prop, and?

PuckHog

Seaman
Joined
Jun 26, 2005
Messages
68
I have a 15' open bow tri-hull which had a '69 Merc 65hp O/B on it. I had a 15 pitch prop and the boat rode great (30 MPH by GPS). I recently put a '78 Merc 80hp on the same boat. First I started with a 19 pitch but couldn't get over 4800 rpm, so I replaced it with a 17 pitch (getting 5200 rpm now). 5200 rpm with a 17 pitch for 80hp on a 15' seems low. Other thing (much stranger) is the way the boat rides now. First, it hooked to the right something awful. A trim tab adjustment later, and that's OK. Thing is the boat is listing heavily to port when it planes and kicking up a lot more wake than it used to. The motors are virtually identical externally and I have the cavitation plate dead even with the bottom of the hull. I've tried adjusting the tilt, and other than affecting bow-rise (as expected) it does nothing for the list. According to any measuremaents I can come up with, the motor is centered and square to the boat. What gives? Sorry for the lengthy entry.
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: prop, and?

Is the skeg below the prop any different, ie bent, etc., as compared to the other engine?<br />Sometimes if the engine tucks too far "IN" it will cause more water contact and may cause some handling differences.<br />Are the props the same manufacture??? I would imagine the 17 has slightly less diameter than the 15, no???
 

PuckHog

Seaman
Joined
Jun 26, 2005
Messages
68
Re: prop, and?

The skeg is slightly different. The 'new' 80hp has a larger (taller?) skeg than the 'old' 65hp, though that could just mean less of the 80hp skeg has been ground off over the years. Both appear to be straight.<br /> The 15 pitch that is on the 65hp (worked fine)has a diameter of 12-1/4" (stock Merc alum.). The 19 pitch I started with on the 80hp (4800 rpm max)has a diameter of 13-3/4" (stock Merc alum). The 17 pitch currently on the 80hp (5200 rpm max) on the boat is a Michigan Wheel Vortex alum with a diameter of ~13-1/4". <br /> I'm going to try different tilt settings this weekend to see what changes are effected (manual tilt/trim) and will give the skeg a much closer look when I get home tonight.
 

PuckHog

Seaman
Joined
Jun 26, 2005
Messages
68
Re: prop, and?

Well, I examined the skeg a bit closer and it had an almost imperceptible curve to it (concave toward port). I was able to very carefully flatten the skeg.<br /> After testing, I had to re-adjust the trim tab (anode) and now the boat cruises straight and level with no effort on the wheel, which allowed me to tilt the motor out a little for a touch more bow lift to wet the hull less at plane and I gained 200 rpm at WOT. Thanks for the help.
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: prop, and?

It's funny how some of the smallest little things can make so much difference ain't it???<br />Good Job!! :)
 
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