High Thrust Prop Update

[pvanv]

Last season, I installed a new Nissan 9.8 on my 8,000-pound, 28-foot sailboat. I also installed the 5" pitch x 8.75" diameter, 4-blade HT Solas/Tohatsu prop. The tachometer showed that I was turning max RPM's in excess of 6,000, maybe up to 6,200 -- just below the rev limiter point. The correct WOT range for that motor is 5,000 to 6,000.

This season, I changed to the 7" pitch version of the Solas/Tohatsu HT prop, to lower the max RPM's, and possibly gain half a knot (hoping to push the heavy displacement boat to hull speed -- 5, maybe 6 knots). As expected, it did knock the WOT RPM's down, with a slight increase in boat speed. What's surprising is how MUCH it slowed the motor down. Now the tach only makes it up to barely 4,900 or 5,000 at WOT.

So that old rule of thumb for changing pitch and how it relates to RPM is not correct when using 4-blade HT props. A 2" pitch change resulted in a 1200 RPM change. Apparently, my ideal pitch will be 6", which is not a stock, off-the-shelf prop.

 

[steelespike]

Re: High Thrust Prop Update

I'm surprised at the 2" pitch change.I don't think its to do with the 4 bld so much as a smaller motor being more sensitive to pitch changes.
I have noticed that smaller motors usually pitch changes are small around 1 inch.You may be able to have a prop shop repitch to the 6"

 

[hwsiii]

Re: High Thrust Prop Update

Steele you are right, that rule of thumb never did work right pvanv, only for a narrow range. The correct formula involves using the old pitch divided by the new pitch as well as the gear ratio and RPM of the motor. As the pitches get smaller the RPM change gets more and more dramatic. It is a lot like the rule of thumb that says diameter does not matter, because the people who designed the prop used the correct diameter for the pitch. There is no such thing as the correct Diameter for a prop, just like there is no such thing as the correct Pitch for a prop, because it can be fit to sooo mant different boats and motors. The only real constraints on propeller Diameter is the size of the opening the prop fits in, the power pushing it and the blade surface area required to utilize that power. You can use either Diameter or Pitch to adjust the RPM of a motor, or you can use the blade geometry of the prop. That is why i designed mathematical formulas to find the correct props for boats. Here is the formula to show the change in pitch for your prop, and I think I have the right gear ratio for your motor at 2.08.

H

 

[hwsiii]

Re: High Thrust Prop Update

Pvanv, here is what you can expect by changing to a 6" Pitch, you will be turning the motor between 5,400 and 5,500 RPM.

H

 

[wca_tim]

Re: High Thrust Prop Update

The other thing to consider in a more global context is that may so called "rules of thumb" regarding propping and speed, are most applicsable to planing hulls rather thn the displacement hull of your situation...

because because the resistance encountered when trying to push a displacement hull through the water is so large, many times when you're comparing props, the "prop slip" component in the tables and formulas reflected above changes rather dramatically for displacement hulls being pushed by relatively small engines where often for planing hulls with a lot more power, slip numbers don't tend to change nearly as much between props...

just my two cents...

 

[hwsiii]

Re: High Thrust Prop Update

How is it going Tim, my fomulas will never be right for Displacement type hull speeds, you are right. LOL I could rewrite a whole new set of formulas, but I am not into displacement type hulls. You know I have been thinking about our discussion of thrust the other day, and I still cannot come up with a formula for having precise Quantitative and Qualitative thrust numbers for every situation. I have enjoyed our discussions Tim, and hope to have many more in the future.

H

 

[pvanv]

Re: High Thrust Prop Update

Many thanks to all, especially for grinding my numbers through your formula, hwsiii.

Yes, I'm thinking that given the high load, the displacement hull, and the whole picture, a 6" pitch will be the ideal setup. Since I now have two props, I will see if my local prop shop is interested in re-pitching one of them to 6". When I showed them the 5" prop over the winter, they were skeptical that the prop would fit correctly "on the block" for them to re-pitch it.

In the meantime, given my options of 4900 RPM or 6200 RPM, I wonder whether I'm better off below or above the factory-designed WOT range. I suppose that slightly lugging is better than rattling the rods off the motor. Of course, I have no guarantee that the factory tach is perfectly calibrated, but I suspect that it is very close.

 

[hwsiii]

Re: High Thrust Prop Update

Pvanv, I would go with the HIGH RPM, you dont have to use them, but with the LOW RPM your motor is lugging at almost all higher speed RPM and that is definitely not good for it. Just my opinion.

H

 

[eipeldau]

Re: High Thrust Prop Update

Hello Pvanv,
I too have a sailboat, pretty much the same length/weight as yours, and I had the same decision to make, regarding lugging or revving my outboard. When you measure your WOT rpm for both your propellers, I suppose you're doing it under ideal conditions, i.e., almost no wind and no waves. When you think of a WOT situation on a sailboat, it surely involves a strong headwind with waves that you need to defeat. Under this situation, at WOT you will not reach the 6200rpm that your 5" propeller gives you under ideal conditions anymore. That's because your boat will not move at 5-6 knots anymore, but at just 2 or 3knots. Your theoretical 6200rpm will automatically drop to, let's say, 5200rpm(your desired target). As an experience, try WOT with your boat docked(make sure your mooring ropes are well tied!!!) and measure the rpms. A storm situation would be something between docked and freely moving with no wind/waves. The bottomline is: the environmental conditions will naturally lug down your outboard. Unless you'd WOT under ideal conditions(that's not the usual case for sailboats), you will never get it to 6200rpm. On the other hand, if you use the 7" propeller, under a storm situation your rpm could drop to something like 4200(really lugging) at WOT. I hope I could make my idea clear to you. Let me know if I can help any further!
Good winds!
Henrique

 

[steelespike]

Re: High Thrust Prop Update

I agree with Henrique.Your motor is better off with the 5" pitch for now
Just try to run it at a more reasonable rpm than the wot(wide open throttle)
I too think it will respond better in a high wind/current ,load situation. Good luck,hope you can maybe get one repitched to 6".

 

[pvanv]

Re: High Thrust Prop Update

Hello Pvanv,
I too have a sailboat, pretty much the same length/weight as yours, and I had the same decision to make, regarding lugging or revving my outboard. When you measure your WOT rpm for both your propellers, I suppose you're doing it under ideal conditions, i.e., almost no wind and no waves. When you think of a WOT situation on a sailboat, it surely involves a strong headwind with waves that you need to defeat. Under this situation, at WOT you will not reach the 6200rpm that your 5" propeller gives you under ideal conditions anymore. That's because your boat will not move at 5-6 knots anymore, but at just 2 or 3knots. Your theoretical 6200rpm will automatically drop to, let's say, 5200rpm(your desired target). As an experience, try WOT with your boat docked(make sure your mooring ropes are well tied!!!) and measure the rpms. A storm situation would be something between docked and freely moving with no wind/waves. The bottomline is: the environmental conditions will naturally lug down your outboard. Unless you'd WOT under ideal conditions(that's not the usual case for sailboats), you will never get it to 6200rpm. On the other hand, if you use the 7" propeller, under a storm situation your rpm could drop to something like 4200(really lugging) at WOT. I hope I could make my idea clear to you. Let me know if I can help any further!
Good winds!
Henrique

Many thanks to all the prop and sail experts, especially Henrique. To a man, everyone, including the Tohatsu Guru, agree that it's better to rev than to lug in this situation. I was thinking the same thing, but wanted to gather opinions from those with perhaps more experience.

One test that I had not considered was checking RPM at 100% prop slip, i.e. while tied to the dock. Then I will have both WOT under "ideal" conditions, and a second "worst case" point of reference RPM. While that probably could not be done with larger motors, at 9.8, I should be able to accommodate the thrust at my own dock, using appropriate dock lines and cleats. Then I will be certain that my day-to-day, "against headwind and waves" operation will fall somewhere between the "best" and "worst" RPM -- in other words, right in the middle of the WOT power band, just where the motor is designed to operate. Decision made, Problem solved.

 

[swiss]

Re: High Thrust Prop Update

Paul make sure you report your WOT testing if you tie up to the Dock, love when theory and reality meet!

swiss