Planing hull into FD

See this thread on boatdesign.net about a man that grew tired (literally) of making eddies aft w his flat submerged transom.

The resistance difference between a FD hull and something else is much greater but what you'll see in this thread is a step in the right direction and is rather well documented for our purposes. The majority of the hull remains a slab sided and flat/straight bottomed hull.

Could even be applicable to Mark's external fuel tank issue.

To see quickly where this thread is going go to posts 67 and 73.

Stern Extension to Help Rowing - Boat Design Forums

"much greater" is a couple of big words...the slower you go the less the resistance is an issue...either hull design or parasitic......that's why even planning hulls with big engines get very good fuel economy numbers when going slow enough....close enough to scare the most stoic FD guy...well... most at least.

My thoughts when I hear something like this, whether its boats or houses, is sell it and buy/build what you really want.

What some of our members are showing some of the non-believers or mis-undestanders...

Is that you can buy a planning vessel, run it at displacement speeds, get good not great fuel economy, yet still have those sprint speeds of versatility that the one design types just can't duplicate.

Sure boats built for a specific purpose will do that one thing well...but many want a more versatile boat and the difference in spending $2000 dollars in fuel or $7000 dollars in fuel on a $225,000 boat may not rock their socks.

This is not directed at you Archie but for several posters here projecting ones' thoughts and opinions about what others CAN do is one thing...but constantly telling them WHAT they should do, or the only best thing is what led me to start the TF bad boys club.

There are very few things in boating that HAVE to be a certain way. The only thing that bubbles to the top in my mind is fun.

I think we all need to go back to the link to the pic of the boat in question from post #1. If the guy wants to clamp a box on the back of his skiff to make it 0.1 kn faster, have at it.

manyboats said:

See this thread on boatdesign.net about a man that grew tired (literally) of making eddies aft w his flat submerged transom.

The resistance difference between a FD hull and something else is much greater but what you'll see in this thread is a step in the right direction and is rather well documented for our purposes. The majority of the hull remains a slab sided and flat/straight bottomed hull.

Could even be applicable to Mark's external fuel tank issue.

To see quickly where this thread is going go to posts 67 and 73.

Stern Extension to Help Rowing - Boat Design Forums

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How much of the speed improvement/drag reduction is due to the change in sectional shape and how much is due to increased length?

Glad to see the BD thread is being read here.

I posted it for those that may learn from it .... not for those that have responded.

Little was actually gained speed wise but if you read the OPs text he said he immediately noticed a difference. I've often said if you try to row a skiff of this basic type (especially w someone in the sternseat) and then row a good FD rowboat the difference will be night and day. I wouldn't bring this up so often if there wasn't so much chatter about fuel efficiency on TF.

This experiment was done badly in that the victim boat should have been shortened and the extension made longer w a much more gradual turn up of the bottom aft. Then there really would be increased efficiency to report.

This is just to show you really can make a FD hull out of a planing or SD hull. And of course the more effort you put into it the better the results will be.

Some boats that are bow heavy could acheive normal trim by turning up the afterplane (sternbottom). And be clear that basically I'm not promoting that anyone tack on an extension as described in the BD thread.

TAD in this thread I'm talking about resistance well below hull speed ... Not speed.

manyboats said:

but if you read the OPs text he said he immediately noticed a difference.

TAD in this thread I'm talking about resistance well below hull speed ... Not speed.

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Okay we'll just call it "a difference".....

I'm saying that he could have cut the boat in half amidships, added 2' or whatever the addition to the stern is, and he would also "Immediately notice a difference". He could also add a bulbous bow and notice a difference, or move his seat forward 1' and notice a difference.

Length is significant......stern shape less so. How much less is controversial among NA's and depends on numerous other factors.

TAD wrote;
"Length is significant......stern shape less so. How much less is controversial among NA's and depends on numerous other factors."

Well if you changed a FD freighter by making it a constant section (midshipp) all the way aft to a vertical and flat transom she'd have trouble making it out of the harbor and she's never cross an ocean. Stern shape would matter extremely then.

As you can see I'm in the stern shape camp. One of my favorite boats is the NT32. If I changed the shape in the stern to a FD form and powered it w a 54hp Isuzu would I get a top speed of hull speed? I'm guessing I'd cruise at 7 knots (or a bit less) on 2gph. Reasonable thinking? Then the boat would become really attractive to me. And it would be much better at following seas and probably handle better in port too. And I'll bet there's numerous other advantages too.

If I was NT .. and offered both SD and FD I'd probably sell many more SD NTs as people that could afford a new NT could afford 10gph easy.

Just for giggles give us an example of two boats ... one w "X" amount of stern shape and one w "X" amount of length. Hmmmmmmm. Stern shape for me especially if I'm pay'in for the moorage.

swing batter...swing!!!! Strike three your out......

OK .....
Too much enthusiasm for me.

psneeld said:

What some of our members are showing some of the non-believers or mis-undestanders...

Is that you can buy a planning vessel, run it at displacement speeds, get good not great fuel economy, yet still have those sprint speeds of versatility that the one design types .....
.

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Isn't this considered blasphemy on a trawler forum?

Lindsey Lord, in his book Naval Architecture of the Planing Hull, had a great idea: A boat trailer which attached to your transom (I know not how exactly) which turned your boat into a displacement boat for long trips, and held extra fuel and supplies for that trip, which then could be detatched at your destination for local cruising, finally reattached & fueled up for the return trip. I don't know if one were ever built.

We had a planing hull vessel that gave a good ride while on a plane out on the briny, but got really bad when we had to throttle back because of rough seas. We had a long miserable and wet ride from then on.

People discuss hulls as if there are only two designs. An awful lot of naval architects must have had an easy life just copying those two.

Walk around ins boatyard and wonder at all the differences

Well I've made the transition from deep v planning sm to fd

Full displacement designs with narrow beams really were a product of necessity. There just weren't any powerplants big enough to put a larger boat on plane. If you look at the old double ended Monterrey or Columbia river fishing boats they were all double ended because the designs started out as rowing sail boats. Today fishing boats that have replaced them are deep V shovel nosed boats like the Radon powered by diesel or big block out drives, that get out there fast and return loaded in following seas on full plane. Form follows function. When I was much younger and worked a regular 40hr plus workweek, my 28' Bayliner on trailer allowed me to cruise all over the western US with a family of four. During the last view years of running my own business my 4788 semi-displacement Bayliner at 18knts let me cruise the Delta and coastal cruise the greater Bay Area on long weekends, Running it slow at displacement speeds while more efficient really didn't work in rough conditions. Now that I'm retired, sort of, my full displacement 7knt boat fits my life style. Form follows function. The only hull shape in my mind that crosses over is a Catamaran, They do the last two well, just a little wide for trailering.

bayview said:

Isn't this considered blasphemy on a trawler forum?

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Posting that without a smiley is one of the things holding you up from being accepted into the bad boy club....

Obviously when trawler owners argue their boat isn't really a trawler...then how can ANYTHING be blasphemy here....

Scary,
Yes .... everything's a product of necessity. A design is an organized solution to a problem. No problem no solution and no design.

Yup .. lack of power promoted almost universal dedication to FD and narrow boats. Speed is easy now and not as sought after.

But if you want high degrees of fuel efficiency, seaworthyness and reasonable motion comfort for time spent in open waters you look to full displacement boats. But our culture leans heavily to "do it all" products. Most TF people find the option of going faster than one knot below hull speed a necessity or nearly so. I've been running FD for some time now and never have a need to go faster than 6 knots. Most people have needs that exclude them from the FD club and it's advantages. One must sacrifice some seaworthyness, efficiency and comfort to enjoy the advantages of faster boats.

But Scary the need of the boat owner should dictate design and I think that's the reason FD trawlers aren't in higher demand judging by their prices on the market. The number of people wanting FD boats seems to be matched well w the number available. So they don't meet the average Joe's needs but that dosn't alter the abilities and capabilities of the type.

These are pleasure boats and the bottom line is what people want. That may be a result of their actual needs or just a whimsical vision.

And I must say at this point that by my holding high the FD hull I'm in turn putting down by the nature of conversation the planing and SD boats. Since 95% of TF members have planing and/or SD boats I'm saying degrading things about the membership here just like if "you" had an American car and I had a Japanese car and I was talk'in up Japanese cars as they were better. I'd be telling you I'm smarter than you ect. So I'm not surprised at the hostility here on this thread directed at myself.

The intended purpose of this thread is to draw attention to the fact that the difference in resistance of SD and FD hulls is considerably more than fly stuff and varies tremendously from boat to boat. And I'm not "selling" FD boats as better over all ..... but in specific ways re specific boats they have small or large advantages depending on individual boats.

By the way my favorite boat is a 12 knot SD hull ..... I just can't afford one.

I think like many here have said in this and other threads....QBBL and transom shape can be fly stuff depending...thus like the one correct statement I've read....that it varies hugely from boat to boat.

A poorly designed FD boat isn't going to be anywhere near "way" more efficient than any other boat...of course "depending on more than a few things".

Use of absolutes on this forum seems rampant and goes to show the lack of stepping back and looking at reality way too often.

The computer world must be like the Disney channel for many...well sit back and enjoy I guess...

manyboats said:

I've been running FD for some time now and never have a need to go faster than 6 knots.

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That's because you can't. I suspect that if you had a boat capable of going 12 or 16 or 25 knots, you would go that fast assuming you could afford the fuel.

People invariably say they don't want something when they don't have one, and then if they get one, they start talking about how they don't know they got along without it. I was that way with an automotive GPS, the iPad and backup sensors on a pickup.

Speed is the same thing. We have a boat that is capable of cruising at 30mph, water conditions permitting. It can also meander along at 5 or 6 mph on the same motor. And how fast do we invariably run it? Thirty mph or as fast as the water conditions will allow. We never run it slow even though it's perfectly happy doing so. If 5 or 6 mph was all the faster it would go, we would, of course, do as you do and say we "never have a need to go faster."

I'm not saying that there not nice aspects to cruising slowly. There most definitely are. But it's been my observation that people with boats that can go fast(er) are run that way unless the owners are unwilling to pay the fuel bill.

Later GBs all have pretty big engines in them. GB42s, for example, have a couple of 400+ hp engines in them. This in a boat that used to have two or even just one 120 hp engine in it. Even the later GB36s have one or more often two engines over 200 hp in them. I see these later boats all the time out in the islands, and invariably they're being cruised at speeds up in the teens. Why? Because they can be, and so their owners/operators do.

Six knots is fast enough. What's the hurry?

markpierce said:

Six knots is fast enough. What's the hurry?

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There isn't one if that's all the faster you can go.

High Wire said:

My thoughts when I hear something like this, whether its boats or houses, is sell it and buy/build what you really want.

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That goes for women as well.

manyboats said:

............ if you read the OPs text he said he immediately noticed a difference............

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You wouldn't expect him to say otherwise would you?

How was the difference measured?

Very good post Marin on relativity.

I love to drive my new Jetta .. even fast. It does it like only a new car can and w more power than I ever thought possible w a two liter car. But I also have an 87 Nissan Altima w 276K on it. They spec out very similar in some very important ways .. 4cyl, 1.8 & 2 liter disp, 4 doors both 5 speeds. They both are super sweet but in totally different places. The Jetta on hard acceleration and smooth speed on a smooth road. The Stanza on slow running as in under 50 like I've got no place to go enjoying every shift. Really like them both. One's fast and ones slow.

With the Willard we never even think of going faster than 6 knots. We just go 6 knots until we are there. Or a bit less or more depending on the tide. But I'd rather (most of the time) be going 12 knots. Not re the time it takes to get places but how wonderfully a boat moves at that speed. Nice and clean and free in the water. Dynamic like flying in three axis banking in the turns .... not like bobbing and rocking seemingly unsure of any destination. Rolling and heaving is replaced by gracious sweeps and curves that is beauty in motion. Full disp is more like riding and driving a bus.

But I get along well at 6 knots since I'm retired and can go cruising for hundreds of miles. Remember my copy "A Long Way Home"? We can go the distance at 6 knots and have never used more power than it takes to go 6.4 knots (2500rpm). We drink a lot of tea on the boat though.

manyboats said:

Remember my copy "A Long Way Home"? We can go the distance at 6 knots and have never used more power than it takes to go 6.4 knots (2500rpm). We drink a lot of tea on the boat though.

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Well, I'll leave it at this: You've done more cool boating and seen more fabulous country at 6 knots than I ever will at any speed, including 25 knots.

Marin I haven't done noth'in compared to many here on the forum but we're going back (finally) into the water and when we're ready we'll head for the south sound to do some lazy anchoring.

manyboats said:

Marin I haven't done noth'in compared to many here on the forum but we're going back (finally) into the water and when we're ready we'll head for the south sound to do some lazy anchoring.

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Yeah but.... Your boating has been up and down the Inside Passage and in SE Alaska. In my estimation, one run on the Inside Passage or through SE Alaska is the equivelent of 30 runs on that ditch they have along the east coast. So by my calculations, you're actually quite aways ahead of the game.

Noobe question here. It looks like the cost of operation (Fuel) is the major deciding factor in most people's choice of hull type. How often do you guys find a situation where you wish you had just a little more speed to get out of a bad situation (bad weather and such)? And if you had that capibility would you only use it sparingly or would you (like me) be tempted to push the throttle up just another notch for the fun of it?

Just trying to learn as much as I can before the process moves from the internet looking stage to the shoe leather on the dock stage.

Thanks
Tom

Exactly why I bought a semi planing boat. The dogma among the trawler fans is that engine size determines fuel use while in reality it is the operator that is the biggest determiner of fuel use. At slow speed fuel is a small part of the cost of boating but a limitless cause of debate.