Need Opinions on Prop Problem....?

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Well-known member
Mar 22, 2006
Reaction score
Franktown, Virginia 23354
I'm observing some interesting behavior of my boat with my new prop....

For those of you who read everything you probably know that I recently switched for the stock F225 prop to a Stilleto 14.25 X 17. Here is the thread with details if you want to review the change. ... highlight=

Here is what seems to be happening: At lower speeds around 21 to 23 mph (all speeds measured by GPS) running in 2 foot chop with tabs down the boat seems to start to slow down and fall of plane with no real decrease in engine rpm.... maybe a couple of hundred rpm ...suggesting to me something is going on with this new prop that I don't recall with the other one....

I'll be running along at 4700 rpm and the boat will slowly start losing speed...drops from 22 slowly down to around 18.... there seems to be a magic speed of around 24 mph and 4900 (keep in mind this is 2 foot chop.. headwind... tabs down) where it does not happen..

Am I describing this well enough?

Hopefully Monday or Tuesday I'll get back out on some calmer waters and play around to see what is going on.

Is it the prop?

Keep in mind that the engine rpm is not dropping off.

Maybe this is all normal... but as I recall with my other prop I could have easily run at around 18 mph on plane...... it's as if the prop is losing its bite.

I've experienced the same 'symptoms' on 3 different boats (one of them a 2120). I'd consider it normal.
Richard, I'd suggest more testing in calmer conditions so you can rule out some of the external variables.

I've seen conditions in the Bay where I was passing the mouth of a river (the Severn for instance), where the boat will dramatically loose speed due to currents caused by an outgoing tide. Engine speed will not change, but the SOG will drop by 5 kts or more.

Of course, running in conditions like that are "real world", and mean a lot when selecting the perfect prop (if there is such a thing).

FWIW... I'm doing the same thing with props right now. :)
You defineately lose speed in a chop but you can determine if it is that or you are just losing "bite". Does the engine kind of sound like it "unloads" some or a lot as you lose speed? If so, I think you are ventilating. I had the same issue with a following sea with my black max prop. It is probably caused by air from the water's surface being drawn into the propeller blades. Rake, cup and other aspects of a propeller can affect this according to Mercs site. (see here: ... ema=PORTAL )

It is ashame, because I think I just sold a prop that would be perfect for your boat/power package. It was a Rev 4 15" prop. I didnt want to pm you about it as didnt want to be pushy and you sounded happy in your original thread. I used to blow out fairly often in a tight chop and in turns. My 17" rev 4 has all but eliminated it. (I have had it happen once about 30 hours). The rev 4, because of its rake and cup, can run closer to the surface preventing the blow out situation. Because of its 4 blades, it really bites the water and gives great traction. Email SIM on THT and see if he can send you one. He quoted me about what I paid for mine ($498 or so) and Glen E has implied that he has a return policy-- I am not sure though.

Like I said, you should be able to determine if you are losing speed due to the chop or you are losing "bite".

Good luck