Correct, but you do not want to over prop, that is the same as lunging the engine all the time. Take the time and get you speeds at more then WOT, there is way more to boating than that. You may find the fastest prop is not the best for getting out of the hole or mid range speed.

Lots of variables with props.

spinning it faster than peak power with less pitch might still be the fastest solution

I had a 28' Advantage that was an 80mph boat. I almost always ran a labbed 28" prop, but the fastest it ever ran was with a stock 26" bouncing off the rev limiter, which was set pretty high. The engine didn't make more power at 7000 rpm, but the boat was faster there and sure sounded good. This was with a hopped up 575SCI

Since torque peaks way below power, lugging an engine puts significant stress on the rotating assembly, as it is subject to much higher forces at WOT when the rpm is too low. E.g. the wrist pins eat into the pistons.

In addition, it increases the risk of knock.

Markus,

I wouldn’t suggest just putting too big a prop on the boat and living with it. I would assume that PQ290 would plane the boat out one time, decide a given set of props were too much and stop.

My point is that, in my opinion, it is far safer to increase the load until you hit your target RPM just at WOT position on the stick, rather than stabbing the throttle with too small a prop. I think a couple seconds of too heavy a load is far less damaging than finding the limits of rod integrity while screaming the engine while under loaded.

Thanks. Brad.

Hogie,

I’m am not an engine guru, by any stretch, but I have always been under the impression that everything beyond peak power is leading into either a lean condition or valve train distress, which is what causes the decrease in power beyond the peak. I have always understood that the the sweet spot between reliability, drivability and performance was to load the engine to tach out just to the end of the apex of the HP/torque curves. Intentionally loading the engine to tach out above or below this RPM is giving something up in either reliability, drivability or performance.

Thanks. Brad.

Peak power is function of flow and valve timing, intake and exhaust runner resonant frequencies. Valve train stability is determined by parts selection. You aren’t necessarily going lean past peak power.

Your fastest prop will probably reach peak speed beyond your peak power because you’re running less pitch which gives the engine effectively more “torque” where the water meets the prop. Assuming your power doesn’t drop off dramatically.

Nearly every stock Merc application has its fastest speed far beyond the peak power and torque points.

Some fast hulls free up at speed, so you really just need engine speed up top to keep them accelerating, more so than power.

I have a line on a pair of B1 props that are 28". I'm just going to go for it and see what happens. I have read through the posts here and now have more questions. Lets just say for the sake of this question that with the 28's it's hard to get on plane but mid range and WOT are great and at WOT the rpm is right where I want it @ 5000. I'm thinking these would be a good match for my boat\power. When you guys say "hard to plane" are we talking 30 seconds, 5 minutes, what is acceptable and when does it start to put unnecessary strain on the engines which is something I never considered. I usually throttle up to about 3000 rpm and it rolls over pretty quickly and I don't use the tabs to get on plane. I assume the 28's will be different.

First off....if the 28's work for what you want you could have them tweaked to help with planing if that's an issue

Second...I would not want the motor to live in the peak torque band...you are asking for drive issues IMO....that's a lot of stress on the drives at peak torque trying to push the boat to top speed.

Third...could always drop a little tab to help get on plane if needed.

As for everything else...if you are worried about hurting those motors at anything over 5K then you should seriously think about rebuilding them with better parts....I know we've been down the long horrifying road of your builds in other threads but to me any BBC that's built with anything better than stock parts should spin 5500+ all day long with ease....a black merc motor can do that with cast internals in some. I wouldn't worry about the motors personally if they were built and tuned right. If you are leaning out up top then you need more jet simple as that as I think I remember you being carb'd...can't remember at this point lol. Break them dudes in and let em eat!

If it was my boat I would prop them to run at 5500-6000 which would be probably about what you have and you'll still have the low end with smaller props and I'm betting higher top speeds...all about what you want out of the boat. Just my thoughts.

Lol!!! Was it necessary to bring up the "long horrifying road of your builds" This rpm thing may just be me. What started the "long horrifying road of your builds" was that everyone saw what they thought was the valve train in distress and the dyno stopped at 5600 so running in that rpm range if there is a problem in the valve train would be a disaster. My builder said no distress and the 2nd shop that did the dyno said no distress. I'm going to see where the 28's land and I'll get back here and post.