Weed eater/whipper snipper engines - Page 2

stacker9000 · 09-07-2009, 05:43 AM

I am new to this hobby and by no means an expert,but the shallow prop setting is called a surface drive and seems to be desireable for higher speeds,but if you can find pictures,you'll see the prop is quite a few inches away from the rear of the boat,yours being as close as it is might cause some kind of cavitation issue.Your prop issues may be balance related,and the odd surface area shape may cause more drag than propulsion,it appears to be bent at almost a 90 degree angle,I would try notching the bottom of a flat piece of material and welding it at a 45 degree angle on the shaft tube,overall prop size would probably depend on the boat length,but I don't know enough to even guess how big to go.
Good Luck!!! CB

stacker9000 · 09-07-2009, 05:54 AM

Sorry, double posted and can't fiqure out how to delete.

valet_dave · 09-07-2009, 11:37 PM

Quote:

Originally Posted by stacker9000

it appears to be bent at almost a 90 degree angle,I would try notching the bottom of a flat piece of material and welding it at a 45 degree angle on the shaft tube,overall prop size would probably depend on the boat length,but I don't know enough to even guess how big to go.
Good Luck!!! CB

Yeah I definitely agree with you on the softer angle. We tried the 90 degree mainly because earlier props (about 10-15 degrees) tended to create a spray of water a few feet in the air, but virtually no useable thrust (it was a good water feature). The 90 degree one tends to throw the water directly behind the boat, but the angle is obviously too flat for the amount of torque and RPM the engine is producing.

We've never been able to get the boat moving enough to actually know if we are doing anything right (obviously not!).

The dilemma is, do I stay with the diameter and keep experimenting with the angle, or start by reducing the diameter. Everything effects the other, so it's hard to know which is more important.

Cheers,
Dave

dwr9 · 09-07-2009, 11:53 PM

First off 120mm is way too huge. I think you should stay to 75mm or less for your diameter. Have you looked at any props before trying to make your own? It will never work with the trailing edge parallel to the hub like that. It has to have some angle. That will just be a paddle wheel trying to push the boat sideways. I'll atach a couple older pics of the transom of my boat showing the prop. It's a 3 blade prop, but it will at least maybe show the basic shape of the blades. Interesting project tho, trying to make your own prop. Please keep us informed how it goes.

valet_dave · 09-08-2009, 03:44 AM

Quote:

Originally Posted by dwr9

First off 120mm is way too huge. I think you should stay to 75mm or less for your diameter. Have you looked at any props before trying to make your own? It will never work with the trailing edge parallel to the hub like that. It has to have some angle. That will just be a paddle wheel trying to push the boat sideways. I'll atach a couple older pics of the transom of my boat showing the prop. It's a 3 blade prop, but it will at least maybe show the basic shape of the blades. Interesting project tho, trying to make your own prop. Please keep us informed how it goes.

Thanks for those photos, that gives me some hope that the drive shaft is not too shallow (that is now impossible for me to change).

I did look at some 75mm props, but the general consensus amongst people who know nothing about boats was that they were only for high speed applications... and our weed eaters were never going to generate that kind of RPM. So we decided that a bigger diameter and greater pitch would make use of the supposed torque. Which of course turned out to be wrong.

I will take your advice and try a much smaller diameter. Updates to follow!

Cheers,
Dave

valet_dave · 09-14-2009, 01:40 AM

Okay, we made another prop over the weekend. This one has about an 82mm diameter and is roughly at a 45 degree angle from the shaft.

We threw it in the water without any radio gear, and it's performance was... underwhelming! At about 40% throttle it glides along at about
5 km/ph with virtually no splash. I'm guessing it's because the prop is now too small.

I'll attach some photos for your further amusement. (Note that the photo of me letting it go in the dam gives the impression of speed/splash only because I had to throw the boat into the water, to clear the bank.)

dwr9 · 09-15-2009, 12:12 AM

82mm is a pretty big prop, but around 80mm is often a good diameter for weed eater motors. I doubt the diameter is your problem. There are dozens of variables that go into prop design and I'm definitely no prop expert. How much does your boat weigh? Doug

valet_dave · 09-15-2009, 12:36 AM

Quote:

Originally Posted by dwr9

82mm is a pretty big prop, but around 80mm is often a good diameter for weed eater motors. I doubt the diameter is your problem. There are dozens of variables that go into prop design and I'm definitely no prop expert. How much does your boat weigh? Doug

The boat is extremely heavy, being all stainless steel construction! I would estimate it's about 7 kilograms. Having said that, this prop does not force the motor to stall or labour much, and there is only a very small amount of "white water" (a few bubbles really)... but I'll report back when I get the radio gear in and give it a run at full throttle.

I also have questions surrounding the cluth... or lack thereof. It "appears" that the Talon weed eater I am using does not have a centrifugal clutch (although I could have sworn the line did not spin at idle before I ripped it to pieces)? The prop spins like mad even when idling, which makes it a little difficult to launch. Does anyone know if it is normal for the cheap weed eaters to have direct drive?

stacker9000 · 09-15-2009, 05:42 AM

I studied a little engineering in high school,more over airplane propellers,but figure the principle is the same,I would say your on the right track with your new prop,its just a matter of slightly altering the pitch to find the right performance and remember balance of the blades is important.As for engines most of the cheaper ones I have found are direct drive,and I believe it is uncommon to find a clutched trimmer engine.
Hope this helps,and good luck!CB