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Old 10-02-2018, 01:36 PM   #21
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Izzy:

Here is what little I know about gear ratios and some rules of thumb I have read about:

You pick the gear ratio that lets your prop shaft spin at about 1,000 rpm at a fast cruising speed. I believe your engine is rated at about 200 hp at 3,200 rpm. So at say a fast cruising rpm of 2,800 you need a gear ratio of 2.8:1 to get 1,000 rpm at that rpm.

Then you work propeller diameter against prop pitch to achieve a prop diameter to pitch ratio of about 1:1, ie "square". That is generally the most efficient prop to use.

Transmissions only come in certain ratios so pick the ratio closest to 2.8:1 and then select a prop diameter and pitch to give you as close as you can to a square prop.


Join boatdiesel and use their prop calculator to play with ratios and prop dimensions and to see what kind of performance you will achieve.


But the best approach is to find a good prop shop to work with to pick the right prop and transmission ratio to get this all to work right for your boat.

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Old 10-02-2018, 02:08 PM   #22
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That makes sense. The max torque is at 1700-1800 rpm 385ft lbs. wouldn’t that be where I want the motor running at max efficiency? Which would make my gear 1.7:1? So again I am by far not an expert but inquisitive. What is of most value torque or HP?
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Old 10-02-2018, 02:38 PM   #23
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Izzy:


These are general rules of thumb and you probably need to read Dave Gerr's Propeller Handbook for more explanation.


Maximum torque is only relevant when your throttle is to the wall and you are accelerating. Otherwise you pull the throttle back when you reach the speed you want to travel and you are then pulling much less hp and torque from the engine.


But I can say this, if you used a 1.7:1 transmission ratio (and I am not sure that they are available that small) then you would have to have a very small diameter prop to load the engine up at wot when the prop is turning at 1,880 rpm and that would be very inefficient.

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Old 10-02-2018, 04:55 PM   #24
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Im so unsure of my choice in gear. So here goes another question related to the gear and engine set up. What should I expect on the cruise side of the equation. My goal is to travel to Bimini and Cuba. I really want to put it on auto pilot at 10 to 12 knots or 7 or 8 knots and burn 1.5 GPH. If I really like the boat I’m going to put the 6.7 Cummins as that is the engine set up of choice.
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Old 10-02-2018, 05:05 PM   #25
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Izzy you might want to start a thread in the Power Systems forum section discussing your project. You will get more responses than in the Welcome Mat.
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Old 10-02-2018, 05:43 PM   #26
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Teach me Jedi Night! Hahahah this is my first forum. I read what I can see on the internet but I’ve never joined. But everyone is friendly and I am enjoying it.
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Old 10-02-2018, 05:56 PM   #27
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I have the same boat,220hp ZF 63 trans 1.75 to 1 with 20 X 20 prop just right
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Old 10-02-2018, 05:57 PM   #28
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sign up with boat diesel.com. Entry level membership is $25.00. Enter your boat and engine information in one of their calculators and it will calculate transmission ratios and propeller sizes for you. Also a lot of knowledgable guys on that site too.
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Old 10-02-2018, 06:22 PM   #29
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Easy peasy. Go to this link then click on New Thread.

http://www.trawlerforum.com/forums/s6/

Then copy and past your text from post #14 plus anything else relevant.
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Old 10-02-2018, 08:36 PM   #30
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You had me at Mainship, locked me in at project. Then you step it up and move from I-6 oiler to a V-8, I believe 2 stroke, diesel.


Yo kay, welcome.
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What a pain in the transom.

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Old 10-02-2018, 08:39 PM   #31
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4stoke diesel
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Old 10-02-2018, 08:40 PM   #32
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But glad that either your as nutty as me or we are on to something here. What do you think of my little project where do you see the pitfalls
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Old 10-02-2018, 09:14 PM   #33
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Blue mainship what do you cruise at and what is your fuel burn with that set up. Not even trying to compare the 5.9 with the 6.5 as I know that engine is the gold standard. But trying to have an idea
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Old 10-03-2018, 08:58 AM   #34
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Quote:
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4stoke diesel


Quote:
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But glad that either your as nutty as me or we are on to something here. What do you think of my little project where do you see the pitfalls



I'm diggin it.
I see no pitfalls as long as you stay ahead of things and think them through. Keep future repair and mainteance in mind.
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Old 10-05-2018, 01:55 PM   #35
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Quote:
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Blue mainship what do you cruise at and what is your fuel burn with that set up. Not even trying to compare the 5.9 with the 6.5 as I know that engine is the gold standard. But trying to have an idea
I cruise at 8.5 MPH 2 gal an hour 1400 RPM, fast cruise 14 MPH 8 gal an hour 2100 RPM.You have to have trim tabs to get MS1 on a plane
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Old 10-05-2018, 07:56 PM   #36
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Wow that’s impressive. I would love to hold out for the Cummins.
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Old 10-05-2018, 09:27 PM   #37
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Of course the keel fairing help a lot to get cleaner water flow to the prop
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Old 10-06-2018, 10:54 PM   #38
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Do you have to go under any bridges in PC? I’m wondering am I going to have to loose the bridge? I know I will have an aluminum radar arch up for sale as that will not clear for sure.
I have to go under the Edgewater bridge by Cousley.
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Old 10-07-2018, 09:57 AM   #39
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No bridges,my boats docked off of ohara and administration st.
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Old 10-07-2018, 10:03 AM   #40
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Do you have to go under any bridges in PC? I’m wondering am I going to have to loose the bridge? I know I will have an aluminum radar arch up for sale as that will not clear for sure.
I have to go under the Edgewater bridge by Cousley.
That looks tight. You might want to measure that at high and low tides.
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