I am confused

A

aholst1

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We have an 89 Santego 2757.
The links Pete posted for specs indicate cruising speed of 20 - 22 at 3200 rpm on 3.7 mercs.
Are you on plane at cruising speed?
With the 4.3s, my cruising speed (on plane) is 29 - 31 at 3800 - 4000 rpm
Am I missing something or are my tachs way off?
 
Are you saying if you drop your RPM's from 3800 you come off plane? What happens when you run at 3200 RPM's?
 
How did you decide 3800+ was your cruise rpm???
 
at 3200 rpm I am plowing water
at less than 3800 I drop off plane...
 
Are yiou using your trim tabs at the lower RPM? You shouldn't have to push your engines that hard to stay on plane unless the boat is really loaded with weight.
 
Trim tabs will effect rpms a little,
WOT is 4800 to 5000
I usually top off my tanks before we head out (100 gals) and 40 gals of fresh water, plus 3 - 5 bodies on board
 
That is a surprisingly high get on plane rpm and speed too. i dont think I have ever seen a boat our size not on plane at 20mph. Are you really plowing water at 20 mph?????
It might be a good idea to use a photo tach to see what’s up though the wot rpm seems within range.
 
Bruce,
That may be the ticket, one thing I forgot to mention, these are NOT the original tachs...
AJ
 
Check the speed with a gps too. we need good tach and speed data. I'll be astounded if your boat falls off plane at 29mph.
 
Those V-6s are pretty high revving. I borrowed an old Chaparral cuddy here at the marina a few weeks ago and was stunned at how high the engine would rev before I settled out on a smooth plane. I don't remember what RPM I was going, but it was higher than I was used to. I figured it was just that old dog of a boat. But maybe there's more to the story...
 
I have the 3.7 Mercs and cruising speed on them is 3600 to 3800. The engine is a WOT of 4800, so you cruise at a higher RPM than engines with lower WOT. my speed at 3600 is 24 to 25 MPH. I have a 1988 2757 Montego with the deep vee hull, not the flat hull (it changed half way thru 1988). The 2767 Santego will cruise a little slower because it is a heavier boat.
 
pete: I simply dont think it is possible for a 27' boat to exceed 10-15 mPH without being on plane no matter how big the engines or how deep the bottom.
 
You're probably right, Bruce - what's that old Displacement Hull equation??? something like the Square Root of the waterline length x 1.34 ???
 
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