Resizing a hull

[bushbunny62]

Howdy Guys
I'm looking at building a A-B Class Runabout that is ten feet long and I want to shorten it down to eight feet long. The planing or wetted surface is only 28 inches wide. The reason I want to shorten it is to lighten it up alittle because I'm going to push it with a stock eight horse power outboard engine. Now there are a few varibles to throw into the mix to see if the shortening will cause any problems with the performance of the hull.
1) 350 lbs minimum weight (driver,motor and hull)
2) stock 8hp with stock prop
3) cockpit size needs to be minimum 3' 6" long, 20" wide



This is the boat I'd like to modify.
Thanks
BB

[Stephen Ditmore]

For every hull and every target speed there's an optimum distance from the center of gravity to the transom. One possible result of being below optimum is "porpoising", so I recommend you find someone who can do an onsent of porpoising calculation. The links below might help.

http://www.dlba-inc.com/Pages/troubles.htm
http://www.hydrocompinc.com/
http://wwwvl.naoe.ish.dtu.dk/Universities.htm

[bushbunny62]

Howdy Stephen
How I'm totally confused everything you mentioned is greek to me. I understand what porpoising means but I don't understand how the hull would be effected when I'm running it underpowered. Normally this hull runs a 15hp outboard engine and at a lot faster speed then the 8hp will push it.
onset of porpoising
what questions or information would I have to give to get an answer on the onset of porpoising?
Thanks BB

[Stephen Ditmore]

If you're not going to go fast porpoising may not be an issue. What you'll need to calculate is how far forward of the transom the center of gravity of the boat, including engine, driver, and fuel, will be after the modification is made. Then you'll need the chine beam, the deadrise, and the running weight of the boat (incl. those things mentioned before). From this an experienced powerboat designer should be able to estimate top speed and check to see if you'll be at risk of porpoising, as well as select the proper propeller.

[gonzo]

An important thing to consider is the deadrise at the transom. When the hull was designed the architect calculated the angle for water entry and exit. If you cut down the hull that relationship will change. Also a longer waterline will give you more speed for the power. I don't see any advantage investing so much time and money. It is probably cheaper and better to build a new hull.

[Tom Lathrop]

I have no experience with this particular type of minimal design but here are a couple of things that should be true.

Based on the normal weight of a boat and engine like this, the weight and CG are going to be dominated by the crew and his/her position. Therefore you are going to be disappointed by the percentage of weight loss resulting from reducing the length. Given the shape of the bow, there is not much reducing that can be done on the planing sections and still be able to get her up on plane from a dead start. Attaining plane will be more difficult with the 8 HP engine than with the normal 15 HP that it was designed for, assuming that you will be using a high pitch prop.

My guess is that it will be better to increase the planing surface than to decrease it if you expect to get up on plane with a small engine. In short, I think you may be going the wrong direction.

Try looking at some other designs that were meant for small engines. The Minimax from Glenn L comes to mind.

Try to talk to some people who race the A & B class boats and/or look up a website for them. I could be off base here.

If we're not reading the initial post wrong.
Does bush bunny wish to build a similar hull to the one depicted - just scaled back to 8 ft long rather than the 10 footer depicted and powered with 8 hp rather than the 15 hp the depicted vessel uses?

If this is the case - wouldn't he just take the existing plans and reduce the scale 1/5th then build his vessel?

No one (in their right mind) would chop 2 feet off that depicted beauty!....would they?

My suggestion if I've assumed right is to get the plans for the 10 footer and just reduce ALL measurements by 20%...course - I know nutting bout boats so - I could be waaaaay off track here.

Cheers!

[Tom Lathrop]

Trouty,

I could not determine which he wanted to do, but it makes little difference to the points that I was trying to make. The boats like he is interested in are very hard to get out of the hole and up on plane due to very small area of the bottom and weight concentrated near the stern. They usually have to lean way out toward the bow and rev the engine full bore to get them up. That is why I said that reducing both bottom area and HP did not seem like the way to go.