15' Runabout idea

Hey Folks, playing around with a design and need some input.

All the dimensions are in planning stages, nothing is set in stone!
15' long, 4' beam, 10° dead rise from transom to bow, 20" transom. 2 seater bench style
Plywood and fiberglass construction.
Ideal top speed 60-70mph (outboard size?)

skeg placement?
internal rib spacing?
Material thickness?
should the cockpit be moved closer to the CG? at WOT i don't want much boat in the water.
what am I missing?
recommended Naval Architecture reading?

Thanks Y'all!

 

Well, I guess 60-70 mph is possible with something like that, on smooth water, but hitting the wake of a passing boat could become a dramatic event. There is a saying about boats that you can have any two of the three, of speed, comfort and economy of operation, with relative ease, but getting all three, is very elusive. You can easily get speed and reasonable economy with that, but comfort (and safety) will be lacking. Is it intended for a lake ? 60-70 mph is very fast, and potentially quite dangerous, you are talking about dynamic forces that are probably 4 times as great as at 30-35 mph, and in a low-to-the water little runabout, 30-35mph will seem quite quick, and any chop will be felt through the 10 degree bottom.

 

its intended for lakes and rivers. Not too concerned with comfort. I've been considering building a crackerbox or something like this.

 

The speed requirement is too exacting, design wise, even minor design flaws will be amplified considerably by such speed, you would need a very well designed boat to have safety, flipping at 60 mph could easily be lethal.

 

ok so where does that leave me? just trying to get some numbers here. the whole idea is to eliminate design flaws through this thread.

 

I think you need to apply a little caution when talking about winding a 15 foot boat out to 60+ mph, as it could get ugly if the boat has a quirk that only shows up at high speed, if it hits a wake at the wrong angle. The boat would need to be quite light, the HP substantial, and getting everything into a well-balanced unit, a challenge. You could be in for endless hours of tinkering to get to where you want, or not get there at all. The difficulty is the speed, if you had more sedate ambitions, what you have drawn would seem OK.

 

Ok

Ok what size outboard?

 

You could drive that boat with a 25 hp motor, and cruise about 25 mph. If you have the need for speed, you would need more like 100 hp to get to 60 mph, and only then with the right prop and engine height. And good nerves. And your helm position would need to go somewhat further forward, to balance a heavy engine.

 

Why do you need to go that fast Garret? Just curious, not criticizing.

 

I dont think anyone needs to go that fast, I just want to. However I don't think its realistic with this design. I think ill just build a crackerbox instead.

 

Good plan Garrett. You already know what to expect from a Crackerbox. You can almost surely get some good tips from other Crackerbox people. You could not have those advantages with a new design.

 

15 feet and 50 inch wide transom. 15 X 4.17 x 2 - 90 = 35.1 HP rating. 60 mph with 35 hp. I doubt it. More like 30-40. As has been said by others small boats like that can get very squirrelly at high speeds. Not much hull touching the water. By the way, I have gone that fast on the water. At 65 in a small boat (a PWC yikes!) it feels more like going 100, and if anything goes wrong, adios baby. I have also ridden in a 40 footer at 60-65 and even then, in a larger boat, it was a scary experience. Speed on the water is no trivial matter.

 

Four foot beam is way small to support a good sized outboard, it'll take a lot of power to get that up on a plane and it'll be near swamping when you're sitting still. A more modern padded V hull would provide a better ride and probably better speed, but that will require more design expertise to get something that will work.

The problem here is that you can go very fast with a small (around 50 hp) motor on a very light boat, but with that light weight comes issues with stability and aerodynamic lift that make going 60 mph in a very light boat a serious undertaking.. Something bigger and a bit heavier is going to take more power to go that fast, but it'll be a lot more stable and less exciting than a very light boat. As an FYI I have a 13.5 foot APBA racing runabout that has a 44 cubic inch Merc with a racing lower unit and I'm in the mid 70's with proper prop and only one person up. With a fishing lower unit and heavier 2 place boat (my boat only weighs 140 pounds with a 150 pound motor) you're going to need a good bit more power to get the kind of speed you're talking about. Doing speed calc's that don't take into account lower unit drag can be very misleading.

I would seriously suggest that you start with a set of plans and modify them to suit your needs. You can use the plans for the bottom, and make it look like anything you want the topsides to look like.. If you start with a good set of plans you'll have all the framing and material thickness as well as robust transom design that is important if you're supporting enough power to go 60-70 mph. If you don't have a lot of experience you will likely end up spending a lot of money for something that could well be very dangerous. Glen L sells plans for their "Rebel" which is 15 foot and can support up to 80 hp. A Rebel with 40 hp does about 35-40 mph. That boat with 80 horsepower is going to be pretty fast, but it might not quite get into the 60 mph range, and of course since the boat was designed for that much power it should be decent handling and not too scary doing it. Rebel has a pretty flat aft bottom that is necessary for high speed, but you also give up some ride quality with that bottom design. There are other plans that you could also buy and if you do some research on the web you will likely find someone who's built one and this will let you know now fast it will go with what power. Here's a link to Rebel build and a link to the Glen L plans page.

15' Rebel - runabout and ski boat-boatdesign https://www.boatdesigns.com/15-Rebel-runabout-and-ski-boat/products/352/

Z Rebel Build - Glen-L.com https://glen-l.com/phpBB2/viewtopic.php?f=2&t=14396

 

Burn crash and die.
You want to go fast put a bike engine on it and a direct drive a 5 speed automatic drive abs tonnel haul