When selecting a prop for my Bass Boat; I knew what the RPM power curve was for my engine; and I selected a prop that would allow my engine to rev to that rpm so that at WOT and trimmed I was operating at the maximum power the engine would produce and not lugging or overreving the engine.

My question is: I have not seen any data on the power curve for a Zenoah 260; Do you guys prop the boat for maximum RPM, how do you know when you have reached the peak of the torque and HP curve.

It would seem you would need a box full of different props.

I have a Tach and GPS; but it is very difficult to get two consistent runs due to water conditions; prop unloading, etc.

Then when you start tweaking the needles you are basically starting over; of course all of this is fun and part of the hobby, just curious as to how some of you more experienced boaters go about this.

I may have answered my own question:

Start with one prop; run and tweak for max.
Change props and see what happens and on and on. One thing at a time.

The engine I am running 260 mod 5, I have had tach readings from 13,500 to 14,400; Boat is an Enforcer Magnum. Props X472 and X470. Avg. speed 35.5 with a best of 39.8.

Which is plenty fast for me; just trying to learn a little more about this hobby, without going broke.

Thanks, Jim

the absolute best way to go about it is find a local club ,there is bound to be someone there with a well filled prop box.

Well, here's my take on it.

First of all, I have never owned an M5 so not sure what kind of power it makes, but I'm sure it's a good engine. That said, I am assuming your are running a tuned pipe on it.

Typical mod engines with tuned pipes like to run in the 16.5k to 17.5k rpm range. (this is with a dry pipe, if you have a pipe with water going INTO it, then it will be slightly less). To me it still sounds you are at least 1000 rpm down from optimal.

Tuning the carburetor is done according to weather conditions more than load conditions, although tuned pipe settings will affect carb setting slightly. In general gas engine carbs are very forgiving and once you get it close and not running it lean or on the edge of lean, you don't really have to mess with them much. So for you basic testing it's almost a set and forget thing...you can come back to it later to fine tune.

Most good mod Zenoahs I've seen actually make there best torque and HP in the 15k range. We usually prop for 16.5 to 17.5 as stated before. So we are indeed over revving them somewhat, but that's good. Boats don't need all that power up top...we need it most when turning and getting back up after slowing down to avoid.

Find a prop that will put you in that rpm range while keeping a fair load on the engine on the way up there and you will have a faster boat I assure you. Adjusting the pipe is a must in tuning also, so you have to optimize it when you try different props, especially if they seem a bit "heavy" or hard to pull...see if you can fix that with the pipe, if not, prop is too much and step down one notch.

That's how I go about tuning my boats anyway. That said, the Magnum is a big boat and fairly heavy so you can't be expecting 60mph...I'm sure it would do a good 45 though. My fullsize boat run about 35 mph and I would pass it easy with my Magnum. I think the wet pipe system is a major factor limiting the rpm of the engine on that setup...it wasn't designed to go fast, was design for big water fun and user friendliness. I had one and loved it!

But to answer you question some of us do use HP and torque curves to help in prop selection. Mix that up with experience and trial and error!

14.4K RPM for a mod engine sounds too low, I ran my Redline 41" sprint cat with a Bob Austin mod 667 prop today, the best speed that I got was 75 mph, Bod did not tell me the pitch of this prop so I could not figure out what the RPM of the Redline motor was.

Theres only one way to tell what rpm you are getting and thats to use a tac,no online gimick will ever give you a realtime rpm rating .Working what hp and prop pitch then feeding it into an online calculater will NEVER give you a real rpm rating nor will it give you a true speed either its all theoretical crap.
Mart

Who said anything about an online calculator... did I miss something or were you just warning not to waste time with those? I agree, they are useless. Power curves (dyno sheets) however are very useful if accurate.

marc wrote:
pretty sure that was directed towards Ken since thats what he uses to try figuring out RPM.


Hey Ken... lets see a video of this Cat with the camera never being taken off of it, so we can see it come in with the GPS reading 75

btw the "M5" Zenoah from WHH is just a mid-mod 260... squish set, intake and exhaust timing adjusted. I would not expect alot of speed, or alot of RPM when propped right out of it. It should be turning more then 14XXX though. My bone stock 260 would turn 15930-15960 consistently with a Austin X472 running right at 50-51mph.

marc wrote:
Sory Marc it wasnt aimed at you but yes i know some use it :? that was my point they are completeley inaccurate.The only way to tell for sure on water is to use a tach.Personally i wouldnt worry too much about it and just tune and tune till you get the speed you want on the water.Regarding the m5 motor its a decent runner but not the most powerfull my mate has a cpl and theyre just a mild mod.Keep tuning it and you will get there.
Mart

INEEDSPOTS wrote:No problem will..75 mph was not a new speed for me, my yellow cat had already been one time last year in July, I'm going to send this prop back to bob to add a bit more pitch cause my engine seems to pull it so easily, will post a vid here once it comes back.