How much horsepower do you need?

We're in the early planning stages of selling our boat on the lake and buying a boat on the southeast coast. As I'm looking at boats for sale to get myself educated on options, I'm leaning towards a trawler.

The thing that I can't figure out is the wide range of power for similar size boats. I'm scratching my head a bit at how some of these boats would even move in a headwind. So I'm clearly misunderstanding something and hoping someone can set me straight.

For reference, my current boat is 38 feet, 27,000 pounds, and has two 420 hp MerCruiser gasoline engines. It's a planing hull and can go pretty fast if I want to burn some fuel. But, I don't normally cruise at more than 8 to 10 kts.

As I'm looking at trawlers, I see some with similar size (diesel) engines to my current boat. For example, a Grand Banks Europa 42, 43,000 pounds, with two 420 hp Caterpillar engines. On the other end of the spectrum is a DeFever (49) weighing 54,000 pounds with two 150 hp John Deere engines. Or, a Selene 40 weighing 40,000 pounds with one 300 hp John Deere engine.

Are the lower powered boats intended to run at no more than hull speed? Still hard to imagine that 300 hp can get a 54,000 pound boat even to hull speed, but obviously it must. Can't imagine being in a current or wind with that little power.

I'm also surprised at how many single engine trawlers there are. With one engine and no thrusters, how the heck do you dock? I'm spoiled now with twin engines and both bow and stern thrusters. I just finished a charter in Florida with twin engines and a bow thruster which I thought was a common configuration. But that's not what I'm seeing now as I look at what's out there.

Appreciate any thoughts.

Hi Frank,

We have 107 horsepower at 2400RPM. We cruise around 7.3 knots 2000RPM, which the engine says is 60% load. Boat is 55,000 pounds. You’ll find that many people believe that typical trawlers are overpowered. That said, there are times it would be nice to pick it up, as you certainly know. We are a displacement hull, so we are not picking up anything no matter the horsepower.

Re the bow thruster, we have one. As our boat pivots at the center, it is easy to bump the stern in by pushing the bow out. No stern thruster desired for us.

IMO thrusters are all the rage in the last 20 years. It used to be you didn’t see a bow thruster on much below 80’. The accessibility of easier-to-operate boats probably got a bunch of folks out on the water that otherwise wouldn’t have come...so a good thing for all of us.

What is your mission for the future southeast boat?

And welcome aboard...

Hull speed requires little power, but it does increase with the size and weight of the boat. There are a bunch of single engine 40-odd foots with 120hp, the do hull speed just fine at like 50% power.

I have a 38' light and sorta skinny (14000lb, 12'beam) with a single 450hp Cummins. I calculate that at hull speed I am using about 35hp. But I like to go 20kts and that takes about 200-220hp.

Nothing wrong with spec'ing an engine that is twice the power of expected duty cycle. Lasts longer, quieter at lower rpm, etc.

And many of these boats were marketed to buyers that wanted a faster cruising speed, even though due to fuel costs later it was simply dang expensive to run that way.

Speed sells. Even if not very practical.

150hp in a 50' boat will move it along just fine at hull speed. A 250hp in the same boat will do it at a lower rpm and be quieter.

Also, many of the desirable engines are straight sixes and tend to be bigger and more powerful than absolutely needed.

You could do the job with one of the little car based diesels, but those would need to run at an annoying high rpm.

A single with a thruster can do just about anything you want it to do, just takes a little getting used to.

We have a 60 fter ex prawn trawler weighing in at + - 65 tonne

She has a NTA855M cummins rated at 350hp but at our usual 7.5knot cruise speed @ 1150rpm uses around 100hp of it.

Our 55ft "sister from another father" ship , re purposed alongside ours at the same time had a 6lxb Gardner rated at 127 hp installed.
I recon she'd be running pretty hard vs us at a fast idle.

My hull is 83x17, semi planing. I can go hull speed (12+kts) with 200hp twins. Hull was built for 900 hp engines and could do 22 kts.

I just read a post elsewhere about an 85 X 20’ boat displacing 320,000lbs powered by a 330hp Cat. 3-1 red. W 58” dia prop.
This computes to less than 1/2hp per ton.
She probably cruises w about 180hp applied.

My 30’ Willard has 37hp and 4hp per ton displacing 8 tons.

That 85’ boat has the lowest amount of power per ton of ant boat that I know of.
Most well designed FD trawlers require about 3hp per ton.

Most trawlers are SD and require considerably (or much more) power.

I'm guessing the OPs current boat handles close quarters like a Dixie cup and can be a handful. Displacement trawlers are usually pretty deep keeled and heavy and are pretty sticky - effected more by current than wind (to a point).

What the OP is struggling with is the divide between semi planing/displacement(SD) hulls and displacement hulls. SD hulls will often have the hp to drive the boat at 15-20 kts at the expense of fuel. Proponents of SD boats rightly point out that at displacement speeds, their boats burn similar fuel to a displacement boat.

Most displacement boats are ballasted to keep weight low and improve stability in open ocean conditions. My Willard 36 for example has a round bottom and carries 7000 lbs of ballast. Her 75 hp Perkins pushes her along comfortably at 7 kts. A sistership went to Hawaii and burned 330 gals over the 2300 nms 14 day journey. But my hull has no flat sections to provide lift so wouldn't plane with a pair of Pratt & Whitney's bolted to her deck.

Most displacement trawlers are single engine boats but not all. As you found, Defever displacement hulls are often twins. Nordhavn and KK have both made twins in smaller boats upon customer request. Above 60 feet or so, twins in displacement hulls are more common, but not dominant. But principle remains. It doesn't take much hp to keep a boat moving at displacement speed.

I'd say the biggest difference between the two hull forms is the mindset of the owner. My friend often cruises his 52 foot power cat at displacement speeds but when he wants to make a scheduled bridge opening, he let's it rip, climbing on/off/on plane through no wake zones and such. For us, not only isn't it an option, but have no desire to travel like that. We enjoy the journey and simply plan accordingly.

Good luck in your deliberations. If life ever returns to normal, you may want to consider chartering and/or attending a Trawler Fest.

Peter

Nomad Willy said:

I just read a post elsewhere about an 85 X 20’ boat displacing 320,000lbs powered by a 330hp Cat. 3-1 red. W 58” dia prop.
This computes to less than 1/2hp per ton.
She probably cruises w about 180hp applied.

My 30’ Willard has 37hp and 4hp per ton displacing 8 tons.

That 85’ boat has the lowest amount of power per ton of ant boat that I know of.
Most well designed FD trawlers require about 3hp per ton.

Most trawlers are SD and require considerably (or much more) power.

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Yikes! That's not a lot of power! Must require a lot of planning and restraint in close quarters. Tough to dig yourself out of a hole by going to reverse with that ratio.

Frank60 said:

For reference, my current boat is 38 feet, 27,000 pounds, and has two 420 hp MerCruiser gasoline engines. It's a planing hull and can go pretty fast if I want to burn some fuel. But, I don't normally cruise at more than 8 to 10 kts.

As I'm looking at trawlers, I see some with similar size (diesel) engines to my current boat. For example, a Grand Banks Europa 42, 43,000 pounds, with two 420 hp Caterpillar engines. On the other end of the spectrum is a DeFever (49) weighing 54,000 pounds with two 150 hp John Deere engines. Or, a Selene 40 weighing 40,000 pounds with one 300 hp John Deere engine.

Are the lower powered boats intended to run at no more than hull speed? Still hard to imagine that 300 hp can get a 54,000 pound boat even to hull speed, but obviously it must. Can't imagine being in a current or wind with that little power.

I'm also surprised at how many single engine trawlers there are. With one engine and no thrusters, how the heck do you dock?

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Some of those (GB) will have semi-displacement hulls and are intended to let you go "faster" (a relative term) if necessary, but under hull speed is common. Some will be displacement hulls (DeFever), and yes, to be run at no more than hull speed.

Docking a single without thruster is just something you learn. Prop walk and spring lines are useful tools. If you can't figure it out, you can usually add a bow thruster, and even a stern thruster... but for many, it doesn't come down to that. You might read some Chapman's, for a start...

-Chris

Depending on your intended use you might consider cruising range. When we were looking for our trawler I noticed that boats with fast hulls and large engines usually have smaller fuel (and water) tanks. Seems contradictory but it’s harder to make a boat go fast with extra tonnage in tanks.

Looking at the OP’s listed boats:

GB 42 - 600 gal/840 hp
Defever 49 - 1,000 gal/300 hp
Selene 40 - 600 gal/300 hp

Stella (45 tons) - 1,600 gal/300 hp (twin JD 150’s)

With smaller tanks the next fuel dock becomes a planning priority when you consider that 10-15% of tank capacity is not usable and additional fuel consumption for a generator.

Even in a hull that's capable of exceeding hull speed, the amount of power that's reasonable varies. Some hulls aren't good at going faster, so it's not worth adding enough power to really plane (because you wouldn't really want to except for a short burst anyway).

Other hulls are good enough at going fast that the bigger engines and sightly higher fuel burn when going slow are worth it. I put my hull in this category. 38 feet, 26k lbs and 680 total hp. Even with the low max continuous output of the gas engines it has, it'll do 17 - 18 kts very comfortably until it's time to refuel and it's not flogging the engines to do it.

Docking wise, some hulls just maneuver better than others. The pivot point varies a bit between boats, as does the amount of prop walk. With faster boats, low speed rudder effectiveness varies widely. Some, like a Grand Banks or my boat have decent size rudders and steer fine at very low speeds. Others have tiny rudders (think Sea Ray) and are maneuvering on engines alone and struggling at low speeds.

As far as thrusters go, the need varies. On my own boat, I have twins but no thrusters. The only time I've ever wished for a bow thruster is backing into a narrow slip with pilings and a strong crosswind. I've done it fine without thrusters (and could manage it with a single given the correct dock side relative to prop walk). But a thruster to fight the wind push on the bow would make it easier. IMO, a stern thruster may be convenient, but unless you have a boat that struggles to pivot, it's rarely necessary. Thrusters are annoyingly loud as well, so it's nice to be used to not having them and be able to just quietly slide in or out of a slip (especially for an early morning departure).

But my hull has no flat sections to provide lift so wouldn't plane with a pair of Pratt & Whitney's bolted Peter[/QUOTE]

Love that line but as a Connecticut native I would have to say the P&W’s would get it done! On the other hand GE’s might come up short?.

My NP 39 had a 230 hp Cummins which I thought was100 hp too much for the way I cruse it. The new NP 45 came standard with a 5.9 250 Cummins. For Some reason the 250 was not available so I have the 6.7, 355 hp. Much more power than I will ever need. The extra 105 hp might give another knot or so on the pins. Maybe 12 knots instead of 10 ish.

It all depends on what you want But to me more hp on my type hull has no real advantage.
Rob

This is one of those posts that my Dad (a life long mechanical engineer) would have loved to join were he alive - so I will post on his behalf! "Horsepower is a measure of the amount of work done over a period of time. Specifically, one horsepower is the amount of effort required to lift 33,000 pounds one foot in one minute. People often assume higher horsepower gives more speed in devices, whether cars or boats. People tend to forget about torque." My 28 foot boat is a classic example of the issue: I have a 350 HP stern drive with twin props on same shaft (counter-rotating), with 6.7 litre engine displacement. This boat (sea ray tells me will do WOT 41mph, yikes). The same boat equipped with a 350 Mercury outboard is actually slower - top speed of about 39 mph, even when the RPM is higher on the outboard and the boat is actually lighter. What's missing on the outboard is "torque". Dad would be proud of this post!! Happy 4th!

Our boat is about 60k with twin 225 hp diesels. It’s too much power for displacement speed but not enough to get on plane, especially if I have full tanks. (600 gal)

I run 1650 rpm and that gives me 8.5 kts. I started to get lots of black smoke and low power because I wasn’t running the engines hard enough. Now what I’ve started doing is when cruising into the current, I’ll run the engines harder to maintain 8.5 kts and that seems to be working.

2000 rpm gives me about 10 kts. I only do that when nobody is around because the wake is enormous.

I don’t have thrusters. Our boat is heavy and has a keel so it doesn’t really get blown around with the wind. It pretty much stays put when docking. But it also doesn’t like to rotate. I’ll split the transmissions and set the rpm to 1000 and just wait. It rotates, just very slowly. I’ll adjust my position fore or aft by adjusting rpm. 1100 rpm forward and 900 rpm reverse will allow the boat to creep forward while rotating.

In the 2 ish years that we’ve had the boat, I’ve only really had trouble docking once and it was recently. I was trying to hack into a tight slip with a 2 kt current that ran the same direction as the slip, and a nasty cross wind. It wasn’t happening. After about 6 failed attempts we went around the corner, dropped the anchor and waited a couple of hours for slack tide. Came in bow first on the first attempt.

I learned that I need to realize that I know my boat better than the dock hands. I listened to them instead of trusting my gut. I knew bow in would have been easier but for some reason they thought stern in was the way to go. Learned a lot about my abilities and the boats limitations. But I didn’t hit anything and didn’t embarrass myself.

I will tell you I will probably never be able to justify the cost of adding thrusters.

Thanks for all of the replies. Very educational. I'm taking a way the following:

1. The lower hp engines will get even these heavy boats to run comfortably at hull speed.
2. Larger engine would do the same at lower rpms and therefore quieter.
3. Boats like the Grand Banks may actually be semi-displacement hulls which would account for the amount of power. I'll have to look at that more carefully. I was assuming that all trawlers were displacement hulls. That doesn't seem to be shown in the listings. So will take a bit more research to look it up.

Regarding the twin vs single and thrusters, I've had boats with and without thrusters. I'm going to stick with twins and at least a bow thruster. I've found a stern thruster useful but not necessary. Docking is the most stressful part of boating for most of us. (Anchoring being the second). I'm just not comfortable maneuvering with a single and no thrusters. Kudos to those that have the skills for that.

Frank,
with your current gas boat at 10 kts, how much gas do you burn? the nice thing about a trawler at 10 kts you might only need to burn around 3 gph.
Many 40-ish ft trawlers with singles do indeed have at least a bow thruster and often both bow and stern.
You should consider pros and cons of single vs twin. Singles often have a full keel protecting the prop against groundings, logs, etc. A single also requires half the spare parts as a twin and typically more room to work in the engine room.
Anyway, go to a Trawlerfest show or other shows and crawl around on different boats and in engine rooms and see what you like.
I know others love their twins but in our case when shopping we only considered single engine boats. as others mentioned, to go hull speed you only need less than 200hp...

I had a physics teacher once who said you could make a washing machine motor pull a freight train, with enough gearing...



Having grown up in the US with my dad's twin engine boats in the 30-50' range, it's a bit amazing that I ended up owning a single engine boat. Part of that had to do with the fact that I wanted a hybrid boat and, in 2010, they were only available with single engines. Part of that also had to do with the cost of diesel, which at that time was about $9-$10 per gallon where I live now, in Sweden. It's a bit better now...a mere $7 per gallon.



I can't begin to offer the sage engine advice most of the --um--not-so-old salts on this forum can. Further to that, I won't jump into the single vs. twin discussion. It's as risky as arguing politics or religion ;-)


BUT, I can give you something to think about as you search for your replacement boat. Depending on the size, weight and hull design of the boat...and the type of cruising you intend to do, you might want to have enough power to push you roughly the same speed as the waves in a following sea (let's say 10-15 kts). Being able to go this speed in a following sea, when an unexpected blow comes up, can offer you the opportunity for a BIG improvement in ride...and safety.



The hull speed of my 33' 15,000# boat is about 7 knots and that's what I cruise at most of the time. At this speed, my VW Marine 165hp turbo diesel burns 1.55 US gallons per hour. Kind of easy on the pocketbook, thankfully. When I want to get a better ride in a heavy following sea, however, I bump it up to 10-13 knots to improve the ride. Then my engine burns about 8 US gallons per hour. So do think about this, if you find yourself considering a boat with power that's capable of hull-speed-only.


As far as thrusters are concerned...
My boat came from the factory with a bow thruster as standard. I didn't have it very long before I realized that its very tiny rudder made it very difficult to maneuver at slow speeds for docking...especially when backing. As such, I added a retractable stern thruster at staggering expense. I have never regretted doing it. Over the years, I have sharpened my single-engine handling skills...and I always try to do everything without thrusters first...but it's nice to know you can bail yourself out of a potential mess with a push of a finger. My point here for you is that if you are considering a single-engine boat and the rudder happens to be on the small-ish side and the prop-walk doesn't help enough, you might want to budget for a stern thruster. Obviously, it would be best if you could spend a lot of time testing the maneuverability of the boat you are considering before buying.


Good luck on your hunt, Frank60!

IRENE said:

Hi Frank,

What is your mission for the future southeast boat?

And welcome aboard...

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Irene, our plan is to be part-time liveaboards. We'll figure out how much time once we get into it. Being on a lake now, we can't go anywhere really. We go out for the day, maybe anchor overnight and come back to the same slip. 'Lather, rinse and repeat'.

We love boats and boating but getting bored with the lake scene. We want to travel the ICW, visit new places and meet new people. We're not sure right now how far or how long we will venture at this point. Maybe we'll even do the loop one day. We'll probably start out spending a month or so on the boat at a time. Come home for a while and then go back.

ScottC said:

The hull speed of my 33' 15,000# boat is about 7 knots and that's what I cruise at most of the time. At this speed, my VW Marine 165hp turbo diesel burns 1.55 US gallons per hour. Kind of easy on the pocketbook, thankfully. When I want to get a better ride in a heavy following sea, however, I bump it up to 10-13 knots to improve the ride. Then my engine burns about 8 US gallons per hour. So do think about this, if you find yourself considering a boat with power that's capable of hull-speed-only.

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ScottC, excellent point. I'll keep this in mind for sure. Thanks!!

I read that Krogens naval architects calculated 35 Hp for the Manatee’s full displacement hull. The first few had 50 HP diesel power most likely to get the diesel into a 75% load area. The last Manatee’s built had 110 Hp engines, maybe as a sales tool or maybe because the engine manufacturers only built increased HP engines. Our Manatee cruises at 7-7 1/2 mph at 2400 rpm burning 1.2 gph , the engine is governed at 2600 rpm.
I would guess the propellers might have been redesigned for each engine HP change. Engines started at 50 HP, 90, 100 and finally 110 HP for the same 99 hulls designed and built.

sledge said:

Frank,
with your current gas boat at 10 kts, how much gas do you burn? the nice thing about a trawler at 10 kts you might only need to burn around 3 gph.
Many 40-ish ft trawlers with singles do indeed have at least a bow thruster and often both bow and stern.
You should consider pros and cons of single vs twin. Singles often have a full keel protecting the prop against groundings, logs, etc. A single also requires half the spare parts as a twin and typically more room to work in the engine room.
Anyway, go to a Trawlerfest show or other shows and crawl around on different boats and in engine rooms and see what you like.
I know others love their twins but in our case when shopping we only considered single engine boats. as others mentioned, to go hull speed you only need less than 200hp...

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Sledge, I'd be embarrassed to tell you how much gas I burn now. I'd be happy to get 6 gph. This is another thing that the boat listings don't tell you. Even if they did, you wouldn't know if it's true or not until you had the boat for a while. Frustrating since this is such an important consideration.

I was not aware of Trawlerfest. Just looked it up and bookmarked the page. I will definitely try to get to one of these. The 2020 shows aren't looking good for me though. I have a wedding to attend in Seattle on Oct 10 which is when the Boston show is being held. Then the Seattle show is a month later on Nov 10-14. Unfortunate timing. Hopefully there will be a show in the southeast sometime in early 2021.

My brother-in-law has a team of two percheron/morgan crosses for hand logging firewood on the farm. Then he bought a mule. Did he 'need' the mule? No, but he always wanted a mule.

This complicates your original question, because he only needed two horsepower, yet felt compelled to acquire the mule.

Perhaps the question should be; how much horsepower do you need, and are you muleheaded?

Frank60 wrote;
“ Thanks for all of the replies. Very educational. I'm taking a way the following:

1. The lower hp engines will get even these heavy boats to run comfortably at hull speed. “

Not so.
Many or most FD boats can’t even reach hull speed. Unless they are overpowered. And the definitely don’t run/cruise at hull speed. Cruising speeds vary at about a knot BELOW hull speed. Our Willard at WOT probably attains hull speed but I’m not sure. Don’t have any reason to know. Other than for testing the propulsion system we never run at hull speed or nearly so. We cruise almost always at 2300rpm and a bit over 6 knots. Rated power comes in at 3000tpm for our Mitsubishi S4L2. Hull speed for our 30’ Willard is 7 knots. Unobtainium basically.

Most all FD boats run about exactly the same speeds .. relatively speaking. It the nature of the hydrodynamics involved. If somebody says their FD trawler runs at hull speed and it’s true, they have a SD hull form .. as almost all trawlers do.

Our 36ft LWL 16ft beam 50,000lb boat has a whopping 108hp (2x54hp) total. We can get to hull speed (~8kn) at WOT, but typically operate about a knot to a knot-and-a-half below that.

We wouldn't trade my engine room space, parts cost, or fuel consumption for a even ONE extra horsepower. Suits us just fine.

Best engine room ever. ��

Ray you could do exactly what you have done since you’ve owned your boat with at least 25hp less.
It’s amazing how little it takes. We have basically 40hp and could easily do with ten less. I like the 40hp but never go above 2500rpm. There we probably make about 25hp.

HiDHo I call my “engine room” a compartment.

MurrayM said:

My brother-in-law has a team of two percheron/morgan crosses for hand logging firewood on the farm. Then he bought a mule. Did he 'need' the mule? No, but he always wanted a mule.

This complicates your original question, because he only needed two horsepower, yet felt compelled to acquire the mule.

Perhaps the question should be; how much horsepower do you need, and are you muleheaded?

Click to expand...

MurrayM, you gave my wife and I a good laugh. Thanks for horsing around!

Nomad Willy said:

Frank60 wrote;
“ Thanks for all of the replies. Very educational. I'm taking a way the following:

1. The lower hp engines will get even these heavy boats to run comfortably at hull speed. “

Not so.
Many or most FD boats can’t even reach hull speed. Unless they are overpowered. And the definitely don’t run/cruise at hull speed. Cruising speeds vary at about a knot BELOW hull speed. Our Willard at WOT probably attains hull speed but I’m not sure. Don’t have any reason to know. Other than for testing the propulsion system we never run at hull speed or nearly so. We cruise almost always at 2300rpm and a bit over 6 knots. Rated power comes in at 3000tpm for our Mitsubishi S4L2. Hull speed for our 30’ Willard is 7 knots. Unobtainium basically.

Most all FD boats run about exactly the same speeds .. relatively speaking. It the nature of the hydrodynamics involved. If somebody says their FD trawler runs at hull speed and it’s true, they have a SD hull form .. as almost all trawlers do.

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This thread has been very educational for me. Thanks to everyone that contributed!! Bigger is not always better.

mvweebles said:

Yikes! That's not a lot of power! Must require a lot of planning and restraint in close quarters. Tough to dig yourself out of a hole by going to reverse with that ratio.

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Sounds like a lot of commercial fishing boats I used to work with carrying amazing loads of ocean clams.

Cant sat what the whole drive train was made of, but many had some variation of a 3208 Cat.

I think some of it might just be the manufacturers providing what sells, even if its not the ideal size engine. The original Nordic Tug 26 came with 50hp, and now the newer ones have 150hp!!