17 Foot Planing Center Console Design

[nacra5.8]

Hi-
I'm designing and building a 17 foot center console design for fishing, and cruising around my bay. Conditions are usually fairly benign, maybe 30 knot max winds, 2-3 foot max chop. I need to be able to carry 8-10 people fairly comfortably (and dry). I am planning on building in strip plank, with the chines in ply (rounded bottom is easier to make in strip,but flat sides easier in ply), and glassed then painted. I really want a deep V hull, not a trihull. I have experience in woodwork, not much in building boats, and very little in composites.

Basically, I would like a few guidelines for a design (I already have a prelim hull design, don't know how to attach), and ideas for building. I am planning on building over male frames CNC cut from DELFTship ( I also have AutoCAD), and then glassing the outside, flipping over, inside, glassing in bulkheads, false floor, transom, and then painting. I have no idea what thickness and types of wood to use, and what sort of layup I would need to use. Thanks.

[Mr Efficiency]

17 feet is way too small for your intended load of 8-10 people. Wouldn't it be a lot easier and cheaper to source a good second-hand glass hull ?

[nacra5.8]

Sorry, I meant 6-8 people. I know and have operated boats of this size with 8+ people. No suitable glass hulls are available cheap in my area, and I have always wanted to build a boat

[Mr Efficiency]

Still terribly crowded with 6-8, and depending on your local jurisdiction, may be beyond legal limits. The time you total up the expenses, I doubt you'll save money. Which leaves the 'fun' aspect of building a boat. Which is OK if the design and structural integrity are up to scratch, which is not guaranteed for an amateur design and build.

[PAR]

Having designed boats specifically for your area, I'm a bit surprised by your general sea state observations. I would strongly suggest you just buy a set of plans as there are several other things in you post that have me wondering "why". Plans for the general size and configuration you're looking for are very plentiful and scantling information is provided with them. In short shooting from the hip, in this sort of thing isn't the wisest thing to do, particularly when farther from shore then you can swim back to.

[tom28571]

To expand a bit on what PAR said, round bottom and deep V don't fit on the same boat very well. Making the bottom round because you might like to do some strip building is not a great idea either. Both the deep V, hard chine and your local waters suggest sheet plywood planking as building material. I also think 17' is small for the loads and waters you want. Look at what the locals are using for the same purpose. Look at the Orca, for instance.

An easy mention of 30 knots and 2' - 3' chop might suggest that you have not been out in 30 kts on SF Bay.

Is there some other reason you are considering a round bottom?

For a center console planing deep V in SF bay, you can get by with 3/8" ply with stringers for the hull or better yet, 1/2" for the bottom, especially if you go to 19 or 20 feet LOA. Best would be a double bottom (torsion box) with either longitudinal or transverse 1/4" bulkheads or a grid system. I prefer the longitudinal bulkheads for better ventilation and water drainage with an aft sump.

It is a very interesting and personally satisfying course you are taking. Don't get tied down to any preconceived ideas too much.

[nacra5.8]

OK guys, thanks, lots of good tips. I am not boating on SF Bay most of the time- usually Tomales Bay, which is much calmer than SF. Round bottom wise, I was thinking it's slightly more efficient, and making hard chines in DELFTship is kind of hard. I did a couple of experiments with the Carlson Hull designer, but it didn't seems as professional as DELFTship. So, for an 18 foot boat, how about 1/4 in ply with inside and out glassed with 12 oz? Longitudinal bulkheads sound nice, but what about transverse bulkheads with drainage holes and stringers?


There go my preconceived ideas, but I guess you guys would know better. Thanks.

[PAR]

1/4" planking, even with 12 ounces on each side is mighty slim, especially once you calculate slamming loads. Unless it's a displacement cruiser (even then it's mighty thin) 1/4" is just not reasonable. Now, if you place 2 layers of 12 ounce biax on the inside and a single layer on the outside, you could make the 1/4" plywood work for you (it would then have the stiffness of 7/16" plywood), up to about 22 - 23 knots, at which point you'll want more stiffness.

Tom has a good point about longitudinal webs under the sole. This is the direction you'll want to transmit your loads anyway. Athwart bulkheads will work, but pound for pound the longitudinals work better for the same weight and loading.

As far as hull form, select your target speed range and choose a hull form, best suited for it. Round bilge shapes are good at lower S.L ratios, but once you start getting over 30 knots, you'll really want to start considering chines for control and efficiency.

In the end, it all depends on your SOR and the goals you've set for the design. Compromises will need to be made, but the hull form often dictates the build method options available. For example if your target speed is 30 knots, the bottom planking should be at least 10% thick then if it was a 25 knot boat, just to absorb the increased bottom loading. These aren't things you should guess at, without a great deal of scantling experience.

[nacra5.8]

OK, I guess I'll go with 3/8 plywood, but still with 2 layers out, 1 or 2 in. Do you guys put one on the bias (one at 0/90 one at 45/45) or both just 0/90? Sounds like I'll go with longitudinal BHs.

I think my optimal max speed would be anywhere from 30-40 knots. I have looked at the Sitka Spruce 18, but really would want to design my own. Deadrise wise, I am thinking around 16-18 degrees, does that seem OK?

Thanks guys, especially PAR

[Mr Efficiency]

I think you owe it to your crew to make safety job #1, you can't walk home if the arse drops out of your design, literally.

[tom28571]

Originally Posted by nacra5.8

OK, I guess I'll go with 3/8 plywood, but still with 2 layers out, 1 or 2 in. Do you guys put one on the bias (one at 0/90 one at 45/45) or both just 0/90? Sounds like I'll go with longitudinal BHs.

I think my optimal max speed would be anywhere from 30-40 knots. I have looked at the Sitka Spruce 18, but really would want to design my own. Deadrise wise, I am thinking around 16-18 degrees, does that seem OK?

Thanks guys, especially PAR

If you are talking about regular glass cloth on the bias, that is not what has been suggested. Biaxial cloth is non woven and many times more stiff than woven cloth and is much to be preferred in this application.

Choosing a hull design based on what a software program can produce is a terrible way to make a design decision.

[Mr Efficiency]

Originally Posted by tom28571

If you are talking about regular glass cloth on the bias, that is not what has been suggested. Biaxial cloth is non woven and many times more stiff than woven cloth and is much to be preferred in this application.

Choosing a hull design based on what a software program can produce is a terrible way to make a design decision.

You mean stiffer as in less flexible ? I am interested in knowing how that could be, or did you mean it has greater strength ?

[tom28571]

If you have used both woven and non woven glass cloth, you would know that I meant it just as written. Non woven cloth with straight fibers is far stiffer than woven cloth where the fibers run back and forth through the opposite strands.

[Mr Efficiency]

Originally Posted by tom28571

If you have used both woven and non woven glass cloth, you would know that I meant it just as written. Non woven cloth with straight fibers is far stiffer than woven cloth where the fibers run back and forth through the opposite strands.

OK, I think I will be looking to learn more about this, I have only used the non-woven glass in sandwich construction where obviously the stiffness is high anyway.

[PAR]

Nacra, guessing at scantlings, particularly for a boat that's expected to reach the speeds you've suggested, is frankly insane.