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Old 01-29-2019, 08:46 AM   #1
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Differences in Hull Operating Characteristics?

Folks as we continue working our way to buying our first slow trawler (have 28' Sea Ray now), we are trying to understand the difference in operating characteristics of FD hulls and Semi-Displacement hulls. We are looking at two boat about the same size (40ft with single screws), the FD boat comes with a single 130HP and the Semi comes with a 350Hp engine, each has bow thruster, stern thruster and otherwise generally similarly equipped - although the FD has stabilizers.

We'd love to hear comments on ride characteristics, docking issues, drafts, anchoring, etc., to the extent there are differences. Our plan is to use the boat to go from Hilton Head up and down the coast Nantucket to Keys, Bahamas, Great Lakes as part of Loop. Would one type of hull be much better for us, or makes no real difference for what we have in mind. Thanks
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Old 01-29-2019, 09:15 AM   #2
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Old 01-29-2019, 09:26 AM   #3
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Quote:
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We'd love to hear comments on:

-> ride characteristics
An SD boat can run at FD speeds, but not vice versa. SD can reduce rolling at speeds above hull speed when it gets rough.

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-> docking issues:
FD vs SD has no bearing on docking. Docking is done at speeds well below hull speed. The more boat above water leaves you susceptible to wind. The more boat below the water leaves you more susceptible to current and tide.
The heavier the boat, the more wind and current is required to move it and the longer it will take to begin moving the boat.

Thrusters are wonderful


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-> drafts:
FD vs SD has no bearing on draft. Draft is what it is (Draft: Water Depth > Draft)

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-> anchoring:
FD vs SD has no bearing on anchoring. [/QUOTE]
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Old 01-29-2019, 09:50 AM   #4
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It’s simple,
Ya wanna go slower and burm far less fuel (or just less) get a FD boat.
Wanna go faster than a FD boat get SD or planing.

Of course very very few really wanna go as slow as FD. So it’s a game of balancing most of the variables.
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Old 01-29-2019, 10:11 AM   #5
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Well first are you sure the FD boat has an FD hull? There are very few production FD trawlers: the Krogen 42 and the Nordhavn 40/43 are two. But given that:

FD hulls are more efficient at or below displacement speed than SD hulls. But the difference is maybe three bucks an hour diesel cost. For example a good rule of thumb is that it takes 1.5 hp per thousand pounds to move a FD boat at its displacement speed. The SD factor is 2.0-2.5.

FD hulls roll more in seas both underway and at anchor. The flattish bottom and hard chine of the SD reduces roll.

FD hulls usually have a higher angle of positive stability. FD hulls often have ballast which helps as well as the hull shape.

Well I must disagree with docking differences. FD hulls generally have bigger rudders which helps docking. SD have smaller ones to reduce drag at higher speeds. SD compensates with twins or bow thrusters which you rarely see on FD hulls.

And disagree a little bit about draft. An FD hull usually has a deeper hull section than SD but not much. The keel on most SD trawlers pretty much evens it out.

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Old 01-29-2019, 10:25 AM   #6
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Mark,
As others have stated, for almost all of the questions you asked, there is no major difference per se between the 2 Hull types (eg. docking, anchoring, etc.). However, SD does allow an operator the ability to go above hull speed to "get to shelter" ahead of a forecast "storm", but probably not a big advantage if you plan ahead and follow the forecasts. Most boats that are advertised as "off shore" capable are full displacement (Nordhavn, Kadey Krogen, etc.), stabilized, and also usually have the necessary tankage for the needed range.
As far as fuel use, at or below hull speeds probably not much difference if at all. We boated all summer this year with friends who own a Krogen (they FD, we SD). At similar speeds, we burn very close to if not the same gph as them. They run at a higher rpm to achieve hull speed than we do, therefore about the same fuel. They have less than half the horsepower that we do.
Pick the boat that meets your needs (and wants) better than the other, or if planning long range cruising where fuel stops are far between, get the one with better tankage, or off shore capability if that is important.

What brands/models are you considering?
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Old 01-29-2019, 10:58 AM   #7
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One of the differences I found when looking was capacities. For semi displacement, there is a much bigger engine(s), often larger fuel tanks, and as a result often smaller water and holding tanks. Simply, if it's going to plane it needs bigger engines and weight becomes a factor. Depending on the design, space is often lost to a larger engine room to house more engine and fuel.

The above isn't always the case as my boat which is a SD hull, still has good water and holding tanks capacity, without giving up too much space for a bit bigger engine room.

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Old 01-29-2019, 11:02 AM   #8
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https://www.yachtsinvest.com/faq/en-...classification


This is a different angle toward assessing yachts.


I'm not as young and tough as I pretended I was. I can have fun, safely, with almost any boat.
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Old 01-29-2019, 11:05 AM   #9
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Willy has it right if slow and economical works for you FD is the way to go. You'll see some comments about FD boats rolling terrible but there's rolling and then there's rolling. My round bilge FD boat rolls but it's a gentle roll that is more akin to a duck, it rolls the same in a one foot chop as it does in six feet. Most flatter bottomed SD boats have more of a snap component when they do roll but they don't roll much in a small sea. In general FD boats are better seaboats if you intend to do any offshore work, I'll get flamed for saying that but that's been my experience.
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Old 01-29-2019, 12:02 PM   #10
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There are a few more minor differences in a FD boat's behaviour.

A FD hull tends to have more of a keel, a narrower aft section, and (as mentioned) a larger rudder. In a following sea they tend to be more stable and do not wallow near as much as an SD hull.
I also find that FD hulls are less wind affected when docking due to the slightly deeper/longer keel and extra weight.
One last thing - a FD hull is less affected in performance when loaded up with extra weight.
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Old 01-29-2019, 12:12 PM   #11
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One issue with FD boats is they're not as common especially in smaller sizes than SD, that's why I'm building my own (plus I'm cheap).
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Old 01-29-2019, 12:42 PM   #12
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People often worry about the difference in fuel cost between the two types of hulls. Really if you run a SD hull ad displacement speeds the fuel cost isn’t much different unless you are obsessed with fuel economy. I find that fuel isn’t the major cost in boating. One main difference is single vs twins. There will be a larger cost with twin engine maintenance. But you have a second engine to get home on. Everything is a tradeoff. You just need to find the tradeoffs that work for you. One factor in our decision is that my wife is not able to sit aboard on some FD hulls for any length of time due to the roll. That is one factor that is a must for us. So before you buy try sitting aboard for a while in differing conditions to make sure you like the ride at anchor, dock as well as underway.
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Old 01-29-2019, 01:50 PM   #13
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One of the boats is a N40 for FD. The SD boats is North Pacific and similar set ups. All good stuff here and really appreciate the comments. Looks like we’ll end up with SD boat- wife not too excited about flopping around the kitchen trying to cook eggs while underway. Thanks all !!
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Old 01-29-2019, 04:32 PM   #14
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One of the boats is a N40 for FD. The SD boats is North Pacific and similar set ups. All good stuff here and really appreciate the comments. Looks like we’ll end up with SD boat- wife not too excited about flopping around the kitchen trying to cook eggs while underway. Thanks all !!
Mark,
IMHO - A stabilized FD hull is a great solution. Active stabilizers will make a world of difference and "flopper stoppers" will do the same at anchor. You get the efficiency, seaworthiness and kindliness of FD without the rolling underway.

We drive an FD hulled boat without stabilizers and it is rolly, gentle and predictable but rolly. Many N40's have either active fins or paravanes (or both).

My advice would be to befriend a few owners and go for some rides. No better way to compare the characteristics of a few brands/models you've zero'd in on.
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Old 01-29-2019, 09:27 PM   #15
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We have a SD m/y. The comments about the different types of rolling are accurate, we came from a sailboat previously.

The m/y gives easily for a few degrees then stiffens up when the hard chines start pushing water. The sailboat would roll more metronomically. So in small chop the SD is preferred.

However when the m/y gets waked hard enough to overcome the resistance of the hard chines the resulting roll is a hard snap roll.

Getting waked by a big sport fish in your FD sailboat isn't fun because it's a big roll but it's a slow roll and you have a big rudder to help stay on course.

Getting waked by a big sport fish in a SD powerboat, especially if they are overtaking, is a violent process. You roll hard and fast, and you have small rudders so staying on course is more difficult.

SD Hull is nearly as efficient as FD if you are willing to run strictly at your best fuel economy speed. I can push 65000lb @2nm/g if I'm willing to do 6 knots. Or (typical) I can get 1.3nm/g running 8+knots with enough forward momentum to keep the boat really stable. Or if we are in a big hurry we can run 14knots at .5nm/gallon plowing a big hole in the water-. But when a big storm is building behind you the cost difference between 6 and 14 knots isn't that important. In the sailboat when the wind was slack and the current was against you and the swells were on the beam and full speed was 4 knots, that was it you had no options. In the m/y we can choose to spend the money to save time and crew comfort. I like having options.

If you are looking at fast trawlers or SD powerboats and want to run them slow, natural (non turbo) diesels are better for this. Turbo diesels get more power from the same displacement, but if they aren't run fast enough to spin up the turbos they will soot up and need more frequent maintenance. Not a deal breaker, but a small advantage to naturals for that application.
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Old 01-29-2019, 09:35 PM   #16
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One of the boats is a N40 for FD. The SD boats is North Pacific and similar set ups. All good stuff here and really appreciate the comments. Looks like we’ll end up with SD boat- wife not too excited about flopping around the kitchen trying to cook eggs while underway. Thanks all !!
That was a quick decision! Lol
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Old 01-29-2019, 10:27 PM   #17
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Comodave wrote;
“People often worry about the difference in fuel cost between the two types of hulls. Really if you run a SD hull ad displacement speeds the fuel cost isn’t much different”.

Not a realistic comparison at all in that a SD hull is quite happy running at hull speed ... maybe even it’s best speed to run. But running a FD hulled boat at it’s hull speed is insane. I think people have a notion that hull speed is the speed FD boats go. Far from it. Most run what their skippers feel is the best speed to run at that is very close to one whole knot BELOW hull speed. Wave making drag of a FD boat at it’s hull speed is extremely high. I’ll say only overpowered FD boats can even achieve their calculated hull speed. The overpowered boats would have over 4hp per ton of displacement. My boat has 5 and I question if I can actually achieve her hull speed of 7 knots.

So running at hull speed is world’s apart for SD and FD boats.

And Dave that’s what trawler forum guys talk about .. fuel consumption.
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Old 01-30-2019, 03:31 AM   #18
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Comodave wrote;
“People often worry about the difference in fuel cost between the two types of hulls. Really if you run a SD hull ad displacement speeds the fuel cost isn’t much different”.

Not a realistic comparison at all in that a SD hull is quite happy running at hull speed ... maybe even it’s best speed to run. But running a FD hulled boat at it’s hull speed is insane. I think people have a notion that hull speed is the speed FD boats go. Far from it. Most run what their skippers feel is the best speed to run at that is very close to one whole knot BELOW hull speed. Wave making drag of a FD boat at it’s hull speed is extremely high. I’ll say only overpowered FD boats can even achieve their calculated hull speed. The overpowered boats would have over 4hp per ton of displacement. My boat has 5 and I question if I can actually achieve her hull speed of 7 knots.

So running at hull speed is world’s apart for SD and FD boats.

And Dave that’s what trawler forum guys talk about .. fuel consumption.
I'm obsessed with fuel consumption, I look with glee at the charts I get on Boatdiesel.com showing my boat doing .2 gallons per hour at 5kts, that's 25mpg in car talk. I don't yet know if I'll actually get that in practice but I like looking at it. Perhaps if I hadn't gone through those three wives I wouldn't need to pay so much attention.
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Old 01-30-2019, 03:40 AM   #19
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Folks as we continue working our way to buying our first slow trawler (have 28' Sea Ray now), we are trying to understand the difference in operating characteristics of FD hulls and Semi-Displacement hulls. We are looking at two boat about the same size (40ft with single screws), the FD boat comes with a single 130HP and the Semi comes with a 350Hp engine, each has bow thruster, stern thruster and otherwise generally similarly equipped - although the FD has stabilizers.

We'd love to hear comments on ride characteristics, docking issues, drafts, anchoring, etc., to the extent there are differences. Our plan is to use the boat to go from Hilton Head up and down the coast Nantucket to Keys, Bahamas, Great Lakes as part of Loop. Would one type of hull be much better for us, or makes no real difference for what we have in mind. Thanks
Hi, hope you enjoy all comments above.
One point that all my far more experienced and knowledgeable fellow members omitted, although AusCan came close is that a single screw full displacement hull generally, but not all ways has the running gear safely protected inside the extension of the keel shoe.
This running gear ( the propeller and rudder) is without doubt THE most important part of a boat that must be considered at all times. This is why we have our faces glued or alarms activated to the depth sounder.
However with a majority of SD hulls the running gear hangs out unprotected and vulnerable.
The beauty of the keel shoe is that you can survive a beaching when miscalculating tides, run over sand bars, escape the dreaded cray pot ropes and as I have run over the top of submerged logs in dirty brown river water.
No doubt I will be corrected by the SD owners with single and double screw that do have keel shoe.
But for someone new to boating, a little careless or distracted at times, a full displacement gives this important peace of mind.
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Old 01-30-2019, 05:08 AM   #20
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Speaking of FD boats that's a very interesting boat you have. Is that an Australian built boat? I like small FD boats but I don't see many in the US.
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