Friday, October 2, 2015

A Lighter, Roomier "Grinder"





 I built Bantam and Grinder, both modified Phil Bolger designs. Grinder turned out very well, but I thought I could do better. My idea was to build a boat similar to Grinder, but much lighter, with a different profile and a bit longer cabin. Since a flat bottom boat gains efficiency as it loses weight, I wanted to try a smaller engine that needed less battery, all Okume construction, a laminated 3/4" hull, and get rid of the heavy shoe Phil had on the clam skiff. I changed the sheer line to something I liked, eliminated the raised coaming of Grinder, and added an extended gunwale to stop spray and provide a walkway and seating all around the boat. The cabin is 9'6" long, and Grub is 21' overall. 
The construction pretty much follows the method I used on Grinder. Here are some build photos.
 









    The engine is a 30 HP Tohatsu EFI four-stroke with both pull start and electric start capability, so I could do without a back up battery. The prop is a four blade 10"x9".
  In place of Phil's 1 1/2" x 16" shoe in the center of the hull bottom, I used twin 1 3/4" skegs. This turned out not to be enough to stop a disturbing "bump and wiggle" hull flexing when underway.
   My immediate fix was a cross batten mid-hull in the cabin, which solved the worst flex, but I still was not happy. I wanted stiffness but not weight, so I took the cabin off after the first season, turned the boat over and put a keel on. The photos will help explain. It's made of left over 3/8" Okume, 1 3/4" high and 16" wide.
                     








Adding the keel completely stopped the annoying bottom flexing. It also had a remarkable effect on the sound and behavior of Grub. Pounding, with the associated noise is so reduced it seems like a different boat. My brother has started an AF4 build, and I am recommending a similar keel.
Here are few beauty shots, and an action video:





Grub gets on plane (video) at @3400 rpm and 9 to 10 knots, and will get to @ 22 knots WOT. It will loaf along at 5 knots at 2,000 rpm very quietly. I'll be working on the cabin enclosure over the winter, with hopes to take another Erie Canal trip in company with Grinder and the new AF4 called "Big Shot".