Rocker, beam and bottom design questions

[slower but cooler]

1) How much rocker, what shape and where do I put it. I've been told "no rocker in the last 6'6" of the boat and square the transom as much as possible". Also was told the rocker should be about a foot wide and about 3/8" in height. My father always talks about one of the race boats he designed but didn't build. He had a cabinet shop make it and they didn't put in any rocker. He said it was glued down and faster at half throttle than full as it would bury the nose. Originally my father deviated from his plans and put extra rocker in. It porpoised wildly and Jack Davidson ended up re-working the bottom. One of the photos shows the double bottom area.
2) This is not a drag or race boat. It's a family ski and cruiser. Her old top end was about 55. She ran the same speed with a 1953 Cadillac 331 motor as she did with a fresh olds 455. New motor will be gm (not sure on big or small block yet) but nothing outrageous. We are seriously considering adding length and beam to the boat. Has anyone done this and what did it do to the handling? The old length was 16' and the old beam about 6' so with two bench seats it was cramped. We are thinking of splitting it down the middle and keeping the shape so it will be longer in the deck but we'll move the dash forward and possibly get some seating along the sides of the engine.
3) I know most of your boats are 18' to 21' but I don't know what the common beam is?
One other thing. A plate is not an option. My father believes plates were put on boats that didn't get the design right and as long as he is alive there won't be a plate on the transom.
.

[V-DRIVE VIDEO]

Hold out 'til Art Anderson (GoFastRacer) sees this. He will be able to answer all of your questions and a few more that he may add in. :)

[GofastRacer]

Hold out 'til Art Anderson (GoFastRacer) sees this. He will be able to answer all of your questions and a few more that he may add in. :)
You sure about that Jerry!..:D :D

[GofastRacer]

Well, if this just a family 55mph cruiser I wouldn't put any rocker in at all make it flat, especially if you're not going to put cav plates on!..BTW, I guess all these flatbottom, runnerbottom, drag, circle, GN V bottom boats with plates that hold records are all setup wrong huh, LOL.. Cabinet shops are not boat builders,LOL, I bet there was a hook in the bottom that's why it drove the nose over at speed, with a rocker you need the power to hold it up on the rocker otherwise it will hop!. Also the strut angle and location and where the motor is positioned at determins the handling characteristics of the boat!...

[Sangster]

Art.. You are slowly becoming my HERO.......

[slower but cooler]

Naturally I'd like to see a little more speed after the rebuild. Honestly 65mph would make us the speed king on most days at the lake we normally run on. 70mph would be the max I'd want to run with this boat. I'm not really sure how realistic this is given the weight of the wood hull. At the original 16' length it would porpoise gently (after the modified bottom). I could level it out by stabbing the gas at the right moment. It was little twitchy on glassy water. The motor will be as far to the rear as possible. Basically if I can re-build this boat with a gentle enough ride (yeah I know it's a flatbottom) and seating for 5-7 comfortably I can probably avoid having to buy a lake volvo (ski boat) and maintaining two boats. I am happy enough at 60-70mph. Having flipped this boat at 40mph (mechanical failure) I really don't have the desire to push the speeds up where you guys are running. Besides I really enjoy crowds walking past $150,000 eliminators to gawk at our little 50 year old homemade boat. Thanks for all the help.

[GofastRacer]

Naturally I'd like to see a little more speed after the rebuild. Honestly 65mph would make us the speed king on most days at the lake we normally run on. 70mph would be the max I'd want to run with this boat. I'm not really sure how realistic this is given the weight of the wood hull. At the original 16' length it would porpoise gently (after the modified bottom). I could level it out by stabbing the gas at the right moment. It was little twitchy on glassy water. The motor will be as far to the rear as possible. Basically if I can re-build this boat with a gentle enough ride (yeah I know it's a flatbottom) and seating for 5-7 comfortably I can probably avoid having to buy a lake volvo (ski boat) and maintaining two boats. I am happy enough at 60-70mph. Having flipped this boat at 40mph (mechanical failure) I really don't have the desire to push the speeds up where you guys are running. Besides I really enjoy crowds walking past $150,000 eliminators to gawk at our little 50 year old homemade boat. Thanks for all the help.
Curious, is this a brand name hull or a one off custom???...

[GofastRacer]

Art.. You are slowly becoming my HERO.......
:D :D

[Marty Gras]

If you move the prop foward far enough, it won't need plates, It won't need an anchor either!

[VDRIVERACING]

A discussion of what constitues "set up right" might go on forever. Cav plates allow the driver to adjust for continuously changing conditions affected by speed, water surface, cargo weight, etc. It's true that they can also be used to compensate for some level of hull/setup problems, although I tend to think of their function as a control surface, not a correction surface.
That aside, your dad is right in stating you do not need them. However, any design has performance limits, so you might consider a conservative approach in what you may achieve handling and speed wise. I mention that because you mentioned some desire to increase top speed. This boat might be a lot happier in the 50 MPH range, than faster. It sounds like you want a familiy cruise/ski boats and that is more than fast enough for grandma in the back seat...

[slower but cooler]

This was a one off custom. The story goes that my father (a mechanical engineer) was designing boats at the same time Jack Davidson was starting to build. Both apparently started with Glen-L plans and then heavily modified them. My father built about 6 boats but only 3 v-drives (2 outboards and one cracker box). Jack built a few more. My father early on would go over to Jacks radiator shop and swap ideas, trace patterns and get advice. The modified bottom design was one which Jack Davidson did in about 1956-1957 so it may have been the same as some really early Sangers. Since this was the last one my father built, our knowledge base is about 50 years old. I figured bottom design ideas may have changed in that time. The boat has been as fast as 60 (on a 50 year old speedo) and rides better at the upper end. That said it would be nice to cruise without fighting porpoising in the 30-40mph range without giving up top end. As far as plates are concerned, I don't think he's trying to argue boats with plates are somehow inferior. I do agree with him that a plate on the rear of this boat would ruin the aesthetics because of the unique lines at the transom. Thanks for all the advice.

[DansBlown73Nordic]

Lets see some pictures before you took it apart....

[GofastRacer]

Both apparently started with Glen-L plans and then heavily modified them.
You happen to know which boat it was taken from by chance???..

[slower but cooler]

This is purely a guess and only speculation but there are several pictures of the "white mist" race boat in our slide collection and it was built about the same time.