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PostPosted: 11 Jul 2016, 10:26 
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Location: CT
There has to be a way of calculating how high to raise the mounting point based on the anticipated top speed of the boat and the amount of offset based on how quickly displaced water will rise to fill the void behind the boat and then based on the speed of the boat @ top speed. I should from this be able to determine how much the water will come up after clearing the transom in order to set the height of the new mount accordingly...but I can't find the information I need.

I tried guestimating by taking measurements off of several different manufacturer's setups but there doesn't appear to be a direct relationship between offset distance from the transom and the distance above the bottom line of the boat that the cavitation plate ends up set to. I thought perhaps this would have been due to the top speed of the boat but the aftermarket manufacturers of these tilt-trim setups have no way of knowing what that would be for all applications.



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2002 Alumacraft 1436LT w/ 1984 Mariner Tiller Converted to Remote & 55# Minn Kota Terrova 12v (removable)

1985 Bass Tracker III - restoration posting coming soon w/ 1988 Mercury 60hp 2-Stroke & 55# Minn Kota Terrova 12v shared w/ my 14'


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PostPosted: 11 Jul 2016, 11:03 

Joined: 23 Oct 2015, 16:29
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Location: Twin cities, Minnesota
Its a waste of time for mfrs to try to make them perfect, it's a whole lot easier for them to make sure the prop/lower unit is low enough that the prop doesn't vent/cav/blow out and most customers will be happy enough


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PostPosted: 11 Jul 2016, 11:10 
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Only one I ever heard of was STILL a ballpark ... as the rig is highly dependent on the propeller being run.

I've done extensive prop testing, one day between 2 boats (22' deep-V CC and a 25' mod-V heavy cabin boat) using the identical OB motor. Same 4-blade prop on his boat added stern lift, but on my boat it added bow lift. One cannot also assess or judge the performance of props of same alleged diameter and pitch between brands, due to balded profiles or treatments that can significantly alter/improve or ruin YOUR performance. Among just a few of the variable are material, blade thickness, rake, cup, and use of vent holes (allow wheels to spool up faster to RPMs, until the force of the water outside the holes essentially 'closes' them and they no longer vent, i.e., read 'slip').

For larger motor and boats the 'golden rule' was 1" of height raised for every foot of setback. Note this implies that one had set the correct motor height set for a specific prop well beforehand.

FWIW my top 2 boating pet peeves are:

  1. Most OBs from dealers or POs are mounted TOO DEEP!
  2. Most boats wear the WRONG prop (too much pitch)!

Dealers are known for #1 to alleviate call-backs from new boaters, who don't realize that yes, on deep-V hulls at speed you may need to trim in the OB, even by a large amount, to cut an aggressive turn without venting.

And for #2, OB makers are known for publishing as favorable speed/efficiency numbers as they can (Yamaha is the prime offender here ...), but rigging lightly equipped boats with the 'fastest' prop it can wear. Then Mr. New Boat Owner goes out, buys that motor, loads his rig down with 100s of pounds of more weight than at which she was tested/rated ... and then complains ...



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#2) 25' Parker refurb from EMPTY hull http://www.classicparker.com/phpBB3/vie ... p?f=15&t=6
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PostPosted: 11 Jul 2016, 11:36 

Joined: 23 Oct 2015, 16:29
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Location: Twin cities, Minnesota
It's not clear to me what your goal is? Is it simply top speed? Is it being able to run shLlow rivers?


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PostPosted: 11 Jul 2016, 11:44 
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Yeah - I went through dialing in the prop last year and I'm currently running my 25hp merc at 5,800 RPM (+/- as the induction tach is all over the place but seems to average out around 5800 last time I took a look at it) and getting 26mph when I'm alone with a 13P factory aluminum prop with a cupped edge applied to fix slipping issues I was having. As for the motor height on the transom, I wanted to raise it after seeing a linked post (I believe it was your write up Dale) showing how the cavitation plate should run just at the surface of the water. Mine is definitely dragging, despite being level to the bottom of the boat, but it still won't hold in a tight turn as I end up blowing it out if I don't throttle down - so I haven't raised it any more. Plus, I'd been toying around with the design for this already so fine-tuning the current setup didn't seem all that important.

I'm playing with making up a power trim for two reasons - first off to see if I can come up with a simple & comparably inexpensive way of doing it for use at high speed, and second because I'm taking the boat up river and I'd like some simple adjustment ability. Right now I'm having to lift the motor and set it back down on one of three differing thickness wooden blocks. I've got several friends who would be interested in doing the same on their boats if I can get the material cost of the setup down to ~$250.



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2002 Alumacraft 1436LT w/ 1984 Mariner Tiller Converted to Remote & 55# Minn Kota Terrova 12v (removable)

1985 Bass Tracker III - restoration posting coming soon w/ 1988 Mercury 60hp 2-Stroke & 55# Minn Kota Terrova 12v shared w/ my 14'


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PostPosted: 11 Jul 2016, 11:52 

Joined: 23 Oct 2015, 16:29
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Location: Twin cities, Minnesota
To me like your primary concern is running shallow. Sounds like what you really need is more cupping and or more pitch so you can raise the motor higher. this will alleviate your blow out problems


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PostPosted: 11 Jul 2016, 11:56 
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^^ not @ all helpful for what I'm looking for, thank you.


Dale - thinking about that 1" of height for EA 1' of setback - if that's the general advice among installers I should be fine setting the slots on my mounts so that at their lowest position the cavitation plate is in its current position relative to the bottom of the boat and with 1" of upward positioning available I should be able to find an appropriate height. If I didn't get anywhere looking into it this would have been what I was going to do, I just didn't want to find I should have been raising it 2" and been S.O.L. & needing to buy more materials.



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2002 Alumacraft 1436LT w/ 1984 Mariner Tiller Converted to Remote & 55# Minn Kota Terrova 12v (removable)

1985 Bass Tracker III - restoration posting coming soon w/ 1988 Mercury 60hp 2-Stroke & 55# Minn Kota Terrova 12v shared w/ my 14'


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PostPosted: 11 Jul 2016, 12:43 
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onthewater102 wrote:
thinking about that 1" of height for EA 1' of setback - if that's the general advice among installers

NOTE that was for V6 OBs on offshore-type hulls and you see those specs bandied about on OB bracket makers, like Artmstrong brackets.

I have no clue about setbacks and heights on smaller OBs and hulls ... strongly suggest you look at specs/info from OB power trim makers like Bob's Machine Shop, Panther Products and CMC for example.



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#1) 1st tin rebuild, 18' Lund viewtopic.php?f=21&t=36583
#2) 25' Parker refurb from EMPTY hull http://www.classicparker.com/phpBB3/vie ... p?f=15&t=6
#3) 16' V-tin rebuild viewtopic.php?f=21&t=36465
#4 Procraft SV14
#5) 16' Starcraft entirely NEW Transom Skins viewtopic.php?f=3&t=37548
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PostPosted: 11 Jul 2016, 12:45 

Joined: 23 Oct 2015, 16:29
Posts: 32
Location: Twin cities, Minnesota
So you got your prop tuned to the motor height you had and now you want to move the motor up ? When you do move the motor up to get the cavitation plate above the bottom of the boat you're going to have to retune the prop most likely. If you're already having Blowout problems in turns raising the motor is just going to make this problem worse


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PostPosted: 11 Jul 2016, 13:04 
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I've got specs on the panther and cmc Pt-35, as well as setups on larger bass boats with substantially larger motors. I haven't been able to find anything on the one's made by Bob's Machine, but as I said, I'm not seeing much consistency between the ones I've looked into and I'm inclined to think they were just assembled to be functional as tilt/trim without much thought given to motor height. For instance, the installation instructions (http://www.cmcmarineproducts.com/userfi ... Manual.pdf) for the CMC PT35 indicate that for its 5.5" of setback the cavitation plate should be between 1" and 3" above the bottom of the boat - that's an incredibly large range for small motor setups so I highly doubt there is much precision to it.

The panther model offers even less info in their installation instructions - the 40 and 55 models have 7" of offset but make no mention of adjusting the height of the motor. http://96bda424cfcc34d9dd1a-0a7f10f8751 ... hansen.pdf



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2002 Alumacraft 1436LT w/ 1984 Mariner Tiller Converted to Remote & 55# Minn Kota Terrova 12v (removable)

1985 Bass Tracker III - restoration posting coming soon w/ 1988 Mercury 60hp 2-Stroke & 55# Minn Kota Terrova 12v shared w/ my 14'


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PostPosted: 11 Jul 2016, 16:50 

Joined: 24 Dec 2011, 22:06
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It is all trial and error, even with the bigger boats. One boat might like 6" of setback and 8" of lift, exact same boat and motor might only like 9" of lift. That's why there's nothing but a "general rule of thumb". Every boat is loaded and driven differently. Every hull is different. Even my hull is slightly different than my friend's and they're supposed to be exactly the same boat. Some boaters want the motor deep so that they can keep the throttle pinned while in a slight turn. Other boaters want every last .01 mph out of their rig. Then another boater might like to run his higher than it should be for that big 30' tall rooster tail like the guy I seen this morning.


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PostPosted: 13 Jul 2016, 20:05 
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If you take some time and go back through posts we have covered this subject pretty extensively. Sometimes I wish we had the ability for "stickies" in this forum.
You can pretty much throw any formula you have heard about out the window for small boats and engines.
The formula of 1" up/1" back is a fable as well as the 1/2" up/1" back. Everything depends on the boat, the boat bottom, the engine, the propeller, and the center of gravity, and what gain the owner wants to see......and that's just the basics.
I remember the advent of the "Gil Bracket"...those set the engine back nearly 30"...think there was a way to bring the engine up 30" or even 15"?
Since there are no real "performance" propellers for small engines you have to realize that your gains will be limited by your propeller until you have that prop modified.
The main thing to remember for anyone getting into this is to make ONE change at a time and document document document what you are doing. If you are running in a current or wind run a two way average. Keep your gasoline within 3 gallons of your base line at all times. If you have installed a set back plate and are working on engine height then start with a baseline performance number, make a change, record speed and RPM and keep making changes until your speed stays the same and the RPM goes up (slippage). Chances are you will not be able to starve the engine for water but keep a tab on it anyway.
Once you have hit that wall you have a decision to make. Modify the propeller and keep going or are you satisfied?
Other factors affect small boats more so than larger ones. Weight is the biggie here. If you change the location of a permanent weight (battery or fuel) you will change everything and usually you will have to start over. Keep records. Weight changes usually require an engine trim adjustment which also changes everything. See where I am going with this? Small boats are fun but can get labor intensive when it comes to tweaking for performance.


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PostPosted: 13 Jul 2016, 23:50 

Joined: 24 Dec 2011, 22:06
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And to answer another question, "why can't dealers just set them up right from the get-go"?

Because they can't. And here's a couple reasons why, from my own experience. Guy comes in, wants a fast duck boat. Ok, 25 Yamaha 3 cylinder on a CMC, re-wired of course, which gives it 5.5" of set back and we raised them so that when the av plate was level, the motor was 2 1/4" raised. This worked pretty good for most owners, based on feedback. They could be lifted to almost 3" and pick up 1 mph but we felt it to be safer to run them at 2 1/4". 13" black steel prop, which put most of these setups on the mid 30 mph range safely and reliably. So that's what we did. Guy comes in 4 days after buying, says it won't start. I pulled the rope, no compression. What he did...was to move the battery and fuel tank to the front of the boat because as it was planing off, he couldn't see over the bow. He didn't like it and moved the battery and tank, and ran it. In doing so, he also "lifted" the motor by offsetting the weight, which made the motor blow out constantly. Burned up powerhead. Yamaha wouldn't pay for it as it was a set-up issue and not a defect (I don't blame them..not their fault). Dealer had to buy him a powerhead. And as you know powerheads are not cheap not even at dealer cost.

Then many many years ago when Mercury had us (dealer I worked for at the time sold Merc and Mariner outboards). I was pretty new, had been there maybe 5 years at the time, 3 years part time. A boat was sold, and I had to prep it. Duracraft 1748 crappie boat, stick steer job, with a Mariner 40 magnum (4 cylinder). Got it rigged and sent it out. Owner called said something was wrong with it. It ran fine for me? He said it was slow...28 mph wax, which I felt wasn't too bad but he felt like he should be seeing closer to 40. Turned out motor was too deep, we lifted it a hole at a time and each hole it'd pick up some speed. By time we had it all the way up, it was in the 35-36 mph range-which made Mr. Customer a lot happier. At that time GPS was nonexistent so we relied on the pitot speedometer and the tach. Now keep in mind we couldn't do the work at the lake...we were 16 miles one way from the closest body of water, which was full of stumps and submerged trees so running faster than 2 mph was impossible. So we had to test at the other lake, which was 23 miles one way. No tools, one man deal. Dunk it, run it, take it back to the shop, lift, re-bolt, go back to the lake. There were 4 adjustment holes so 4 trips back & forth. This looked kind of bad on the dealer, myself as a greenhorn tech, and the dealer's manager.


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PostPosted: 14 Jul 2016, 09:13 
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...so, returning from left field...let me simplify my question - how fast does water flow to fill a void? I've found calculators for flow rate in a pipe - but not on open water.



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2002 Alumacraft 1436LT w/ 1984 Mariner Tiller Converted to Remote & 55# Minn Kota Terrova 12v (removable)

1985 Bass Tracker III - restoration posting coming soon w/ 1988 Mercury 60hp 2-Stroke & 55# Minn Kota Terrova 12v shared w/ my 14'


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PostPosted: 14 Jul 2016, 10:07 
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onthewater102 wrote:
... so, returning from left field - how fast does water flow to fill a void?

You do realize that water, fluid mechanics or hydrodynamica calculations are some of the toughest to predict, never mind solve?

If truly laminar, you could project a consistent flow, with same expectations if turbulent flow only. You'll have a mix, as vortexes form to fill the resultant 'hole in the water', which results in nonlinearity in the filling, where oscillation from the inflow will likely create the largest nonlinear disparity. That's all I recall from my physics and mechanics (theory, not wrench turning ones ... lol!) classes.

But that's a long way around of saying ... I have no freakin' clue :roll: !



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#1) 1st tin rebuild, 18' Lund viewtopic.php?f=21&t=36583
#2) 25' Parker refurb from EMPTY hull http://www.classicparker.com/phpBB3/vie ... p?f=15&t=6
#3) 16' V-tin rebuild viewtopic.php?f=21&t=36465
#4 Procraft SV14
#5) 16' Starcraft entirely NEW Transom Skins viewtopic.php?f=3&t=37548
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