Hull Shapes----Show us your girl's bottom

Woodsong wrote:
Keith- you've got/had some blisters my friend!!
Here is Living Light. I do wish they had carried the keel further aft to better protect the shaft. Someone may know why that was not a good reason???

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*Yea, the PO told me she had blisters, but I gasped when we hauled her during the survey. Negotiated a price reduction, then spent $25,000 to fix them right. Complete hull peel, post cure with the HotVac system, then re-laminate. Nary a bubble since.

The American Tug hull was designed by Lynn Senour originally as a shrimping boat hull.* Needed to be no more than 34ft, beamy for a lot of fish, and fast enough to get out and back quickly to be as productive as possible.* TOMCO picked up the mold and the AT 34 was born.*

Mike wrote:
Bobc -

Ahh,* a Jensen Marine DF 34. Great boat.

When I switched from sail to power that is the boat I wanted, but there are only a few here on the east coast. I ended up with a Jensen Marine DF 38 - called a PassageMaker 40 -* and it is just a slightly larger version of your boat with a tri-cabin arrangement. Exact same hull.

I call it the "little DeFever"* because quite a few of my cruising buddies from Defevercruisers have 41's and they kid me about being "little."* Funny thing though, when we raft up the boats are the same size.

Mr.DeFever calls them all "the 38"

Mike
Palm Coast FL

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So your's is the little DeFever.* Why am I suddenly having feelings of inadequacy?*

Mike, we seem to be on the same page, I just made the switch from sail back to power.* I wanted a boat that I felt would be just as comfortable in rough stuff as the Cape Dory I had, and just as easy to single-hand. * I also went with the <s>baby</s> <s>nano</s> pico Defever because I wanted to fit in 35-36' covered moorage. **

Great Laker, as you probably know, your AT34 was built in La Conner, very close to where I keep my DF34.* Nice boat!* Lot's of room inside for a 34, especially in the stateroom, and they still have a usable cockpit.* The AT's seem to be pretty popular here.* I like them better than the smaller Nordic Tugs which are also made locally.* The boat they bring to the boat shows has plexiglass engine hatches on the bridge.* They're kind of cool.

Davis/Defever trawler hull shape

Well, if Moonstruck is not even semi-planing, then I assume she is regarded as true planing hull.* If so then what does that make me old CHB, because she's not that much different, give or take an odd chine strake or two.....sort of...

*


-- Edited by Peter B on Thursday 1st of September 2011 06:38:08 AM

Peter B wrote:
Well, if Moonstruck is not even semi-planing, then I assume she is regarded as true planing hull.* If so then what does that make me old CHB, because she's not that much different, give or take an odd chine strake or two.....sort of...

*

*

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*Peter B,

I will give my opinion, and wait to see if Eric (Trawler Forum R & D Dept.) throws cold water on it.* First the odd "chine strakes" you mentioned are probably the "lifting strakes" that help get the boat on plane by raising the bow.* They do not carry to the transom.

Your hull, to me, looks like a classic semi-displacement bottom.* Hard chines and flat sections aft.* The wide flat stern should help reduce "squatting".* Put enough power in that hull and she should plane.* However, at lower speeds she is fairly efficient, but not as efficient as a full displacement hull shape.* Grand Banks seams to have started the trend of semi-displacement hulls on trawler style yachts.* Most other Oriental constructors followed suit.* Defevers, Willards, and those based on lobster style hulls mostly have rounded or soft chines.* Even with soft chines the lobster style hulll is semi-displacement as they will plane with enough power.* Most of this style are run at above theoretical hull speed.

Now to how your hull differs from Moonstruck's, your hull has a deaper forefoot that tapers up toward the stern.* You keel starts a the forefoot and ends at the nearly flat section aft.* As Capt. Mike said, Moonstrucks keel runs nearly parallel to the waterline all the way to the stern.* Instead of a "keel" like yours the bottom carries a 16 degree dead rise to the stern.* This and a difference of approx. 880 hp gives Moonstruck a 27 knot cruise speed.


-- Edited by Moonstruck on Thursday 1st of September 2011 07:56:23 AM

Ahhh. Sounds like I've been summoned for cold water dowsing. Photographs are tricky to use to analyze hull shapes. Better than nothing to be sure but can lead one astray. Quarter beam buttock lines tell how well a boat will or won't plane. Remember?... this is a imaginary line fore and aft and parallel to the keel from amidships (roughly) to the transom. But buttock lines can be drawn or imagined that are very close to the keel or very close to the chine. Pure planing boats like yours Don have aft buttock lines all parallel and strait running aft to the transom. You will see that Peters hull has a buttock line at the chine that is as a planing hull but his buttock line at or close to the keel is comparatively steep. That is it runs aft from amidships rising a great deal by the time it gets to the transom. So the inboard buttock lines are not like a planing hull at all. They are rather like a displacement hull. A pure FULL displacement hull will have ALL aft buttock lines running aft all the way to the water surface with NO transom below the water line. Keith's Krogen comes very close. Close enough to be called a full disp hull but actually is not 100% *...more like 97%. But there is an advantage to Keith's 97 in that he can run at hull speed or slightly over better than a 100% hull like a Willard Vega. Another important part of the picture here is the straightness of the buttock lines. Keith's BLs are not too far from strait. I call this type of hull a banana hull whereas the bow is the stem end of the banana and the other end is the stern. My Albin 25 was such a boat. My Willard has anything BUT straight BLs. Very convex. Getting back to Peter's CHB it has a pronounced rise to it's inner BL and a strait BL at the chine. That makes the Quarter beam BL half way between and it has considerable rise. The QBBL is usually giv'in for analysis for this reason. Giv'en enough power (lots) it will go considerably above hull speed but never plane in the way Don's boat does. Peter's CHB is closer to a FD hull than a IG or a GB because the inner buttock lines are steeper and as a result the QBBL is also steeper. So the planing/disp question is answered mostly by the QBBL but with as much transom below the WL as the GB, IG and CHB have they will never come close to the performance of a full disp hull below hull speed. Remember most FD hulls don't cruise at hull speed * ....that is their top speed unless they are over powered ...as most are. So you see Don semi disp hulls really don't have flat runs aft but "flattish" and a bit warped so the bottom is tilted upwards towards the stern. They are a bit like a motorsailer. Not good planing boats and not good disp boats. BUT the vast majority of trawler men are a bit like Marin. He loaths 7 knots and is willing to burn much more fuel to go 8. These semi disp boats are perfect for all such skippers.*

More on hulls.

Look at Jason's hull pics. See the first pic shows his BL at the keel as very steep and convex whereas his chine is almost flat and parallel to the WL. I think Peter's hull is much closer to what the pics show clearly on Jason's hull. Perhaps CHB made more than one hull shape. That's not uncommon. And consider that if the bottom buttock lines curved up to (or above) the WL so there was no submerged transom then that hull would be a full disp hull. Note also that Larry's American Tug shows almost a flat and straight aft run such that his buttock line even right next to the keel is almost straight and flat (very very low angle). I even think I see a bit of hook in the bottom aft of the keel. Speaking of keels if one was to remove it I think anyone would call the AT a planing hull. The keel really has nothing to do w the planing/disp hull question except in a planing mode few NAs would go w that keel because of all the drag. Marin's GB has a little straighter run aft at the keel (that's really where most of these boats differ) that would tend to put it in more of a planing configuration but his boat has a bit less submerged transom which should give it better disp dynamics but give it enough power the boat should squat and assume a more planing shape. That's why (I think) the GBs do fairly well w more power and speed but still perform as well as their brothers at disp speeds.

Don. The lifting strakes have three jobs. In order of importance. They reduce wetted surface (very important in a planing hull). They provide a bit of lift at speed and knock the spray down close to the water where it has a long way to go to get over the rail. No the CHB won't really plane. Actually the lobster hulls are quite efficient at lower planing speeds but cause too much drag and wetted surface at higher planing speeds. I'd say most lobster boats are not semi-displacement but planing hulls intended to be run at slower planing speeds. I think a lobster boat otherwise like yours (disp ect) would be more efficient than your Eastbay at 18 or 20 knots and less efficient at 27. Just a guess. Most people think (incorrectly) that a big trawler like keel makes a hull semi-disp. Not so in my opinion. Has all to do w the hull shape without the keel. You're last paragraph is basically spot on. So I don't think I threw any water on your post. I have offered what I think I know to sharpen up the slightly fuzzy and incomplete parts of your basically good comment. I'm open to facts or opinions that could change any of the above.

nomadwilly wrote:
BUT the vast majority of trawler men are a bit like Marin. He loaths 7 knots and is willing t more fuel to go 8. These semi disp boats are perfect for all such skippers.*

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Actually I loath eight knots every bit as much as seven.* If fuel prices were not the factor they are we'd have a boat that did 25 knots.* And while it's the accepted term, semi-displacement is not correct.* That's like saying semi-dead.* The term used by the naval architects I've read articles by--- like Tom Fexas--- is semi-planing.

* Semi-displacement is a marketing term coined to help create the image of boats like CHBs, IGs, etc as being close to the "rugged working trawlers" that the manufacturers think their buyers want to pretend they have.* American Marine, creator of the Grand Banks line of boats, never used the term "semi-displacement" to my knowledge, nor did they ever refer to their Grand Banks line as "trawlers."* Their Grand Banks line was always refered to as "diesel cruisers."

Marin,

*The GB line I'm sure will be void of such sins as you say. I am inclined to say semi means "partly" and to apply that word to the planing issue that would mean partly disp or partly planing. Your boat is (as it sits and as you use it) beyond a doubt partly planing as you cruise above hull speed and clearly much more than partly disp. The DeFever 34 is more like a displacement hull than the GB but clearly is not and would thus be partly planing also. When you have a boat that runs well at 18 knots but has a hull that has some part of it's shape that is partly displacement and for that reason that hull could'nt run w Don's planing hull at 30 knots *.....that hull would be "partly" disp or semi-disp. But the word semi-disp brings up an image more like Peter's CHB. Anyway most all trawlers are not FD or planing so are of the huge grey area in between. So for simplicity's sake I'll accept semi-planing or semi disp as meaning the same and inclusive of 95% of all the boats on this forum. And GB was right on w "Diesel Cruisers"!!

The builder describes the Coot as a "pocket cruiser."* Naval architect George Buehler (he modified plans of another designer to create the Coot) describes it as a "trawler yacht ... designed and built along real workboat ideas."* To me, the Coot's bottom lines are very similar to those of many*working*steel trawler fishing vessels.* Under full power with an 80-HP engine, she doesn't exceed hull speed (less than 8 knots).




-- Edited by markpierce on Thursday 1st of September 2011 01:12:43 PM

Still haven't caught up here.

Dude, * Never hit a net but your'e right Willy may go right over. Some of our gillnets here in SE are VERY long. I've never come really close to hitting one though. I see the boat at the end of the net and can usually see the white floats over the stern. When I see that I keep looking till I see the big orange ball at the other end of the net. Frequently takes quite awhile.

I see no cup in the Manatee's hull.

Mike, I've always wanted to see your boat's hull in photographs.

Woodsong, * If your keel was longer you have increased directional stability. Boat would slide sideways less readily in cross winds but when you wanted to swing your stern over w a blast of thrust at full rudder you'd get less swing. The DF 34 is a heavier deeper boat than the Monk. There is less submerged transom in the water and a deeper forefoot. The DF will ride smoother, go straighter generally and in quartering seas and it's general stability will depend more on it's beam than the hard chine. The hard chine soft chine is less of an issue than most folks seem to think. At rest the boat will have a bit easier motion in a beam sea w the soft chine like when fishing. With the same power and enough to exceed hull speed the Monk will be faster because of her lighter displacement and lower wetted surface. When the wind's play'in w ya in the harbor the DF will be noticeably friendlier. The DF would be more efficient at 7 knots Probably the Monk at 9 knots. Does that help?

Mark I think you have the near perfect trawler hull. You buttock line at the chine says it all. Perfect speed should be almost a knot under hull speed. "Under full power with an 80-HP engine, she doesn't exceed hull speed (less than 8 knots)." Sounds like she's perfectly powered for a FD hull. If I had your boat I'd prolly cruise at 7.2 knots unless there was an ultra smooth spot right below or above that point. If I had my choice of all the boats on this forum I'd take yours. I may be slightly influenced by the new part.

nomadwilly wrote:
So for simplicity's sake I'll accept semi-planing or semi disp as meaning the same and inclusive of 95% of all the boats on this forum.

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Eric--- It's a nit pick, no question.* But Fexas' explanation is that there are several kinds of planing.* You can be on the full plane, a partial plane, a mushing plane (what in floatplanes is termed "on the hump," and so forth.* But displacement either is or isn't.* Your hull is either capable of going at or below displacement speeds only, hence a displacement hull, or it's not.* If it's going faster than displacement speed, it is achieving at least a small degree of planing, hence "semi-planing."

nomadwilly wrote:
*If I had your boat I'd prolly cruise at 7.2 knots unless there was an ultra smooth spot right below or above that point.

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*Need about 92% power (2200 RPM) for that speed.* At 75% (1800 RPM) the speed is about 6.5 knots.* I usually don't like to go over 2000 RPM, giving me*about 6.8 knots: that's one knot less than maximum speed! *I'm still in the breaking-in period for the engine.* Between 1600 and 2000 RPM the engine and drive train sound "sweet" (happy at work without strain)*to me.

*


-- Edited by markpierce on Thursday 1st of September 2011 03:10:10 PM

Not sure what you call this bottom, but when we found her, she could now get out of her own way. When a new set of props, clean bottom and alittle TLC to the engines she is runing fine now. Heading out for a 25mile run tomorrow night up into the Delta for the long weekend

Afew more shots ,

Mark,

I think you've got rpm and power/load all mixed up. 92% power is where you burn 92% as much fuel as your engine will burn at WOT and I think that's at 2500rpm when your'e propped right. 2200rpm is probably more like 60 or 65hp. At 1800rpm your'e 700 down from max. My Mitsubishi at 700 down is producing about exactly half power and your JD should be at about 60% power (maybe under 50hp) so 55hp looks more likely. I don't like to go over 500rpm down either (2500 for me) but the engines should be happy 2 to 300 down. My max is 7 knots at 3000rpm and the highest cruise (2500) gets me 6.4 knots. 2300 gets 6.15 and that's my usual cruise. Run your Flo-scan numbers Mark and tell me what you get.

*Dswizzler,

Can't really see your hull shape * ....too dark. Nice big boat though.

Marin,

Your NA is very correct but you are not. Overpowered FD hulls (most are overpowered) can and will go over hull speed. My Willard will go 8 knots w 80hp. That's what one of the guys on Willard Boat Owners group on yahoo says. He has an 80hp 4cyl Cummins and I've seen a picture of his boat underway and he's throwing the wake if a 50 footer in that picture. So FD hulls can (w great effort) go over hull speed but hull speed should be their top speed and cruise close to a knot less. And there are full disp hulls that aren't quite 100% "full" disp like Keith's Krogen and Marks Coot. Both have a little submerged transom but I consider them both for all practical purposes full displacement. And my Willard is not semi-disp. Carl's boat is also 100% FD but given enough power it too would exceed hull speed. This is of course is conversational correctness and not dock talk.

Eric--- Yes, I realize that if you put enough power in something it can be driven to higher speeds that it would normally ever go.* Witness some of the naval ships with massive amounts of power that, when applied, can drive the ship at high speeds but the stern is actually lower than the level of the surrounding ocean.* Staggeringly inefficient but efficiency is not the priime objective with these ships.

When I said "at or below hull speeds only," I meant under normal operation.

Mike-- Very helpful "lesson" for people like me with a minimal to nonexistant knowledge of how hulls really work. One question I have is what does S/L ratio mean? I've looked superficially on the internet and find plenty of references to the S/L ratio but nothing that says what the "S" and the "L" stand for. Is it speed to length? Or what?*

And how does the term "S/L ratio" apply to the characteristics of a hull?* In other words, what physical*characteristics of a higher S/L ratio*determine that a hull can be driven faster?* I can see the logic of it in your drawings but I'm curious about the actual reasons.

Thanks much,

nomadwilly wrote:
Mark,

I think you've got rpm and power/load all mixed up.

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*You're right.* The governor limits RPMs to 2400.* The Flowscan isn't yet calibrated: it reads high.* It shows about 5 GPH at 2400 RPM and 2.5 at 2000.

Thank you Mike. Wonderful drawings. Wish I could do that and if I could I wouda. Pics can explain some things much better than words. I'm known for drawing much on paper napkins in restaurants. It's good that you used the three examples and perfect examples. I've been trying to get this quarter beam line thing across for several years.*

Marin the SL ratio = Speed/Length Ratio. S/L ratio = speed over the water line length. Hull speed is where the wave of the boat (crest to crest) is the same length as the boat. So w my Willy w a WLL of 27.5' the square root of that WLL is 5.243. Hull speed is 1.34 X the square root of the WLL so we have 1.34 X 5.243 = 7 knots. A S/L ratio of less than 1.34 is under hull speed for any vessel as these bow generated waves travel at a fixed speed. They differ in length. An aircraft Carrier has a hull speed of about 50 knots. That's because the wave will be about 1000' long. Obviously length is golden. Visualize your boat going along making a wave. All things that move in the water make waves that travel away like when you throw something in the water and the little waves travel outward one after another ....the first 2nd and on and on until the waves are so weak they can't be seen. As you proceed at hull speed the bow wave is the instigator and the 1st following wave will be right at your stern and if your WLL is 50' the 2nd following wave will be 50' behind the stern. And if it's calm you can see numerous following waves astern all traveling at the same speed as you. If you speed up a bit the waves will still be going at the same speed because of the chacteristics of waves but they will be longer. Aboard a fast OB the 1st following wave may be several boat lengths behind. On a displacement vessel it's most efficient to run a tad bit slower than the 1st following wave so it pushes you ahead slightly. Some large and slow vessels frequently operate on the 2nd or third wave back to establish a returning wave under the stern. With our short boats the 1st returning following wave (sea) is the strongest so we can take maximum advantage of it. With a QBBL that has an angle approximating the shape of the following wave (perhaps 5 or 7 degrees) the wave will do a better job of pushing us along. I don't know how square sterns and flat runs aft affect the waves ability to help us along. I suspect a shape like the wave would be best however a big wide stern would increase lift aft and possibly help even more. Other types of explanations will be more clear than mine to many just because they are worded differently. I respond to physics more than math and numbers so if someone could expand this on a numerical plane it would be very helpful to some or many.

Eric--- Great explanation, thank you, and great photo. It really illustrates what you're talking about. You're hired.

Nice drawings and explanation, Mike.* I used the picture below in another post.** From the angle of the chines, it looks like the GB32 in the yard next to me has the potential for going faster -- given enough power.
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Absolutely wonderful explanation, drawings, and follow up. *Thanks guys. *Here's an example of a full displacement Krogen doing what Eric is talking about (riding your own 1st wave).

*

Excellent explanations Eric and Mike.* Thanks!

Mike--- So, is your hull, which is very handsome in its lines I think, considered a displacement hull or semi-planing? It appears to me that the afterbody of your hull is more rounded than that of our GB, for example, which is downright flat*albeit slightly Veed.


-- Edited by Marin on Friday 2nd of September 2011 08:22:07 PM

The Krogan in healhusler's picture shows a boat sitting proudly between the crests of her own wave. A bit over driven as one can see her following wave is aft of her stern. Probably maxed out at about 9.5 knots. Notice how the inertia of the bow wave pulls the water away from the hull amidships exposing a bit of her normally submerged hull sides. I love these Krogen's. Now that I've seen (thanks Mike) her whole hull I lust after these DF 34s too. But in a race I'd say the GB has the advantage. I'm fuzzy about at what speed the GB would start to have less drag though* ...prolly at about 9 or10 knots. Mike, your avitar shows a boat w almost no submerged transom. Is that the DF 34? And is the buttock line at the keel quite convex?

nomadwilly wrote:
The Krogan in healhusler's picture shows a boat sitting proudly between the crests of her own wave. A bit over driven as one can see her following wave is aft of her stern. Probably maxed out at about 9.5 knots. Notice how the inertia of the bow wave pulls the water away from the hull amidships exposing a bit of her normally submerged hull sides.

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*We're about a knot below hull speed here, avoiding trying*the climb out of the "valley."



*


-- Edited by markpierce on Saturday 3rd of September 2011 07:26:43 PM