-
Long time reader first time writer.....
I have a 21' shallow V (with small pad) skiboat 350MAG MPI and Bravo 1 drive. Like every other person I'm looking for more speed.......currently the boat runs a 23" LaserII @ 4800 - 4900 RPM to 66MPH GPS (23" Revolution 4 @ 4600 - 4700 RPM to 65MPH GPS), engine is stock and exhaust exits thru prop.
My initial question is this, currently the prop shaft runs almost 8" below the pad and I was interested to hear any opinion on what effect an IMCO 3" shorter lower would have ?, and what performance increase if any I could expect ?, the only requirement for the boat is to be able to pull a slalom (single ski) skier out of the water the rest can be top end speed.
I realise that raising the "X" has very different effects on very different hulls but the performance sterndrive industry is almost non exsistant in my part of the world so any help would be much appreciated.
Regards
CC
-
What kind of boat is it? Your X-dimension is probably so low because the shallow V won't let the manufacturer mount the engine any higher. Is the cav plate dragging in the water?
To be honest, without some kind of experiences from folks who have done the same (maybe from the manufacturer?), then it's a crap shoot on how any gains/affects you will see.
The only adverse affect would be that if the X is too high, the prop will pull air when coming out of the hole and blow out. However, with the shaft 5" below the bottom, I doubt that would happen.
-
Thanks for the reply,
The boat is an Australian built Skicraft Excel S21, operating weight approx 3200 lbs.
I have contacted the manufacturers in the past but it appears that other than running one with a 502 and Blackhawke drive that is where the information dries up......
I'm sure you are correct when you suggest the drive height is a compromise of engine mounting height etc.
The lower ventilation plate on the drive runs almost parallel to the pad(perhaps an eighth of an inch above if I was being generous).
Again I realise it's a bit of a guessing game but I figure there must be some general rules that carry over from one shallow V with pad to another ??
From general reading it sounds as if setting a propshaft around the 4 - 5 inch mark below the bottom will allow the use of surfacing props like the Bravo 1??
Thankyou again, any help is appreciated......
-
From general reading it sounds as if setting a propshaft around the 4 - 5 inch mark below the bottom will allow the use of surfacing props like the Bravo 1??
That makes sense that someone would try a Blackhawk- they have a much shorter leg, as they are made to surface.
The Bravo 1 props aren't really surfacing props. I am not an expert, but with the shaft 4" below the bottom, the prop height is still very conservative. I don't think you would have any blowout coming out of the hole, or carving a hard, slow turn.
I think Shockwave makes a boat similar to that? Maybe you could PM "shockwavebd" on this forum? He might be able to tell you where they set the X on their 21' skier for a point of reference.
-
Maybe these will help..........
-
CC , transom angles play a role in X dimension measuremment. Go to this link , save target as .It will tell you what your optimum X fix is .Page 12 on .
www.boatfix.com/merc/Install/gas/86017211.pdf
-
Spectras only,
Thankyou for your post, I appreciate the thought but I have read the Mercruiser manuals a number of times, I understand how to calculate an X dimension & what mercruiser recommends, what I don't know is with the current selection of props out there and the huge amount of testing that has been done what will (may) happen with a 21' long shallow V (with pad) 350MAG MPI if I raise the propshaft from 8" below the bottom to 5" below the bottom ??
I can't be the only person with a boat of this size and generic hull design that has considered this ??
Yet again, thankyou any help or suggestions are very much appreciated.
Regards
CC
-
As per the manual , the maximum is 3" you can raise for a Bravo. I 've remeasured my X dim last night , and it's 1 1/8 higher than standard for Bravo . Boats like yours with a modified V were running Alphas in the past giving you
max 2 1/2 to play .
