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Prop relationship to bow high-low

Old 08-27-2020, 03:18 PM
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Default Prop relationship to bow high-low

I have a new 24 bay boat set up like this:
250SHO
10" set back
4 blade Yamaha HS4 21-T x 15 prop
T Top
Trolling motor
5 batteries in the console.
The boat seems to want a touch of tab to run right and with out porpoising and I thought the 4 blade would lift the stern.

What do you think about the pitch? Is it too much? Does that affect the bow lift?

Last edited by Bfish; 08-28-2020 at 04:29 AM.
Old 08-27-2020, 03:21 PM
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Pitch should be selected for WOT rpm.

Ideas:
Move weight forward.
Raise motor if possible without losing water pressure at WOT trimmed out.
Add a wedge plate if needed (first check that the motor is set to go all the way to max negative trim when trimmed all the way down.
Old 08-27-2020, 04:00 PM
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Which one are you trying to achieve , bow lift or stern lift?
Old 08-27-2020, 04:16 PM
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The pitch of a propeller is the distance the prop will move in one revolution if it were moving through a soft solid, like a screw through wood. For example, a 21-pitch propeller moves forward 21 inches in one revolution. Here's a good article that might help you.

https://www.louisianasportsman.com/f...rms-for-props/
Old 08-27-2020, 04:37 PM
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Originally Posted by Bfish View Post
Trolling mother
Sorry about your Mom. LOL
Old 08-27-2020, 04:43 PM
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I gotta ask.... why 5 batteries? That's roughly 200 lbs.
Old 08-28-2020, 04:27 AM
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Originally Posted by Lucky Bum View Post
I gotta ask.... why 5 batteries? That's roughly 200 lbs.
3 group 24's for the TM and 2 group 27's for the house and engine.
Old 08-28-2020, 04:28 AM
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Originally Posted by Unevensteven View Post
Sorry about your Mom. LOL
Ha! Need to edit that she was a fine woman.
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Old 08-28-2020, 07:06 AM
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Pitch will work like gearing, with the inches of pitch being how far it would theoretically move forward with one rotation, as Stoots mentioned above.With your typical load and calm seas, put the hammer down and see what you get for max RPM's, and what your speed is. This gives you a baseline. If that RPM number is close to the upper end of the max operating rpm range for you outboard, it's good in terms of pitch.

You have a four blade prop, which has a lot of surface area. Surface area helps decrease slip. If you have a 20" pitch prop (to keep the math easy) with zero slip it would go forward 20 inches for one revolution. If it is slipping 10%, it would only go forward 18 inches. If it slipped 50% it would only go 10 inches.

Slip also varies by speed. The faster you are going through the water, the less slip % you will have. So your boat with your prop will have a varying amount of slip, which decreases the faster you go. Now if your prop slip numbers are high, increased surface area, more blades, and blade shape helps. But everything is a compromise, and more blades and more surface area can hurt your top end as the slip decreases and the drag increases. So calculating prop slip would tell you if you could benefit from more or less surface area.

Next, leave the tabs all the way up "bow up". Tuck the motor all the way in. (Trimmed in as far as you can go.) Get up to your normal cruise speed, and try trimming out a little. Wait 30 seconds, and see how it handles. Now trim it out a little more. See how that works. Keep doing that until you feel that it's out too much, then go back to the spot where it was happy.

Now, do you still have porpoising? While running, is the top of the anti-ventilation plate visible, or is it completely buried? It should be running at or pretty close to the top. You can have a friend take a picture. That way, when yo stop, you can gauge how far the plate was from the top of the water. If it was four inches under, you might benefit from raising the engine a bit. Once you do, repeat all the above tests and see if you are better or worse.
Old 08-28-2020, 07:45 AM
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The more rake a prop has, the more bow lift it will provide and the better it is likely to behave at higher engine height relative to water.
https://www.mercuryracing.com/prop-s...-3-blade-rake/
Old 08-28-2020, 09:29 AM
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There are both stern lifting and bow lifting designs of props - consult an expert such as Prop Gods.
Old 08-28-2020, 09:54 AM
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Your engine is set back 10" and you are carrying all the weight in the console and aft of console plus the windage of the t-top on 24'. That leaves a light bow with a motor that has a lot of leverage.
I've found that it's a natural tendency to trim up too soon as you are running at close to cruise speed. As outlined above, get up to cruise RPM with the motor at either negative or neutral trim. You can pick the bow up a little from there with positive trim, but just a little to high at cruise RPM will start the porpoising. At that point you can accelerate a little, or trim back down. See what running flat, as in not bow high, does for your speed and fuel.
Hull shape has a lot do with tolerance to being too bow high, some boats run great with bow up, others will start to pitch from the lightest wind on the bow or smallest waves and then keep doing it until tab or trim fixes it.
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Old 08-28-2020, 04:21 PM
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Originally Posted by Clinker View Post
There are both stern lifting and bow lifting designs of props - consult an expert such as Prop Gods.
Can they do both at the same time ? Was my question for the solution for OP’s problem?
Old 08-28-2020, 04:27 PM
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One of the other issues is I can only get it up to 5500 rpm and maybe 50mph. I'm going to try some props and think something about 19p would be better. Powertech or Merc rev4. I want to stay with 4 blade
Old 08-28-2020, 08:17 PM
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A prop either lifts the stern or it doesn't. If it doesn't, that's how you get "bow lift".
In other words, if you want the bow up, you want a prop with as little lift as possible.

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