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post #11 of 23 Old 05-18-2013
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Re: Prop size for 4-107

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Originally Posted by pyewackette View Post
Gotcha, thanks for the info I will see what I can do to find a prop matching what you say.

Also, the ratio is 2:51 to 1.
P,

If your current prop is in otherwise good condition, you may want to check with a prop shop and see if they can flatten your pitch down closer to optimal. If the 16" diameter doesn't have clearance issues, then getting it reset will likely be a whole lot less expensive than buying a whole new wheel.

Just a thought...


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post #12 of 23 Old 05-18-2013
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Re: Prop size for 4-107

Pye:
Porfin has a good idea there.
Keep us in the loop on this please. Props are a bit of a black art and I'm always interested in learning more about prop sizing.

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post #13 of 23 Old 05-18-2013 Thread Starter
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Re: Prop size for 4-107

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Originally Posted by PorFin View Post
P,

If your current prop is in otherwise good condition, you may want to check with a prop shop and see if they can flatten your pitch down closer to optimal. If the 16" diameter doesn't have clearance issues, then getting it reset will likely be a whole lot less expensive than buying a whole new wheel.

Just a thought...
Thanks for the thought, I'll see what I can do.
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post #14 of 23 Old 05-18-2013
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Re: Prop size for 4-107

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Originally Posted by bobperry View Post
What you have now is what we call a "square prop" i.e. diameter and pitch the same and they seldom work well.
There's an interesting point. The same thing applies to engine bore & stroke ratios - when they are the same it is also called a "square" engine and it seldom works well either.

I understand that sail plans where the area is equally split between main & jib ("square"?) also don't generally work well.

Any math wizards here who can explain this curiosity?

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post #15 of 23 Old 05-19-2013
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Re: Prop size for 4-107

JonB:
I don't know about "square" sail plans. I'm not sure I would go along with that theory.
But for props diameter is the more important number so it figures you should give up pitch for more diameter. You size or diameter first then pick the pitch.

The way I size a prop is a method I have used very succesfully for many years. I pick up he phone and I dial:425-355-3669

This is the phone number for Max-Prop. I ask for Fred or Fredrique. They both know props well. Then I do exactly what they tell me. It never fails.

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post #16 of 23 Old 05-19-2013 Thread Starter
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Re: Prop size for 4-107

Interesting new development since the subject of resizing a prop came up. When I originally looked at the prop it had 16x16 stamped on the outer hub, but the inner hub had a 16x16 stamped but the last 6 was vague. It was still a 6 but it had scratch marks. Wen the subject of resizing came up I went to look again, still hard to make out, so I went to an old survey and there I found that the surveyor indicated a 16x12 prop. I went back the prop and took a wire brush to it and I can now see a faint stamp of 2 over the last six.

So, I am now assuming that the prop is a 16x16 re-sized to a 16x12. Have any idea how that effects things?
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post #17 of 23 Old 05-19-2013
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Re: Prop size for 4-107

Ok we know a few things:
I figured a 2.57 to 1 reduct gear and you have a 2.51 to 1, close enough.
Your prop is either a 16 by 16 or a 16 by 12.
16 by 12 is much close to my recommended 15 by 10 than 16 by 16.
But while it is clearly still too much prop I'm not sure that it is big enough to cause the problem that you are experiencing. But it is obvious by your symptoms that you are over propped.

Find a reputable prop shop. They can measure and tell you excatly what the dia and pitch are of your existing prop. Ask them if they can resize the prop to 15 by 10. Try that. Or buy a new three blade.

If you really love your boat and you have a birthday coming up, or better yet, your wife has a birthday coming up, giver her a new Max-Prop for her birthday. It's a lot like jewelry. She'll love it. Max-Prop will send you the new prop, already adjusted for what they know to be right for your engine and gearbox. You will love it and you will have far better performance in reverse due to the syrmetrical blade design of the Max-Prop.

And it all ended happily ever after.

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post #18 of 23 Old 05-19-2013
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Re: Prop size for 4-107

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Originally Posted by bobperry View Post
JonB:
I don't know about "square" sail plans. I'm not sure I would go along with that theory.
Interesting. It was something I read years ago and it was not explained, just one of those "That's just the way it is, nobody knows why" kind of comments.

Just out of curiosity, have you ever designed a "square" sail plan?

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post #19 of 23 Old 05-19-2013
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Re: Prop size for 4-107

JonB;
I never really thought about it. I design the rig prportions by eye with a target SA/D in mind. General rig proportions will be a fucntion of the type of boat I am after and the position of the keel.

So,,,I pulled out my own book and went through it looking at th sloop rigs I have designed. In almost every case of a masthead rig the fore triangle is much bigger than mainsail area. The closest I found to a "square rig" was, MOBISLE, LOON and WHITE EAGLE. They are not strictly square but they are close.
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Mobisle sail plan.jpg   White Eagle sail plan.jpg  

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post #20 of 23 Old 05-19-2013 Thread Starter
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Re: Prop size for 4-107

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Originally Posted by bobperry View Post
Ok we know a few things:
I figured a 2.57 to 1 reduct gear and you have a 2.51 to 1, close enough.
Your prop is either a 16 by 16 or a 16 by 12.
16 by 12 is much close to my recommended 15 by 10 than 16 by 16.
But while it is clearly still too much prop I'm not sure that it is big enough to cause the problem that you are experiencing. But it is obvious by your symptoms that you are over propped.

Find a reputable prop shop. They can measure and tell you excatly what the dia and pitch are of your existing prop. Ask them if they can resize the prop to 15 by 10. Try that. Or buy a new three blade.

If you really love your boat and you have a birthday coming up, or better yet, your wife has a birthday coming up, giver her a new Max-Prop for her birthday. It's a lot like jewelry. She'll love it. Max-Prop will send you the new prop, already adjusted for what they know to be right for your engine and gearbox. You will love it and you will have far better performance in reverse due to the syrmetrical blade design of the Max-Prop.

And it all ended happily ever after.
Here is what I plan to do; the bottom paint I use works really well, except for the prop and shaft. I assume it is because I don't change the zincs soon enough, the bottom paint wears off quicker on the metal than the hull. So, I have always wanted a second prop so that I could switch them off mid way through the year. Grind and paint the second one and have it put on. So, switch props as they get more difficult to keep clean.

If I buy a second used prop, 15x10, I will have one to switch on and off to clean and repaint. There is a local boat yard that has lots of used props for sale.

Besides, I can tell you right now, the prop as jewelry idea, it ain't going to fly with my wife. So, this is the best I can come up with for now. But, now that you mention it, she wants a Electro Scan waste system, so I can used that and claim it's like jewelery. Works for me.
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