Differences in Hull Operating Characteristics?

[greysailor]

I`ve had 2 SD boats, one had chine slap one did not. Recently inspecting a boat I saw that the chines started above waterline and there were waves going under the chines, which I figured would "slap". Raised it on TF, and yes, those boats get chine slap. So check the boat, it may be easily predictable.

X2..... I have a Cabo sportfish on each side of my boat. When little wavelets blow in with the wind or from the nearby boat ramp, those Cabos sound like horses walking on a wood floor![emoji34]

 

[Simi 60]

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

When you are buying fuel at 50% of what most of the western world pays I guess you would think that way.

If you were paying twice as much would you think differently?

 

[dhays]

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, if you have any questions on the North Pacific, let me know. I’ve owned a NP 43’ for a few years and there are a couple other NP owners here.

I run slow most of the time, usually burning 2.0-2.2 gph. Not a lot worse than a FD hull with a smaller diesel. FWIW, I couldn’t be happier with our choice of boats.

 

[bogranjac1]

When you are buying fuel at 50% of what most of the western world pays I guess you would think that way.

If you were paying twice as much would you think differently?

I'm paying AU $1.35 per litre Diesel x 3.78 litres to USA gallon = AU $ 5.10 ( US$ 3.71 )

So what our brothers in the freezing northern hemisphere paying at an average gas station?

Sorry about the thread drift, sometimes I just can't resist.

 

[Fish53]

I'm paying AU $1.35 per litre Diesel x 3.78 litres to USA gallon = AU $ 5.10 ( US$ 3.71 )

So what our brothers in the freezing northern hemisphere paying at an average gas station?

Sorry about the thread drift, sometimes I just can't resist.

I think to some the fuel usage issue between FD and SD hulls is a significant part of choosing. I pay $2.15USD for a gallon of gas and $3.01USD for diesel at the pump. I have no idea what marine fuel costs as I don't buy it and fuel my boat from the tank in the back of my truck. My whaleboat I'm hoping will use between .25 and .5 US gallons an hour and I would be curious what your boat uses as it's the same size. When I had a 26' SD lobster boat with a Volvo tamd41a in it trolling for tuna at 5kts used about a gallon an hour. This brings me to another hull configuration question, my brief research indicates that a transom can create a significant amount of drag that a double ended boat doesn't have, this is at slower speeds of course. Does anyone have any views on this?

 

[ranger58sb]

When you are buying fuel at 50% of what most of the western world pays I guess you would think that way.

I'm paying AU $1.35 per litre Diesel x 3.78 litres to USA gallon = AU $ 5.10 ( US$ 3.71 )

So what our brothers in the freezing northern hemisphere paying at an average gas station?

Not close to 50%. Don't know today's typical marine rate, nor local land-based gas station rates, but our last load of marine diesel from our fuel dock was USD $3.15/USG on 11/16/2018. Haven't heard about any huge fluctuation since then, though...

-Chris

 

[North Baltic sea]

When you are buying fuel at 50% of what most of the western world pays I guess you would think that way.

If you were paying twice as much would you think differently?

Hi,

Agree 100%, here the gallon costs 6.5-7 $ and it really matters, I wouldn't be surprised if the price rises over $ 10 / gal in the next few years, because here we are serious about reducing carbon dioxide emissions and the price increase is an effective way to curb consumption.

NBs

 

[Fish53]

Hi,

Agree 100%, here the gallon costs 6.5-7 $ and it really matters, I wouldn't be surprised if the price rises over $ 10 / gal in the next few years, because here we are serious about reducing carbon dioxide emissions and the price increase is an effective way to curb consumption.

NBs

Back in 2008 when I still had my 38' fishing boat I was going through 400 gallons every three day trip @ $4.50 a gallon, if we get back up toward $5.00 again a few attitudes about fuel may change. The boat I ran in Alaska then typically used 24,000 gallons a trip, we always thought about fuel.

 

[Nomad Willy]

I don't recall anyone mentioning this detail....sorry if I missed it.


On a SD hull (and our current planing hull) the hard chines do create quite a bit of water slap. This makes sleeping for some difficult. I believe (but could be wrong) that FD hulls don't have this characteristic.

They can.
A FD boat much like a hard chine sailboat w the transom above the water and the exposed bottom at a shallow angle will “wave slap”. With a real shallow angle the wave slap can be really bad. Like thump thump instead of slap slap. Or even bang bang.

 

[Nomad Willy]

I think to some the fuel usage issue between FD and SD hulls is a significant part of choosing. I pay $2.15USD for a gallon of gas and $3.01USD for diesel at the pump. I have no idea what marine fuel costs as I don't buy it and fuel my boat from the tank in the back of my truck. My whaleboat I'm hoping will use between .25 and .5 US gallons an hour and I would be curious what your boat uses as it's the same size. When I had a 26' SD lobster boat with a Volvo tamd41a in it trolling for tuna at 5kts used about a gallon an hour. This brings me to another hull configuration question, my brief research indicates that a transom can create a significant amount of drag that a double ended boat doesn't have, this is at slower speeds of course. Does anyone have any views on this?

I do,
It depends on the speed mostly,
Good skippering of a FD trawler would include a speed very close to one knot below hull speed.
Only 1/2 a knot below hull speed (faster) and the FD boat will be very inefficient probably burning more fuel than a typ SD hull w the same disp and speed.
Now consider the FD boat running 1/2 a knot slower (1.5 knots below HS) and the FD hull may be 2-3 times more efficient.
Just a small change in speed changes the efficiency a lot.

 

[Fish53]

I do,
It depends on the speed mostly,
Good skippering of a FD trawler would include a speed very close to one knot below hull speed.
Only 1/2 a knot below hull speed (faster) and the FD boat will be very inefficient probably burning more fuel than a typ SD hull w the same disp and speed.
Now consider the FD boat running 1/2 a knot slower (1.5 knots below HS) and the FD hull may be 2-3 times more efficient.
Just a small change in speed changes the efficiency a lot.

I don't want to stray too far from the thread but you make good points about the efficient operation of a FD hull. Many seem to believe that operating at hull speed is the goal. While you want your boat to be capable of hull speed with a little in reserve below hull speed is by far the most efficient. I focus my cruising speed at an SL ratio of 1 which is simply the square root of my waterline length expressed as knots. For my boat that drops my speed from approximately 6.5kts. to 5kts. but reduces fuel burned by more than 50%.

 

[Simi 60]

I'm paying AU $1.35 per litre Diesel x 3.78 litres to USA gallon = AU $ 5.10 ( US$ 3.71 ).

That's cheap but that's at a service station and surely not on the water.
Best we can get on the water here is $1.52/litre and gets worse the further north you go

 

[bogranjac1]

I think to some the fuel usage issue between FD and SD hulls is a significant part of choosing. I pay $2.15USD for a gallon of gas and $3.01USD for diesel at the pump. I have no idea what marine fuel costs as I don't buy it and fuel my boat from the tank in the back of my truck. My whaleboat I'm hoping will use between .25 and .5 US gallons an hour and I would be curious what your boat uses as it's the same size. When I had a 26' SD lobster boat with a Volvo tamd41a in it trolling for tuna at 5kts used about a gallon an hour. This brings me to another hull configuration question, my brief research indicates that a transom can create a significant amount of drag that a double ended boat doesn't have, this is at slower speeds of course. Does anyone have any views on this?

Fish53,
My 3gmd20 Yanmar has a maximum 3400 RPM to develop 20 H.P.
2000 RPM @ 60% of available RPM = 6 knots @ 1.3 Litres an hour
At This is my preferred cruising speed.
2400 RPM @ 70% of available RPM = 6.5 knots @ 2.0 Litres an hour.

 

[Fish53]

Fish53,
My 3gmd20 Yanmar has a maximum 3400 RPM to develop 20 H.P.
2000 RPM @ 60% of available RPM = 6 knots @ 1.3 Litres an hour
At This is my preferred cruising speed.
2400 RPM @ 70% of available RPM = 6.5 knots @ 2.0 Litres an hour.

Do you know what your boat weighs? Mine I believe will end up about 3 tons dry.

 

[bogranjac1]

Do you know what your boat weighs? Mine I believe will end up about 3 tons dry.

Fish53,
My boat weighs2000 kg and has a waterline of 7..25 metres.
I estimated the hull speed around 7 knots.

 

[Vignette]

Forget all the other criteria if it’s a Trawler go with stabilizers everything else pales in comparison

 

[Rick Hudson]

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.

Have to agree that the best of both worlds (from a stability point of view) is an FD hull with fins. We put some on our Cheer Men 38, along the chine. It made no difference to the speed & fuel consumption, but really took the twist out of crossing wakes or being beam on to waves. Of course, physics is physics, so you still rise and fall, but that porpoising action has gone (except when the sea is on the aft quarter, when there's no obvious improvement in stability).

 

[Kit_L]

Interesting thread, to be sure. I would like to add that there is more than one FD hull type and the differences can have significant effects on performance.

Sea Biscuit (Cecil Boden design; said to be Sharpie-influenced, but designed and built locally) has a flatter aft section for about 1/3 of the boat's 40' length. The deadrise in this hull is perhaps 10° (I will measure this off the plans and get back with this info.) and it varies up to the high bow with its sharp entry. She weighs 15 tons, but only draws 4'.

On our maiden voyage back from Qld, as I wrote elsewhere here, we covered 550nm in 62 hours, and used ~650 litres of fuel. We rode the East Australia current most of the way, and it was running at 2–3kn South, around 35nm offshore, on average.

This boat behaves as Willy described above (talking about SD hulls): in confused sea states, the roll accelerations are fast—completely different to the fishing boats I have been on. Overall, a much smaller roll amplitude, but much faster. As well, as she was originally designed as a cray boat, with a fish well a little ahead of amidships, she is relatively tail heavy (fuel tanks in rear compartment; engine in the next) so her pitching rotation point is closer to the 2/3rds mark, not amidships. This means that the forward cabin is moving too much to be rested in—my co-skipper and I used the wheelhouse bunk for our off-watch rests as a result. I plan to move the house battery bank forwards to help this, and perhaps add another water tank too, also forwards; there's plenty of room.

At rest, anchor or dock, she does not roll like a round-bottom FD hull, either, and she is resistant to being waked. I have not taken her offshore in anything more than 2m swells, but the rolling is way less than the fishing boats of similar size I mentioned above. At the dock, the hull is quiet, in terms of wave slap.

A note on fuel consumption WRT FD hulls; on flat water, Sea Biscuit (120hp Perkins, 28" prop) can be pushed to 8.2kn or so—but then her wake is significant and she is sitting in a trough. At the "one Kn below hull speed" that Willy recommended above (which corresponds reasonably closely to Captain Beebe's 1.1 S/L ratio, so about 7kn) SB makes zero wake, and this corresponds to about 1,250rpm on the Perkins. We ran her at this rpm for most of the trip, raising this to 1,450 for the last leg south of Sydney, where we had less EAC assist.

So, although SB is a FD hull, in many ways this design is more similar to the SD hulls I have been on (but without the sprinting speed!).

 

[Joe Pica]

Full displacement Hulls are not all the same. For example the Great Harbour Trawlers are Lou Codega designed full displacement hulls without a full keel. Instead they have a tug /commercial design with skegs and a hard chine that is inherently stable. These are very heavy boats resistant to snap rolls and they settle softly at anchor when the other round bilged hull continue to pendulate thru a large arc like a metronome. The N-37 (37' long at waterline with a 16' beam) that I cruised over 40,000 miles weighed loaded by hoist scale at 48.000lbs.. The boat was driven by 2 small naturally aspirated diesels 54 hp each. Cruise was at 8mph (7 knots) burning < 2 gal an hour combined for both. Total burn over the cruise hours of 6000 hrs was 2.3 gals per hour total for both engines the 8kw generator and a 28,000 btu hydronic diesel heater used all winter for two years. I found that the stability of the hull form was very good and the boat behaved especially kindly in a following sea without any indication of sensitivity to broaching. Note that an N37 cruised to Hawaii from Jacksonville FLa thru the Panama canal. Another one from Jacksonville to Bermuda and then to Newport. The point is that different displacement type hulls forms can behave differently dependent upon the individual design configuration of the hull.

 

[Nomad Willy]

Joe,
Judging from the shape of those propellers I'm guessing you're putting a lot of power into them. But otherwise I'da guessed the engines were small.
Why the very long bow pulpit?

 

[Joe Pica]

Big anchor pulpit extends only 2.5' beyond bow plumb. Don't want the 74lb Rocna anchor swinging to hit the bow. The props are 24×13 slow turning by a 54hp diesel for each.

 

[syjos]

If we could find two single engine boats with the same length and engine with one being FD and the other SD, the single engine FD hull by design will be more economical at the same speed than a single engine SD hull.

The FD hull is usually narrower than it's SD counterpart.

The FD hulls forward section and bow is narrower and sharper to knife through the water instead of creating lift on the SD.

The FD hull gets narrower from the middle of the hull to the stern, which makes the hull more efficient. The SD hull is pretty straight to the transom for lift.

The FD hull usually has minimum transom under water since the rear section starts to rise about half way from the stern. Less transom under water is more efficient. The SD is flatter back there to create lift.

The SD hull has chines which do create more drag than not having chines. The bottom of a FD hull is a smooth, curved tapered shape for least resistance.

The FD hull is usually designed specifically for efficiency and the result is less HP to move it forward.

The planning hull is designed to go as fast as possible with given HP.

The SD hull is a combination of the above two hulls. Good fuel economy at displacement speeds but the ability to get up on a plane. There is a small compromise, the SD hull will burn a little more fuel than the FD hull.

 

[Nomad Willy]

syjos,
How much more efficient the FD hull is depends hugely on many elements of the two boats design. And usually TF members think of both boats traveling at hull speed. IMO a FD hull will be less efficient than a SD hull running at the same hull speed w the same displacement. SD boats are designed to be driven at hull speed or more whereas FD boats are running way to fast for any decent efficiency at hull speed. Some FD boats can’t even attain hull speed. There’s no point in running that fast even if they can.

But there’s so much over-lap in subtle design features that saying FD this and SD that sets up a trap for anybody trying to compare. Many boats are in the vicinity of right between FD and SD. If you had such a boat it would only burn “a little more fuel than a FD hull”. A little bit. Very safe statement. However a boat that is so FD that it showed no signs of SD design it wouldn’t even look like a trawler. It would look like a sailboat. It would probably be a sailboat. And it would be somewhere in the vicinity of three times as efficient. My own opinion is that the average SD trawler will be 2X as efficient w equal displacement and WLL running at one knot below hull speed ... where most wisely run FD hulled boats run. But it’s just a guess. And w a lot of boats it may be 1.25 times as efficient.

As to FD hulls having “minimum transom under water” is closer to no transom underwater.

Hard chines and soft chines are as likely to be found on FD hulls as not. There are billions of FD hulls w big time hard chines and of course there are lobsterboats .. that are soft chine planing boats.

But your post is IMO one of the best posts on the subject that I’ve read. A lot of truths and general statements than can be read, understood and taken to the bank by the average TF member. Cheers

 

[Sea Word]

My boat is a SD shape, hard chine, with puny motors and outdrives. I wish it was FD because the lack of keel makes it track like crap and I worry about the drives getting damaged. I may even put keels on it someday to mitigate this faults, but weary it may have adverse effects elsewhere.

 

[Moby Nick]

We've owned our round-bilged SD Albin-25 for going on 8 years and I'd like for someone to tell us when to expect the "rolling" to start? The Albin's motion on the sort of water we enjoy embarking or anchoring on does not impart discomfort due to "rolling". Maybe that is due to our previous 28" yawl which was extremely lighter displacement that other sail cruisers (even lighter than the Albin) in order to facilitate long-distance trailer towing. The yawl's hull had a draft of 6" and was utterly flat in both beam and length and would react noticeably to ANY wave higher than a ripple. The only way to reduce "snap roll" was to put the hull underway under a press of sail. Compared to out previous Shearwater Yawl, the Albin is very comfortable on the water.

 

[Nomad Willy]

Some learned people think the definitions of planing, SD and FD lay in their speed potential and have nothing to do (more or less) with the hull shape. The boats are classified or thought of as being in a certain speed/length ratio. As I recall a speed length ratio of 1 is hull speed. 7 knots for my 30’ boat w a WLL of 27.5’. A SLR of 2 would be 14 knots and three 21 knots.

They assign a range like 1.4 to 2.2 as being the range of SD boats. 2.2+ would be planing and below 1.4 FD. I offer this as sort-of an example. Been a long time since I’ve addressed speed/length ratios.

Using the SLR to talk about the speed of SD boats has a lot of merit. It tends to settle arguments whereas other other roads to classification generally create more discussions and arguments. Attainable speeds have too much to do w power, size of props, gear ratios and other related matters. To me it’s all about hull form or shape. But the most important element of hull form is the quarter beam buttock line .. QBBL. It’s about the angle of the bottom in a certain part/place on the after end of the bottom of the hull. Few understand it here and most go bananas when I mention it. But it does show that the most important part of hull shape is in the stern having little to nothing to do w the shape of the bow.

Anyway there are numerous ways to classify FD, SD and planing hulls. There’s a lot of naval architects and otherwise learned people re boat design on boatdesign.net and as I recall there’s a lot of disagreement there even despite all the learned and articulate people.

Probably the reason hull definitions come up on TF fairly often is because most all trawlers are SD. One could loosely say planing hulls and FD hulls are sort-of or rather obvious. But SD hulls are sort-of or rather obviously oddballs. They are kinda this and kinda that. And kinda is usually rather hard to define.