Semi-Displacement Hulls?

What were some of the better built semi-displacement hulls, say... pre-1990's?

Most of the SD hulls common to rec trawlers have a relatively straight run aft. And a straight run aft is a promenent feature of planing hulls. But they're considered SD because of their big keel and relatively heavy weight.

One that is much closer to a FD hull is the CHB.

I took this pic a few days ago in the yard while this CHB was being hauled for a potential purchase.
Notice the bottom curve's up almost to the WL at the transom. This is a relatively unusual feature of a SD trawler. There is very little submerged transom in this design. She's not noticeably slimed down beam wise aft so she's as roomy as most others w the exception of her shallow Lazerette she's as big as her middle of the road SD trawler cousins.

The advantage of this design variant is considerably less drag at hull speed and more importantly at a knot below. So for the same displacement, WLL, and beam She's more efficient. More range and less speed. Another advantage is better directional stability especially in following seas. My opinions all.

This is a design feature though .. not a build quality assessment.

Semi displacement is not very good hull shape for fuel efficiency.
That rocker in the hull where it it rises still leaves an immersed transom, so it sucks the water (creates resistance to forward motion ) as it moves through the water since it is still FLAT across the back. Then when you try to go fast it sucks it down much more and they can not get over the hump, so you end up just plowing and throwing up lots of water and uses up huge amounts of fuel like a FD hull does trying to go faster.

You can not go fast as in a planing hull either. So you sort of get the worst of both.
My boat is a semi displacement, I find I end up going at displacement speeds as the fuel burn is ridiculously higher than a planing hull would be at higher speeds and the speed is no where close to what a planing hull can do, so just negatives all around.
If your willing to go very slow, then it is still not as efficient as FD hull, but most likely people don't know the difference. Boat designers sure churn out lots of them, maybe they are cheaper to make than FD boats. If you like the FD hull shape, and can not afford a trawler, just get a used sailboat with fixed keel, remove the rigging and use it as a power boat.

Semi displacement boats are frequently a compromise that is a disapointment.

If you want to go fast there are many sport fish with OK accomidations that aren't too squirely at displacement speeds.

Within the capacity of an AP to steer.

The concept sounds great ," I can put put at cheap fuel burn and then "outrun any storm", a huge wake at WOT at 12-14K seldom can.

I would lean towards a SD hull is not as efficient fuel wise, but there are small plusses for it over either planing or displacement hulls.

Like the glass half full or half empty discussion.....

Every boat is a compromise.....its all about which way is better for you.

sdowney717 said:

So you sort of get the worst of both.

Click to expand...

It's funny, or strange rather, that you mention that. I've heard them described as the best of both worlds. Fuel efficient at cruise speeds but fast enough to outrun weather if needed.

sdowney717 said:

If you like the FD hull shape, and can not afford a trawler, just get a used sailboat with fixed keel, remove the rigging and use it as a power boat.

Click to expand...

Ohhh if it were only that easy! LOL

toocoys said:

Ohhh if it were only that easy! LOL

Click to expand...

It is...more than a few do it.....some take it to extremes and remodel quite a bit.

Get a sailboat with extensive rig damage from a storm for pretty cheap and if it doesnt have water dsmage it could be turn key.

Some sailboats can enclose or alter their cockpits (center or aft or even pilothouse) to be comfortable.

psneeld said:

It is...more than a few do it.....some take it to extremes and remodel quite a bit.

Get a sailboat with extensive rig damage from a storm for pretty cheap and if it doesnt have water dsmage it could be turn key.

Some sailboats can enclose or alter their cockpits (center or aft or even pilothouse) to be comfortable.

Click to expand...

I know it’s physically easy, but the reason we done have a sail boat in the first place is because my partner doesn’t like the cave feeling of the interior, or the lack of deck space and confined cockpit areas, center or otherwise.

I would rather a center cockpit sailboat than anything else on the water, but because of compromises I’ll never be able to sail unless I do it alone.

Fuel efficiency for semi-displacement hulls at displacement speed, has a great deal to do with the engines. In a 45' boat, if you want to plane (sort of), you'll probably need 500 to 700 horse power. In displacement (7 knots), you'll probably burn 3 to 4 GPH. My 45' semi-displacement boat with 135 HP will cruise 7 knots at 2 GPH. A full displacement 45' boat will probably cruise 7 knots at 1.5 to 1.75 GPH.

With the right engine, semi-displacement can be reasonably efficient, but then you loose your 12 to 14 knot top end. You have to decide what your priorities are.

Ted

toocoys said:

It's funny, or strange rather, that you mention that. I've heard them described as the best of both worlds. Fuel efficient at cruise speeds but fast enough to outrun weather if needed.

Click to expand...

Marketing hype from people who make semi displacement hulls to their gullible customers.

toocoys said:

I know it’s physically easy, but the reason we done have a sail boat in the first place is because my partner doesn’t like the cave feeling of the interior, or the lack of deck space and confined cockpit areas, center or otherwise.

I would rather a center cockpit sailboat than anything else on the water, but because of compromises I’ll never be able to sail unless I do it alone.

Click to expand...

I have been on an Island Packet 43, and boy was that a nice layout, did not feel claustrophobic or tight. Sailing and power cats are nice, but hard to find a slip to fit.

Anybody here have a naval architect's technical description of "semi-displacement" ? I have never seen one. Some function of Speed/Waterline (S/L) or Displacement/Waterline (D/ L)?

caltexflanc said:

Anybody here have a naval architect's technical description of "semi-displacement" ? I have never seen one. Some function of Speed/Waterline (S/L) or Displacement/Waterline (D/ L)?

Click to expand...

Cal,
It’s expressed in numerous ways.
The way I prefer for a long time in the past is a very simple but not very accurate method. Observe the amount of flat transom that is underwater at rest. If more than an inch or two (depending on boat size) one would be look’in at a SD.

A much better way that I hear on Boat Design .net is to observe the angle of the bottom aft of a bit behind midships. As in X number of degrees. Many here dislike the expression “Quarter Beam Buttock Line” but that’s what it is. It’s a line at the “quarter beam” like half way between the keel and chine running fore and aft. A qbbl of 0 degrees would indicate a planing hull. A line of 2,3 or maybe 4 degrees is SD and 5+ degrees Displacement (depending on draft) and with the transom out of the water or in some cases (mostly heavy boats) with a small amount of thranom submerged .. like an inch or two on a 35’ boat would be FD.

But there really isn’t a black and white .. this boat is FD .. this boat is SD. The above is sort of a rule of thumb with a grey area between. The whole boat, disp, shape, power, the size or design of the keel all enter into it. There are SD trawlers that have a planing hull shape but are diesel powered w a fairly large rudder and a big keel that are clearly SD but would be a planing hull w the features mentioned above quite different. I would be fairly or somewhat comfortable calling the old 34’ CHB a FD boat (see my original post) but an IG would not. They are much closser to a 0 degree qbbl. The shape of a planing hull. Dress it up the IG hull as a planing boat and it is. Twin gas engines, almost no keel, much lighter weight w small tanks, smaller rudders for less drag at speed ect ect but w the same hull form/shape the IG could be a planing boat. So a hull w/o the big keel, heavy diesel engine/s, big fuel and water tanks you’d have a planing boat. But the same hull form w/o the heavy trawler features is known in dock talk and on brochures is known as a SD boat.

But for those with minds that require a black and white definition you will be forever frustrated. You’ll adopt a theroy and the one day (maybe in a few days) you’ll see a boat that clearly dosn’t fit your theory. So some would call the CHB 34 a SD and some a FD. Both would be right in the grey scope of boat type/hull type talk.

A nice big old dismasted sail boat could easily be converted to a power cruiser , but the keel will probably have to be lightened a good deal.

The mast and rigging removal could make the boat too stiff for comfort.

A snap roll can create a vomitorium, weather its too stiff from form , a flat bottom or hard immersed chines or just too much weight in the keel.

An old center boarder might be best as the weight is shallow.

The big hassle will be resale, remember Winny the Poo!

FF said:

A nice big old dismasted sail boat could easily be converted to a power cruiser , but the keel will probably have to be lightened a good deal.

The mast and rigging removal could make the boat too stiff for comfort.

A snap roll can create a vomitorium, weather its too stiff from form , a flat bottom or hard immersed chines or just too much weight in the keel.

An old center boarder might be best as the weight is shallow.

The big hassle will be resale, remember Winny the Poo!

Click to expand...

What he said X2.......

IMO it’s hard to do but if one starts w a good (appropriate) sailboat I think it can be done. A non-typical sailboat hull is a good start. But it would take time to find such a boat. I bought Willy (my trawler) before I found a really good sailboat to dismember.

Not being a NA I don’t know most of the features needed. One would be full ends. The 27’ Albin is such a boat but they have the prop aft of the rudder. And so it goes.

I’ll try and post something I picked up on facebook about an interesting conversion.

https://www.facebook.com/photo.php?fbid=10212404096964877&set=pcb.1943433729041918&type=3

toocoys said:

What were some of the better built semi-displacement hulls, say... pre-1990's?

Click to expand...

Getting back to this innocent (?) question, if it means heavy boats with the ability to get on plane to, say around 150% of theoretical hull speed, or attain an S/L of over 200 (which pretty much defines a planing boat) , but maybe not as much as 300, then on my list #1 (surprise!) would be Hatteras and #2 Grand Banks.

caltexflanc said:

Getting back to this innocent (?) question, if it means heavy boats with the ability to get on plane to, say around 150% of theoretical hull speed, or attain an S/L of over 200 (which pretty much defines a planing boat) , but maybe not as much as 300, then on my list #1 (surprise!) would be Hatteras and #2 Grand Banks.

Click to expand...

Glad to see one person actually answer the question. I'd second those two, toss in perhaps some of the tug models and a lot of Sea Rays out there.

And boatloads of lobster hull designs.....

Grand Banks!

sdowney717 said:

Sailing and power cats are nice, but hard to find a slip to fit.

Click to expand...

Not to mention hard to find the money for! Have you priced used Cat's lately?

Having a catamaran is the only way that I would EVER get my partner to sail, but affording a cat is a whole nother ball game. It'll never happen.

BandB said:

Glad to see one person actually answer the question. I'd second those two, toss in perhaps some of the tug models and a lot of Sea Rays out there.

Click to expand...

Not any "tugs" nor anything close to a "semi-displacement" Searays I can think of pre-1990's. And he was looking for top quality.

psneeld said:

And boatloads of lobster hull designs.....

Click to expand...

For that time period, I can only think of pure planing boats. Those things can haul some serious ass. Who are you thinking of?

caltexflanc said:

For that time period, I can only think of pure planing boats. Those things can haul some serious ass. Who are you thinking of?

Click to expand...

What are YOU thinking of? What lobster hull/downeast is a planing boat?

Most all Lobsterboats are planing hulls.
Just because they have draggy rounded bilges just means they plane a bit slower. The rounded chines allow the slipstream to suck the boat down and increase the angle of attack and wetted surface. But the straight run aft allows them to plane.

'"Just because they have draggy rounded bilges just means they plane a bit slower."

Or need a tad more power to get to the same speed.

As many use recycled automobile V8s , more power is no problem.

The more comfortable roll when stopped working pots is worth a bit more fuel to get home.

Ellis Downeast Semi-displacement Hull Design - Ellis Boat Company ...

If one stops and thinks about a day fishing boat, the whole concept of semi displacement comes to light.

A reasonably fast boat to get to/from the fishing grounds, yet capable of hauling loads somewhat efficiently.

Sure many modern designs lean towards planing...it seems they have generally grown in size and horsepower too it seems. Lower fuel prices and competition seems to be the driving force.

Meanwhile back at the ranch.... after 29 posts I’ve been able to amass two models... CHB which I could possibly afford, and Grand Banks which is still just a dream.