Humble design critique request

Stitch and glue open fishing skiff. 5 oz Dynel in epoxy over 9mm hull, 12mm bulkheads and seats. Flotation fore and aft, built in lockers under side seats which are bonded to the hull.

2" x 1 1/4" inwales/grabrails not shown.

LOA = 14' 10"
LWL = 11' 10"
Beam = 5' 6"
Draft = 5"
Displacement at DWL = 716#
Cp = .66
Weight with 15hp motor = 360#
Expected top speed with 15hp = 20kts

Reading Larsson and Eliasson suggests that the formulae aren't really optimized for small boat design, so I'm soliciting some subjective help.

The basic parameters of this boat are based on the feedback from the retail channel and designed to a price point which is comparable to similar sized aluminum skiffs.

There is nothing so novel about this design that I anticipate any obvious nasty performance habits, but I'd appreciate any suggestions.

On edit: I can see that my model was ambiguous in the stern. I cleaned it up a bit to hopefully make it a bit clearer.

 

Take the camber out of the bottom and increase a vee to about 14 to 16 degrees .In a harbour chop will make it a little softer riding at 20 knots !!
Watch for any rocker in the keel as it could porpose if you go any faster or even at 20 it might !

What do i know ???

 

Do I understand you correctly? I should make the aft section of the bottom flat (remove camber), and increase the deadrise forward?

Currently the keel is flat for 2/3 of lwl. I'm concerned about torturing the bottom panel if I bring the forefoot much farther forward, particularly if combined with increasing the v-angle.

How accurate is savitsky at predicting porpoising in small boats?

 

just remove the rounded camber from the keel to the chine thats there !!

 

I found the drawing a bit unclear, is there supposed to be athwartship camber in the bottom panels aft ?

 

You're right. It was unclear. I've uploaded a new lines drawing.

(Is there a way to get Free!ship+ to not show interior furniture through the hull in profile and body plans?)

 

Lengthen the waterline and increase wave deflecting ability by changing the bow profile. Narrow the planning surface and make it a straight flat run.

 

I'm affraid it is still unclear. The colors make the chines of your boat nearly invisible. You should uncheck the button titled "Show plates with fill color", which appears in the upper left corner of the "Linesplan" window:


In this way only black and white lineplans will remain, which are easier to comprehend.

 

Lines drawing sans color.

I wish there were a way to hide layers in the linesplan.

 

The benefits would be a softer ride in a chop, a longer waterline to allow a slightly faster trolling speed and less resistance at pre-planing speeds by getting most of the transom out of the water, and less tendency to porpoise, right?
I note that most aluminum fishing skiffs have very wide planing surfaces aft, I assume that this is for manufacturing reasons, because it's cheaper to build with four panels rather than five (or six)... or is there an upside to what they do?


I'm primarily a builder, not a designer, so I appreciate your insights.

How do you see the conflict between optimized engineering and perceived customer value? In the words of one salesman we talked to; " *Shrug* People buy boats like this (smokercraft alaskan 15). They want wide, deep and inexpensive."

I tried to bring that message to a boat that could comfortably and safely do 20kts with 15hp.

 

You need to rethink the design. It is overly heavy and has too wide a planing surface, the bottom is also a bit thin but if you don't put much power on it could be ok.

I think you would do well to look at Payson's Diablo. It is lighter, and faster than what you have started with. Moreover, the bottom is thicker and stronger, and it uses more glass on the bottom and sides than you have specified. Your bow is very full and this will result in pounding if it gets even a bit rough. Diablo has a narrower entry and a won't pound as much as your design.

I also think you are a bit heavy, the hull weight of the Diablo is only 150 pounds.

This exercise is proof that it's a lot harder to start from scratch than it is to adapt something that is already proven and works.

Lastly, most 15 hp motors are really 20 hp motors with inlet restrictors and smaller carbs. If you are paying for the weight of a 20, you might just as well put a 20 on it and go a lot faster.

 

Optimistically, the bare hull should weigh about 266 pounds. The general hull form, length and design target is more similar to a Devlin Candlefish at 320 pounds (although the Candlefish 16 is slightly bigger).

http://store.devlinboat.com/candlefish.aspx

I love the Diablo, and if I were building this for myself, it'd probably be more like that - minimum wetted area when modestly loaded, (although I don't think I could build it to 150 pounds or with only 2 gallons of epoxy. Think about it; the Diablo takes four sheets of 6mm ply and 3 sheets of 12mm ply. That's 250# of plywood. Modern marine ply is much heavier than it used to be.)

Like I mentioned, I'm designing this to a price point (and the 5hp difference costs $500) and is heavily biased to what retailers are telling me their buyers want; a wide, deep, burdensome utility skiff.

I very much appreciate everyone's advice. I will narrow the bottom and flatten it aft, and lengthen the waterline by making a more plumb bow with a finer entry. I'm confident that's doable without breaking hull panels.

Thanks again.

 

Lumberjack__jeff,
Why are you looking to make your design flatter aft? Is it as a result of "michael pierzga's" comment? "Narrow the planning surface and make it a straight flat run." I think he was referring to the type of boat that he has that is basically a semi-dory multiple chine boat w a very narrow bottom. Unless you narrow the bottom a LOT I don't think his suggestion applies. I'm fairly certain he meant w a very narrow bottom like his and Diablo's. Sorry in advance if I'm wrong. So if you don't go to the semi-dory Swampscott type I think you should either increase deadrise or go to a flat bottomed boat. The only thing I don't like about your boat on post 9 is that it's very flat w too little deadrise.

 

A compelling case has been made for making a finer entry with increased deadrise forward and a longer waterline.

As you note, this isn't going to have a narrow bottom like a Diablo, so I intend to make comparatively small changes to the aft sections, (with an eye on improved performance with modest hp) and I'm now leaning against making it flat. The keel adds significant strength to my 9mm ply bottom.

Regarding the merits of a fine entry in a fishing skiff I think that the Bateau FS14 is a good example of the opposite.

I think that, although my initial design might not be optimized, it's not an outlier either.

 

Version 2.0
Length: 14' 10"
LWL: 12' 5"
Beam: 5' 5 3/4"
Draft: 5"
Displacement at DWL: 659#
Cp: .60
Wetted surface area: 44 ft^2
Total weight with 115# long shaft 15hp motor: 374#

This iteration is also acceptably efficient, it should do 25 kts with 15hp, and it should even row tolerably.

It also looks better. Thanks for your help.