Increasing Hull speed with OB...

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RogerHM

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Oct 5, 2008
62
Grampian 23 Bobcaygeon ON
What modifications can be made to my hull to increase Hull Speed when motoring? (I have a 9.9 Merc Big Foot with a Thruster.) Boat is a Grampian 23, with a keel-centre-board.
I'm thinking extending the stern bottom??? Maybe replacing boat with a Chrysler 26 with a swing keel that could be easier modified... And with a larger motor???
I know the hull-speed formulae et al. But when I see a Macgregor going by at 15 - 20 MPH, and I'm doing 5 - 6, I gotta admit to envy :) Roger
 
Ross

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
That fast Mac is a planing motorboat with sails added. You have a sail boat with a motor attached.
 
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Larry14133

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Jul 17, 2006
75
Oday 302 Port Henry
Hull Speed

Hull Speed = 1.34*(lwl)1/2. LWL = Length at water line.

The only option for increasing hull speed of a displacment hull is to increase the lenth at the waterline.
 
Ross

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Larry14133 said:
Hull Speed = 1.34*(lwl)1/2. LWL = Length at water line.

The only option for increasing hull speed of a displacment hull is to increase the lenth at the waterline.
Click to expand
That reads 1.34 times the square root of the water line length.
 
kenn

kenn

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Apr 18, 2009
1,271
CL Sandpiper 565 Toronto
RogerHM said:
... when I see a Macgregor going by at 15 - 20 MPH, and I'm doing 5 - 6, I gotta admit to envy
Click to expand
when you're reaching in an 8-10 kt breeze and you're overtaking the same Mac, how do you feel then? ;)

I don't want to reopen the "MacX/Edge vs the world" thread here. The choice can be made on sailing preference: if you want to enjoy sailing in ideal conditions, but still have the option of motoring quickly, the MacX or Edge is a good choice. If you really enjoy sailing in most conditions, and the motor is your fallback for safety, docking or when the wind quits, then your Grampian is the better choice.
 
dserrell

dserrell

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Sep 8, 2009
171
Island Packet 31 Cutter/Centerboard Federal Point Yacht Club, Carolina Beach, NC
1
 
Last edited:
Stu Jackson

Stu Jackson

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Feb 26, 2004
23,353
Catalina 34 224 Maple Bay, BC, Canada
The Grampian, from what I've read, is a very well built boat. Your other choice is less. Stay with what you have as far as boat quality is concerned.

Live fast, sail slow!

Or is it Live slow, sail fast!

Either way...
 
B

Benny

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Sep 27, 2008
1,149
Hunter 320 Tampa, FL
The constant of 1.34 was derived many years ago to quantify the effect of hull shape on speed. This constant was derived as an average to accomodate the hull shapes of the time. It was derived for much larger boats and since that time hull shapes have been significantly modified. It follows that applying the formula to your particular boat will most likely render an inaccurate result. Close enough? Perhaps the same as saying 1 knot of speed for approximately every 4.4 ft of boat length.
 
Ross

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
can't post a picture

You want this kind of ability?
 
dserrell

dserrell

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Sep 8, 2009
171
Island Packet 31 Cutter/Centerboard Federal Point Yacht Club, Carolina Beach, NC
Great picture Ross! Wonder how fast a Mac sails? Fair weather?

David
 
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MoonSailer

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Jun 7, 2007
875
Pearson- 323- Mobile,Al
Many sailboats will plan especially the smaller "board boats" My Laser and Bucaneer 18 both planned and went faster than hull speed. On some days I actually passed smaller motorboats in the Laser. Wind surfers have exceeded 30 knots!!! So really the only way to go fast on a monohull sailboat is to have a light planning hull. Well there are other expensive designs.
http://www.sailtexas.com/modernyachtdesignfinalb.html
 
Joe

Joe

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Jun 1, 2004
8,323
Catalina 27 Mission Bay, San Diego
As was stated eariler, the mac is not a sailboat. It's a water ballasted powerboat hull with a light weight sail rig stuck on top. The very last thing in your sailing world that you should do is compare your fine sailing vessel with that boat..........

If you're jonesing to go fast on the water under power, get a power boat and have your fun. Don't mess with the fundamental design of your boat because you see some yayhoo pounding across a flat water lake or bay. When the wind pipes up and the waves get bigger, you'll see him slowing down to keep from shaking is rig right off the roof.

The formula for hull speed was used primarily to determine optimal steam engine size for a full displacement hull back in the day. The designers did not want to overpower the vessels because of the extra fuel costs of larger engines were exponential when compared to any increase in speed above the formula.
 
M

MoonSailer

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Jun 7, 2007
875
Pearson- 323- Mobile,Al
My link above describes how some boat hulls do go a little faster than hullspeed by using a big flat stern.
 
R

RogerHM

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Oct 5, 2008
62
Grampian 23 Bobcaygeon ON
"WOW!" Thanks mates! Never expected such an amazing, informative response!
I DO LUV my Gramp! Especially with its unique modifications, found on no other boat in the area, that serve me well...
But, ya know, there's always a better way! I just don't have the time or $$s... Maybe next-life a multi??? Roger
 
A

alexco38

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Sep 25, 2008
1,096
CS 30 Toronto
If you are handy enough, any boat can be made to fly. How about put in a set of hydrofoil underneath and a big kickass motor? Just kidding.

Enjoy what you got.
 
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Steve Dion

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Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
Have you checked which prop you have? Many of the OB mfg have different props for different applications. Check it out, maybe you need a different prop for a little more poop!
 
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