08 mercury 60hp on 15' aluminum, harf time matching up a prop

[Wood Slayer]

I have a 15' 52" ambush aluminum tiller handle boat that I mounted a 08 mercury 60hp 4 stroke bigfoot on. The max rpm range for the motor should be 6000. Here are my specs
19p aluminum prop no trim 6400rpms
19p 13 1/4" aluminum prop trimmed is 7200rpm.
The propeller is a new cheap propeller ($40 on sale).
So this prop is giving me 46.4mph with good handling. I know that prop design is endless. I was wondering if someone can lead me in the right direction, i understand there is progressive style props, chopper... with lowering my rpm, gain a little more speed. But keep a decent Hole shot.
Thanks in advance guys!!
Richard

 

[Texasmark]

For me, all bets are off with a bigfoot. It's a larger hp lower unit with a smaller hp powerhead. It's made to push. Go to the top of this page and the Boat parts and accessories. Go to that page and look for props and thumbing through the prop pages, look for prop parameter explanations or something like that. They will answer all your questions. Just stay in the pusher prop category and stay out of the high performance stuff. You definitely don't want any "Rake", nor "Porting".....holes in the side of the prop barrel, or high pitch. Cupping is good usually.

I guess you have me confused. 60 hp on that boat with a standard 60 hp lower unit would be a good match. 60 hp on a 20 foot toon with a big foot lower unit would be a good match.

Let's play with some numbers and see where the dust settles. My 90 Merc ran a 13+ diameter prop and had a 2.33 gear box. I don't know if your big foot has the 90 LU or the 115 which is a 2:1.

Using the 90 with 2.33 I'm going to the prop slip calculator and see what comes out. Inputs:

Prop pitch: 19
Rpms: 6400
Actual Speed? don't know which prop is the 46.4 number...will use it on both props and see what happens.
Gear ratio: 2.33:1
Speed (in a solid): 49
Actual 46.4
Prop Slip: Calculated 5% Highly unlikely with no trim correction unless trim manual setting is optimum and prop is a high performance SS Raked, and cupped.



Prop pitch: 19 different prop, properly trimmed
Rpms: 7200 either because of prop characteristics, other than pitch change, or reduced boat friction due to proper trimming allowing for more top end which should have brought the speed up also. But since I have only one speed number will use it.
Gear ratio: 2.0:1
Speed in a solid: 65
Actual 46.4
Prop Slip: Calculated 29%.....no way on a 15' alum tiller boat.

-------------------------------
So here we are at 2 extremes and I don't believe either.

Need to know which prop you were running for the 46.4 mph reading.

Also want pictures of props, side and rear views. Don't remove from engine if not already mounted. Just shoot as is.

Also pictures of your first run with "no trim".....side view, camera 3' off the ground pointed level and directly at the gap between the LU and the transom, ensuring the LU and transom are in the picture with a little bit of the bottom.....say 1' forward of the transom included in the shot. This will show me the relationship of engine angle to hull.

Last, picture of your optimum trim position run trim angle taken from same spot.

With that information I can probably help you....

PS....one more last: Get the gear ratio of the LU. It will be prop turn ? what the crankshaft turns.....will take 2 complete revolutions of the crankshaft for the prop to turn one revolution for the 2:1 gearbox.

If a 2.33 it will be 1/ 2.33 = 2 turns of the crankshaft for 0.85 revolution of the prop.

You want an answer. You do the work on your part and I'll respect your effort and do my best to get you a usable answer. This is not a textbook problem. As I said, something isn't right here.

 

[Wood Slayer]

Texasmark,
Thanks for your reply. I have all the photos you requested, im unable to load them as it says my size limit is to large, im working on getting that corrected. The 46.4 is with the 19p prop, also it took 2.3 turns of the flywheel to get the propeller to turn 1 full time. Im working on getting these pics posted.

 

[Wood Slayer]

Also, the 46.4 was with the boat trimmed. Without trim i was running somewhere in 38mph.

 

[Wood Slayer]

Here is a link to the photos you requested

 

[GA_Boater]

Welcome aboard, Wood.

If you keep turning 7200 RPM, you will be spending major bucks to fix a blown motor. You need more pitch or the tach is off.

You need to have 3 posts ( you have that now) to post photos. Here are the photo attaching methods that work here, Option 3 is the easiest and best way to show decent sized pics. http://forums.iboats.com/forum/forum-rules-and-guidelines/9938375-photo-upload-tutorial

 

[Wood Slayer]

I have the last remaining photos as well. I will get them posted asap. Here is some more specs on my setup 15' 52" ambush aluminum boat.
6" jackplate
550lb boat
Im 190lb
6 gallons of fuel
1 battery.
Electric trim, big tiller handle
All weight is in the rear of the boat.

 

[Wood Slayer]

Thank GA boater!
Yes, im not a fan of turning 7200 rpms. Im working on getting this corrected. This is a new setup to me, ive ran it 3 times. Im dealing with a tiny tach. So im not sure how reliable they are. I will re-route my wiring for it tomorrow, to ensure that the wires are not laying on top of any other wires. Thank you for the information.
Im a first time poster, but long time stalker.

 

[Wood Slayer]

Here are the additional pics

 

[Texasmark]

Jack plate puts the engine farther aft meaning it needs to run higher, not lower. For rule of thumb for me is if you aren't blowing out periodically, you are too low. Depth is a SIGNIFICANT factor in drag and drag limits mph period.

If you want high performance you need a high performance prop. You don't have that. For a cheap prop Turning Point Hustler (in aluminum for about $100 here on iboats) is the best bet for that. You get everything but thin blades that a SS can give you and that's a few mph at most. It's ported too so it improves your hole shot while if you get the higher pitch for WOT performance, you get the best of both worlds.
------------------------------------
Going back to the numbers with your new info:

Untrimmed:

Using the 90 gearbox with 2.33

Prop pitch: 19
Rpms: 6400
Actual Speed: 38
Speed (in a solid): 49
Prop Slip: Calculated 22% Way too much for that boat.

Using the 115 gearbox with 2.0

Prop pitch: 19
Rpms: 6400
Actual Speed: 38
Speed (in a solid): 58
Prop Slip: Calculated 34%......no way!


Going to stop right here. You have an instrumentation problem. I have used this go-fast.com, BAM prop slip calculator for years and it works considering slight variations caused by uniqueness of the rig involved.

For that rig your prop slip should be more around 12% with that prop and setup the way you are.
----------------------
If I were going to design a "pusher" engine it would have a shallow pitch, large diameter prop, turning slowly as compared to a "high speed", high pitched, tiny prop at high rpms. So let's pursue the "pusher" theme.

The product literature for that 60, using 1994 data, standard lower unit, is 10" diameter prop, pitch TBD, 1.64:1 gear ratio (high speed prop vs engine rpm) topping out at 5500 max recommended engine rpm.

Going in the opposite direction for pushing, I'd opt for the large diameter (13" which the 4" diameter LU 75 thru 125 hp Mercs use) highest gear ratio.....lowest prop speed vs engine rpm which is the 2.33 that the 90 LU uses.

With that thought let's run some numbers:

Gear ratio: 2.33
Engine speed 5500 rpm max
Prop pitch 19
Speed in a solid: 42
Reasonable prop slip for that boat: 12%
Actual speed: 37

That is dead-nuts-on one of your numbers (38 roughly, your input) for untrimmed speed but your tach. says you are turning 6400. Time to get a new tach or set your current tack to 6P.

Get that fixed and we will continue. If in fact your tach is accurate then your speed information is incorrect....GPS is the best for actual speed.

The accurate numbers of speed and rpm have to be available to make a logical determination as to where you currently are and where you need to go in prop selection for your desired performance.

I still need to see your wake picture. Again, I want the shot where the wake converges behind the boat. The only part of the boat I would like in the picture is just the top rear of the engine cowling to give me a reference point. I mainly want to see wake. That will help me understand how your current setup is working.......Trim to best speed, at WOT for the shot(s).

 

[Sea Rider]

Is that a 5-6 K wot rpm range OB ? What's a no trim position ? when trimmed, is OB trimmed to run parallel to water level, that's at 90?. With current load that OB should rev between middle to max wot range, distribute some weight forward. Will need to up some prop pitches to lower wot rpm or expect a kaboom if running full hammer dowm...

Happy Boating

 

[Wood Slayer]

Thank you to all for the replies! I will gather the wake pictures on this following monday.
The speeds posted are gps so i know they are accurate. I can not say the same for the tiny tach rpm meter. I do have the wires routed for that ziptied to some additional wires under the cowling.
I will reroute the wiring to the otherside of the powerhead and run the same course on the water and document the results.
A little back history on myself:
Previous owner of 8 boats, my current boats i have are a -2003 Trition Tr-21 with a yammy 225 ox66 Throphy plus 26p- gps 68.8
-2016 Ambush 15' with the mercury 60hp Bigfoot results are not finished.
I purchased the big foot because i had a steal of a price at $2100, it needed a adapter plate replaced.
A last question, should I look into replacing the LU to get a correct gear range of lets say 1.83---2.0? Before pursuing a prop change?
I currently have the motor mounted on its highest hole, jackplate is at its highest setting, and i also drilled new holes to get the jackplate even higher.
The cavitation plate is 1" below the transom. My wake line is low as well.
Thank you again guys!

 

[Wood Slayer]

Sea Admiral,
Thanks for the reply. Yes the motor is trimmed to run parallel. It is a electric trim. Yeah I was thinking about just propping up to a 21p and being done with it.
But, thats no fun. Im really just trying to get this rig setup and dialed in. Im planning on eventually switching to a ss prop. But im waiting until I get good consistent results. Thanks for the reply!
Richard

 

[Wood Slayer]

My apologies for these mini responses. After re routing my tach wires I happened to take 3 seconds and read the manual. I originally never setup the tach. I had it reading 1 spark per revolution.
I now have it set up for 1 spark every 2 revolutions. TexasMark, you were spot on. I will get back when I have accurate data.

 

[Texasmark]

Is this your boat:

----------------------------I borrowed this from another site's forums-------------------------------------

"Let's start on the bottom and work up.

The hull of the AMBUSH is a modification of Legend Craft's Sport V model that has been around for years. We added a reverse chine that actually is reversed, or turned down. The keel is actually a keel that is pressed one half on one side, one half on the other and then tig welded to form a keel. It stops about 10 inches from the transom for cleaner water flow to the prop. The strake's, as I call them, are spaced closer together and the press makes a flat surface with distinct angles turning up. The strake's are folded before the transom and welded shut at an angel to the transom. It is a 5 degree v-hull. The rake of the hull is more gradual to the bow and matches the curve of the keel up to the bow cap.

There are 5 longitudinal ribs in every bottom width of boat we build. 48-52-56-60 inches. They are NOT pressed or forced into the boat!

The bow cap is a bow cap.

The front deck has a straight drop down for the low deck. It is considered a mid deck by some but it's a low as we will build due to storage underneath. The High deck will be the same length with even more cubic storage space.

Side covers angle almost straight down to the floor. This keeps maximum floor space available.

The rear seats are two separate pods that are connected by a cross bench that creates the full seating that is standard. A "bulkhead" is added for closing the area for the gas tank and battery. It can be left off, if preferred, to create more access. If pods are ordered then the cross bench is simply not added. This makes a "custom" back seat possible at no extra charge.

The Transom is 15 degrees and comes in 20 or 15 inches. Nothing new. The transom angles to the side are flat with no break in them. Nothing fancy just easier to replicate from boat to boat.

Ah yes, the floor.....

Our floor will be welded to the ribs. NO RIVETS!

The gunwale, or gunnel to us rednecks, will be off set to the outside of the boat to protect the sides. It will have a bull nose on the outside and curved top. The accessory channel is angled towards the inside and the channel width is 3/8" wide. Under that will be a wiring channel for adding any electrical pieces to the bow. Again, nothing new but it will be proprietary to the Ambush and also available on Legend Craft cross rib models as an upgrade.

We are still offering most of the models that were previously built by Legend Craft. The main difference being, we are not a complete custom shop. We intend to offer dealers quality boats at a reasonable price that can be ordered for stock with options that are similar to any other manufacturer.

Custom configurations will still be available for cross ribbed Legend Crafts, but will not be the sole focus of the business.

I hope this answers the questions that have been presented.

Thanks,

Bart."


--------------------------------------------------------------------------------------
Elsewhere on the www I found that the bigfoot is based on the 115 engine which means yes to the 4" dia gearbox, and yes to the 13 ? dia prop but it also means 2:1 gear box......just another piece of info.

1. This needs clarifying: Do you have a 15 or 20" transom. Per the sales brochures, the Merc 115 only comes in 20 or 25". Will assume big foot is for a 20" transom. If you have a 15" transom on your Ambush 1552 then that could explain why you have to jack things up so far to get the Anti Vent (cavitation...your words) plate in proximity to the bottom of the hull. You have 5" of excess midsection you have to overcome.

If this is indeed the case then you might choose to pursue your comment: "A last question, should I look into replacing the LU to get a correct gear range of lets say 1.83---2.0? Before pursuing a prop change?"
"I currently have the motor mounted on its highest hole, jackplate is at its highest setting, and i also drilled new holes to get the jackplate even higher."

You may be opening a can of worms in this pursuit in considering what it cost and effort it will take to install another midsection and lower unit, unless you drop in an OEM 60 hp standard set of parts which will be the 1.64 gear ratio and 10" prop and there is still the cost associated with that.

2. Do you have the "Elite Transom? If so, the jack plate does not provide setback. The hull extends well out beyond the added transom (extended) feature so forget having to jack up just because you have setback.....there isn't any.

3. Prop tuning, is the last part of the "setup" according to the old salts that haunt this site....as the saying goes. What you tune in is what you go with. Getting one right, say in alum, and then switching to SS opens you up for who knows what new questions. My best guess, if you are determined to do that would be to spend another $100, when we get the setup right and buy a Turning Point Hustler alum 3 blade which iboats sells and which is about as close to a SS as you can get in a performance "stock" prop.

4. Ok, let's play with some new numbers:

If your tach was reading 2x rpm, then your trimmed out sweet spot would yield 7200/2 = 3600 rpm for 46.4 mph with a 19P prop and a 2.0 gearbox.

Running the numbers gives you a theoretical, in a solid speed of 32 mph. Stop right there. Not the right input.

I don't know if 4 stroke outboards trigger off the flywheel magnets and associated stator (pickup coils) like 2 stroke engines do or use a cam shaft geared output device at a ?:1 ratio to the crank shaft (like automobile OHC 4 stroke engines do). Even though your engine is a 4 stroke, it could still fire every revolution as the trigger mechanism is probably like 2 strokers, magnets imbedded in the flywheel that pass a magnetic iron core coil and as they pass they generate a trigger.

Regardless, 1:1 is 7200 and 1:2 is 3600. I proved above that 3600 is just not the right answer as it will only get 32 mph in a perfect solid.....no slip. 7200 doesn't work either as I proved that earlier today in my replies above. Your wake photo that you said you will get Monday will verify whether or not you are running in the 30's or the upper 40's.....wake tells all.


On your tiny tach.........don't know about that. What's the tolerance and max rpm rating of it? I have another idea. Go out to your car and hook it to a spark plug wire on your car. Get a helper and have them rev the engine to 3000 on the car's tach. What does the tiny tach read?

Will stay tuned.

 

[Wood Slayer]

Mark,
Your good! I will do my best to address your points/questions.
1: yes this is my boat and manufacturer, no on the elite model

2: i have a 15" transom, the motor is 20". the jack plate raises 5", i also increased it 1 1/2" with new holes, this includes mounting the motor on its highest setting.

3: If I can get a aluminum prop dialed in, then that will get me at a great starting point for ss. I will be happy to purchase the Hustler prop as long as i can focus on the correct pitch.

4: i called around my local marinas today in correlation of a new LU, no success. I'm on the market for a used 1:83, merc changed the LU design in 07.

5: i went with a 1:2 setup on the tach settings per a post I found in the internet stating a 4 stroke hits once per cylinder every two revolutions. Is this true, Im not sure! I cant stand behind the tach, unfortunately all I have is a diesel vehicles. I have the tach wire wrapped around my #1 spark plug wire.

6: im pretty sure the tach reads to 9999 rpms, i was lead to this style tach based off other "old" forums ive read.

Conclusion:
Im not certain of my tach setup at this time, i will get a friend to come over with his truck so I can get it dialed in correctly before my outing on monday.
I will be certain to get all requested info/pics for you. Thank you for your time and diligence! I will post some additional pics here is just a moment.
Richard

 

[Wood Slayer]

Here are some additional photos, i was off on my speed as well. The accurate speed was 43.6mph.
I also included the sources of the tach and where I recieved the info on spark per revolution.
Mark,
Is there anything else I will need to gather? Im aware of a additional cost for a LU, im willing to purchase one if it will give me overall beneficial results. Im assuming that the results would be significant, am I being naive ? Im just not willing to pay $1600 right now for the LU. I will keep digging for one.
Richard

 

[Texasmark]

Mark,
Your good! I will do my best to address your points/questions.
1: yes this is my boat and manufacturer, no on the elite model

2: i have a 15" transom, the motor is 20". the jack plate raises 5", i also increased it 1 1/2" with new holes, this includes mounting the motor on its highest setting.

3: If I can get a aluminum prop dialed in, then that will get me at a great starting point for ss. I will be happy to purchase the Hustler prop as long as i can focus on the correct pitch.

4: i called around my local marinas today in correlation of a new LU, no success. I'm on the market for a used 1:83, merc changed the LU design in 07.

5: i went with a 1:2 setup on the tach settings per a post I found in the internet stating a 4 stroke hits once per cylinder every two revolutions. Is this true, Im not sure! I cant stand behind the tach, unfortunately all I have is a diesel vehicles. I have the tach wire wrapped around my #1 spark plug wire.

6: im pretty sure the tach reads to 9999 rpms, i was lead to this style tach based off other "old" forums ive read.

Conclusion:
Im not certain of my tach setup at this time, i will get a friend to come over with his truck so I can get it dialed in correctly before my outing on monday.
I will be certain to get all requested info/pics for you. Thank you for your time and diligence! I will post some additional pics here is just a moment.
Richard

Going to walk down your list:

1 and 2. With No on the elite model, does the hull NOT extend past the engine, aka is a normal transom with the jack plate attached. If so, then your 1 ? inch extra, after your 5" plate adjustment to bring you to 21 ? is the way to go. Shouldn't need more than that.

3. Again, if you want to dial in an alum. the Hustler is the ticket. Right now what you need to know is what pitch and we are trying to solve for that.

4. If you are going to change LU, an OEM regular LU for the 60, 20" height should drop right in. You should do that before you spend any money on the one you have.


-------------------
Let's say you do get a 1.83. Let's say your engine will run your lightweight boat up to 5500 rpms. Let's see what kind of pitch in a 10" prop that will take using a realistic 12% slip: Starting with a 19P your numbers are 54 max and 48 realistic.


Ha! Just what I thought. I go to the prop selector under Boats and Accessories at the top of the page and punch in your 2001 and above, 60 hp (non Bigfoot) and plug in the 19 and guess what? The selection stops at 17P. However, if I go back to the chart and punch in "with Bigfoot" the selection goes out to 26P As I assumed the 60 with a conventional lower unit is not made for light fast hulls like the higher hp engines that are running the 13" diameter props and up. Therefore you can't get to where you currently are

So, back to the numbers. Running your best in class 17P on your proposed conventional LU, 5500, 1.83, 12% slip = 48 mph in a solid, 42 @ 12% slip. You are better than that as you are!

This time using your current 115 hp 2:1 gearbox, with the max available 26P just for a data point, 5500 rpm = 68 mph max and 60 with the 12% slip. If your hp exceeds the drag sufficiently, you could do that with your current engine and a new prop.....but that number I'd bet is pretty optimistic.....You don't know till you try it, but I don't want you having "visions of sugar plums" when they are impossible to obtain.

Now, do you still want to pursue your standard lower unit?
--------------------
5. 4 strokes have (1) intake cycle (? revolution), (2) compression cycle where fuel air is compressed another ?, (3) Bang and compression stroke another half cycle (4) exhaust stroke, last ? for a total of 2 complete revs for one Bang! 2 strokers just halve the numbers. Change all ?s to ?s and you do the job in one complete rev. 2 strokers put magnets in the flywheel and pickup coil on a bracket under the flywheel. On my 3 cyl the magnets were 120 degrees apart. Every rev all cylinders get whacked 3 times but only one occurs when the piston is at the end of the compression ? stroke. The other plug firings are wasted but no big deal.

If the 4 stroke is a conventional OHC design, the cam shaft, running at ? the crank shaft speed would be used to drive a distributor firing mechanism to trigger the spark plugs and that would be only one spark per pair of crankshaft revolutions per plug. If the 4 stroke uses distributer less electronic ignition with a separate coil per plug then the crankshaft will have a mark of some kind that passes a sensor and the sensor would fire the plugs every time the crankshaft rolled which would put a spark on the plug every rev and would be wasted every other rev since the camshaft would only set up the compression stroke valving every other crankshaft rev. In short you need to know.

However, as I indicated for you, where you are now, 7200 nor 3600 are usable numbers. They just don't fit the rest of the equation.

6. Great on the 10k max reading. You still need to run the calibration test I asked for using your car's tach which I see you will do on Monday.

 

[Wood Slayer]

Mark,
If the OEM is only going to provide me with less speed or very minor improvements, then no. I dont want to invest into a new L/U.
Now with that said, I dont believe my setup will give me the 60mph range. But, if I can get into the 50+ I will be a very happy Waterfowl hunter!!
So, im including some additional pics that May be helpful to you so you understand my setup.
I also found some additional props to try out. What I have are listed below.
15p 3 blade aluminum with a few chips and scrapes.
19p 3 blade aluminum (original one)
23p 4 blade ss powertech, blades are in fair/good condition. Im going to try and find time to get to the river today. First I will get the tach calibrated!
I will report my data with all 3 props. If I cant get out today then it will be monday.
Richard,
P.S. Im a little confused about the L/U. My serial number pulls up a 2.33 L/U. You keep refering to it as a 2.1. Is it the same?

 

[Texasmark]

Mark,
If the OEM is only going to provide me with less speed or very minor improvements, then no. I dont want to invest into a new L/U.
Now with that said, I dont believe my setup will give me the 60mph range. But, if I can get into the 50+ I will be a very happy Waterfowl hunter!!
So, im including some additional pics that May be helpful to you so you understand my setup.
I also found some additional props to try out. What I have are listed below.
15p 3 blade aluminum with a few chips and scrapes.
19p 3 blade aluminum (original one)
23p 4 blade ss powertech, blades are in fair/good condition. Im going to try and find time to get to the river today. First I will get the tach calibrated!
I will report my data with all 3 props. If I cant get out today then it will be monday.
Richard,
P.S. Im a little confused about the L/U. My serial number pulls up a 2.33 L/U. You keep refering to it as a 2.1. Is it the same?

Published data that I found on the www said that the 60 hp 4 stroke bigfoot used the 115 lower unit. 115 means 2:1. The 90 hp, like my last engine used 2.33.
------------------------
Forget the 15. It will only make things much much worse! I'll run some numbers on your SS 4 blade. That's a nice HP prop and probably too much for you and you don't want the 4th blade in your way....it's a liability unless you have a heavy boat you need to get on plane.....but we'll see
------------------------------

I really like your setup. The last pics you sent told me a lot. I wouldn't touch that setup with a 10' pole. Now let's work the prop and get some reliable numbers for rpm and lower unit gear ratio.
-------------------------------------
Get this data point: Mark any prop's position. Turn the flywheel 2 complete revolutions. If a 115 with 2:1 the prop will have rotated 1 complete turn. If a 2.33 it won't make but .85 a turn. Then we will have that put to bed. Send data back when you get it.

One last question. You are wanting all this speed from a 15' boat with a 60 hp engine. You are working for 50 mph. That speed possibly could be realized with just you in the boat with some gas and a battery and a prop pitched for that load. When you load up your boat with "dekes" and guns and ammo. and camo. and rations and all that your speed is going to crater! I'd hang on to your current prop for a backup and let's see what it's really doing for you....is it putting the engine rpms where they belong!