Engine height ??

[newellchunker]

I've just bought a 1987 Larson DC-225 with a '93 Mercury 200 HP Offshore. The engine is bracket mounted and is propped with a Merc. SS Mirage 17P. At WOT trimmed with just me (300 lbs.) and around 40 gals. fuel it will only turn around 5300-5400 rpms. Is there a formula for raising the engine so many inches for so many inches of offset from transom? The engine is currently mounted on the bracket on the top holes (flush w/Bracket). When I tried to trim her out more she just porposed..sp?

 

[Dhadley]

Re: Engine height ??

Very, very general rule of thumb -- for every 5" you go back you can go up 1".

 

[Texasmark]

Re: Engine height ??

One thing I learned from jacking (slightly) was that the higher the engine the less control on bow lift at hi speed. I didn't look but it seems that if the anticav plate is in the water you can trim her hard and she will respond hard (bow lift). But once you get that plate up you can move your engine tilt significantly more with significantly less effect on lift.<br /><br />Not that that's a big deal, as lift definitely improves performance.<br /><br />With the engine jacked up (as mine is) you can still have too much trim out and not enough speed. Increasing speed stops it. Like I was setup at WOT and backed off a couple of thousand. Next thing you know she's a bouncing. Either speed back up or cut the trim till she stops.<br /><br />So I don't know it you will be able to do yourself any good, but what have you lost if you try it?<br /><br />Mark

 

[brownies]

Re: Engine height ??

When my motor was ALL the way down on the jackplate (boat was that way when I bought it). I knew no better and ran it that way. <br /> There was lots of trim adjustment available, and seemed like lots of bow lift possible.<br /> Running as fast as it would go, trimmed as much as possible, You would "feel" like you were in a big tall 4wd. (bow was WAY UP THERE and It seemed the whole boat was up there also).<br /> No noticable difference in speed or rpm with just me or me and a 180lb passenger...or with me and two other passengers (330lb).<br /><br /> Now that I've moved my motor up as far as it will go and still have water pressure. There is not much trim adjustment available. Bow does not lift like it used to. Even with this limited trim adjustment, it quickly reaches a point of instability.<br /> I now loose 5mph with a 150lb passenger, no loss in rpm though.<br /><br /> I'm working way back down with the motor now, small increments.<br /><br /> I'm not real sure what I have done, but, it's just not making any sense to me....<br /> All you can do is try different heights and see where it performs best.

 

[Texasmark]

Re: Engine height ??

I think the cav plate was acting as a lift fin (if you will) and the more you trimmed out the more it dug, hence the more the bow lift.<br /><br />With it above the hull and out of the water, you are totally dependent upon propeller thrust angle which isn't nearly as effective as the former.<br /><br />What you didn't mention is the difference in speed. Your "quickly reaches a point of instability" comment tells me that you have significantly reduced your drag and your speed is higher, sooner, as your boat is on top now whereas you were stern plowing before?<br /><br />Just a guess. Knowledgeable guys should pick up on this shortly.<br /><br />Mark

 

[walleyehed]

Re: Engine height ??

This is where Top quality SS props come into play...a prop like a Raker or Turbo Lightning, possibly even a Mazco which are all designed to be run at higher surfacing conditions, will give better results.<br />Remember, don't get stuck on one brand...My std ole' Raker has more bow lift than a Tempest...a Turbo Lightning or Solas Scorpion has even more lift.