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Offshore V design help, please - 1/23/2012 3:46 PM   
mudhenk27


 

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Status: offline 		I'm an experienced designer of model sailboat hulls, but I'm making my first attempt at a powerboat hull.  It is intended to be kind of a "sleeper"....looking like a lobsterboat above the water, but with the underwater lines of an offshore racer.  That probably sounds odd, but a friend brought a lobsterboat model to the pond last season, and I'd like to give him a bit of a surprise when this one takes off

Based on what I've studied, I've drawn a hull in FreeShip that is 1300mm (51") LOA, and 350mm (14") beam on the waterline.  The V is about 20 degrees at the stern.

What surprises me is two things:
1) that the displacement seems lighter than what would be needed to float the weight of the hull + gas engine
2) that the CG of the hull is 490mm from the stern, when the rule of thumb that I see here seems to be more like 400mm or less.

I realize that a planing hull is a different ballgame from my sailboat displacement hulls, so any help from you powerboat designers would be greatly appreciated!

Thanks...........Bill


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RE: Offshore V design help, please - 1/23/2012 5:24 PM   
expresscraft


 

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Status: offline 		on most of our deep vees the cg is 29-30 percent

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RE: Offshore V design help, please - 1/23/2012 11:57 PM   
mudhenk27


 

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Status: offline 		Thanks, Expresscraft...it was actually your reply to my post on the Homelite thread that led to this.

I see that I can get a CG at 30% by making the hull depth shallower toward the bow, and also narrower at the waterline toward the bow...I've attached another screenprint of the plan where I've done that.  But that makes me wonder if 30% is really important or just a matter of style?  Does the bow shape help get the boat up on plane, or is it irrelevant?  Looking around, the long shallow, narrow entry seems common on Offshore V boats....but Dave Marles's Sigma 51 looks deeper in the bow.  Is that because of his FSRV roots?

Any thoughts, design freaks?

Thanks for helping me understand.............Bill


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RE: Offshore V design help, please - 1/24/2012 9:08 PM   
expresscraft


 

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Status: offline 		if the cg is to far forward the boats bow will dig in and bow steer, or flop into the corners, if its further back the boat will run loose, not so much with a homelite so if further back works you might get away with it, i could not open your file so im talking in general, if your cg was back with a powerhouse motor you would get blow overs, different bows due make a difference

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RE: Offshore V design help, please - 1/25/2012 8:29 AM   
MrMikeG


 

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Status: offline 		I would make the widest point further forward, yours looks like an arrow and will do funny things in the corners.
Homelite 28% CG, stock Zenoah 30% cg mod Zenoah or QD 32-33% CG.
50" boat & Homelite I would be going 28% CG and a biggggg prop.

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RE: Offshore V design help, please - 1/25/2012 2:45 PM   
mudhenk27


 

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Status: offline 		Thanks for the help, guys.....I'll work on it and post again to see if you like it better.

Bill


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RE: Offshore V design help, please - 1/25/2012 4:00 PM   
Johnnysplits


 

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quote:

ORIGINAL: mudhenk27 underwater lines of an offshore racer.  The V is about 20 degrees at the stern.





On offshore race boats it's 28 degrees.

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RE: Offshore V design help, please - 1/25/2012 5:59 PM   
mudhenk27


 

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Status: offline 		OK guys,  I've simplified things a bit to concentrate on the part of the boat that's in the water....I'll worry about the superstructure later.  I got rid of the arrowhead shape.  The boat is 51 inches long, and 14 inches wide at the waterline.  Deadrise at the stern is still 20 degrees, steepening a bit going forward.

I'd appreciate your assesment of the shape...does it have the right sort of look now?

Johnnysplits, 28 degrees sounds steeper than a lot of the stuff I've read here...is that on a specific boat, or a general rule of thumb?

As it stands, the displacement is about 3 pounds...which sounds light to me to float a gas engine, and the CG is 41%.  Of course, that's the static hull dimensions, and doesn't take into account the dynamics of power and planing...so I could just mount the motor in position to give me 28% when I build.  Is this fairly typical of the breed, or am I missing something in the concept?

Thanks again...I appreciate your helping me learn.

Bill


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RE: Offshore V design help, please - 1/25/2012 11:59 PM   
MrMikeG


 

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Status: offline 		That looks better to me.

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RE: Offshore V design help, please - 1/27/2012 1:40 AM   
Johnnysplits


 

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quote:

ORIGINAL: mudhenk27

Johnnysplits, 28 degrees sounds steeper than a lot of the stuff I've read here...is that on a specific boat, or a general rule of thumb



Yeah, 22 is more like it. My bad.

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RE: Offshore V design help, please - 1/27/2012 4:35 PM   
mudhenk27


 

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Status: offline 		Thanks to your help, I think I have it figured out.  This version seems right to me...but I've been wrong about that before, so your comments would be appreciated.

I modified the bow for a bit finer entry, but the real "ah-ha" came after studying some more.  Looking at pics, I finally realized that these boats don't sit level on the water when static...rather, they sit stern down...then level out under power when planing.  So I rotated my hull drawing, and wadayano, the CG came out at 35%....finally in the ballpark.

Now to refine it and make it look like an innocent lobsterboat above the waterline....

Thanks..........Bill


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