How much horsepower do you need?

[markpierce]

When cruising one knot below hull speed (at 42% power, based on fuel consumption, not RPM from an 80-horsepower engine), looking over the railing watching the water pass by, it seems fast enough.

I disdain going "bouncy, bouncy" on a fast boat. ... With some waves in the inland waters here, I need to slow down from cruising speed to avoid burying the bow.

Smooth waters are always welcome.

 

[Nomad Willy]

Sure glad so many responded to the question of power required. Frequently there is a big difference between power required and power needed. I tried really hard to procure the real power requirement and still failed .. but not by a lot. 20 .. 25% at the most. But these are pleasure boats and there one would think actual powered required should not be very much in question. Power wanted or desired is really more applicable. But reference point are very desireable when trying to decide what engine to install in one’s boat.

 

[Simi 60]

HP is an interesting thing

These 2 100ft Sydney ferries only had 28 and 36 HP in steam and did 11 knots but when replaced with diesel needed 450hp to do the same



https://en.m.wikipedia.org/wiki/Karingal_and_Karrabee

 

[Insequent]

HP is an interesting thing

These 2 100ft Sydney ferries only had 28 and 36 HP in steam and did 11 knots but when replaced with diesel needed 450hp to do the same



https://en.m.wikipedia.org/wiki/Karingal_and_Karrabee

That seems pretty extreme, and with Wikipedia who knows? What they don't provide is the torque info, or gear reductions or prop sizes.

Carroll Shelby: Horsepower sell cars, torque wins races.

A popular adaptation is - horsepower sells boats, torque moves them.

 

[Pete Meisinger]

BandB wants to go 50 KN ? I can't even begin to imagine it. In the olden days (my youth) a FAST ski boat did about 40 mph. Then came the Hydrodynes with twin tower of power mercs which did 50+ mph. That was fast then and it's fast now!

Give me my six to eight KN at 4 kpg all day long, thanks..

pete

 

[Simi 60]

That seems pretty extreme, and with Wikipedia who knows? What they don't provide is the torque info, or gear reductions or prop sizes.

Carroll Shelby: Horsepower sell cars, torque wins races.

A popular adaptation is - horsepower sells boats, torque moves them.

I went searching for it because my memory was jogged on a ferry with similar numbers when having a discussion with the designer on powering a powercat I was building.
This wiki link I guess, confirmed that they existed.


But what your comment points out is.........it's not just about HP

Carroll Shelby....just watched Ford vs Ferrari, great movie, Christian Bale plays a stellar part as usual.

 

[mvweebles]

G'Day Frank... I've owned my 38' Europa trawler for over 8 years. It has a 2002 235hp single turbo diesel, 26,000 lbs with both bow and stern thrusters. It will cruise at 7-8 kts in most weather. The only problem I encounter is high seas here on Chesapeake Bay then I must slow down. I only burn 2 to 2.5 gph so quite economical. Good boats are out there so keep looking. (mine is for sale so send me an email to: twoflew@gmail.com and I'll give you more of my perspective. Cheers, John

Single plus thrusters is a very nice combination on a boat. She'll move at dock like a show pony, and very efficient for long range coastal cruising. Too bad more builders don't go this direction.

Good luck with selling her. I love a sedan layout for cruising. Not ideal for overnight passages which I do, and I'd still take a sedan due to nice flow of living space.

I'm happily married to my Willard 36. But if I were ever to cheat on her.....got a link to your sale site?

Peter

 

[Codger2]

Speed sells. Even if not very practical..

I agree with Ski on the speed thing and sold my little trawler about 5 years ago for a larger twin, bow thruster and 20KTS (WOT). Although, I cruise it at 10 knots most of the time, if I'm wanting to get to my destination faster I push her up to 18 knots. Having that ability to adjust to a sea state and go faster if desired is absolutely fantastic! If your heart is set on a 50-60 foot boat that will go 18 knots, they are certainly out there but very pricey! Best of luck in your search!

 

[BandB]

BandB wants to go 50 KN ? I can't even begin to imagine it. In the olden days (my youth) a FAST ski boat did about 40 mph. Then came the Hydrodynes with twin tower of power mercs which did 50+ mph. That was fast then and it's fast now!

Give me my six to eight KN at 4 kpg all day long, thanks..

pete

Wifey B: I was once on a boat that would run up to 90 mph (78 knots). When it topped the 60 knot level it got a bit scary as control diminished. On Lake Norman there's always been a battle between Nascar owners and drivers and others to have the fastest boat. There is a Youtube video of Rick Hendrick going 125 mph on his boat. Display on video and GPS speed. No thanks. Our boat when we lived on Lake Norman was a 50 knot boat and we normally cruised from 25-40 but occasionally around 5 to 10. 50 knots in a small boat feels fast. Ours was 30'.

We have a 39' Contender CC that is for employee use, not ours, and it tops out at 57 knots. Now, I'll tell you in a center console that feels fast. Now they typically don't run over 40-45 knots which is still faster than our fastest boat.

Why do I like speed? 1-it's just fun. To get out on the water and go fast, just fun. 2-To get somewhere faster. 500 nm from our home to Myrtle Beach. At 7 knots and 8 hours a day you're talking 9 days. At 20 knots, you're talking just over 3 days. At 35 knots, you're talking 2 days. Now 45 knots doesn't really make much difference. About 1 1/2 days then.

Now, before the "but you miss everything", perhaps fast in one direction and slow in the other.

One place we like speed is if we decide we want to go spend time on the Chesapeake. Well, it's something like 850 nm to Annapolis, about 750 nm to Norfolk. If it takes us two full weeks of hard cruising to get there, that's a lot of time. If we can get there in 5 days then much more feasible when we only have 6 weeks carved out for it. Speed is very helpful when your cruising area you want to explore is distant.

Still not for everyone but when one is considering 7 knots vs. 15 it's worth the thought.

 

[Nomad Willy]

Below are many comments that I have on posts in the thread.
Your post/posts may be among them.

Post #1 ... Out of the chutes is Frank60 ( the OP) and he makes reference to a GB 42 power. He reflects on one GB42 w 840hp. We all know they came stock w two power options. One is/was with a 120hp FL. The range of 120hp to 840hp is too wide. The boat is OK w 120hp but I think almost no one on this form would call 120hp as ideal power. And probably only Wify B would go for the 840hp boat. But it's very rare for one boat to have such an offering.
If the boat had 840hp the stern sections would show a considerably straighter run aft from amidships. And the CG would probably be further aft too.
So the boat has the wrong stern for either 120hp or 840hp. See picture at bottom of post.

Post #12;
I'll suspect there are flat sections in your NP. Haven't seen any hull section pics so don't really know but it's rare for FD boats to look like a NP and be FD.

Post #13;
Mark P writes a good post about powering reflecting on the description of HPO. Work over time. But I think he goes off the track on torque. Some fall into this concept. Torque is an instantainious element and does no work. One dosn't drive a heavy boat w/o doing work. And RPM has nothing to do w what we're talking about. It's about HP. 140ho at 3300 rpm is the same as HP at 2300rpm.

POst #24;
By boathealer is a good example of a FD boat in action. He's a little lower powered than me at 4.25hp per ton. I'm at 5. So we both could do w about 20% less power and still have sufficient reserve. My cruising speed comes in at 2300 and the S4L2 Mitsubishi is rated at 3000rpm. I say RPM as there is no such thing as "revolutions per minutes" or rpm's. Ray cruises (probably) at about 50% power.

Post #30;
Benthic2 observes the role played by marketing in the blame for overpowering. People like power in their cars and a small car has lots more power than a 30' displacement boat. Show him a properly powered boat and he'll say "no way" and walk.
Re the original NT I think they came w a 36hp Volvo. Guessing 3 cylinders.

Post #31;
Good post by Wayfarer about what is needed. Shake the need for speed. Don't need it as hundreds of thousands of sailboats make the case for excessive power clear.

Post #32;
boat poker's post #32 shows what a FD boat is made of .. in a picture. And I'll add another. Both show that the bottom comes up to the water level to allow the water coming out under the stern is flowing cleanly w little drag.
See picture at bottom.

Post# 36;
Al from Ketchikan saying his boat was designed by Monk and at that time "fuel efficiency was the driving force for economical, cost saving hulls". I say that w a wide boat and almost fully submerged transom that economy was maybe in the far distance of his mind, put certainly not primary. If so the stern of Monk's boats would be quite different. I love the Monk boats though.

Post #37;
Dave, ... I agree. Just right.

Post#38;
KynotYet Makes the point that if you stray too far from norms the numbers we're talking about may not apply. Boats of 12' compared to ships of hundreds of feet aren't comparable. An aircraft carrier can do 54 knots but I have yet to see one plane. Yes. I have a square sterned canoe that will go about 14 knots.

Post #40;
AusCan writes "I may use up to about 30hp pushing a headwind with a dirty bottom." The other 12hp never gets used. He has the same engine as we do and only slightly different hull. Even though he has the same engine his is rated at 5hp more. Same engine but mine is rated by industrial standards so mine is rated at 37hp.

Post #43;
Read link on post #34.

Post #48;
Jay N writes about extreme comparisons that don't apply. A mouse running across the road may have little feet going very fast but he doesn't make it. If battleships had legs they wouldn't go that fast but they went 28 knots. It may be the Reynolds number stuff that governs this ..... but it happens.

Post #60;
Wify B paints the picture we all need to read re powering. Smart girl .. yes I stewed about this a lot repowering Willy and missed the mark of optimum power. But if I had JUST MISSED it .. one more hp down and I'd have been underpowered. So unless we're very high risk takers we need to choose an engine powerful enough so we won't wind up under the mark. Very good point as an over powered boat is useable as you wish to use it but an underpowered boat will be .. incapable of what you expect.

Post #61;
Mark points out that speed is relative re observing wakes. I read a lot into wakes and love the beauty in their dynamic dance of what appears to be flowing waster. He also has a well powered boat and Mark's Rule of a knot below hull speed is golden ... only for FD boats though. SD boats can gracefully go faster depending on their hull shape and power. Thanks Mark for the golden rule Mark.

 

[stubones99]

The fastest I've ever been on the water was on one of our SeaDoos with a 135 hp engine... 76 mph. Do it 2-3 times and overhaul the pump, due to the poor engineering, too little oil in the pump housing, and poor seals around the jack shafts...
Yes, it was fun, but could have been deadly.
Yes, I'll never buy a bombardier product.

Dad said never go faster than you want to hit something...

 

[BandB]

The fastest I've ever been on the water was on one of our SeaDoos with a 135 hp engine... 76 mph. Do it 2-3 times and overhaul the pump, due to the poor engineering, too little oil in the pump housing, and poor seals around the jack shafts...
Yes, it was fun, but could have been deadly.
Yes, I'll never buy a bombardier product.

Dad said never go faster than you want to hit something...

Wifey B: That's like scary fast on a SeaDoo. Our Jet tenders will run 40 knots and that's fast. I know there are faster Waverunners, but it reminds me of people talking about skiing barefoot because of the speed you have to go. Those who do, tell you the water is very hard when you hit it at 50 mph. I've known a lot of people suffer ear damage and other issues over high speed PWC crashes.

 

[stubones99]

It was too fast, in retrospect...
I couldn't read the gauges due to the air pressure on my eyes. I read the top speed readout after I got done with the speed run.
The lake was glassy smooth when I started... but didn't stay that way
Once I found out that a speed run equals a pump rebuild, I stopped doing that.

 

[BandB]

It was too fast, in retrospect...
I couldn't read the gauges due to the air pressure on my eyes.
The lake was glassy smooth when I started... but didn't stay that way
Once I found out that a speed run equals a pump rebuild, I stopped doing that.

When I was young there were hydroplanes that would come out on the lake most Sunday evenings after the traffic cleared, typically around 7:00. No way I would have ever gotten in one of them. To me the funniest part was that just sitting on their own, they wouldn't float.

 

[markpierce]

Lots more time to take corrective action going seven knots versus 70.

 

[BandB]

Lots more time to take corrective action going seven knots versus 70.

Wifey B: A lot easier to dodge all you seven knotters when doing 70.

 

[Steve91T]

My truck is a 99 F350 with a 6bt 5.9 Cummins. It’s been upgraded with a larger turbo and injectors and will hit nearly 50 psi of boost and is making somewhere around 450hp. It’s a fun engine.

I’ve always thought that a pair of these engines with compound turbos would be perfect for the boat. They’d be happy with the small turbo slightly spooled at 1500 rpm but then you could push the power up and really go.

But....

I’m really happy at 8.5 kts.

So maybe a could or 120’s is all I’d need.

I like my 225’s. Tons of torque for docking. It really moves a lot of water at 1000 rpm. But like I said early, it’s too much power for 8.5 kts and not enough to go much faster.

Of course I’d have to win the lotto to actually change these engines.

 

[Nick14]

Dad said never go faster than you want to hit something...

That is the best way of thinking about speed I've ever heard. Your father was a very wise man.

 

[Art]

That is the best way of thinking about speed I've ever heard. Your father was a very wise man.

"Dad said never go faster than you want to hit something..."

I want to hit nothing at any speed!

 

[Nomad Willy]

It was too fast, in retrospect...
I couldn't read the gauges due to the air pressure on my eyes. I read the top speed readout after I got done with the speed run.
The lake was glassy smooth when I started... but didn't stay that way
Once I found out that a speed run equals a pump rebuild, I stopped doing that.

Yes but perhaps the majority of boat maneuvers close in can’t be done at that speed/throttle. In my old Albin Club there was an old guru that preached to do all docking maneuvers at an idle. ..... yea sure .....
It would be like someone learning to fly ...... and never leaving the flight pattern at his home base airport. 10,000hrs of that and I wouldn’t fly 10 miles up the road with him.
IMO one needs to nibble at the raged edge a bit to learn and experience one’s boundaries and how to do it. How long and consuming what distance does it take to get into reverse and stop? Experience is the only way to learn it.

 

[Nick14]

"Dad said never go faster than you want to hit something..."

I want to hit nothing at any speed!

OK, picky picky with the semantics. Not too many people want to hit something (except maybe my wife when it comes to hitting my head when I talk too much about boats). If you want to put it in the proper King's English, how about -

"Never go faster than you're willing to hit something."

 

[ghostrider454]

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.

Short answer is YES
We docked just fine on one engine with none of the extras on my last vessel. It's called good seamanship and having four line handlers.

We delivered two Diesel Ducks last year from Asboat in Turkey. One went to Rio and the other to Punta Arenas. You are fighting headwinds and currents all the way. Only had to refuel twice enroute. That's a 13.4m steel vessel with a single JD 130kW engine displacing 27,946 kg.

The current vessel has twin 1,705 kW (2285hp) engines which powers this 35m x 15.4m x 5.5m LNG powered tug at 13.5kts with a 70mt bollard pull.
We made our first trip from Lisbon to Godthab and Iqaluit in moderate ice conditions in two weeks with two fuel barges. We averaged 10kts the distance. Now back at sea, heading to Resolute and hoping the ice is gone. If ice has dissapated, we will make the final push thru the NW Passage by October. Otherwise we will turn around at Resolute and go to Akureyri.

 

[rgano]

OK, picky picky with the semantics. Not too many people want to hit something (except maybe my wife when it comes to hitting my head when I talk too much about boats). If you want to put it in the proper King's English, how about -

"Never go faster than you're willing to hit something."

Since you never expect to hit anything, the whole thing is just nonsensical.

 

[stubones99]

He also said "some people would be upset if you hung them with a brand new rope..."

 

[boatpoker]

22,000 hrs in our single engine log. Never understood the need for twins or bow and stern thrusters and we ddock just fine. it's all about understanding the phyics of what going on around you and using it to your advantage, It takes practice but once you get a feel for the dynamics you can pilot anything.

Out Benford 38. trawler withaa 55hp diesel runs just fine between the abacos and Toronto. A round trip costs us about $1300.00 in fuel at 7.2 knots.

There is absolutely no need for more than 75hp in a 40' displacement trawler.
Anything more will just make you a giant wake machine.

Unfortunately "trawler" has come to mean anything the marketing department ssys it is. i'e' "fast trawler" ..,. oxymoron.

 

[Mako]

It helps if you define what "ton" you are using as there are four out there (roughly):
- short ton of 2,000 lbs (American usage)
- metric ton of 2,200 lbs
- long ton of 2,240 lbs
- tonne, a measure of the number of casks of wine your boat can hold

In the metric world, I've always seen boats designed to metric tons. In America, it seems that the bulk of naval architure formulas are expressed in long tons.

 

[boatpoker]

- tonne, a measure of the number of casks of wine your boat can hold
.

Actually "tun" , a measurement of volume, not weight. A tun was cask of 100 cubic feet of Portuguese port wine. Centuries later devolved into tonne, ton.

Transport Canada Appointed, IMO tonnage measurer.

 

[Insequent]

It helps if you define what "ton" you are using as there are four out there (roughly):
- short ton of 2,000 lbs (American usage)
- metric ton of 2,200 lbs
- long ton of 2,240 lbs
- tonne, a measure of the number of casks of wine your boat can hold

In the metric world, I've always seen boats designed to metric tons. In America, it seems that the bulk of naval architure formulas are expressed in long tons.

A tonne is 2204.6 lbs. You're a cask or two short for your metric ton!

 

[markpierce]

What's a couple of hundred pounds or so on a well-founded boat? Individual guests can exceed that weight.

 

[Mako]

Mark it’s more related to calculated power required. Let’s say you follow the general rule of 4hp per ton for a trawler. Does a 30 “ton” boat need a 120hp or 132hp engine? That difference doesn’t sound like a lot but could be what puts you over the rating of your preferred M-1 engine, or having to go from a T to a TA model. Perhaps I’m splitting hairs, but I’m a NASA engineer so I’m allowed to [emoji846]

Anyway, a lot of things we discuss here on the forum are theoretical. Eg., do you really think we need to argue 100 threads over a 20kg Mantus versus a 25kg? In the end, that clay or sand or silt on the bottom is going to decide whether you drag or not! Or perhaps it’s that extra 1/4 “tun” of wine [emoji485]