40 something foot yachts - engine variables

[Action]

Thanks for the reply. I bet the green has a lot to do with performance too!

 

[Iggy]

What kind of hp would be necessary to go North from the Gulf?
Or is it just better to go down stream?
Where else would one encounter a current of significance that could not be waited out, like a tide?

Every boat is different on this topic of current. At least once a year I go though the Cape Cod Canal. Currents can range from 0 to 4, some days as high as 5 knots.

I went against the current one year with a 30' twin engine boat. After that day, I will never do that again! Never mind with my 40' trawler. I did nothing but burn up fuel for 7 miles of canal! After that I time it to go with the current.

Lets say I was doing 7 knots and the canal is 7 nautical miles and your math is right. It must be close to it. Now I hit a 3 knot current head on and now I am doing 4 knots. Now it will take me 1hr and 45 minutes. To go with the current burning fuel at a 4 knot current rate with a 3 knot push to do 7 knots.

I don't know if your math is correct. I just know I will not do that again unless I have to.

 

[sunchaser]

I have twin Cummins 330hp that I run at 1800-2000rpm when heading to Catalina. The last 15 minutes of the 2 way trip I run at 2400-2600 to help keep injectors clean.

I was not aware that injectors got “dirty” . I am aware that poor fuel filters or bad fuel affect injectors on the Cummins Diamond series 330.

Provided your oil temps are 190 F or so at 1800 RPM 2600 is maybe fun but not necessary. In general being over propped, bad exhaust design or poor maintenance are the three areas Tony Athens says to be religious about on the 330s.

When used as a genset the 6BT will run for many thousands of hours at 1800 RPM with nary a hiccup. But, it is hard to over prop a genset .

 

[Hippocampus]

Going back to the HP decision. Spent decades with engines around 100hp. And that was sufficient to cross oceans. However that was in sail. When leaving sail listened to people I trust who consistently told me go SD not FD unless I was going to continue with blue water. SD gives you more options and more freedom.

This has been shown to be true. Shorter weather windows required. More ability to play tides and currents but also the ability to ignore them when you want. Locks and bridges much less a timing concern.

They also consistently told me to go common rail. In sail I was willing to accept a turbo but not a computer I couldn’t fix at sea. They said that attitude is ridiculous for a coastal boat. With common rail the concerns about running at low load are markly diminished. Similarly the twin v single is a tempest in a teakettle as you’re not away from service support and are within tow range in the very rare case of need. So need for a twin and avoidance of common rail is way overstated. A very different risk than if blue water.

Finally asked the OPs question. Consistent feedback was you don’t need the massive HP until you do. That’s been proven to be true as well. Already have met the elephant several times. Idiots violating rules of the road and you need to scoot out of the way. Times fighting wind, waves and current and needing to make headway and have good control. Times when the forecasts have been wrong and you need to get inside NOW.

I’m not running around on plane. Those times at WOT are few and far between. But do run at above hull speed multiple times. So I have a Cummins QSC at 540hp in a NT42. Is it too much? Yes most of the time. Would I want less? Most definitely no as having that power improves quality of life, useful miles traveled per day, and safety.

If I went back to blue water it would be a different calculus. Would use Gerr’s formula to calculate need and add 10-20%.

 

[Mako]

Your last sentence is interesting because the genera rule I learned from my naval architecture class for powering heavy displacement commercial vessels is cruise speed at 80% power.

I prescribe to that, but very few here on the TF do.

 

[Action]

Your last sentence is interesting because the genera rule I learned from my naval architecture class for powering heavy displacement commercial vessels is cruise speed at 80% power.

I prescribe to that, but very few here on the TF do.

Thanks.

I suppose there is a formula for everything.
How would that 80% be calculated on the water?

And lets throw an example - data from a YW listing. Have no idea if it is accurate
42 CHB Ponderosa Sundeck
Waterline length 38 feet
Beam 13.67 feet
Draft 3.5 feet
displaces 26,000 pounds
Twin Perkins T6.3544M stated hp 165
Have no idea about prop as it is not given
Not sure what else is needed.

if 80% then 165 x 2 = 330 x 80%= 264hp

How does one know when they get to 264hp?

Assuming 80% is ideal. OK my word not what you stated. Let me know if that is inaccurate.
Is there a down side to going slower?
Or going faster?
Also would have to assume a clean hull and engines in good working order.

 

[Mako]

Bear in mind that heavier vessels are less affected by wind and wave action, so less reserve power is required. Also if these vessels generally stick to crossing oceans then they are also infrequently affected by having to fight against tidal currents.

They are also not affected by marketing campaigns. If the builder of a 40ft powerboat states that his vessel is perfectly powered for an economical cruise of 7.5 knots, with a Vmax of 8.0 knots, how many boats is he going to sell? Few if any. That why the term “semi-displacement” was coined, to get you into the 10-12 knot range, but not into the Magnum / Cigarette / Donzi category.

Lots of factors out there when it comes to deciding how much power you need. Of course this is only applicable for a repower or a new build. Otherwise you get what came with your boat.

So to say that a boat is “overpowered” is of little value unless you clearly define what the performance and operational parameters are. It’s SOR (statement of requirements).

 

[Mako]

Oh, silly me I didn’t answer your question, you calculate your engine’s power based on fuel burn and manufacturer’s power curves and accurate measurements (FloScan or similar).

 

[Action]

Thanks.

The above vessel was not built or marketed when new to cross oceans. Anything I buy will not be used in that manner. In my ignorant opinion, I am unwilling to pay for a vessel that would be equipped to do that. I have a rough idea of what might be necessary to do that kind of travel. It has an appeal however there is a price to pay for that kind of travel.

Actually, I do have some choice in engines with used boats.
Most that I have seen are twin diesels. There have been a few twin gas and a few rare single diesel.

Even in the twin diesel which is predominate, the choices range from twin NA Lehman at 120hp each to twin 3126 Cat at 420hp each. The older ones tend to be lower HP range than newer. However they all fall in a window of 25 to 35 years of age in the 40 to 44 foot aft cabin. Which triggered my question and the start of this thread. There is a spread of 240 hp to 840hp which I view to be rather large!

Is there a point to all the extra hp?
Or is it just to move a 42 foot aft cabin over 20kts?

 

[Turtle Blues]

Umm, well, summed up, Yeah after about 12kts that is the point!

 

[Comodave]

Everyone is different. If we weren’t then everybody would be driving the same car, etc. Some people are happy at 7 knots and some others are happy at 20 knots. That is pretty much why the horsepower differs so widely. Boat builders try to build what the customers want and in the last little while it appears that people have wanted faster boats. There isn’t a lot of logic to it and trying to understand can drive you nuts. Just accept it and if you want a slower boat then you will likely be looking at boats from the 70s and 80s. Or if you buy a newer boat plan on running it slow and compensate by running it up occasionally to blow out the engines. I would advise on buying the best condition boat that fits in your budget and then be happy…

 

[OldDan1943]

My N46 had a John Deere 120. I would run it WOT until the temp got high then I would slow down a bit until the temp was “normal” and I was content.

 

[Action]

Thanks so much.

I believe I have my answer.

 

[mmullins]

This trend isn't just apparent in boats. For example, a 1976 Honda Accord came with 68hp. The current Honda Accord has 195hp.

 

[PierreR]

Prior to buying a larger boat, the fuel bill was a big deal. Once I bought the boat the fuel bill seems quite reasonable. I put 475 hours on the boat this year and the fuel bill is less than the winter storage.
I have twin Detroit 453's

 

[ksanders]

I reliably get 1.75 NMPG at 7-8 knots and really like the ability to cruise at 15 knots when need be, for example to make port or an anchorage before dark.

 

[sunchaser]

The popularity of a 500 HP EV is amazing. Even the politicos love them giving all sorts of largesse to allow reasonable purchase.

Fast cars, fast boats and fast women are intoxicating. But they all come at a price. If you can’t afford them turn on the TV or go to the movies where all abound.

Since Adam bit the Apple mankind has lusted for speed whether chariot races or the Reno air races. It is in our genes.

 

[Mako]

I reliably get 1.75 NMPG at 7-8 knots and really like the ability to cruise at 15 knots when need be, for example to make port or an anchorage before dark.

So each of your engines is running at about 40hp. That’s barely idle speed. Are you reaching operating oil temperature - do you run up your engines hard every few hours?

 

[ksanders]

So each of your engines is running at about 40hp. That’s barely idle speed. Are you reaching operating oil temperature - do you run up your engines hard every few hours?

My engines hold right at the thermostat temperature.

My normal cruise RPM is 1250-1300 RPM.

No i do not intentionally run my engines hard every few hours, but I do run faster from time to time when the situation warrants.

I will paraphrase quote Tony Athens, probably the most knowledgable person on the planet regarding real life operation of the B series Cummins engine.

He has Never seen a b series engine fail due to low loading. Never.

 

[Jeff F]

So each of your engines is running at about 40hp. That’s barely idle speed. Are you reaching operating oil temperature - do you run up your engines hard every few hours?

Those engines, and most since, have coolant/oil heat exchangers. So at prolonged low output oil temperature gets up to at least coolant temps. I think it's a non issue then.

I cruised with a 6bta averaging 1.5 gph and never thought it did the engine any harm. Do the same with my current JD, often running all day at 1100 rpm.

Low oil temps may be a concern where there is raw water oil cooling, which may include the Perkins and I think includes the Lehmans as well.

Good thread. Think the OP asked lots of sensible questions.

 

[jbloyd]

In looking at "trawler" yachts for sale there seems to be a fairly wide range of engine HP choices. Not that one has a lot of choice in the used boat market.

What I don't exactly understand is twin engine HP ranges from 300 to maybe 800+. And I assume most of these hulls are semi-displacement hulls.

What possible use is 700hp twins in say a 42' aft cabin?
To move that type of yacht 8 to 10 kts, isn't 500 hp or less sufficient?

Twin 120HP are more than sufficient if you are not trying to plane your condo.

 

[Action]

Twin 120HP are more than sufficient if you are not trying to plane your condo.

I have come to that conclusion.
And should I want to travel at far greater speed I have an (older) express cruiser that can travel at significant double-digit speeds for that want should I have it.

As an interesting note and may be a lil off topic (however it is my thread) my September/October Boat US magazine came in today. On page 24 there is an article stating that proposed legislation is out there to expand the speed limit restricted areas on the Eastern seaboard because of the North Atlantic Right Whale. The speed limit would be to 10kt for vessels that are 35 feet long or longer. Article does not state how far off shore. But would expand from the current 10 areas to basically Maine to Florida.

If the powers to be get their way no ones gonna go faster anyway.
I don't understand all of that. As I have not lived on the East coast. Would like to visit some day via the water.

 

[Jeff F]

You said at the beginning you didn't want to go fast.

I meant to jump in earlier on the current question. I've done the trip north from the Gulf to Chicago and plan to do it again next spring. It's not a hp thing unless you want/need to cover ground fast. I've got a single 135 hp in a light 50 ft and have never felt the need for more hp in order to handle the adverse current or for any other reason.

But that said, chugging along an endless channel at 5 knots over the ground isn't everyone's cup of tea, and marinas are few and far between.

 

[Action]

You said at the beginning you didn't want to go fast.

Correct and that is still my position.

Speed is not the goal. Which means a greater HP than a pair of Lehmans or Perkins would be the desired power package. Actually, a single would be what I would really want. That power choice would take out half of the maintenance and engine parts & labor required. Sadly a single diesel is an extremely rare thing in a 40 to 45 foot aft cabin.

There are lots of boat choices with twin Lehmans or Perkins power options. Knowing that there are other choices in much high HP ratings, my question to start this thread becomes why. The answer I have come to is speed. Which seems silly to me. Speed in a trawler. Speed in a vessel that was designed to move slow.

The land equivalent might be a motor home with enough power that one could smoke the back tires. Rather silly in my opinion.

 

[Jeff F]

Actually, a single would be what I would really want. That power choice would take out half of the maintenance and engine parts & labor required. Sadly a single diesel is an extremely rare thing in a 40 to 45 foot aft cabin.

I've seen a few in steel or aluminum. This one sold, but a good example. https://www.trawlerforum.com/forums/showthread.php?t=66575

 

[Action]

Thanks

 

[mmullins]

Correct and that is still my position.

Sadly a single diesel is an extremely rare thing in a 40 to 45 foot aft cabin.

Grand Banks 42 classics can be found with a single diesel as can Selene's 38 Voyager.

 

[sunchaser]

Action
What is your budget?
Are you looking for a fixer upper?
If a fixer upper do you have relevant skills?
What are your cruising plans?
Where will your boat be moored?
How many miles per year are you anticipating?

Lastly, if you had the funds would you buy a Nordhavn 41?

 

[OldDan1943]

How many friends do you have?

 

[mmullins]

Action
What is your budget?
Are you looking for a fixer upper?
If a fixer upper do you have relevant skills?
What are your cruising plans?
Where will your boat be moored?
How many miles per year are you anticipating?

Lastly, if you had the funds would you buy a Nordhavn 41?

I think the OP wanted an aft cabin. But, that being said, I believe I could get real flexible on my "wants" list if I could afford a Nordhaven 41!