Feedback on my first hull design

Hi Vince,

I like your design concepts, you have a good eye. Jets require an uninterupted flow. Deep Vee hulls normally have two jets, one on eithor side of the Vee. Your boat is small and will only have one engine and one jet. You can reduce the Vee as the hull transitions aft and have a small flat section for the jet drive on the centerline. Just be aware that you need a transition for water to happily go into the jet.

It has already been mentioned, but the longitudinal center of buoyancy of the hull needs to coincide witht the longitudinal center of gravity. You have an evolving design concept. Get the size of the engine and fit it into your design along with your general arrangements. Odds are you will tweak your hull a bit before you are happy. After that you nee to make a weight take off to locate the center of gravity. I can send you a sample spreadsheet if you don't have one.

I designed a 28-ft boat with a rounded transom that looks similar to your original concept. In order to keep the boat from being "tippy" I kept the waterline beam constant from midships aft to the transom. Making a small model is a great idea.

I don't think I have offered you any new advise or insight, I hope it is helpful.

Best Regards,

Roger

 

First "Design"

Hi Vince,

Perhaps you could take a moment and visit these two sites where you can go over, in detail, the form and behavior of a moderate vee planing hull as you are desiring for your interpretation.

http://www.fishyfish.com/tolmanskiff.html

http://www.skiffkits.com/

These two sites have tons of info and build sagas for several boats of this type, The Tolman Skiff. The Tolman was developed to handle the constantly changing sea states found off the coast of Alaska, from smooth water to heavy chop and swells. They plane easily, are very stable in a wide variety of conditions and can be built lightly to yield excellent fuel mileage.

I'm not saying your hull should mimic the Tolman, but the form is a standard and perhaps the place you want to be is not far from this form.

Chris

 

As a big fan of both diesel and jet I am all in favour of this combination. Engine placement will be fairly flexible as you can just stick in a shaft of the needed length to move the engine forward. But bear in mind that diesels are heavy (although not necessarily so, look at something like a Yanmar 6LP series if you can find one, they're lighter than the eqivalent Merc gas motor) You will probably be looking at about 900 to 1200 pounds of motor. Jets are also heavy- the unit itself is heavier than a sterndrive, and it's also full of very heavy water. (Figure maybe another 250-350 pounds or so for the pump.) So it would do to have some idea of these weights before you finalize your hull design; most manufacturers' web sites have them listed. Roger's advice about getting a smooth water flow to the jet is good; I'd take his words to heart and try to get a nice place for the inlet if this is the route you're going.

 

I don't know what is meant here by 'good water flow to the intake' and I doubt that anyone besides the person saying it does either. I have seen jet drive boats which cut a flat off the v wide enough for the manufacturer's intake to be mounted on and then in front of that just taper the sides of this flat to the centerline in about a foot and a half or two feet and it has worked just fine. You don't need to do anything fancier than that unless you are planning to go very very fast and very very fast means a over a hundred knots. You can get more authoritative advice from the manufacturer or dealer than you will by trying to conjure up what ' good flow to the intake' means.
I would like to comment on one other thing here also. What constitutes a design? A boat builder who just sets up a keel and centerline members and puts up some molds and wraps battens and starts making frames and planks etc, etc, and ends up with a boat; are we going to tell him his boat is not a design? He never had a drawing even, much less pages of specifications and calcultations. So are we going to tell Vince, who at least had a drawing that his drawing is not a design? It most certainly is a design. So please get off it. Don't go telling someone they have not got a design when they most certainly do. I knew a very fine boat builder who ALWAYS called the plans he got from designers 'the funny papers'. And I don't think he was necessarily overdoing it, at least not all the time anyway. And in case you think I'm trying to discredit designers, well I do design work and I can take all this I am saying in stride. And even smile afterward. I think some of us who know quite a bit get a little pompous at times. We need to remember that even the boat designer that knows the most doesn't know everything.
I'm off my high horse now.
I appreciate all of you.
Gilbert

 

Gilbert,

My son used to draw boats all the time when he was 10 years old. These were designs?

When I was young, I read Buck Rogers comics about rocket ships to other planets. These were designs?

Chester Gould had Dick Tracy wearing a two way wrist tv/radio and flying about on the moon in a personal hovercraft. These were designs also?

I spent six years in college learning to be a design engineer. Now you tell me that this time and effort was wasted and I could have just started out by laying out some vacuum tube and transistor symbols on paper and it would have been just fine.

I don't know about pomposity, but I do recognize BS when I see it. In the morning I may wish that I deleted this before posting it but right now, here it goes.

 

Holy Crap ....

Hello...

What have you miscreants done to Tom....

Tom - come on over - I just bought a case of 8 hour Tylenol Arthritis 500 mg tablets - and I got a new power planer...

What could happen...

SH.

 

Hey Sean,

Depending on what we use to wash down those pills, we could lop of a few digits.

 

re: jets

Could a custom jet intake be fabricated to fit on a vee hull? I dont see any reason why not. Otherwise, Maybe two smaller engines and two jet drives could be a good option? Maybe two 125's or 150ies even. Or just split the power of one larger engine into two jet drives.

I like jets for a lot of reasons. The flexibility in engine mount is one of the main reasons. and I've always just liked jets.
I dont like stern drive, I think the COB would be way too far aft.

As for design, I think people are getting confused between boat design and hull design. In this thread I posted the approximate lines for the design of a hull. I gave the specs of this hull and asked for comments. I am looking for fundamental flaws in the design I'm portraying.

In similar aircraft forums for instance, If somebody posts the approximate lines for wings, and says "comment on my wing design" I can see quite quickly, just from looking at the profiles, if the wing meets this persons requirements or not. Say someone is designing a jet fighter, with a fat, asymetrical aerofoil, thats a pretty obvious flaw in the design (notice I used the word design) and I would comment on this.

Lets clear this up. I beleive anything which represents a final product in some way, shape or form, is to be considered a design.

de·sign (dĭ-zīn')

v., -signed, -sign·ing, -signs.

v.tr.

To conceive or fashion in the mind; invent: design a good excuse for not attending the conference.
To formulate a plan for; devise: designed a marketing strategy for the new product.
To plan out in systematic, usually graphic form: design a building; design a computer program.
To create or contrive for a particular purpose or effect: a game designed to appeal to all ages.
To have as a goal or purpose; intend.
To create or execute in an artistic or highly skilled manner.
v.intr.
To make or execute plans.
To have a goal or purpose in mind.
To create designs.
n.

A drawing or sketch.
A graphic representation, especially a detailed plan for construction or manufacture.
The purposeful or inventive arrangement of parts or details: the aerodynamic design of an automobile; furniture of simple but elegant design.
The art or practice of designing or making designs.
Something designed, especially a decorative or an artistic work.
An ornamental pattern. See synonyms at figure.
A basic scheme or pattern that affects and controls function or development: the overall design of an epic poem.
A plan; a project. See synonyms at plan.

A reasoned purpose; an intent: It was her design to set up practice on her own as soon as she was qualified.
Deliberate intention: He became a photographer more by accident than by design.
A secretive plot or scheme. Often used in the plural: He has designs on my job.
[Middle English designen, from Latin dēsignāre, to designate. See designate.]

Hell yes, why not?

I'm sure a lot of thought went into these designs. The person creating them would have tried to think of something which might, possibly, be able to fly to other planets. plausible or not, they were still designs.

Now, I would like to use this thread to receive feedback on the approximate lines of a hull I would like to build. Can we stick to that?

 

Vince, that is exactly what was going on before you started whining about the feedback.

 

Thanks for your productive feedback.

 

terminology can be specific

Vince just as you noted, there are various specific uses of the word "design" many of which are synonymous or at least related in concept. What, earlier in the thread, you were corrected on is the specific use in the marine field, the term is most often used to express the entirety of a finished product which includes more, by far, than you've resolved in the preliminary sketches. In point of fact your concept has been modified by what you've learned from many remarks posted here. Those in the business my say you were in the "design cycle" implying you were working toward a "design". The usage of the word 'design' in marine planning parlance implies a (completed) suite of graphically expressed and mathmatically reasoned documents the result of which can be assembled by others with sufficient skill and knowledge into the vessel expressed by the 'design'. Its another specific use, like the many instances of synonyms you listed, thought of as a PACKAGE of information, not often used without qualifier(s) until that stage. Qualifiers often include,
concept sketches (of a design), design statement (for a design),
preliminary sketches (of a design), preliminary drawings (of a design), lines drawings (for the design), construction drawings and so forth.

We can debate the usage all day...but that is how I understood it was used in "that" post. It (that usage) seemed in complete context to my experience, most likely used in this way by someone trying to 'tell' you something, hoping you have 'ears to hear', hope so.

A "jet" (poor term; they pumps) is most often NOT installed on center line in a hull with deadrise much above 10 or 12 degrees because of the 'jet plate', to use the manufacturers' term for the flat plane of the mechanical attachment to the hull of the pump inlet, most often including the inlet grill.

If you have too great a deadrise when you begin to transition to the inlet grill you can cavitate the pump. If you have a 250 or 300 hp engine under load from an axially flow pump that suddenly unloads- the pump and engine can over speed, less likely with a governor controlled diesel but not best practice. When you cavitate a pump you lose steering and headway, and the boat may need more than a few seconds to regain speed. A pump gives thrust in direct proportion to the water leaving the impellers, so if you unload one you have to spend a few minutes getting your 'head-of-steam' back, they don't accelerate quite like prop driven boats.

To explore this transition you can create a bottom prism of the proposed deadrise and begin intersecting the V at the width of the 'jet plate' inlet. Say 24" or 28" wide - where will this plane intersect the V of the bottom? That distance above the keel line is the minimum transition for the water flow. Let's say a 5" (or so?) ht. change is required to get a 2' wide 'jet plate' into your V bottom; now if you have a plate that is 3' fore and aft then at some point (3' plus) forward you have to begin to taper the 'jet plate's' plane into the V of the prism of the hull. The deeper the V the greater the distance of the transition without creating an abrupt step in the bottom shape.

What it means to give "good water flow" is to make sure this transition is fair, not a step edge, is smooth and not abrupt so the water which is essentially being compressed by the hull's passage doesn't release at the transition like it does at the trailing edge of the transom. That is when you'd cavitate the pump.

Also, "jet" boats with deep V's may need to have attention paid (during the many helical routes around the "design cycle"- not the same as "beating around the bush" but similar) to the effects of turning the boat. Even outdrives which are much deeper in the water than a jet pump inside the hull can caviate, or suck air, in a turn; on some boats. If the hull doesn't roll 'down' into the inside of a turn somewhat, there is a tendancy of the chine to keel section of the hull to lift the outside bottom plane near too, or above the surface of the water and this would certainly vent the pump suction. This is another reason some jet hulls have less V -to avoid cavitation in a turn at speed because the deeper V hull can create more relative vertical movement of the keel plane in relation to the chines and the surface of the water.

For these reasons several welded aluminum "jet" boats of my experience had the reverse chine flat- straight and parallel to the keel at the inner chine in plan view, while the outer chine-to-topsides joint tapered aft. This very slight taper (1.25" for every 7' of chine aft the mastersection) made a very large difference in roll attitude (10 degrees without 18 with) in turning and was considered important to help keep the pump suction 'wet' on these boats as they manuevered up rivers at speed.

Hope to have gotten back 'on point' with useful experience to incorporate in your project's ongoing design cycle. I'd like to suggest if you have not already found them, Van Dam (www.vandamwoodcraft.com/ ) boats as the have designed and built some extraordinarily comely "jet" powered boats with lots of tumblehome.

Cheers,
kmorin

 

Thanks for your contribution kmorin. Very much appreciated.

I would gladly adopt the term "design cycle" it really doesn't matter.

In terms of "jet drive" or "pumps" or whatever you want to call it, I suspect with a 20degree vee, these would not be a good option. I dont like the idea of stern drive simply because the weight is too far back. What would be the recomendation?

In terms of the hull "design cycle" - I haven't read any real critacism of the new "shape" - does this mean I am on the right track.

Still two questions - What do people think of the strakes? good idea or no?
What is the main purpose of a non-trip chine and do you think I should incorporate it into my "hull shape"

Thanks.

 

I'm pretty new too, but the V looks good and chines are fine. Maybe fiberglass would be nice, but wood would look very pretty. Looks beamy, right? Maybe this could work as a small trawler hull, perhaps.

 

Personally, Vince, I'd go with a mid ( or mid/aft) mounted engine and a drive out to a prop and rudder. Check out the Volvo Penta site for the shaft angles on they're inboard diesels.

Strakes are advantageous, but without doing CFD on them it's hard to say where they should be, or what geometry. Just have a look at other boats with the same sort of dead-rise and copy them. Deadrise can be measured with a protractor, a line and a fishing weight. Then just work out where they are as a fraction of the chine width.

The non-trip chine I'm not so sure about, personally, I think it's a fashion thing. There is certainly advantage in flattening the chine forward to deflect spray, but aft I'd just use a radiused corner on the chine. You certainly don't want an absoulutely sharp edge, but a 1" radius would be adequate I suspect.

Good Luck,

Tim B.

 

hull shape

Vince, it would be typical for a designer to sketch and calculate then sketch again incoporating the improvements they've learned were needed when they did their displacement, waterplane and CG, CB, LCB, VCB and stability calcs for the frist sketches. Then back to the drawing board for another go and recalc, and so on.

When you use your waterplane calcs to find out what it takes to immerse an inch of hull and combine it with the current center of bouyancy (CB, or C sub-b) you might choose then to locate the engine more fore or aft. You may also decide to add a bit of beam aft to carry the engine without drawing the other few inches implied by any location for the engine wt. This is explaned as a trial-result-&-correction cycle, and constitutes marine design, hence the name ****gn-cycle.

Back to your hull, stub shafts made like automotive torque tubes, can be incorportated with stern drives so the engine can be located foreward rather than exclusively close coupled to the transom housing and outdrive input shaft. So an engine with outdrive doesn't absolutely have to be hard against the transom. This means you could design with another drive and still attain balance.

Most planing boats designed and built today in GRP and aluminum, have a reverse chine used for several purposes. It deflects spray forward, and helps define the running waterline in plan view aft. Tank towing models has revealed the optimum aft angle is about 5 deg down for best lift combined with lowest drag release, and these tests seem to indicate that low drag release is attained with an almost sharp edge for speeds 15 to 45 below that rounder shapes are more effective and above things get honed sharp.

Longitudinal external strakes or spray rails help planing hulls in several capacities but at slow speeds are more drag than gain. The lower surface is most often horizontal to 5 deg down, and if the deadrise changes (warped bottom hulls) the 'lift' strake or spray rail will be 'molded' to compensate for the varying deadrise.

They also help to reduce wetted area by deflecting the wake wash at higher speed, and depending on the horsepower to wt ratio and therefore the top speed, well placed strakes can give a reduction in beam of the running waterline reducing the aspect ratio of this shape and resulting in a few more mph top end. They are structural contributions on metal boats and are even used as a welded in heat exchanger where in the coolant is circulated inside the volume of the hollow strakes outside the hull plate.

They also add to lateral plane or a 'virtual' keel plane. If you had 2 or 3, 1-1/2" or 2" high vertical surfaces on each side of the hull that amounts to almost 6" of keel bar or plate when turning. This has the tendancy to keep the stern of boats, with less deadrise, from skidding or slewing in a turn at speed. On the other hand there is no actual keel hanging down from the center of the hull increasing draft and grounding the hull in shallow water, so the outer or vertical surface of 'lift strakes' or spray rails also act to steady the helm as you manuever.

Placement is the subject of more calculations as these horizontal surfaces lift (vector the hull upward) as a function (not linear) of the velocity of the boat over water. Since water gets 'harder' the faster you strike it (Reynolds number goes up) the lift increases as you accelerate, but since its not one mile an hour of speed gives 1 more lb of lift per square foot of surface, again, you need to go around and around making approximations, recalculating and refining your 'design'. There is no flat answer to where to put lift strakes unless all other details like hp/wt, waterplane characteristics, Cb, CG, are well known and solidified.

The reasons for lift strakes are generally positive the placement and proportional beam (relative to bottom BOA, of max chine width) of the resulting surfaces are not hard and fast but the result of iterations of the design cycle.

Next, lift strakes can be run as functions of the (at rest) waterline or more aligned to butt lines. Each has adherants, it seems to me more aligned by the material of the hull than dramatic differences in results.

My understanding of non-trip chines is to avoid rolling the hull over side too- when it skids in a turn at high enough speeds combined with enough slew or side drift to built up water along the topsides above the chine. If I understand this design feature I'd expect to see it on boats with very small deadrise combined with little or no liftstrake vertical surface (the virtual keel above) AND those boats traveling above 30 mph (maybe this should be above 40) when turning. I think most V bottom boats will heel into a turn, even cambered (convex chine-to-chine sections in body plan) bottoms will roll into the turn so their chine acts as the keel during the change in direction. So I don't see the advantage of non-trip chines unless you'll be building a very shallow, highspeed (above 40) hull with a CG, 2' or higher above the running waterline. Hydroplanes, sleds, airboats all have used these chine arrangements to reduce tripping but I don't recall seeing them on any V bottom boats.

I did see them on a boat from New Zealand, I believe it was called the "overshot jet" (I've most likely spelled this incorrectly and may not recall that design well). In that case th e boat was very shallow, almost flat chine to chine, carried lots of people up high (very high CG) and the pump gave good speeds (likely low 50's) which the operator used to thrill the passengers by turning extremely short radiused turns in a box canyon amid rapids. In this boat slewing the stern about the bow in an almost single length turn was a desired feature of the boat's performance, and the water that would have built up along the topsides as the boat spun about it's mastersection might have allowed the boat's high CG to roll it over this wave. Instead the designers had included a plate that was about 45 deg to vertical along the after chine so the hull would slip water under it as it slewed around in a circle.

I don't think a runabout would benefit from this chine arrangement unless you anticipate the types of speeds, turning at those speeds and bottom shapes (shallow) that typically characterizes their use.

The reason to seriously consider logging others' bottom designs as a reference is the somewhat tedious and repetitive math iterations needed to rationally place the lift strakes if you choose to use them. Maybe you can define some rules of thumb from others, but the time to draw and calculate, then redraw and recalculate is the same for any new design. As the lift formula has to have values 'assigned' so perhaps a spread sheet approach will serve you best, of course you might just take a look at what others have done and be guided by that?

Cheers,
kmorin