Question for people with "fast trawlers"

bayview said:

The term hull speed should be abolished from any forum that seeks to educate people.

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Not at all IMO.
The speed to run is a very important issue and more highly related to boat design that most boaters knowledge. And boat design is not as easy as how to tie a boline.

There is the run what ever speed you want crowd or as fast as you can afford fuel.
There's a lot of non black and white in hull speed discussion but the more you dig the more black and white it gets. The subject of hull speed is very clear though compared to anchor performance. The door you open to get into it is a simple formula. And it explains why aircraft carriers can run in the vicinity of 50 knots.

I have a center console that will do 40 knots. I probably run it at hull speed more than half the time. I'm still a sailor trapped in a powerboat. I enjoy going slow.

Nomad Willy said:

Not at all IMO.
The speed to run is a very important issue and more highly related to boat design that most boaters knowledge. And boat design is not as easy as how to tie a boline.

There is the run what ever speed you want crowd or as fast as you can afford fuel.
There's a lot of non black and white in hull speed discussion but the more you dig the more black and white it gets. The subject of hull speed is very clear though compared to anchor performance. The door you open to get into it is a simple formula. And it explains why aircraft carriers can run in the vicinity of 50 knots.

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Exactly! And, a very good example for the importance of boaters being able to learn exactly what determining "Hull Speed" represents.

MOF Eric... I believe some posts ago it is you who began this HS importance discussion. Thank you for doing that.

I can not understand why people chose to [wish to] push hull speed to the curb [into the drink - so to say - lol]. Especially when that calculation is what I believe to be one of if not the most important mathematical scale of hull design, power options and boat performance.

bayview said:

The term hull speed should be abolished from any forum that seeks to educate people.

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I can't agree.

Even though the term isn't entirely accurate, I think it is still useful. I don't understand hull design at all, so I think of hull speed as simply that point where the distance between bow and stern wave equal the LWL. A better way to look at this may simply be the upper limit of displacement speed.

In my own simple mind, I think of anything beyond displacement speed as requiring more power to overcome the effects of this wave trough. A SD hull can do it, a planing hull certainly can do it. There are other ways to accomplish it as well but for most of the boats we have they will fall generally into one of those three categories.

Many boats don't have a sharp break at the hull speed in the power/speed curve. Even so, I think that understanding the concept of hull speed still is useful for most operators know.

Think of it this way, is it good for an operator of a planing hull to understand the mechanics of how a boat planes? Sure. If they simply think of the throttle as a speed lever why not just drive the boat right on the edge of the step? The same is true, to a lesser degree for those of us that operate FD hulls. Isn't it good for us to understand, at least a little bit, what happens as we move out of the displacement speed range?

Nomad Willy said:

...

I can't vote in your poll because I'm never at hull speed.

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With a FD boat I'm never at hull speed either. But I did vote - "30% and under" includes 0% I assume.

Richard

About 50/50. Best economy is ~7kt single engine so do that if I have time. Otherwise, 15kt. Top speed ~18kt. Not retired.

Can't exceed hull speed. Normally cruise at one know below. Occasionally go hull speed, usually when fighting a strong current, but this results in a 135 percent increase in fuel rate.

Nomad Willy said:

And it explains why aircraft carriers can run in the vicinity of 50 knots.

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And why submarines can outrun them

Not suggesting it's not useful theory. But for someone with a SD boat and a high hp/disp configuration - which is any fast trawler - it's nothing more than a footnote.

I am running at hull speed 30percent or less since I am mainly going out for weekends and couldn't go many places at hull speed.
At 7 knots, I burn 2.9 gph and at 14 knots burn 12 gph. So when just cruising around I go slow but when going to a destination, I go fast. It costs me an extra 6 gallons an hour to go the same distance in half the time. I'm wondering if I get a bigger Trawler after I retire, will I be able to adjust to 8 knots? I think so if in the right boat.

The other issue for us, is noise. Conversation in our boat is somewhat difficult when traveling at higher RPMs. And, we have Yanmars, which seem to be relatively quiet, from what I have observed. We like easing along at 7 or 8 knots and not having to shout.

But, we are also retired, and rarely on a schedule or in a pinch for time. I realize not everyone has that luxury.

I should have asked for daytrippers and not liveaboards.

Big difference in leaving the house with a half hour drive to the boat @ 8am than leaving @ 8am .

I find my day trip range to be a 25 mile radius of where I keep my boat.

I've also only had it 2 months and haven't even slept on the hook yet .

all too often hull speed is used as a synonym for cruising speed or an efficient speed. It is neither. This very poll implies it is a desirable speed. My answer to the poll was never but that was not an option.
Flank speed of carriers has nothing to do with what we discuss here other than as an example of maximum speed increased with length and unlimited budgets.

As a replacement with a more descriptive term for efficient cruising I suggest, for your next target of attack, bow down speed.

bayview said:

all too often hull speed is used as a synonym for cruising speed or an efficient speed. It is neither. This very poll implies it is a desirable speed. My answer to the poll was never but that was not an option.
Flank speed of carriers has nothing to do with what we discuss here other than as an example of maximum speed increased with length and unlimited budgets.

As a replacement with a more descriptive term for efficient cruising I suggest, for your next target of attack, bow down speed.

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You knew what was meant by the poll question and never is less than 30% the last time I checked. I think hull speed is used here as per the basic well known formula but then people know that generally the efficient speed is slightly under hull speed. Regardless, if it's hull speed or 50% or 150% of hull speed, we don't run it.

Bay,
Or boat level speed.
Quite a few boats here run bow down because they are not trimmed right. Too much weight fwd or too much weight everywhere. The pointy end does not support weight like the aft end.
I made the aircraft carrier remark and it was simply to point out the wide range of hull speeds.

But you're so very right about "hull speed" seeming a desireable speed. But hs IS a desirable speed for perhaps most real trawlers on this forum. Certainly not for FD hulls or planing hulls. For them it's actually a stupid speed. But there's so many grey zones between types some slip through. But for hulls between a NT and a GB hull speed is very often very good a speed IMO. For those that don't understand it hs is applicable to all FD and SD or SP hulls and even planing hulls in the same way. Of course it isn't.

But again as you point out hs is a buzz word on the docks or floats and those that use the word seem to be old salts so all the newbies start using the hs expression very often so-as to become old salts. We're like wolves and like to be part of the pack. A quick way in is to use the lingo in any sport or activity.

But to your point there are so many here that don't understand hs and throw the expression around like they know all about it, it could be better if no one ever mentioned it. But maybe it's OK as most have a vague idea what hs is. But to be even 1/2 a knot wrong is way off. And lastly to your point nobody is going to stop using the expression because it gets them closser to being an old salt. The holy grail.

So buck up, listen to and talk about hull speed.

We are enroute from Muskegon, MI to Port Washington, WI.
Two hours in....at 7.3 kts- we'll see how long before or if I can resist the throttles.
It is abeautiful day. 1ft waves, light N wind.

schaefferoil said:

We are enroute from Muskegon, MI to Port Washington, WI.
Two hours in....at 7.3 kts- we'll see how long before or if I can resist the throttles.
It is abeautiful day. 1ft waves, light N wind.

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Enjoy!

Willy, you are right on many points however we often hear of a newbie planning his boat purchase based on the assumption that he can run at hs and pondering the fuel use at that speed.

Boat level speed works for me. as does zero bow lift speed.

Nomad Willy said:

Bay,
Or boat level speed.
Quite a few boats here run bow down because they are not trimmed right. Too much weight fwd or too much weight everywhere. The pointy end does not support weight like the aft end.
I made the aircraft carrier remark and it was simply to point out the wide range of hull speeds.

But you're so very right about "hull speed" seeming a desireable speed. But hs IS a desirable speed for perhaps most real trawlers on this forum. Certainly not for FD hulls or planing hulls. For them it's actually a stupid speed. But there's so many grey zones between types some slip through. But for hulls between a NT and a GB hull speed is very often very good a speed IMO. For those that don't understand it hs is applicable to all FD and SD or SP hulls and even planing hulls in the same way. Of course it isn't.

But again as you point out hs is a buzz word on the docks or floats and those that use the word seem to be old salts so all the newbies start using the hs expression very often so-as to become old salts. We're like wolves and like to be part of the pack. A quick way in is to use the lingo in any sport or activity.

But to your point there are so many here that don't understand hs and throw the expression around like they know all about it, it could be better if no one ever mentioned it. But maybe it's OK as most have a vague idea what hs is. But to be even 1/2 a knot wrong is way off. And lastly to your point nobody is going to stop using the expression because it gets them closser to being an old salt. The holy grail.

So buck up, listen to and talk about hull speed.

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Eric - I believe although we may own very different hull-shape boats that you and I are on the same page regarding importance of "Hull-Speed" as an important determining factor.

IMO... HS math formula accurately determines water drag against a hull's lwl by showing when that hull will reach break-even speed where it stops "slipping" through the water. Thereafter the hull begins to either "push" the water if it's displacement or to climb up on it's own bow wave to reach some level of plane if it's semi displacement or to go completely atop and over its bow wave if it is a planing hull.

Additionally, it is clearly evident that at speeds just over hull speed, but not up on plane, for SD or P hulls, is just about the least economical speed a boat can travel. Also D hulls should never bother to try to surpass HS... because that is simply not possible without enormous power and fuel cost for a knot or two faster of pushing an ever bigger bow wave.

I believe that for most hulls of each design type: By calculating hull speed and then cruising at 10% below hull speed is just about the best mix for fuel economy as well as relatively good amount of miles traveled. Of course one could cruise even slower and probably attain even better fuel mileage but time taken to reach a point may become exaggerated. Of course, slower the speed travel through water surface means that tide times and resulting currents become important factors too.

Example: Our Tollycraft planning hull has calced hull speed of 7.58 knots.

- At 10% hull speed reduction of 0.758 knot the cruising speed becomes 6.822. When traveling just below hull speed I maintain from 6.5 to 7 knots with both engines running. That gives me just over 2 nmpg.

- At further reduced speed of 4.5 to 5 knots with just one engine running our Tolly approaches 3 nmpg.

- At planning speed of 16 to 17 knots she gets 1 nmpg. But, we get there quickly!

There is a trade off for everything... including speed traveled as compared to fuel used, as compared to hull shape that is in place.

Of course the most fuel-efficient boats are those with full displacement and single screw having just enough power to reach hull speed.

Happy "Hull Speed" Daze! - Art

Bayview,
At what speed does the bow start rising?
I used to think it was at hs in the 70's on a FD hull. Wrong.
But at what speed does that happen?
For best economy it may be a very good speed. It is of course when the boat starts climbing the backside of their own bow wave. Is it different on different boats? As to a SLR of course.

Art, I'm in general agreement with everything you say, with one quibble.

Additionally, it is clearly evident that at speeds just over hull speed, but not up on plane, for SD or P hulls, is just about the least economical speed a boat can travel.

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Agree that this is the case for planing hulls, but disagree that it's true for SD.

With planing hulls there's a clear hump between displacement speed and planing speed, and it's not a place you want to linger, as you point out. Best strategy is to operate either below HS or > 2x HS.

But SD boats don't really plane, and in fact are designed to operate pretty much anywhere in the range. You can happily run a SD design anywhere in the 1-2x HS range, and in fact that's where most owners run them. If you graph speed vs power within this range it's almost linear, unlike a planing design.

Art wrote;
"Additionally, it is clearly evident that at speeds just over hull speed, but not up on plane, for SD or P hulls, is just about the least economical speed a boat can travel. Also D hulls should never bother to try to surpass HS... because that is simply not possible without enormous power and fuel cost for a knot or two faster of pushing an ever bigger bow wave."

First of all one should not even try to get to HS on a FD boat .. much less surpass it. Since I'm in salt water I don't even know if I can attain HS because I'm never in water not moving from tides. One of the reasons I want to go to Lake Washington is to find my true speeds. Going there (HS) or attaining it has no value. That is actually possible though. A FD hull going faster than HS is more of a reality than most think. There's a Willard 30 that has a big engine that claims 8 knots on the yahoo WBO.

And secondly a good SD hull boat may find a bit over HS is his best speed. I'm not talking about an IG that has the lines of a planing hull I'm talking about a SD hull that's much closser to a FD hull. That's the idea of a SD hull ... to go a little faster than a FD boat. That may be 1 knot faster or 5 knots faster. It depends whether they are more related to a SD hull or a planing hull. A wide range actually.

Jeff F said:

Art, I'm in general agreement with everything you say, with one quibble.



Agree that this is the case for planing hulls, but disagree that it's true for SD.

With planing hulls there's a clear hump between displacement speed and planing speed, and it's not a place you want to linger, as you point out. Best strategy is to operate either below HS or > 2x HS.

But SD boats don't really plane, and in fact are designed to operate pretty much anywhere in the range. You can happily run a SD design anywhere in the 1-2x HS range, and in fact that's where most owners run them. If you graph speed vs power within this range it's almost linear, unlike a planing design.

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I had a 1998 34 Mainship great boat ...
I do not think you mean this the way it was written
"You can happily run a SD design anywhere in the 1-2x HS range, and in fact that's where most owners run them. If you graph speed vs power within this range it's almost linear"

If that were the case your fuel burn per mile covered would not change from hull speed up to 2X hull speed. That was certainly not the case with my Mainship powered by the 165 Perkins.

Jeff F said:

Art, I'm in general agreement with everything you say, with one quibble.



Agree that this is the case for planing hulls, but disagree that it's true for SD.

With planing hulls there's a clear hump between displacement speed and planing speed, and it's not a place you want to linger, as you point out. Best strategy is to operate either below HS or > 2x HS.

But SD boats don't really plane, and in fact are designed to operate pretty much anywhere in the range. You can happily run a SD design anywhere in the 1-2x HS range, and in fact that's where most owners run them. If you graph speed vs power within this range it's almost linear, unlike a planing design.

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Point taken and I believe you are correct... as memory serves me regarding cruise speeds used on SD hulls back in the 1960's / 70's. I misspoke on previous post!

Jeff,
IMO a true SD hull has some rocker to the bottom. At SD speeds there's no need to have a straight run on the bottom. It's actually a negative to some degree. But some designers use it to gain other things like great stability at the dock. Your's is not one of those. Your early MS is optimized for flexibility as are many other trawlers that are closser to Art's planing boat than a true SD boat. The NT 32 has some rocker and is great about a knot above hs. Your MS is ok there too but not nearly as efficient as the NT.

The SD boat is much more complicated hull-wise than either a FD or a planing hull. An undesirable thing about SD hulls is that they are the least understood and the most prevalent here on TF.

smitty477 said:

I had a 1998 34 Mainship great boat ...
I do not think you mean this the way it was written
"You can happily run a SD design anywhere in the 1-2x HS range, and in fact that's where most owners run them. If you graph speed vs power within this range it's almost linear"

If that were the case your fuel burn per mile covered would not change from hull speed up to 2X hull speed. That was certainly not the case with my Mainship powered by the 165 Perkins.

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I should have been clearer. The line isn't flat - MPG goes down as speed goes up - but the slope is relatively constant. With a planing boat you might get better MPG at 2x HS than 1.5x HS, which was Art's point I think. But that's not true of SD designs. Make sense?

I could do better with a whiteboard

Actually let me take one more crack at this

If you graph speed vs. power in this range it is a fairly smooth exponential curve.

There. Think I'm happy with that. Thanks for calling that out.

Our boat:
-Cocktail speed, ~5-6kts, 50%, boat rides very flat in the water. Many evenings on the water at this speed, See my avatar.
-Hull speed, ~10kts, 25%, boat rides a tiny bit bow high. Usually to get home or to an anchorage before dark.
-Fast Cruise, 17-22kts, 15%, running on plane. Usually only done if we're headed somewhere that's further than a few hours away.
-WOT, 30.3kts, 5% or less, at 65GPH, can you say "NEVER"?

My WAG vote was 70%, but in thinking about it, 80% may be closer. I have a planing hull so my numbers probably aren't really on the OP's mark anyway. 7-8 knots is kind of a fast idle and extremely economical - easy conversation, relaxing, doesn't scare the wildlife, and very comfortable. The boat has a real sweet spot (fuel burn and ride), planing, at about 14 knots. Unless I'm running from weather, I rarely exceed 14-15 - maybe 17-18. She's an old girl with all original drive train. However, once per year, on her birthday, she gets WOT for a little while just to prove she can still put out the factory advertised 27 knots. I refrain from checking the FloScans on that run.

Those that go above hull speed and push walls of water aside sure ruin the enjoyment of others.

Lots of fast boats heading north, some doing under 10 and we barely feel them pass. Others doing closer to 20 of the same design roll the bejesus out of our 65 tonne 60 ft bulk, you should see what happens to the smaller boats.
The owners of the wave producing POS have no idea or are simply to ignorant to care.

We do 8 knots with zero wash. We can go faster, but others enjoyment would be compromised by the wash we produce so don't.
$$ saved is a bonus.

I have no issue with speed, in the right hull.
We had a 60 ft power cat go past a few days ago doing 20+ and almost zero wash. Marvelous display of efficient design.
Should be more of it.

Simi 60,
Not for me. I like watching them and pick'in up a little fun from them.
Also I can have a pretty big wake roll under Willy and if I judge the angle of the wake to the boat and the distance between crests Willy sometimes dosn't even roll a bit. It's a challenge. But I frequently get multiple wakes all at once wrong.