Having problems finding exactly the right prop

stevens · Jul 23, 2005

Hi guys, I am having problems finding the exactly right prop and kindly ask for your input. Thanks to all those who have helped us so far. Here's the situation: We have a 21' daycruiser with a 120 hp Mercury turbodiesel engine coupled to an Alpha outdrive with a gear ratio of 1:2. The engine is rated for 4400 rpm, and the rev-limiter kicks in at 4450 rpm. The boat now has Smart Tabs, which have helped on the holeshot and on the porpoising. The boat will plane at 12 knots and be cleanly on plane at 15 knots. We use the boat mostly for carrying the family of 4 plus all our stuff around in the local archipelago. We usually like to go at either 5 knots (no wake zone) or 15-25 knots (slow planing speeds), and the wife and kids aren't very comfortable going much faster than that, at least not when there are waves. No skiing or such, at least not yet, but I occasionally get to go flat out for a while when we want to get somewhere in a hurry, or when I am alone in the boat. The boat is stern heavy and needs to be trimmed exactly right so that it won't either plow or porpoise. Pictures of the boat on: http://home.broadpark.no/~ssjoborg/ http://home.broadpark.no/~ssjoborg/Smart Tabs Installation/index.html The props we've tried so far: It came with a 14.5" x 23" Black Max, an unvented three-blade aluminum prop. With this prop, it turns 4300 rpm and does about 35-36 knots on a good day. With a full load, it does about 31-32 knots. OK ride but I can't trim it much out before the boat starts to porpoise. Kind of "mushy" feel to it as well. I then tried a 14.25 x 21" Ballistic, unvented three-blade SS. Great holeshot, but it went straight to the rev limiter at 4450 and 34.5 knots with a light load on board. Obviously too small a pitch. It also ventilated very easily, making it hard to trim far out, and it would blow out a lot. I moved on to a 14.25 x 23" Turbo, unvented three-blade SS. Great prop in every way, except the pitch was too high. I could barely get it to 4100 rpm and 36.5 knots with little load, and 3950-4000 and 32-34 knots with a full load on board. Holeshot was also a problem as it would take several seconds to reach the rpms where the turbocharger kicks in and take-off would begin. The ability to trim out was great, and I had to really push it before it would ventilate. I've now sold both the Ballistic and the Turbo, but am still not quite satisfied with using the original Black Max alu prop. Given this, I am wondering what to get next. I am thinking either a 22P vented three-blade Turbo (Walleyehed's original recommendation) or a 21P vented four-blade Turbo. Does that sound right to you, or would you suggest something else? As you can tell, we don't want to fiddle around with this much more, but would still be open to one more try. Any help or advise would be appreciated.

craze1cars · Jul 23, 2005

Re: Having problems finding exactly the right prop

You liked the 3 blade Turbo but need about 350 to 450 rpm depending on your load. So theoretically a 21P in that same product would do that for you, but would put you very close to your rev limiter with a light load and would certaily give you max holeshot. A 22P would put you a bit short of your rev limiter. A 4 blade? A total unknown as you haven't tested one yet...higher risk of it being completely different from anything you've tried. I'd buy a 21P AND a 22P Turbo 3 blade, test them both, and return the one I liked the least.

stevens · Jul 23, 2005

Re: Having problems finding exactly the right prop

Thank you, that sounds like good advise. However, there is a slight obstacle to your plan in that I will have no possibility of a return. You see, I'll be buying the prop somewhere in the US and have it shipped to over here (half the price of buying locally). If it doesn't work out, I'll have to sell it on the local equivalent to ebay. So, I better get it right the first time! (eh, that would be the third time...) Any reason why a 21" Turbo would have slightly lower rpm & go faster than a same pitch Ballistic? Or, would it?

Speedwagon · Jul 23, 2005

Re: Having problems finding exactly the right prop

Do you not have a place that sells props there? I would think that after shipping, it might be cheaper to just buy a prop locally, than to have 3 of them shipped overseas.

tommays · Jul 23, 2005

Re: Having problems finding exactly the right prop

i never found and exactly right one just a coupple of different ones depending on how i amd going to use the boat that day on any given day a prop that gives your correct WOT is going to be affected if your weight changes + OR - 100 kilos or its windy or calm in my case i hate the canvas my wife loves it up it makes a big differece in how the boat handles and it loads the motor a lot more tommays

stevens · Jul 23, 2005

Re: Having problems finding exactly the right prop

Edit: It really is worth it to ship the prop over if I can find one at the right price in the US, as new props are very expensive over here. But, the whole process is a hassle, especially if I must sell it again, so I'd appreciate your help in getting it as right as possible the next time. Again; any reason why a 21" Turbo would have a lower rpm & go faster than a same pitch Ballistic? Or, would it?

dingbat · Jul 23, 2005

Re: Having problems finding exactly the right prop

Try a Stiletto from Overtons. They have a liberal return policy and it will cost you only you the price of shipping to exchane the prop for another. overtons

shep70057 · Jul 24, 2005

Re: Having problems finding exactly the right prop

That same size turbo in the smaller pitch will let your motor gain approximately 400 RPM's. Each inch in pitch will be approximately 200 RPM's. So if you liked the 23 pitch turbo but it was too much prop, the 21 will get you to the high end of the rpm range of your motor. If this winds up being the right prop for you, bring it to a local prop shop and have them drill vent holes so that you can get to your powerband to help holeshot although going with a smaller pitch prop will help it also.

umblecumbuz · Jul 24, 2005

Re: Having problems finding exactly the right prop

After reading all this, what's wrong with Kenny's original recommendation of a 22P vented 3 blade Turbo? Or for that matter, a 22P 3 blade Stiletto? And I do what you're doing Steve. I get my props and other stuff sent from the States. Here, they've never heard of prop shops, and only a few lads on the whole island know what you're talking about when you talk vented props, selective cupping, etc. The dealers certainly don't. It's a big uphill learning curve! As it happens, I had a problem similar to yours, so I listened to this forum, especially Kenny (Walleyehed), and got lucky. I hit the right prop (Stiletto) first time, and I'm well pleased.

stevens · Jul 24, 2005

Re: Having problems finding exactly the right prop

Stillfishing - glad to hear your Stiletto worked out well. Would you know how the Turbo and Stiletto are supposed to differ? Here, there is only one very reputable prop shop, and they are on vacation for the whole month of July....... No, I don't think there's anything wrong with the suggestion of a 22P. I just haven't found one yet on ebay, where I stumbled across the 23P first. I am also starting to think that a vented 22P might be the right compromise between holeshot & top end speed. While it theoretically would only take me to 4300 at the most, it will at least not hit the rpm-ceiling. I am afraid, though, that it too will be too heavy for the engine. But, I am hoping someone like Walleyehed or Dhadley will reappear soon to correct me if I am wrong on this. Still vacation time in the US, I guess. Thanks again for all the help so far.

ufm82 · Jul 26, 2005

Re: Having problems finding exactly the right prop

I'll try one more time. The differences in brands have a lot to do with performance. Rake, blade thickness, cup, pitch progression etc. all have an influence on performance. Are the diameters all truly identical? Blade surface area? There are a lot of variations between brands. I'm not going to try to argue again with walleyehead but venting won't make a noticeable difference on a stern drive. That's not just my opinion either. But if you want to try one, buy a prop that you can change the port size on, like a PVS Mercury prop. Nice looking girls you have on your boat. I'm impressed. UFM82

stevens · Jul 26, 2005

Re: Having problems finding exactly the right prop

Nice looking girls you have on your boat. I'm impressed.

Yeah, right, and it's sunshine 365 days a year, gas is cheap and the beer is always cold around here.... Thanks for your input, though. I find this whole subject to be increasingly confusing. It comes down to trial and error, I suppose. The following listing on ebay (e-bay link) (item no. 4562370132) was the one that really made me realize that this is not an exact science. If he can't turn a 20" 4-blade Turbo with his 200 hp engine, what does that imply for my 120 hp? So, I've posed the same initial question to Tech Support at Stiletto Propellers and look forward to hearing what they say as well.

ufm82 · Jul 26, 2005

Re: Having problems finding exactly the right prop

Keep in mind that even when people think they can't turn a prop, it may not be the prop. Engine performance issues, improper use and even poor maintenance on the boat itself can have an influence. Also, although I can turn a 23 Tempest at 5,600 RPM in the Gulf of Mexico, I can only turn it to 5300 here in Ohio. (800 feet above sea level.) It would be even worse if I went to Colorado and tried to boat there. He even said he "wasn't developing enough power to turn it efficiently". That could be a myriad of things. Weight in the boat, load, water conditions, weather, temperature, humidity, etc. all cause problems with finding the "perfect" prop. I have come by my knowledge thru trial and error. I will say that although the boats aren't identical, I did run a 20' cuddy a few years back that ran a 200 hp Mercruiser I/O. I ran both a 23 Laser II on it and a 21 Hi-Five prop and both ran right at 50 mph at WOT. The Hi-Five gave me a killer holeshot and great handling while the Laser got me to top speed faster. It did allow me to trim up further than the Hi-Five did and that was a neat feeling, having the boat come up out of the water. With yours being a lower hp powerplant that is strong on torque, I am still thinking a large diameter prop would be good, like a Mirage. Good luck- I hope you find your prop without spending a ton of cash. UFM82

umblecumbuz · Jul 26, 2005

Re: Having problems finding exactly the right prop

Hi Stevens, I'm not a prop expert, and I can't say with authority whether a Turbo will perform better than an equivalent Stiletto. But ... the Stiletto I bought hit exactly the right spot for my boat. I looked at that ebay link. A lot depends on the guy's hull, his motor setup and condition, so there's no way of making a judgement based on the information he gives. We really DO have 365 days sun every year here, which is why there's no set boating 'season' - we're out all year round (current sea temp is 25 degrees!). Gas is genuinely very cheap, and the beer is free. I lied about the gas.

tommays · Jul 26, 2005

Re: Having problems finding exactly the right prop

there are probably at least 50 different 21 in props on the market and then there is mercs custom shop which costs a lot but at this point sounds cheeper than what your trying now Contact Scott Reichow at 920-924-2037 for further information regarding Mercury Racing Lab Finished Propellers. scott_reichow@mercmarine.com tommays

stevens · Jul 27, 2005

Re: Having problems finding exactly the right prop

I've now received an OK offer for a 22P 4-blade vented Turbo. How do you think that would work? I am afraid that it might still be too much to turn. A year ago, a guy called Tom2697 posted the following:

Each inch of smaller pitch will increase rpm by about 200. Losing the 4th blade should also gain you another 100 rpm. You also need to look at the diameter of the props. Every 1/4 inch larger in diameter lowers the rpm by about 100 rpm.

If this is right, I should end up at 4200, which is a bit low, I guess. Another 1-200 would have been OK, so a 21P four-blade or a 22P three-blade? Remember, the 23P 3-blade Turbo took us to 4100 rpm and 36.5 knots with little load, and 3950-4000 and 32-34 knots with a full load. The rated RPM is 4400. The 21" 3-blade Ballistic maxed out.

stevens · Jul 27, 2005

Re: Having problems finding exactly the right prop

OK, just spoke with tech support at Precision Propellers. They said: - to go from a 3-blade to the same prop in a 4-blade, you need to drop one inch of pitch to remain at the same rpm. - a 22P four-blade will turn about the same as a 23P three-blade. - every inch you drop in pitch will increase RPM by 150-200, but this is not an exact science. With me turning 4100 with a 23P three-blade Turbo, I could then get either: - a 20P four-blade topping out at 4300 - a 22P four-blade to remain at the same RPM of 4100 - a 21P three-blade hitting the redline of 4450 rpm - a 22P three-blade topping out at 4300 rpm I guess these are now listed in order of speed as well, from slow to fast. The Turbos aren't factory vented (so that could be a shop-job next winter if needed), and the Stilettos don't come in a three-blade 22P. I guess it boils down to either an unvented 22P Turbo or a factory-vented 21P Stiletto. Decisions, decisions!

walleyehed · Jul 27, 2005

Re: Having problems finding exactly the right prop

It's VERY easy to vent the prop... I guess I should say I feel a bit bad about the 23 turbo not working as you wanted... The posting of dropping 1" in pitch when goingto a 4-blade is really only true if the manufacturer of the 3-blade is refering to HIS 4-blade as well. If we change manufacturers, I've seen 3 blades in a certain pitch pull harder(lower rpm) than another manufacturers 4-blade of the same stated pitch. The best info we have is the data you have provided with the 3 props, and it's GOOD data. The question came up of stiletto vs. Turbo...well, the Turbo is a bit more aggresive design, and to quote Dhadley(hope he don't mind!) "The Turbo has a bit more technology built into it"...not only compared to stiletto, but most mass-produced SS props as well. I'll do anything I can to help you get the right prop, and we will get it...you're just going through the "fine-tune" stages, and it ain't always a perfect prop the first time. we'll get it nailed....

stevens · Jul 27, 2005

Re: Having problems finding exactly the right prop

Hi Walleyehed, and welcome back to the discussion. No, please don't feel bad about the 23P not working out. You first suggested a 22P, it was just me who stumbled across the 23P on ebay and didn't have the patience to wait. So, what do you all think - should I try the 22P 4-blade Turbo available now, or rather go for a 22P 3-blade Turbo or maybe even a 21P 3-blade Stiletto? Maybe if I'll wait long enough, they'll be on an off-season 3-for-1 sale and I could get them all ....

umblecumbuz · Jul 27, 2005

Re: Having problems finding exactly the right prop

Quote from Tech Support at Precision Propellors:- every inch you drop in pitch will increase RPM by 150-200, but this is not an exact science.

Too true. It's a bit like quoting deadrise as though it was a fixed figure, when we should understand that deadrise varies depending where it's measured along the hull. The pitch of a prop varies depending where across the blade it's measured. This is just one of the reasons why props from different manufacturers vary a lot even if they're quoted as all having the same pitch.

Having problems finding exactly the right prop

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