25 seafari questions

[Fr. Frank]

Here's what I figure on speed for your 25' Seafari, based upon your hull shape and bottom paint, with the right prop, + or - 10%. These are approximate results I got with a 1973 25' Wellcraft Nova II conversion from twin 165 hp sterndrives to single outboard on a Gil bracket to twin outboards on Gil Brackets. The boat belonged to the marina where I worked, and we changed it around quite a bit. It had K-planes already installed. All the results below were without using the planes.

We tried a single 235 Evinrude 1st. That was a pig. Then we tried a single 250 hp V8 Evinrude. We blew the powerhead in under 30 hours. Then we mounted the 275 3.4L Mercury. We ran that for a couple of months, then we switched to twins. Of the above, only the V8 Evinrude was a new motor. The BB Mercs were rebuilds from the Marine Patrol.

Original twin 165 hp Mercruisers cruised at 28-30 mph, 4-5 seconds to plane, WOT around 41-42 mph.
Single 235 hp Evinrude V6 cruised at 23-25 mph, 8-10 seconds to plane, WOT around 28-30 mph.
Single 250 hp Evinrude V8 cruised at 29-30 mph, 7-8 seconds to plane, WOT around 34-35 mph.
Single 275 hp Mercury BB V6 cruised around 29-30 mph, 5 seconds to plane, WOT around 38-39 mph.
Twin 275 hp Mercury BB V6 cruised around 34-35 mph, 3-4 seconds to plane, WOT around 46-48 mph.

Either the single 250 hp V8 or the 275 hp V6 had acceptable performance. Note that even twin 275 hp could not get the boat above 50 mph. I remember the single Mercury 275 used less fuel (23 gph) at near WOT (5200 rpms than the Evinrude 250 at cruise rpms of 4300 (24 gph).

While I suspect you'll use less fuel than the above, performance should be comparable.

Your major hurdle will be managing your CG.