Gear ratio....i'm so lost

[JoeM]

There is no real performance curve nor numbers for the little 3.0l mercruiser, it doesnt exist.
I have already researched that. It is not there, and, that little engine does not make 135hp. nor 140hp, nor 130hp.
it is reliable, and makes torque, it is a good engine, but the HP numbers don't exist.
that's because it doesnt make much HP. but, it will make torque.
I have the old version (non Vortec) and it makes about 112HP.
That is, if you assume for gasoline to provide about 10HP/per gallon/per hour. and that is generous.
My 3.0L engine burns 8.0 gallons per hour balls-to-the-walls.
That indicates to me 80HP.
At cruise, I burn 5.0 gallons per hour, and that tells me 50HP.
There is only so much BTU's of energy per gallon of gasoline, and as such, my little 3.0L engine is telling me that it is 112HP at the flywheel.
If you start doing the math with 112HP at the flywheel, then DEDUCT transmission losses, then DEDUCT strut bearing losses, and DEDUCT all the other losses, what you end up with is the numbers posted by my fuel flow measurements, AND that of Glen-L who designed it.
And at the end of the day, it is about 80HP AT THE PROP. hence my numbers of 8.0 gallons per hour, @ 10 HP per gallon per hour.

If you build your 27' True Grit, and install a 3.0L VORTEC engine, you may go up to 115HP @ flywheel, but not much beyond that.
It is a good engine, BUT, don't let the salesman sell you into 130HP. it doesn't exist.
It is a good engine, but only buy into 115HP.
It will make torque though.

Joe

If your engine will turn 4800, I assume it's not diesel. At 10 knots, that's about 65% of WOT. That is about right for gasoline. Diesels at 80-85%. The prop estimates should be good as starting point. Cruise RPM should bee about 2875.

Bill

thanks guys. I took way too long in my responses and ended up losing everything I typed up. Lots of calculations that I should have done on paper and transferred to the post instead of working it out as I type . I'll retype it all later.

[JoeM]

Time to try this again.

I ran some numbers at Victoria Props.

LWL: 25' Beam: 8' Draft: 1.4' Displacement: 7,000 Max Engine HP:112 Ratio: 2:1 Number of bearings: 2

Results: With 3% loss at gearbox and 1.5% loss per bearing per their built in numbers.

Max HP at prop: 105.3 Max Torque at prop: 230ft/lbs

Displacement Hull Speed: 6.7kts (7.7mph) This is using the traditional displacement formula of 1.34 times the squareroot of LWL( 1.34x 5) being a semi-displacement hull speed could be more but I figured this is a good place to start.
Min Hp required for Hull speed: 15.3HP @ prop
Max Speed with 112 hp engine 13.13kts(15.1mph)
HP at prop needed to get 10kts: 46hp

They recommended props 3blade 15.1'x9.5' 4 blade 14.2'x9.3'

I recalculated with their value of needing only 15.3hp@prop for running at minimum hull speed
Found if I ran at 20% power I get ~ 21hp at prop with engine RPM of 960 and prop RPM of 480.
I would burn ~2.1gph and get about 3.67mpg with a max range of 579.86 miles with my planned 158gal fuel capacity.

I also calculated their need of 46hp at prop for 10 kts and found that I need to run at about 40-50% to get that value. I ran the numbers at 40,50, and 60% assuming I need up to 60% power to reach 10 kts(11.5mph).

40%
HP at prop: 42.12
4.2gph
2.74mpg
432.9 mile range

50%
HP at prop:52.65
5.3gph
2.17mpg
342.86 mile range

60%
HP at prop 63.18
6.32 gph
1.82mpg
287.56 mile range

[JoeM]

Veering off from the original reason for this thread, I have some thoughts after all that.

I might not be able to hit my targeted 18-20mph with this particular engine and hull. So I have to ask myself if am I comfortable with my top speed possibly only reaching 15mph? And I am. I will be within my 200-300 mile range that I wanted. My "usual" trips are about 20-40 miles out to the grounds and I plan on leaving the night before so I'm in the "zone" for grey light/early bites. Having a minimum 2-4 hour run to and from the grounds, while less than ideal especially for the return trip, is made up for (in my mind) by the comfort that the hull and my layout will give. I will only run at night with 2 people so with a perceived max of 5 other people on the boat(only 3-4 fishing at once) I only need room for max of 3-4 people to sleep at a time.

I also realize that since this hull is not a pure displacement hull my calculations may be off and I could hit a higher top speed and the HP requirements might be different as well.

edit: while my "usual trips" are going to be 20-40 miles to and from, I do want to the capability for longer range or duration trips which Is why I wanted a minimum of 200-300 mile range.

[Triple-d]

Hi Joe
I have a 27' True Grit hull with a 170 hp cummins mercruiser diesel with a bravo 2 leg. Hull weighs 8500lbs with a 67 gal fuel tank. My top speed is 19.5knots, 9.8gph. I cruise at 2800rpm which is 61%eng load, 12.7knots and 4gph. Idle is 3.4 knots and .2gph. Too fast for our fishing trolling so had to put a 15hp kicker motor on. Hope this helps and good luck with your build. Don

[kens]

JoeM,
Sorry, but I'm not agreeing with those numbers.
you should figure a 16" dia prop, and personnaly, I would squeeze in a 17" (leaves you the 4-blade option)
I am running the 3.0L with 2:1 gear
I have hit 30mph on a good day, you should easily hit 20.
When you stretch that design, it is a semi-displacement.
If you used displacement hull numbers, it wont work.
Use semi-displacement formula.

I have contacted some prop shops that were touted a 'Gurus', and they told me my prop was cavitating, all sorts of reasons my boat wont go.
I got another prop shop and all is well.....

[JoeM]

Hi Joe
I have a 27' True Grit hull with a 170 hp cummins mercruiser diesel with a bravo 2 leg. Hull weighs 8500lbs with a 67 gal fuel tank. My top speed is 19.5knots, 9.8gph. I cruise at 2800rpm which is 61%eng load, 12.7knots and 4gph. Idle is 3.4 knots and .2gph. Too fast for our fishing trolling so had to put a 15hp kicker motor on. Hope this helps and good luck with your build. Don

Thanks for the info! Glad to hear another successful True-Grit builder is around and with all the builds i've been looking at I saw your pics on the Power Requirements thread but didn't remember that it was a True Grit design. My apologies, as she's a beauty. Your swim platform and transom door are good ideas and executed nicely.

With rough calculations figuring a 10% loss(not sure what i/o's lose in efficiency) at 61% engine load you're putting out about 93.3hp to get 12.7kts and 4gph.

JoeM,
Sorry, but I'm not agreeing with those numbers.
you should figure a 16" dia prop, and personnaly, I would squeeze in a 17" (leaves you the 4-blade option)
I am running the 3.0L with 2:1 gear
I have hit 30mph on a good day, you should easily hit 20.
When you stretch that design, it is a semi-displacement.
If you used displacement hull numbers, it wont work.
Use semi-displacement formula.

I have contacted some prop shops that were touted a 'Gurus', and they told me my prop was cavitating, all sorts of reasons my boat wont go.
I got another prop shop and all is well.....

Kens, I welcome your disagreement as it's helping me understand everything much better, may not seem like i'm getting it though .

I wasn't too sure of those numbers but figured it was a place to start. That online calculator didn't have the ability to change the displacement hull numbers to semi-displacement.

Also, while our hulls are similar, I believe they are different in a specific way related to planing. If I recall from my reading on these forums, this was discussed before. I thought your aft deadrise on the double eagle was at 1* where the TrueGrit is closer to 5*. I might be wrong but I thought someone described the difference as being the double eagle was a semi-planing/near-planing hull and the True Grit was a semi-displacement hull. The result being that it takes less power to get on plane than the True Grit. I could very well be wrong as i've read so many threads in such a short time.

[kens]

Yeah, Double Eagle planes, but, I also look at what my 3.0L will do, and look at Ray Macke with a outboard on his.
If Glen-L recommended a 16" prop , then trust Glen-L, not the prop shop you called.
There is something that is a constant in the gear ratio thing, and it kinda weird, but it works out that way, and that is:
As boat speed goes down, prop diameter goes up.
Sounds weird?
That is because (for the same given engine) as boat speed goes down, this means the boat size is bigger, and remember what we discussed earlier:
as boat size goes up, gear ratio gets deeper, the bigger the ratio the bigger the prop.
Glen-l you mentioned wants a 16" wheel, your prop shop said 15"
I was alerted when they said 15", I know this is going the wrong way.
I get up on plane, and I turn a 15x16.
If you are NOT going to get up on plane then your diameter MUST be bigger than mine !!!! this is just the physics of it.

It is too early to fret over prop size now anyway. All you need at this point in the build is an idea of your engine you want to use, and the strut & shaft requirements. If you want the 3.0L mercruiser, on a 27' True Grit, then choose a gear of 1.8 ~ 2:1 ratio. Use 1 1/4" shaft. Use enough strut to swing 16" with full tip clearance, and swing 17" dia at tight clearance. Select rudder accordingly.
These are the hard hardware items that get fixed into the build.

Prop selection is at your discretion at sea trials
BTW: (that little engine will in fact turn a 17" 4-blade) that's an option to keep in mind

[PeterG]

A bit behind the posts on this thread, but saw kens mention above that power/rpm curves are not available for the GM 3.0L marine engine. But, I found this from GM:
http://www.gmpowertrain.com/Libraries/M ... .sflb.ashx

Okay I be quiet and read the rest of the posts...

[JoeM]

A bit behind the posts on this thread, but saw kens mention above that power/rpm curves are not available for the GM 3.0L marine engine. But, I found this from GM:
http://www.gmpowertrain.com/Libraries/M ... .sflb.ashx

Okay I be quiet and read the rest of the posts...

I spent a long time looking for it. Using so many different variations of search terms. Thank you sir!

[JoeM]

Yeah, Double Eagle planes, but, I also look at what my 3.0L will do, and look at Ray Macke with a outboard on his.
If Glen-L recommended a 16" prop , then trust Glen-L, not the prop shop you called.
There is something that is a constant in the gear ratio thing, and it kinda weird, but it works out that way, and that is:
As boat speed goes down, prop diameter goes up.
Sounds weird?
That is because (for the same given engine) as boat speed goes down, this means the boat size is bigger, and remember what we discussed earlier:
as boat size goes up, gear ratio gets deeper, the bigger the ratio the bigger the prop.
Glen-l you mentioned wants a 16" wheel, your prop shop said 15"
I was alerted when they said 15", I know this is going the wrong way.
I get up on plane, and I turn a 15x16.
If you are NOT going to get up on plane then your diameter MUST be bigger than mine !!!! this is just the physics of it.

It is too early to fret over prop size now anyway. All you need at this point in the build is an idea of your engine you want to use, and the strut & shaft requirements. If you want the 3.0L mercruiser, on a 27' True Grit, then choose a gear of 1.8 ~ 2:1 ratio. Use 1 1/4" shaft. Use enough strut to swing 16" with full tip clearance, and swing 17" dia at tight clearance. Select rudder accordingly.
These are the hard hardware items that get fixed into the build.

Prop selection is at your discretion at sea trials
BTW: (that little engine will in fact turn a 17" 4-blade) that's an option to keep in mind

Thanks Kens!

I actually didn't call them, so don't fault them for the recommendations. I used their online calculator on my own to get a rough idea. IMO it's too early for me to be bugging shops about specifics that may or may not change as I go through the build.

I did a little more reading on 3 vs 4 blade, and for my purposes I'm starting to lean towards that.

I do agree this is WAY too early in the process to lock myself into a prop or worry about specifics on it. I'll be lucky if I need to worry about this in 4 years. I am just working through the planning phase as I gather money and materials though.

I like your recommendations and for now am planning on going down that route.

[kens]

Hi,
That is the first power curve of any sort I have for that engine. did you see the fine print at the bottom?
the one that says 'power may vary'
I did call a marinizing company and spoke with their engineer. he said that engine wont make 140hp. under his breath he said it is 120 tops.
It is a good engine, but it is not 140hp.

If you build that True Grit to assume a 16" prop, and assume a 1 1/4" shaft, and your engine beds are on standard 24" centers, then you could fit any engine in there, gas, diesel, or whatever gear ratio your engine calls for.
The ultimate goal is correct shaft RPM, if you got enough HP for 20mph then your shaft RPM window is 2000rpm.
For 2000 shaft rpm window, a slow speed diesel is 1.5:1, and a gas engine is 2:1.

[PeterG]

True the fine print on the GM brochure, they are couching their claims as they don't control what the subsequent vendor (like Mercruiser) will equip the engine with. Too many variables such as a carburetor or fuel injection, intake and exhaust efficiency and ignition system used, or installed accessories like the alternator or power steering for a stern drive. And you could have the best bench-tested setup giving the 140 hp and still have losses or lower power output due to the installation in the boat. One of the reasons power at the prop is a much more useful number for boats