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Thrall
05-19-2010, 12:18 PM
Not for my boat, but a question.
Guy at work has a Supra he bought new a couple years ago.
This winter he was telling me he thought he hit something b/c every blade on his prop has a fairly sharp bend "back" (towards the front of the boat) at the end of the blade, maybe 1/2 to 3/4" from the outside edge, just at the tip.
Prop is an OJ 14.xx X 14.
He took it in to a prop shop and the guy was like "yeah, you hit something for sure" then someone popped out of the back room and said that was a custom job for more hole shot.
Seems counter-intuitive as it's effectively reducing the prop dia and cup.
Said the boat runs great, no vibrations, good hole shot, 42 mph top end on a 340hp 22' Launch.
Anyone seen this before?

FrankSchwab
05-19-2010, 01:25 PM
Hopefully Eric will be along shortly.

Might be some attempt at mimicking the winglets (turned-up tips) on modern jet wings.

/frank

vision
05-19-2010, 02:57 PM
My bet is that it is normal for that prop. But Eric will give us the correct answer.

Thrall
05-19-2010, 02:58 PM
My bet is that it is normal for that prop. But Eric will give us the correct answer.

This definately does not look normal. Don't have pic to post, but have seen pics and it literally looks like the ends of the blades are folded close to 90deg.

Jesus_Freak
05-19-2010, 11:28 PM
Hopefully Eric will be along shortly.

Might be some attempt at mimicking the winglets (turned-up tips) on modern jet wings.

/frank

Except that the flow on modern jet wings is primarily aligned with the winglets, while the flow leaving the prop blade could have a strong radial component (perpendicular to the winglets).

Yes, Eric, what do you think? ;)

FrankSchwab
05-20-2010, 01:30 AM
But the winglets are designed to reduce wingtip vortices - partially caused by air sliding radially down the wing; the differences between upper and lower surface generating a rotation. Note that the wing on a jet is not straight; it's swept back; the sweep also helps to create this flow.

Which description, remarkably, matches well with a prop with the tips bent forward. The direction of lift from the prop is forward also; similar to an upward pointing winglet where the lift of the wing is upward.

I'll stop now.

Where's Eric?

/frank

EJ OJPROP
05-20-2010, 06:41 AM
Eric had an outing with his son. Without seeing the "custom Job" it is difficult to respond to. Thrall, if your guy wants to send it in I will be happy to take a look at it. I would say though that a bend at the tip of the blade would not help low end.

Jim@BAWS
05-20-2010, 07:34 AM
I have some land in South Florida for sale REAL CHEAP that has a perfect shallow lake to run that prop in.

Cant wait to hear the end of this story.

06197ttlq9-footer
05-20-2010, 08:51 AM
O.K. I'm no prop expert...But I'm fluent in SMARTA$$! Dude from the back of the shop was being a SMART A$$. Making fun of the guy who customized his prop "himself" by hitting some thing hard enough to bend the tips of the blades. We use that term for anytime you screw up boat truck car etc. You show your buddy the bent prop and he says "nice Work" you say it's custom. Works especially good when you crash something and now pieces are missing. LOL

my guess is he hit something while in REVERSE.

PICS?!?

Thrall
05-20-2010, 09:14 AM
I know it sounds weird, and no I don't have pics. I still don't think it's right either.......does sound like he got into something in reverse.
Eric, I agree, this does not look like it would help anything, low end included, however, he's convinced he got something better than stock and apparently the guys at the store were serious about it being a custom mod to his prop, otherwise I'd think they would like to get some of his $ to straighten it.
He still claims 42 mph GPS on flat water with a 22' wakeboat, so it doesn't appear to be hindering performance.
Only reason I could think this would be done intentionally would be to gain clearance to the hull, but any newer wakeboat should be able to run a 14-1/2" dia prop, right?

EJ OJPROP
05-20-2010, 09:23 AM
Clearance would not be an issue on the Supra. If the prop is the original that came on the boat then the "custom" work was completed on a ramp or a trailer. I have seen plenty of damaged props that have what you could say is a uniform bend, but a bend is a bend.

Jim@BAWS
05-20-2010, 10:16 AM
We had a prop in here not to long ago that had a whole blade missing.

We offered a new customer a 25% discount

It was 75% balanced

5280Hawk
05-20-2010, 10:20 AM
Most bends are repairable right?

EJ OJPROP
05-20-2010, 10:26 AM
Most are, depending on how far the blade is bent. If the blade is bent all the way into the hub area we typically will not repair it.

06197ttlq9-footer
05-20-2010, 10:40 AM
Here are some pictures of some great custom work....Gotta love google :) does your buddy's prop look like this?

mrprostar
05-20-2010, 10:57 AM
Here is my custom prop job. Really didn't take that long either. :rolleyes:

Thrall
05-20-2010, 11:17 AM
Clearance would not be an issue on the Supra. If the prop is the original that came on the boat then the "custom" work was completed on a ramp or a trailer. I have seen plenty of damaged props that have what you could say is a uniform bend, but a bend is a bend.

I believe you. He may need some convincing. I'm going to send him your info so he can here it from "the man."
I told him no way I thought that was right, but someone convinced him it was legit.

Thrall
05-20-2010, 11:19 AM
Here are some pictures of some great custom work....Gotta love google :) does your buddy's prop look like this?

Hahaha..no! Looks much more "professional.":rolleyes:
I only asked this queston here because I didn't buy the custom work story either and if Eric says it's messed up then it's messed up.
Still don't understand why the shop didn't offer to fix it for him.

EJ OJPROP
05-20-2010, 11:31 AM
I won't say it's messed up until I see it. I am saying I would not bend the trailing edge back to gain low end. Am very interested in seeing the custom work though.

Thrall
05-20-2010, 12:09 PM
I wouldn't think you'd put a sharp "lip" or bend anywhere or any direction to improve performance. I don't remember much from fluid dynamics classes other than smoother is generally better.

Jesus_Freak
05-20-2010, 12:57 PM
But the winglets are designed to reduce wingtip vortices - partially caused by air sliding radially down the wing; the differences between upper and lower surface generating a rotation. Note that the wing on a jet is not straight; it's swept back; the sweep also helps to create this flow.

Which description, remarkably, matches well with a prop with the tips bent forward. The direction of lift from the prop is forward also; similar to an upward pointing winglet where the lift of the wing is upward.

I'll stop now.

Where's Eric?

/frank

Great points. I could pick at the semantics of your details, but dont have the time/energy. In short, I agree with you. I propose three issues:

1) The "radial" flow leaving the prop is larger (relative to the axial component) on a prop than a typical airplane wing, including Goertler effects.

2) I have seen cases, at certain approach Reynolds numbers, where winglets actually cause more harm than good for aircraft.

3) Large obstructions on the suction surface of prop blades most likely wont help anything. Let's just say that I have more than a "gut-feel" on this. ;)

FrankSchwab
05-20-2010, 01:49 PM
You win.

(The message you have entered is too short. Please lengthen your message to at least 10 characters.)

Jesus_Freak
06-09-2010, 12:53 PM
Here is a link to recent journal article that pertains to this thread: http://apps.isiknowledge.com/InboundService.do?Func=Frame&product=WOS&action=retrieve&SrcApp=Alerting&UT=000277444800004&SID=2EFoGoFlOi6ceenG213&Init=Yes&SrcAuth=Alerting&mode=FullRecord&customersID=Alerting&DestFail=http%3A%2F%2Faccess.isiproducts.com%2Fcus tom_images%2Fwok_failed_auth.html

I dont know what all they talk about (havent ordered the article), but the short version is that they are putting spoilers on the edges of prop blades and playing with the designs to find the optimum under a certain set of conditions.