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Old 09-17-2021, 09:11 AM   #1
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Hull speed/ horsepower questions

My wife and I are on our first official road trip to look at a trawler. Iíve been a registered user here for a few years in anticipation. We have been sailers for over 40 years, And Iíll keep our coastal and easily singlehanded sailboat for the foreseeable future. Hereís my questions, which illustrate my sailing background.
Do power vessels have a ď hull speedĒ and is it the same formula as with a sailboat? My brain doesnít function well in this arena.
Iím mostly just curious about the above, but here is my main question.
The Grand Banks 42 we will see today has twin Ford Lehmanís each of 120HP. They list a cruising speed of nine knots. In my very limited knowledge that sounds about right.
The other Grand Banks 42 has twin CAT 3208ís stating 375 HP. It says a cruising speed of 18 knots? That sounds quite fast for a trawler regardless of all the additional horsepower. I also know they all have optimum RPMís, giving the balance between speed and fuel consumption, but sure that will differ between the two as well. Any follow up will be helpful, including thoughts on what optimum cruising speed and fuel burn would be.
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Old 09-17-2021, 09:29 AM   #2
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There is a theoretical formula for hull speed = 1.34*sqrt(LWL). That is simply a geometry/hydrology based formula that figures the speed at which the boat speed matches the bow wave speed.

Some hulls, so called displacement hulls react very negatively to running beyond their bow wave. The force required to push them up an over the bow wave rises dramatically. Others such as semi-displacement hulls are more linear- more force produces more speed. Planning hulls sort of jump over the bow wave and once past it they ride higher in the water so the force to keep moving sort of levels out.

There is no most efficient speed of a boat. The slower a boat goes the more efficient it is up until the parasitic engine losses add up to more than the reduction in force required to go slower. That is probably near idle so maybe 3-4 kts is the most efficient speed.

That boat with the twin 3208s can be almost as efficient at 8 kts as the one with the twin Lehmans at the same speed. The only difference being that the 3208s have 10 liters of displacement which produces more parasitic losses than the 6 liter Lehmans.

If you want to go 15 kts or better, buy the boat with the 3208s. If only you want to go 7-8 kts, buy the one with the Lehmans all else being equal. But it will take a whole lot more fuel to go 15 than 7-8 kts.

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Old 09-17-2021, 09:30 AM   #3
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Hull speed absolutely still applies to powerboats. The Grand Banks hull is pretty much a very draggy planing hull (some would call it semi-displacement), so it'll plane given enough power. Because of that, it can exceed hull speed, it just requires significantly more power than it would to travel at or below hull speed (as it needs to climb the bow wave and get up on top of the water).
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Old 09-17-2021, 09:40 AM   #4
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Yes the same formula works close enough for hull speed. 1.34 times square root of the waterline length. If you use 1.1 or 1.2 times instead of 1.34, will give you a economical cruise speed estimate in knots. So with the Lehman’s, whatever speed you get at 1800 rpm will get you 2 gpm each for total of 4 gph. 9 knots sounds optimistic for an easy cruise. That’s more than hull speed. Miles per hour maybe. Plenty of GB42 owners here will chime in.
Now the Cats will be around 10 gpm total around hull speed. 18 knots? Maybe when new and empty. We chartered a 42MY with 375 Cats and kept it below 1500 rpm. 200 miles and 30 generator hours used 271 gallons of fuel!!!
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Old 09-17-2021, 09:52 AM   #5
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Hull speed is often discussed, and High Wire makes a good point about using a constant of 1.1 as opposed to 1.34 for the most economical and efficient speed. 1.1 is what I have been using.
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Old 09-18-2021, 05:48 AM   #6
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You'll also have much more room around the Lehmans to do maintenance and daily checks.
Something else to consider.
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Old 09-18-2021, 06:38 AM   #7
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We have a GB 36 with a single Lehman 135. We cruise at about 7.5 knots pulling an inflatable. (This leaves current out of the equation) Engine is turning about 1750 RPM at less than 3 gallons per hour. These are comfortable speeds for both the engine and hull. I would estimate that you can add a knot or 2 for the 42 with twin Lehmans with a very slight increase in fuel consumption. The Lehmans are bullet proof if cared for reasonably well and the maintenance is simple and straight forward.

My neighbor has a 42 with the high HP Cats. He cruises at the speeds mentioned above and gets comparable fuel economy. Anything more requires a Lot more throttle and therefore fuel. LOTS OF FUEL! My guess is at the time GB bowed to consumer demand for faster boats and low fuel prices. In my opinion these hulls although hard chined and theoretically designed to plane are meant to cruise at or under hull speed. I guess it would be nice to have the extra HP / speed that the Cats offer to get out of harm’s way. However, in my opinion the maintenance costs of the turbo's and after coolers on those big Cats cancel out the benefits.
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Old 09-18-2021, 06:53 AM   #8
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Additional thoughts...... I think that as a former sailor you will appreciate a 9 knot cruising speed. Anything more defeats the purpose of these boats. If you do want to cruise at 15 knots there are more efficient options / boats available.
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Old 09-18-2021, 08:06 AM   #9
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Yes, power boats have a hull speed. The 1.34 X square root of the length of the water line is a decent starting point. Efficiency is a whole different subject relative to speed. My very simple rule of thumb:

"It takes a lot of energy to make water stand up".

Wake could be described as the wasted energy from pushing the hull through the water. If your boat lacks a real time fuel consumption gauge, look at your wake as you incrementally increase speed. It's not hard to figure out at what point the efficiency is diminishing.

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Old 09-18-2021, 11:01 AM   #10
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My fuel consumption more than doubles going from one knot below hull to hull speed. So, I generally cruise one knot below.
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Old 09-18-2021, 11:18 AM   #11
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With the consequent trade off in maintenance costs and complexity, it would be nice to know that additional speed was available for when it was necessary (weather, darkness approaching, etc). That may depend entirely on your usage patterns.
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Old 09-18-2021, 01:53 PM   #12
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Unless you absolutely have to have the extra speed go with the Lehmans. They are bulletproof and very reliable. As said they will be waaaay easier to maintain that the 3208s. The 3208 is a great engine in the NA version. When they get turboed and aftercoolered they can be rode hard and put away wet and not last as long as they could. The problem is you donít know how the PO ran them.
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Old 09-18-2021, 02:48 PM   #13
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I think sellers tend to often over-sell their cruise speed. Of course there are some captains who just like spending lots of money on fuel.

the GB 42 has a longer LWL than I would have imagined, but even so, I think a comfortable, efficient cruise speed is likely to be closer to 7-7.5 knots rather than 9 kts.

My boat has a LWL of 38.25í. A good efficient cruise speed for me is just about 7 knots. That turns out to be about 2 gph on my single 5.9l cummins.
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Old 09-18-2021, 05:15 PM   #14
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We had a 41í President with SP225 Lehmans. It would do 17 knots at WOT. We cruised at 9 knots at 1600 RPMs. It was roughly 10 MPH which is used here on the Great Lakes. Easy number to calculate navigation with. We burned about 7 to 8 GPH but I really didnít keep a real close record of the fuel burn. I never measured the LWL so I am not sure what the calculated hull speed was, didnít really care.
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Old 09-18-2021, 06:12 PM   #15
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This may help clear things up a little.

https://vicprop.com/displacement_size_new.php
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Old 09-18-2021, 06:26 PM   #16
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Quote:
The 1.34 X square root of the length of the water line is a decent starting point.
This is just a starting point. It makes no allowance for hull shape or fineness ratio.

A 30 foot trawler and a 30 foot scull will have the same theoretical hull speed using this formula.

What is "hull speed" anyway? Some people say it is the fastest you can go without generating a wake. Technically the definition is the speed where the wavelength of the bow wave is the same as the hull length.
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Old 09-18-2021, 09:06 PM   #17
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If you wish to travel at hull speed, the vessel with the 2-120's is vastly overpowered. The one with the 2 Cats which is said to make 18k cruise, proves that it is a planing hull b/c it is cruising at 2X+ its' hull speed and that is the very definition of full planing.
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Old 09-18-2021, 10:19 PM   #18
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Quote:
Originally Posted by Brooksie View Post
If you wish to travel at hull speed, the vessel with the 2-120's is vastly overpowered. The one with the 2 Cats which is said to make 18k cruise, proves that it is a planing hull b/c it is cruising at 2X+ its' hull speed and that is the very definition of full planing.
Perhaps in the case of that particular boat. But in my example above, a rowing scull can go multiples of its hull speed and it is not planing.

Wikipedia:

Quote:
Wave-making resistance depends on the proportions and shape of the hull: many modern displacement designs can exceed their hull speed even without planing. These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly used by canoes, competitive rowing boats, catamarans, fast ferries. For example, racing kayaks can exceed hull speed by more than 100% even though they do not plane.
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Old 09-19-2021, 05:49 AM   #19
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"I think sellers tend to often over-sell their cruise speed."

Their simple method is to give the seed in land MPH instead nautical.

Since may ACW charts are in land miles its easy to do.

For most folks miles per day that is comfortable, is a better concept of speed .
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Old 09-19-2021, 08:45 AM   #20
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Wow. Such helpful polite and thoughtful replies! I’m just now catching up on reading these because we have been on a three day road trip looking at numerous GB 42’s.

If somewhat flexible on the ptice(meaning some reasonable dickering room) we will go after the one with the 120 Lehmans. My only concern, and I’m doing some research today to decide how BIG a concern it should be, is when original fuel tanks were replaced it went from 600 total storage to 400. I see no upside to that except for weight reduction and that is probably a ridiculous consideration coming from an old sailor where 1,500 lbs makes a huge difference. Just thinking the lessening range might rear its head more often that I would like, especially with what I read about fuel quality and price in the Bahamas.
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