"Overpowering?"

Powderpro

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When I was considering an engine for my boat I was looking at an 6.7 cummins at 480hp or the 8.3 at 500hp. I did go with the 8.3 and the marginally higher horsepower but it was not for the "extra hp" reason. Most of the new diesels (under 8 litres) seem to be high rpm, 3000+ engines with a block that is not sleaved. Additionally the horsepower means very little, what is most important is torque and at what rpm it is delivered. In the case of 6.7 compared to the 8.3 the horsepower difference was only 20 but there is nearly 400 lbs of torque separating the two.
Just another spin, pun intended, on the thread.

Not trying to start on argument, but the 6.7 480hp Cummins is 1,165 ft/lbs peak torque at 2,000 rpms, the 8.3 500hp Cummins is 1,327 ft/lbs peak torque at 1,800 rpms. That is 162 ft/lbs torque difference, not nearly 400 lbs as you claimed. I personally think you made the right decision going to the 8.3 500hp. In most cases, the 8.3 500hp is the better choice between those 2 engines. I guess if you were concerned about the extra weight and size of the 8.3, then the 6.7 would be a better option, but the 8.3 is going to deliver better performance in a downeast boat.
 

Powderpro

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There is no replacement for displacement.;)

I've installed a 6.7 liter Iveco 420hp at 3,000 rpm, 2.5:1 reduction, 28" prop.

I've also installed a 9.0 liter John Deere 500hp at 2,400 rpm, 2:1 reduction, 28" prop.

Shaft speeds on both boats were the same, boats were about the same size at 37' and 38', the boat with the 9 liter John Deere weighed about 1,500lbs more.

To get 18-19 knots, the 6.7 Iveco had to be ran at about 77-79% load.

To get 18-19 knots, the 9.0 John Deere had to be ran at about 58-60% load.

Fuel burn at 18-19 knots was about the same on both boats. The larger John Deere did not burn a noticeable amount of more fuel to go the same speed as the smaller Iveco engine.

There are a lot of factors that should go into deciding what engine is best for you.
 

DIRTY RIGGERS

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Not to beat a dead horse,

Found a few numbers on propeller load specific fuel consumption:
Cummins 6BT lbs/hp/hr .430 @ 2000 96hp, .408 @ 2200 127hp, .399 @ 2400 165hp, .397 @ 2600 210hp.

In this case, running at 1/2 the max hp uses 10% more gal/hp/hr with the same engine. Doubling the size of the engine then running at 1/2 load would be much worse, maybe 20% more fuel.


Brooksie. I currently run a pair of cummins 210 motors in my charter boat. Not a downeast but none the less the motors still burn x amount of fuel when propped correctly. I don't usually add up my exact fuel burn I just log it in average gph burned for the total number of hrs that the engines have run during the day b/c we run and troll and idle etc...

I was curious where you got those numbers for the 6BT that you posted above because they are fairly different than the print out I have from cummins by a good amount.

Lets say you are going to make a 200nm haul at 2000 rpms and the boat will run 20kts at this speed. Using my cummins numbers a pair of these engines should be burning 5.9gph each using approx 200hp or 11.8gph for the pair. This burn would equal about 1.7nmpg at a speed of 20kts.

By using the numbers that you posted above it would mean that these motors should be burning a pretty good amount less. Not a bad thing at all. I just would like to know your thoughts on the numbers that I posted above from cummins vs the numbers that you got from cummins.

When I did the math using your numbers I came up with this...
2000 7/.430 = 16.3 30.6/7= 4.4gph
2200 7/,408 = 17.15 34.3/7= 4.9gph
2400 7/.399 = 17.5 35/7 = 5gph
2600 7/.397= 17.6 35.2/7 = 5.02gph

If I did the math incorrectly then let me know and I will delete it so I don't confuse anyone else and drop the whole thing. Basically I am just looking for an independent viewpoint.

The numbers above would indicate that at 2000 rpms the 210 cummins would be burning 4.4gph at 2000rpms using 127hp or about 2.27nmpg/2.6smpg. This is significantly better than the numbers I posted and if this was the case then on a trip of 200nm the fuel burned would be 88gallons using your numbers and 120 gallons using my numbers. A difference of over 30 gallons. Not a huge amount but still it all adds up.

So basically how accurate can these really be. I know it is very dependent on prop slip, sea state, and many other factors to factor in but on paper which cummins numbers are more accurate for planning purposes?

cummins.jpg
 
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Brooksie

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I took my example fron Cummins data sheet bulletin 4000130 3/6/2000 which claims to use 3.0 propeller curve which is pretty standard in the industry. I used the chart because the graph part isn't as clear.
I don't know if this accounts for the difference but in your math, 16.3 x 2 hours should equal 32.6 not 30.6 which would be 4.66 not 4.4
I don't think your math really shows us anything anyway.. doubling the hp produced, per gallon, per hr because you are making a 2hr run, isn't the correct approch it only makes the engine look 2X as efficient.
So I would say, using your charts, making a 2 hr run @ 2000 rpm burning 5.9 gph takes 11.8 gallons. And, the propeller, is using 96hp (not 127) at this rpm so 96/5.9 = 16.27hp/gal/hr (and at 7 lb/gal that equals .430 lb/hp/hr which agrees with the specific consumption on the chart)
"How accurate can they be?" Very, for that 3.0 multiplier propeller curve. Would another multiplier work better in your case? Maybe.
 

DIRTY RIGGERS

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Without risking derailing the thread I figured I would at least ask. I've used the cummins data sheets mainly for estimating and never came up short on a run using the sheet I posted. So I guess factoring in the prop curve would be more accurate than the method I've been using. I will look into it further and try to score a copy of that sheet you mentioned.

I was just using the 2 motors and time of the run as an example since I usually run a twin diesel boat I thought it would make sense for me to try and figure it out that way. As for my math... lol. I never was a math wiz. I probly just hit the wrong button on the calculator to get that number or just stupid. It was all just quick math to try and learn a little something from this thread and I think I did so thanks for the insight.
 

Brooksie

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Without risking derailing the thread I figured I would at least ask. .

Not at all, this is what this thread is about. And this forum is about learning from each other not "data sheets". Besides, derailing be damned...
 

DIRTY RIGGERS

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I hear ya. That's why I like this forum.

As for overpowering... Most fellas down my way tend to think that my boat is under powered b/c it cruises at 20kts and the newer models cruise at 30kts b/c they r pushing twice the power. I laugh b/c we only have to run 2-3 miles for most of the fishing we do. 20 miles at the most to get on the swordfishing grounds. My engines are were made in 1989 and still run everyday. Plus I think 20kts is fast. I grew up going 12kts on my dads boat. haha.

When we do run to the islands during the fronts in the winter for wahoo trips I typically just set the auto pilot and let her eat at 17kts anyways. 4-6 is common. It's bumpy but at 17kts I drink my coffee and watch the sheets of water hit the enclosure right in my helm chair. Them 40kt center console boys wish they were riding on my boat nice and dry. They always tuck in behind my wake on the way home anyways. It's funny to see those boys doing 17kts in a triple outboard boat struggling to keep it on plane... lol!!!
 

DIRTY RIGGERS

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Hey fellas...

Just wondering who has the most accurate prop calculator/power required for down east semi displacement hulls? I've tried to use the one on the boatdiesel website but was wondering if there is another one out there that is trusted. The boat diesel calculator doesnt like to allow the semi displacement boats to go past 13kts no matter how much power you add on.

I'm trying to get some round numbers for an upcoming project. The hull will be a 30 repco. Dont have the hull yet but searching daily. I already have an engine for her in my garage. Now I just need to get the prop in the ballpark with the gear I have. The engine is a 170 yanmar and has a 2:1 gear. I could find a 1.5:1 if needed but it would take some hunting so I wanna try to make the 2:1 work.

I was figuring just in my mind really... that I will need a roughly 18" prop if the repco will have room to swing that size. Probably just gonna run a 1.25" good quality shaft since the 170 isnt a huge torque monster. Time will tell! Can't wait to get to cuttin!
 
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Brooksie

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Hey fellas...

Just wondering who has the most accurate prop calculator/power required for down east semi displacement hulls? I've tried to use the one on the boatdiesel website but was wondering if there is another one out there that is trusted. The boat diesel calculator doesnt like to allow the semi displacement boats to go past 13kts no matter how much power you add on.

I'm trying to get some round numbers for an upcoming project. The hull will be a 30 repco. Dont have the hull yet but searching daily. I already have an engine for her in my garage. Now I just need to get the prop in the ballpark with the gear I have. The engine is a 170 yanmar and has a 2:1 gear. I could find a 1.5:1 if needed but it would take some hunting so I wanna try to make the 2:1 work.

I was figuring just in my mind really... that I will need a roughly 18" prop if the repco will have room to swing that size. Probably just gonna run a 1.25" good quality shaft since the 170 isnt a huge torque monster. Time will tell! Can't wait to get to cuttin!

Did you look at the 30' Repco hull that was in Chatham/Orleans listed on Craigs for 4500? It had prop, running gear, steering, blown? engine, hauler, maybe a tank? Oh no, I guess not I just noticed So Fl address.
 

Powderpro

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Dirty Riggers- I have used the boatdiesel prop calculator on a 34 Calvin Beal and on a 38 Wesmac. When I input my information, I use "Average Planing" as the hull design, and it has always been real close to accurate on the speed and the pitch of the prop. For the purpose of the calculator, I think most downeast boats would be "Average Planing" or "Slow Planing".
 

plowin

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You are correct powderpro I misspoke in regards to my torque numbers. But I'm still all set with 3000 plus rpm engines without sleeves. Regardless thanks for the correction.
 
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eyschulman

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My two cents: With the new common rail electronic diesels I am not so sure the old rule of running a diesel at 80%+ load for its health still stands. Yes run them up for a few min. at the end of the day but lighter loads should not be a problem because the motors injection and electronics will adjust the amount of fuel burnt. For example my JD 6068AFM75 motors are rated at 330 hp at 2600RPM a M 4 rating the same motor with a different chip or electronic program can be rated M2 or M1 the big difference is the top RPM and fuel burn. I and others including some knowledgeable mechanics believe the same effect can be had by using the throttle. In other words I can run my motor like an M1 just keep the RPM and load at lower levels say 50-60% load. Why have the higher rating? Because when I want the extra zap all I have to do is push the throttle forward.
 

DIRTY RIGGERS

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Dirty Riggers- I have used the boatdiesel prop calculator on a 34 Calvin Beal and on a 38 Wesmac. When I input my information, I use "Average Planing" as the hull design, and it has always been real close to accurate on the speed and the pitch of the prop. For the purpose of the calculator, I think most downeast boats would be "Average Planing" or "Slow Planing".

Hey thanks powederpro. I was wondering if that would be closer but didn't wanna get too overzealous on the numbers. Would you say they are more of the slow planing w/o lift rails and avg planing with lift rails? I know every hull is different and it's hard to generalize. Just trying to get close with a prop mostly without going through a bunch first. I've played that game with a few science project inboard boats with unusual power and it is a pain in the ass.

Brooksie. I did see that hull but haven't spoke to the owner. I'm down here in S FL but am looking to have a hull shipped down if I find the right one. There's a few for sale up there right now in all sorts of different shapes. I'm not in a big rush but would really like to score one before the new year if I find one.
 
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Powderpro

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Dirty Riggers- I don't think lifting rails have anything to do with it, my 34 Calvin did not have rails, and it was a rocket. It has more to do with the design of the hull. I would consider the Calvin Beal and Wesmac to be one of the faster, more efficient DE boats. They are both a modern design and I have found them to be very good boats. When I used "Average Planing", the calculator was right on for the cruise and top speed, but it was a little overzealous on the pitch. For example, the Calculator said 33" pitch for my Wesmac, and 31" would have been perfect. On my Calvin, the calculator called for 28" of pitch, and 27" would have been perfect. So in both cases, the top speed was real accurate, and the pitch was only off 1"-2".

I'm not real familiar with the Repco 30, I'm assuming it's an older design? In any case, I would think that you would be safe going with "slow planning" to get you close on your prop. Remember, the more accurate you are on the actual weight of the boat, the more accurate the calculator will be. Good luck.
 

Powderpro

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You are correct powderpro I misspoke in regards to my torque numbers. But I'm still all set with 3000 plus rpm engines without sleeves. Regardless thanks for the correction.

Do you have a 33 Flowers with a QSC 500hp? What kind of performance numbers do you get? I bet that boat really goes good. I built a 34 Calvin Beal with the QSC 500hp and it went very well.
 

DIRTY RIGGERS

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Yea it's an older design. One of the first solid glass lobster boats built back in the 70's I'm pretty sure. We are trying to keep the total weight under 7500lbs which I think is quite possible with all the composite materials and stronger lighter fiberglass weaves out there. We are going to use divinicell foam for the house and trunk cabin and we plan to record the washboards as well. Keeping the weight down should give us a decent cruise but I'm not looking for a rocket ship. The main concern is efficiency and keeping the hull at the speed where she rides best which I am thinking will be in the 11-14kt range. Not sure if lift rails would help at this speed or not so we will try it out without them first. I just wanna see how she runs and how the water flows across the hull at these speeds before going with the lift rails. Spray rails will be added though.
 

FPTMarineDiesel

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My two cents: With the new common rail electronic diesels I am not so sure the old rule of running a diesel at 80%+ load for its health still stands. Yes run them up for a few min. at the end of the day but lighter loads should not be a problem because the motors injection and electronics will adjust the amount of fuel burnt. For example my JD 6068AFM75 motors are rated at 330 hp at 2600RPM a M 4 rating the same motor with a different chip or electronic program can be rated M2 or M1 the big difference is the top RPM and fuel burn. I and others including some knowledgeable mechanics believe the same effect can be had by using the throttle. In other words I can run my motor like an M1 just keep the RPM and load at lower levels say 50-60% load. Why have the higher rating? Because when I want the extra zap all I have to do is push the throttle forward.
Hello EY Schulman,

If I may interject a little, for the most part you correct in all that you state, however no high speed diesel engine provider has a way at the moment to bypass air around the CAC and at light load or no load situations the air coming into the combustion chamber in some if not all marine environments is quite cool. A diesel engine needs heat in the cylinder to remain efficient in combustion and even though the jacket water temperature may be stable the incoming air is robbing heat energy and can over time create issues with ring packs and cylinder wash down, even on C.R. engines with electronic control. For sure some manufacturers can adjust dynamically better than others with more ability to correct for different environmental conditions. Today the components of the engine are much more durable and reliable for most diesel engine producers than they were 15 years ago, mainly due to emissions regulatory demands, this makes it more important to keep them loaded in the combustion chamber and up to working temperature for optimum efficiency and life expectancy. I guess my point is that no diesel engine like to be set at low speeds, with light to no load. The most ideal application for a diesel engine is generator power where an engine sits at a stable speed loaded at 60% or better. Just some things to keep in mind when owning a diesel, running them easier all of the time is not always better and not necessary for electronic engines where combustion is optimized as you state at higher speeds when loaded.
 

Brooksie

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FPTMarineDiesel; no high speed diesel engine provider has a way at the moment to bypass air around the CAC and at light load or no load situations the air coming into the combustion chamber in some if not all marine environments is quite cool. .[/QUOTE said:
I think Volvo did this on some of their models and may still do it. In marine use air is being taken from a warm engine compartment unlike over the road and other uses. Then again it is passing over a CAC that could have 32' water in it; maybe Volvo just bypassed the water not the air? I don't see where Volvo's CAC bypass did any good but maybe in the nordic countries?
 

FPTMarineDiesel

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I think Volvo did this on some of their models and may still do it. In marine use air is being taken from a warm engine compartment unlike over the road and other uses. Then again it is passing over a CAC that could have 32' water in it; maybe Volvo just bypassed the water not the air? I don't see where Volvo's CAC bypass did any good but maybe in the nordic countries?
It is a very good idea indeed and I have never seen it on a Volvo, what model do you speak of? Interesting. It keeps heat energy from being saturated and removed by cold intake air when there is no boost air being added or generated. The CAC's are extremely efficient for the high power density, and the HXC are also so even though the engine temperature may seem stable, the in cylinder combustion temperatures are not, especially in the colder climate regions.

Thanks for the competitor info. I used to work on some Volvo product and had not seen that option on them.
 

Brooksie

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It is a very good idea indeed and I have never seen it on a Volvo, what model do you speak of? Interesting.

Volvo model.. don't know. Maybe I dreamed it. Anyway it could be done with "off the shelf" boost or temperature controlled bypass valve, w/o electronics (my fav way), in the raw water CAC piping.

If I were doing my old engine, I might operate a ball valve bypass manually to test the idea then boost controlled so there would be no charge cooling at low power regardless of engine temp.
 
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