What type of hull is this

[SithLord]

Hi folks,

Wondering if you can help me classify this hull type. Would you say it's a planing hull or semi-planing (semi displacement)?

It's a 50ft flybridge I'm considering buying. I previously owned a similar size boat (an old Ocean Alexander) which was definitely semi-displacement, but while there's similarities in hull shape between the two; the ocean had a significantly deeper keel... maybe twice as deep as this.

But my understanding if it was a full planing hull, wouldn't a keel of this size just create unncessary drag?

 

[Comodave]

It looks more planning to me. The keel is pretty small. What is it powered with? That may tell the story.

 

[SithLord]

2 x Volvo TAMD74 EDC 480hp

 

[Comodave]

Well it may fall into a gray area. My 41’ Formula has twin 450hp Cummins in it. But a 50’ with just a bit more hp probably will plane but not really fast. Guessing top around 27 knots and a cruise around 20 knots. Guessing. If you get it let me know how far off I am…

 

[SithLord]

Yeah will do. My understanding is a semi-planing boat doesn't ever completely plane. There's some lift to exceed the speed limitations of a displacement hull. And keeps good sea keeping ability and fuel efficiency at hull speeds which was the case with my previous Ocean

 

[psneeld]

Not sure why people think there is a clear line or gap between the hull definitions.

Sure, there might be "points" or sections on graphs/charts that professionals might say "the numbers" define the categories, but with real life boats, a picture doesn't tell much. It might be definitely one or another or all it might do is show a hull that could be either depending on just a variable or two..... like HP or props.

I had a 37 foot sportfish that had a bigger keel proportionally than those pics and it was definitely run on plane all the time. However, if I backed it off just a couple hundred RPM where many would probably cruise at (though the milage was worse), it would settle enough that many would think it was a semi-displacement. The hull didn't change, so what was it?

 

[SithLord]

Yeah the other thing is I haven't seen it in person yet. There could be a bit of optical illusion going on in the photo with how big the keel actually is. i.e. if looking at the distance between the travel lift slings.

 

[Bmarler]

That little keel isn’t going to induce much drag, but it will certainly help with tracking.
I think the pictures are deceiving, and in the flesh it will look different.
I would definitely have a look at the hardware for the swim platform. There’s some rusty stuff and possibly some crevice corrosion going on.

 

[ranger58sb]

2 x Volvo TAMD74 EDC 480hp

Hard to tell, but like Scott said, it's often more of a continuum than a hard "this or that."

Without knowing boat details, other than the "50ft flybridge" thing, I'd guess those (that horsepower) might have been base engines and the boat won't be any kind of speed demon. (But then too, you're here in trawlerforum so maybe speed doesn't matter much to you anyway.)

If my guess about base engines is close, perhaps that boat was also offered with optional power, and something like 650-700 hp each might have made the boat faster, maybe more of a "planer" than a "semi-planer." You might be able to check in places like the Powerboat Guide for a slightly more basic build info...

-Chris

 

[SithLord]

Yep you got that right. I have no interest in going fast burning 170-200L /hour. Can't afford it. I can install Starlink and work onboard. I don't need to get anywhere fast. But full displacement trawler style vessels in my region are crazy expensive. With a full keel it probably needs stabilizers etc etc. It's finding the balance and thats semi-displacement... but I don't know if thats what I found here. Hence the thread.

 

[heysteve]

Also look at the rudders: the smaller they are the faster the target cruise speed. A generalization like everything else, but it seems to hold true.

 

[rslifkin]

I'd call that a planing hull, but probably one that's not meant to cruise north of 20 kts or so and is also capable at lower speeds (due to some keel to help tracking and big enough rudders). My boat also fits into that category and we're happy with running it mostly at 6.5 kts although it will cruise happily at 17 kts if we want to burn the fuel.

Honestly, in my mind, a lot of what people call "SD" hulls on trawlers are really just planing hulls that don't have enough power to actually plane. In my mind, given enough power, if the hull goes through a distinct plowing phase above hull speed and then breaks onto a clean plane, it's a planing hull regardless of what it looks like. A true SD hull should exceed hull speed without that bow-up plowing range.

As far as keels and big rudders on a planing hull, they do increase drag. And at high enough speeds a keel starts to cause handling problems. But on a planing hull with a lower target cruise speed, the penalty isn't huge and it improves handling dramatically at lower speeds.

Here are a couple pictures of the hull form on my boat and one showing the wake running at 17 kts (clearly a clean planing wake). Quite a few people have seen the hull and thought it's either SD or displacement (as it doesn't look like a planing hull underwater), but based on how it behaves I'd call it a planing hull. In our case we have a fairly deep forefoot, convex forward sections, a narrow reverse chine up forward that slowly twists out and is reduced to a simple hard chine with no flats by about 2/3 aft from the bow. The hull also has a little bit of rocker towards the center, but not out at the chines. Big trim tabs to get enough lift at the stern (due to the lack of chine flats) and relatively low deadrise aft, plus a significant keel (just over a foot at its deepest point) and decent size rudders.

For perspective, our boat is 38 feet, about 27,500 lbs loaded and has twin 340hp engines. Fast cruise is 16 - 18 kts, at WOT it'll do a bit over 25 (I know it has hit 27 - 28 earlier in its life when it was a bit lighter).

 

[SithLord]

After looking more and more I think we need to disregard my photos entirely. If you look at the bottom of the port and starboard rudder and how they would align. Everything is off with this pic. I'll try to request better photos and update here.

 

[SteveK]

Honestly, in my mind, a lot of what people call "SD" hulls on trawlers are really just planing hulls that don't have enough power to actually plane. In my mind, given enough power, if the hull goes through a distinct plowing phase above hull speed and then breaks onto a clean plane, it's a planing hull regardless of what it looks like. A true SD hull should exceed hull speed without that bow-up plowing range.

I think the same. The same hull with a bit more power or different props and a big difference in top end. They all go trawler speed.