Gear Ratio

[Quadruplecccc]

I have a 1976 merc. 233hp with #1 drive pushing a 1976 24' Crestliner CC at 33mph (gps) wot.

I thought the old motor was getting tired and not pushing as hard as it should. Years later it was finally time to drop a rebuilt engine in her. Interestingly, the same top end was achieved.

Could it be that someone swapped an outdrive with a gear ratio that was too high ? Top rpms get to about 38-3900 rpms with a 17 pitch prop. The boat runs great but was wondering why some boats seem to just fly by me with similar features.

How fast should this boat really go? How do I check the gear ratio and what I should be using in a boat this size?

Just some more things to kick around! Thanks!

 

[HT32BSX115]

Re: Gear Ratio

Howdy,


Well, what is your drive ratio & did you use a known tach?


If you are indeed turning only 3900 RPM, that is too LOW.

You are lugging the engine and should prop it lower or change the ratio.


Your 233 hp engine is what? A V-8 or inline 6? what is the correct rated WOT RPM?


Cheers,


Rick

 

[Quadruplecccc]

Re: Gear Ratio

I'm on my 2nd tach and don't know what gear ratio I have. Thats why I asked the question. 17 pitch prop seems low enough but even going to a 15 pitch wont get me to 4400rpms Like it should be getting.

 

[Quadruplecccc]

Re: Gear Ratio

Oh, forgot to mention its a ford 351 windsor v-8

 

[HT32BSX115]

Re: Gear Ratio

Ok,


Well I am guessing that a 351 would probably want to turn up around 4800 like most small v-8s.

You can calculate your drive ratio easily by pulling the plugs, turning the engine several turns and have someone count turns of the prop. use a calculator......divide (the engine turns will be 1.5-2 times higher than the prop turns) eng by prop and you get ratio.

then you can look up someplace what the ratio should be and decide if you have the right drive for your engine. If you do, then try a different prop.

Make sure your 2nd tach is accurate too!


A 17p prop *should* be ok but you won't know until you know your ratio.

Also, you need to be sure that you're not "dragging an anchor" I.E. marine growth on the bottom or waterlogged hull etc......

 

[Howard Sterndrive]

Re: Gear Ratio

The Fords don't spin as fast. Recommended WOT rpm is 3800-4200 for the 233.

 

[Quadruplecccc]

Re: Gear Ratio

I agree with you Howard S. I'm pushin about 3900rpms at WOT. Just seems to me that 33mph is a little slow. But then again it is a heavy old boat. Does 33mph sound right to you??

 

[45Auto]

Re: Gear Ratio

A brand new Sea Ray 24' Sundancer with a 260 HP V8 barely breaks 40 MPH. With 230 HP, I would expect mid to high 30's on an equivalent brand new boat. With a 34 year old boat and 230 HP, 33 MPH seems pretty reasonable.

 

[Quadruplecccc]

Re: Gear Ratio

Well, then It seems I may have been looking for trouble when there is none. Works for me!!