Prop Testing on an Underpowered boat

[nodima]

There have been several threads around unpowered Quabbin boats, so I thought getting another data point out there might help someone. Today, I had a chance to test two props on my Lund.

Background: the boat is a 2004 16' Alaskan tiller, with a 25 Merc bigfoot. It was loaded with me (185 lbs), a half tank of fuel ~10 gal, 2 batteries, bow mounted TM, and misc supplies and gear (unknown weight).

First prop was a new 4 blade 12 x 9 alum. With this prop, I saw 17 knots, but only 5100 RPM's. I tried this several times on different tilt settings, and this was the best performance.

Then I switched to a 10.88 x 11 alum 3 blade prop. I got basically full RPM's (~5650) and saw 19.4 knots with the same tilt setting that worked best with the other prop. I did not test other tilt settings with this prop. In the past this prop has produced 20 knots and change for me--but the load could have been much less.

I was really surprised that the extra inch of diameter and the 4th blade would not gain RPM's despite having 2" less pitch.

Initially, the hope was that the 12x9 prop would help run the boat better with additional people aboard, but it does not look like this is a better real world solution than is my existing 10.88x11. I did notice that the 12x9 FELT like the boat was not as deep in the water, and it may have been marginally faster to plane, but I did not time it.

Last weekend, I had the boat on the CT river, and with one other guy (~180 lbs) and similar gear, was going upstream at about 12 and downstream at about 15.5. It was very windy, and the downstream run was right into it. Given the strong currents this would extrapolate around 14 knots, which is a knot and a half LOWER than what I have seen with the 10.88x11.

Next step is to see if I can exchange the 12x9 for either a 12x8 4 blade or for a 9 pitch 3 blade model. Any advice as to which might be a better choice?

Thanks

 

[hwsiii]

Re: Prop Testing on an Underpowered boat

What year is the motor and what is the WOT RPM and gear ratio. I show 6,000 RPM and gear ratio of 2.4, but that is wrong.


H

 

[nodima]

Re: Prop Testing on an Underpowered boat

WOT on the motor should be 5750 according to the sticker, and you are really close on the gear ratio 2.42 is the sticker on the lower unit. Motor is a 2005

thanks

 

[hwsiii]

Re: Prop Testing on an Underpowered boat

Nodima, either that 4 blade prop has a lot of cup on the trailing edge, or the numbers you gave me are wrong, maybe because you only tested the prop in one direction and that was with the current, i did notice you did another test running both directions though, so you understand the theory. Cup on the trailing edge of a prop adds theoretical pitch to a prop and makes it act like it has more pitch than it does.
The 4 blade shows a negative 9% slip and that is impossible, and the three blade shows 8% prop slip and that is excellent. Here are the numbers.


Prop Slip

Neither one of your two choices are a good idea, and here is why. You have a Bigfoot, it is designed for large diameter props and for use on heavy weight boats to push slower but with enough surface area to push that weight. You have a 2.42:1 gear ratio designed to handle the extra surface area for these props and heavy loads and the prop aperture is large enough for these large diameters. Most of the reason for using 4 blade props is for more blade surface area and you are able to just use a larger diameter prop which has more blade surface area without having to have a lot of rake or other blade geometry features to do this. You have a relatively small HP engine to turn these larger diameters, so even with a 3 blade prop you will get good prop slip numbers, and your lower unit is larger in size than a normal 25 HP engine so it creates more drag than a normal lower unit would.
So even changing to a 12 x 8" 4 blade you will not gain any speed with the rise in RPM to about 5,700 RPM you will get beause of the lower pitch . And if you change to 10 1/2 x 9 in the 3 blade you will just overrev the engine to approximately 6,800 to 7,000 RPM and still not gain any speed, except the engine may blow up, and it certainly won't last long.


Prop Change

The first thing I would do is raise the motor as high as you can on the transom without creating ventilation problems when you make faster turns to the right and left.This will require some kind of spacer between the top of the transom and where the motor mount sits. If you are able to gain enough RPM to make it worthwhile you will then want to add something like an aluminum cap over the transom to be able to permanently mount the motor in that position. With a bigfoot this is definitely a place where you can gain speed because of less drag from the lower unit. Try it only with the 3 blade because that is all you need, then if you are able to get enough higher RPM you can try changing to a 10 3/8 x 12" pitch Michigan vortex to get more speed. Theoretically it will require a loss of about 450 RPM from the 11" pitch you are running now, but by moving the motor up i expect you to gain at least 200to 300 RPM and that will help offset the 450 RPM loss and should gain you extra top end speed, but I am not positive your prop slip won't go up some.

Hope this explains my reasoning, but if you question it just give those other props a try and we will see what happens, you really have nothing to lose except a little time, just be careful and don't overrev the engine too high.


H

 

[nodima]

Re: Prop Testing on an Underpowered boat

Thanks, that is a lot of great info for me to chew on.

The test where I got the numbers was actually on a small lake, with little wind.

Great stuff. Thanks for you time, I really appreciate it.

 

[hwsiii]

Re: Prop Testing on an Underpowered boat

You are very welcome, I hope this helps you make a decision of what is probably the best course of action for you.


H

 

[nodima]

Re: Prop Testing on an Underpowered boat

It will--that said, the main body of water I like to access is currently closed to all private vessels due to an over reaction to zebra mussels found in a different body of water in my state (MA), and if the worst case scenario comes to fruition, I may just bite the bullet and sell the 25 and buy a 50 (Max for the hull) and cause myself some new issues.

Again, thanks a lot!

 

[hwsiii]

Re: Prop Testing on an Underpowered boat

You are welome.


H