Feedback on my first hull design

ok I'm no expert but here's some observations. I've designed and built few smaller boats so take that into consideration.

Have one panel change from the front bow flare to the aft tumblehome can be done but when you're assembling you will need many forms/stations to maintain the proper shape and have it look the same on both sides.

The small chine you have looks to have some significant bend to it. Thinner wood can take that bend but anything over 1/4" (6 mm) and you'll have to start thinking about steaming or something to get that shape.

The tumblehome in the back looks pretty extreme. More so than the run-abouts I've seen. along with the extreme transom rake angle toward the boat...you could get rocked and rolled pretty good by following seas.

But you're on the right track. Looks similar to some lines I played with. Best thing to do is to keep working with it. Maybe even start fresh on another file to see if any fresh ideas pop in your head. For me, starting over often brought out things I wouldn't have seen just revising the current file.

Once you have the design you think you like, do a 8' scale model to see how things come together. Just stitch it up...no need for fiberglass. That will reveal a lot of little details you didn't think of.

But as a first attempt, it looks real good!

 

Hi Vince,
If you've finalized the shape you want I would call it a design. Or maybe even if you haven't finalized it.
Generally your ideas look pretty good. I would like to make a few comments though.
What you are attempting to do is not real easy to pull off. But I am not trying to discourage you. I would just encourage you to do a lot of tinkering with the drawings until you think it's as good as you can make it.
While a radio controled model would give you a lot of great information, they are quite expensive. So don't build one if you are on a tight budget until you have gotten things as good as you think you can make them. And then you might be as well off to put the money in the boat.
Become an avid student of all the runabout designs you can find. If you can find out what their good points are and their bad points from people that have used them you will be in a lot better space to design one than asking folks on this forum who may or may not have designed or used one.
Now for the few comments I have on your drawings. In the first drawing you posted, if you are going to build it of wood, the dramatic upturn of the chine at the bow will create problems for construction if you are going to use a chine batten. Let it meet the stem considerably lower if this is the case.
Myself, I would recommend that the beam of the bottom at the transom not be less than 90% of the maximum beam of the bottom. If you use beveled chines maybe you could go a little less than 90%. I am not saying it will not work to have it narrower.
For the shallow deadrise hull I would find the lightest engine that would give me the performance I wanted. This could mean a pretty small engine if you are not wanting to compete with drag boats and such.
Good luck,
Gilbert

 

Hi VinceS,

Playng with lines on gut feel and "instinct" is a lot of fun. What makes it really satisfying and powerful is when you start moving lines because your calculated numbers tell you so.
What you have now is (as mentioned) a picture of a hull that you like, and thats not a bad place to start.

So, you have a basic design brief (2 foot chop, 6 people etc), and a hull similiar to what you want.

Now comes the challenging part. You need to make your boat shape fit your design brief, and along the way fill in the rest of the design detail. By "boat shape" I mean everything from hull lines, stringer locations, to helmsman ergonomics and so on.

If you enjoyed drawing the hull, and you are serious about designing a good boat, then you will enjoy doing a little number crunching in order to prove and refine your design. Its basic math, and it actually means something (as opposed to the math in school that was largely meaningless)

Read up about it. A good book to start with is:

"Small Boat Design for Beginners" - Frank Bailey
ISBN 0 589 50203 4

In your position it would be well worth getting a copy and spending a few nights reading it.

Good Luck,
Rob

 

First of all, I'm quite disappointed at the negative and discouraging feedback. I am here to develop my skills and receive feedback on my design (sorry, should I say “not really a design at all but some computer lines drawn on paper”) – I have committed considerable funds to this project however purchasing marine design applications or paying a marine architect to design the boat for me (which in itself would defeat the purpose) is just not in the budget and can’t be justified. I am doing considerable research and am paralleling this with feedback and advice from others.

Ok, now that the cobwebs have been swept, this is what I’m hearing.

If I want to deal with a choppy lake I need to increase the deadrise. I have settled on 15 degrees as there will generally NOT be a 2 foot chop. Just want to design for the potential issue. I also get the impression that 15 degrees would provide appropriate handling at a reasonable speed.

I am doing some more research into the chine – non-trip chine etc. Is seems to be an important aspect which needs a bit of extra time.

I think Tom Lathrop may have overestimated my goals in terms of speed. I’m not looking for anything beyond 40mph, even 30mph is sufficient.

As for the construction and power, I have already decided plywood is not my thing. I expect to be constructing this boat from a nice wood with some fibreglass for strength. I would like to maintain the classic runabout look, maybe modernise it a bit.

Please pour the feedback on in thick layers. Please avoid criticism which isn’t constructive.

Thanks all for your help so far. Tom Lathrop: despite my bite, I appreciate your feedback, please continue.

Vince

 

Before I read the three messages perceeding my reply, i would just like to note I didn't read these before constructing my reply - appologies if i have overlooked something or given unfair judgement.

 

a possible drawback to tumblehome-
stability. If you have an increase in draft from higher payload- more people onboard, carry extra provisions, fuel water, etc. the waterplane area will decrease with increasing sinkage. Immersed volume increases and since BM = I/V, and I decreases while V increases, your transverse stability goes down. Also you can have a similar dynamic effect in rolling as the knuckle dips below the water surface.

These are not unworkable issues, but necessary to be aware of.

 

Maybe it's just me but I don't see where the discouraging feedback is.

Reading through all the responses I see some suggestions offered along with encouragement.

 

Vince,

Anything I wrote was given with the best intention and I stand by every part of it. If you were looking only for reinforcement of your ideas, either you or me is in the wrong place. If such critiques are viewed as "negative" or "discouraging" you will never make it as a good designer. Honest critique is the only thing worth a damn when you ask for opinions. The rest is just flubber.

I would have been misled into very wrong conclusions if my requests for opinions of those who knew far more than me about design ideas had only been approval. The best advice is still to study as much as you can. Otherwise even the best answers will be misunderstood.

 

reading with ears to hear

Vince, unless you get lines like- " WOW look at this guy- he's not on the right path.." and " WHY YOU @@##!"*%$, you call that a DEEESINE?" then its seems like its up to you to read the posts with more "hearing".

I'd suggest you read the post about the comparison to your work as "in the circles of those who do this for a living, my estimation is; you are at a very preliminary stage of development. In fact "we" don't even use that term "design" until there are many more settled decisions, the stage you are at is called in the industry a preliminary sketch, or perhaps a concept sketch."

That's how I read it, and I hope you could find "ears to hear".

I hope you'll continue to research small planing boats and learn to design one that both encompasses your visual aesthetics with a soundly designed hull for the waters you plan to cruise. In the mean time you might gain information more rapidly with a more "grasshopper" inclined (apprentice, learner, student-like) interpretation of the posts, and less idea that you're being impugned.

With that said, the chine in profile has no need to hook upward toward the sheer in the last foot or two of its run. This makes an 'unbuildable' shape, gains nothing for entry and is not considered 'fair', so that line's finish at the bow, in profile, could use your continued attention with an eye to refinement.

The chine(s) reverse for about a 1/3 of the hull and then become plumb aft amidships toward the transom. This has no function in a planing hull that I'm aware. I have seen hulls where the reverse chine is limited to the forward 1/3 of the hull but they were all metal and I think it will be a builder's problem in wood. It is generally accepted that if you reverse the chines (step outward in section horizontally) that you're doing so to reduce spray forward and to increase area and possibly vector the chine's wake release, aft.

The plumb chine plate aft would give more performance if it were between horizontal to 5 deg downward and in plan view, taper (as mentioned above by others) slightly aft amidships but not more than a few percentage of the overall width of the chine plate. In this case the inner is almost strictly parallel to the keel in plan view. One reason is that the warped shape is actually more drag than a constant V and if you add to that a tapering surface INSIDE the wake release point, at the outer chine, you will add MORE drag to that surface with this shape.

If the bottom is tapered more than 8 or 10 % in the run of a planing boat they 'squat' -or run with a bow high attitude (greater than 5deg pitch) because the loss of area is roughly equivalent to a rounded butt line, in profile view, of the same surface.

Some tapering of the outer chine in plan is very desirable as it gives a shorter turning radius to the hull since it will allow a greater roll into the turns; deeper roll than a completely parallel chined boat will.

Already mentioned is the aspect of deadrise at 1/3 aft, or the master section, as this is most often the section at the leading edge of the running waterline. Therefore the reason it has been mentioned above about the difficulty of designing a well found and reliably controlled warped bottom is because as speed increases on any planing surface (conventional bottoms) this line moves aft and therefore the warped bottom presents dozens -hundreds of different "bottom" configurations to the water. Designing for one of these bottom to waterline instances is "a full day's work for a grown man" as they say of labor tasks in the building trades. I will echo the advise above about close attention to this aspect of your design, and recommend that you research and document as many other boats as possible for references about the forward and aft deadrise angles in your warped bottom.

This point is where you will acutally 'take' the water at speed in a chop, so this where the "ride" comes from. If it is less than 18 or 20 degrees, at this point, and your boat is less than a 1500lb. all up wt, you will "beat your eyes out" in a small short 2' chop (less than 30 peak to peak for your 20'er) at 25 mph. Fact. You can ease the ride by increasing wt, up to a point or you can sharpen a 10 or 12 degree bottom or slow down. I believe once you've gone around this item in the design cycle a few times you'll 'see' more of what

So, I hope you can hear more than one person saying (with repect and consideration for your enthusiasm) "from our experience"; you are embarking on a challenging design project since the warped bottom is more complex than the monohedron or constant deadrise hulls to design well. There are elements of these hulls which have caused some problems in the past and you may care to watch these areas as they arise in your project.

If you haven't already, may I suggest that you subscribe to, or acquire back issues of, the Classic BoatING Magazine (underline to emphasize the name), they're at classicboating@webtv.net phone (262) 567-4800 in Oconomowoc WI. They are exclusively involved the mahogony runabouts you've taken as inspriation.

wishing you fair lines, a sweet sheer and an interesting building project, leading to a beautiful, strong and reliable boat.

Cheers,
kmorin

 

Thanks to all replies, please don’t think I've had a sook, this is not the case. It is very hard to judge emotions on the interweb, I think I'll leave mine out of it from now on. I will however note that there are some members on the forum (not point the finger at anybody in this post) that take an elitist attitude which can be a turn-off for many potential home builders. To reiterate, not anybody in this thread.

I hear and respect every bodies suggestions. I don’t have time to write a full-blown reply at the moment but i will advise that I have taken it back to the drawing board. I have gathered that I am on the right track, but there are some major factors I need to take a look at before I continue. In my sketches at home, I have increased the deadrise to 15 degrees, I might take this up to 20. I am also investigating chines, specifically non-trip chines (which so far, present themselves as basically chopping the chine so the hull transitions from 20degree deadrise, to a 45 degree angle at the chine)

In terms of research, I have been looking into a boat building project extensively for 6 months, however the design of the runabout has only occupied a month of my spare time thus far.

Thanks again, will write longer and present some scans when I have some more time.

 

What we have here is a f a il yre to comunicate.

 

On the deadrise- I know it's a lot harder to figure out how a 'warped' or 'modified' vee with constantly changing deadrise will behave. What I'd suggest is to take your protractor and plumb-bob to the local boat ramp and get some idea what kind of angles are common in the boats you find there (and what their captains think of them). You'll probably find that a lot of runabouts in the 16-25 foot range have a much shallower deadrise at the transom than at the 1/3 mark. The substantial warp is more complex than a deep V, yes, but if you're maxing out at 30-40mph the aft sections will never be out of the water anyway and so the lower deadrise will cut time-to-plane, give you a lower minimum planing speed (very important in this type of boat) and improve wake and fuel efficiency. So I think it's worth looking at a few hulls, and doing a couple laps of the design cycle to refine your bottom configuration some more. The chines will of course be difficult. I'd look to inboard ski boats for inspiration here, as well as runabouts. Chine design on ski boats tends to be very carefully refined because of the need for minimum wake and perfect tracking and handling. So look at a few, get some photos and angles, you may find some ideas you could include.

 

thanks again for all the advice.

This is a picture of my interpretation of what I'm hearing.

I'm not yet convinced on the non-trip chine, and also I've had a bit of a look into strakes.

This is what I've come up with in my head.

20 degree constant deadrise (not sure exactly how to fit this into the hull design I’m looking for, I'm playing with a lot of stuff on paper)
45 degree non-trip chines, two strakes spaced evenly between the keel and non-trip chines. When I say "spaced evenly" I would space them at 1/3rd intervals at the widest point of the hull (or the bit of the hull which makes contact with the water while on a plane) and continue these parallel to the transom. I’m debating whether or not the strakes should curve up slightly at the bow. I’m leaning towards just tapering them into the hull until they disappear.

I decided 20 degrees deadrise is about as much as I'm prepared to have. To compensate for the increased deadrise I decided to include strakes to help with handling, reduce drag and help get the thing up on plane quicker. Also it could be useful to divert spray. The only other advantage I can think of is a tiny bit of extra buoyancy down lower.

The learning curve sure is steep when you have people criticism your methods. Thanks everyone!

 

Starting from scratch.

So I obviously needed to go back to the drawing board with my first idea, so here is my second.

This is actualy depicting about 24 degrees deadrise, which is way more than I am looking for (I just checked it then, but feel like posting it as is so far anyway) - I will take it down to 19-20 degrees and re-post.

also, due to the limited number of lines I can stick into hull design (10, I was hoping for 14) I cant model the barrel back as it sits in my mind at the moment.

I have decided not to include the non-trip chine (or my interpretation of a non-trip chine) - from the reasearch I've been doing over the last few days, I have been unable to find a runabout with a non-trip chine. Most hydroplanes have them, but my aim isnt to go above 100mph.

As for propulsion, I have been flipping some ideas around in my head and came up with the novel idea of a 250HP desiel driven Jet Drive. This should take her to 40mph quite comfortably - I also have a preference for desiel. Another reason I like the idea is the lack of bits sticking out, and more importantly, I can mount the engine further forward with a drive shaft extending to the unit at the back. The other option I was considering is a stern drive, I/O drive thingo. But I am not a big fan of mounting the engine so far aft. If both of these ideas are not sensible (please pummel them to death if you feel the need) I will have to stick to the traditional propulsion method.

My paper draft of this is coming along quite well. 19.6 degree dearise, two strakes, gorgeous barrel back etc.

 

Basically, as the Australian guy said, stick to the books and follow your insticts. Exactly designing your boat is not a priority. If it is approximately close to ideal mesures, it will still be a very good boat that you'll enjoy. Your schetches show clean lines, and that is more important. Pay attention to ballance of empty and full boat, keep weights distributed evenly and your boat will be just fine. Every speed requires different angle of something, every chop requires different deadrise. Your design can only cover a small segment of all conditions at sea.