High-Thrust Prop?

[pvanv]

This may be a little off-topic, but here goes:

I am using a Nissan NSF 9.8 A3 long electric remote on a 7,000-pound sailboat. It replaces an old, tired, points-and-condenser 2-stroke Evinrude 15.

I bought the rather radical-looking High-Thrust 4-blade 5-pitch prop that Tohatsu sells for this motor. The low-speed maneuvering is spectacular. Much better that the old 15 with a "sail" 3-blade 5-pitch prop. And it backs up -- something that a typical 3-blade prop does not do well on a sailboat. I've got to say that overall, everything is a great improvement. Lower vibration, much quieter, starts and runs very smoothly, and double the fuel economy.

Of course, "high speeds" are not part of this picture. The hull speed of this displacement-hull boat is about 6 knots. It doesn't go faster than that unless it's surfing.

My tach shows that I am turning 6200 or so at full throttle, while making a boat speed of 4.5 knots. I would like to go 5 or 6 knots, but don't want to sacrifice too much of that wonderful low-speed responsiveness. Stands to reason that I probably want the RPM down about 750 or so at WOT. I am thinking that I could stand some more pitch. So then the question becomes, how much more?

My local prop shop says that they aren't sure exactly how much to increase the pitch, but that they can re-pitch my prop, say to 6 inches, for a good price.

I see that Tohatsu now sells a 7-pitch version of the High-Thrust 4-blade prop, in addition to the 5-pitch that I have. Will that be too much pitch? Maybe I should buy a new 7-pitch prop, and if that lowers the RPM too much, or hurts the low-speed thrust too much, then I will know that the "magic" pitch is 6. Of course, then I will have an extra prop...

 

[mthieme]

Re: High-Thrust Prop?

No matter how much math you see cross this post, you've already answered your question.
Each setup is unique and it boils down to trial an error to get what's good for you. Unless someone has the same exact setup or been through the same exact thing.
An option used in my neck of the woods is to simply borrow a prop and test. Since your setup is really unique, I doubt this is an option.
I suspect you're going to wind up with a spare prop (or two!).
You could always eBay the extra(s).

 

[bowman316]

Re: High-Thrust Prop?

i ended up buying 2 extra props for my boat, until i got the right one, and i sold one on e-bay, so now i only have 1 extra prop. You should always have a spare prop, unless you want to see if you can sail back to the dock.

 

[BillP]

Re: High-Thrust Prop?

Normally to get higher hull speed while maintaining the same max rpms...increase the pitch and decrease the diameter. There is no magic formula for this, especially on a 7k displacement hull with small high rpm engine. Winds and waves play a major factor in hull speed when having a rig like yours...power (less prop slip) is needed to slug though. That's where your large diameter with flat pitch will give best results. You may be able to just increase the pitch with same diameter and see if the motor is still in the motor mfgs WOT rpm range.

 

[pvanv]

Re: High-Thrust Prop?

Since the price of re-pitching and a new prop are about the same in this case (well under $100), I think I'll get the 7" pitch and see what happens. Seems pitch changes are always a bit of a crap-shoot, as every rig is a bit different. But the tach does indicate that more pitch is in order. I can foresee three possible outcomes of going from the 5 to the 7:

1- It might give me more top speed without too much loss of low end. This is what I'm hoping for.

2- It might kill the low end. If so, I'll have it re-pitched to 6, so it works more like the 5. Then I'll use whichever one gives the best overall performance.

3- At this low horsepower, it might not make much of a difference at all. This would be the worst scenario. Then I need to decide whether I want to keep a desk ornament of the spare prop or dump it on eBay.

I suppose a spare prop is nice to have, but I haven't needed one yet. The boat draws more than 4', so the keel would ground before the prop hit the bottom. I have sailed for many years without any motor at all. Getting back to the dock without the aux CAN be an issue, depending on the weather. Then too, I suppose it's possible for the rubber hub to fail due to age and strain one day...

 

[BillP]

Re: High-Thrust Prop?

30+ yrs ago I had a rig similar to yours...a little heavier but same situation. 10k lbs (H28 ketch) with a (pos) 9.2hp Chrysler sailboat version with 25" shaft extension. I tried several props and eventually ended up with a stock prop. It loaded the engine but I didn't care because it gave best overall speed with less fuel burn. It ran hull speed at about 2/3 throttle. The biggest problem was keeping the prop in the water if I needed to motor when rough. It had plenty of power for normal motoring but making headway against 20kt winds and 3' chop was hopeless.

bp