mercruiser drive ratio choice

hello there:

need some help.

have a 22' boat with a mcm260 engine. 1.98:1 pre alpha drive.

both of which did not come with the boat.

mercury says boat came with a 1.50:1 ratio.

need to put a new lower unit on a was wondering what ratio to order.

would like to run a 21 or 23" stainless cleaver prop.

boat weighs 3700 lbs. with a 1/4 tank of fuel.

expect loaded weight to be around 4300 to 4500 lbs.

any ideas or advise would be greatly appreciated.

regards

yukonmicro

You need to change the drive to a 1.50:1 ratio.The 1.98:1 drive is for the 2.5 and 3 liter engines. The only gear change choice you have in the lower unit is a standard lower or a High altitude lower. You should take the cover off the upper and count the teeth on the gears to be sure of your ratio.

I would get a newer drive. The pre alphas are not a durable as the newer ones.

Once you get the correct ratio drive you should start with a 14.5x19p and adjust prop size until your WOT is around 4800 rpm. You can pick up a good used drive for $400-$650

need to put a new lower unit on a was wondering what ratio to order.

Click to expand...

Ayuh,... The ratio is changed in the Upper....
Alpha1s, 'n pre-alphas are All the same lower ratios...

the boat had a 22" michigan stainless on it and seemed to accelerate ok.

not sure of the WOT as the tach did not function but it sounded good....may have just been the thru hull exhast...lol

with this prop it broke free on high speed turns.

the prop has some damage as does the cavitation plate along with 4 holes from a prior whale tail..

should i maybe order a 1.62 ratio to allow me to use the 21 or 23" cleavers???

yukonmicro said:

would like to run a 21 or 23" stainless cleaver prop.

Click to expand...

why?
19P prop is usually the most efficient - that's why 17,19 and 21 are most popular

Check w/SEI drives. They have a quality product for a good price w/a 3 yr. "no fault failure" warranty.

would like a good cruise with consideration of top speed along with this boat will often be in rough water conditions.

when my wifes not in the boat i ilke to go fast.

cleavers supposedly have low drag / torque to turn.

had a pair of these on a past boat and they worked very well.

swapping drives is more costly than swapping props. Generally there is no way to calculate pitch more than ballpark.
Propping based on WOT rpm trial and error is what most do... kind of hard to work it backwards.

My advice would be to put a 1.5 or 1.47 (V8) ratio drive on.
Only the V8 drive ratio will spin the water pump impeller fast enough to ensure proper cooling of 260HP

I wouldn't be surprised if the temp climbed under full load with 1.98.... the impeller is not huge on an alpha. I suppose if the impeller was in brand new condition, it would be ok... but the impeller is turning somewhat slower in a 1.98 drive...which is fine for cooling 140HP

Unless you have an unusually large engine in a small boat, or an unusually small engine in a large boat, or you're at high altitude, best to stick to stock ratios

what would you define as high altitude??

yukonmicro said:

what would you define as high altitude??

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this type of drive ratio is higher than 1.50.

since you operate in rough water you would require a 1.50 ratio as this operation requires more power from the engine. having a tach that works is what I would repair before using this boat. good way to damage the engine and you have now IDEA how this power plant is operating.

with the WOT RPM then you can determine how you must proceed.

260HP with 4500lb 22ft boat is very good power.