Planing hull question

[TheFisher]

When designing a hull, should the line from the bow to the stern be level or should it have a slight angle to compensate for the bow rise. If the bow is expected to rise 3 degrees, should the keel at the transom be 3 degress higher than the keel at the bow? A rough picture has been included to show what I mean.

Fisher,

When a boat like this planes, it gets its support from the aft planing sections. So if these sections rise as in the second drawing, the bow will lift more in direct proportion to the aft section rise. In other words, nothing is accomplished. The trim angle of the boat is determined by the aft hull shape not the at rest attitude. There are reasons to give the aft hull such a rise, like getting the bow high to meet waves. Some offshore sportfishers are even given a bit of aft rocker to force the bow even higher.

This is the simple answer. There are always many inter-related factors that add up to the final performance. It usually depends on the performance goals and where and under what conditions the boat is to be used.

Tom Lathrop

[TheFisher]

Tom,

Thanks!

The hull in question is for a 27 foot offshore center console fishing boat.
The hull will have a 17 or 18 degree dead rise at the transom.
7inch wide chine flats and 2 lift strakes per side.

I need to have it handle some rough waters and be as dry as possible. I don't want the bow pointed to the sky though!

How do I determine these factors?

I think its time for MMD or another more experienced designer to come in and speak. I have not studied this particular situation enough to offer other than general advice. What you propose sounds perfectly normal but more deadrise in the forward sections might be called for.

Tom Lathrop

[TheFisher]

Thanks. If anyone else can offer more I would appreciate it.

BTW, the proposed deadrise at the entry is 52 degrees.

[Timm]

If I were designing this boat, I would make the keel line parallel to the waterline and perfectly straight from somewhere around station 3 or so aft to the transom. I would also make sure all the butts were as straight and level as possible back aft. If the buttock lines are flat, you will still get 3-5 degrees of trim as the boat squats aft.

Be careful with the chine flats being too wide as this could cause the boat to ride rough. I generally do not have the chines enter the water in profile view before station 3.5 - 4, and always include a downangle of 5 degrees or so. Some like to put more, but I was taught that anything over about 7 degrees is no more effective.

Good luck with your design!

[TheFisher]

Thanks Timm,

Great! I currently have the keel staright and parallel to the waterline from station 3 aft!!

This board has been a wealth of information to me and has brought about many design changes as I have learned more.
For example, I increased the rake at the bow and narrowed the transom waterline beam relative to amidships, for handling in following seas.

I'll reduce the width of the chine flats to 5" and make sure they don't enter the water until well after station 3. I think I have them entering after station 4 right now. I did include a down angle of almost 10 degrees. I will change it to less than 7 degrees.

I am awaiting the arrival of Dave Gerr's book on boat strength in order to figure my scantlings and get a better idea of what my hull weight will be. My initial thoughts on the hull sides is 3/8" plwood core with 2 layers of 12 oz. biaxial glass inside and out with epoxy resin. The hull bottom will be 3/4" plywood core with 2 layers of 12 oz biaxial glass inside and out. The chines and keels will have a total of 8 layers of glass.


Once I have the design the way I like it I may have to pass my design onto someone who can perform the computer modeling for hydrostatic calculations etc. The only software I have is Greg Carlson's Hull program.

30 miles offshore is not a good place to find out your design is no good. Other lives besides mine will depend on the vessel being right!!!

Any NA's or experienced designers interested in doing this please drop me a quick note.

All constructive criticism is welcomed.

BTW, I can provide more specific details on the boat if necessary.

[Timm]

Fisher,

If you can create a traditional lines plan, then there is no need to model the hull. Simply divide it into equal stations about the waterline and figure the area of each section below the water. That info can then be put into Simpson's Rule and the hydrostatics calculated. I have a spreadsheet I can send you if you send me your email address.

As you are building in plywood, you may want to be careful building chine flats. It may be easier to build the hull without the flats and then add them on after the hull is planked. It would probably be much stronger as well. Then you could glass over the whole thing. Good luck!

[TheFisher]

Timm,

I would appreciate the hydrostatics spreadsheet. I can create the lines if needed. Please send it to Vince at uniquecrystal dot com

I have a displacement vs vessel weight question in another post.

[Humberto Rodrìg]

Originally posted by Timm
Fisher,

If you can create a traditional lines plan, then there is no need to model the hull. Simply divide it into equal stations about the waterline and figure the area of each section below the water. That info can then be put into Simpson's Rule and the hydrostatics calculated. I have a spreadsheet I can send you if you send me your email address.

As you are building in plywood, you may want to be careful building chine flats. It may be easier to build the hull without the flats and then add them on after the hull is planked. It would probably be much stronger as well. Then you could glass over the whole thing. Good luck!

Please send a copy of your spreadsheet to HumbertoR92@hotmail.com