WideOpen Throttle (WOT)

Report the performance of your boat, supply as much information as possible (weight, HP, prop, speed, etc.).
Subject: Make the Glen-L design name the subject. If the design already has a posting, add your information to that post.

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Al
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WideOpen Throttle (WOT)

Postby Al » Thu Feb 26, 2009 3:29 pm

When selecting a prop, we are told to always select a prop that will permit operating at WOT. My question is why ? Who operates an engine (any engine for that matter) at WOT ? If for example I can run at 30MPH using a 13" prop at 3000 rpm, why would I want to use a 12" prop at 4200 rpm ? An engine is designed to work so what's the problem operating it at a lower rpm ? Can someone please explain this "wide open throttle" thing to me ? Thanks

Al
building Audeen

PS Does anyone have a good LH 12 or 13 " prop for sale ? :)
You know that you're having a good day when you don't accidently drill a hole through the bottom of your boat !

Nova SS
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Postby Nova SS » Thu Feb 26, 2009 3:34 pm

thats a good queston. My guess on this is so you cannot ever over rev the motor. If you prop it so you can only go to max rpm at WOT then you will never be able to over rev it. just my thought on this.

53Evinrude
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WideOpen Throttle (WOT)

Postby 53Evinrude » Thu Feb 26, 2009 4:53 pm

On a related note IMHO why aren't boats geared more toward overdrive to keep the engine RPM down & the fuel economy up? Seems like all I read about is reduction gearing.

Also, I swear I sometimes hear a boat going across the lake that sounds like it's "going through the gears". Am I nuts?

On an unrelated note, I've also wondered why production vehicles of all types aren't geared higher. My 85 Escort 5 speed got 30 mpg at 70 mph due to high gearing. Now they advertise 28 mpg like it's something to be proud of???

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Bill Edmundson
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Postby Bill Edmundson » Thu Feb 26, 2009 4:56 pm

If you use too small of a prop, you can over rev the engine. If You use too much prop then you can overload the engine. Both, conditions will damage the engine.

There are some 2 speed marine gears.

Bill
Last edited by Bill Edmundson on Thu Feb 26, 2009 4:57 pm, edited 1 time in total.
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Mr Hot Rod
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Postby Mr Hot Rod » Thu Feb 26, 2009 4:57 pm

Al and I are almost neighbors, so I'll take a stab at it !

Can't say it any better, so here's an explanation from the Velvet Drive Installation Manual :

The propeller is selected to load the engine and still permit full power to be developed. The propeller must allow the engine to come up to rated speed. It is incorrect to use a propeller so large that the engine will be overloaded, because this will not only reduce the power delivered to the propeller shaft, but more importantly it will cause abnormally high loading within the engine. This can result in destructive pressures and temperatures which cause premature bearing and valve failure. For ski towing, it is best to select a propeller which will permit the engine to maintain rated RPM when under load.

The book Inboard Motor Installations written by Glen L. Witt and Ken Hankinson covers this topic and a whole lot more !
It can be ordered through Glen-L's Bookstore : http://www.boatdesigns.com/products.asp?dept=34

Hope this helps !
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53Evinrude
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WideOpen Throttle (WOT)

Postby 53Evinrude » Thu Feb 26, 2009 8:45 pm

There are some 2 speed marine gears.

Then could it be assumed that 3 or 4 speeds would be better? I think the boat I've heard "shifted" more than twice. Sounded like it was moving pretty good.

The whole RPM/prop sizing thing seems very mystic to me. Guess that's why you're the engineer & I'm not!

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Bill Edmundson
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Postby Bill Edmundson » Fri Feb 27, 2009 4:47 am

The 2 speed marine gears, I think, are for something like a large off-shore fishing boat. One gear for running and one for trolling.

My guess is you're hearing a big turbo-charger kick in. I love the sound when mine kicks.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

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Dave Grason
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Postby Dave Grason » Fri Feb 27, 2009 6:07 am

Hey, let's muddy the water a little more on the prop thing.

I had a buddy with a 16ft Bayliner powered by a Force 115hp outboard. Almost from the minute he bought the boat, he talked excitedly about getting a stainless steel prop for it. He extolled the virtues of these props as being such performance boosters.

NOT.

When he got the SS prop on, the boat did not perform as well as the stock aluminum prop. Top end speed was no better. But from a dead stop the boat really struggled to get out of the hole and onto a plane. Both props were the exact pitch - identical in all respects except for the material they were made from. So why the difference in performance?

Simple. From a dead stop, when the throttle was punched, the aluminum prop's blades would deflect and bend. This was like downshifting in a car. It actually changed the pitch to a lower value and help the boat jump out of the hole. Wow, you learn something new everyday. The SS prop was too stiff and strong to allow that.

And NO, they didn't give him his money back on the SS prop. Later another prop shop gave him credit on trade stuff. So things worked out but that was a lot of effort to get nowhere.
Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

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Mark F.
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More Power!

Postby Mark F. » Fri Feb 27, 2009 9:52 am

For those of us with small boats and underpowered outboard engines, operating at WOT gives us the most power. I tried three different props on my 12' v-hull with a 25 hp motor. The 9" pitch aluminum prop gave awesome acceleration and along with that came the ability to pull skiers out of the water more effectively. However, it was very easy to overspeed the engine since there was a lot of throttle still available at WOT rpm. This acceleration came at the expense of top-end speed, in this case a 6 mph penalty. The 15" pitch stainless prop gave the highest top-end speed, but acceleration was slooooow. The engine couldn't produce the torque to push that large pitch effectively. I finally settled on an 11" pitch stainless prop. With nobody in tow, it is possible to overspeed by 200 rpm. However, with a person in tow, it will not overspeed. This prop also has great acceleration, which makes it a fun prop for just doing donuts and putting the boat through its paces.

Each prop has a specific design that makes comparison between props of the same pitch, but different design a little bit of an "apples to oranges" situation. I stepped up to the Turbo Hot Shot stainless prop and was very pleased with its cornering and acceleration characteristics. Homebuilt hulls of odd sizes present a difficult situation for even an expert dealer to make a good recommendation on their first attempt. Turning Point recommended a 16" or 17" pitch prop in my situation. Their computer model was way off since even a 15" pitch prop was a dog on my hull.

We plan to mess around pulling up multiple skiers this coming season. We will run the 9" prop all day. It has the potential for overspeed, but you just have to train the driver to monitor the tachometer. The overspeed limiter is in their right hand. My 15 year old son has throttle control down to an art when we have a prop installed that can potentially overspeed the engine.

Another consideration is your type of engine. If you are talking about an inboard with a V-6 or V-8 block, you have a lot of torque available and may want to be "nice" to your engine by selecting a prop that will run at a lower rpm. In the small two-stroke outboard category, it might be better to just consider it a big, expensive weed eater that is meant to be run at WOT and choose your prop accordingly. We run ours hard with Yamalube and it's amazing how much fun even a 25 hp motor can be!

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Al
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WOT (and props)

Postby Al » Fri Feb 27, 2009 3:40 pm

Great discussion. Thanks to all. I think that I may have it figured out with all of your help.
Also enjoyed studying your photo's Bill and your interview with Gayle.
I have also followed your progress Duane and fine building job on the "Hot Rod".
I got hung up a little bit with an engine rebuild last summer. Following an early spring test run of the engine I should be back at Audeen construction very soon.
You know that you're having a good day when you don't accidently drill a hole through the bottom of your boat !

Nova SS
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Re: WOT (and props)

Postby Nova SS » Fri Feb 27, 2009 3:46 pm

Al wrote:Great discussion. Thanks to all. I think that I may have it figured out with all of your help.
Also enjoyed studying your photo's Bill and your interview with Gayle.
I have also followed your progress Duane and fine building job on the "Hot Rod".
I got hung up a little bit with an engine rebuild last summer. Following an early spring test run of the engine I should be back at Audeen construction very soon.


Hey Al, I WISH the Hot Rod was mine but I think Paul would have someting to say about that...Good luck with your Audeen.

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Al
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WOT

Postby Al » Sat Feb 28, 2009 5:21 am

Oops! :oops: Sorry Paul. The boats are relatively easy to identify but the names a little more difficult.
You know that you're having a good day when you don't accidently drill a hole through the bottom of your boat !

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Bill Edmundson
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Postby Bill Edmundson » Sat Feb 28, 2009 7:14 am

Al,

Thank you. If I wasn't such a slow talking southerner, I could have probably done the presentation in an hour. I think it's the first time in my life that I've talked for 1 1/2 hours.

Glad someone got something out of it.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

upspirate

Postby upspirate » Sat Feb 28, 2009 8:24 am

some performance boats with V-drives use a two or three speed modified trans

Nova SS
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Postby Nova SS » Sat Feb 28, 2009 1:02 pm

upspirate wrote:some performance boats with V-drives use a two or three speed modified trans


Yep...x2 :)


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