Hull speed/ horsepower questions

[Scooby5959]

What is the real question

My wife and I are on our first official road trip to look at a trawler. I’ve been a registered user here for a few years in anticipation. We have been sailers for over 40 years, And I’ll keep our coastal and easily singlehanded sailboat for the foreseeable future. Here’s my questions, which illustrate my sailing background.
Do power vessels have a “ hull speed” and is it the same formula as with a sailboat? My brain doesn’t function well in this arena.
I’m mostly just curious about the above, but here is my main question.
The Grand Banks 42 we will see today has twin Ford Lehman’s 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 3208’s 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 RPM’s, 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.

I don’t know exactly what you’re asking . if you are asking what engines are the most inefficient For this type of hull . I would probably have to say Any twin engine would be a bad choice. For a variety of reasons that has been discussed many times . but if you’re looking to have the most efficient power plant single engine is the correct answer. My choice would be an engine with enough horsepower to push the boat along at seven or eight knots and a reserve of a couple of knots for when you need extra power . Also an engine that at least has removable cylinder sleeves so you can rebuild the engine in the boat. Anything else would be a bad choice for this type of hull design .If you want or need more speed . then look for a boat that is designed to plane .your run-of-the-mill sports Fisher come to mind. I’m sorry if this isn’t what you wanted to hear it is what it is .you can’t have your cake and eat it too

 

[BruceK]

A trawler is nice to be onboard when run at hull speed. Powering up my Cummins 210s periodically as advised by my mechanic, there is lots of noise, vast quantities of water get moved, speed rises about 20%, and it`s not a pleasant experience. The return on hugely increased fuel use is miserable.
Lehmans are a great simple engine almost anyone can fix,which barring mishaps will run to high hours, and parts are readily available. In good condition, at idle, they sound beautiful while visiting the ER.
In times of urgency you might run them hard to achieve speed. But generally, a trawler is happiest at hull speed, and you probably will be too. There are plenty of good "sports boats" for those who want to go harder and faster and some of them are very acceptable boats.
I hope the search goes well,a GB42 is a good choice. Engines are one of several factors leading to selecting a particular boat, other factors could outweigh a pre determined engine preference, an open mind is a good thing.

 

[Giggitoni]

Is that what it costs to replace the fuel tanks? Do the engines need to come out? I assume stainless water tanks have the same issue just smaller?

I replaced the tanks in my GB42 about four years ago. Removing my Cat 3208 naturals was not an option. Had the old leakers cut up and replaced with three aluminum tanks on each side. Reduced my total gallonage from 600 to 535. Cost was a little less than $19k.

 

[Scooby5959]

I had one of these engines

I replaced the tanks in my GB42 about four years ago. Removing my Cat 3208 naturals was not an option. Had the old leakers cut up and replaced with three aluminum tanks on each side. Reduced my total gallonage from 600 to 535. Cost was a little less than $19k.

I owned a 3208 cat in a L 8000 Ford Cabover,Local delivery truck. Easiest starting diesel I think I’ve ever owned .however smoke like crazy on start up It had 120,000 miles, I was told they all do that something to do with only having three rings on the Pistons I don’t know.I guess it’s a Ford/cat engine they made over 1 million of them put them in everything from fire trucks to Boats,They are considered by many to be a throwaway Engine ,No cylinder sleeves ,However they can be long lived especially the naturals,The turbos didn’t fair as well . I can’t imagine having two of them in a trawler , but why not . not very fuel-efficient either . they quit making them in 99. However parts are readily available and you can buy a reman for around 12,000 which is pretty cheap today

 

[FF]

Hull speed is always an approximation as it is when you run out of power to climb your own bow wave.

The bow goes up, the stern goes down and that's as fast as you can go.

At easy cruising speed the hollow alongside the hull will be less pronounced, as you wont be climbing the bow wave.

After a boat gets skinny ,( better than 6-1 Length to Beam}, the waves created are smaller so the top speed is higher , welcome to multihull land.

The hassle is narrow boats have more wetted surface , so the top speed might be higher but low speed is less efficient .

They try to avoid this with complex super light construction ,which is very expensive.

For most heavy 3-1 L/B ratio boats OC Divers boat fuel burn seems typical,

6.5 or 7K would be the most economical cruise.

A great autopilot makes this easy to live with.

As always weather its the voyage or the destination, that decides weather you need another 300-600HP

"1.2 GPH @ 6 knots
1.5 GPH @ 6.5 knots
2.0 GPH @ 7 knots
3.7 GPH @ 8 knots"