Quote:
Originally Posted by dave s
Thanks Bushwacker!
The 5 degrees is only for the bulkhead; if I make a bracket, where the motor mounts will stay at 12 degrees.
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Ah, that makes sense - thought it was strange that what I thought was transom core was so far forward of aft end of hull!
Quote:
Originally Posted by dave s
. . . After owning and fishing a few cutout 25" transom boats. I would really feel safer with the full transom . . .
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Amen brother! After making one return trip from the Bahamas in 6-8' following seas in my boat with a 20" transom cutout, I can relate to that!
Quote:
Originally Posted by dave s
Since I'm redoing the whole boat anyway, I can move a lot of weight forward.
Do you think a hull extension will help with the Cog balance at speed, compared to a regular bracket, where the tub is raised up a few inches?
I'm thinking besides the static flotation, the extension will help lift the rear a bit while running.
I'm not so concerned with top speed, since most days I'm running in 2-3' chop.
So So the supposed speed benefit of the raised tub, isn't as much a factor for me.
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Moving weight forward to maintain CG is the key to good performance with a bracket, especially on a smaller boat.
Regarding a hull extension, Carl Moesly's comment, when I asked him what he thought about the bracket on my boat, was "Why not just make the hull longer?!" So yes, a hull extension will not shift the CG like a bracket will. It will provide most of the benefits (except for WOT speed increase) of a bracket with none of the negative effects, so it's the best although most difficult/expensive option. Even with a maximum flotation bracket like a Hermco, the flotation tub will be out of the water when you're on plane, so that flotation goes away, causing a significant aft shift in CG.
Suggest you send a PM to Blue Heron, as he's thinking of making a hull extension for a 20 I/O CC model he bought a few months ago. He found a technical paper and some software from some folks at Stevens Institute, where they modeled and tested deep V hulls comparing baseline, bracketed, and extended hull configurations. The model predicted and hydrodynamic testing confirmed that the bracketed model runs with a higher trim angle (bow higher out of the water) and has a higher minimum planing speed than either the baseline or extended hull models. This explains the higher WOT speed for bracketed models, but it also implies a harder ride because the sharper sections of the V are not in the water at cruising speed. This theory and test data match my own experience, and is why I've been preaching in this post that, on a 20, you really need to avoid using a heavy 4-stroke motor on a bracket! Potter's arguments in favor of a bracket regarding safety and WOT speed/economy are correct AS FAR AS THEY GO, BUT THEY DON'T TELL THE WHOLE STORY! If you can't shift weight to maintain CG location with a bracket, I believe the modifications you will have to make to an get acceptably low min planing speed and soft ride at cruising speed will reduce prop efficiency and increase drag enough that you'd get BETTER CRUISE performance with the motor mounted on the transom!
I could not shift weight on my Seafari without making unacceptable mods to the boat, so even my DI 2-stroke motor is too heavy for use on a bracket. I had to add some band aids like a stern lifting 4B prop and a Doelfin to get my min planing speed and ride back to what it was with a 300 lb motor mounted on the transom. The 4B prop really helps you hang on plane down to 12 mph, but it has more blade drag so gets lower MPG and WOT speed than a good 3B prop. The Doelfin also produces lots of stern lift, but at the cost of extra drag, and my tests with the I-Command system, which calculates instantaneous MPG from a GPS spedo and fuel flow from engine computer, indicate that it costs me 0.1-0.2 mpg at cruise. I left it off for my recent 90 mile trip up to the Long Point gathering for that reason, since most of the trip was in the ICW where I didn't have to slow down to 12-15 mph to punch thru 2-3' square waves like I've often had to do on the Little Bahama Bank!
The bottom line is the benefits of the solid transom, big swim platform for diving, and extra room/storage in the boat provided by the bracket are big enough to me to justify a slight reduction in cruise performance, so I'd do it again, although I might consider using the ~50 lb lighter 125 hp "115" E-TEC that wasn't in production when I bought the 150. Optimum cruise on the 165 hp "150" is only about 35% throttle and it only needs 45% to climb on plane, so I'm convinced the "115" would still have adequate power but would burn about 30% less gas. However I've consistently averaged 4.4 mpg with the "150" on trips of 200-700 miles with heavy cruising loads, so I can't really squawk too much about fuel burn with the current configuration, considering that it has so much margin for even heavier loads if necessary!