Why Aren't Semidisplacement Hulls Considered True Ocean Going Boats?

Baker said:

PS.... Most pilot boardings are done with the ship anchored or at least hove to.

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Not around here... full speed ahead, near as I can tell.

-Chris

My boat with and without stabilizers

I took the following video while bringing back our, new to us, OA 456 with stabilizers, in 3 - 5-foot seas. Oops, just told that the 39-second clip is too large.

A full planing hull will have less interior space than the so called SD hills. The big sportfishers need speed to get out to the canyons and stream which can be 100 miles plus offshore.
As with many things form follows function. You'll also notice that a lot of them have no pulpit or forward windows or railings which are all prone to breaking when going 20+ knots into seas.

Sealife said:

I believe the original poster was asking about ocean or blue water experience.....ICW not so much. BTW its Kadey Krogen. Until youve been hundreds of miles offshore, its hard to describe it to others. The sheer size and weight of the water affxts both hulls very differently. We are not talking about a steep chop or boat wake on the ICW

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Depends on the boat and conditions. Generalizations aren't much good in this context. For instance you can find much steeper seas closer to shore. Witness Trying to cross the Gulf Stream with a northerly blowing.

Planing, Full Planing, Semi-Planing, Semi-Displacement. All nomenclature. They are all hybrids. Every hull design has it's own differences.

BandB said:

Planing, Full Planing, Semi-Planing, Semi-Displacement. All nomenclature. They are all hybrids. Every hull design has it's own differences.

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Agreed. But nobody answered the question. If you are designing a boat that will plane and is powered to plane, why would you make it more of a semi boat instead of a planing boat? Grand Banks went away from the Semi planing boat to a modified V....around 10 years ago? I imagined for the reasons I am getting at....likely efficiency since people want to go fast. I think even Mainship went with a Modified V on a few of their "trawlers" before they went Tango Uniform. Nordic and American Tug are semi boats powered to plane. Why would they not go the route of GB and (maybe) MS????

".likely efficiency since people want to go fast,,"

The people that want to go fast need smooth water to do it.

The ride on a boat at speed brakes people before it brakes the boat.

$8000 Stidd seats only work for a half hour or an hour before even Navy Seals (folks in great condition) have to slow down.

The deep V is one attempt at softening the hull ride. But probably hydrofoils are the best answer.

There is lots of ongoing research in giving a less destructive ride to people in open water , but no great results, yet

PBB did a few articles on the research

Baker said:

Agreed. But nobody answered the question. If you are designing a boat that will plane and is powered to plane, why would you make it more of a semi boat instead of a planing boat? Grand Banks went away from the Semi planing boat to a modified V....around 10 years ago? I imagined for the reasons I am getting at....likely efficiency since people want to go fast. I think even Mainship went with a Modified V on a few of their "trawlers" before they went Tango Uniform. Nordic and American Tug are semi boats powered to plane. Why would they not go the route of GB and (maybe) MS????

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My point is that you're not talking semi vs. planing. You're talking about a wide range of hull types. You mention modified V. I'd add steps. Throw in various other variations. I would design the hull to maximize performance and efficiency and ride while within the intended use of the boat. If I was designing for a group cruising at 15-20 knots then I'm not concerned about how it might perform at 30 knots if powered for it. I'm not going to be powering it for faster.

If I'm William Garden, I want it to go as fast as I can make it given the size and type boat I'm designing.

I think Nordic and American Tug stay where they are for quality of ride but also because they don't believe their buyer is interested in greater speed. Grand Banks had their hand forced a bit by an ex- or theirs, Marlow. Marlow designed for more speed. Then GB's 15 knot cruise was deficient. So, now you have a 43' that WOT is 24 knots and cruise is 20 knots with twin 480 hp and a 54' with WOT of 20 knots and cruise of 17 knots with twin 715 hp. They hit a bit of a barrier there though. Adding more hp produces very little so they've reached the limitations of their new hull design. If they wanted to reach a cruise of 25 knots, then they must redesign, which is basically what they appear somewhat in the process of doing. Meanwhile their sales are in the Palm Beach line where the 55' achieves WOT of 31.8 knots and cruise of 27 knots with Volvo IPS 800's.

They are all designing to a consumer's desires which none of them know for sure. American Tug believes their customer values comfort and economy and is very happy with a WOT of 16-17 knots and a cruise of 12-13 knots with many of them running most of the time at 7-8 knots. The other part is what would they sacrifice in changing the hull to get a bit more speed. They simply don't believe their consumer wants more.

Beneteau with Swift Trawler obviously felt there was a demand for more speed pretty comparable to Grand Banks, just a little faster. Maybe the demand is for even more speed than they offer. I would be curious as to how their boat might perform with more hp and has their hull neared it's max or is there much more that one could get with more hp.

FF said:

"
The people that want to go fast need smooth water to do it.

The ride on a boat at speed brakes people before it brakes the boat.

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Not always true. Boats designed for speed and rough conditions often handle it better with greater speed. You don't have a requirement for smooth water. Ever seen a Fountain or Cigarette or various offshore racing boats perform? With faster boats the best speed in rough seas is partly determined by the wave separation and timing with the waves. In a 44' Riva, heading into 4' at 6 seconds and it's going to be more comfortable at around 30 knots than it would be at 15 and at 30 knots or so will pretty much smooth the waves out.

I think Nordic and AT could put more power in their boat and get more speed without redesigning. But yes, they are still balancing speed versus efficiency....and also they generally believe in single engines so that is a design issue.

If you are designing a boat that will plane and is powered to plane, why would you make it more of a semi boat instead of a planing boat?

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I thought I had answered. Interior volume is the answer, the trade off being speed. Secondarily, slightly more efficiency at hull speed and below. So on something like my Hatteras, or the original Flemings (which have an almost identical hull form, having seen both out of the water within 100 yards of each other) 14-16 knots planing cruise speed instead of say 20-25, and typically smaller engines HP wise.

caltexflanc said:

I thought I had answered. Interior volume is the answer, the trade off being speed. Secondarily, slightly more efficiency at hull speed and below. So on something like my Hatteras, or the original Flemings (which have an almost identical hull form, having seen both out of the water within 100 yards of each other) 14-16 knots planing cruise speed instead of say 20-25, and typically smaller engines HP wise.

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I don't buy it. I have seen G I G A N T I C planing boats with tremendous beam and interior space. Your boat is a perfect example. Your boat does not lack interior volume and it is not a semi planing hull. Ask Hargrave or whoever designed it. I would think it to be modified V.... Yes I know it is not cut and dry but Hatt has never been known for their semi hulls. In fact Your boat goes slower(15kts) simply due to a lack of power....not hull form. If your boat had 1500hp per side it would be perfectly happy cruising in the 20s. If you overpower a semi planing hull it will start to bow steer....IOW, all of the lift is in the aft part of the hull and the forward part has very little lift. This was an issue on the Mainship Pilot 2s with the 6 cylinder Yanmar...allegedly.

Gordon J said:

I have an Ocean Alexander semi displacement hull with stabilizers. I can tell you that the stabilizers work great and the ride is as comparable or better than that of a full displacement hull with stabilizers.

I have just completed a trip both down the ICW with a full displacement Katie krogan and return with my ocean Alexander, and the ride on my ocean Alexander is every bit as good.

Gordon

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We're not talking about the "ride" or comfort in the yachting world. We're talking about seakeeping abilities in the ocean far offshore in 10 to 20' seas and worse. You can putz around in the ICW or a few miles offshore in benign conditions and have your yachty ride (like you're house) but that says nothing about serious ocean travel in stroms and such in the ocean.

For smaller boats (not ships) the FD hull is best. You can row an aluminum planing skiff if you pay attention to your heading but a FD whitehall row boat is far supperior. If it becomes really rough the skiff becomes unsafe and the propper rowboat remains under control. Same w ideal offshore travel in boats and for many of the same reasons. Directional control is probably the biggest reason most of the time. The FD boat is better at going where it's pointed and in really big water there's nothing better than being able to control your boat.

THr first few repsonses pretty say it all.

THD and Ski articulated it well.

Not much to ad, other than a semi planing hull is weight sensitive, as you are using power to get out of the water, therefore a full fuel load becomes self defeating.

Baker said:

Why would you choose to design a semi planing hull that you intend to do planing speeds and take the efficiency hit???...instead of designing a planing hull???

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I pondered that exact thought when I went shopping for a late model OA 42 Sedan. (SD hull) My old OA 42 Sedan had a modified V hull and was faster than the SD. It didn't carry the weight that the SD carries and was a pig at trawler speeds. The SD hull on the newer OA boats (42s) are a wonderful compromise of having a boat that will cruise at 8-18 knots.

This short video of the OA 42's hull is quite revealing. The best look at the running hull is at the 1:45 mark of the video.

Wxx3 said:

a semi planing hull is weight sensitive, as you are using power to get out of the water, therefore a full fuel load becomes self defeating.

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What? A properly designed and powered boat isn't going to be defeated by carrying a full load of fuel.

Wxx3 said:

Not much to ad, other than a semi planing hull is weight sensitive, as you are using power to get out of the water, therefore a full fuel load becomes self defeating.

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And that is not the case with any hull with respect to adding power? I have not noticed any "weight sensitivity" on my SD hull. It carries significantly more weight than than the original Modified V and after climbing the wave (adding more power as you say) you can back off the power and ride the wave! I'm not saying that the SD hull compares "efficiency wise" with the FD. I am saying that the advent of the SD hull was a big plus in designing a boat that can do a multitude of missions. Not just low speed blue water cruising.

Baker said:

PS.... Most pilot boardings are done with the ship anchored or at least hove to.

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Not true, at least not on the west coast when entering ports. Leaving ports - different story, in which case they're already docked or anchored.

Sd

Donsan said:

Don't forget stabilization. Most true blue water cruisers have some form of stabilization that don't work well or are inefficient on a semi displacement boat.

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Nomad Willy said:

We're not talking about the "ride" or comfort in the yachting world. We're talking about seakeeping abilities in the ocean far offshore in 10 to 20' seas and worse. You can putz around in the ICW or a few miles offshore in benign conditions and have your yachty ride (like you're house) but that says nothing about serious ocean travel in stroms and such in the ocean.

For smaller boats (not ships) the FD hull is best. You can row an aluminum planing skiff if you pay attention to your heading but a FD whitehall row boat is far supperior. If it becomes really rough the skiff becomes unsafe and the propper rowboat remains under control. Same w ideal offshore travel in boats and for many of the same reasons. Directional control is probably the biggest reason most of the time. The FD boat is better at going where it's pointed and in really big water there's nothing better than being able to control your boat.

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My comment only dealt with whether stabilizers were as effective in SD hulls. I made no comment about blue water capabilities. I could care less since I am done with blue water. I put thousands of blue water miles under my sailboat keel. Now I am ready for something new, and yes, more comfortable.

Gordon

Gordon,
Narrower FD boats respond better to paravane stabilizers.
I belive you'd be more comfortable w a FD hull.
Have you read my post #49 ?
Perhaps it's flawed.
If so how so?

Nomad Willy said:

Gordon,
Narrower FD boats respond better to paravane stabilizers.
Have you read my post #49 ?
Perhaps it's flawed.
If so how so?

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Gordon isn't talking about paravanes. You keep reverting.

Nomad Willy, why would I use paravanes when I can use hydraulic? I am not a nautical engineer and take no issue with your opinions.

Gordon

Codger2 said:

This short video of the OA 42's hull is quite revealing. The best look at the running hull is at the 1:45 mark of the video.]

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Hmm. Walt,...I think I might have underestimated that hull. Of course, it's very calm water, but that baby is nicely trimmed. Anyway, I hate those videos that show glimpses of things and fade the frames "artistically" so you can never gain a point of reference on much of anything. Thankfully, they didn't do that to the shot underway before you could actually see something. Nice boat though.

Baker said:

I don't buy it. I have seen G I G A N T I C planing boats with tremendous beam and interior space. Your boat is a perfect example. Your boat does not lack interior volume and it is not a semi planing hull. Ask Hargrave or whoever designed it. I would think it to be modified V.... Yes I know it is not cut and dry but Hatt has never been known for their semi hulls. In fact Your boat goes slower(15kts) simply due to a lack of power....not hull form. If your boat had 1500hp per side it would be perfectly happy cruising in the 20s. If you overpower a semi planing hull it will start to bow steer....IOW, all of the lift is in the aft part of the hull and the forward part has very little lift. This was an issue on the Mainship Pilot 2s with the 6 cylinder Yanmar...allegedly.

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Sorry Baker but you are dead wrong about my boat. I got to compare its hull to a couple hundred other boats of all kinds during a couple of hurricane haul outs. Like the Fleming for instance it has a much fuller and deeper hull than boats the same size that can go twice as fast. Someday I will dig up the file of photos I took.

I may have a picture around of a Hatteras 58 LRC out of the water and the subtle difference between my hull and its.

My Hatteras achieved its very large interior via high freeboard and an 18'2" beam that extended for a good deal of the length. I have posted a picture of it next to a Fleming 55 that illustrates this dramatic above the water line difference. Another factor was that Hargrave molded the 1050 gallon fuel and 210 gallon holding tanks into the bottom center of the boat.

Paravanes and fin stabilizers mechanically operated work the same way dynamicly so re this discussion are the same. They act as an ofset plane to the direction of movement. Offset when the boat rolls.

We actually have been arguing about the wrong thing. Re the effectiveness of stabilizers the question of FD or SD is mostly irrelevant.

One should say instead that boats w strong righting moments will find stabilizers less effective. Roughly speaking it's mostly the beam of a vessel that's the problem. SD or FD have little to do w it. Weight and inertia are involved in no small way too.

Imaging an unloaded barge w stabilizers. With this image anyone can see the stabs would be very ineffective. Then imagine a narrow, heavy and round bottomed boat w stabilizers. Very strong effect. The barge has an extremely strong righting moment and the narrow boat's righting moment is weak. I don't see where there's any room for discussion much less argument. Boats w a strong righting moment are not going to do as well w stabilizers ..... and that's just a fact as I see it.

For those not familar w the expression "righting moment" it's a force that tries to right a boat that is found to be tipped, rolled or otherwise (as they say) "not on an even keel". The rolling force that causes the boat to right itself. Of course when a boat is sideways on the face of a wave the boat (via it's righting moment) tends to right itself to the face of the wave. And the job of the stabilizers is to oppose this force so the boat won't jerk itself to a new and different position. "Snap roll". The stabilizers don't stabilize at all ... they just oppose the rate of movement to the tendency to roll.

If I could post a short 39-second film, you would see how effective fin stabilizers are on my OA 456. Alas, I am unable to figure it out.

I understand you to say that the rounder, deeper hulls require more stabilization than SD hulls, and I think that is right. In the end, however, stabilizers make both hull types more comfortable, unless you like roll. I would have hated being on the Kady Krogen 48 without stabilizers.

Gordon

Gordon

Nomad Willy said:

Dos'nt make it seaworthy.

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Of course it doesn't since "seaworthy" is an insurance/legal term and not a marine term. "Seaworthy" defines the specific condition of a specific vessel for a specific voyage and includes a determination of the qualifications and competence of the master and crew for that specific voyage.

Gordon,
You probably would'nt like my Willard. The only stabilizer I have is the substantial keel. But I would hate to think of the roll w/o the keel.

if I were looking for smaller...

Nomad Willy said:

Gordon,
You probably would'nt like my Willard. The only stabilizer I have is the substantial keel. But I would hate to think of the roll w/o the keel.

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I am sure your boat would be fine. My OA 456 also has a keel, although not quite as large as yours (Proportionally), but still enough to reduce roll.

Gordon