Prop/motor height setup

[CaneCutter79]

My boat is a 79 model with a tri-hull up front. The back portion of the hull is not as deeply "V" shaped as newer bass boats so it sits a bit flatter and has no contour towards the front of the bow. In other words, the keel is pretty much level and straight until it reaches the bow where it begins to taper off and upward. So, I cannot get a lot of bow lift due to the hull design.

Here are some pictures of my setup. The motor is mounted right on the flat style transom so it cannot go any lower and I suspect it should be lower because in sharp turns and WOT, the prop slips and has "blowout". I can only trim the motor up just a little bit before the prop slips.

What I have shown in the pics is the trim settings at WOT. This is just above all the way down ont the trim. Does the motor height appear to be too high or too low? I feel it's too high. I cannot use a jackplate to lower the motor as it is as low as it will go. Currently, the bottom plate rides just above the water's surface when on plane at WOT.

ALSO...

Which plate is my cavitation plate? I have to plates. The bottom plate holds the hydrofoil (which helps hole shot). Everyone has told me to loose the hydrofoil but the boat actually runs faster with it and the hydrofoil is out of water and above the surface at WOT so I cannot see how it's causing drag or causing the prop to slip at WOT. If the hydrofoil is above the surface, is the motor too high?

One last thing....

In theory, prop blades should always should be under the water surface correct? If your prop blades are above the water plane/surface, that will create slip and prop issues correct?

I believe this sketch roughly shows how my prop is placed at WOT.

 

[Tacklewasher]

Re: Prop/motor height setup

Guessing you have a standard shaft motor on a boat built for a long shaft.

No question your anti-ventilation plate is pretty high up in relation to your boat. So your prop will ventilate and you have a potential overheating problem as you may have your water intake too high.

 

[sho3boater]

Re: Prop/motor height setup

I can only say what I would do; I'd silicone the top water holes shut and put a water pressure gauge on it. Then I'd find a bow lifting SS prop for it. That kind of prop with a lot of rake will surface (actually partially surface) and lift the bow way more than a stock type prop and bite better. I'm not sure what you have on there look that prop up and find out. I have an old laser 2 I ran on a previous boat that worked very nice, but there are others. On that boat the motor was raised 2" on the transom, it was an older vee with no pad and a bow rider. It could run a lot of trim, but hardly any at all with a stock alum prop. The blades will have a lot of rake (angled back) and blades will be pointed/more pointed and not as wide front to rear. Most bass boat props are this style because they carry loads and need a lot of lift. You could also try a 4 blade, they tend to lose a little on top speed but gain more bite in corners and holeshots. Either way you need a prop that lifts a lot. A SS prop will work better the blades are thinner and designs are better.

The foil is not safe to run if it is below the surface at planing speeds, unless it is a really slow boat. It will not hurt to have it, but if you jump a wave and come down on it they can slap the bow harder. I have only needed them on underpowered boats, most of my boats would plane faster with some trim. It is mounted on the cav plate, that is always over the prop. Try to move anything heavy to the rear of the boat, or out of it of course.

If you bought the boat like that and that is an aluminum prop, odds are the owner took off the good prop before he sold it. I've done it.

 

[CaneCutter79]

Re: Prop/motor height setup

The prop I have is a SS 19-pitch with lots of cup on the blades. I agree, the 4-blade SS prop with bow lift would help get a better bite and bow lift.

Water pressure is not a problem. The "pee stream" is strong and the intake gets plenty of water.

I feel strongly that the motor is about 1-2 inches high but I wanted other's opinions and not just what my gut tells me.

 

[hwsiii]

Re: Prop/motor height setup

ane, take a piture this way so we an see how high it is. And it is the plate with the hydrafoil on it.

H

 

[Bifflefan]

Re: Prop/motor height setup

for me, i think you have a standard shaft and need a long shaft, but..
what may be going is, because of this you can use a jack plate, just set it as low and possible. then you can use the added lenght to get the prop further back so as not to be running (so much) in the hole the bottom of the "V" makes.
In other words let the water come off the bottom and then close back up before you chop it up with the prop.

the jack plate should set the motor back 6"s. From what ive read this is a good thing. especially for yours.

 

[sho3boater]

Re: Prop/motor height setup

I agree a jack would be ideal and (if your transom is solid) the setback will also help.

But if you are trying to make that setup work, having the motor high is a good thing for the right prop. You need to find out if it is too high for a good prop. That prop does not look like it has a lot of rake but hard to tell in photos. The cup helps bite but the rake does more for lifting the bow.

Issue with the water is when you run WOT and trim it up you can lift the holes out of the water. It can be hard to see the pee stream at top speed. The boat I had was set about that high, and I could run the prop nearly half out of the water and actually lose water pressure...and get my best speed that way. Yes, it really needed a jack then you can run less trim. It torque steered badly with excess trim.

It is a balancing act; you lower the prop and use more trim for more lift but then get more LU drag. That is why a setback helps to balance the boat better and use less trim. Depends on how much lift your prop makes, and how much that hull needs.

 

[kick bass]

Re: Prop/motor height setup

I can't tell a lot about your prop from the pictures. do you have a number on it? it looks like a painted aluminum prop. I can't see any rake in it. the motor is to high. do you have a water pressure guage on it? evinrudes have a low water pick up, but it still looks like it is border line. A high Rake prop would help, they like to run high in the water, but still you need to check and see if you can move your engine back, with a jack plate. what rpms are you running and what speed with a GPS? Can you look at the transom close and see if there was a jack plate on the boat? the fin you have on it helps with hole shots because it is forcing water down to your prop, which is to high.

I have given a lot of thought to this, and your transom looks to short for a long motor and to long for a short motor. what brand boat is this? The only thing I can figure is it had a jack plate. What you have will work, but you will have to install a jack plate. Even if you put a raker prop on your boat, it still will blow out in curves and out of the hole. you have to get the engine down. OK what about the aluminum piece that is on the transom? was this an ad on? has it had some type of transom work?

 

[CaneCutter79]

Re: Prop/motor height setup

I agree a jack would be ideal and (if your transom is solid) the setback will also help.

But if you are trying to make that setup work, having the motor high is a good thing for the right prop. You need to find out if it is too high for a good prop. That prop does not look like it has a lot of rake but hard to tell in photos. The cup helps bite but the rake does more for lifting the bow.

Jack plate isn't an option. The boat is 30yrs old and it was not designed for using a jackplate. Plus it's not worth the expense of buying a jackplate to gain a few mph. What I am looking for is the right prop adjustments.

I will take a photo looking at side of the motor and transom as described above and post it later.


I need to update this tread with new info!


My boat mechanic was nice enough to let me borrow (free of charge) a brand new 21-pitch 3-blade SST (OMC) prop just to see if my prop was the problem. The 21pitch slipped worse than my 19 and my 19 has a little more cup on the blades than the 21. As far as RPM and mph, I cannot give accurate RPM range but it sounded the same RPM range as my 19. I only got 36-37mph from the 21 and my 19 will get 39-40 easy. The 21 was maybe 100-200 rpm lower than my 19 but not more than that. The 21 started with slight cavitation soon after it was on plane even before I trimmed the motor up. That told me that I need a prop with a lot of bite, not a lot of rake. My stern rides lower in the water due to three batteries, (2) 6gal cans of fuel, and the motor and it makes a fairly good wake. You can see the wake coming off the sides of the hull in this quick video. Notice the pee stream is strong.
http://www.youtube.com/watch?v=8TK3XQnfCJ4

Does this information provide better information for my situation? Sorry I cannot give accruate RPM and water pressure readings but I do not have those gauges or access to any. My boat mechanic tells me I should just rebuild my prop and add lift cup to it and that will help stop the prop from slipping. For $115, that's certainly the cheapest option vs. $300 for a new prop.

I know I will NEVER get more trim from the current setup but with the right prop for my boat, I will gain a slight bit of trim and a lot more bite. If the stern was lifted a bit more, I could gain a few mph with more bite and less drag. That's really all I want at this point.

 

[corm]

Re: Prop/motor height setup

Hi,

After looking at the vid it confirmed in my mind what I saw in your pictures.
Your transom has been plated which raised up your transom height. An OMC when dropped flat on the transom like yours is always sticks down in the water to far and will spray a lot of water around. You all have seen these boats running on the water. They look like they have something caught on the front of the LU spraying water up the transom and around the engine..
So your going to have to pull the engine remove the plate and see if you can figure out a different way to plate it. It may have cracking under the plate.
I would bet you will be able to drop your engine a good inch. Also that transducer will disrupt the water flow to your prop too. Causing cavitation.
The black teflon coated SS OMC props are pretty good blades. It will cavitate when trying to lift a heavy in the bow tri-hull boat up. Two things will fix that with out putting a set back plate on it. 1, a prop with more blade area to hold the bow up and not slip while running partially in the air or 2. dropping the engine down in further in the water.
I realize this all sounds preposterous. So I want you to call up the boat company and ask them if the plated their transom on this boat and what the proper transom height was on this boat. Then measure yours to see if I am right.

This may be of interest to you. It is for Merc's but it is about the same thing for OMC.
http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html
Also CMC makes a 2.5" set back for $90.
OOPs quick search at Bobs gave me this http://www.bobsmachine.com/
go down to the mid of the page where it says parts and click on set back spacers there you have some. 2' , 3/4" or 5/8" nice ones. These guys make great stuff.

 

[hwsiii]

Re: Prop/motor height setup

ane, there appears to be many people trying to help you with this problem and with very good ideas. I will tell you that not having a tahometer is definitely false eonomy, and no one an tell how many RPM a motor is running by ear. I will agree that you an tell to a degree that one prop is turning more RPM than another, but that is it. You NEED a tahometer for your motors health, otherwise you are working in the dark.

With that being said you need a Solas Amita 4 blade 19" pith prop to lift the stern of your boat and gain more speed. You an buy it right here at Iboats heaper than adding up to your present prop, and it will give muh more stern lift than adding up to the prop you presently have. Here is the information for the prop here at Iboats.


Iboats prop

Price: $98.99
You Save: $37.01
Compare: $136.00
Discount: 27.3% Off
Add Amita Aluminum Propeller 2413-125-19 To Your Cart
Propeller Specifications:
SKU:#2413-125-19
Manufacturer: Solas
Brand: Amita
Material: Aluminum
Diameter: 12 - 1/2?
Pitch: 19?
Blades: 4
Rotation: Standard (Clockwise)
Usually Ships Within: 1 Business Day


H

 

[CaneCutter79]

Re: Prop/motor height setup

Thanks all. I've been given a lot of information. Out of all of it, I have come to the following conculsions:

- I need a tachometer to more accurately monitor my motor for good motor health and for more accurate readings (I'll bet $50 my motor is turning 5,500rpm give or take 100)
- Due to the aluminum transom bracing that was added about 5 years ago (before I bought the boat), it has raised the motor up 1" - 1-1/2" and creating 50% of my problem (not many caught that alum. bracing....thanks!)
- The boat company ran out of business back in the early 80's so I have no spec information on the boat
- My motor is too high
- My prop is out of round by 1/8" (helping cavitation)
- My prop does not have enough lift for it's current height
- My boat needs a shallow water prop (flats boat perhaps)
- An offset bracket for $100 could offer a lot of motor height flexability to get my motor back to factory designed spec and offer 2-3mph gain just from the offset of the motor


All in all, I stand to gain 5mph by selecting the right prop and utilizing an offset bracket to adjust my motor height. Considering I loose 5mph now when the prop does slip, I could gain another 5mph if I had a prop that did not slip and lifted the boat more out of the water. Overall a gain of 5-8mph on a 30yr old 15ft tri-hull would get my top end around 45-48mph with the right chop. That is truely impressive!

Thanks again! (My tachometer is bought and on it's way)

 

[CaneCutter79]

Re: Prop/motor height setup

I took the boat out yesterday with a light load. I ran 39mph (GPS) at 5,150rpm on smooth water. With a bit more chop I could have held 5,200rpm at 40mph (GPS). I originally predicited 5,500 so I was only off by 300rpm or so. Not bad for judging rpm's by my ear vs. using a tachometer. So for those who said you can't judge by your ear, I was pretty close. :laugh:

The boat revs up around 3,800-4,100rpm on hole shot (when you hammer the throttle) and eventually climbs to 5,150 as you trim out the motor.


So based on some online prop calculators, a good 19" 3-blade will give me the best top end with a moderate hole-shot. A better performing prop like a Raker or Turbo will give me more lift and better grip towards the surface vs. a deep running SST prop. I might gain 2mph by changing to a quality prop.

My prop measurements puts the center of the prop shaft 5-1/2" below the center of the pad and the transom plate is 23" tall from the deepest point of the hull to the top of the transom plate. It's 1'-5" on the outside corners (not that it matters)

All in all, I need a better griping prop. A jackplate won't work for me because of the way the motor sits on the transom. I would not be able to trim the motor up due to interference with the steering components and the cowling. A 4-6" plate would put these parts right on the top of the transom when the motor is tilted up, unless I could raise the motor about 2-3" and I don't see how that's an option.

 

[sho3boater]

Re: Prop/motor height setup

It depends on how high your bow is, if it needs to be higher you will never see a good top speed until it is lifted. I guess I would be on the search for a lifting prop to try. If it worked you are good, if it still does not lift the bow enough then I'd look at the transom and how I could lower it. Also you can try a diameter prop that just fits it will hold more water, if you have a choice anyway. I have a vee hull that needs a lot of lift, the motor is 2" high on the stock transom. It ran 4600 with a stock prop and hardly any bow lift (because the motor is that high) and with laser2 of same pitch it ran 6K rpm. The laser is 20p SS prop and stock alum 21, they ran same mph at 4600rpm. The difference is I could trim the laser way up surfacing and it bites (going straight) and the stock prop I can't it blows out with a little trim. I would get a rooster high as the motor.

 

[CaneCutter79]

Re: Prop/motor height setup

Just to update, I got a 3-blade Turbo1 17-pitch and I'm running the same RPM's, TONS of bow lift, and about 1-2mph slower. The speed reduction is probably due to loading the motor to achieve so much bow lift. Currently, I can run full trim and about half of the hull is completely out of the water. About 1/3 of the the rear hull is running on the pad.

So, I got lift but no speed increase. I got a new prop and the RPMs stayed the same. From the many posts I have read, I thought the RPMs would actually increase by lifting the hull more.

So what is the rule of thumb for lifting the motor? Is it +/-100RPM for every 1" of lift? If so, I can lift up to 2 inches on the factory transom. Is it worth it to do this? I'm told the Turbo actually prefers less water and performs better when it breaks the surface a bit.

 

[Chris's CVX16]

Re: Prop/motor height setup

Since your already at a higher than stock height for the motor raising any further and you should get a water pressure gauge first.

The 17" pitch is your speed reduction vs your old prop.

At this point I think I would just leave well enough alone. Try it out for a few months and see how much better this setup is vs. your old setup.

 

[CaneCutter79]

Re: Prop/motor height setup

Since your already at a higher than stock height for the motor raising any further and you should get a water pressure gauge first.

The 17" pitch is your speed reduction vs your old prop.

At this point I think I would just leave well enough alone. Try it out for a few months and see how much better this setup is vs. your old setup.

Actually, the motor is as low as it can go. I can raise it up (2) notches (2inches) but that's all.

If I raise the motor, I'll be sure to get a fuel gauge. SOmeone told me that I NEED to raise the motor to keep from lugging the motor down and extend motor life. If the motor is at it's optimum RPM range of 5,000rpm, why is this considered "lugging"? The motor specs from what I was told tested the horsepower rating at 5,000rpm. So if I'm getting 85hp at 5,000, wouldn't I just be winding the motor higher above 5,000rpm and burning more gas? Sure, I'll gain 1-2mph but I'll burn a lot more gas.

Is that wrong?

 

[hwsiii]

Re: Prop/motor height setup

Cane, the following information was given by Dhadley and this is what you should try next.

Dhadley

The SST II should have pretty good cup but the T-1 will indeed run a higher X dimension. Raise the motor and retest, you have a good baseline now. You've made one change at a time which is the correct procedure.

CaneCutter79

If I raise the motor, I'll be sure to get a fuel gauge. SOmeone told me that I NEED to raise the motor to keep from lugging the motor down and extend motor life. If the motor is at it's optimum RPM range of 5,000rpm, why is this considered "lugging"? The motor specs from what I was told tested the horsepower rating at 5,000rpm. So if I'm getting 85hp at 5,000, wouldn't I just be winding the motor higher above 5,000rpm and burning more gas? Sure, I'll gain 1-2mph but I'll burn a lot more gas.

Is that wrong?
Today 11:07 AM

Yes, you are wrong. Your motor specs actually call for maximum recommended RPM of 4,500 to 5,500, 5,000 RPM is where the maximum HP is made. With today's fuel, you need to raise your RPM so that your motor does not build up carbon deposits in the cylinders. These are outboard mechanics recommending this procedure, they tear these motors down regularly and get to see the results of NOT being able to turn maximum RPM and what it does.


Correct Anti Ventilation Height

This is what you are looking for when you look at the motor running in the upper 30% of maximum RPM, the Anti Ventilation plate should be able to be seen quite clearly, forget about how high the hydrafoil looks like in height, the vent plate is what counts if it is high enough the hydrafoil is. And you will burn no more gas because the motor will have less load on it at the higher RPM.

By raising the motor you have less lower unit drag and a commensurate rise in speed and RPM.


H

 

[CaneCutter79]

Re: Prop/motor height setup

Cane, the following information was given by Dhadley and this is what you should try next.

Dhadley


CaneCutter79


Yes, you are wrong. Your motor specs actually call for maximum recommended RPM of 4,500 to 5,500, 5,000 RPM is where the maximum HP is made. With today's fuel, you need to raise your RPM so that your motor does not build up carbon deposits in the cylinders. These are outboard mechanics recommending this procedure, they tear these motors down regularly and get to see the results of NOT being able to turn maximum RPM and what it does.


Correct Anti Ventilation Height

This is what you are looking for when you look at the motor running in the upper 30% of maximum RPM, the Anti Ventilation plate should be able to be seen quite clearly, forget about how high the hydrafoil looks like in height, the vent plate is what counts if it is high enough the hydrafoil is. And you will burn no more gas because the motor will have less load on it at the higher RPM.

By raising the motor you have less lower unit drag and a commensurate rise in speed and RPM.


H

Cool. That's good information. I'm quite sure the vent plate is just above the water surface as you have shown in your pic. The foil is at least 3-4" above the pad so the vent plate should be up as well when you look at my pics of my motor. I'll double check that though.

What you said about burning deposits makes sense. I just wanted to double check before I go through the effort of raising the motor.

So, what's the best method for raising the motor? I don't have a lift handy and I'd hate to pay my mechanic $40-60 just to raise it. I can do it myself if I had a hoist or some way to lift the motor.

 

[hwsiii]

Re: Prop/motor height setup

Cane, the way i do it is I block up the lower unit with wooden blocks on each side of the lower unit by the skeg, and then I undo my bolts and I have someone crank the front roller down to raise the bow which takes the load off the motor and I take the bolts out and then have them crank it to the hole I need to reach and reinstall the bolts. Just be careful.


H