Stability of Slow Cruising a Deep Vee Hull in Rough Water

[David_Old_Jersey]

Originally Posted by wmcruiser

HI, i have just carried out a similar conversion-
I have a rinker fiesta vee cruise 23ft 6000lbs that had a 5.0l v8 petrol and removed (due to corrosion of unit) and repalced with 1.8 ford diesel direct to stern drive.
the engine transplant went well, but prop resizing took some doing as getting low enough pitch to obtain RPM-
ended up with outboard prop repitched by local engining company to 14 x7P.(as not using through prop exhaust refitted with waterlock set up to transom exhaust)
this gives max rpm 3200 (9Kts)and cruisng of around 6-8 (2600rpm)kts depending on tide etc.
fuel burn between 5-9 ltres/hr depending on conditions but much less 4lt/hr @ 4-6 kts.
Good to fair weather/sea conditions(force 1-3) handles very well but may add a rudder safe unit to help with steerage.
bad sea condition (f 4-5 wave roll height 1-1.5M) which i got caught in which wasnt palnned or predicted before leaving - very interesting she powered though ok but ride was very rough/rolling as managed min 6kts speed but really need more to push though. got back safely but would recommend it for faint hearted (or intend to do it again !!!!).
next thing to go is to look at ballst issue as bow now heavy due to 200kg engine weight loss at stern. this may improve speed a little on good days allowing bow to lift more than it does.

I appreciate that the above a different boat to OP's, but as I hadn't seen this thread for a while (and recently was pondering over the outcome - cheers to OP for updating folks )....I thought I would look back to see what I said originally about the proposed conversion (from fast planing to slow displacement speeds):

Originally Posted by David_Old_Jersey

My gut says that the boat will be inefficient as a displacement boat and that therefore the 4108 may struggle to acheive 7 - 8 knots.

Will she be stable in rough water? - not as stable as when travelling at 30 knots (kinda like the riding a bicycle thing) and will probably make a fair bit of leeway as little or no keel, plus the windage....will she be dangerously unstable? I dunno, but I would start cautiously! but I doubt she would be ideal for heavy weather, so would plan my trips accordingly - not to say don't do the conversion, really depends on how the boat is to be used.

I think that prediction pretty much in the ballpark........




(I also see that I got sidetracked in the thread - again . kinda funny to see what got me all excited a year or 2 back, especially as can't remember it ).