Prop to pad Explanation

[Loxboy1]

Hi everyone. Im in the process of testing props. My motor is on a hydraulic jack plate and the hull has a pocket. Reading around the fourms i seen alot of talk about prop to pad height. Can someone please explain this to me and inform me as to where more or less i need to be?

[Gorilla0178]

Do you have a low water pickup? What kind of props?

[Spdrcr29]

Level the boat, level the prop shaft, 3.5 inches below pad is a decent starting point on the average vee hull. Start raising it incrementally until you lose speed, get scared, or get close to minimum recommended water pressure. If you get scared, ease off the throttle and start dropping the jackplate, don’t chop the throttle back.

Smaller, lighter hulls, motors with low water pickups (LWPs) and notched hulls/setback brackets can generally run higher (smaller prop to pad #), some can even run above the pad height.

[nitro_rat]

"prop to pad" is referring to what's known as the "x" dimension. there are some good diagrams on here to clarify but basically it's the difference between a straight line projected back from the bottom of the pad of the boat and a straight line drawn through the centerline of the prop shaft.

[Gordon02]

Here are some drawings I made to help explain the terms-

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[Loxboy1]

Ahh okay so i wasnt far off with my thinking. I have a low water nose cone that i just had done so ive been messing with props, im picking up a 24p tempest plus tomorrow to try this weekend. I had tried a 26 tempest plus and spun it to 5200. I need 5800. I knoticed that once i brought the jack plate up to 5+ i lost speed so im guessing that means im to far above the pad.

[Loxboy1]

What is a good starting refrence, or do i just keep jacking untill i get where i want to be. This is on a 20’ flats boat with a 225 efi

[moatorbotin]

Ahh okay so i wasnt far off with my thinking. I have a low water nose cone that i just had done so ive been messing with props, im picking up a 24p tempest plus tomorrow to try this weekend. I had tried a 26 tempest plus and spun it to 5200. I need 5800. I knoticed that once i brought the jack plate up to 5+ i lost speed so im guessing that means im to far above the pad.
you're heading in the right directions. As mentioned above, lift the motor until you lose speed or water pressure, or the boat handles poorly. It's all experimentation, what hull and motor combo are you running? That info will be helpful and someone here likely has a similar set up and can give you some starting points.

Sometimes the experiementatin is the fun part.

[Loxboy1]

you're heading in the right directions. As mentioned above, lift the motor until you lose speed or water pressure, or the boat handles poorly. It's all experimentation, what hull and motor combo are you running? That info will be helpful and someone here likely has a similar set up and can give you some starting points.

Sometimes the experiementatin is the fun part.

its a 1996 2020 action craft with a 1996 225 efi merc. Bobs machine 6inch set back jack plate, nose cone was done by a guy in Mims FL

[Gordon02]

I'm not a Tempest user, so my comments are based on appearances alone. Those look like a general purpose prop. They don't have much rake and are not known for providing much bow lift. This prop/speed thing is addictive!

Let's get the money pit started....Jackplates add set-back for more leverage of the outboard. More leverage rearward allows less leverage downwards (allowing you to raise the propshaft because it replaces having the prop leverage further below the pad/surface). Typically those of us playing with height and speed adjustments on a Pad/V will want to use high-rake, hi-lift props. I don't remember the Tempest being among those providing either. I'm a big Trophy fan - they're durable, great holeshots, and run within a few mph of the best chopper/over-hub models.

Here is another drawing to think about......Rake, Lift, setback, ect are all good until the driver "over-trims". You want 100% of the thrust going straight ahead, not pushing the back down and front up. You are trying to fly the boat, not plow with it! Gordon

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[donmac]

"X" dimension, that's a term I've never associated with outboards! I always thought the "X" dimension was the crankshaft centerline height on an I/0!

[XstreamVking]

I'm thinking that raising the prop shaft to the moon will not help that particular hull. You need some prop in the water to give it bow lift. Raising it for shallow water operation is fine. For speed it will not help imo. Put the prop in the water and run the tempests and trophy.s. Your motor pulling a 24p prop to 5800 should net low to mid 60's. That's IF you can get the bow to lift and eliminate some hull drag. jm2cts.

[moatorbotin]

I'm not a Tempest user, so my comments are based on appearances alone. Those look like a general purpose prop. They don't have much rake and are not known for providing much bow lift. This prop/speed thing is addictive!

Let's get the money pit started....Jackplates add set-back for more leverage of the outboard. More leverage rearward allows less leverage downwards (allowing you to raise the propshaft because it replaces having the prop leverage further below the pad/surface). Typically those of us playing with height and speed adjustments on a Pad/V will want to use high-rake, hi-lift props. I don't remember the Tempest being among those providing either. I'm a big Trophy fan - they're durable, great holeshots, and run within a few mph of the best chopper/over-hub models.

Here is another drawing to think about......Rake, Lift, setback, ect are all good until the driver "over-trims". You want 100% of the thrust going straight ahead, not pushing the back down and front up. You are trying to fly the boat, not plow with it! Gordon

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I've always thought the trophy holeshot was not so great, mid range porpoise on two different hulls (progression 24LD, and new style Vegas XT) was annoying, and top end was decent.

[Radiant]

The flats boats can be tricky to set up. I’m imagining your action craft isn’t the lightest, but I don’t see why low 60s isn’t possible. What kind of load are you running? Fishing (trolling motor, live wells, etc.)? I’m not sure if a nose cone will benefit your setup. Did you try blocking off intake holes first?

Although I’ll share my experience. I have a 20ft lake and bay hull which has a pad and when properly set up, is a fast hull. With a 2.4 200 and a nose cone, I found that the old 22 chopper I had would get me about 69 mph and 7000 rpm, prop had no cup left and was slipping a bit. With a better prop to get my revs down I’m sure low 70s was more than attainable. People say the nose cone hurts speed under 85, but i found that the higher I raised the jackplate, the more stable the boat felt and the faster I could go. The motor was a lot higher than a stock case would’ve allowed.

I sold the 200 and am not running a 135, 2.0 liter with a stock case. I’m spinning a 25 tempest which has decent bow lift. With over 30 gallons of fuel, I ran 60 mph. Boat wasn’t so stable but I was able to drive it to get that number. If it had solid mounts it would be much better, you can see how rubber mounts move around. I’m sure I can get another mph or two.

It depends on your setup. You can make the nose cone work like I did, I wish I could’ve tried the 200 with a standard lower and scoops or holes blocked off. You might have to keep the motor pretty low to provide the leverage to bring the bow up, if you try running the motor high, and trimming up, I imagine the bow won’t come up.

Keep us updated

[Dave Strong]

You are not going near fast enough to need a coned lower, any positive trim will kill speed.

Dave

[FUJIMO]

"X" dimension, that's a term I've never associated with outboards! I always thought the "X" dimension was the crankshaft centerline height on an I/0!
that is correct. nothing to do with o/b's.

[FUJIMO]

https://www.screamandfly.com/attachment.php?attachmentid=441046&d=1515598370 (https://www.screamandfly.com/attachment.php?attachmentid=441046&d=1515598370)

[FUJIMO]

I'm thinking that raising the prop shaft to the moon will not help that particular hull. You need some prop in the water to give it bow lift. Raising it for shallow water operation is fine. For speed it will not help imo. Put the prop in the water and run the tempests and trophy.s. Your motor pulling a 24p prop to 5800 should net low to mid 60's. That's IF you can get the bow to lift and eliminate some hull drag. jm2cts.
times two.

[Loxboy1]

You are not going near fast enough to need a coned lower, any positive trim will kill speed.

Dave

well i needed a low water pickup. I spent weeks looking for options and it was eather a bobs or sending it to this guy in FL, his work was much cleaner so i sent it to him. Im not looking fir speed just get the boat running right with the propper rpms

[big tom]

Not close to the same set up, but I was going to do tempest's on my 31 twin cc then I called BBlades. Raised my motors up 1 notch. Went from offshore series 4 blade 22p @5800 62 mph to 24p BBlade o/b series 5800 69-70 mph 1in dia larger prop. Worth looking at. they are a little pricey..

[donmac]

Not close to the same set up, but I was going to do tempest's on my 31 twin cc then I called BBlades. Raised my motors up 1 notch. Went from offshore series 4 blade 22p @5800 62 mph to 24p BBlade o/b series 5800 69-70 mph 1in dia larger prop. Worth looking at. they are a little pricey.. the BBlades work good! picked up 5mph [73-78] on a Nordic 22 changing from Bravo 1 ,no other changes!

[big tom]

I also gained big in the mid-range cruise. 4000 rpm 43-45 mph depending on amount of fuel im carrying.

[Radiant]

What’s the name of the guy who did your cone?

[Loxboy1]

Update!!

Disclamer:: i was running in a fresh water canal with just me on board and about 20gallons of gas.

24 tempest plus, jack plate raised to 4 with positive engine trim, 5800 rpms 58mph. Blowing a real nice rooster tail, However, steering is extremly tight, i experienced this with the other props i tried last weekend with the jack plate that high as well.
i also didnt get much bow lift that i could tell.

just for compairison i put my 19p mirage plus back on. Boat jumps on plane in seconds. Jack plate up at 2 with slight positive trim 57mph at 5800~5900rpms, i have to be carefull with this prop as i can go past max rpms if im not paying attention. Much better bow lift. No rooster tail at all. And steering is soft as silk.

[XstreamVking]

That's good for that 2020. They are designed to plane fast and run flat. You have a lot of wetted surface.

[Loxboy1]

That's good for that 2020. They are designed to plane fast and run flat. You have a lot of wetted surface.

Buy why am i getting the mph/rpm from a 24 and a 19? Does this mean my hull likes more prop in the water? Should i be looking into a 4blade??

[Dave Strong]

Try a Trophy.

Dave

[nitro_rat]

Buy why am i getting the mph/rpm from a 24 and a 19? Does this mean my hull likes more prop in the water? Should i be looking into a 4blade??

prop slip is a factor. this will lower speed/increase rpm's. also not all props run true to pitch number so you 19 could measure 20 or 21 and your 24 could measure 23 or 22. If the props are of unknown provenance they could have been worked to a different pitch in the past and not marked...

slip is probably the main factor in your case.

[Radiant]

Sounds like you’re running good with the mirage. Maybe try the next pitch up. The tempest with the jackplate higher and trimmed up is probably just slipping. When you drop it in the water to about the same level as the mirage, how fast are you running and at what rpm?

[Loxboy1]

Try a Trophy.

Dave

what pitch?

[Loxboy1]

Sounds like you’re running good with the mirage. Maybe try the next pitch up. The tempest with the jackplate higher and trimmed up is probably just slipping. When you drop it in the water to about the same level as the mirage, how fast are you running and at what rpm?

if i run the tempest at the same height as the mirage. Its like 4900~5000 wide open.
Ive been trying to prop the boat to be able to run with the jack plate up more but if all im doing is slipping, then i dont see the point.

[Dave Strong]

what pitch?

24, sometimes takes alot of trying to find the right one. Start taking notes on what prop, rpm, speed and handling.

Dave

[Loxboy1]

The pick where the prop shaft is level with the pad is where i ran the tempest, the other 2 are where i ran the 19 mirage

[Loxboy1]

24, sometimes takes alot of trying to find the right one. Start taking notes on what prop, rpm, speed and handling.

Dave

why would the steering pretty much feel locked up? Even with the tempest at 2 on the jack plate the steering is easy once i bring it up its super hard

[XstreamVking]

The ideal is to have the motor in a neutral trim position. All the power is going fwd. and pushing the boat. When you trim a long gearcase it causes drag. (This is one reason you want a bow lifting prop.) Just experiment with it and just like you are doing note where and when things (good and bad) happen. This is the fun part of boat ownership.

[Dave Strong]

why would the steering pretty much feel locked up? Even with the tempest at 2 on the jack plate the steering is easy once i bring it up its super hard

Normal, just part of running. Sometimes 4 blade props have less torque steer than 3 blades. I get torque steer running tempest but have non with a Trophy.

Dave

[Loxboy1]

Normal, just part of running. Sometimes 4 blade props have less torque steer than 3 blades. I get torque steer running tempest but have non with a Trophy.

Dave

im going to start looking around for a trophy

[Gordon02]

Harder steering or a hard pull to the left felt at the steering wheel is coming from the almost surfacing prop blades - it's the "paddlewheel" affect.

Picture an old river steamboat with the large paddlewheel pushing the boat up and down the river. Your prop's blades are getting a good bite at the bottom, but finding turbulence on the top of the rotational circle. As this happens on standard rotating prop, the prop is trying to walk the boat's stern/rear hard right - just like the steamboat's wheel. This is all happening at the rear of the outboard too, so the stress is transferred to the steering wheel. Gordon

[Gordon02]

Over-trim is ugly and causes ugly things...... Besides laminar flow being disrupted, the feed to the prop is disturbed, and the thrust result is not efficiently applied to moving the boat forward.

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[Loxboy1]

Over-trim is ugly and causes ugly things...... Besides laminar flow being disrupted, the feed to the prop is disturbed, and the thrust result is not efficiently applied to moving the boat forward.

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thanks for this!!!

question, if im switching from a 24p 3 blade do i need to go lower on the pitch when going to a 4 blade? Or is it the same? Props are measured by how many “inches” the would travel forward in one revolution correct? So if im running a 24p 3 blade then i would still need a 24p in a 4 blade?

[Dave Strong]

thanks for this!!!

question, if im switching from a 24p 3 blade do i need to go lower on the pitch when going to a 4 blade? Or is it the same? Props are measured by how many “inches” the would travel forward in one revolution correct? So if im running a 24p 3 blade then i would still need a 24p in a 4 blade?

No Trophy is smaller diameter.

Dave

[Gordon02]

The pitches are the same regardless of 2, 3, or 4 blades. The 4 blades will have a higher "developed blade area" meaning that there is more surface area to push/propel the boat. This term is different than pitch and diameter yet both have a direct relationship -

Looking at a propeller from the rear and looking at the theoretical circle, the amount of blade area is how much of that circle is filled. Naturally a 2 blade of the exact pitch would have less "blade area" than an identical pitched 3 blade would have, and the 4 blades will fill that circle even more. These factors impact a prop's ability to lift a heavy boat out of the hole and carry the bow at speed. I wrote an article many years ago for Bass & Walleye Magazine on this subject of Developed Blade Area. I'll try to post it below - Gordon


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[Jimboat]

"prop to pad" is referring to what's known as the "x" dimension. there are some good diagrams on here to clarify but basically it's the difference between a straight line projected back from the bottom of the pad of the boat and a straight line drawn through the centerline of the prop shaft.

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[XstreamVking]

From what i'm seeing here, a 21p Mirage + will be the prop. About 3'' below the bottom for speed, deeper for cruising.