one of the cross bars for comfort. This is another Ultra Prop that I bought used. I have all the pitch blocks for this prop so experimenting is not a problem. The 2" would also get the prop out of my transom. Maybe I'll just go to 11 right away and take all the blocks along. They are easily changed and I can do it on the river at camp. I do want to keep all the length I can in the prop.

Just for past info, I was running the same prop at 48" and 18* before I switched to the stick prop. That prop got the boat out of the water very fast but was low on top end. I want to get this prop some where in the middle. of the stick prop and the old Ultra Prop at 48x18*.


Duane, these props are made to cut down, they even give you cut marks on 1/2" increments. When you cut, you cut and allow the mark to show then use a file to finish up to make sure they are all cut exact.

[Papee]

OK so after more think and figuring.

I was running the 48x18 before and needed more prop.

Check my math here:
that setup would be the same at 55x11

If I am correct there and need the 1" of clearance for the cage, I'm only clearing at about 1 1/2 I'd feel better at 2 1/2"

I'd end up with a 57" prop and at 57x11 that would be equal to a prop only two inches longer than the original if you follow me. So 57x11 may be my best starting point? This would be equal to adding 2 inches to my original setup and bringing my rpms down approx 400 rpm from the original setup which at that ?? rpm, I was running out of gas.

I didn't have the tach when I had the original setup so I have no rpm reference.

Just a note, This is a direct drive setup and I want to keep the original carb on it for a reference for people who choose to run a direct drive and not mod the motor. I have decided to keep this boat around for a spare boat for my new excuse to spend more time on the river. IE second boat for a guide service. After fine tuning this thing I'm just having a hard time letting it go since it works so well.

[Duane Scarborough]

I'll split with you Chuck LOL I'll cut 1" and go to 12 hows that? The prop is a little too close to one of the cross bars for comfort. This is another Ultra Prop that I bought used. I have all the pitch blocks for this prop so experimenting is not a problem. The 2" would also get the prop out of my transom. Maybe I'll just go to 11 right away and take all the blocks along. They are easily changed and I can do it on the river at camp. I do want to keep all the length I can in the prop.

Just for past info, I was running the same prop at 48" and 18* before I switched to the stick prop. That prop got the boat out of the water very fast but was low on top end. I want to get this prop some where in the middle. of the stick prop and the old Ultra Prop at 48x18*.


Duane, these props are made to cut down, they even give you cut marks on 1/2" increments. When you cut, you cut and allow the mark to show then use a file to finish up to make sure they are all cut exact.

Papee,

See, I learn something new every day.

Thanks,

Duane

[Papee]

I'm having the same trouble here as Buster, we just aren't having the weather to run. After all the going over in my head, when I went out to do the work I rechecked the prop clearance and decided to leave the length for now then put in a set of 10 degree shims. I hooked it up to the Jeep this time so I didn't have my ARV out in the middle of the street again.

I got the rpms back up to 3000-3100 on the trailer using a 59x10* setup. Looks like we will finally have some nice weather for a while starting Monday. We had a local tornado warning on Thursday which rarely happens around here. I am real close to the Appalachian Mountains and at the foot of another(Berry Mountain).

[Papee]

Ok, I had a few minutes today to get out and do a test. It wasn't very good conditions because we had some pretty good winds but knowing how my boat acted before helped.

Now for the confusion, the prop seems to be making more thrust, I can't stow my flotation devices where I had been, I had to catch one on them during the ride.

It also seems to have changed the way the boat runs on the water, it doesn't seem as free as it was before. I may be pushing too hard on the nose now, I had put some shims in to keep the nose down in the wind with the stick prop,

I am getting on plane about the same as the stick prop right now. I can run about 22 mph at 2200 rpm and top speed in the wind was about 26.2 mph at 2500-2600 rpm, this is the top rpm I can get.

Looks like I'll have to start cutting, I don't want to go any lower tan 10* on the pitch. I also can only go to 9* with the prop. Most of my evaluations come from watching the spray and where it was on the boat compared to the stick prop and how the boat felt in a turn.


My thoughts on adjustments"
Cut the blades 1" going down to a 57" prop.
Take 1/8 inch out of the shims so it doesn't push so hard on the front and also take a few more washers along to add just in case I went the wrong way.

Pushing on the nose may have taken some of my speed so engine shims may help that some and cutting the prop should get my rpms back up and help it get out of the water a little faster.

[Duane Scarborough]

Papee,

You know that I have a foot in mouth problem, but here goes:

I would NOT cut the prop yet.

You said: "I got the rpms back up to 3000-3100 on the trailer using a 59x10* setup."

You also said: "Now for the confusion, the prop seems to be making more thrust, I can't stow my flotation devices where I had been, I had to catch one on them during the ride.

It also seems to have changed the way the boat runs on the water, it doesn't seem as free as it was before. I may be pushing too hard on the nose now, I had put some shims in to keep the nose down in the wind with the stick prop,

I am getting on plane about the same as the stick prop right now. I can run about 22 mph at 2200 rpm and top speed in the wind was about 26.2 mph at 2500-2600 rpm, this is the top rpm I can get."

It seems to me that if your RPM goes down when on the water, compared to what you've got in a static on the trailer test, it's not a prop pitch problem. I think that you should have seen the opposite.

I'd play with shimming it before I started cutting a good prop.

(Swallowing foot now).

Duane

[Papee]

I'm as far as I want to go with prop shims at 10 degrees so the only thing left is to cut the blade.

Changing the engine shims will not get me any more RPMs. I'm looking at two different problems, the prop is to big now, I can't go down and more in prop shims so this means taking some off the prop.

It's not really plowing just riding a little hard on the nose( I think) I may be wrong and have that backwards. I'll have to see when I get it out and experiment with the engine shims to see which way I need to go. These adjusts are a very fine adjustment and most people wouldn't even notice. Knowing how my boat runs when it is set up right helps me with these adjustments.

Keep in mind that running direct drive, a 52" 2 blade prop is recommended and I am at 59 inches with 3 blades now. I run a top rpm of not more than 2600. I would like to get at least 3000 rpms. I'm beginning to think I read the rpms wrong when I tested it on the trailer. I am looking for a specific performance from the prop which is right between my first setup which got me out of the water like a rocket, and the stick prop I just took off. The stick was fine getting out of the water but I'm looking for an end performance between the two. I want to get out of the water a litter faster and cut my top speed some.

The fastest my boat has gone is 38.3 mph by GPS. I'm looking to get out of the water fast and have a top speed of about 30-33 MPH

My guess is I'll probably end up with a 56" prop to meet these spec I'm looking for.

[Papee]

Well I'm happy now, I had a chance to go out for a few hours this morning. I end up at 57x10*.
It's gets out of the water faster than the stick prop, cruises easily at 25 MPH at 2500 RPM and the fastest speed I got was 31.9 at 3200 RPM.

I kept hearing this weird noise I hadn't heard with the stick prop, I finally figured out it was my exhaust! All I heard before was prop. Now it's time for some mufflers.

[Sandman]

Nice Papee... now it's time for a new video and some more pics

[Duane Scarborough]

Well I'm happy now, I had a chance to go out for a few hours this morning. I end up at 57x10*.
It's gets out of the water faster than the stick prop, cruises easily at 25 MPH at 2500 RPM and the fastest speed I got was 31.9 at 3200 RPM.

I kept hearing this weird noise I hadn't heard with the stick prop, I finally figured out it was my exhaust! All I heard before was prop. Now it's time for some mufflers.

Papee,

I'm glad you got it figured out. It sounds like you've accomplished your goal. Yay !

I take it that this ultra prop must be considerably quieter than the stick prop was.

For what it's worth, I've been told that when you get the right prop, and the right mufflers, at an idle you're going to hear another strange sound.

That'd be the sound of the valve train. (valves, lifters, etc.)

I haven't heard that myself, because I haven't ran mine with mufflers yet, but Les says it so.

Thanks,

Duane

[Papee]

I don't know how you could hear any of that over the magneto LOL That thing makes some noise, it took some getting use to. When I first started running it I was always looking back.

[airhead]

Hey Papee,

Glad to hear that your happy with your prop but as usual I have a question. You might remember i told you that i ordered a 56" 3 blade composite from competition. Well i actually measured it today while working on the cage and it's a 52 x 18*. I'm not sure what happened but what kind of rpm's were you turning with the 2 blade 52 and do you think i should give them a call and try to get the 56" ?

Thanks

[Duane Scarborough]

Papee,

Somehow my reply got lost when Airhead was also replying.

Oh well. I hope to be able to hear the valve train over the mag. I was warned that the mag was noisy, so I was not surprised. When it's running, all I hear is exhaust, so far.

One of MY goals is to have a relatively QUIET airboat.

Thanks,

Duane

[airhead]

Sorry about that Duane.

[Duane Scarborough]

No worries, mate. You didn't do it.

Duane

[Papee]

Hey Papee,

Glad to hear that your happy with your prop but as usual I have a question. You might remember i told you that i ordered a 56" 3 blade composite from competition. Well i actually measured it today while working on the cage and it's a 52 x 18*. I'm not sure what happened but what kind of rpm's were you turning with the 2 blade 52 and do you think i should give them a call and try to get the 56" ?

Thanks

I didn't have the tach on then but you should be pretty close with that. It may need to go down some. Just give it a try and see what happens, Bob will exchange the shims as long as they are in good shape. The only spares I have here are 9.10,11,12,13, and 18.

I wouldn't cut the blade at all at that length but just for reference, cutting the overall length 1" did more than taking away 1 degree of shim on mine. Estimate the smaller shims if needed to the low side. That prop will be great for that beast if you set it for low end. I had mine so it would spin up fast and the boat came out of the water fast but got only 21mph. but it also did 21 with two people in it.

[airhead]

Thanks Papee,

I don't know what happened on the prop but i guess i will just bite the bullet and give it a try. I can always save my pennies and buy more blades i guess Glad you're happy with yours and keep us informed if you try something different.


Jim