Prop selection chart?

Is it possible that a manufacturers prop selection chart could be off or they just don't account for the possible performance of all props? Case in question: I have a brand new Alumacraft 1442 NCS flat bottom-230lbs, a Tohatsu 20hp 4 stroke-114lbs, myself-135lbs, my 2 young sons-80lbs, anchors and gear-50lbs. Total of 609lbs. According to Tohatsu's prop chart this puts me at a "moderate" load which they claim rates a 9 or 10ish pitch prop to run right. I'm running a SS 9 1/4 x 10 and I'm hitting the rev limiter at this load. Could the performance of this prop be that much higher to where they just don't account for it and I need an 11 pitch if I stay with this brand or am I reading this wrong? Please help this problem has been plauging me for a while now and I thought the new boat I just bought weighing a little more than my old one would help to balance it out but apparently not.

Re: Prop selection chart?

Just to add in a little more info, with the above stated load and the new Solas Saturn 9 1/4X10 that I'm running on it, it was hitting the rev limiter at over 6300rpms consistently everytime. It didn't do this with the older version of the Saturn 9 1/4X10 I had before unless I ran really light (by myself with almost no gear). That prop was damaged so I ordered a new one and received the "new saturn". With this prop now I've had to move the throttle linkage set screw so that the butterfly valve only opens to about 80% just to get the rpms to run at 6100, which is still at the very high end of the WOT range. I'm just confused because everything I'm reading and hearing says that I should be running right with my 10 pitch but the data I've actually gathered and the reality of what is actually happening would suggest otherwise.

Re: Prop selection chart?

This may be a dumb question but are you sure the new prop is a 10 pitch?(they can be made wrong,boxed wrong,stamped wrong...)
If you still have the old prop hold them together and see if theres any difference.
Sounds like you plan on having a moderate load most times?(if you went with the 10" pitch seeing as you could hit the limiter before)
Prop charts will get you in the ballpark,a good working tach will decide the proper prop.

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Its not a dumb question. I did compare the 2 props next to each other. The blades appear to be pitched at the exact same angle. The blades on the "new saturn" have a slightly larger ear curve to them, the flange effect at the exhaust is not as large and the whole prop itself feels a bit lighter than the old one. The brochure from Solas that came in the box says the new saturn was made to accommodate heavier loads. The new one is stamped TA9 1/4X10 on the side and the old one just 9 1/4X10. If this new prop were pitched for 11 it would probly be just about right but I don't want to drop a chunk of money on another one and find out that I receive the old one in 11 which might be too much pitch. The part numbers are the same from what I can tell. I'm still just baffled at how the prop chart could be that much off, even considering a stainless over aluminum. Tohatsu says an 11 pitch on the 20hp should be used for loads under 500 lbs. I'm definately over that

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I wouldn't be all that baffled as you say,your only a pitch off(ballpark).
Did the prop calculator take in the type boat your using?
A deep V will use a different prop compared to a flat bottom of the same weight.
If it were me,I'd rather try a new prop(11)then use my outboard with a governer...could you borrow a friends or see if a seller would let you try one out first before buying?

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I have a Merc 20 HP on a 14' MV flatbottom and carry about the same loat as you are. I use a 10"X 13 pitch on mine and it runs within the limts great. See if you can use a prop for testing and go from there. What is the recommed WOT RPM for your motor? One inch of pitch increase should lower your RPM around 500 RPM.
Oldman570

Re: Prop selection chart?

Flat bottom boats can be fast and will generally plane quicker and faster than softer shaped hulls.
Also if you think about it there must be a point where a light load and a moderate load are only a pound apart.

Re: Prop selection chart?

Thanks for the info guys. I suspected that the Tohatsu prop chart may not be "solid numbers" since it only goes off of weight, but I wanted to hear some other opinions to make sure that I wasn't way off base on this. With a total load of just over 600 lbs, it was hitting the rev limiter at around 6340rpms with the carb fully open. I had to adjust the throttle linkage set screw to where the carb only opens about 80% just to get it to run at 6100rpms with this load. This is the max that the motor is recommended to run at. My thinking now especially after hearing what some of you have said is that I'm definately ok to move to an 11 pitch ad this should allow me to be able to use full range of carb operation once again and still be somewhere near the upper range of rpms. Maybe I can keep the old prop and use it for when I know I'm going to carry heavier loads. Is there some type of quick disconnect cotter pin that allows easy change of proppellers without having to trash the old one and bend a new one each time?

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I change props fairly often and use the same cotter pin almost all season...1/2 tops,unless I lose it in the grass.
I'd say an 11 but maybe even a 12 pitch prop...
I drop from a 13 to a 11 pitch and only loose alittle less then 500 rpm.

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I think the most I can get for the 20hp is an 11 1/2 pitch but I would hesitate to go past 11 anyways cause I don't want to sacrifice too much acceleration. Right now its very responsive and jumps up on plane in a heart beat so I can probly sacrifice a little of that if it means running within range. If the 11 pitch can even drop it 200-300rpms than it should do the trick