In depth - Prop depth question? (punn intended)
 

I can definately see how propshaft depth in relationship to the bottom of the boat, or other parts of the running surface of the boat (like our stepped transoms), could have an effect on prop bite or slip. This measurement can be measured in a variety of different ways.

Question: How do you think boat weight plays a part in prop depth in relation to the water and prop slip?

ie. two Nordic Heat's with the same x-dimension. One weighs 10,000 lbs. the other weighs 100 lbs. The heavy boat would have the prop deeper in the water than the light boat. The heavy boat probably would not have problem grabbing water. The light boat would now have a prop that was more in the air, making slip very high. The light boat would have less drag for sure because less boat was in the water, but the slip would be really high because the prop was so high.

I am thinking that changing the prop depth with shorties or spacers needs to be adjusted in some relation to how deep the prop is in relationship to the water, which has some to do with the weight of the boat, and some to do with how high the boat sits on top of the water while planing.

I know that when I went to an all carbon hull on my pro jetski, the boat would sit almost 1.5 inches higher in the water than my other boat with a production hull. I had to manufacture a much deeper scoop on the intake to the jet pump in order to get the carbon boat to hook up.

I think that there are several reasons why some of Heats like prop spacers, and some don't. One is propably x-dimension, the other is probably boat weight.

So do you think these lighter (wood-free) Nordics are going to need some prop depth adjustment because they sit higher in the water?

I would love to hear some thoughts on this.

Chris

To continue the saga:

Boat was un-runable with stock prop depth of 3.5 inches from bottom of keel to prop shaft centerline.
Boat ran much better with 1" spacer making prop depth 4.5 inches from bottom of keel to prop shaft centerline.

Third outing in my new Heat.

Ran my boat with 3/4 of a tank in each side (more fuel than I have ever run), and six people in the boat. Two guys in the buckets, and four skinny ladies on the rear bench. ie... more weight in the rear. Fairly smooth water. Slip went from .143 to .124

Progress:

Initial boat speed = 78.4 mph 19.2% prop slip
1 inch spacer speed= 80.1 mph 14.3% prop slip
3/4 tanks and six people = 82 mph 12.4% slip

Any thoughts?

Try a P5x ?
Try a 1.5" spacer?

Chris

Here it is :eureka:.............
Steve

Odd..................

Are you still sitting too high? It just seems odd to me that could be the case but give all the extra weight and the added speed would certainly seem to be the case.

What is the motor weight compared to mine?

My slip is down around 8-9% and I can't swing the big wheels.
I'm on the verge of getting ready to order a 1" shortie and a 1/2" spacer at this point and I'm at 4.5" to CL of the prop shaft.

Could I be sitting that low, no way..

Southocg,

I think it also has to do with the balance of the boat. My boat is a bow-rider, and therefore has more weight on the nose than yours. The fact that I can put weight in the rear seat, and go faster, indicates to me that a deeper prop depth might be in order. I need more bow lift for sure. According to Mercury Racings website your Illmore motor weighs about 200 + pounds less than my 600sci. But you also have a wood transom, and I have a composite transom, which is lighter. Your boat has got to be lighter overall, but probably has a greater percentage of that weight at the rear of the boat, pushing your prop deeper in the water. You also have more horsepower, and therefore are able to lift the nose of the boat easier, which also puts the ass end down more while running. Steve Z explained to me that it is like a teeter-totter, with the pivot point being the step in the hull.

I am also going to try a P5x at Steve Z's recommendation. What was your experience with the P5x ?

Chris

P.S. I definately feel that you need to go up 1/2 inch

I'll give you a call later today.

Jim

Update:

My best speed to date has been 81.6 with a 30 B spinning at 5100 with weight in the ass end. (four skinny ladies)

Tried a 27 P5x today and went 79.7 right at 5400 rpm. Really liked the way the P5x carried the boat. My speed at 3k and 4k rpm was up about 3-4 mph. I am going to try a 28 p5x which should bring my rpm down to about 5200, and gain some more speed.

Chris

P. S. still thinking about dropping the drive another .5 over the 1" spacer I have installed because the boat picked up 1.5 - 2 mph with weight in the ass end when spinning the 30 B. Best speed with a light load and the 30 B was 80 mph.

I'll try to call you tonight. I have been at Powell and Havi for the last 2 weeks. But got your message.

Good talking to ya,

Chris

Interesting thread.

My prop depth was too deep. I recently changed to an IMCO shorty.
My boat seems to now run as fast with 3 people + fuel in the boat as it does when I'm solo on fumes.
This wasn't the case before.

I've heard of people adding weight in the back of the boat on either side of the motor.
I can see how it would change the balance, help keep the rear down, and let the bow fly a bit more.
Haven't tried it myself yet, but is an interesting concept.

I'm in a 26 outlaw with a 600sci.
With no steps, I really need the bow up to run.