Ride comfort - displacement vs Semi-Displacement

[ranger58sb]

Sounds people here on TF don't have an answer to the original question.

More like 4,374 answers, few alike... as with many of boating's typical "it depends" stuff.

It's difficult to quantify variable stuff, like the way individuals react to boat motion. What's a definition of "comfortable?" Ask 100 boaters and get 300 answers... and maybe only a few -- or even none -- might have exactly the same comfort level and tolerances as you.

You'll have seen comparisons citing boat lengths, too. A 37" SD versus a 49 FD; a 30, a 50, a 70 something; et cetera. Well, that's often just the best we've got, given most haven't been on every boat of every length in every sea state... (But of course a 70 anything might well be more comfortable than a 30 anything. That's almost a "Duh!")

Anyway, I know how long my (current) piece of string is, but...

One thing that seems to have been posted a few times is that sometimes for the SD hull you have to go faster to get comfort.

We've found that to -- sometimes -- be the case, both with SD and planing hulls. Doesn't work all the time; seems often a matter of adjusting speed to wave height/period. Tacking can help sometimes, too, but that's also a strong maybe.

Wetted surface length can be another factor. Used to be here on the Chesapeake, a 46' hull was often recommended, because that length could often reasonably span three of the choppy short-period wavelets we often get here. Which will sometimes beat your teeth out...

I can predict many here would agree that stabilized boats are likely to be more comfortable than non-stabilized boats. Without regard to SD vs. FD.

Many might also agree that non-stabilized boats, no matter whether SD or FD, will be uncomfortable in some sea states... and the only significant difference will be individuals' personal perception in any given specific situation.

Something like that...

-Chris

 

[rslifkin]

For the purpose of this discussion, what's the definition of semi-displacement? A 12 knot cruise? A 15 knot cruise?

I've been taking the broad definition of it. Anything from a slightly tweaked FD hull that can exceed hull speed by a couple knots up to a slow planing hull designed to cruise at 20 kts or less (once you push past that the game starts to change as big rudders are too draggy, keels cause handling issues, etc.).

 

[O C Diver]

For the purpose of this discussion, what's the definition of semi-displacement? A 12 knot cruise? A 15 knot cruise?

My definition of semi displacement or semi planing woud be the ability of a traditional hull ( not a catamaran or other) to reach twice the square root of the waterline length. So a 49' waterline length boat should be able to reach 14 knots. To accomplish this, the hull needs large relatively flat areas in the stern, a bow that can climb up on plane (usually a shallower draft and no bulbous bow), and obviously the propeller (drive train) to meet the speed.

Ted

 

[Jeff F]

For the purpose of this discussion, what's the definition of semi-displacement? A 12 knot cruise? A 15 knot cruise?

I'd say it's in the range of 1.3 - 2.0 SL. So 9-12 knots on a 40' boat.

 

[Jeff F]

I can predict many here would agree that stabilized boats are likely be more comfortable than non-stabilized boats. Without regard to SD vs. FD.

My claim is that FD requires stabilization more than SD, because SD offers additional form stability through speed that isn't available with FD.

Another note - I most likely will be getting a 1980s boat, whether the hull is a displacement or SD may turn out to just be a "thats what the boat has" thing. If you look up the hull difference is always list the displacement has "more comfortable". But that is like saying "better" with no definition.

I disagree. There are very few 1980s FD boats in existence. I think on examination you'll find that FD boats represent a tiny portion of the market.

Driving a SD design slow does not turn a boat into a displacement design. Boats like the Hatteras LRC, DeFevers, etc are SD designs by my definition.

 

[Mac2]

Let me ask this another way. Percentage wise, how often do those of you that have SD hulls, have to slow to displacement speeds due to the conditions? Those of you with tons of experience (think delivery captains), are there times you would leave port with an FD hull, but not an SD hull (from the perspective of an average boater). Obviously if you have the ability to wait for a weather window this is not relevant, but I would think it interesting to understand any limitations-if any.

 

[Don L]

That might be of interest. I bet the answer is zero.

The basis in my original question really has to do with the cost of operation and fuel costs. I long was able to figure out that a SD boat can improve its' ride with more speed.

I thank everyone who played on the thread. I am going to stop watching and move along to other things that are part of my decision of getting back int boating and choosing a boat.

 

[mvweebles]

......are there times you would leave port with an FD hull, but not an SD hull (from the perspective of an average boater).

Good comparison. Let's take an older KK42 vs a similar vintage and similar condition GB42 going from Santa Barbara CA to San Francisco CA, about 300 miles uphill. A couple days non-stop.

I cannot think of a condition where I would leave with a KK42 but would stay in port with a GB42. They are both strong boats and capable of making the run in equal comfort. If there's a difference, it's slight.

Peter

 

[rslifkin]

Good comparison. Let's take an older KK42 vs a similar vintage and similar condition GB42 going from Santa Barbara CA to San Francisco CA, about 300 miles uphill. A couple days non-stop.

I cannot think of a condition where I would leave with a KK42 but would stay in port with a GB42. They are both strong boats and capable of making the run in equal comfort. If there's a difference, it's slight.

Peter

I agree. The KK42 would likely survive somewhat worse weather, but I don't expect the threshold where the crew is miserable to be all that different.