What makes a boat plane?

I am getting ready to begin building a boat that is not designed to get up on plane. Something like a Godzilli 16 or Bo-Jest. I want to modify the hull so it can get up on plane for more speed. I am considering having the plans modified. So my question is what aspects are best to make a boat lane? Particulary considering these will be mods to a displacement hull.

Chris

 

The designers of neither of these boats ever intended for them to plane and they would be out of their element if you tried. You can ask the designers and I would advise going by their suggestions. Anyway, a planing tugboat seems out of character.

 

I am wondering if anyone has experimanted wth adding lift blocks to the bottom of a hull and how well it worked. I have talked to one of the designers. Great guy, Sam, from Devlin. He said he would be able to re-design the hull. I am seriously considering this as I really love their designs. I agree that a fast tug may be a little out of character, but I have 3 young kids (who love waterskiing and tubing) and money for 1 boat.

 

A displacement hull won't plane. Planing hulls have rather straight buttock lines and the structural integrity to take the pounding at high speeds.

 

Take a look at Phil Bolger's Microtrawler. For example: http://4dw.net/cosailor/ontario/mt.htm

It looks like a tug but has a planing hull shape that will meet your speed requirement with ease.

 

A displacement hull WILL plane. If you put a big enough engine in it, a displacement hull will become AIRBOURN.

http://foxxaero.homestead.com/indrad_034.html

I've seen people who didn't know the difference between the two different hull forms blow engine, after engine, after engine wondering why they couldn't get their boat to go any faster....I forgot what my point was...

 

I should start by saying that I'm not familiar with the boats you refer to... but
I'm under the impression that the key thing with planing craft is flow separation at the aft end... For a boat to plane it needs to have a transom... A boat with round stern buttocks will likely have difficulties planing due to a lack of separation... the water will follow the shape of the hull up above the level of the surrounding water, create a large wake and tend to cause the boat to trim...

I'm also under the impression that boats intended to plane have different characteristics that allow them to remain stable at high speed when there are significant dynmic lift forces on the hull (from the water it encounters)... they tend to have low centres of gravity, hard chines (possibly also to aid with flow separation), and relatively flat bottoms (with varying amounts of deadrise, but not rounded buttocks/sections/waterlines as are common with displacement craft). The position of the LCG is also quite critical with planing craft.

I think it's entirely possible to have a sucessful planing craft with a style modelled after a displacement craft, but imagine it might be quite involved and not very fruitful to try and modify an existing displacement vessel to plane... Might be easier to find an existing planing hull and try and modify it to the styling of the displacement craft you like... Having said this, I'm not familiar with the boats you refer to...

Just my two cents... best of luck with your project...

 

Ian Nicholson...

Hello...

There is a big thick book entitled - 'Hydrodynamics of Planing Craft' - just can't recall author...

See http://www.jbmarine.biz/Web/JBReviews.html ...

Getting something to get over its bow wave involves many dynamics - power and weight - trim - CG and others...

Getting something that does not have a displacement curve favouring the aft sections - which should be a good flat run of the 'buttocks' - to plane is an odd pursuit in the first place...

And so am I...

This is where I get shamed - any reverse curvature in the aft sections will induce a negative pressure area - hence the flat run of what you see as common power boats - there are exceptions in some 'picnic boats' that have tucked the above waterline stations into a barrel shape...

SH.

 

A friend of mine built a little tug some years back - I think it may have been one of Jay Benfords designs - she was about 14 ft long. The bottom was completely flat - save for a small keel running the full length. A vertical stem and near vertical sides. Quite a pretty (if somewhat useless! ) little boat.
Anyway, it had an 8hp o/b hidden under the rear cockpit seats. It very nearly planed with this...and when he whacked a 15hp motor on it, it planed quite effortlessly. I suspect its handling may have been a little suspect - particularly in a seaway.... still, it just goes to show, you can make a tug boat plane.
He upgraded recently - to a 57ft live aboard...

 

A displacement hull won't plane. Overpowering will result in hull failure. Calculate the power to push a tug at 3 times the hull speed, and there is no power plant that would fit without sinking the boat.

 

Well Gonzo, I used to think that, until I was hit by a gust when running free in my 17 ft modified Whitehall. It took off like a scalded cat, and since then I have found that a 10 h.p. outboard will have the same effect. The hull is essentially a double ender at the waterline.


What seems to be happening, is that I have a long, lean hull, very stiff and light (strip/epoxy), with a long enough area of parallel buttocks from about midsections to (say) 4 ft from the stern, that enough power (sail or motor) gets her up nicely. The very fine entry ensures that when her bow lifts, its weight prevents too nose up a stance. The clean separation aft does not seem to know that it should be a transom.

I guess that light weight is the secret here.

Comments? Tony.

 

IT ALWAYS comes down to, horsepower to weight, and available money. leaveing out the non-planers. Grumann Canoes ( aluminum ) make great planeing boats. You have not lived or almost died till you ride one with a raceing outboard motor on one. 18 to 30 hp on one will definatly open your eyes up!!

 

Double enders plane. I never claimed they didn't. As for your whitehall planing I have a couple of questioins: first, what is a scalded cat in knots? Second, how do you determine the boat is on a plane and not in displacement mode?

 

WE were keeping up with cars on a 2 lane road and when we tried to turn back it was chattering so fast sideways I backed off the throttle like a chicken.Cold sweat the first time. Great fun straight, with the decent keels they have. No fun in a turn. I just slid sideways, never felt like I had any real control.

 

Well now, here's a seasonal question - how fast is a scalded cat? Presumably we have to settle on units first. How about kittens? Fur balls? We can't use knots - far too pedestrian...

Anyway. Let's assume that my nominal full displacement speed is 1.4 X 4 = 5.6 kts (since my waterline length is about 16 ft). Then if I find myself grossly exceeding that (say a rough guesstimate of 10 knots plus, based on time and distance across a known piece of water) then most people would say like me that I am probably planing. Marchaj would disagree, and call this semi-planing. I take his point (there is a good discussion in his first book, with some fine photos), but let's not be picky.

What cannot be achieved with a true double ender is wake separation at the transom, so we can't use the appearance of that to determine a change of state.

Tony.