Quote:
Originally Posted by 23Seacraft79
bushwacker, did you do the single or twin hermco bracket? if so, what engine/shaft length did you go with??
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I used a single bracket. I believe the twin bracket is only applicable for the 23 or a boat with an 8' beam, so don't think it was intended for a 20 with a 7.5' beam. I have a 25" motor but, as explained below, I've now raised it a total of 5 3/8" above where it was originally mounted, so a 20" E-TEC would probably work, although it would have to be mounted in next to the lowest hole with the powerhead 5" closer to the water. Might be different for a Suzuki which runs a very large prop that just barely clears the AV plate, so a Zuke might have to run deeper. (These comments only apply to a 20' hull; I have no clue about the bigger bracket on a 23!) For best performance on most motors and props, you generally want the Anti-Ventilation plate just above solid water when you're up on plane, although some props can run higher than others depending on the design. Don Herman mounted my bracket right where you want it, i.e., as low as possible for maximum flotation, and the swim platform height is perfect, about 2" above the water, but almost awash when a couple of guys get on the platform.
However my AV plate was totally buried even with motor raised as high as it would go, so Don redrilled the motor mount holes to raise motor another inch to the very top of the mount pad; can't raise it much higher because lower bolts would be right in plane of the swim platform. AV plate was still buried at 3.25" above keel (1st 3 pics below), so I added a
Vance Mfg. JPL4300 manual jackplate which had the smallest setback of any I could find.
I raised motor with jackplate to where the AV plate is now about 6 1/8" above keel and AV plate is finally above water! (last 2 pics) I picked up about 100-150 rpm, which let me turn a better prop (heavily cupped 4B SS BRP Cyclone 14.5" x 15p ) up to 5450 rpm with max load (optimum is 5300-5500 rpm for my specific model motor). That prop change raised WOT speed about 3 mph and increased cruise mpg by 0.2-0.5 mpg! My previous PowerTech 15x15 4B SS prop will spin 100-150 rpm faster but acts like it has less pitch than the Cyclone, evidently due to cupping differences, since both have the same blade area as determined by comparing circumference of blades. Both props run fine at the highest location, with no tendency to ventilate during sudden hard turns, although I can't/don't have to trim out as far as I did with motor mounted lower settings. I do see some effects of the extra 3" of setback of the jackplate because boat has an annoying tendency to porpoise, so I'm currently investigating some schemes that will allow same motor height but with minimal setback.
One thing I've concluded from all this is that the Trade Factor of raising motor 1" for each foot of setback, often mentioned on the internet, is TOTALLY
BOGUS, at least for my setup! In my case, the Trade Factor works out to be: 6.125" increased motor height/(33" setback/12") = 2.23" height increase/foot of setback, or more than
2X the commonly quoted number!