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Old 06-10-2024, 15:38   #1
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Prop sizing

I am noticing my boat can exceed max rpm and is not providing as much prop walk as I would like for docking.

I assume I am under propped? Should I increase diameter or pitch, or both?
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Old 06-10-2024, 16:01   #2
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Re: Prop sizing

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I am noticing my boat can exceed max rpm and is not providing as much prop walk as I would like for docking.

I assume I am under propped? Should I increase diameter or pitch, or both?
Talk to someone or two that makes props. Experience counts.
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Old 06-10-2024, 20:03   #3
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Re: Prop sizing

Need more data to determine if you are under-propped. Starting with the standard: what engine speed (RPM, actual at the engine) is reached at WOT (wide open throttle) with clean bottom and prop in calm water? And then what boat speed (through the water) is reached?

But most people try to minimize prop walk. I'm not sure I've ever heard of anyone trying to increase it. While it can be handy at times if you know how to use it, in general it reduces your control, especially to the other side.
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Old 06-10-2024, 20:26   #4
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Re: Prop sizing

It gets hard to standing turn in to stronger winds. While I’m in reverse I get blown bow down so I’d like pretty strong prop walk to fight it.

I hit about 6 kts at 2800 and about 6.5 kts at 3000 in really calm conditions. I sail about a kt faster is strong wind.

WOT I hit close to 3500
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Old 06-10-2024, 22:10   #5
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Re: Prop sizing

Helps to know what prop is on there and what engine you have and how the current prop fits in the aperture. There may not be room for a larger prop. I am assuming you have a 3 bladed right? When I sailed the DE38 I don't recall how much prop walk it had, but I could always make adjustments by giving a quick blast over the rudder to kick the stern around. Don't change a prop for how it performs in reverse in a wind. I often had to back out of a fairway if the wind was giving me trouble.
Yes, talk to a prop shop.
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Old 07-10-2024, 05:09   #6
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Re: Prop sizing

I’m getting ready to haul out for season (and work), but memory is I don’t have a lot of room in diameter. Boat has a Yanmar 4jh4ae.
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Old 07-10-2024, 05:15   #7
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Re: Prop sizing

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Old 07-10-2024, 05:28   #8
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Re: Prop sizing

I’m probably little chicken in reverse too...

I did make the turn with a 15kt side wind blowing me bow down the fairway, but I had to really put the coals to it in reverse and froward.

I came out not so hard initially, but realized I wasn’t going to get bow across the wind, so I went in reverse really hard then forward really hard. WOT both ways.

Getting some more speed built up in reverse gave me more time with my forward burst over rudder before I started making headway.

I do think if wind is coming hard down fairway I’m going to just dump the coals to it right off… generate as much walk as I can, build decent speed up in reverse… then drop the hammer in forward.

If that doesn’t work I can always stop, let myself go bow down and start reversing out.
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Old 07-10-2024, 06:08   #9
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Re: Prop sizing

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I do think if wind is coming hard down fairway I’m going to just dump the coals to it right off… generate as much walk as I can, build decent speed up in reverse… then drop the hammer in forward.

If that doesn’t work I can always stop, let myself go bow down and start reversing out.
When I read the above comment I wonder if THOMAS1985 might be using the term “prop walk” to mean something completely different then most sailors would understand.
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Old 07-10-2024, 06:13   #10
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Re: Prop sizing

What do you mean?

I’m referring to boat stern pulling to port in reverse.
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Old 07-10-2024, 06:20   #11
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Re: Prop sizing

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I’m probably little chicken in reverse too...

I did make the turn with a 15kt side wind blowing me bow down the fairway, but I had to really put the coals to it in reverse and froward.

I came out not so hard initially, but realized I wasn’t going to get bow across the wind, so I went in reverse really hard then forward really hard. WOT both ways.

Getting some more speed built up in reverse gave me more time with my forward burst over rudder before I started making headway.

I do think if wind is coming hard down fairway I’m going to just dump the coals to it right off… generate as much walk as I can, build decent speed up in reverse… then drop the hammer in forward.

If that doesn’t work I can always stop, let myself go bow down and start reversing out.
That last bit is a key. For every boat there's a certain amount of wind where you just won't be able to get the bow up into it in a tight space no matter how much throttle you use, you'll need more room to work. And boats tend to be more directionally stable stern to the wind, so it's sometimes easier to just back into the wind until you get to a spot with more room to maneuver.

As far as prop selection, it looks like Yanmar rates the 4JH4AE at 3000 RPM. So a conservative prop selection would probably have you hitting somewhere around 3200 with a clean bottom at WOT in open water.

From the pictures it doesn't look like you have much room to add diameter, so most likely the answer will be a bit more pitch. A prop design with more blade area would also bring RPM down a little and improve thrust at low speeds, but it will also increase drag under sail unless you change to a folding or feathering prop. Talking to a prop shop (and giving them information on current and target RPM, what the current prop is, etc.) will be worthwhile, they'll likely have some good guidance.


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When I read the above comment I wonder if THOMAS1985 might be using the term “prop walk” to mean something completely different then most sailors would understand.
I don't get the impression he's confused there. In my experience with most boats, leaning on the throttle hard at low speed or putting it in reverse while moving forward generates more prop walk than reversing gently. Situations that cause the prop to slip more in the water lead to more sideways movement for a given amount of forward or backwards movement.
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Old 07-10-2024, 13:13   #12
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Re: Prop sizing

Looks like that prop is as big as you’re going to get, (maybe a little too big, hard to tell) and I’d guess the pitch is probably correct too. As rslifkin says, there are some things you may just not be able to do if there’s too much wind.
A full keel and heavy boat; I don’t think boats like that get a whole lot of prop walk, but I haven’t tested too many.
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Old 07-10-2024, 13:48   #13
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Re: Prop sizing

My boat definitely handles far better out in the big stuff than in the marina

I may just have to give up and concede I’m coming out stern first in strong wind blowing down the fairway
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Old 07-10-2024, 14:45   #14
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Re: Prop sizing

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My boat definitely handles far better out in the big stuff than in the marina

I may just have to give up and concede I’m coming out stern first in strong wind blowing down the fairway
No shame in that.
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Old 07-10-2024, 16:01   #15
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Re: Prop sizing

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I don't get the impression he's confused there. In my experience with most boats, leaning on the throttle hard at low speed or putting it in reverse while moving forward generates more prop walk than reversing gently. Situations that cause the prop to slip more in the water lead to more sideways movement for a given amount of forward or backwards movement.
I certainly could be wrong, but it sounded to me like he was equating prop walk with reverse speed as opposed to the tendency to turn.

I have a lot of experience with boats that have very poor rudder authority in reverse at speeds you would want to be moving at in close quarters. With a boat like that prop walk is really a significant advantage when you plan ahead and know how to use it. Reducing prop walk for those boats actually make them a lot less predictable to handle.

Prop walk: It's not always bug, it can be a feature!
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