How Much Horsepower

[stevensibs]

I am close to the point in my project where I must choose a replacement diesel engine for the 160 Cummins straight 6 that was in the boat, I assume from the factory. It was a normally aspirated diesel with a Twin Disc 1.97:1 gear. I was looking at the Cummins factory reman 6BT 210 hp engine but I prefer the Lugger that Northern Lights builds. The L1066T has 175 hp @2400 rpm, 165 hp @ 2400 rpm, and 135 hp @ 2200 rpm. These rpm's seem high to me because engine logs that came with the Bristol showed 8 knot cruise at 1600 rpm which is what I want. The next engine up that Lugger makes is 250 hp, 200 hp, and 185 hp, respectively to the rpm's. Anybody know how I can determine what engine to buy and how much horsepower is required? The trawler is 42' full displacement weighing 17 tons gross.

 

[Tom.B]

I freely admit that I don't know much about, and have done no research on, the selection of new diesel engines. My advice is only this: When/If the time comes for us to replace a motor, I'd give a lot of weight to the post-purchase service and the company that is easy to contact and talk to. As shallow as it sounds, I feel like the "pleasantries" of company employees says a lot about many other aspects of the company. That's all.

 

[O C Diver]

If you desire to run the new engine at the same RPM with the same transmission ratio as the old one, it's best to compare horsepower and torque graphs for the old and new motors. While my guess is you will be fine with either the 210 Cummins and 175 Lugger as I believe they both have developed all there torque by 1,600 RPM, compare the HP graphs.

Ted

 

[FF]

The usual displacement rule of thumb is 3HP/tonne for normal cruise 5hp ton max required..

A marine ton is 2240 lbs so if your boat is 17 tons about 50- 60 hp at cruise is all you need.

JD has a small 4 banger that would come really close.

IF it cant crank 60 hp at 1600 ,(use 24/7 hp graph) a slight change in tranny ratio (if that is being replaced will handle it at perhaps 1800, these engines are smooth as sewing machines so even 2000rpm would not be bad.

The other way of handling a higher engine RPM to produce required HP is with a different prop.

This can add efficiency as the diameter might stay the same with only a pitch change.

Again a book ,or the boat diesel computer would refine the numbers.

If 17 tons is by ACTUAL weight on a scale recently , fine , but remember cruising boats get heavier from added toys over time.

And the USCG document Tons number is NOT WEIGHT , but a volume measure.

FF

 

[rwidman]

If you're having an engine installed, the company doing the work can probably guide you in selecting the appropriate engine. If you're buying one in a crate from an Internet vendor, that's probably not the case.

That said, there is a lot to be said for either rebuilding the original engine or installing an exact replacement (new or remanufactured). Everything will fit perfectly, the prop and transmission will be the correct pitch and ratio, there's no need to replace gauges and controls, etc. You could save thousands of dollars and dozens of hours.

Also, the manufacturer of the boat had the expertise to select an engine that would best power the boat efficiently in the first place. Unless the boat didn't perform to your expectations with the "stock" engine, there's no compelling reason to change unless you just want to.

 

[twiisted71]

a NA 6B Cummins would be hard to beat in my opinion. If maintained I'd imagine it should outlast the boat and be as reliable as ANYthing available. I'd love to find one for my boat and get rid of the Perkins powerhouse.

 

[Larry M]

In November 2010, I received a quote to replace our SP135 with a L1066T. The link below and post # 7 is the discussion and quote. Hobo is 43 feet with 37 feet of water line and displaces 44,000 lbs.

http://www.trawlerforum.com/forums/s6/replacing-ford-lehman-sp135-2263.html

 

[Chrisjs]

We displace about 35 tons and have a 280HP Cummins 6BTA (turbo). Typically we run at 1,400-1,700 rpm (2600 max). We are close to full displacement hull and depending on conditions usually run at 7-8kt give or take. Plenty of power and could most likely make do with less. Fuel consumption is about 2.5gpm (3nmpg). With 2,500 hours, no problems with engine/turbo. Engine is mated to Twin Disc MG5050.

 

[Aquabelle]

I used the calculators available on BoatDiesel.com to establish your hull speed as being 8.5kts....and the power needed to achieve that is, as FF predicted in post #4 above, just 65hp at the flywheel. Of course you don't want to cruise at full engine WOT so you need to find an engine that develops 65hp at the engine (flywheel) RPM that equates to the 1600 at-the-prop rpm that you say you would be happy with. (Often, that seems to mean an engine rated at about x3 the hullspeed hp....so your thinking in the range 160-210hp would seem right.) And as rwidman says in post #5, you need to be sure the dimensions of the new engine are compatible with what you are removing. Key question: Are you planning to keep the existing gearbox or replace it with the engine?

 

[Nomad Willy]

I could care less what the engine rpm is. It means nothing. One must match the engine speed and ideal prop dia and pitch. The Dia, pitch and type/style of prop should be decided on first relative to the power required for the speed wanted for the boat. Hull speed is not a factor to be considered for semi-planing boats. Only full disp craft.
Once you come quite close on the propeller choose an engine. As I've said rpm should have NOTHING to do w your selection ... unless you can't afford to change your gear ratio to match the engine to the prop. Consider all the other usual things for engine selection like weight, size, fuel efficiency, price, smoothness, serviceability, after sale service and all the other details. Try to be objective and give all engines available a good look. I passed on Volvo because of their high parts cost. Have seen/heard several wonderfully sweet Volvo engines since and wonder if I made a mistake.
Remember that any brand, any rpm and turbo or not will last and give excellent service for as long as you run your boat and still be in excellent condition when you sell as long as you load your engine properly, warm it up slowly and do all the fluids and adjustments.
I think it's fun to engine shop and wish I could do it again.

 

[twiisted71]

Hull speed is not a factor to be considered for semi-planing boats. Only full disp craft.

Is there another term used for SD and planing hulls that refer to the speed just before the bow begins to rise and water begins to cleanly leave the stern?

 

[stevensibs]

Thank you all for the quick responses. First to GonzoF1; I am considering manufactures positions world wide and their history of service and parts. OCDiver, As the engineroom is bare, all I have to start with is that old Cummins 160 and Twin Disc. They both went to scrap but I kept the logs for the company that sells me an engine. FF, there are no toys on this vessel...no plumbing, no electrics, no tanks, nothing. Building it from keel up. To RWideman, can't rebuild, it's gone. And I'm doing everything myself. I never was on this boat in the water, she was on the hard. To Twisted71, I agree; the Cummins 6BT is a marvelous engine. I kinda want an all Lugger engine room tho, Northern Lights genset and main. To LarryM thanks for the link. It helped. Chrisjs, thanks to you too, I put your numbers in my notes. Aquabelle, it will be a new tranny, existing shaft and wheel, tho I may have to repitch. Manyboats: want to come and do mine? lol. Thank you all for the input, I have to make a decision soon because as I am in the fiberglass stage of this build, I must also prepare the engine beds. Tanks are coming soon then it's engine time. Still don't know which way to go.

 

[Marin]

Have you talked to Northern LIghts/Lugger? They have a pretty good reputation for customer service before and after the sale. A good friend of mine was the head of their engineering department for several decades. He recently retired but if you like I can ask him who at the company today he would recommend as a contact to ask the sorts of questions you have.

 

[Nomad Willy]

Marin,
That's a given as they are all good engines. None are better than others except for some details. Northern Lights build fine products but they are 96% parent engine. One of the NL engines is a Toyota ... The smallest I believe.

 

[stevensibs]

yes as a matter of fact, just the other day. They were very nice to talk to. The reman Cummins with two panels and harness is 18k. 2 year warranty. I bet the Lugger is twice that tho.

 

[Marin]

Eric--- Pretty much every engine that goes in a recreational boat under perhaps 60 feet or whatever is a marinized version of somebody else's base engine. The exceptions are those companies who decided the market was profitable enough for them to marinize their own engines--- Cat, John Deere, Cummins, and Volvo to name ones I am familiar with. But Lehmans, Perkins, Luggers, Sabres, etc are all based on someone else's engine. Lugger uses a lot of John Deere base engines as well as engines from Japan and Germany.

But the reputation of the engine has more to do with just the reputation of the base engine or the company that made it. The marinization can be designed and executed in a high-quality way or not so much. The Ford Lehman 120, for example, has a badly designed and manufactured raw water pump drive coupler. I know this because the fellow who designed it way back when told me. It seemed a good design at the time, he said, but time has proven it not to be.

So while Lugger may not make their core engines, what they do to them after they get them from Deere or whoever I think has a significant impact on the longevity and reliability of the engine. Plus of course, if you buy a Lugger engine you are not going to get service or support directly from John Deere, you're going to get it from Northern Lights/Lugger. So the quality of their service and support operations and staff will have a major impact on the experience you have with their products.

 

[FF]

I bet the Lugger is twice that tho. __________________

Depends on your EGO.

The way to get a JD or Lugger is to decide which you basic engine need and wait till JD TRACTOR has a sale , on factory rebuilt , with NO core required.

THe rebuilt may last longer than a new engine as the block will have moved about during its life , and re machining will produce a unit with much of the heat stress distortions removed.

A Twin Disc , also used or rebuilt will complete the package.

There are many aftermarket sources for wet exhaust manifolds if that is how the boat is set up.

New build dry stack and keel cooled will offer far less maint required , and loads easier to live with in cold areas.

If you need 65 hp an engine no larger than 120, (at its 24/7 rating) would be advisable to assure proper loading and no slobbering.150 if you have large hyd. requirements underway or a large cruising generator .

Our system for replacing an engine that is usually far smaller than the add dept specified for the first one, is to use a similar swept volume at the same cruise shaft RPM.

This allows the existing prop to be reused.

FF.

 

[stevensibs]

I need to speak with FF some more, I thinks.

Steve

 

[FF]

One of the biggest mainization costs is a wet exhaust manifold.

This is claimed to help lower the engine room temperature and allows a wet exhaust.

My preference is dry stack, but if a wet exhaust is required Custom Marine Inc. - It's All About Performance - Header Applications, Tailpipes, Accessories

and others can supply a wet section for an exhaust.
Marine Exhaust Systems of Alabama, Inc.

www.[B]mesamarine[/B].com/
http://www.mesamarine.com/
The stock manifold gets a riser , drops down and then the wet section is installed.

Any number of sources will sell a lace on hi temp insulation kit for the exhaust manifold to your specifications.

Hopefully good engine room ventilation is part of the swop, most stock boats stink at providing enough air.

The fewer "marine" parts the lower the cost.

IF you are willing to go outside the box , the international 360 and 466 (6 cyl inline) are built with electronic timing , or straight mechanical.The boat choice for under 1000 hours a year.

These are smooth quiet engines that are sold at remarkable low prices with minor use.$3k to $5K and all have SAE bell housings so the TD tranny is easy to fit.

Many working lobster boats and others that can use 100-150 Cont hp have discovered these robust engines.

With a good used T/D the installation could be under $5K to 8 K complete.

While a good auto wrecker can provide a late model engine with few miles , the best I have heard of is a Skoolie bus takeout.

The Feds allow the skools a new bus every 12 years or so , however the highway funds are also robbed for ongoing maint.

At the 10-11 year point many will receive new engines , as therre "free amnd the mechs find R&R very easy and profitable.

Purchase one of these at auction and for a grand or two you have a nice new engine , and all the electronics if you prefer the fuel saving of the black box.

Boating only needs to be expensive if you must Strut The Dock , and do not have some mechanical skills.

FF

 

[stevensibs]

Thanks FF. Another avenue for me to consider. The new Lugger and one panel came in at 43k. The recon Cummins is 18k with two panels and 35' of harness for the flybridge and 15' harness for the pilothouse. All in all, a pretty good price and it's in my budget for the refit. I know they are reconditioned in Mexico; I'm not sure if that's bad or good. I've looked at them on the pallet, all gleaming white and nice looking with 2 year warranty or 2000 hours. One can't beat the parts supply and service centers of Cummins' network...anywhere I cruise I will find service. This choice certainly isn't about ego; I am refitting this trawler from the keel up, doing it right (imho) and want no trouble with the motor. The genset I have decided on is Northern Lights so naturally I looked into Lugger for propulsion. Cummins does have a recon engine of 180 hp @ 2200 rpm which I think more closely resembles the 160 hp I took out.
If you have any other thoughts I'd love to hear from you and the other guys that answered this post. Thank you all...

 

[Nomad Willy]

As to the power requirement consider carefully how much of the power you used on the old engine and how that loaded the engine. How important engine loading is, is not really clear but engine manufacturers have the best information .... especially the engine you will have. I personally prefer 55 to 65 % of WOT determined by a percentage of the maximum fuel that the engine can burn at rated rpm. Get that max fuel burn number from the engine manufacturer. A 120 Lehman is 6 gph for example. As I recall most of our engines are most efficient at about 1800 to 2000 rpm. Over propping may produce even more efficiency but has it's own problems. Many do it though. Running your engine about 600 rpm down from it's rated speed would be a good load to shoot for if you can attain max rater rpm .... And you should be able to do so. It's quite likely a smaller engine would be better for you as most boats are over powered. I think a smaller engine running at a greater load is better but many disagree w me. I also think one needs very Little (if any) extra power for bad weather ect. The most objective people to talk to about this aren't on the forum ..... Manufacturers!

 

[stevensibs]

Very insightful...I agree with you that a smaller DIESEL engine running for long periods at 65 to 85 per cent of it's rated power is better than a larger engine with added horsepower that never gets used. I never got to see this boat run; the nice old folks that I bought it from lived on her for some 20 years then put her up on the hard waiting to be sold. I considered rebuilding the old Cummins but could not get parts or support down the road. I still may spring for the Lugger but it messes up my budget fierce. My feeling is anything from 160 to 210 hp will do the trick and as you say I can dial it in by bending the wheel.

 

[koliver]

That sounds like a question for the Forum on "Boatdiesel.com" Spend $25 for a membership. It will be the best investment you can make before repowering.

 

[THD]

You might want to do a search for recent articles by Steve D'Antonio. He wrote recently (in Passagemaker I think) that virtually all displacement hulls are way overpowered. He is pretty compelling on the issue. You also may be able to find it on his website www.stevedmarineconsulting.com. I have talked to him on several occasions and he is a pretty helpful, accomodating guy. It may turn out you only really need a much smaller powerplant than you think.

 

[stevensibs]

I will join Boatdiesel.com and I have corresponed to Steve D. Good ideas.

 

[THD]

Keep us posted on what happens. It will be interesting to see what you come up with ccompared to what you originally had.

 

[FF]

the nice old folks that I bought it from lived on her for some 20 years then put her up on the hard waiting to be sold.

If those nice old folks kept a log book , simply use the GPH as an estimate of the power required.

With a large engine that is probably not efficient at cruise I would use each 1 GPH to estimate 15 hp at the prop.

FF

 

[Nomad Willy]

Twiisted71'
No. And I was of the opinion that hull speed was attained just as the bow started to rise for decades. It seemed obvious to me but if you calculate your hull speed and run your boat in near calm conditions it becomes clear that the bow starts to rise long before hull speed is reached. Iv'e known for a time now that the best speed to run a FD hull and probably a SD hull also is well below hull speed. I know of no specifics but .9 knots below hull speed must be about right. Prismatic coefficient, aspect ratio and many other things enter into it but if you like rules of thumb 90% of hull speed would seem to be about the best speed for economy. Could'nt go far wrong. Perhaps Skipper Dude is listening. On BoatDesign.net one person proclaimed that a boat was planing when the water cleanly leaves the stern. Getting back to the bow rising something more specific may be happening than w hull speed. The boat is clearly balanced on the bow wave and the following wave before the bow starts to rise and after it does the bow is beyond a doubt starting to climb or try to climb the bow wave. The surfing action I alluded to before must drop off rather quickly after the bow starts to rise. I think you are right that this is an important speed to recognize and try to understand. It would be nice to know how 90% of hull speed compares w the exact speed when the bow starts to rise. Anyone have any knowledge or opinions here. Perhaps someone could make some runs and come up w numbers that would apply?

 

[Marin]

Eric--- I don't know squat about hull design and coefficients and buttock lines and so forth. But I can give you an observation I deliberately made one day some years ago when we left Bellingham with the bay in absolute glassy conditions.

I was curious for no particular reason that I can recall so I taped a pencil horzontally to the steering cable raceway that goes up to the flying bridge infront of the helm so I could line up the pencil point with the top fo the bow rail. Keeping those things in alignment I then began to add power from our idle speed. And you know what? The bow started to come up immediately as observed againt Lummi Island across the bay. Not a lot and as I added power on up to our cruising rpm of about 1600 the bow did not rise significantly, but it rose a bit more with every addition of power.

I attributed this to the simple increase of hydrodynamic pressure against the hull. It's going faster, the pressure is greater against the hull, particularly its forward section, and the force has to result in something, so the bow comes up.

At 1600 and at the pitch the props were then (their pitch is different today) we got 8 knots. Which I believe is a wee bit over hull speed for this boat.

I've observed the same thing in the floatplane. As you idle around at what we call "displacement" speed, as soon as you start adding power, even just a bit, the nose starts coming up. Oviously when you go to runup power and later takeoff power the nose comes way up. But the nose starts rising even with an additional 50 rpm over idle. Again, I attribute this to the simple increase in hydrodynamic pressure against the floats.

 

[Nomad Willy]

Marin,
Never heard of a boat that rose in the bow just above idle. Perhaps you've just got too much power.
As to the airplane that's obvious .. You increase throttle any amount and you increase prop wash and that goes over the horizontal stabilizer and the negative incedense therein pushes down on the tail and since it is attached to the fuselage the "bow" starts rising immediately.