just a couple of amateur questions

i am working on designing a small (18' LWL) low speed (15 knot) runabout... so.... variable deadrise? is it worth the loss of efficiency? does it work? anyone know of any designs i could reference? lifting strakes, aka spray rail, how many? what angle? how big? what works, what doesn't? (lets say between 6'-7' WL beam).... i've heard of the 18 degree min for strakes... is it worth it to go to 18 degrees for effieciency/comfort, or go w less for lift??? hey, thanks for checking out the thread, any tips for a rook would be appreciated

 

A low speed anything will require flatter deadrise, not steep deep V configurations. A deep V needs lots of power, just to get it up on plane. With variable deadrise I'm assuming you mean a warped or double wedge bottom. This would be a much more suitable semi displacement speed hull form.

The best advise you could take to heart, is develop your shapes around well established hull forms, of known properties. Taking big swings at things you don't completely understand will result in a lot of frustration come launching day. After you've got some design experience under your belt and have a feel for how things work, you can then try stretching the envelop a bit with different approaches to common issues.

 

do you want it to plane???? do you want it to plane BARELY if not loaded, or do you want it to plane with authority?

inboard or outboard?

 

Clearly 15 knots on an 18' LWL is a 3.5 S/L, so she's up on plane, but not quite yet hauling the mail. We don't have enough information about the design to really be especially helpful other then recommending reference material and additional study.

 

yeah, i would like it to plane... and needs to be light too, if i can.... upper of 2700

 

what kinds of water? Open ocean? Large lake? small pond?
How far at a time do you want to drive it? Are you driving it around for pleasure or a specific job?
If it is for "fun", you can live with a flatter bottom then if you had to be in it 2 hours a day going to work or something.

 

15 knots is hard on a small boat unless it celarly plane. Shoot for 19knots on power and prop, that should get you over hump. Then rest of design is easier. Designwise go for something like Caroline Skiff to get up on plane easy.

 

I beg to differ. 15 knots is an easily attained objective - all you need is a low deadrise, lightweight boat (low bottom loading) with the LCB located in the right place.
Based on yf's comments however, I would suggest forgetting about designing such a craft yourself. Go buy a set of stock plans - there are any number available that will suit your needs and (assuming you buy from a reputablesource) you will be building a proven product. Remember it takes just as much effort to build a poorly designed boat as a well designed one....

 

The problem with small boats like this is that, adding a second passenger, cooler, food and little fishing gear, the boat now does Hull speed and is too heaviy to plane.

 

3.5 S/L is well into plane mode and there is no reason that adding a crew member or cooler full of beer should knock it off of plane. This would be true of lower S/L numbers, where the available power just wasn't enough to get her fully up.

As Will and I have both suggested, moderate deadrise and an easy run (plus other things) will permit an 18' boat to get up an scoot along nicely.

Again we'll need a lot more information about the project to offer much more. HP, weight, maybe some sketches, etc.